CA1038976A - Time division multiple access satellite communications system - Google Patents
Time division multiple access satellite communications systemInfo
- Publication number
- CA1038976A CA1038976A CA219,732A CA219732A CA1038976A CA 1038976 A CA1038976 A CA 1038976A CA 219732 A CA219732 A CA 219732A CA 1038976 A CA1038976 A CA 1038976A
- Authority
- CA
- Canada
- Prior art keywords
- time
- station
- stations
- satellite
- delay
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
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Classifications
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B7/00—Radio transmission systems, i.e. using radiation field
- H04B7/14—Relay systems
- H04B7/15—Active relay systems
- H04B7/204—Multiple access
- H04B7/212—Time-division multiple access [TDMA]
- H04B7/2125—Synchronisation
- H04B7/2126—Synchronisation using a reference station
Abstract
TIME DIVISION MULTIPLE ACCESS
SATELLITE COMMUNICATIONS SYSTEM
Abstract of the Disclosure A satellite communications system in which messages from a plurality of stations are received at a synchronous satellite in time multiplex without overlapping. The system is so arranged that the apparatus to insure absence of time overlap in received messages at the satellite is relatively simple compared with the prior art closed-loop synchronizing systems. In particular the guard time is controlled to be at least equal to the double hop propagation delay variation eliminating the need for accurate measurement of propagation delay. Furthermore, as a result of the absence of critical propagation delay measurement, the acquisition process does not requires special purpose apparatus.
SATELLITE COMMUNICATIONS SYSTEM
Abstract of the Disclosure A satellite communications system in which messages from a plurality of stations are received at a synchronous satellite in time multiplex without overlapping. The system is so arranged that the apparatus to insure absence of time overlap in received messages at the satellite is relatively simple compared with the prior art closed-loop synchronizing systems. In particular the guard time is controlled to be at least equal to the double hop propagation delay variation eliminating the need for accurate measurement of propagation delay. Furthermore, as a result of the absence of critical propagation delay measurement, the acquisition process does not requires special purpose apparatus.
Description
Satellite communications systems have now been in use for a number of years. In such a system, an earth station transmits to a synchronous satellite which receives the message and retransmits it back to another earth station. The satellite then acts as a transponder in a communication link. Due to the effort and expense involved in placing a satellite into orbit it is generally desired that such satellites be utilized to the maximum. As a result, systems have been envisioned in which a number of earth stations can communicate with a satellite in a time multiplexed mode of operation. Although a satellite may contain a number of transponders we will, hereinafter, consider only one such transponder or a satellite with only one trans-ponder. In the time division multiple access ~TDMA) mode a number of earth stations can sequentially transmit to the satel-lite. Each station transmits in a burst mode. If these messa~e bursts are received at the satellite sequentially, with no over-lap, they may be retransmitted by the satellite without garblin~
or distortion. It should, however, be apparent that while it is necessary for the satellite to receive the transmission sequen-tially, this does not necessarily mean that the earth stations must transmit in a like manner. Due to the different propagation delays from different earth stations to the same satellite, the relative transmission periods may be different when viewed from an earth station. And of course, if the multiple transmissions from multiple éarth stations overlap in time when they are re- `
; ceived at the satellite, the result will be garbled and distorted ; at least to the extent of the time overlap. The prior art recog-nized this problem and has solved it to the extent that TDMA
communications systems are in operation. One such solution is 3~ disclosed in Gabbard patent 3,562,432 for "Synchronizer for Time Division Multiple Access Satellite Communications System". As disclo~d in the afore-mentioned patent, each ground station receives a reference or marker signal and its own trans~ission period or time slot is determined by delaying a predetermined amount from receipt of the reference marker. Furthermore, the afore-mentioned system recognizes that due to differences in propagation delays between widely spaced earth stations, errors may be introduced into the system. As a result, a feedback arrangement is provided whereby each earth station receives not only the reference marker but also its own transmissions. In this way the propagation delays from that earth station to the satellite can be accurately measured and appropriate corrections made. Furthermore, to pro~ide for tolerances in the system, a guard time is placed into the cyclical time frame during which time, as viewed at the satellite, no messages are received. Any departure from nominal conditions, up to the extent of the quard time, is absorbed with no resulting message overlaps. ~ecause such a synchronization system requires feedback from the satel~ite for its error corrections other apparatus is required to perform the acquisition process. At the time acquisition is initiated, of course, the ground station will have no feedback signal on which to base corrections. Therefore, a special low level signal is transmitted so as not to interfere with trans-missions from other earth stations. ~fter receipt of the special low le~el signal at the satellite it is retransmitted and recep-tion at the transmitting station then enables the synchronizing apparatus to make the appropriate corrections so that when transmission is initiated the message signal will not overlap in time any ot~er message signal recei~ed at the satellite.
Since these systems must necessarily measure propagation delays in the nanosecond range they are quite sophisticated and compli-cated.
More particularly the prior art TDMA systems operate in a cyclical time frame wherein a plurality of messaqe bursts . - ' .
or distortion. It should, however, be apparent that while it is necessary for the satellite to receive the transmission sequen-tially, this does not necessarily mean that the earth stations must transmit in a like manner. Due to the different propagation delays from different earth stations to the same satellite, the relative transmission periods may be different when viewed from an earth station. And of course, if the multiple transmissions from multiple éarth stations overlap in time when they are re- `
; ceived at the satellite, the result will be garbled and distorted ; at least to the extent of the time overlap. The prior art recog-nized this problem and has solved it to the extent that TDMA
communications systems are in operation. One such solution is 3~ disclosed in Gabbard patent 3,562,432 for "Synchronizer for Time Division Multiple Access Satellite Communications System". As disclo~d in the afore-mentioned patent, each ground station receives a reference or marker signal and its own trans~ission period or time slot is determined by delaying a predetermined amount from receipt of the reference marker. Furthermore, the afore-mentioned system recognizes that due to differences in propagation delays between widely spaced earth stations, errors may be introduced into the system. As a result, a feedback arrangement is provided whereby each earth station receives not only the reference marker but also its own transmissions. In this way the propagation delays from that earth station to the satellite can be accurately measured and appropriate corrections made. Furthermore, to pro~ide for tolerances in the system, a guard time is placed into the cyclical time frame during which time, as viewed at the satellite, no messages are received. Any departure from nominal conditions, up to the extent of the quard time, is absorbed with no resulting message overlaps. ~ecause such a synchronization system requires feedback from the satel~ite for its error corrections other apparatus is required to perform the acquisition process. At the time acquisition is initiated, of course, the ground station will have no feedback signal on which to base corrections. Therefore, a special low level signal is transmitted so as not to interfere with trans-missions from other earth stations. ~fter receipt of the special low le~el signal at the satellite it is retransmitted and recep-tion at the transmitting station then enables the synchronizing apparatus to make the appropriate corrections so that when transmission is initiated the message signal will not overlap in time any ot~er message signal recei~ed at the satellite.
Since these systems must necessarily measure propagation delays in the nanosecond range they are quite sophisticated and compli-cated.
More particularly the prior art TDMA systems operate in a cyclical time frame wherein a plurality of messaqe bursts . - ' .
- 2 -- ~ .. ~, - : . . . , ~ :
' ' " ~
10389'76 each from a different station are interleaved at the satellite.
Since the guard times are period~ of non-transmission, system efficiency can be defined as the ratio of burst duration to the sum of burst duration and associated guard time.
To achieve high efficiencies the guard time was re-duced to the extent such reduction was possible in view of sys-tem tolerances.
These foregoing systems operate quite well in their intended en~ironment, however, due to the complexity and asso-10 ciated cost thereof, they are restricted to ground stations in which hea~y traffic may be expected so as to justify the compli-cated synchronizing apparatus. The experience with satellite communication systems, however, has led to a demand for greater use of such communication systems. In particular, there is a demand for "thin" traffic systems. Generally these systems are envisioned in which hundreds, or even thousands, or outlying ground stations can communicate with one or a few main earth stations. In such systems the traffic to or from any one particular outlying ground station may be light and thus the term "thin". The few main stations may well have heavy traffic demands. Since the hundreds or even thousands of outlying stations will generally have light traffic, it is not economically - justifiable to burden each of these stations with its own syn- `~
chronizing and acquisition apparatus as referred to above.
~he present invention provides a TDMA satellite com-munications system in which hundreds or even thousands of out- ;- `
lying stations can communicate with one or a few main earth ~ ;
stations. In particular, the present invention is directed at ;
a method of, and apparatus for, insuring that the multiple transmissions do not overlap in time when received at the syn-ch~onous satellite. Furthermore, the present invention accom-plishes the foreg~ing without the necessity for complic~ted, '~ :
~ 3 ~
~r . . . - - ~ .
. - . , - ~ , . ., -~038976 costly and sophisticated synchronization and acquisition ap-paratus. To the contrary, however, the apparatus which is pro-vided to insure the absence of time overlap of signals at the satellite is relatively simple. Simply stated, applicants have recognized that the maximum propagation delay variation for any particular system can be quantified that is, it can be predicted.
In mobile thin route stations the delay variation is due to the unknown geographic location of the mobile station (e.g., planes and ships) plus the 24-hour variations in geostationary satellite position relative to any earth reference. In networks of geographically-fixed stations only the satellite 24-hour motion effect contributes to the variation since the average geographic effect can be predicted and accounted by fixed delay compensa-tion particular to each station location. In a thin traffic system each of the stations will be operating at relatively low transmission rates and therefore the propagation delay variation can be absorbed by increasing the guard time between message bursts. In this manner the timing apparatus at each of these stations insures that no time overlap of transmission will occur at the satellite. In effect, the system is open-loop, in that it does not require feedback for its operation. In this manner the complicated and sophisticated precise time measuring circuit-ry can be eliminated. ~ -More particularly in the system of the present invention ~-a master station communicates with a plurality of earth stations through a satellite relay. The master station time frame con-sists of a signaling time slot including a unique word or refer-ence marker, two or more digitized voice channels and a plurality, such as eighty, teletype channels. Master station signaling transmissions may be recei~ed at all stations. Each of the out-lying stations is capable of transmittin~ on a voice channel or on one of the teletype channels. lt should be understood that - 4 ~
!
, -more than one master station may be employed and the number of voice and teletype channels may be varied as will be explained hereinafter. Since a number of stations may be operating simul-taneously on teletype transmissions a TDMA scheme is used. Each station transmits in a burst mode and the apparatus of the present invention insures the message bursts will interleave and not overlap when received at the satellite relay. The master station signaling time slot makes available to all outlying stations the identity of the unused teletype time slots. Each : 10 of the teletype time slots is defined as beginning a predeter-- mined delay after receipt of a reference marker. As will be understood by those skilled in the art the predetermined delay is different for each of the different time slots. Each time slot also includes a guard time and the length of the guard , time is determined to insure that the message bursts from the different stations interleave at the satellite relay.
For a nominal system, with equal propagation delay from satellite to all stations and under ideal conditions, no guard time is required. However, the variation in propagation delay from satellite to closest station as compared to propaga-- tion delay from satellite to furthermost station means that the ~ -furthermost station recei~es the reference marker ~ milliseconds after the closest station does. If both stations times from the marker the furthermost station would transmit "late".
Furthermore the time for the message burst to reach the satellite relay from the furthermost station is ~ milliseconds longer than -from the closest stations introducing a further variation of .
- Thus if the two stations were transmitting in adjacent time slots with no guard time, the furthermost station's message burst would o~erlap 2 milliseconds into the message burst of the ; closest station. To pre~ent this a guard time is introduced with a duration at least equal to 2 milliseconds where is the maximum propagation delay variation from the satellite to the plurality of stations.
The acquisition of a time slot is simplified by reason - of the fact that each station desiring to transmit has informa-tion available as to which time slots are available, if any. To acquire that time slot it is only necessary for the station to transmit, in the chosen time slot, its identity and the informa-tion that is required to acquire that time slot. If it is still available when the signal is received at the master station the time slot assignment will be made and the station will receive a go ahead signal. If, by chance, two stations simultaneously attempt to acquire the identical time slot their requests will overlap, be garbled and no time slot assignment will be made.
However, this is extremely unlikely and is of minor significance for the stations may then attempt to obtain other time slots.
It will be appreciated that the special acquisition apparatus has - been eliminated.
Although eliminating the precise time measuring equip-ment and special acquisition apparatus is desirable this must not ; occur at the expense of rendering the system uneconomic. The --primary economic factor aside from equipment cost is efficiency which has previously been defined. To insure reasonable efficiencies the message burst is timed to be at least equal to the guard time. To recapitulate, each transmitting station is assigned a time slot of duration T. The station transmits a message burst of duration tB with a guard time G such the T =
tB + G and G =, 2 where F is the propagation delay ~ariation.
Furthermore, tB ~ G to maintain reasonable efficiencies. An upper bound is placed on tB for the following reasons. Since a -
' ' " ~
10389'76 each from a different station are interleaved at the satellite.
Since the guard times are period~ of non-transmission, system efficiency can be defined as the ratio of burst duration to the sum of burst duration and associated guard time.
To achieve high efficiencies the guard time was re-duced to the extent such reduction was possible in view of sys-tem tolerances.
These foregoing systems operate quite well in their intended en~ironment, however, due to the complexity and asso-10 ciated cost thereof, they are restricted to ground stations in which hea~y traffic may be expected so as to justify the compli-cated synchronizing apparatus. The experience with satellite communication systems, however, has led to a demand for greater use of such communication systems. In particular, there is a demand for "thin" traffic systems. Generally these systems are envisioned in which hundreds, or even thousands, or outlying ground stations can communicate with one or a few main earth stations. In such systems the traffic to or from any one particular outlying ground station may be light and thus the term "thin". The few main stations may well have heavy traffic demands. Since the hundreds or even thousands of outlying stations will generally have light traffic, it is not economically - justifiable to burden each of these stations with its own syn- `~
chronizing and acquisition apparatus as referred to above.
~he present invention provides a TDMA satellite com-munications system in which hundreds or even thousands of out- ;- `
lying stations can communicate with one or a few main earth ~ ;
stations. In particular, the present invention is directed at ;
a method of, and apparatus for, insuring that the multiple transmissions do not overlap in time when received at the syn-ch~onous satellite. Furthermore, the present invention accom-plishes the foreg~ing without the necessity for complic~ted, '~ :
~ 3 ~
~r . . . - - ~ .
. - . , - ~ , . ., -~038976 costly and sophisticated synchronization and acquisition ap-paratus. To the contrary, however, the apparatus which is pro-vided to insure the absence of time overlap of signals at the satellite is relatively simple. Simply stated, applicants have recognized that the maximum propagation delay variation for any particular system can be quantified that is, it can be predicted.
In mobile thin route stations the delay variation is due to the unknown geographic location of the mobile station (e.g., planes and ships) plus the 24-hour variations in geostationary satellite position relative to any earth reference. In networks of geographically-fixed stations only the satellite 24-hour motion effect contributes to the variation since the average geographic effect can be predicted and accounted by fixed delay compensa-tion particular to each station location. In a thin traffic system each of the stations will be operating at relatively low transmission rates and therefore the propagation delay variation can be absorbed by increasing the guard time between message bursts. In this manner the timing apparatus at each of these stations insures that no time overlap of transmission will occur at the satellite. In effect, the system is open-loop, in that it does not require feedback for its operation. In this manner the complicated and sophisticated precise time measuring circuit-ry can be eliminated. ~ -More particularly in the system of the present invention ~-a master station communicates with a plurality of earth stations through a satellite relay. The master station time frame con-sists of a signaling time slot including a unique word or refer-ence marker, two or more digitized voice channels and a plurality, such as eighty, teletype channels. Master station signaling transmissions may be recei~ed at all stations. Each of the out-lying stations is capable of transmittin~ on a voice channel or on one of the teletype channels. lt should be understood that - 4 ~
!
, -more than one master station may be employed and the number of voice and teletype channels may be varied as will be explained hereinafter. Since a number of stations may be operating simul-taneously on teletype transmissions a TDMA scheme is used. Each station transmits in a burst mode and the apparatus of the present invention insures the message bursts will interleave and not overlap when received at the satellite relay. The master station signaling time slot makes available to all outlying stations the identity of the unused teletype time slots. Each : 10 of the teletype time slots is defined as beginning a predeter-- mined delay after receipt of a reference marker. As will be understood by those skilled in the art the predetermined delay is different for each of the different time slots. Each time slot also includes a guard time and the length of the guard , time is determined to insure that the message bursts from the different stations interleave at the satellite relay.
For a nominal system, with equal propagation delay from satellite to all stations and under ideal conditions, no guard time is required. However, the variation in propagation delay from satellite to closest station as compared to propaga-- tion delay from satellite to furthermost station means that the ~ -furthermost station recei~es the reference marker ~ milliseconds after the closest station does. If both stations times from the marker the furthermost station would transmit "late".
Furthermore the time for the message burst to reach the satellite relay from the furthermost station is ~ milliseconds longer than -from the closest stations introducing a further variation of .
- Thus if the two stations were transmitting in adjacent time slots with no guard time, the furthermost station's message burst would o~erlap 2 milliseconds into the message burst of the ; closest station. To pre~ent this a guard time is introduced with a duration at least equal to 2 milliseconds where is the maximum propagation delay variation from the satellite to the plurality of stations.
The acquisition of a time slot is simplified by reason - of the fact that each station desiring to transmit has informa-tion available as to which time slots are available, if any. To acquire that time slot it is only necessary for the station to transmit, in the chosen time slot, its identity and the informa-tion that is required to acquire that time slot. If it is still available when the signal is received at the master station the time slot assignment will be made and the station will receive a go ahead signal. If, by chance, two stations simultaneously attempt to acquire the identical time slot their requests will overlap, be garbled and no time slot assignment will be made.
However, this is extremely unlikely and is of minor significance for the stations may then attempt to obtain other time slots.
It will be appreciated that the special acquisition apparatus has - been eliminated.
Although eliminating the precise time measuring equip-ment and special acquisition apparatus is desirable this must not ; occur at the expense of rendering the system uneconomic. The --primary economic factor aside from equipment cost is efficiency which has previously been defined. To insure reasonable efficiencies the message burst is timed to be at least equal to the guard time. To recapitulate, each transmitting station is assigned a time slot of duration T. The station transmits a message burst of duration tB with a guard time G such the T =
tB + G and G =, 2 where F is the propagation delay ~ariation.
Furthermore, tB ~ G to maintain reasonable efficiencies. An upper bound is placed on tB for the following reasons. Since a -
3~ station transmits at a constant rate ~during actual trans-mission) the information content of a message burst is directly proportional to message ~urst ~uration tB. Each station must be ... ~ ":
provided with apparatus to store an amount of information equal to at least the informational content of a message burst.
Therefore increasing message burst duration tB directly in-creases the extent of storage apparatus required at each station.
As a result a balance must be maintained between the competing interests of maximizing efficiency while minimizing the extent of the required storage apparatus and to achieve this we prefer to have tB 3 G = 2~. It should be understood that the guard time G may be increased over 2 (although 2~ is a minimum) and that tB may be less than or exceed G (varying the efficiency of the system) within the scope of the in~ention.
In describing the present in~ention is preferred em-bodiment will be disclosed comprising a mobile communications system in which one main earth station can communicate with hundreds or even thousands of outlying earth stations each loca-ted aboard a vehicle or movable platform (e.g., ship, plane, or oil-drilling rig). As the description proceeds it will become apparent to those skilled in the art that many modifications can be made i~ the preferred embodiment, that is, a number of main earth stations may be empl~yed, the outlying stations need not be aboard vehicles but may be located on ground (along an oil pipeline, for example), the particular format of time frames may be varied as well as the operating rates of the different trans-mitters and communications links. Further, other ~ariations not specifically mentioned will also occur to those skilled in - the art. --- Brief Description of the Drawings In describing a preferred embodiment of the present invention which is disclosed in this application reference will be made to the accompanying drawings ta which like reference ~ -characters identify identical apparatus and ~n which:
Figure 1 represents a preferred master station . - . . - . - , - . . . .
-- ~038976 cyclical time frame;
Figure 2 is a representation of a preferred TDMA cycli-cal time frame; and Figure 3 is a block diagram of a portion of the appar-- atus at an outlying station, including the apparatus of the pre-sent invention which insures proper interleaving of message - -bursts.
Detailed Description of the Invention Figure 1 illustrates a preferred embodiment of the master station time frame. This time frame consists of four portions, 11 through 14, some of which are themselves subdivided into further portions. The master station transmits a 2-phase .
PSK at a 48,000 bits per second rate in a time division multi-plex mode. The 48,000 bits per second consist of 8,000 bits of network signaling, portion 11; 16,00~ bits of PCM voice channel A, portion 12; 16,000 bits of PCM voice channel B, portion 13;
and finally 8,000 bits of teletype, portion 14. This teletype data portion of the time frame may be preferably subdivided into 80 channels of each 100 bits.
Referring now to portion 11, the signaling portion, it will be seen from Figure 1 that the initial portion llA com-prises a unique word of N bits. Although the unique word llA ~
is illustrated as being "lumped", that is a consecutive block ~ '~r of N bits, it may well be distributed, one bit per frame for N
frames. However pro~ided, a unique word defines a TDMA time re-ference. This unique word requires the same auto correlation, cross-correlation and redundency properties needed for previous closed-loop TDMR synchronization systems. In systems where there is only one master station, no multiplexing control is re-quired for the master stations other than the contro~ apparatus which provides the format illustrated in Figure 1. ~urthermore, the prior art contains adequate teachings of suitable control - 8 ~
.
'' , apparatus to provide a message format as illustrated in Figure 1.
In situations where there are two or more master stations it may become necessary to employ the apparatus of the present invention when the transmission of the master stations are to be received in TDMA by the same satellite transponder. The appara-tus of the present invention which insures no time overlap of message bursts at the satellite will be described with reference to Figure 3. ~he apparatus at the master station which provides for network signaling and which makes channel and time slot assignments is conventional and need not be described here. See, - for instance, Puente et al, U.S. patent 3,564,197.
Figure 2 illustrates the outlying station message burst format, Whereas the time frame of the master station had a one second period, the TDMA teletype signaling time frame is 8 seconds in duration. During this 8~second period of time a maximum of 80 message bursts is provided each in its own time slot. Each of the B0 time slots then is 100 milliseconds in duration. Altho~gh there are 80 time slots available in the TDMA system, as is well known to those skilled in the art, less than 80 stations may be transmitting simultaneously. The re- -maining time slots would then be unused and available for a station that desires to transmit. Each time slot is shown as comprising three portions, a guard time 15, a preamble 16 and a message 17. In a preferred embodiment the outlying station transmission can comprise 2 16 kilobits/second modulated carriers.
One carrier is continuous mo-de voice and the other is a 16 K
bits/s TDMA c~annel carrying 80 independent 100 bit per second channels which are used for signaling and/or teletype traffic.
The format for the continuous mode voice channel at each of the outlying stations is not illustrated as it forms no part of the present inventio~ and is convent~onal. ~he ~DM~ channel, in particular, the apparatus for insuring that the various message ~; .
bursts from the plurality of outlying stations are received at - the satellite in non-overlapping and interleaved relation is the apparatus which forms the present invention. This apparatus will be described more particularly with reference to Figure 3.
Utilizing the 16 kilobit per second rate, as an ex-ample, a preamble portion 16 which is used for modem timing recovery and unique word transmission comprises 160 bits. The message burst comprises 800 bits. For purposes of description we may consider tB to be the duration of the preamble and the message burst.
The preferred format, that is, an 8-second time frame pro~ides a reasonable balance between two competing con-siderations. In the first place, once the burst duration is -fixed, increasing the time frame increases the number of time slots a~ailable for distribution. ~owever, if the number of time slots is increased by increasing the period of the time frame, then the bit rate must also be increased if the system is to serve a 100 bit per second teletype channel. This increases the storage apparatus required at each station. On the other hand, if the time duration of the time frame is required to one second then, for example, with the same bit rate and message burst duration, only 10 time slots are available for distribution.
The ~-second time frame with the 800 bit message burst and 50 millisecond message burst duration is a reasonable compromise.
Those with ordinary skill in the art will understand how the message burst duration, bit rate and number of time slots can be varied to suit the needs of the communications system. It should be understood, however, that the present invention is not res-; tricted to the exemplary showings of Figure 2.
3~- Figure 3 illustrates the apparatus of the present in-vention which provides that the plurality of message bursts from p~ur~lity of outlying stations will be recei~ed at the 1~-,:
10;~8976 satellite interleaved and non-overlapped time relationship. Fig-ure 3 illustrates the apparatus at each of the stations which ~;
controls the messaye transmissions so as to achieve the objects of the present invention. Each outlying station comprises, in a receiving portion, an RF channel receiver 39, and a reference unique word detector 21 which is coupled to the demodulator 40.
The demodulator 40 also feeds the received data to conventional receiver apparatus (not shown). An OR gate 22 couples the re-ference unique word detector 21, to a burst delay counter 23.
The reference unique word dete~tor 21 when detecting receipt of a unique word produces a sync pulse, which is coupled through OR gate 22 to reset burst delay counter 23. Burst delay counter 23, which may be a conventional multistage counter, receives in addition to the sync pulse, a scaled control clock pulse train -from pre-scaler 24. A control clock at the outlying station feeds signals of a predetermined repetition rate to clock pre- - -scaler 24, which is merely a divider. The output of clock-pre-scaler 24 is a pulse train of scaled control clock pulses at a repetition rate which bears a predetermined ratio with the repetition rate of the control clock pulses. The scaled control clock pulses operate burst delay counter 23. A plurality of stages of burst delay counter 23 feed a decoder 25. Decoder 25 operates in response to a particular combination of output siqnals and provides, when such a combination exists, a start- ` ~ -of-burst pulse to flip-flop 32 and burst duration counter 33 ~urst duration counter 33 may be a conventional multistage counter. The~~particular output configuration of burst delay counter 23 that decoder 2~ operates in response to will be described in more detail below. Burst delay counter 23 also provides an output to delay modulo detector 30. Delay modulo detector 30 responds to a different combination of signals from burst delay counter 23. When th; latter combination of signals . :
is present, the delay modulo detector 30 provides an output pulse to OR gate 22 which is capable of resetting burst delay counter 23 in the same manner as the sync pulse resets it.
Flip-flop 32, which receives the start-of-burst pulse from decoder 25, provides an output to modulator 36 and data buffer and control 35. The burst duration counter 33 also re-ceives the start-of-burst pulse and is reset thereby. A decoder 38 monitors the changing configuration of the output of burst duration counter 33 and, in response to a predetermined output combination pro~ides a reset signal to flip-flop 32. The burst duration counter 33 is operated by the same scaled control clock pulses which operate the burst delay counter 23.
Data buffer and control 35 receives three inputs in addition to the input from flip-flop 32. Data buffer and con-trol 35 receives continuous data from a 100 bit per second tele-type channel. Data buffer and control 35 also receives a con-tinuous clock to clock into the buffer the continuous data re-ferred to above. As is well known by those skilled in the art, the data buffer and control has the continuous data read in at a rate of 100 bits per second from the conventional teletype channel. Howe~er, when transmitting, data is read out of the data buffer and control 35 at a much higher rate, in this pre-ferred embodiment at 16,000 bits per second. Therefore, data buffer and control 35 also receives a transmit burst clock at the 16,000 bit per second rate. Information pulses when read out of data buffer and control 35 are pro~ided to modulator 36.
Msdulator 36 also recei~es the 16,000 bit per second burst cloc~ pulses and pro~ides the modulated output to transmit RF
channel 37 which couples the modulated data signals through an antenna for transmission to satellite 34.
~ he operation of the apparatus thus far referred to is as follows; A master station transmission, whose format is - 12 ~ .
illustrated in Figure 1, is received by RF channel 39 and de-modulated in demodulator 40. These components are conventional in the art and more detailed descriptions thereof are deemed unnecessary. The refexence unique word detector 21 detects the presence of unique word llA and provides a sync pulse responsive thereto. A sync pulse coupled through OR gate 22 resets burst delay counter 23. Burst delay counter then counts in response to scaled control clock pulses from clock pre-scaler 24. When burst delay counter 23 reaches a count corresponding to the count to which decoder 25 is responsive, decoder 25 produces a start-of-burst pulse. This pulse has two effects. In the first place, this pulse sets flip-flop 32 to provide an output to modulator 35 and data buffer and control 35. This output marks the begin-ning of the burst window and the beginning of the message burst.
At the same time, burst duration counter 33 is reset and it begins to count in response to scaled control clock pulses from clock pre-scaler 24. When the output of burst duration counter -33 matches that of decoder 38, an output is provided to reset flip-flop 32. This action removes the signal from the Q output .~ .. . , ~ . .
of flip-flop 32 thus terminating the burst window and terminating the message burst from the particular station involved.
Delay modulo decoder 30 is also permanently connected to burst delay counter 23 so as to respond to a count in burst delay counter 23 corresponding to 1000 milliseconds or 1 second.
When delay modulo decoder 30 receives a counter from burst delay counter 23 corresponding to 1 second it produces an output sig-nal, which is~coupled throu~h OR gate 22 to reset the burst de-lay counter 23. Should, for some reason, the unique word refer-ence mar~er not be received or not be properly decoded by refer-ence unique word detector 21, theoutput of delay modulo decoder 30 will serve to reset burst delay counter 23. ~he decocer 30 is responsive to a count corresponding to 1 second corresponding - 13 _ , .' . '' ,.
- : : , i. ~ ,, -, . .
to the master station time frame period. Those skilled in the art will understand how decoder 30 may respond to any count corresponding to the master station time frame period.
It should be apparent that the particular configuration to which decoders 25 and 38 respond respectively control the initiation and termination of the transmission. Thus, as one example, the decoder 25 can be arranged to respond to burst delay counter 23 counting up to the equivalent of 40 millise-conds to initiate transmission for a first time slot. Decoder 38 can be arranged to respond to a count in burst duration counter 33 corresponding to 60 milliseconds. The 60 millise-conds comprise the sum of the preamble portion 16 and message portion 17. For the next time slot decoder 25, at another station, is arranged to respond to a burst delay counter 23 out-put corresponding to 140 milliseconds and decoder 38 responds to a count corresponding to 60 milliseconds. For a further time slot the decoder 25, at still another station, is arranged to respond to a count in burst delay counter 23 corresponding to 240 milliseconds. And, in general, for time slot N the de-coder 25 is arranged to respond to a count in burst delay counter23 corresponding to l(N-l)lO0 + 40l milliseconds. Each decocer 38 is arranged to respond to a count in burst duration counter 33 corresponding to 60 milliseconds. Since burst delay counter can only count up to a count corresponding to 1 second, N may then vary from l to lO.
In actual implement~tion, each of decoders 25 and 38 may be A~D ga~es connected to the burst delay counter 23 and burst duration counter 33, respectively. The decoder 38 may be so connected to the respective stayes of burst duration counter 33 so as to respond to a count corresponding to 60 milliseconds.
~he decoder 25 may be connected to burst delay counter 23 through manually controlled switching contacts selectively operable to .' -. . , :
:
engage the various staqes of burst delay counter 23 so as to respond to a count corresponding to [(N-1)100 + 40] milliseconds where N can be in the range from l to l0.
Other apparatus may be provided to selectively connect decoder 25 to the various tages of burst delay counter 23 as re-cited above, in response to time slot assignments made by the - master station and transmitted during master station time frame ~ -- 11. Such apparatus is well known to those skilled in the art and a detailed disclosure thereof is deemed unnecessary. See for instance U.S. patent 3,564,147.
; The apparatus described above is sufficient for those . . .
cases where the TDMA frame illustrated in Figure 2 has a dura- -tion which is less than or equal to the master station time frame, illustrated in Figure 1. As described above, however, the TDMA time frame illustrated in Figure 2 has a duration of 8 ~ -seconds whereas the master station time frame has a duration of 1 second. Therefore, the TDMA time frame cycles once for every 8 cycles of the master station time frame. In order to properly detect every eighth master frame, super frame detector 26, OR
qate 27, frame counter 28, decoder 29, and frame modulo decoder 31 are pro~ided. Super frame detector 26 is connected to the modulator 40 as is reference unique word detector 21. An output - . .
of super frame detector is coupled through OR gate 27 to reset frame counter 28. Frame counter 28 may be a conventional multi- '~
stage counter. Frame counter 28 is operated by the scaled control c~ock pulses from clock prescaler 24, A decoder 29 is connected to separate stages of frame counter 28 so that it can respond to a predetermined count of frame counter 28. The manner in which decoder 29 is connected so as to respond to the predeter- ' mined count, and the manner in which the predetermined count is determined will be explained later. Upon reaching this prede-term~noa count, however, the output of decoder 29 ~ pro~ided `.:
:
. ' ;' ', ..
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~038976 as an input to decoder 25.
Frame modulo decoder 31 is also connected to frame counter 28 to respond to a different predetermined count of frame counter 28. When frame modulo decoder 31 detects this predetermined count it provides an input to OR gate 27 which ; services to reset frame counter 28 in the same manner that the super frame detector 26 output causes frame counter 28 to be re-set.
The manner in which decoder 25 and decoder 29 cooper-ate to select a predetermined time slot will now be explained. ~ -Decoder 25, being controlled by burst delay counter 23 which is reset at every period of the master station time frame, is there-fore reset every second. Decoder 25 may then select one of the 10 time slots occurring in any one second period of time. De-coder 29 selects one of the 8 groups of 10 time slots which occur in a single TDMA time frame. Thus, for instance, if a station is to transmit within time slot 14, decoder 25 would be set to respond to the count corresponding to N equals 4, and decoder 29 - would be set to respond to a count corresponding to the second of the 8 frames. Alternatively, if a station were to transmit in time slot 44, the decoder 25 would be arranged to respond to the same count corresponding to N equals 4, and the decoder 29 would be arranged to correspond to a count corresponding to the fifth one second interval of time subsequent to the super fra~e detector output. The manner in which decoder 29 is arranged to respond to the different predetermined counts can be the same as that ex-plained for de-coder 25, that is, ani AND gate hard-wi-res to switching contacts which are manually settable. Alternatively, this connection can be automatically made by electronic switching units responsive to signal impulses from the master station, as i~ well known in the art. If apparatus is aYaila~le to automati-cally set decoders 25 and 29 it is tben possible to also ... .
~ .`' ''' :. - - ~.. ~ .
~038976 automatically set decoder 38. The advantage thus gained in that the message burst duration may then be variable for different time slots. Thus the output of decoder 29 forms one input to decoder 25, such that decoder 25 produces an output if a signal is received on each of its inputs including the input from decoder 29.
Certain other apparatus would also be preferably used, although not specifica?ly illustrated. For instance, the delay modulo detector 30 and frame moduls detector 31 can supply miss-ing reference unique words or super frame words if that becomesnecessary. However, if the reference unique word or super frame words are absent for an extended period of time the outlying station at which these signals are not received may become out of sync due to variations in locally generated clock signals as opposed to the master station clock. To this end, a counter may be arranged so as to count the instances of missing unique words or super frame words and to shut down this station if a prede- -termined count is exceeded. Of course, if during the counting process a unique word or super frame word is received, then the counter is reset.
In order to acquire a time slot, in those cases where the outlying stations decoders 25 and 29 are manually selectable to different time slots, the operator must have information as to which time slots are available. To this end, the master ~tation, within time frame portion 11, transmits information as to the status of each of the time slots. A decoder at each of the outlying stations makes this information visiblé by indicator 7ights or otherwise. Therefore, when a station desires to transmit, the operator merely properly sets decoder 25 and 3~ decoder 2g to se~ect a time slot that is available. The data -buffer and control 35 has permanently stored therein information corresponding to the preamble portion 16 of the message burst.
.' ~-- - - , . . . . .
-:. .. . . - -;, . ~: -- .
This includes the station identity. Thus, when the operator selects the settings for decoder 25 and 29, the station identity is sent in the selected time slot. If no other station has - requested this particular time slot the master station providesa go ahead signal which is received and decoded at the outlying station. When the go ahead signal is received it is only then necessary to connect the continuous data line to the data source and transmissions will occur automatically in the proper time slot.
With the apparatus thus far explained, the decoders 25, 29 and 38 will provide that in any 100 millisecond time slot, a station may transmit in a 60 second portion thereof. ~hus, the - guard time of 40 milliseconds is provided. In the foregoing description, the guard time occurs during the first 40 milli-seconds of the station's 100 millisecond time slot, and the infor-mation transmission or message burst occurs during the later 50 millisecond portion. However, it is within the scope of the present invention to provide the guard time in the last 4~ milli-second period of the time slot and the message burst in the first 60 millisecond portion. To this ena, the decoders 25 and 29 may be arranged to respond to a count corresponding to (N-l) 100 milliseconds. In combination with the decoder 38 arranged to allow transmission for only 60 milliseconds, the 40 millisecond guard time will be provided in the last 40 milliseconds of the 100 millisecond time slot. Although~ as has been explained above, the guard time may occur either at the beginning or the end of the time slot~ it 1s essential that the arrangement throughout a partic~lar system be uniform. Although the present application - utilizes as e~emp~ary a 10~ millisecond time slot with a 40 millisecond guard time, those skilled in the art will understand that the time slot duration, TDMA time frame and master station time frame may all be varied to suit the particular circumstances.
1();18976 What is essential is that each message burst from each of the - stations has an associated guard time which is no less than the - duoble hop propagation delay variation within the system.
- Furthermore, although the guard time may occur either prior to or subsequent to any message hurst, the location of the guard time in the time slot must be uniform throughout the system.
In operation, the transmission of the various outlying stations are separated by a nominal separation equal to the guard time. This guard time is selected to be at least equal to the double hop propagation delay variation ~ for the system.
A reasonable value for the delay variation ~ is 20 milliseconds and therefore a reasonable period for the guard time is 40 milli-seconds. It should be apparent from the foregoing that with the guard time specified above, the various transmissions from ~ , the outlying stations will not overlap and will interleave at the satellite for a retransmission to the master station.
.:
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provided with apparatus to store an amount of information equal to at least the informational content of a message burst.
Therefore increasing message burst duration tB directly in-creases the extent of storage apparatus required at each station.
As a result a balance must be maintained between the competing interests of maximizing efficiency while minimizing the extent of the required storage apparatus and to achieve this we prefer to have tB 3 G = 2~. It should be understood that the guard time G may be increased over 2 (although 2~ is a minimum) and that tB may be less than or exceed G (varying the efficiency of the system) within the scope of the in~ention.
In describing the present in~ention is preferred em-bodiment will be disclosed comprising a mobile communications system in which one main earth station can communicate with hundreds or even thousands of outlying earth stations each loca-ted aboard a vehicle or movable platform (e.g., ship, plane, or oil-drilling rig). As the description proceeds it will become apparent to those skilled in the art that many modifications can be made i~ the preferred embodiment, that is, a number of main earth stations may be empl~yed, the outlying stations need not be aboard vehicles but may be located on ground (along an oil pipeline, for example), the particular format of time frames may be varied as well as the operating rates of the different trans-mitters and communications links. Further, other ~ariations not specifically mentioned will also occur to those skilled in - the art. --- Brief Description of the Drawings In describing a preferred embodiment of the present invention which is disclosed in this application reference will be made to the accompanying drawings ta which like reference ~ -characters identify identical apparatus and ~n which:
Figure 1 represents a preferred master station . - . . - . - , - . . . .
-- ~038976 cyclical time frame;
Figure 2 is a representation of a preferred TDMA cycli-cal time frame; and Figure 3 is a block diagram of a portion of the appar-- atus at an outlying station, including the apparatus of the pre-sent invention which insures proper interleaving of message - -bursts.
Detailed Description of the Invention Figure 1 illustrates a preferred embodiment of the master station time frame. This time frame consists of four portions, 11 through 14, some of which are themselves subdivided into further portions. The master station transmits a 2-phase .
PSK at a 48,000 bits per second rate in a time division multi-plex mode. The 48,000 bits per second consist of 8,000 bits of network signaling, portion 11; 16,00~ bits of PCM voice channel A, portion 12; 16,000 bits of PCM voice channel B, portion 13;
and finally 8,000 bits of teletype, portion 14. This teletype data portion of the time frame may be preferably subdivided into 80 channels of each 100 bits.
Referring now to portion 11, the signaling portion, it will be seen from Figure 1 that the initial portion llA com-prises a unique word of N bits. Although the unique word llA ~
is illustrated as being "lumped", that is a consecutive block ~ '~r of N bits, it may well be distributed, one bit per frame for N
frames. However pro~ided, a unique word defines a TDMA time re-ference. This unique word requires the same auto correlation, cross-correlation and redundency properties needed for previous closed-loop TDMR synchronization systems. In systems where there is only one master station, no multiplexing control is re-quired for the master stations other than the contro~ apparatus which provides the format illustrated in Figure 1. ~urthermore, the prior art contains adequate teachings of suitable control - 8 ~
.
'' , apparatus to provide a message format as illustrated in Figure 1.
In situations where there are two or more master stations it may become necessary to employ the apparatus of the present invention when the transmission of the master stations are to be received in TDMA by the same satellite transponder. The appara-tus of the present invention which insures no time overlap of message bursts at the satellite will be described with reference to Figure 3. ~he apparatus at the master station which provides for network signaling and which makes channel and time slot assignments is conventional and need not be described here. See, - for instance, Puente et al, U.S. patent 3,564,197.
Figure 2 illustrates the outlying station message burst format, Whereas the time frame of the master station had a one second period, the TDMA teletype signaling time frame is 8 seconds in duration. During this 8~second period of time a maximum of 80 message bursts is provided each in its own time slot. Each of the B0 time slots then is 100 milliseconds in duration. Altho~gh there are 80 time slots available in the TDMA system, as is well known to those skilled in the art, less than 80 stations may be transmitting simultaneously. The re- -maining time slots would then be unused and available for a station that desires to transmit. Each time slot is shown as comprising three portions, a guard time 15, a preamble 16 and a message 17. In a preferred embodiment the outlying station transmission can comprise 2 16 kilobits/second modulated carriers.
One carrier is continuous mo-de voice and the other is a 16 K
bits/s TDMA c~annel carrying 80 independent 100 bit per second channels which are used for signaling and/or teletype traffic.
The format for the continuous mode voice channel at each of the outlying stations is not illustrated as it forms no part of the present inventio~ and is convent~onal. ~he ~DM~ channel, in particular, the apparatus for insuring that the various message ~; .
bursts from the plurality of outlying stations are received at - the satellite in non-overlapping and interleaved relation is the apparatus which forms the present invention. This apparatus will be described more particularly with reference to Figure 3.
Utilizing the 16 kilobit per second rate, as an ex-ample, a preamble portion 16 which is used for modem timing recovery and unique word transmission comprises 160 bits. The message burst comprises 800 bits. For purposes of description we may consider tB to be the duration of the preamble and the message burst.
The preferred format, that is, an 8-second time frame pro~ides a reasonable balance between two competing con-siderations. In the first place, once the burst duration is -fixed, increasing the time frame increases the number of time slots a~ailable for distribution. ~owever, if the number of time slots is increased by increasing the period of the time frame, then the bit rate must also be increased if the system is to serve a 100 bit per second teletype channel. This increases the storage apparatus required at each station. On the other hand, if the time duration of the time frame is required to one second then, for example, with the same bit rate and message burst duration, only 10 time slots are available for distribution.
The ~-second time frame with the 800 bit message burst and 50 millisecond message burst duration is a reasonable compromise.
Those with ordinary skill in the art will understand how the message burst duration, bit rate and number of time slots can be varied to suit the needs of the communications system. It should be understood, however, that the present invention is not res-; tricted to the exemplary showings of Figure 2.
3~- Figure 3 illustrates the apparatus of the present in-vention which provides that the plurality of message bursts from p~ur~lity of outlying stations will be recei~ed at the 1~-,:
10;~8976 satellite interleaved and non-overlapped time relationship. Fig-ure 3 illustrates the apparatus at each of the stations which ~;
controls the messaye transmissions so as to achieve the objects of the present invention. Each outlying station comprises, in a receiving portion, an RF channel receiver 39, and a reference unique word detector 21 which is coupled to the demodulator 40.
The demodulator 40 also feeds the received data to conventional receiver apparatus (not shown). An OR gate 22 couples the re-ference unique word detector 21, to a burst delay counter 23.
The reference unique word dete~tor 21 when detecting receipt of a unique word produces a sync pulse, which is coupled through OR gate 22 to reset burst delay counter 23. Burst delay counter 23, which may be a conventional multistage counter, receives in addition to the sync pulse, a scaled control clock pulse train -from pre-scaler 24. A control clock at the outlying station feeds signals of a predetermined repetition rate to clock pre- - -scaler 24, which is merely a divider. The output of clock-pre-scaler 24 is a pulse train of scaled control clock pulses at a repetition rate which bears a predetermined ratio with the repetition rate of the control clock pulses. The scaled control clock pulses operate burst delay counter 23. A plurality of stages of burst delay counter 23 feed a decoder 25. Decoder 25 operates in response to a particular combination of output siqnals and provides, when such a combination exists, a start- ` ~ -of-burst pulse to flip-flop 32 and burst duration counter 33 ~urst duration counter 33 may be a conventional multistage counter. The~~particular output configuration of burst delay counter 23 that decoder 2~ operates in response to will be described in more detail below. Burst delay counter 23 also provides an output to delay modulo detector 30. Delay modulo detector 30 responds to a different combination of signals from burst delay counter 23. When th; latter combination of signals . :
is present, the delay modulo detector 30 provides an output pulse to OR gate 22 which is capable of resetting burst delay counter 23 in the same manner as the sync pulse resets it.
Flip-flop 32, which receives the start-of-burst pulse from decoder 25, provides an output to modulator 36 and data buffer and control 35. The burst duration counter 33 also re-ceives the start-of-burst pulse and is reset thereby. A decoder 38 monitors the changing configuration of the output of burst duration counter 33 and, in response to a predetermined output combination pro~ides a reset signal to flip-flop 32. The burst duration counter 33 is operated by the same scaled control clock pulses which operate the burst delay counter 23.
Data buffer and control 35 receives three inputs in addition to the input from flip-flop 32. Data buffer and con-trol 35 receives continuous data from a 100 bit per second tele-type channel. Data buffer and control 35 also receives a con-tinuous clock to clock into the buffer the continuous data re-ferred to above. As is well known by those skilled in the art, the data buffer and control has the continuous data read in at a rate of 100 bits per second from the conventional teletype channel. Howe~er, when transmitting, data is read out of the data buffer and control 35 at a much higher rate, in this pre-ferred embodiment at 16,000 bits per second. Therefore, data buffer and control 35 also receives a transmit burst clock at the 16,000 bit per second rate. Information pulses when read out of data buffer and control 35 are pro~ided to modulator 36.
Msdulator 36 also recei~es the 16,000 bit per second burst cloc~ pulses and pro~ides the modulated output to transmit RF
channel 37 which couples the modulated data signals through an antenna for transmission to satellite 34.
~ he operation of the apparatus thus far referred to is as follows; A master station transmission, whose format is - 12 ~ .
illustrated in Figure 1, is received by RF channel 39 and de-modulated in demodulator 40. These components are conventional in the art and more detailed descriptions thereof are deemed unnecessary. The refexence unique word detector 21 detects the presence of unique word llA and provides a sync pulse responsive thereto. A sync pulse coupled through OR gate 22 resets burst delay counter 23. Burst delay counter then counts in response to scaled control clock pulses from clock pre-scaler 24. When burst delay counter 23 reaches a count corresponding to the count to which decoder 25 is responsive, decoder 25 produces a start-of-burst pulse. This pulse has two effects. In the first place, this pulse sets flip-flop 32 to provide an output to modulator 35 and data buffer and control 35. This output marks the begin-ning of the burst window and the beginning of the message burst.
At the same time, burst duration counter 33 is reset and it begins to count in response to scaled control clock pulses from clock pre-scaler 24. When the output of burst duration counter -33 matches that of decoder 38, an output is provided to reset flip-flop 32. This action removes the signal from the Q output .~ .. . , ~ . .
of flip-flop 32 thus terminating the burst window and terminating the message burst from the particular station involved.
Delay modulo decoder 30 is also permanently connected to burst delay counter 23 so as to respond to a count in burst delay counter 23 corresponding to 1000 milliseconds or 1 second.
When delay modulo decoder 30 receives a counter from burst delay counter 23 corresponding to 1 second it produces an output sig-nal, which is~coupled throu~h OR gate 22 to reset the burst de-lay counter 23. Should, for some reason, the unique word refer-ence mar~er not be received or not be properly decoded by refer-ence unique word detector 21, theoutput of delay modulo decoder 30 will serve to reset burst delay counter 23. ~he decocer 30 is responsive to a count corresponding to 1 second corresponding - 13 _ , .' . '' ,.
- : : , i. ~ ,, -, . .
to the master station time frame period. Those skilled in the art will understand how decoder 30 may respond to any count corresponding to the master station time frame period.
It should be apparent that the particular configuration to which decoders 25 and 38 respond respectively control the initiation and termination of the transmission. Thus, as one example, the decoder 25 can be arranged to respond to burst delay counter 23 counting up to the equivalent of 40 millise-conds to initiate transmission for a first time slot. Decoder 38 can be arranged to respond to a count in burst duration counter 33 corresponding to 60 milliseconds. The 60 millise-conds comprise the sum of the preamble portion 16 and message portion 17. For the next time slot decoder 25, at another station, is arranged to respond to a burst delay counter 23 out-put corresponding to 140 milliseconds and decoder 38 responds to a count corresponding to 60 milliseconds. For a further time slot the decoder 25, at still another station, is arranged to respond to a count in burst delay counter 23 corresponding to 240 milliseconds. And, in general, for time slot N the de-coder 25 is arranged to respond to a count in burst delay counter23 corresponding to l(N-l)lO0 + 40l milliseconds. Each decocer 38 is arranged to respond to a count in burst duration counter 33 corresponding to 60 milliseconds. Since burst delay counter can only count up to a count corresponding to 1 second, N may then vary from l to lO.
In actual implement~tion, each of decoders 25 and 38 may be A~D ga~es connected to the burst delay counter 23 and burst duration counter 33, respectively. The decoder 38 may be so connected to the respective stayes of burst duration counter 33 so as to respond to a count corresponding to 60 milliseconds.
~he decoder 25 may be connected to burst delay counter 23 through manually controlled switching contacts selectively operable to .' -. . , :
:
engage the various staqes of burst delay counter 23 so as to respond to a count corresponding to [(N-1)100 + 40] milliseconds where N can be in the range from l to l0.
Other apparatus may be provided to selectively connect decoder 25 to the various tages of burst delay counter 23 as re-cited above, in response to time slot assignments made by the - master station and transmitted during master station time frame ~ -- 11. Such apparatus is well known to those skilled in the art and a detailed disclosure thereof is deemed unnecessary. See for instance U.S. patent 3,564,147.
; The apparatus described above is sufficient for those . . .
cases where the TDMA frame illustrated in Figure 2 has a dura- -tion which is less than or equal to the master station time frame, illustrated in Figure 1. As described above, however, the TDMA time frame illustrated in Figure 2 has a duration of 8 ~ -seconds whereas the master station time frame has a duration of 1 second. Therefore, the TDMA time frame cycles once for every 8 cycles of the master station time frame. In order to properly detect every eighth master frame, super frame detector 26, OR
qate 27, frame counter 28, decoder 29, and frame modulo decoder 31 are pro~ided. Super frame detector 26 is connected to the modulator 40 as is reference unique word detector 21. An output - . .
of super frame detector is coupled through OR gate 27 to reset frame counter 28. Frame counter 28 may be a conventional multi- '~
stage counter. Frame counter 28 is operated by the scaled control c~ock pulses from clock prescaler 24, A decoder 29 is connected to separate stages of frame counter 28 so that it can respond to a predetermined count of frame counter 28. The manner in which decoder 29 is connected so as to respond to the predeter- ' mined count, and the manner in which the predetermined count is determined will be explained later. Upon reaching this prede-term~noa count, however, the output of decoder 29 ~ pro~ided `.:
:
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~038976 as an input to decoder 25.
Frame modulo decoder 31 is also connected to frame counter 28 to respond to a different predetermined count of frame counter 28. When frame modulo decoder 31 detects this predetermined count it provides an input to OR gate 27 which ; services to reset frame counter 28 in the same manner that the super frame detector 26 output causes frame counter 28 to be re-set.
The manner in which decoder 25 and decoder 29 cooper-ate to select a predetermined time slot will now be explained. ~ -Decoder 25, being controlled by burst delay counter 23 which is reset at every period of the master station time frame, is there-fore reset every second. Decoder 25 may then select one of the 10 time slots occurring in any one second period of time. De-coder 29 selects one of the 8 groups of 10 time slots which occur in a single TDMA time frame. Thus, for instance, if a station is to transmit within time slot 14, decoder 25 would be set to respond to the count corresponding to N equals 4, and decoder 29 - would be set to respond to a count corresponding to the second of the 8 frames. Alternatively, if a station were to transmit in time slot 44, the decoder 25 would be arranged to respond to the same count corresponding to N equals 4, and the decoder 29 would be arranged to correspond to a count corresponding to the fifth one second interval of time subsequent to the super fra~e detector output. The manner in which decoder 29 is arranged to respond to the different predetermined counts can be the same as that ex-plained for de-coder 25, that is, ani AND gate hard-wi-res to switching contacts which are manually settable. Alternatively, this connection can be automatically made by electronic switching units responsive to signal impulses from the master station, as i~ well known in the art. If apparatus is aYaila~le to automati-cally set decoders 25 and 29 it is tben possible to also ... .
~ .`' ''' :. - - ~.. ~ .
~038976 automatically set decoder 38. The advantage thus gained in that the message burst duration may then be variable for different time slots. Thus the output of decoder 29 forms one input to decoder 25, such that decoder 25 produces an output if a signal is received on each of its inputs including the input from decoder 29.
Certain other apparatus would also be preferably used, although not specifica?ly illustrated. For instance, the delay modulo detector 30 and frame moduls detector 31 can supply miss-ing reference unique words or super frame words if that becomesnecessary. However, if the reference unique word or super frame words are absent for an extended period of time the outlying station at which these signals are not received may become out of sync due to variations in locally generated clock signals as opposed to the master station clock. To this end, a counter may be arranged so as to count the instances of missing unique words or super frame words and to shut down this station if a prede- -termined count is exceeded. Of course, if during the counting process a unique word or super frame word is received, then the counter is reset.
In order to acquire a time slot, in those cases where the outlying stations decoders 25 and 29 are manually selectable to different time slots, the operator must have information as to which time slots are available. To this end, the master ~tation, within time frame portion 11, transmits information as to the status of each of the time slots. A decoder at each of the outlying stations makes this information visiblé by indicator 7ights or otherwise. Therefore, when a station desires to transmit, the operator merely properly sets decoder 25 and 3~ decoder 2g to se~ect a time slot that is available. The data -buffer and control 35 has permanently stored therein information corresponding to the preamble portion 16 of the message burst.
.' ~-- - - , . . . . .
-:. .. . . - -;, . ~: -- .
This includes the station identity. Thus, when the operator selects the settings for decoder 25 and 29, the station identity is sent in the selected time slot. If no other station has - requested this particular time slot the master station providesa go ahead signal which is received and decoded at the outlying station. When the go ahead signal is received it is only then necessary to connect the continuous data line to the data source and transmissions will occur automatically in the proper time slot.
With the apparatus thus far explained, the decoders 25, 29 and 38 will provide that in any 100 millisecond time slot, a station may transmit in a 60 second portion thereof. ~hus, the - guard time of 40 milliseconds is provided. In the foregoing description, the guard time occurs during the first 40 milli-seconds of the station's 100 millisecond time slot, and the infor-mation transmission or message burst occurs during the later 50 millisecond portion. However, it is within the scope of the present invention to provide the guard time in the last 4~ milli-second period of the time slot and the message burst in the first 60 millisecond portion. To this ena, the decoders 25 and 29 may be arranged to respond to a count corresponding to (N-l) 100 milliseconds. In combination with the decoder 38 arranged to allow transmission for only 60 milliseconds, the 40 millisecond guard time will be provided in the last 40 milliseconds of the 100 millisecond time slot. Although~ as has been explained above, the guard time may occur either at the beginning or the end of the time slot~ it 1s essential that the arrangement throughout a partic~lar system be uniform. Although the present application - utilizes as e~emp~ary a 10~ millisecond time slot with a 40 millisecond guard time, those skilled in the art will understand that the time slot duration, TDMA time frame and master station time frame may all be varied to suit the particular circumstances.
1();18976 What is essential is that each message burst from each of the - stations has an associated guard time which is no less than the - duoble hop propagation delay variation within the system.
- Furthermore, although the guard time may occur either prior to or subsequent to any message hurst, the location of the guard time in the time slot must be uniform throughout the system.
In operation, the transmission of the various outlying stations are separated by a nominal separation equal to the guard time. This guard time is selected to be at least equal to the double hop propagation delay variation ~ for the system.
A reasonable value for the delay variation ~ is 20 milliseconds and therefore a reasonable period for the guard time is 40 milli-seconds. It should be apparent from the foregoing that with the guard time specified above, the various transmissions from ~ , the outlying stations will not overlap and will interleave at the satellite for a retransmission to the master station.
.:
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Claims (9)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. In a time multiplex communication-system wherein a plurality of stations with a propagation delay variation ?
transmit in a TDMA mode to a satellite, a method of ensuring non-overlapping reception at said satellite without measuring actual propagation delay comprising the steps of:
(a) transmitting to each of said stations a reference marker, (b) transmitting from each station that has information to transmit in a time slot beginning a predetermined time subsequent to receipt of said marker, said predeter-mined time different for each of said stations, (c) timing said transmissions to last for a predetermined time less than the duration of said time slot so that a guard time is provided, (d) selecting said guard time to be at least equal to twice said propagation delay variation ?.
transmit in a TDMA mode to a satellite, a method of ensuring non-overlapping reception at said satellite without measuring actual propagation delay comprising the steps of:
(a) transmitting to each of said stations a reference marker, (b) transmitting from each station that has information to transmit in a time slot beginning a predetermined time subsequent to receipt of said marker, said predeter-mined time different for each of said stations, (c) timing said transmissions to last for a predetermined time less than the duration of said time slot so that a guard time is provided, (d) selecting said guard time to be at least equal to twice said propagation delay variation ?.
2. The method of claim 1 wherein said transmissions are timed to have a duration equal to or greater than said guard time.
3. The method of claim 1 wherein at each said station said guard time G precedes said transmission.
4. The method of claim 1 wherein at each said station said guard time G succeeds said transmission.
5. The method of claim 1 wherein said time slot has a duration T, said transmission has a duration tB and said guard time G?2.epsilon. such that T=(tB+G) and tB?G.
6. A TDMA communication system for transmitting a plurality of burst mode messages from a plurality of stations for non-overlapping reception at a satellite transponder which eliminates the necessity for measuring propagation delays at each of said stations comprising:
marker reception means at each said station for de-tecting receipt of a time reference marker, the time of receipt of said marker varying as the distance from each said station to said satellite, first and second delay means at each station, said first delay means settable at each station to a preassigned time slot of predetermined duration for each said station's transmission, transmitter means at each said station initiated at the expiration of said first delay, said second delay means at each station for controlling the termination of said transmitter means, said second delay means terminating said transmitter means 2.epsilon. prior to termination of said time slot duration at said station where ? is the max-imum propagation delay variation among all said stations.
marker reception means at each said station for de-tecting receipt of a time reference marker, the time of receipt of said marker varying as the distance from each said station to said satellite, first and second delay means at each station, said first delay means settable at each station to a preassigned time slot of predetermined duration for each said station's transmission, transmitter means at each said station initiated at the expiration of said first delay, said second delay means at each station for controlling the termination of said transmitter means, said second delay means terminating said transmitter means 2.epsilon. prior to termination of said time slot duration at said station where ? is the max-imum propagation delay variation among all said stations.
7. The system of claim 6 in which said predetermined duration is equal for all stations.
8, The system of claim 7 in which said predetermined duration is equal for all stations and the difference between said predetermined duration and the quantity 2.epsilon. is at least equal to 2.epsilon.,
9. In a time multiplex communication system in which a plurality of stations may transmit information messages for reception at a satellite, said information message from each station being transmitted in a TDMA burst mode and wherein trans-missions to and from said satellite are subject to propagation delays equal to a minimum propagation delay plus a variable pro-pagation delay d in the range O?d?.epsilon., each of said stations transmitting in a preassigned time slot of duration T in a cycli-cal time frame, each said time slot beginning a predetermined time after receipt of a marker, apparatus for ensuring non-over-lapping receiption at said satellite of said message without re-quiring actual measurement of propagation delay comprising:
(a) means for detecting receipt of said marker, (b) first delay means responsive to said means for detecting for producing a first signal, (c) second delay means responsive to said first signal for producing a second signal a predetermined time after receipt of said first signal, (d) transmission means for information trans-missions, 'e) control means responsive to said first and second signals for initiating said trans-mission means in response to said first signal and for inhibiting said transmission means in response to said second signal where said second delay means has a delay no greater than T-2?.
(a) means for detecting receipt of said marker, (b) first delay means responsive to said means for detecting for producing a first signal, (c) second delay means responsive to said first signal for producing a second signal a predetermined time after receipt of said first signal, (d) transmission means for information trans-missions, 'e) control means responsive to said first and second signals for initiating said trans-mission means in response to said first signal and for inhibiting said transmission means in response to said second signal where said second delay means has a delay no greater than T-2?.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US441710A US3922496A (en) | 1974-02-11 | 1974-02-11 | TDMA satellite communications system with guard band obviating ongoing propagation delay calculation |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1038976A true CA1038976A (en) | 1978-09-19 |
Family
ID=23753980
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA219,732A Expired CA1038976A (en) | 1974-02-11 | 1975-02-10 | Time division multiple access satellite communications system |
Country Status (3)
| Country | Link |
|---|---|
| US (1) | US3922496A (en) |
| CA (1) | CA1038976A (en) |
| GB (1) | GB1487689A (en) |
Families Citing this family (29)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4071711A (en) * | 1974-08-02 | 1978-01-31 | Farinon Electric Of Canada Ltd. | Telephone subscriber distribution system |
| JPS52122027A (en) * | 1975-12-10 | 1977-10-13 | Nec Corp | Burst synchronizing method and unit of time-sharing pluralistic connec tion communication system |
| US4210780A (en) * | 1978-03-27 | 1980-07-01 | The Mitre Corporation | Multiple access digital communications system |
| US4320503A (en) * | 1979-08-06 | 1982-03-16 | Rca Corporation | Synchronizing transmissions from two earth stations to satellite |
| US4298979A (en) * | 1979-09-27 | 1981-11-03 | Communications Satellite Corporation | Decoding TIM bus structure |
| US4292683A (en) * | 1979-11-06 | 1981-09-29 | Satellite Business Systems | Guard band reduction in open loop TDMA communications |
| EP0035232B1 (en) * | 1980-02-29 | 1985-05-15 | International Business Machines Corporation | Method and apparatus for the initial acquisition of synchronization for a station in a tdma satellite communication network |
| US4398291A (en) * | 1980-06-23 | 1983-08-09 | Nippon Electric Co., Ltd. | Satellite communication system for switching formats with reference to super-frame time slots |
| FR2526250B1 (en) * | 1982-04-30 | 1988-05-13 | Labo Electronique Physique | METHOD FOR AUTOMATIC TIME SETTING OF STATIONS IN A MULTIPLEX TRANSMISSION AND DATA PROCESSING SYSTEM |
| JPS5922464A (en) * | 1982-07-29 | 1984-02-04 | Fuji Xerox Co Ltd | Timing synchronization circuit |
| US4630267A (en) * | 1983-12-23 | 1986-12-16 | International Business Machines Corporation | Programmable timing and synchronization circuit for a TDMA communications controller |
| US4675863A (en) * | 1985-03-20 | 1987-06-23 | International Mobile Machines Corp. | Subscriber RF telephone system for providing multiple speech and/or data signals simultaneously over either a single or a plurality of RF channels |
| JPS62147822A (en) * | 1985-12-23 | 1987-07-01 | Nec Corp | Time division multiple access communication equipment |
| US4805189A (en) * | 1986-02-13 | 1989-02-14 | Signatron, Inc. | Signal processing system |
| US4825448A (en) * | 1986-08-07 | 1989-04-25 | International Mobile Machines Corporation | Subscriber unit for wireless digital telephone system |
| JP2723691B2 (en) * | 1991-04-30 | 1998-03-09 | 日本電気株式会社 | Variable timing signal generation circuit |
| ES2078150B1 (en) * | 1993-06-25 | 1998-01-01 | Alcatel Standard Electrica | SUBSYSTEM OF COMMUNICATIONS BETWEEN BASE STATIONS AND BASE STATION CONTROLLERS IN COMMUNICATION SYSTEMS TO GUSTS. |
| US5546383A (en) | 1993-09-30 | 1996-08-13 | Cooley; David M. | Modularly clustered radiotelephone system |
| JP2800679B2 (en) * | 1994-06-01 | 1998-09-21 | 日本電気株式会社 | Communications system |
| US6563856B1 (en) | 1998-07-08 | 2003-05-13 | Wireless Facilities, Inc. | Frame synchronization and detection technique for a digital receiver |
| US6788917B1 (en) | 2000-01-19 | 2004-09-07 | Ericsson Inc. | Timing systems and methods for forward link diversity in satellite radiotelephone systems |
| US7042367B2 (en) * | 2002-02-04 | 2006-05-09 | Halliburton Energy Services | Very high data rate telemetry system for use in a wellbore |
| US9048964B2 (en) | 2012-10-26 | 2015-06-02 | Deere & Company | Receiver and method for receiving a composite signal |
| US8942264B2 (en) | 2012-10-26 | 2015-01-27 | Deere & Company | Receiver and method for receiving a composite signal |
| US8781039B2 (en) | 2012-10-26 | 2014-07-15 | Deere & Company | Receiver and method for receiving a composite signal |
| US8942157B2 (en) | 2012-10-26 | 2015-01-27 | Deere & Company | Receiver and method for receiving a composite signal |
| US10693574B2 (en) | 2015-07-02 | 2020-06-23 | Qualcomm Incorporated | Method and apparatus for efficient data transmissions in half-duplex communication systems with large propagation delays |
| CN108259079A (en) * | 2017-12-29 | 2018-07-06 | 中国电子科技集团公司第二十研究所 | High speed moving platform TDMA satellite communication synchronisation control means based on ephemeris |
| US20220006514A1 (en) * | 2018-09-27 | 2022-01-06 | Telefonaktiebolaget Lm Ericsson (Publ) | Systems and methods for timing adaptation for satellite communications |
Family Cites Families (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US2406165A (en) * | 1944-07-01 | 1946-08-20 | Rca Corp | Communication system |
| US2723309A (en) * | 1950-03-10 | 1955-11-08 | Int Standard Electric Corp | Telephone system |
| US3529243A (en) * | 1967-10-11 | 1970-09-15 | Us Army | Synchronous tactical radio communication system |
| US3593138A (en) * | 1968-07-31 | 1971-07-13 | Nasa | Satellite interlace synchronization system |
-
1974
- 1974-02-11 US US441710A patent/US3922496A/en not_active Expired - Lifetime
-
1975
- 1975-02-10 CA CA219,732A patent/CA1038976A/en not_active Expired
- 1975-02-11 GB GB5819/75A patent/GB1487689A/en not_active Expired
Also Published As
| Publication number | Publication date |
|---|---|
| GB1487689A (en) | 1977-10-05 |
| US3922496A (en) | 1975-11-25 |
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