CA1199682A - Frequency hopped single sideband mobile radio system - Google Patents
Frequency hopped single sideband mobile radio systemInfo
- Publication number
- CA1199682A CA1199682A CA000423000A CA423000A CA1199682A CA 1199682 A CA1199682 A CA 1199682A CA 000423000 A CA000423000 A CA 000423000A CA 423000 A CA423000 A CA 423000A CA 1199682 A CA1199682 A CA 1199682A
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- CA
- Canada
- Prior art keywords
- carrier
- frequency
- signal
- hopped
- hopping
- 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.)
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Classifications
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
- H04B1/68—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission for wholly or partially suppressing the carrier or one side band
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B7/00—Radio transmission systems, i.e. using radiation field
- H04B7/24—Radio transmission systems, i.e. using radiation field for communication between two or more posts
- H04B7/26—Radio transmission systems, i.e. using radiation field for communication between two or more posts at least one of which is mobile
- H04B7/2628—Radio transmission systems, i.e. using radiation field for communication between two or more posts at least one of which is mobile using code-division multiple access [CDMA] or spread spectrum multiple access [SSMA]
- H04B7/2634—Radio transmission systems, i.e. using radiation field for communication between two or more posts at least one of which is mobile using code-division multiple access [CDMA] or spread spectrum multiple access [SSMA] for channel frequency control
Abstract
FREQUENCY-HOPPED SINGLE SIDEBAND
MOBILE RADIO SYSTEM
Abstract The present invention relates to a frequency hopped single sideband (SSB) mobile radio system implemented by hopping the carrier frequency of an input signal (s(t)) every ?seconds. The hopping is controlled by a carrier-frequency hopped sequence (fi(t)) generated by a carrier-frequency-hopping generator. When employed in a frequency-hopped SSB transmitter, the carrier sequence functions to modulate the input signal, "hopping"
it to a different carrier frequency every ? seconds. The carrier frequency-hopped SSB receiver employs the identical carrier sequence as used by the transmitter to demodulate the transmitted carrier-frequency-hopped SSB signal, thereby recovering the original single sideband signal (s(t)). By frequency hopping the carrier signal of an SSB signal, the present invention mitigates the effects of co-channel interference and frequency selective fading inherent in prior art SSB cellular mobile radio systems.
MOBILE RADIO SYSTEM
Abstract The present invention relates to a frequency hopped single sideband (SSB) mobile radio system implemented by hopping the carrier frequency of an input signal (s(t)) every ?seconds. The hopping is controlled by a carrier-frequency hopped sequence (fi(t)) generated by a carrier-frequency-hopping generator. When employed in a frequency-hopped SSB transmitter, the carrier sequence functions to modulate the input signal, "hopping"
it to a different carrier frequency every ? seconds. The carrier frequency-hopped SSB receiver employs the identical carrier sequence as used by the transmitter to demodulate the transmitted carrier-frequency-hopped SSB signal, thereby recovering the original single sideband signal (s(t)). By frequency hopping the carrier signal of an SSB signal, the present invention mitigates the effects of co-channel interference and frequency selective fading inherent in prior art SSB cellular mobile radio systems.
Description
~ 1 --FREQUENCY-HOPPED SINGLE SIDEBAND
MOBILE RADIO SYSTEM
Background of the Invention 1. Field of the Invention The present invention relates to a frequency-hopped single sideband mobile radio system, and more particularly, to a frequency-hopped single sideband system for use in a cellular mobile radio environmen-t wherein the single sideband signal changes its spectral location every ~ seconds and the sequence of frequency bands occupied by the single sideband signal is controlled by a suitable scrambling code.
MOBILE RADIO SYSTEM
Background of the Invention 1. Field of the Invention The present invention relates to a frequency-hopped single sideband mobile radio system, and more particularly, to a frequency-hopped single sideband system for use in a cellular mobile radio environmen-t wherein the single sideband signal changes its spectral location every ~ seconds and the sequence of frequency bands occupied by the single sideband signal is controlled by a suitable scrambling code.
2. Description of the Prior Art It is well known that in a cellular mobile communication system increasing the density of users can be achieved by employing spread spectrum modulation techniques rather than conventional FM techniques. A spread spectrum communication system derives its name from a coding technique which translates the usual narrowband information spectrum into a wideband (spread) spectrum which resembles that of noise. There are two spread spectrum systems known in the prior art, namely, phase reversal (or higher or pseudo-random phase modulation) and frequency hopping~
~ne frequency hopping arrangement ls disclosed in U. S. Patent 4,276,652 issued to A~ M. McCalmont on June 30, 1981, and relates to a signal scrambling system in which the frequencies of the information signal band are inverted by heterodyning the information signal with a varying beat frequencyO In accordance with the McCalmont arrangement, the beat frequency varies in a sequence of small incremen~s which, in their totality, provide lar~e overall frequency excursions and which, individually, are sufficiently small to avoid the generation of unduly strong spurious signals within the information signal band~
A system employing both the phase reversal and frequency hopping techniques is disclosed in U. S.
Patent 4,193,030 issued to G. Rabow et al on March 11, 1980. This system applies the coding techniques of the prior art to select the frequency of transmission in a pseudo-random fashion. However, within each frequency hop interval, the signal is further modulated by selecting its phase as 0 degrees or 180 degrees, also in pseudo-random fashion.
The above-described systems, as well as many other prior art systems, employ frequency modulation (FM) techniques~ Although frequency modulation systems are wide-spread and highly successful, the ever-increasing demand for higher system capacity, together with a shortage of available frequencies in the microwave spectrum, has led to a search for a more efficient modulation scheme. Th;s search has concentrated on single sideband modulation (SSB), a well-known technique wherein the speech signal is linearly translated in frequency to a spectral location suitable for radio transmission. Unfortunately, with the use of SSB modulation, co-channel interference becomes extremely disturbing to those using the system.
One technique for suppressing co-channel interference in SSB systems is disclosed in U. S. Patent 4,019,140 issued to R. Swerdlow on April 19, 1977. As disclosed, intelligible crosstalk, or co-channel interference, is substantially eliminated by phase-modulating the AM carrier-wave prior to its modulation by the baseband signal. Such a system, however, would require extensive modification and would be able to accommodate only a small number of users if it were to be used in a cellular mobile radio system employing frequency reuse, and would also subject stationary users to disruption in communications due to frequency-selective fade problemsO
A problem remaining in the prior art~ therefore~
is to provide a method for employing single sideband modulation in a cellular mobile radio system which may æ
serve a large number of users without incurring severe co-channel interference, while having the ability to reduce the probability of complete disruption in communications for those mobiles that are either stationary or at low speeds as found in dense traffic conditions in urban areas.
Summary of the Invention In accordance with an aspect of the invention there is provided in a system for communicating a message signal (s(t)), a transmitter responsive to said message signal for producing as an output a carrier-frequency-hopped single sideband signal s'hop (t)), wherein said transmitter comprises carrier-frequency-hopping generating means for generating as an output a unique sequence of sep-arate possible carrier frequencies (Fl, F2....,FN);
modulating means responsive to both said message signal and said unique sequence of carrier frequencies for moAu-lating said message signal in accordance with said unique sequence of carrier frequencies and producing as an output of said transmitter the carrier-frequency-hopped single sideband signal; and a receiver responsive to said carrier-frequency-hopped single sideband signal produced by said transmitter and receiving therefrom said message signal, wherein said receiver comprises carrier-frequency-hopping generating means for generating as an output said unique sequerlce of separate carrier frequencies generated by said transmitter carrier-frequency-hopping generating means, said receiver carrier-frequency-hopping generating means also capable of producing as an output an assignment code (c(t)) associated with said unique sequence of carrier frequencies wherein said transmitter carrier-frequency-hopping generating means is responsive to said assignment code generated by saic] receiver carrier-frequency-hopping generating means for producing as an output said unique sequence of carrier frequencies related thereto; and demodulating means responsive to both the received carrier-frequency-hopped slngle sideband signa~ (glhop(t)) and 6~:
- 3a said unique sequence of separate carrier frequencies for demodulating said received carrier-frequency-hopped single sideband signal in accordance with said unique sequence of separate carrier frequencies and producing as an outut of said receiver the message signal applied as an input to said transmitter.
The problem remaining in the prior art has been solved in accordance with the present invention, which relates to a frequency-hopped single sideband mobile radio s~stem, and more particularly, to a frequency-hopped single sideband system for use in a cellular mobile radio environment wherein the single sideband signal changes its spectral location every ~ seconds and the sequence of frequency bands occupied is controlled by a suitable scrambling code.
It is an aspect of the present invention to provide a means for employing a single sideband trans-mission scheme in a mobile radio environment which overcomes the crosstalk interference problem inherent in multiple signal single sideband systems. In accordance with the present invention, the crosstalk interference problem is overcome by hopping the carrier frequency of each single sideband signal, changlng the interference from the reception oE an unwanted speech signal to that of noise with random-like properties, where the reception of the interfering noise is subjectively more acceptable than attempting to hold a conversation while hearing other conversations in the background.
It is another aspect of the present invention to provide improved transmission and reception capabilities of a stationary mobile unit which is situated in an area subjected to a deep communication fade which completely disrupts the communication. Since fading is a frequency dependent condition, the application of frequency-hopping in accordance with the present invention, increases the likelihood of maintaining communication, albeit with some degradation in the recovered speech quality, as the probability of repeatedly hopping into a deep fade condition is small. Frequency hopping ensures that no mobile unit is ever in a permanent deep fade, provided the mobile is not in a completely hostile environment, such as a tunnel, etc.
It is a further aspect of the present invention to provide a single sideband system that offers a degree of communication privacy to the users of the system by controlling the sequence of hopped carrier frequencies by a suitable scrambling code.
Other and further aspects of the present invention will become apparent during the course of the following description, and by reference to the accompanying drawings.
Brief Description of the Drawings Referring now to the drawings, where like numerals represent like parts in several views:
FIG. l illustrates an exemplary frequency-hopping single sideband transmitter for inclusion in a mobile unit formed in accordance with the present invention, FIG. 2 illustrates an exemplary division of N channels and N carrier hopping frequencies for an allocated handwidth of B Hz for use in association with the present invention;
FIG. 3 illustrates the movement of the spectral locations of two arbitrary carrier frequency-hopped SSB signals for successive values of ~, emanating from an exemplary transmitter formed in accordance with the present invention;
FIG. 4 illustrates an exemplary base station capable of receiving and demodulating a plurality of M carrier-frequency-hopped single sideband signals in accordance with the present invention; and FIG. 5 illustrates an exemplary hexagonal cellular configuration of a mobile radio system capable of employing the present inventionO
3t6~3~
Detailed Description For the purposes of illus~ration, the present invention will be discussed from the viewpoint of signal flow from a mobile unit to a base station. It must be appreciated, however, that communication will, in fact, also take place in the opposite direction, that is, with a base station transmitting a signal to a mobile unit, and remain within the scope of the present invention.
A block diagram of an exemplary frequency-hopped single sideband transmitter 10 for use, for example, in a mobile unit of a mobile radio system is illustrated in FIG. 1. In operation, an input signal s(t), for example, a speech signal, enters transmitter 10 and is applied as an input to a first bandpass filter 12. First bandpass filter 12 functions to limit the output signal therefrom to a predetermined frequency range fa to fb and produce as an output a filtered speech signal r(t). Assuming r(t) is allowed to reside in a frequency band of Bc Hz, extending from 0 to 4 kHz, for example, the input speech signal may be bandlimited by first bandpass filter 12 to the band fa=0.3 kHz to fb=3.3 kHz, for example, where these values of fa and fb are typical for telephony applications.
In accordance with the present invention, the filtered speech signal r(t) from first bandpass filter 12 is subsequently applied as a first input to a multiplier 14 where a second input to multiplier 14 is a carrier-frequency-hopping sequence fi(t). Carrier frequency sequence fi(t) is generated by a carrier-frequency-hopping generator 16 included in transmitter 10. In general/
generator 16 is under the control of a carrier-frequency-hopping generator 24 included in a base station 30 (not shown), which sends a command to the mobile incl~ding transmitter 10 initializing generator 16 and dictating the carrier-frequency-hopping sequence to be used by the mobile during transmission. Pseudo-random generator arrangements are well-known in the art, and any one may be employed to produce a pseudo-random sequence of carrier frequencies that contain the set of carrier frequencies {Fl,F2,...,FN}.
These pseudo-random sequences of carrier frequencies are utilized to hop the input signal s(t) around its allotted frequency band. For example, if the total bandwidth allotted to a cell in a system formed in accordance with the present invention is B Hz, and an exemplary filtered speech signal r(t) having a bandwidth of fb-fa, located within the bandwidth of Bc Hz, the total bandwidth B could be divided into N channels, where N=B/BC. In accordance with the present invention, therefore, generator 16 functions to produce a pseudo-random sequence of N distinct carrier frequencies ~Fl, F2,...,FN}, where an exemplary carrier frequency Fi is the carrier frequency that positions an SSB signal into the ith spectral channel in the bandwidth B. ~n illustration of the relationship of B, ~c~ N and the set of carrier frequencies {Fl, F2,---,FN} is shown in FIG. 2. FIG. 2 also includes a detailed illustration of the spectral location of an SSB signal for T seconds when the carrier frequency hopped sequence has a value of Fi.
In operation of the present invention, generator 16 produces one carrier frequency every T seconds, where one exemplary sequence may be fi(t)={F2, F4, F6, Fl, FN,...}. Therefore, the output of multiplier 14 will be a double sideband, suppressed carr;er signal shop(t), whose spectral location is altered every T
seconds in accordance with the carrier frequency sequence fi(t)~
Prior to transmission, the output signal shop(t) from multiplier 14, is applied as an input to a second bandpass filter 18, as shown in FIG. 1. Sesond bandpass filter 18 comprises the same bandwidth as first bandpass filter 12, that is, from fa to fb, but where the passband frequencies are moved to occupy the frequency region Ei~fa to fi+fb. Second bandpass filter 18 functions to filter out one sideband of double sideband output signal shop(t), for example, the lower sideband~ to produce as an output of transmitter 10 a carrier-frequency-hopped single sideband signal s'hop(t). Accordingly, in order to allow the frequency band of second bandpass filter 18 to track the movement of the single sideband signal, the output of carrier-frequency-hopping generator 16 is also applied as an input to second bandpass filter 18. An exemplary arrangement of a bandpass filter capable of performing the tracking function of second bandpass filter 18 is disclosed in U. S. Patent ~,232,26~ issued to G. Willoner on November 4, 1980, which teaches the formation of a digitally programmable active RC bandpass filter with constant absolute bandwidth.
In a mobile radio system, a given transmitter 10 may be in such a location that the signal it desires to transmit will be subject to a deep fade condition, that ls, where a specific RF bandwidth is subject to severe attenuation. This problem is avoîded in the present invention by controlling the output of generator 16 in a manner whereby successive carrier-frequency-hopping output signals are in non-contiguous channels, i.e., more than one channel separates a successive carrier frequency and in general, the frequency hopping distance will encompass many channels, and ideally exceed the coherence bandwidth, as defined in "Microwave Mobile Communications", W. C. Jakes, ed. Wiley, 1974. An illustration o~ two exemplary output signals from multiplier 1~ is illustrated in FIG. 3, where as shown for successive time slots 1, 2,..~,L, each signal hops a distance greater than one channel.
In the application of the present invention in a cellular mobile radio system, there will be a plurality of M (generally not greater than N) mobile units in communication with a single base station 30 in a particular cell, where an exemplary base station 30 is illustrated in FIG. ~. Each mobile in a particular cell comprises a separate one of a plurality of M transmitters 101-lOM, where each transmitter functions as described hereinabove in association with transmitter 10 of FIG. 1. In 619~
accordance with the present invention, each mobile unit 101-lOM communicates with cell base station 30 via a distinctly different carrier frequency sequence fl(t)-fM(t), respectively. To ensure that each sequence is, in fact, unique, the plurality of M carrier-frequency-hopped sequences fl(t) through fM(t) are determined by a single synchronized carrier-frequency-hopped generator 2~ located at base station 30, as shown in FIG. 4.
Carrier-frequency-hopping generator 24, as mentioned hereinbefore in relation to transmitter 10 of FIG. 1, broadcasts a plurality of assignment codes cl(t)-cM(t), such that transmitters 101-lOM utilize the identical carrier-frequency-hopped sequences as receivers 201-20M, respectivelyO That is, assignment code cj(t) related to carrier-frequency-hopped sequence fj(t) is broadcast to transmitter lOj when receiver 20j is also assigned carrier-frequency-hopped sequence fj(t).
For an exemplary mobile comprising transmitter lOi, therefore, received assignment code ci(t) may be thouyht of as an address which is related to carrier-frequency-hopped sequence fi(t). Therefore, generator 16 produces as an output sequence fi(t), which as described hereinabove, is applied as an input to both multiplier 14i, and second bandpass filter 18i, and functions as described hereinabove in association with transmitter 10 of FIG. 1.
Each of the separate carrier-frequency-hopped sequences fl(t) through fM(t) is also utilized by base station 30 to correctly demodulate the plurality of M carrier-frequency-hopped signals shop l(t)-s~op M(t) which it receives from mobile units comprising transmitters 101-lOM, respectively. As shown in FIG~ 4, each of the separate carrier-frequency-hopped sequences fl(t)-fM(t) is applied as an input to a separate one of a plurality of receivers 201-20~ included in base station 30, where receivers 201, 20i, and 20M are illustrated in detail.
In operation, receiver 20i functions to demodulate the received carrier-frequency-hopped signal G~3~
shop i(t). Since the signal produced by transmitter 10i travels through the communication medium before being received by receiver 20i, some signal degradation will occur, and, therefore, the signal received by receiver 20i, shop i(t), is an approximation of the transmitted signal shop i(t). Upon reception, received carrier~frequency-hopped SSB signal shop i(t) is applied as an input to a first bandpass filter 22i, where first bandpass filter 22i like second bandpass filter 18i of transmitter 10i, is capable of passing only the portion of the received signal in the frequency band fa~fi(t) to b+fi(t) where fa~ fb~
and fi(t) are the frequencies described hereinabove in association with transmitter 10 of FIG. 1. In order for the sequence of frequencies fi(t) used by irst bandpass filter 22i to correctly filter the received signal, carrier-frequency-hopped generator 24 included in base station 30 must operate in synchronization with transmitter 10i, producing the same carrier frequency in every T
seconds as employed by transmitter 10i. One exemplary synchronization method is disclosed in U. S.
Patent 3,641,433 issued to Ro Mifflin et al on February 8, 1972, which describes a transmitted reference synchronization system.
Therefore, assuming that carrier-frequency~
hopping generator 2~ does, in fact, apply the same set of frequencies in synchronization with transmitter 10i, the output of first bandpass filter 22i will be a received filtered carrier-frequency-hopped SSB signal ri(t).
SSB signal Pi(t) is subsequently applied as a first input to a multiplier 26i, where the second input to multiplier 26i is the carrier-frequency-hopped sequence fi(t) produced by carrier-frequency-hopping generator 2~. Multiplier 26i together with a second bandpass filter 28i whose frequency passband is fb~fc Hz, functions to demodulate the carrier-frequency-hopped SSB signal ri(t). Since the sequence of carrler frequencies used by transmitter 10i enable the baseband voice signal si(t) to be SSB modulated and ~g%
frequency hopped, and they are identical to those used by receiver 20i in the demodulation process, the output signal si(t) of second bandpass filter 28i is the recovered speech signal corresponding to the original signal si(t). As mentioned hereinabove, the correspondence between Si(t) and si(t) is limited by transmission channel and circuit imperfections.
Since each separate receiver 201-20M functions as described hereinabove in association with receiver 20i, base station 30 is capable of producing M separate output signals gl(t)-sM(t) which correspond to the M separate input signals sl(t)-sM(t) modulated and transmitted by transmitters 101-lOM, respectively.
A simple diagram of a cellular mobile radio system comprising hexagonal cells is illustrated in FIG. 5, where it is to be understood that the hexagonal cellular configuration is exemplary and for illustrative purposes only, since in the practice with the present invention, the individual cells may comprise any configuration known in the art. Assuming that the total available system bandwidth is equal to BT, it is common practice in a hexagon cellular system to partition the banc]widt~ BT into equal segments, assigning a different segment to each cell of a set of contiguous cells, where in FIG. 5 a cluster of seven such cells, each allocated separate bandwidths, Bl-B7 is shown. If, in the example shown in FIG~ 5, more than seven cells o a predetermined size are required to accommodate the user area demand, some frequency bands must be reused. The arrangement illustrated in FIG. 5 reuses all seven frequency bands and attempts to keep the distance between cells usinq the same frequency band large in order to restrict mutual interference. For example, the cells using frequency band Bl are spaced a distance D apart, where this distance is measured from the respective centers of the cells, as shown in FIG. 5 A mobile unit traveling in a central cell~
denoted A, using frequency band Bl will receive co-channel %
interference from each of the outlying cells also using frequency band Bl. In studying co-channel interference, the six closest co-channel cells, labeled Al-A6, cause the dominant co-channel interference. If carrier-frequency-hopping is not used, the mobile in cell A will receiveinterference in the form of intelligible crosstalk from cells Al-A6. However, when carrier-frequency-hopping is employed in accordance with the present invention, each cell in the set of cells Al-A6 will perform carrier-frequency-hopping with different and distinct sets of carrier frequency sequences. Thus, the mobile in cell A, upon receiving separate interfering signals from cells Al-A6, will demodulate these signals with the unique carrier frequency sequence belonging to cell A. Consequently, the lS mobile in cell A will recover the desired signal, and the interfering signals will be demodulated as incoherent signals. Thus, because the carrier frequency sequence unique to the mobile in cell A is incorrect to demodulate the signals from cells Al-A6, all of the demodulated interference signals, when added together will become more noise-like and in the case of speech transmission, perceptually more acceptable than a single coherent interfering speech si~nal.
As the mobile in cell A leaves this cell and enters one of the adjacent cells, for example, a cell which employs frequency band B3, the mobile ceases to communicate via the base station in cell A and begins to communicate via the base station in its new cell. The mobile may or may not maintain the same carrier frequency sequence -either alternatlve is possible in accordance with thepresent invention. That is, in a set of seven contiguous cells employing di~ferent frequency bands, the mobile may retain the same carrier frequency sequence and merely change the spectral location that the sequence is used in as the mobile travels from cell to cell. Alternati~ely~
each base station may transmit a different carrier frequency sequence assignment code to the mobile as it enters its cell. In either case, information must be "handed-off" between the base stations in cell A and the new cell the mobile enters. There exist many methods of completing this handing off which are well known in the art, where one such arrangement is disclosed in U. S.
Patent 3,71~,574 issued to T. Baba et al on January 30, 1973 which relates to a mobile communication system.
Although frequency hopping does not alter the signal-to-noise ratio of SSB systems, computed using long-term averaging, it does have important perceptualadvantages. For example, consider a stationary mobile, e.g., parked or in congested traffic, operating in a SSB system that does not employ frequency hopping. Suppose the mobile is subjected to a deep fade that results in a complete disruption of communication. Since such a fade i5 frequency dependent, if frequency hopping is applied in such a situation, the probability that communication will be restored increases, since the probability of repeatedly hopping into a deep fade condition is small. Of course, if the mobile was not in a deep fade but receiving excellent communications, the effect of frequency hopping would degrade the recovered speech although the degradation would be minimal. What frequency hopping ensures, therefore, is that no mobile will ever be in a permanent deep fade.
Further, in cellular mobile radio systems employing frequency reuse, co-channel interference exists in the form of cross-talk among the mobiles in all of the cells using the same segment of the frequency band. By applying frequency hopping, in accordance with the present invention, the co-channel interference is changed from cross-talk to random-like noise, as each mobile demodulates using its unique frequency hopping sequence. Additionally, as each signal broadcast by a mobile transmitter is "hopped" around the N channels at a rate of one hop every seconds, only the intended receiver knows the hopping sequence, or code, employed and is able to demodulate correctly the transmitted signalO Therefore, a single 6~32 sideband transmission system formed in accordance with the present invention will deter eavesdropping, provi~ing a measure of security over the communication path. Lastly, as all such transmission systems are in general s~lbjected to fading, well known methods, such as the inclusion of pilot tones can, in general, be added to this transmitted carrier-frequency-hopped SSB signal shop(t) as it is employed in accordance with the present invention.
~ne frequency hopping arrangement ls disclosed in U. S. Patent 4,276,652 issued to A~ M. McCalmont on June 30, 1981, and relates to a signal scrambling system in which the frequencies of the information signal band are inverted by heterodyning the information signal with a varying beat frequencyO In accordance with the McCalmont arrangement, the beat frequency varies in a sequence of small incremen~s which, in their totality, provide lar~e overall frequency excursions and which, individually, are sufficiently small to avoid the generation of unduly strong spurious signals within the information signal band~
A system employing both the phase reversal and frequency hopping techniques is disclosed in U. S.
Patent 4,193,030 issued to G. Rabow et al on March 11, 1980. This system applies the coding techniques of the prior art to select the frequency of transmission in a pseudo-random fashion. However, within each frequency hop interval, the signal is further modulated by selecting its phase as 0 degrees or 180 degrees, also in pseudo-random fashion.
The above-described systems, as well as many other prior art systems, employ frequency modulation (FM) techniques~ Although frequency modulation systems are wide-spread and highly successful, the ever-increasing demand for higher system capacity, together with a shortage of available frequencies in the microwave spectrum, has led to a search for a more efficient modulation scheme. Th;s search has concentrated on single sideband modulation (SSB), a well-known technique wherein the speech signal is linearly translated in frequency to a spectral location suitable for radio transmission. Unfortunately, with the use of SSB modulation, co-channel interference becomes extremely disturbing to those using the system.
One technique for suppressing co-channel interference in SSB systems is disclosed in U. S. Patent 4,019,140 issued to R. Swerdlow on April 19, 1977. As disclosed, intelligible crosstalk, or co-channel interference, is substantially eliminated by phase-modulating the AM carrier-wave prior to its modulation by the baseband signal. Such a system, however, would require extensive modification and would be able to accommodate only a small number of users if it were to be used in a cellular mobile radio system employing frequency reuse, and would also subject stationary users to disruption in communications due to frequency-selective fade problemsO
A problem remaining in the prior art~ therefore~
is to provide a method for employing single sideband modulation in a cellular mobile radio system which may æ
serve a large number of users without incurring severe co-channel interference, while having the ability to reduce the probability of complete disruption in communications for those mobiles that are either stationary or at low speeds as found in dense traffic conditions in urban areas.
Summary of the Invention In accordance with an aspect of the invention there is provided in a system for communicating a message signal (s(t)), a transmitter responsive to said message signal for producing as an output a carrier-frequency-hopped single sideband signal s'hop (t)), wherein said transmitter comprises carrier-frequency-hopping generating means for generating as an output a unique sequence of sep-arate possible carrier frequencies (Fl, F2....,FN);
modulating means responsive to both said message signal and said unique sequence of carrier frequencies for moAu-lating said message signal in accordance with said unique sequence of carrier frequencies and producing as an output of said transmitter the carrier-frequency-hopped single sideband signal; and a receiver responsive to said carrier-frequency-hopped single sideband signal produced by said transmitter and receiving therefrom said message signal, wherein said receiver comprises carrier-frequency-hopping generating means for generating as an output said unique sequerlce of separate carrier frequencies generated by said transmitter carrier-frequency-hopping generating means, said receiver carrier-frequency-hopping generating means also capable of producing as an output an assignment code (c(t)) associated with said unique sequence of carrier frequencies wherein said transmitter carrier-frequency-hopping generating means is responsive to said assignment code generated by saic] receiver carrier-frequency-hopping generating means for producing as an output said unique sequence of carrier frequencies related thereto; and demodulating means responsive to both the received carrier-frequency-hopped slngle sideband signa~ (glhop(t)) and 6~:
- 3a said unique sequence of separate carrier frequencies for demodulating said received carrier-frequency-hopped single sideband signal in accordance with said unique sequence of separate carrier frequencies and producing as an outut of said receiver the message signal applied as an input to said transmitter.
The problem remaining in the prior art has been solved in accordance with the present invention, which relates to a frequency-hopped single sideband mobile radio s~stem, and more particularly, to a frequency-hopped single sideband system for use in a cellular mobile radio environment wherein the single sideband signal changes its spectral location every ~ seconds and the sequence of frequency bands occupied is controlled by a suitable scrambling code.
It is an aspect of the present invention to provide a means for employing a single sideband trans-mission scheme in a mobile radio environment which overcomes the crosstalk interference problem inherent in multiple signal single sideband systems. In accordance with the present invention, the crosstalk interference problem is overcome by hopping the carrier frequency of each single sideband signal, changlng the interference from the reception oE an unwanted speech signal to that of noise with random-like properties, where the reception of the interfering noise is subjectively more acceptable than attempting to hold a conversation while hearing other conversations in the background.
It is another aspect of the present invention to provide improved transmission and reception capabilities of a stationary mobile unit which is situated in an area subjected to a deep communication fade which completely disrupts the communication. Since fading is a frequency dependent condition, the application of frequency-hopping in accordance with the present invention, increases the likelihood of maintaining communication, albeit with some degradation in the recovered speech quality, as the probability of repeatedly hopping into a deep fade condition is small. Frequency hopping ensures that no mobile unit is ever in a permanent deep fade, provided the mobile is not in a completely hostile environment, such as a tunnel, etc.
It is a further aspect of the present invention to provide a single sideband system that offers a degree of communication privacy to the users of the system by controlling the sequence of hopped carrier frequencies by a suitable scrambling code.
Other and further aspects of the present invention will become apparent during the course of the following description, and by reference to the accompanying drawings.
Brief Description of the Drawings Referring now to the drawings, where like numerals represent like parts in several views:
FIG. l illustrates an exemplary frequency-hopping single sideband transmitter for inclusion in a mobile unit formed in accordance with the present invention, FIG. 2 illustrates an exemplary division of N channels and N carrier hopping frequencies for an allocated handwidth of B Hz for use in association with the present invention;
FIG. 3 illustrates the movement of the spectral locations of two arbitrary carrier frequency-hopped SSB signals for successive values of ~, emanating from an exemplary transmitter formed in accordance with the present invention;
FIG. 4 illustrates an exemplary base station capable of receiving and demodulating a plurality of M carrier-frequency-hopped single sideband signals in accordance with the present invention; and FIG. 5 illustrates an exemplary hexagonal cellular configuration of a mobile radio system capable of employing the present inventionO
3t6~3~
Detailed Description For the purposes of illus~ration, the present invention will be discussed from the viewpoint of signal flow from a mobile unit to a base station. It must be appreciated, however, that communication will, in fact, also take place in the opposite direction, that is, with a base station transmitting a signal to a mobile unit, and remain within the scope of the present invention.
A block diagram of an exemplary frequency-hopped single sideband transmitter 10 for use, for example, in a mobile unit of a mobile radio system is illustrated in FIG. 1. In operation, an input signal s(t), for example, a speech signal, enters transmitter 10 and is applied as an input to a first bandpass filter 12. First bandpass filter 12 functions to limit the output signal therefrom to a predetermined frequency range fa to fb and produce as an output a filtered speech signal r(t). Assuming r(t) is allowed to reside in a frequency band of Bc Hz, extending from 0 to 4 kHz, for example, the input speech signal may be bandlimited by first bandpass filter 12 to the band fa=0.3 kHz to fb=3.3 kHz, for example, where these values of fa and fb are typical for telephony applications.
In accordance with the present invention, the filtered speech signal r(t) from first bandpass filter 12 is subsequently applied as a first input to a multiplier 14 where a second input to multiplier 14 is a carrier-frequency-hopping sequence fi(t). Carrier frequency sequence fi(t) is generated by a carrier-frequency-hopping generator 16 included in transmitter 10. In general/
generator 16 is under the control of a carrier-frequency-hopping generator 24 included in a base station 30 (not shown), which sends a command to the mobile incl~ding transmitter 10 initializing generator 16 and dictating the carrier-frequency-hopping sequence to be used by the mobile during transmission. Pseudo-random generator arrangements are well-known in the art, and any one may be employed to produce a pseudo-random sequence of carrier frequencies that contain the set of carrier frequencies {Fl,F2,...,FN}.
These pseudo-random sequences of carrier frequencies are utilized to hop the input signal s(t) around its allotted frequency band. For example, if the total bandwidth allotted to a cell in a system formed in accordance with the present invention is B Hz, and an exemplary filtered speech signal r(t) having a bandwidth of fb-fa, located within the bandwidth of Bc Hz, the total bandwidth B could be divided into N channels, where N=B/BC. In accordance with the present invention, therefore, generator 16 functions to produce a pseudo-random sequence of N distinct carrier frequencies ~Fl, F2,...,FN}, where an exemplary carrier frequency Fi is the carrier frequency that positions an SSB signal into the ith spectral channel in the bandwidth B. ~n illustration of the relationship of B, ~c~ N and the set of carrier frequencies {Fl, F2,---,FN} is shown in FIG. 2. FIG. 2 also includes a detailed illustration of the spectral location of an SSB signal for T seconds when the carrier frequency hopped sequence has a value of Fi.
In operation of the present invention, generator 16 produces one carrier frequency every T seconds, where one exemplary sequence may be fi(t)={F2, F4, F6, Fl, FN,...}. Therefore, the output of multiplier 14 will be a double sideband, suppressed carr;er signal shop(t), whose spectral location is altered every T
seconds in accordance with the carrier frequency sequence fi(t)~
Prior to transmission, the output signal shop(t) from multiplier 14, is applied as an input to a second bandpass filter 18, as shown in FIG. 1. Sesond bandpass filter 18 comprises the same bandwidth as first bandpass filter 12, that is, from fa to fb, but where the passband frequencies are moved to occupy the frequency region Ei~fa to fi+fb. Second bandpass filter 18 functions to filter out one sideband of double sideband output signal shop(t), for example, the lower sideband~ to produce as an output of transmitter 10 a carrier-frequency-hopped single sideband signal s'hop(t). Accordingly, in order to allow the frequency band of second bandpass filter 18 to track the movement of the single sideband signal, the output of carrier-frequency-hopping generator 16 is also applied as an input to second bandpass filter 18. An exemplary arrangement of a bandpass filter capable of performing the tracking function of second bandpass filter 18 is disclosed in U. S. Patent ~,232,26~ issued to G. Willoner on November 4, 1980, which teaches the formation of a digitally programmable active RC bandpass filter with constant absolute bandwidth.
In a mobile radio system, a given transmitter 10 may be in such a location that the signal it desires to transmit will be subject to a deep fade condition, that ls, where a specific RF bandwidth is subject to severe attenuation. This problem is avoîded in the present invention by controlling the output of generator 16 in a manner whereby successive carrier-frequency-hopping output signals are in non-contiguous channels, i.e., more than one channel separates a successive carrier frequency and in general, the frequency hopping distance will encompass many channels, and ideally exceed the coherence bandwidth, as defined in "Microwave Mobile Communications", W. C. Jakes, ed. Wiley, 1974. An illustration o~ two exemplary output signals from multiplier 1~ is illustrated in FIG. 3, where as shown for successive time slots 1, 2,..~,L, each signal hops a distance greater than one channel.
In the application of the present invention in a cellular mobile radio system, there will be a plurality of M (generally not greater than N) mobile units in communication with a single base station 30 in a particular cell, where an exemplary base station 30 is illustrated in FIG. ~. Each mobile in a particular cell comprises a separate one of a plurality of M transmitters 101-lOM, where each transmitter functions as described hereinabove in association with transmitter 10 of FIG. 1. In 619~
accordance with the present invention, each mobile unit 101-lOM communicates with cell base station 30 via a distinctly different carrier frequency sequence fl(t)-fM(t), respectively. To ensure that each sequence is, in fact, unique, the plurality of M carrier-frequency-hopped sequences fl(t) through fM(t) are determined by a single synchronized carrier-frequency-hopped generator 2~ located at base station 30, as shown in FIG. 4.
Carrier-frequency-hopping generator 24, as mentioned hereinbefore in relation to transmitter 10 of FIG. 1, broadcasts a plurality of assignment codes cl(t)-cM(t), such that transmitters 101-lOM utilize the identical carrier-frequency-hopped sequences as receivers 201-20M, respectivelyO That is, assignment code cj(t) related to carrier-frequency-hopped sequence fj(t) is broadcast to transmitter lOj when receiver 20j is also assigned carrier-frequency-hopped sequence fj(t).
For an exemplary mobile comprising transmitter lOi, therefore, received assignment code ci(t) may be thouyht of as an address which is related to carrier-frequency-hopped sequence fi(t). Therefore, generator 16 produces as an output sequence fi(t), which as described hereinabove, is applied as an input to both multiplier 14i, and second bandpass filter 18i, and functions as described hereinabove in association with transmitter 10 of FIG. 1.
Each of the separate carrier-frequency-hopped sequences fl(t) through fM(t) is also utilized by base station 30 to correctly demodulate the plurality of M carrier-frequency-hopped signals shop l(t)-s~op M(t) which it receives from mobile units comprising transmitters 101-lOM, respectively. As shown in FIG~ 4, each of the separate carrier-frequency-hopped sequences fl(t)-fM(t) is applied as an input to a separate one of a plurality of receivers 201-20~ included in base station 30, where receivers 201, 20i, and 20M are illustrated in detail.
In operation, receiver 20i functions to demodulate the received carrier-frequency-hopped signal G~3~
shop i(t). Since the signal produced by transmitter 10i travels through the communication medium before being received by receiver 20i, some signal degradation will occur, and, therefore, the signal received by receiver 20i, shop i(t), is an approximation of the transmitted signal shop i(t). Upon reception, received carrier~frequency-hopped SSB signal shop i(t) is applied as an input to a first bandpass filter 22i, where first bandpass filter 22i like second bandpass filter 18i of transmitter 10i, is capable of passing only the portion of the received signal in the frequency band fa~fi(t) to b+fi(t) where fa~ fb~
and fi(t) are the frequencies described hereinabove in association with transmitter 10 of FIG. 1. In order for the sequence of frequencies fi(t) used by irst bandpass filter 22i to correctly filter the received signal, carrier-frequency-hopped generator 24 included in base station 30 must operate in synchronization with transmitter 10i, producing the same carrier frequency in every T
seconds as employed by transmitter 10i. One exemplary synchronization method is disclosed in U. S.
Patent 3,641,433 issued to Ro Mifflin et al on February 8, 1972, which describes a transmitted reference synchronization system.
Therefore, assuming that carrier-frequency~
hopping generator 2~ does, in fact, apply the same set of frequencies in synchronization with transmitter 10i, the output of first bandpass filter 22i will be a received filtered carrier-frequency-hopped SSB signal ri(t).
SSB signal Pi(t) is subsequently applied as a first input to a multiplier 26i, where the second input to multiplier 26i is the carrier-frequency-hopped sequence fi(t) produced by carrier-frequency-hopping generator 2~. Multiplier 26i together with a second bandpass filter 28i whose frequency passband is fb~fc Hz, functions to demodulate the carrier-frequency-hopped SSB signal ri(t). Since the sequence of carrler frequencies used by transmitter 10i enable the baseband voice signal si(t) to be SSB modulated and ~g%
frequency hopped, and they are identical to those used by receiver 20i in the demodulation process, the output signal si(t) of second bandpass filter 28i is the recovered speech signal corresponding to the original signal si(t). As mentioned hereinabove, the correspondence between Si(t) and si(t) is limited by transmission channel and circuit imperfections.
Since each separate receiver 201-20M functions as described hereinabove in association with receiver 20i, base station 30 is capable of producing M separate output signals gl(t)-sM(t) which correspond to the M separate input signals sl(t)-sM(t) modulated and transmitted by transmitters 101-lOM, respectively.
A simple diagram of a cellular mobile radio system comprising hexagonal cells is illustrated in FIG. 5, where it is to be understood that the hexagonal cellular configuration is exemplary and for illustrative purposes only, since in the practice with the present invention, the individual cells may comprise any configuration known in the art. Assuming that the total available system bandwidth is equal to BT, it is common practice in a hexagon cellular system to partition the banc]widt~ BT into equal segments, assigning a different segment to each cell of a set of contiguous cells, where in FIG. 5 a cluster of seven such cells, each allocated separate bandwidths, Bl-B7 is shown. If, in the example shown in FIG~ 5, more than seven cells o a predetermined size are required to accommodate the user area demand, some frequency bands must be reused. The arrangement illustrated in FIG. 5 reuses all seven frequency bands and attempts to keep the distance between cells usinq the same frequency band large in order to restrict mutual interference. For example, the cells using frequency band Bl are spaced a distance D apart, where this distance is measured from the respective centers of the cells, as shown in FIG. 5 A mobile unit traveling in a central cell~
denoted A, using frequency band Bl will receive co-channel %
interference from each of the outlying cells also using frequency band Bl. In studying co-channel interference, the six closest co-channel cells, labeled Al-A6, cause the dominant co-channel interference. If carrier-frequency-hopping is not used, the mobile in cell A will receiveinterference in the form of intelligible crosstalk from cells Al-A6. However, when carrier-frequency-hopping is employed in accordance with the present invention, each cell in the set of cells Al-A6 will perform carrier-frequency-hopping with different and distinct sets of carrier frequency sequences. Thus, the mobile in cell A, upon receiving separate interfering signals from cells Al-A6, will demodulate these signals with the unique carrier frequency sequence belonging to cell A. Consequently, the lS mobile in cell A will recover the desired signal, and the interfering signals will be demodulated as incoherent signals. Thus, because the carrier frequency sequence unique to the mobile in cell A is incorrect to demodulate the signals from cells Al-A6, all of the demodulated interference signals, when added together will become more noise-like and in the case of speech transmission, perceptually more acceptable than a single coherent interfering speech si~nal.
As the mobile in cell A leaves this cell and enters one of the adjacent cells, for example, a cell which employs frequency band B3, the mobile ceases to communicate via the base station in cell A and begins to communicate via the base station in its new cell. The mobile may or may not maintain the same carrier frequency sequence -either alternatlve is possible in accordance with thepresent invention. That is, in a set of seven contiguous cells employing di~ferent frequency bands, the mobile may retain the same carrier frequency sequence and merely change the spectral location that the sequence is used in as the mobile travels from cell to cell. Alternati~ely~
each base station may transmit a different carrier frequency sequence assignment code to the mobile as it enters its cell. In either case, information must be "handed-off" between the base stations in cell A and the new cell the mobile enters. There exist many methods of completing this handing off which are well known in the art, where one such arrangement is disclosed in U. S.
Patent 3,71~,574 issued to T. Baba et al on January 30, 1973 which relates to a mobile communication system.
Although frequency hopping does not alter the signal-to-noise ratio of SSB systems, computed using long-term averaging, it does have important perceptualadvantages. For example, consider a stationary mobile, e.g., parked or in congested traffic, operating in a SSB system that does not employ frequency hopping. Suppose the mobile is subjected to a deep fade that results in a complete disruption of communication. Since such a fade i5 frequency dependent, if frequency hopping is applied in such a situation, the probability that communication will be restored increases, since the probability of repeatedly hopping into a deep fade condition is small. Of course, if the mobile was not in a deep fade but receiving excellent communications, the effect of frequency hopping would degrade the recovered speech although the degradation would be minimal. What frequency hopping ensures, therefore, is that no mobile will ever be in a permanent deep fade.
Further, in cellular mobile radio systems employing frequency reuse, co-channel interference exists in the form of cross-talk among the mobiles in all of the cells using the same segment of the frequency band. By applying frequency hopping, in accordance with the present invention, the co-channel interference is changed from cross-talk to random-like noise, as each mobile demodulates using its unique frequency hopping sequence. Additionally, as each signal broadcast by a mobile transmitter is "hopped" around the N channels at a rate of one hop every seconds, only the intended receiver knows the hopping sequence, or code, employed and is able to demodulate correctly the transmitted signalO Therefore, a single 6~32 sideband transmission system formed in accordance with the present invention will deter eavesdropping, provi~ing a measure of security over the communication path. Lastly, as all such transmission systems are in general s~lbjected to fading, well known methods, such as the inclusion of pilot tones can, in general, be added to this transmitted carrier-frequency-hopped SSB signal shop(t) as it is employed in accordance with the present invention.
Claims (11)
1. In a system for communicating a message signal (s(t)), a transmitter responsive to said message signal for producing as an output a carrier-frequency-hopped single sideband signal s'hop (t)), wherein said transmitter comprises carrier-frequency-hopping generating means for generating as an output a unique sequence of separate carrier frequencies (fi(t)), from a plurality of possible carrier frequencies (F1, F2...,FN);
modulating means responsive to both said message signal and said unique sequence of carrier frequencies for modulating said message signal in accordance with said unique sequence of carrier frequencies and producing as an output of said transmitter the carrier-frequency-hopped single sideband signal; and a receiver responsive to said carrier-frequency-hopped single sideband signal produced by said transmitter and receiving therefrom said message signal, wherein said receiver comprises carrier-frequency-hopping generating means for generating as an output said unique sequence of separate carrier frequencies generated by said transmitter carrier-frequency-hopping generating means, said receiver carrier-frequency-hopping generating means also capable of producing as an output an assignment code (c(t)) associated with said unique sequence of carrier frequencies wherein said transmitter carrier-frequency-hopping generating means is responsive to said assignment code generated by said receiver carrier-frequency-hopping generating means for producing as an output said unique sequence of carrier frequencies related thereto; and demodulating means responsive to both the received carrier-frequency-hopped single sideband signal (?'hop(t)) and said unique sequence of separate carrier frequencies for demodulating said received carrier-frequency-hopped single sideband signal in accordance with said unique sequence of separate carrier frequencies and producing as an output of said receiver the message signal applied as an input to said transmitter.
modulating means responsive to both said message signal and said unique sequence of carrier frequencies for modulating said message signal in accordance with said unique sequence of carrier frequencies and producing as an output of said transmitter the carrier-frequency-hopped single sideband signal; and a receiver responsive to said carrier-frequency-hopped single sideband signal produced by said transmitter and receiving therefrom said message signal, wherein said receiver comprises carrier-frequency-hopping generating means for generating as an output said unique sequence of separate carrier frequencies generated by said transmitter carrier-frequency-hopping generating means, said receiver carrier-frequency-hopping generating means also capable of producing as an output an assignment code (c(t)) associated with said unique sequence of carrier frequencies wherein said transmitter carrier-frequency-hopping generating means is responsive to said assignment code generated by said receiver carrier-frequency-hopping generating means for producing as an output said unique sequence of carrier frequencies related thereto; and demodulating means responsive to both the received carrier-frequency-hopped single sideband signal (?'hop(t)) and said unique sequence of separate carrier frequencies for demodulating said received carrier-frequency-hopped single sideband signal in accordance with said unique sequence of separate carrier frequencies and producing as an output of said receiver the message signal applied as an input to said transmitter.
2. A communication system formed in accordance with claim 1 wherein the message signal (s(t)) is multiplied with a sequence of pseudo-random carrier frequencies such that said message signal can only be recovered by a receiver that has knowledge of said sequence of pseduo-random carrier frequencies.
3. A communication system formed in accordance with claim 1 wherein the modulating means comprises a first bandpass filter responsive to the message signal for producing as an output a filtered message signal (r(t)) equal to the portion of said message signal appearing in a predetermined frequency range (fa-fb);
a multiplier responsive to both said filtered message signal and the unique sequence of separate carrier frequencies for producing as an output a carrier-frequency-hopped message signal (shop(t)) equal to the multiplicative product thereof; and a second bandpass filter responsive to both said carrier-frequeney-hopped message signal and said unique sequence of separate carrier frequencies for removing unwanted components from said carrier-frequency-hopped message signal and producing as an output of said transmitter the earrier-frequency-hopped single sideband signal (s'hop(t)); and the demodulating means comprises a first bandpass filter responsive to both the received carrier-frequency-hopped single sideband signal and the unique sequence of carrier frequencies for producing as an output a filtered carrier-frequency-hopped single sideband signal (r(t));
a multiplier responsive to both said filtered carrier-frequency-hopped single sideband signal and said unique sequence of carrier frequencies for producing as an output a multiplicative product thereof (d(t)); and a second bandpass filter responsive to said multiplicative product signal for removing unwanted components therefrom and producing as an output of said receiver the message single applied as an input to said transmitter
a multiplier responsive to both said filtered message signal and the unique sequence of separate carrier frequencies for producing as an output a carrier-frequency-hopped message signal (shop(t)) equal to the multiplicative product thereof; and a second bandpass filter responsive to both said carrier-frequeney-hopped message signal and said unique sequence of separate carrier frequencies for removing unwanted components from said carrier-frequency-hopped message signal and producing as an output of said transmitter the earrier-frequency-hopped single sideband signal (s'hop(t)); and the demodulating means comprises a first bandpass filter responsive to both the received carrier-frequency-hopped single sideband signal and the unique sequence of carrier frequencies for producing as an output a filtered carrier-frequency-hopped single sideband signal (r(t));
a multiplier responsive to both said filtered carrier-frequency-hopped single sideband signal and said unique sequence of carrier frequencies for producing as an output a multiplicative product thereof (d(t)); and a second bandpass filter responsive to said multiplicative product signal for removing unwanted components therefrom and producing as an output of said receiver the message single applied as an input to said transmitter
4. A transmitter for communicating a message signal (s(t)) wherein said transmitter is responsive to said message signal for producing as an output a carrier-frequency-hopped single sideband signal (s'hop(t)), wherein said transmitter comprises carrier-frequency-hopping generating means for generating as an output a unique sequence of separate carrier frequencies (fi(t)) from a plurality of N
possible carrier frequencies; and modulating means responsive to both said message signal and said unique sequence of separate carrier frequencies for modulating said message signal in accordance with said unique sequence of carrier frequencies and producing as an output of said transmitter the carrier-frequency-hoped signal sideband signal, where the modulating means comprises a first bandpass filter response to the message signal for producing as an output a filtered message signal r(t) equal to the portion of said message signal appearing in a predetermined frequency range fa-fb);
a multiplier responsive to both said filtered message signal and the unique sequence of separate carrier frequencies for producing as an output a carrier-frequency-hopped message signal (Shop(t)) equal to the multiplicative produce thereof; and a second bandpass filter responsive to both said carrier-frequency-hopped message signal and said unique sequence of separate carrier frequencies for removing unwanted components from said carrier-frequency-hopped message signal and producing as an output of said transmitter the carrier-frequency-hopped single sideband signal (s'hop(t)).
possible carrier frequencies; and modulating means responsive to both said message signal and said unique sequence of separate carrier frequencies for modulating said message signal in accordance with said unique sequence of carrier frequencies and producing as an output of said transmitter the carrier-frequency-hoped signal sideband signal, where the modulating means comprises a first bandpass filter response to the message signal for producing as an output a filtered message signal r(t) equal to the portion of said message signal appearing in a predetermined frequency range fa-fb);
a multiplier responsive to both said filtered message signal and the unique sequence of separate carrier frequencies for producing as an output a carrier-frequency-hopped message signal (Shop(t)) equal to the multiplicative produce thereof; and a second bandpass filter responsive to both said carrier-frequency-hopped message signal and said unique sequence of separate carrier frequencies for removing unwanted components from said carrier-frequency-hopped message signal and producing as an output of said transmitter the carrier-frequency-hopped single sideband signal (s'hop(t)).
5. A receiver capable of receiving and demodulating a carrier-frequency-hopped single sideband signal (shop(t)) and recovering thereform a message signal (s(t)) wherein the receiver comprises carrier-frequency-hopping generating means for generatinq as an output a unique sequence of separate carrier frequencies related to the received carrier-frequency-hopped single sideband signal; and demodulating means responsive to both said received carrier-frequency-hopped single sideband signal and said unique sequence of separate carrier frequencies for demodulating said received carrier-frequency-hopped single sideband signal in accordance with said unique sequence of separate carrier frequencies and producing as an output of said receiver the message signal related thereto, wherein the demodulating means comprises a first bandpass filter responsive to both the received carrier-frequency-hopped single sideband signal and the unique sequence of carrier frequencies for producing as an output a filtered carrier-frequency-hopped single sideband signal (r(t));
a multiplier responsive to both said filtered carrier-frequency-hopped single sideband signal and said unique sequence of carrier frequencies for producing as an output a multiplicative produce thereof (d(t)); and as second bandpass filter responsive to said multiplicative product signal for removing unwanted components therefrom and producing as an output of said receiver the message signal related thereto.
a multiplier responsive to both said filtered carrier-frequency-hopped single sideband signal and said unique sequence of carrier frequencies for producing as an output a multiplicative produce thereof (d(t)); and as second bandpass filter responsive to said multiplicative product signal for removing unwanted components therefrom and producing as an output of said receiver the message signal related thereto.
6. In a communication system employing single sideband modulation a plurality of M mobile units, each mobile unit capable of accepting a separate message signal (s1(t)-sm(t)), wherein each separate message signal is modulated by a separate of a plurality of M distinct carrier-frequency-hopping sequences (f1(t)-fm(t)) to produce as an output a separate one of a plurality of M
unique carrier-frequency-hopped single sideband signals (s' hop,1(t)- s' hop, M(t)); and a base station responsive to and capable of demodulating said plurality or M unique carrier-frequency-hopped single sideband signals and recovering therefrom a plurality of M separate message signals (s1(t)-sm(t)).
unique carrier-frequency-hopped single sideband signals (s' hop,1(t)- s' hop, M(t)); and a base station responsive to and capable of demodulating said plurality or M unique carrier-frequency-hopped single sideband signals and recovering therefrom a plurality of M separate message signals (s1(t)-sm(t)).
7. A communication system in accordance with claim 6 wherein each mobile unit of the plurality of M
mobile units comprises a carrier-frequency-hopping generator means for generating a unique carrier-frequency-hopped sequence; and the base station comprises a carrier-frequency-hopping generating means for generating both the plurality of M distinct carrier-frequency-hopping sequences employed by said plurality of M mobile units and a plurality of M assignment codes (c1(t)-CM(t)), where each assignment code is associated with a separate one of said plurality of M distinct carrier-frequency-hopping sequences, said base station capable of transmitting each assignment code of said plurality of M assignment codes to a separate one of said plurality of M carrier-frequency-hopping generating means of said M mobile units for assigning the unique carrier-frequency-hopping sequence thereto.
mobile units comprises a carrier-frequency-hopping generator means for generating a unique carrier-frequency-hopped sequence; and the base station comprises a carrier-frequency-hopping generating means for generating both the plurality of M distinct carrier-frequency-hopping sequences employed by said plurality of M mobile units and a plurality of M assignment codes (c1(t)-CM(t)), where each assignment code is associated with a separate one of said plurality of M distinct carrier-frequency-hopping sequences, said base station capable of transmitting each assignment code of said plurality of M assignment codes to a separate one of said plurality of M carrier-frequency-hopping generating means of said M mobile units for assigning the unique carrier-frequency-hopping sequence thereto.
8. A communication system in accordance with claim 7 wherein the base station carrier-frequency-hopping generating means is capable of generating the plurality of _ 19 _ M distinct carrier-frequency-hopping sequences such that at any instant of time each mobile unit in communication therewith is employing a different carrier frequency.
9. A communication system in accordance with claim 7 wherein the base station carrier-frequency-hopping generating means is capable of generating M distinct carrier-frequency-hopping sequences wherein for any carrier-frequency-hopping sequence two consecutive carrier frequencies in said carrier-frequency-hopping sequence are separated in frequency by a predetermined amount to reduce the probability of fading in both carrier frequencies.
10. In a communication system employing single sideband modulation a plurality of mobile units, each mobile unit capable of accepting a separate message signal, wherein each separate message signal is modulated by a separate one of a plurality of distinct carrier-frequency-hopping sequences and said plurality of mobile units is capable of producing as an output a plurality of unique carrier-frequency-hopped single sideband signals; and a plurality of base stations, each base station encompassing a distinct geographic area and contiguous base stations alloted a separate portion (B1-B7) of a predetermined total frequency band wherein each base station comprises a carrier-frequency-hopping generating means for generating a plurality of distinct carrier-hopping frequencies such that each carrier element of each sequence of said plurality of distinct sequences resides in said alloted portion of said total frequency band such that each base station is capable of communicating with any mobile units located in its associated geographic area.
11. A communication system in accordance with claim 10 wherein a set of base stations (A1 -A6) alloted the identical portion (Bj) of the predetermined total frequency band are located a predetermined distance (D) apart, each base station in said set of base stations generates a unique plurality of carrier-frequency-hopped sequences such that interference between the separate pluralities of carrier-frequency-hopped sequences produced by said set of base stations is in the form of a pseudo-random signal.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US06/363,273 US4479226A (en) | 1982-03-29 | 1982-03-29 | Frequency-hopped single sideband mobile radio system |
| US363,273 | 1982-04-29 |
Publications (1)
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|---|---|
| CA1199682A true CA1199682A (en) | 1986-01-21 |
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| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000423000A Expired CA1199682A (en) | 1982-03-29 | 1983-03-07 | Frequency hopped single sideband mobile radio system |
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| US (1) | US4479226A (en) |
| EP (1) | EP0104206B1 (en) |
| CA (1) | CA1199682A (en) |
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| GB (1) | GB2118401B (en) |
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| FR2518338A1 (en) * | 1981-12-15 | 1983-06-17 | Thomson Csf | METHOD FOR SYNCHRONIZING THE TRANSCEIVER-RECEIVER STATIONS OF A FREQUENCY HOPPER NETWORK AND POST FOR CARRYING OUT SAID METHOD |
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- 1983-02-28 DE DE8383901055T patent/DE3367145D1/en not_active Expired
- 1983-03-07 CA CA000423000A patent/CA1199682A/en not_active Expired
- 1983-03-24 GB GB08308159A patent/GB2118401B/en not_active Expired
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| EP0104206A1 (en) | 1984-04-04 |
| GB2118401B (en) | 1986-07-30 |
| EP0104206B1 (en) | 1986-10-22 |
| GB8308159D0 (en) | 1983-05-05 |
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