GB2313985A - Spread spectrum communication systems - Google Patents

Abstract

A spread spectrum communications system utilizes a modulated spread spectrum pilot signal. Phase-frequency tracking is facilitated by de-spreading n chips of an m chip symbol of the pilot signal and de-spreading the remaining n - m chips of the symbol, thereby generating intra-symbol samples. To provide optimum immunity to co-channel interference the spreading codes used to spread the spectrum of the said pilot signals are arranged to be a set of codes which are orthogonal with respect to each other, and that m chips of each code are orthogonal with respect to the corresponding m chips of any other code of the set and that the remaining n - m chips of each code are orthogonal with respect to the corresponding n - m chips of any other code of the set. Baseband signals at output of converter 2 are applied to despreading or rake receivers 3, 4 associated with data stores 5, 6 including the despread codes. After frequency / phase synchronisation is achieved, a phase / frequency tracker 10 is connected to the output of receiver 6. The tracker 10 may be of the type disclosed in figure 2 ( not shown ).

Description

IMPROVEMENTS IN OR RELATING TO SPREAD SPECTRUM COMMUNICATIONS SYSTEMS The present invention relates to spread spectrum communications systems, and in particular to spread spectrum communications systems which include pilot signals to provide a means for synchronising terminals operating within such systems.

In spread spectrum communications systems, the bandwidth of a data bearing radio signal is spread by arranging for data to modulate a pre-determined spreading code. The resulting signal is arranged to modulate a carrier signal in accordance with a predetermined modulation scheme, thereby generating a signal suitable for radio frequency transmission. The bandwidth of the resulting signal is thereby spread by an amount proportional to the length of the spreading code within a data symbol. At a receiver the signal is de-modulated and de-spread using an identical code to that used in the transmission process.

In order to provide effective communications, a receiver for detecting spread spectrum signals should be provided with a means for acquiring and tracking the phase and frequency of a transmitted signal. One technique for achieving phase and frequency acquisition of a transmitted signal, is to furnish the system with a pilot signal. The spectrum of such a pilot signal is arranged to be spread using a spreading code as with other signals within the system. By acquiring a phase and frequency reference from the pilot signal, terminals operating within the system are provided with the necessary phase and frequency information with which to de-modulate and de-spread transmitted signals.

A pilot signal within a spread spectrum system may be either modulated or un-modulated. A pilot signal may be modulated in order to confer information common to terminals operating within the system.

It is highly desirable for spreading codes within a spread spectrum system to be orthogonal with respect to each other. This provides a means whereby a transmitted signal may be recovered at a receiver in the presence of like modulated contemporaneously transmitted spread spectrum signals, albeit with different spreading codes. A set of such codes are known to those skilled in the art as 'Walsh codes'. 'Walsh codes' are known to be self orthogonal, that is that any member of the code set is truly orthogonal with respect to any other member of the code set.

A method and apparatus for phase and frequency acquisition of an un-modulated pilot signal is disclosed in the Applicant's co-pending patent application, serial No. 9522780.7.

An improved method and apparatus for phase and frequency acquisition of a modulated pilot signal is disclosed in the Applicant's co-pending patent application, serial No. 9600404.9.

The present invention aims to confer advantages to a spread spectrum system furnished with a modulated pilot signal.

According to the present invention, there is provided a spread spectrum communications system comprising at least one modulated spread spectrum pilot signal for providing terminals within the system with a means for phase and frequency acquisition, the terminals being provided with phase and frequency tracking apparatus comprising means for de-spreading m chips of a symbol of the said pilot signal from the remaining n m chips of the symbol, characterised in that a spreading code used to spread the spectrum of the said pilot signal is one of a set of pilot codes which are substantially orthogonal with respect to each other, and in that m chips of each code are substantially orthogonal with respect to the corresponding m chips of any other code of the said set and in that the remaining n - m chips of each code are substantially orthogonal with respect to the corresponding n - m chips of any other code of the said set.

The improved phase-frequency acquisition apparatus for a modulated pilot signal is arranged to generate a plurality of samples per symbol of the spread spectrum pilot signal. By ensuring that the spreading codes are not only orthogonal with respect to each other but that m chips of each code are orthogonal with respect to m chips of any other code and likewise that the remaining n - m chips are orthogonal to the remaining n - m chips of any other code in the pilot code set, the plurality of samples per symbol required for phase-frequency tracking a modulated pilot signal will be provided with optimum immunity to the presence of like modulated contemporaneously transmitted signals.

Users of the spread spectrum communication system communicate data through traffic channels. The spectrum of signals communicated on the said traffic channels may be spread using codes from a further set of spreading codes in accordance with principles hereinbefore described.

The set of spreading codes used to spread the spectrum of the pilot may be selected and arranged to be substantially orthogonal to the said further set of codes for the traffic channels.

The set of pilot codes may be further selected and arranged so that the n chips of any code of the set of pilot codes and the remaining n - m chips of the code are substantially orthogonal to any code of the further set of codes.

The set of pilot codes may be further selected and arranged so that the m chips and the remaining n - m chips of any code of the set of pilot codes may be substantially orthogonal with respect to any part of any code in the further set of codes.

One embodiment of the present invention will now be described by way of example only with reference to the accompanying drawings in which, FIGURE 1 is a schematic block diagram of a receiver for detecting and demodulating data transmitted in accordance with the spread spectrum system, FIGURE 2 is a schematic block diagram of a combined demodulator and phase-frequency tracker.

The general form of receiver suitable for detecting and recovering data communicated in accordance with the spread spectrum system is shown in Figure 1. An antenna 1, for detecting radio signals, is shown connected to a combined radio frequency receiver and down converter 2, hereinafter referred to a receiver/converter. The receiver/converter 2, is assumed in this illustrative embodiment to perform all operations necessary for receiving and down converting radio signals detected by the antenna 1, so that signals fed to units 3, 4 connected thereto are representative of baseband signals corresponding to the detected radio signals. The baseband signals are received by two despreading receivers 3, 4. Such de-spreading receivers 3, 4 are well known to those skilled in the art and may, for example, be Rake receivers. The de-spreading receivers 3, 4 are respectively connected to data stores 5, 6. The data stores 5, 6 each contain data representative of a set of spreading codes corresponding to those used to spread the bandwidth of signals to be de-spread.

The outputs 7, 8 from the de-spreaders 3, 4, communicate with a phase-frequency tracker 10, via a switch 11. The switch 11 operates to connect the input 9, of the phase-frequency tracker 10, to either the output of the first de-spreader 3, or the second de-spreader 4, under control of a switch controller 12.

The phase-frequency tracker 10, is shown in detail in Figure 2 where parts corresponding to those shown in Figure 1 bear the same numerical designations. The phase-frequency tracker 10, operates to detect and recover the phase and frequency of the received base band signal fed from the de-spreading receivers 3, 4, shown in Figure 1. The received signal samples Yn enter the unit 10, via the conductor 13, and are fed to a multiplier 14. The multiplier 14, multiplies the received sample Yn with a signal fed back from an output 15, of the phase-frequency tracker 10, via a conductor 16. The signal fed back from the output 15, of the phase-frequency tracker 10, represents a phase and frequency error in the received signal, hereinafter known as a phasefrequency error. The effect of this multiplication is to remove the phase-frequency error from a received signal so that the signal samples at the output of the multiplier 14, may be coherently demodulated by units 17,18, to which the signal is fed.

The symbol averaging unit 17, operates to produce a single sample corresponding to each received symbol, by averaging a plurality of signal samples associated with a single received symbol. As will be hereinafter described, when detecting a pilot signal, two samples per symbol are generated, whereas in normal operation only a single sample per symbol is generated. Hence the symbol averaging unit 17, operates to provide a single sample for each spread spectrum symbol, whether a single or a plurality of samples per symbol are communicated ro it from the multiplier 14.

Information is subsequently recovered by the coherent demodulator 18, to which the signal from the symbol averaging unit 17 is fed. Demodulated data is subsequently output for further processing and utilisation via the conductor 19.

The remaining elements within the phase-frequency tracker 10, serve to ascertain and track the phase-frequency error in the received signal, so that this can be removed in accordance with the operation of the multiplier 14. The method and apparatus for phase-frequency tracking is described in detail in our co-pending patent application serial numbers 9522780.7 and 9600404.9. For continuity, a brief description of the operation of the phasefrequency tracker 10, will now be given as hereinafter follows.

< {yn#y*n-1} = < {yn} - < {yn-1} ...... ....................(1) A frequency discriminator 20, operates on signal samples fed from the multiplier 14, to form a signal appertaining to the anti-correlation between a present sample Yn and a previously received sample Yn- 1 in accordance with equation 1, where in equation 1LA is the argument of complex sample A. The output 21, of the frequency discriminator 20, forms a signal representative of a complex number, the phase of which is calculated in accordance with the phase difference between the current sample Yn and the previous sample Yn l, and incorporates effects due to noise, modulation and phase rotation due to phase and frequency errors. Under the control of a second switch controller 22, the output of the frequency discriminator 20, is communicated to the accumulator 23, via either a phase shift remover 24, or by-passing the phase shift remover 24, in accordance with the position of a switch 25. The accumulator 23, operates to form a signal at its output 15, representative of the sum of all previous m output samples from the frequency discriminator 20, in accordance with equation 2.

The signal generated at the output 15, of the accumulator 23, represents the accumulated change in phase corresponding to the last m samples from the frequency discriminator 20, over an extended phase range, that is, without restriction to the range +s radians normally associated with phase measurement. As will be appreciated by those skilled in the art, in this mode of operation the signal generated at the output of the accumulator 22, will be representative of the frequency error in the received signal and, by feeding this signal back to the multiplier 14, via the conductor 16, the frequency error will be removed from the received signal sample.

In a situation where the frequency error has been reduced to a level whereby this error can be unambiguously resolved from the phase of the received signal sample, the phase-frequency tracker 10, operates without the frequency discriminator 20, and accumulator 23, in the aforementioned manner. In this case, the phase error may be calculated by an argument function 26, which receives a signal sample from the averaging unit 17, and generates data appertaining to the argument of the signal sample.

The argument function 26, thereby generates a signal representative of a measure of the phase error in the received signal sample over an un-extended phase range, that is, in the range +s radians. This phase error signal is fed to the accumulator 23, via a multiplier 27, and is combined into the signal representative of the phase-frequency error generated at the output 15, of the accumulator 23, and fed back to the multiplier 14.

Figure 2 also shows a fader controller 28, connected to the output 15, of the accumulator 23, which operates to generate two signals represented as y and 1- y. The fader controller 28, communicates the two signals y and 1- y to the accumulator 23, via conductors 29, 30. The fader controller 28, operates to weight the contributions made to the phase-frequency error signal generated at the output 15, of the accumulator 23, by the argument function 26, and frequency discriminator 20, in combination with the accumulator 23, so that the phase contribution provided by the argument function 26, is weighted by y, whereas the frequency contribution provided by the accumulator 23, is weighted by 1 - y. The fader controller 28, operates in combination with the accumulator 23, to the effect that when the phase-frequency error is outside a range whereby it can be unambiguously resolved from an argument estimate by the argument function 26, then y is zero. In this case the contribution to the phase-frequency error signal is entirely from data appertaining to the frequency error generated by operation of the frequency discriminator 20, in combination with the accumulator 23. Where, however, the phase-frequency error signal can be unambiguously resolved from an argument estimate from the argument function 26, then y takes on a value in the range between zero and one in proportion to the level of phase error, which in the extreme case results in a value of 7 equal to one, which has the effect of generating a phase-frequency error comprised entirely of an argument estimate contribution.

The phase-frequency tracker 10, as so far described provides a means for phase-frequency tracking a spread spectrum pilot signal where such a signal is un-modulated. Where, however, the pilot signal is modulated, further steps must be taken in order to recover the phase and frequency of a signal, which have the effect of mitigating against phase rotations introduced by a modulation scheme. Such steps and corresponding apparatus are disclosed in our co-pending patent application serial number 9600404.9 and will be briefly explained as follows. The phase-frequency tracker 10, for a modulated pilot signal is arranged to provide two samples for each spread spectrum symbol. This is achieved by correlating the first 2 chips 2 of each symbol against the corresponding 2 chips of a local 2 reference code, thereby generating a sample corresponding to the first 2 chips of each symbol. The operation is repeated with the 2 last 2 chips thereby generating a symbol sample for the last half 2 of the symbol. The result of this de-spreading process is to produce a stream of samples corresponding to two samples per spread spectrum symbol. It will be appreciated by those skilled in the art that there will be no substantial change in phase as a result of a modulation process between samples within a symbol, whereas the phase between the last part of the previous symbol and the first half of the next symbol will be effected by a phase displacement in accordance with the modulation process. Such samples which correspond to successive samples within a symbol are hereinafter known as intra-symbol samples, whereas successive samples which cross a symbol boundary are known as inter-symbol samples. By providing the phase-frequency tracker 10, with a means for selecting and operating upon intra-symbol samples only, the phase-frequency tracker can operate in accordance with the principles as hereinbefore described for an un-modulated pilot signal. The second switch controller 22, in Figure 2 therefore operates to provide the accumulator 23, with intra-symbol samples only. The inter-symbol samples may be utilised by removing the effect of phase rotations caused by the modulation process, and this is performed by the phase-shift remover 24, in combination with the de-modulator 18.

In operation, the receiver shown in Figure 1 will function in either of two modes. In synchronisation mode, the receiver performs phase-frequency tracking of the pilot signal as hereinbefore described by generating two samples per symbol. In this mode, the de-spreader 3, operates with reference to data representative of a set of spreading codes stored in the data store 5, and the switch controller 12, operates to switch the twice oversampled de-spread pilot signal samples to the phase-frequency tracker 10.

Once phase and frequency acquisition have been achieved, the switch controller 12, operates the switch 11, so that signal samples from the de-spreader 4, are fed to the phase-frequency tracker 10. In this mode, the de-spreader 4, is operating to despread and de-modulate data communicated by a spread spectrum communications signal appertaining to a traffic channel of the spread spectrum system. In this mode, the de-spreader 4, operates to de-spread a received signal in accordance with data representative of a set of spreading codes held in the data store 6, and generates a single sample per spread spectrum symbol. The signal samples are then passed to the phase-frequency tracker 10, which serves to de-modulate the received signal samples and recover data carried by a signal transmitted on the traffic channel.

In accordance with the principles hereinbefore described, the receiver shown in Figure 1 operates to detect wanted signals in the presence of like modulated contemporaneously transmitted signals. Therefore for the phase-frequency tracker 10, to operate most effectively, the spreading codes used for the pilot signal, which are held in the data store 5, should be not only orthogonal with respect to each other, but that the first 2 chips of any code 2 should be orthogonal with respect to the corresponding n2 chips of 2 any other code, and similarly orthogonality should exist between the remaining n chips of any code and the corresponding chips of any other code.

Walsh codes are known to be self orthogonal. However, only a sub-set of such codes provide the necessary orthogonality between the first and last n chips required for intra-symbol 2 sample generation as hereinabove described. Therefore, by providing the data store 5, with such a sub-set of Walsh codes, the spread spectrum system may operate with the phase-frequency tracker 10, as hereinbefore described in the presence of like modulated contemporaneously transmitted pilot signals.

Furthermore, spreading codes of the pilot signals should also be orthogonal to the spreading codes used to spread the spectrum of signals transmitted on traffic channels of the spread spectrum system. Correspondingly, the n chips of the spreading codes used 2 for the pilot signal should also be orthogonal to the corresponding n2 chips of any of the spreading codes used to spread the 2 n spectrum of the traffic channel signals. Likewise the remaining n2 2 chips should be orthogonal to the remaining n chips of the spreading codes of the traffic channel signals.

The spread spectrum communication system as hereinbefdre described may be provided with traffic channels which have a lower data communication rate than that of the pilot signal.

Furthermore, in order to reduce complexity, a data communication rate of the pilot signal, may be an integral multiple of that of the traffic channels. For example, the data rate of the pilot signal may be 256 Ksymbols/s, whereas the data rate of the traffic channels may be 64 Ksymbols/s. In this case the respective correlation periods over which the de-spreader 3, for the pilot signal and the de-spreader 4, for the traffic channel signals will be 3.906cos and 15.625cos. To provide two samples per spread spectrum symbol of the pilot signal, which are correspondingly generated from the first n2 chips and the remaining n chips as hereinbefore 2 2 described, the de-spreader 3, will have a correlation period of 1.953cos, which is one eighth of the correlation period of the traffic channel signals. For this reason the set of codes used for the pilot signal which are held in the data store 5, may be further reduced to provide that the first and the remaining 2 chips of 2 each code are orthogonal to any corresponding part of a traffic channel code. In this example the n chips should be orthogonal to 2 any of 8 chips of any L-symbol spreading code used for the 8 traffic channel signals, held in the data store 6, for any possible phase reversals which may occur as a result of transmission.

It will be appreciated by those skilled in the art that alternative embodiments are possible which fall within the scope bf the present invention. In particular it will be appreciated that intra-symbol samples of a spread spectrum symbol may be generated by correlating any combination of m-chips out of the nchips of each symbol, with the corresponding m-chips of a local reference code.

Claims (7)

WHAT WE CLAIM IS:

1. A spread spectrum communications system comprising at least one modulated spread spectrum pilot signal for providing terminals within the system with a means for phase and frequency acquisition, the terminals being provided with phase and frequency tracking apparatus comprising means for despreading m chips of a symbol of the said pilot signal from the remaining n - m chips of the symbol, characterised in that a spreading code used to spread the spectrum of the said pilot signal is one of a set of pilot codes which are substantially orthogonal with respect to each other, and in that m chips of each code are substantially orthogonal with respect to the corresponding m chips of any other code of the said set and in that the remaining n - m chips of each code are substantially orthogonal with respect to the corresponding n - m chips of any other code of the said set.

2. A spread spectrum communication system as claimed in Claim 1, comprises at least one traffic channel wherein the spectrum of signals communicated on the said traffic channel is spread using codes from a further set of spreading codes.

3. A spread spectrum communication system as claimed in Claim 2, wherein the set of pilot codes is selected and arranged so that any code of the set of pilot codes is substantially orthogonal to any code of the further set of codes.

4. A spread spectrum communication system as claimed in Claim 2 or Claim 3, wherein the n chips of any code of the set of pilot codes and the remaining n - m chips of the code are substantially orthogonal to any code of the further set of codes.

5. A spread spectrum communication system as claimed in Claim 4, wherein the n chips of any code of the set of pilot codes and the remaining n - m chips of the code are substantially orthogonal to any part of any code of the further set of codes.

6. A spread spectrum communication system as claimed in any preceding Claim, wherein the number of chips m, is substantially equal to the number of chips n - m, of each spread spectrum pilot symbol.

7. A spread spectrum communication system as hereinbefore described with reference to the accompanying drawings.