GB2189969A - Code tracking circuits for spread-spectrum receivers - Google Patents

Abstract

A code tracking circuit operates by cross-correlation of a received wideband signal against a locally generated tracking code sequence. When the local sequence is identical with the embedded sequence in the received signal, the correlation characteristic is the same as the code autocorrelation function. A local code sequence generator 40 includes delay elements 42, 44 to provide 'early', 'prompt' and 'late' phases of the local code sequence which are phase multiplexed by a phase sequencer 28 and switch 26C to provide the tracking code sequence. The cross- correlation is effected by a multiplier 14, a narrowband filter 16 and an AM demodulator (including an AGC amplifier 18 and a linear detector 20), and the cross-correlation output is applied via switches 26A and 26B respectively to a lock comparator 50 and to a loop integrator circuit including an amplifier 30. The three multiplexed samples ('early', 'prompt' and 'late') from the detector 20 are then used to provide a lock indication by the lock comparator and to control the code sequence generator 40 via the integrator amplifier 30 and a voltage controlled oscillator 38. <IMAGE>

Description

The output of the detector 20 represents the degree of cross-correlation between the inputs of the multiplier 14. This output is fed selectively in inverted or non-inverted form through switch arrangements 26A and 26B respectively to a lock circuit (to be described later) and to a loop integrator comprising an operational amplifier 30, a capacitor 32 and resistors 34, 36, connected as shown. The switch arrangements 26A, 26B are under the control of a phase sequencer 28, whose operation will be described later.

The output of the loop integrator amplifier 30 is fed to a voltage controlled oscillator 38. The output of the oscillator 38 is fed to a code sequence generator 40 which provides the code sequence required for despreading the received signal. The code sequence available directly from the output of the generator 40 appears on an 'early' line, and this code sequence is also passed through first and second delay circuits 42,44 which respectively provide delayed code sequences on 'prompt' and 'late' lines. The 'early', 'prompt' and 'late' code sequences are selected by a further switch arrangement 26C also under the control of the phase sequencer 28. Thus only one of the three respectively-phased code sequences is selected at any moment by the switch arrangement 26C and this is fed to the other input of the multiplier 14.The phase sequencer 28 causes the switch arrangement 26C to scan through the three phases of code sequence and thereby constitute the tracking code sequence. The 'prompt' output of the first delay circuit 42 is fed to an output terminal 46 and the corresponding 'prompt' code sequence is used in the receiver to de-spread the received signal. Each of the delay circuits 42,44 is preferably arranged to provide a delay of 0.5 chip, although any delay between the limits of 0 and 1 chip could theoretically be provided.

The output of the detector 20 is also fed to a lock circuit for providing a lock indication when synchronisation has been achieved. The output of the detector 20 is fed both directly and via an inverting and amplifying circuit providing a gain of -2 through the switch arrangement 26A to the lock circuit which is constituted by a comparator 50 connected to the switch arrangement 26A by means of a resistor 52 and a capacitor 54 connected between the inverting input of the comparator 50 and reference ground. A reference voltage VT is applied to the non-inverting input of the comparator 50. The output of the comparator appears on an output terminal 56 as a 'lock' indication.The switch arrangement 26A is operative to apply the output of the detector 20 directly to the lock circuit during the 'early' and 'late' phases of the phase sequencer 28, and to apply the output times -2 during the 'prompt' phase.

In operation, the input terminal 10 receives a wideband IF input which includes a code sequence embedded in the spread-spectrum radio channel. As stated above, it is essential in a spread-spectrum recover to make provision for acquisition of the code sequence in order to de-spread the radio signal and subsequently demodulate the narrowband carrier.

The circuit operates by cross-correlation of the received wideband radio signal against a locally generated tracking code-sequence. When the local sequence is identical with the embedded sequence in the radio signal, the correlation characteristic is the same as the code auto-correlation function (see Figures 2 and 3). When the local code-sequence generator is delayed twice as shown in Figure 1, for example by 0,5 chip each time, the three phases of the local code sequence are therefore available, namely 'early', 'prompt' and 'late' phases. These are multiplexed by the phase sequencer 28 and switch arrangement 26C, and correlated against the wideband signal, by multiplication in the multiplier 14 and subsequent AM demodulation following narrowband IF filtering.It is important that the code-phase slices taken by the phase sequencer 28 each manifest the correct cross-correlation with the embedded code in the received signal. This may be simply ensured by commutating the code-phase at integer multiples of the code-sequence duration.

Alternatively the code-sequence and phase sequence should be anharmonically related, in order to ensure that the long-term expectation value of the cross-correlation of sliced and embedded codes is the same as the code auto-correlation function. This latter technique puts extra constraints on loop bandwidth and is therefore not preferred.

The de-spread signal from the multiplier 14 passes through the narrowband filter 16 which rejects uncorrelated signals in the radio channel.

The level of the narrowband signal will then represent three multiplexed samples of the crosscorrelation between local and embedded codes. The levels are resolved by the AM demodulatorwhich is shown in Figure 1 as comprising the AGC amplifier 18 and the linear detector 20. The AGC time constant (set by the values of the resistor 22 and the capacitor 24) must be long compared to the phase-sequence cycle time, so that the AGC operation cannot defeat the multiplexed amplitude information.

Two out of the three amplitude samples are used for code-tracking. The amplitude sample selected during the 'early' phase E is inverted with respect to the sample selected during the 'late' phase L. The resultant amplitude difference L-E is integrated in the loop integrator including the amplifier 30 and applied to the voltage controlled oscillator 38 which clocks the local code-sequence generator 40. Figure 2 illustrates the tracking behaviour of the system.

Diagrams (a) to (c) show the 'early' E and 'late' L sampling of the auto-correlation function when the local code is running respectively 'too early', 'correct' and 'too late'. The diagram (d) shows the resultant phase discriminator characteristic L-E.

This has a linear operating region of +0.5 chip (or +n radians) of the code-clock. An equivalent phasedetector constant K can be determined, based on the performance of the linear detector. The result is a second-order phase-locked-loop, wherein: SPECIFICATION Code Tracking Circuits for Spread-Spectrum Receivers The present invention relates to code tracking circuits for spread-spectrum receivers, and in particular to such tracking circuits using phase control of the code.

There has been significant recent interest in the development of spread spectrum transmission/ reception systems. A direct sequence spread spectrum system may be viewed as an overlay to a conventional radio communication system. The original narrowband radio signal is multiplied (modulated) by a deterministic digital code sequence to produce a wideband radio signal. The increase in bandwidth (spreading) results from the high code-bit rate of the spreading sequence. The wideband signal may be simply de-spread at the receiver by multiplication with an identical code sequence to restore the narrowband radio signal for subsequent demodulation.

An alternative approach is to spread the narrowband baseband data signal prior to modulation by the radio carrier to produce a wideband baseband signal which is subsequently modulated onto a radio carrier. This is a radio overlay to a baseband spread spectrum system.

A significant advantage of spread spectrum systems is that multiple transmission of a number of signals on one radio carrier can be achieved by the use of sets of codes having low cross-correlation.

Such sets of codes are termed orthogonal codes.

Thus it is possible to achieve minimum interference between separate transmissions on the same radio carrier frequency and spread over the same frequency by selecting different codes from such an orthogonal set.

Further details of spread spectrum systems, their use and their implementation can be obtained from the appropriate literature, for example "Spread Spectrum Systems", R. C. Dixon, John Wiley & BR< Sons, 1976.

Upon reception of the spread-spectrum wideband signal, de-spreading is effected by using the same code sequence as that used in transmission, the code sequence needing to be synchronised to the code sequence embedded in the received signal.

Thus a search process is necessary for acquisition of the code sequence since the receiver will have no way of obtaining synchronisation information other than that forming part of the received signal.

Various methods of achieving code synchronisation have been proposed but each of these methods has involved certain disadvantages. For example, in one proposal a tracking loop is provided to effect the search for synchronisation but the loop is stressed in one direction. Therefore even a very short-term loss of received signal will be sufficient to cause the loop to move in the stressed direction and to lose synchronisation.

According to the invention there is provided a code tracking circuit for a spread-spectrum receiver, the circuit comprising: means for cross-correlating a received spreadspectrum wideband signal with a tracking code sequence to provide a cross-correlation signal and an inverted version thereof; a code sequence generator operable to generate a code sequence for de-spreading the received wideband signal; and phase multiplex means for receiving the generated code sequence and operable to supply sequentially 'early', 'prompt' and 'late' phases of the generated code sequence as the tracking code sequence to the cross-correlating means, and for modifying the code sequence generator timing by the cross-correlation signal when the tracking code sequence is 'late' and by the inverted crosscorrelation signal when the tracking code sequence is 'early'.

The use of a phase multiplex technique embodying the invention leads to a number of advantages.

Firstly, since a phase-multiplex technique is utilised for providing cross-correlation information, only a single code tracking path and crosscorrelating arrangement needs to be provided. A non-multiplex system would require replication of these components which, in the preferred embodiment, include a multiplier, a narrowband IF filter, an AGC amplifier and a detector. Thus a significant cost saving can be achieved.

Secondly, a parallel tracking scheme would require very close gain-matching and this is not necessary in the present phase multiplex technique.

Thirdly, the use of three-phase multiplexing allow a definite 'lock' indication of synchronisation to be obtained without the need for stress in the tracking loop. This reduces the possibility of loss of synchronisation during interruptions in the received signal. It is therefore an ideal technique to use when the signal is deliberately interrupted, such as in a time-division-duplex radio system.

The invention will now be described, by way of illustrative and non-limiting example, with reference to the accompanying drawings, in which: Figure lisa circuit diagram of a code tracking and lock circuit for a spread-spectrum receiver, according to an embodiment of the invention; Figure 2 is a series of diagrams showing the code tracking characteristics of the circuit shown in Fig. 1; and Figure 3 is a series of diagrams showing the code lock characteristics of the circuit shown in Figure 1.

Referring to Figure 1, a phase-multiplex code tracking and lock circuit embodying the invention is shown. The code tracking and lock circuit is for use in a spread-spectrum receiver and it receives a wideband IF input from the preliminary receiver sections at a terminal 10. The input from the terminal 10 is fed via a wideband IF filter 12 to one input of a multiplier 14 which also receives at another input a tracking code sequence which will be described in greater detail later. The output of the multiplier 14 is fed via a narrowband IF filter 16 to an AM demodulator shown for example in the form of an AGC amplifier 18 and a detector 20 with a time constant circuit including a resistor 22 and a capacitor 24 connected as shown.

looD bandwidth.

damping factor, wnRPC1 = 2 where Kv is the voltage controlled oscillator gain constant, R1 is the resistance of the resistor 34, R2 is the resistance of the resistor 36 and C1 is the capacitance of the capacitor 32.

The loop bandwidth con must be small in comparison to the phase-sequencing rate, otherwise an unwanted phase-modulation will result.

All three amplitude detector samples are used for 'lock' indication. However, as mentioned above, the 'prompt' phase P is inverted and given twice the weight of the 'late' and 'early' phases Land E. One consequence of this form of weighting is that DC offsets from the detector 20 are effectively cancelled in the resultant signal L+E-2P. Figure 3 illustrates the build-up of the lock characteristic L+E-2P shown in diagram (d), derived from the diagrams (a) to (c) showing the auto-correlation function sampling when the local code is running respectively 'too early', 'correct' and 'too late' in similar manner to diagrams (a) to (d) of Figure 2.

The characteristic L+E-2P has a pronounced negative peak at zero relative phase. This signal is filtered by the time constant of the resistor 52 and the capacitor 54 and compared in the comparator 50 with the reference threshold VT in order to validate the 'lock' condition. This time constant must necessarily be long compared with the phasesequence cycle time.

Since a phase-multiplex technique is used in the code tracking and lock circuit of Figure 1, it is not necessary to replicate the components in the AM demodulation section, namely the multiplier 14, the narrowband IF filter 16, the AGC amplifier 18 and the detector 20, as would be necessary for each phase in a non-multiplex system. Also, the close gainmatching requirements of a parallel tracking scheme are eliminated. Furthermore, the threephase multiplexing technique allows a definite 'lock', indication to be provided without the need for stress in the tracking loop. Since this enables the receiver's code sequence generator to idle at the correct frequency in the absence of signal, this reduces the possibility of loss of code-lock during interruptions in the radio signal. As stated above, it is therefore an ideal technique in situations involving deliberate interruptions of signal, such as in a time-division-duplex radio system.

Claims (9)

1. A code tracking circuit for a spread-sprectrum receiver, the circuit comprising: means for cross-correlating a received spreadspectrum wideband signal with a tracking code sequence to provide a cross-correlation signal and an inverted version thereof; a code sequence generator operable to generate a code sequence for de-spreading the received wideband signal; and phase multiplex means for receiving the generated code sequence and operable to supply sequentially 'early', 'prompt' and 'late' phases of the generated code sequence as the tracking code sequence to the cross-correlating means, and for modifying the code sequence generator timing by the cross-correlation signal when the tracking code sequence is 'late' and by the inverted cross correlation signal when the tracking code sequence is 'early'.

2. A circuit according to claim 1, wherein the phase multiplex means comprises delay means connected to the code sequence generator and operable to provide 'early', 'prompt' and 'late' code sequence outputs, and a phase sequencer for switching between the code sequence outputs to provide the tracking code sequence and for switching between inverting and non-inverting outputs of the cross-correlating means.

3. A circuit according to claim 2, wherein the delay means comprises first and second delay elements connected in series to an output of the code sequence generator, the output of the generator providing the 'early' code sequence, the output of the first delay element providing the 'prompt' code sequence, and the output of the second delay element providing the 'late' code sequence.

4. A circuit according to claim 1, claim 2 or claim 3, wherein the cross-correlating means comprises a multiplier for multiplying the received wideband signal with the tracking code sequence, a narrowband filterforfiltering the output of the multiplier, and an AM demodulator for demodulating the filtered output of the multiplier.

5. A circuit according to claim 4, wherein the AM demodulator comprises an AGC amplifier and a linear detector.

6. A circuit according to any one of the preceding claims, wherein the code sequence generator is clocked by a voltage controlled oscillator which receives a control voltage from an integrator operable to integrate the cross-correlation signal when the tracking code sequence phase is 'late' and to integrate the inverted cross-correlation signal when the tracking code sequence phase is 'early'.

7. A circuit according to any one of the preceding claims, including a lock circuit for providing an indication of the generated code sequence being locked in synchronisation with the received wideband signal, the lock circuit comprising a comparator operable to receive the crosscorrelation signal when the tracking code sequence phase is 'early' and 'late' and to receive an amplified version of the inverted cross-correlation signal when the tracking code sequence phase is 'prompt', and to compare these signals with a reference level to provide a lock signal.

8. A circuit according to any one of the preceding claims, wherein the phase multiplex means is arranged to commutate the code sequence phases at an integer multiple of the code sequence duration.

9. A code tracking circuit for a spread-spectrum receiver, the code tracking circuit being substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings.