GB2233860A

GB2233860A - "Extending the range of radio transmissions"

Info

Publication number: GB2233860A
Application number: GB8916097A
Authority: GB
Inventors: Andrew William Jeffries; William Basil Harrold
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1989-07-13
Filing date: 1989-07-13
Publication date: 1991-01-16
Anticipated expiration: 2009-07-13
Also published as: GB8916097D0; GB2233860B

Abstract

In a communications system employing HF radio transmission a message to be transmitted is subjected to time spreading modulation prior to transmission. The time spreading modulation results in processing gain at the receiver with either resultant transmission range increase, which is particularly beneficial in the case of inefficient antennae, or limited transmitter power, or permitting lower power to be used for a predetermined transmission range. A message 5 at a first bit rate is multiplied at 6 by a pseudo random spreading sequence 7 at a higher bit rate, and the resultant data stream is modulated at DPSK modulator 4 prior to transmission. <IMAGE>

Description

COMMUNICATIONS SYSTEMS.

This invention relates to communications systems and in particular to extending the range of radio transmission, especially HF radio transmission.

It is often desirable for HF transmission to be performed from devices with poor antenna efficiency.

This means that the maximum effective radiated power may be very limited, with consequent restrictions in the transmission range. It is sometimes possible to communicate over long ranges with low transmitter powers at HF. Such occurrences are rare, however, and the coverage of HF communication is much improved if the power can be increased to compensate for high propogation loss. Selection of the optimum transmission frequency can improve the transmission range, however even then inefficient antennas may still restrict the transmission range.

According to one aspect of the present invention there is provided a communications system employing radio transmission and wherein a message to be transmitted is subjected to time spreading modulation prior to transmission.

According to another aspect of the present invention there is provided a communications system employing radio transmission and including a transmitter and a receiver, wherein the transmitter includes means whereby a message to be transmitted is subject to time spreading modulation, and wherein the receiver includes means to despread the message as received at the receiver, said time spreading modulation resulting in processing gain at the receiver.

According to a further aspect of the present invention there is provided a method for transmitting a message employing radio transmission and including the step, prior to transmission, of subjecting the message to time spreading modulation.

According to yet another aspect of the present invention there is provided a method of transmitting a message by radio transmission between a transmitter and a receiver, including the step, prior to transmission, of subjecting the message to time spreading modulation at the transmitter and including the step at the receiver of despreading the message as received, said time spreading modulation providing processing gain at the receiver.

Embodiments of the present invention will now be described with reference to the accompanying drawings, in which Fig. 1 illustrates, schematically, a basic embodiment of a transmission system; Fig. 2 illustrates, schematically, a basic embodiment of receiver system for use in an HF communication system including the transmitter of Fig. 1; Fig. 3 illustrates, schematically, a more specific embodiment o= transmission system, and Fig. 4 illustrates, schematically, an embodiment of receiver for use in HF communication system including the transmitter of Fig. 3, Referring to Figs. 1 and 2 a basic HF communication system, which enables improved range of communication to be achieved without increasing transmitter power, will be described.The improvement in communication range is achieved by use of a time spreading modulation technique, as will be be apparent from the following. This increases the range without increasing the transmitter power albeit at the expense of an increased message duration, but in certain applications this is of no consequence. The signal modulation and bandwidth is identical to currently used HF transmissions.

The transmission system of Fig. 1 includes a standard DPSK (Differential Phase Shift Keying) HF transmitter (block 1) with an antenna 2. DPSK is widely used at HF to combat the short coherence time of the channel. The data passed to the transmitter 3 from the DPSK modulator 4 is appropriately processed, as will be apparent from the following. The actual bit operations for this processing can be performed by any suitably programmed standard communications manager computer, such as a microprocessor.

As an example, suppose the system is required to transmit a 200 bit message. A normal DPSK modem transmitting at 75 bit/s would take 2.67 seconds to transmit this message. In the system of Fig. 1, however, the 200 bit message emerges from block 5 at 0.75 bit/s and is multiplied at 6 by a 20,000 bit pseudo random spreading sequence at 75 bit/s from a source thereof 7.

The resultant 75 bit/s data stream is passed to DPSK modulator 4. As will be apparent this data stream will now take 267 seconds (4.5 minutes) to transmit. The signal (message) has been spread in time by a factor of 100.

This time spreading leads to the same theoretical receiver processing gain as spectrum spreading. In this particular example the processing gain should be 20dB. Thus a signal can be received at a signal-to-noise ratio 20dB lower than usual. The transmission bandwidth and baud rate can be conventional values, consequently existing technology can be used in the radio receiver to give a baseband output at 75 samples /s. The receiving system of Fig. 2 comprises a receiver B with baseband output and a DPSK detector and despreader 9, the latter being described in greater detail hereinafter. Basically, the despread signal is coherently combined from the 100 bauds that are used to transmit each information bit. At no time is coherence required except between bauds (1/75 of a second) and this degree of coherence is normally assumed for conventional DPSK at 75 bit/s.Finally the detector produces the 200 original information bits at 0.75 bit/s. Receiver synchronisation is guided by parameters taken after despreading, this being the same as for spread spectrum systems.

The figures quoted above for data rates and processing gains are only examples. The basic techniques can, however, be applied. to most data rates and modulations.

A more specific example of the system will now be described with reference to Figs. 3 and 4 whic relate to a binary DPSK case. The transmitter system of Fig. 3 is a conventional DPSK modulator which operates at a chip rate f . The information bit stream (data at c rate fb) is combined at exclusive OR gate 10 with a spreading sequence generated at rate fc. c There will be k = f /f c k = fc/fb chips per information bit. When the signal is despread in the receiver the theoretical processing gain will be equal to k.

To express the transmitter function th mathematically, we assume D(n) is the n information bit, c(m) is the m bit in the spreading sequence (m = o to k-l) for this information bit and g(t) is the transmitted pulse shape. Then for binary DPSK, the baseband representation of the transmitted signal is given by: s(t) = ( x(p) + x(p-l) g(t-pTc) where x(p) = D(n) + c(m) p = m + kn + means exclusive OR T = 1 /f c c Alternatively, the modulation could be generalised to use M-ary DPSK. The differential encoder 11 and the BPSK modulator 12 comprise the required binary DPSK modulator. The output of modulator 12 is applied to a standard HF radio transmitter 13 with antenna 14.

The receiver of Fig. 4 includes a standard HF radio demodulator and channel filter 15. The following assumes that the receiver has already attained synchronisation. An incoming HF signal is passed through a matched filter and shifted te baseband at 15. Complex phasors are produced at a rate fc. c Each phasor is multiplied by the complex conjugate of the previous phasor, by means of complex conjugate forming element 16, delay 17 and complex multiplier 18, this being a method of attaining the differential phase. This first difference stream is then derotated at complex multiplier 20 by a despreading power sequence exp(-jt c(m)) provided by block 19. At this stage, phasors from the same information bit period can be coherently added at block 21 to attain the processing gain.For a binary DPSK signal the recombined phasor should have an argument of either 0 or 180 degrees. Thus data decisions can be made by comparing the real part of the recombined phasor with zero at block 22.

If the received base band signal is r(t), then the output of the digital radio receiver of Fig. 4 for th th the m chip in the n information bit will be:

where m = p - kn The decision variable Zn for each information bit is given by:

summing over the chips is differentially coherent addition and will bring the signal out of the noise. We re-er to this process as reconination. The complex argument of Zn is compared with the appropriate decision sectors for M-ary PSK. For binary DPSK this reduces to a comparison of the real part of Zn with zero, as mentioned above.

Since the received signal-to-noise ratio is potentially a factor k lower than usual, chip and bit synchronisation must be done post recombination. This can be achieved using a matched filter synchronisation procedure similar to those found in many spread spectrum receivers. Synchronisation block 23 takes timing (metric) from the addition block 21 and from this produces frame clock signals for the despreading sequence block 19 and chip clock signals for the demodulator and channel filter block 15.

Thus the coverage (range) of HF transmissions using inefficient antenna can be significantly improved by the use of time spread modulation. Only trivial modifications need to be made to a conventional DPSK transmitter, which latter is very simple and hence suitable for use on small, power-limited, equipments.

The receiver is modified in comparison with a conventional DPSK receiver in order to perform the necessary digital signal processing. The time spreading modulation results in an arbitrarily large processing gain at the expense of increased transmission duration.

If the duration is doubled, for example, there will be an extra 3dB processing gain. It should be noted that this is achieved without resorting to very narrowband transmissions which are more sensitive to frequency offsets. It is an advantage of the time spread modulation system that a very narrowband modulation is not produced at any stage. This means that the signal will be no more sensitive to frequency offsets and phase noise from the oscillators and the medium than a conventional modulation. This is one of the reasons why simply reducing the baud rate cf a conventional modulation would not work i.e. would not result in an extension of range.

The use of time spreading modulation enables increased range to be achieved with an inefficient antenna or limited transmitter power, due to the inherent processing gain at the receiver. Alternatively it enables transmission to take place at a lower power than hitherto in order to achieve a particular range.

Claims

CLAIMS.

1. A communications system employing radio transmission and wherein a message to be transmitted is subjected to time spreading modulation prior to transmission.

2. A communications system employing radio transmission and including a transmitter and a receiver, wherein the transmitter includes means whereby a message to be transmitted is subject to time spreading modulation, and wherein the receiver includes means to despread the message as received at the receiver, said time spreading modulation resulting in processing gain at the receiver.

3. A communications system as claimed in claim 1 or 2 wherein the radio transmission is HF radio transmission.

4. A communication system as claimed in claim 2 or claim 3 as appendent to claim 2 wherein the transmitter includes means whereby the message which is initially at a first bit rate is multiplied by a spreading sequence at a second bit rate, the second bit rate being higher than the first bit rate, the resultant data stream at the second bit rate being modulated prior to transmission.

5. A communications system as claimed in claim 4 wherein the spreading sequence is pseudo random.

6. A communications system as claimed in claim 4 wherein the receiver includes means whereby the received data stream is demodulated and despread, to provide an output comprising the message at the first bit rate.

7. A communications system as claimed in any one of claim 2 to 6 wherein the transmitter is a DPSK (Differential Phase Shift Keying) HF transmitter.

8. A communications system as claimed in claim 7 as appendent to claim 6, wherein the receiver includes a corresponding decoder, wherein the means to perform despreading follow the decoder, and including means to sum coherently phasors from the same information bit period, whereby to produce a decision variable for each information bit, and means to compare the complex argument of the decision variable with the appropriate PSK phase sectors.

9. A communications system as claimed in claim 8 wherein the transmitter is a binary DPSK HF transmitter and the comparing means compares the real part of the decision variable with zero.

10. A communications system as claimed in any one of claims 2 and 4 to 9 wherein the time spreading modulation and the resultant receiver processing gain enables increased transmission range to be achieved with an inefficient antenna or limited transmitter power.

11. A communications system as claimed in any one of claims 2 and 4 to 9 wherein the time spreading modulation and resultant receiver processing gain enables a predetermined transmission range to be achieved with lower power than a conventional system.

12. A communications system employing HF radio transmission substantially as herein described with reference to and as illustrated in Figs. 1 and 2 or Figs. 3 and 4 of the accompanying drawings.

13. A receiver suitable for use in a communications system according to any one of claims 1 to 12.

14. A method for transmitting a message employing radio transmission and including the step, prior to transmission, of subjecting the message to time spreading modulation.

15. A method of transmitting a message by radio transmission between a transmitter and a receiver, including the step, prior to transmission, of subjecting the message to time spreading modulation at the transmitter and including the step at the receiver of despreading the message as received, said time spreading modulation providing processing gain at the receiver.

16. A method as claimed in claim 15 wherein the radio transmission is HF radio transmission.

17. A method as claimed in claim 15 or 16 wherein the message is at a first bit rate and is time spread modulated by multiplying it by a spreading sequence at a second bit rate, the second bit rate being higher than the first rate, and including the step of further modulating the resultant data stream prior to transmission.

18. A method as claimed in claim 17, including the step at the receiver of demodulating the received message prior to despreading.

19. A method as claimed in claim 18 wherein the further modulation is provided by a DPSK modulator.

20. A method of transmitting a message by HF radio transmission substantially as herein described with reference to Figs. 1 and 2 or Figs. 3 and 4 of the accompanying drawings.