WO1999023508A1

WO1999023508A1 - Radio telemetry system

Info

Publication number: WO1999023508A1
Application number: PCT/GB1998/003290
Authority: WO
Inventors: Andrew Bateman; Simon James Jones; Paul George Turner
Original assignee: Wireless Systems International Limited
Priority date: 1997-11-03
Filing date: 1998-11-03
Publication date: 1999-05-14
Also published as: FR2770671A1; GB2331202A; GB2331202A9; GB9723181D0

Abstract

A radio telemetry system for collecting data from multiple remote units (7-11) via respective multiple radio channels in which channel frequencies and bandwidths are allocated according to channel transmission losses measured by reference to pilot signals (P1, P2) in each channel as measured at the remote units. Preferably, each remote unit is allocated a block of contiguous channels (Figure 2d) so that channels either side of a central channel (fu) are available to transmit data from other remote units (8, 10, 12) in the manner of a repeater unit. Preferably, the system also supports voice transmission on one or more channels using a different modulation scheme to that used for data transmission.

Description

RADIO TELEMETRY SYSTEM

This invention relates to a radio telemetry system for collecting data from an array of multiple remote units, for example, for collecting seismic data simultaneously from seismic sensors deployed at remote locations in the field.

The system is a radio system which supports multiple closely spaced narrow band channels across the radio spectrum, typically, at 20 kHz channel centre spacing with up to 1000 data channels.

In order to accommodate attenuation differences along the different transmission paths between the remote units and a common base station, the transmit power of each remote unit is variable and is set so as to ensure as far as possible a substantially optimum level of power reception at the base station on each channel.

However, according to one aspect of the invention, variations in the received power levels of different channels due to path losses can be accommodated by allocating channel frequencies according to received power level so that higher power channels are located adjacent one another in the frequency spectrum, away from lower power channels, thereby minimising the effect of cross-channel interference.

In an embodiment of the invention in which a channel supports an uplink for transmitting data from the remote unit to the base station, and also a downlink for transmitting control data from the base station to the remote unit, the received power level of the downlink at the remote unit is preferably used to control its transmit power level on the uplink.

The power level of the channels are preferably measured by reference to particular pilot signals incorporated in each channel .

According to another aspect, the power characteristic of each channel is made as linear as possible so as to limit cross- channel interference.

According to yet another aspect, the bandwidth of the channels is varied to suit the quality of the channels in terms of received power level, a reduced bandwidth being used where possible with an increased symbol rate made possible by the higher quality of the channel so as to maintain the overall required data rate. In this way, the number of channels provided by the system over a given radio frequency spectrum can be maximised.

According to yet another aspect, each channel comprises one of a plurality of contiguous channels located towards the centre of the total frequency band of the remote unit, the channels either side of the central channel being available to transmit data collected by other remote units so that the remote unit can be used as a repeater in the radio transmission path.

If a remote unit is used as a repeater, it is coupled via a data link to a similar remote unit which receives data on multiple channels from said other remote units and transfers this data via the data link to the remote unit in its analogue base band form for transmission onwards . Onward transmission of the data on said contiguous channels is effected over a frequency band which may be separated from the frequency of the multiple received channels to avoid receiver de-sensitisation by transmission on one of the channels. This frequency separation is especially important, bearing in mind that both units linked to form the repeater are preferably transceivers and support half duplex bi-directional communications, on both uplinks and downlinks. Typically, a separation of at least 2MHz is maintained between transmission and receive frequencies of the repeater. The downlink supports various control functions, including the allocation of channel frequencies to be used by remote units and repeaters .

According to yet another aspect of the invention, one of the channels of the telemetry system is utilised as a voice channel .

Preferably, both voice and data channels operate simultaneously, but each employs a different modulation scheme; for example, QAM constellations for the data channels and a linear modulation scheme for the voice channel .

The voice channel may be incorporated on a downlink to the remote units, and may share this channel with control signals. Also, an uplink may be adapted to allow either voice or data to be sent to the base station, or to carry both voice and data simultaneously in separate sub-bands.

The base station may then be used to relay voice messages on to other remote units on a downlink. Additionally, the system may be controlled to support half duplex voice communication. The invention will now be described by way of example, with reference to the accompanying drawings, in which:

Fi^gure 1 is a schematic diagram of a radio telemetry system for collecting seismic data from an array of data collection units;

Fi^gure 2A to 2D show schematic representations of various transmission channels associated with a typical data collection unit, comprising a command channel downlink, Figure 2A; a repeated commend channel downlink, Figure 2B_; a data uplink, Figure 2C; and a repeated data uplink, Figure 2D;

Figure 3 is a schematic representation of the distribution of channel frequencies according to channel transmission losses;

Figure 4 is a schematic representation of the frequency spectrum of a data channel and the pilot tones provided in the data channel to monitor transmission loss; and

Figure 5A to 5D is a schematic representation showing how the frequency spectrum of a data channel can be varied to suit different quality channels. As shown in Figure 1, the system comprises a plurality of remote units each comprising a seismic array 1 to 6 connected to a respective radio transmitter 7 to 12. The transmitters are arranged, in use, to transmit data to a base station 14. Most transmitters, such as 7,9,11, are arranged in the line of sight of the base station 14, but in some cases an obstruction 16 prevents direct communication between the transmitters 8,10,12 and the base station 14.

The transmitters 7,9,11 located in the line of sight to the base station 14 are each allocated a uni-que channel frequency for the transmission of data on an uplink to the base station. Each channel is operated as a half duplex, bi- directional channel, the transmitters 7,9,11 being transceivers which allow receipt of control data from the base station 14 on a downlink. The control data serves to control the allocation of channel frequencies, the transmit power level of the transmitters, and switching between uplinks and downlinks .

The frequency spectrum of a single channel is shown in Figure 4, and has a nominal central frequency RF and two pilot signals PI and P2 located either side of RF in the channel passband. These pilot signals may be pure tones, but could also be modulated tones or otherwise narrow bandwidth signals. The frequency spacing between the pilot signals PI and P2 is predetermined and is used in the remote unit for frequency locking. The remote unit searches for these two pilot signals PI and P2 , and the difference between these frequencies are measured, compared with the expected frequencies, is used to produce an error signal to adjust a local clock, such as a voltage controlled crystal oscillator.

The pilot signals Pi and P2 are also used to determine the transmission loss in each channel by comparing the amplitude of at least one pilot signal in the downlink, with a reference amplitude, and then adjusting the transmitting power of the remote transmitter 7,9,11 to compensate for transmission losses as far as possible, and thereby ensure adequate power reception at the base station 14. If the power received at the base station 14 is still not acceptable because of a malfunction at the remote unit or a non- reciprocal transmission path characteristic, the base station sends a power correction on the down-link.

At the same time, the transmission losses of the different channels are used to control the allocation of frequencies to the channels for different remote units so that those remote units with significant differing transmission losses are allocated channels at opposite ends of the frequency spectrum, as illustrated schematically in Figure 3. In the drawing, the channels f₇, f₉, fu, for the transmitters 7,9,11, are assumed to have low transmission losses, and are grouped together towards one end of the spectrum, and the channels f₈/ fio, fi2, for the transmitters 8,10,11, served by repeaters are assumed to have high transmission losses, and are grouped together towards the opposite end of the spectrum.

Those transmitters, 8,10,12, shadowed by obstruction 16 communicate with the base station 14 via a repeater 18. The repeater comprises a first unit 20 in radio communication with the transmitters 8,10, and 12, and a second unit 22 in radio communication with the base station 14. In this example, both the first and second units 20 and 22 include radio transceivers. The units 20 and 22 are connected together via a data link 24. The data link 24 is a base band relay cable, such as a cable containing a plurality of electrically conducting wires although other data transmission media may be used. The units 20 and 22 are arranged such that one unit acts as receiver whilst the other acts as a transmitter. Thus the repeater supports half duplex bi-directional communication. This ensures that the shadowed remote units 8,10 and 12 can both send data to the base station and also receive commands from a command down link channel of the base station via the repeater .

The receiver and transmitter typically operate at different frequencies. However, since both operate concurrently, there is a danger of receiver desensitisation as the receiver ' s blocking performance may be exceeded by the proximity of a local transmitter operating at a different, but possibly nearby frequency. This problem may be alleviated by the use of directional antennas, but it is further overcome by physically separating the first and second units (i.e. the receiver and transmitter) by a sufficient distance in order to ensure that the strong out-of-channel signal received from the transmitter has become sufficiently attenuated not to degrade the receiver's performance in respect of the in- channel signals received from the transmitters 8,10 and 12. Thus the cable link 24 serves both to enable the units 22 and 20 to be positioned at respective locations such that each has good line of sight communication with the transmitters 8,10 and 12, and the base station 14 respectively, and also serves to separate the units sufficiently such that cross talk between them is within acceptable levels . The second unit 22 also may include the facility to receive data from a local seismic sensor array 26 and to transmit this data to the base station.

Figures 2a to 2d schematically illustrate the frequency allocation in the repeater system shown in Figure 1. The base station 14 can transmit command data to the radio transmitters 8,10 and 12, and also to the or each repeater 18. Thus the control functions can include the allocation of repeater receive and transmit frequencies , switching between serving up links and down links, waking up or powering down repeaters and setting transmit power levels. The command channel down link has a frequency spectrum centred around nominal centre frequency f_d. The second element 22 of the repeater is in line of sight communication with the base station 14 and receives the command channel data. This data is then frequency down converted and transmitted at base band frequencies over the data cable 24 to the first unit 20 of the repeater. The first unit 20 up-converts the command channel data to a repeated command channel down link having a frequency spectrum centred around nominal frequency f_d' . Additional repeaters (not shown) may receive signals from and transmit signals to the first repeater 18 and are arranged to be responsive to the command channel at frequencies f_d and f_d' . Similarly, the transmitters of the seismic data array may include receiver elements also responsive to signals received on any one or more pre-allocated command channel frequencies including the frequencies f_d and f_d' .

Each of the transmitters 7 to 12 associated with a seismic array is arranged to transmit on a respective frequency. As shown in Figure 2c, the six transmitters schematically illustrated in Figure 1 (8,10,12,30,32 and 34) transmit on six closely spaced channels adjacent a notional seismic data up link frequency f_u. These channels are frequency down converted by the first element 20 in order that they can be transmitted along the data link 24. Thus, the signal transmitted along the data link comprises a plurality of closely spaced channels. This can be regarded as frequency division multiplexed data transmission. The second unit 22 receives the multiplexed signals and up converts them to a plurality of closely spaced channels centred around a repeated seismic data up link frequency f_u' as shown in Figure 2d. The data received from the local seismic array 26 is also up converted and in this example is retransmitted at the centre frequency f_u' with the additional repeated channels being allocated around the centre frequency.

Given that each of the transmitters 8,10,12,30,32 and 34 operates on a respective channel, each located adjacent a centre frequency, it will be appreciated that the respective incoming channels can be frequency converted merely by mixing with a local oscillator. This allows multiple independent incoming channels to be easily and inexpensively converted to another frequency for transmission over the data link 24, and similarly, frequency mixing can again be used to up convert the incoming data for re-transmission to a base station or transmission to a further repeater.

The unit 22 also demodulates any signal received from the base station in order to check for command signals sent from the base station. The repeater operates in half-duplex mode and consequently the repeater is arranged, primarily, to listen to the base station in order to receive commands therefrom.

When the base station is ready to receive information, it sends a command code to each of the remote units instructing them to transmit data, and also instructs the repeater to change operation so that it listens to the remote units and retransmits to the base station until the end of a predetermined period, when it reverts to listening to the base station. It is thus possible to provide a frequency translating radio repeater for use in a remote data acquisition network .

Although the data channels as illustrated in Figure 4 have a fixed bandwidth, the system can be modified to vary the bandwidth of individual channels to suit the received power level of the channel at the base station. Thus, better quality channels with lower transmission losses will support a larger number of symbol states in a QAM or QPSK modulation scheme, and thus a narrower channel bandwidth can be used to achieve the required data transfer rate for the system.

For example, Figure 5A shows a 20 kHz channel configured to transmit data at a rate of 60 kbps using a 16 symbol QAM modulation scheme. Figure 5B shows a 10kHz channel configured to transmit data at the same rate of 60 kbps, but using a 256 QAM modulation scheme which can be supported on a higher -quality transmission channel configured to transmit data at the same 60 kbps rate using a QPSK (4-QAM) modulation scheme. Finally, . Figure 5D shows how adjacent channels of 20 kHz bandwidth each can be used together, with a QPSK modulation scheme in each, to achieve the same data rate of 60 kbps over the same overall bandwidth of 40 kHz as in Figure 5C. It will be appreciated that a seismic telemetry system with up to a 1000 remote units transmitting simultaneously requires an effective frequency planning algorithm to allocate channels from available bandwidth in such a manner that the system performance and bandwidth efficiency are optimised.

Firstly, the frequency planning algorithm must avoid those channels that are occupied by predetermined primary users, i.e. users other than the remote seismic units and base station.

Secondly, the channel frequencies and bandwidth are allocated according to channel transmission losses as described above.

Thirdly, each repeater is allocated a block of contiguous channels (e.g. up to 7 channels) for the uplinks from the multiple shadowed remote units being served, and a similar block of contiguous channels frequency translated for the repeated uplinks. Also, channels allocated to remote units served by a repeater are re-used elsewhere in the system where propagation conflict is not possible. The frequency planning algorithm is an iterative one which repeats the channel allocation process and monitors system- wide performance until satisfactory performance is achieved.

Claims

1. A radio telemetry system for collecting data from multiple remote units, via respective multiple radio channels allocated within an overall radio spectrum, channel frequencies being allocated according to the received power level in each channel so that higher power channels are located adjacent one another in the frequency spectrum, away from lower power channels, thereby minimising the effect of cross-channel interference.

2. A system as claimed in claim 1, in which the transmit power of each remote unit is variable and is set so as to ensure as far as possible a substantially optimum level of power reception at the base station on each channel .

3. A system as claimed in claim 2, in which a channel supports an uplink for transmitting data from the remote unit to the base station, and a downlink for transmitting data from the base station to the remote unit, the received power level of the downlink at the remote unit being used to control its transmit power level on the uplink.

4. A system as claimed in claim 2 or 3 in which the power level of the channels are measured by reference to particular pilot signals incorporated in each channel .

5. A system as claimed in claim 4 in which the pilot signals are also used for frequency locking by the remote units .

6. A radio telemety system for collecting data from multiple remote units via respective multiple radio channels in which the power characterstic of each channel is made as linear as possible so as to limit cross-channel interference .

7. A radio telemety system for collecting data from multiple remote units via respective multiple radio channels, the bandwidth of the channels being varied to suit the quality of the channels in terms of received power level, so that the bandwidth is reduced when an increased symbol rate is supported by higher quality channels so as to maintain an overall required data rate.

8. A radio telemetry system for collecting data from multiple remote units via respective multiple radio channels, each channel comprising one of a plurality of contiguous channels so that channels either side of the said channel are available to transmit data collected by other remote units .

9. A radio telemetry system for collecting data from multiple remote units via respective radio channels, one of the channels of the system being utilised as a voice channel .

10. A system as claimed in claim 9, in which both voice and data channels operate simultaneously, each employing a different modulation scheme.

11. A system as claimed in claim 9 or 10, in which the voice channel is incorporated on a downlink to the remote units, and shares this channel with control signals.

12. A system as claimed in any of claims 9 to 11, in which an uplink may be adapted to allow either voice or data to be sent to the base station, or to carry both voice and data simultaneously in separate sub-bands.