GB2203303A - Radio frequency tracking loop - Google Patents

Abstract

A costas-loop carrier-recovery circuit includes digitisers (20, 21), correlators (18, 19) and a peak-tracking circuit (22) in each I & Q arm. The correlators produce a peak output which indicates the degree of correlation between the input pseudo-random code and a reference code. The peak tracker holds the values (23, 24) which are multiplied together (16) to produce the loop error voltage which will drive the VCO (13). The invention is applicable to direct-sequence, binary-phase-coded, spread-spectrum systems. <IMAGE>

Description

RADIO FREQUENCY TRACKING LOOP This invention relates to a radio frequency tracking loop which can be used in e.g. direct sequence binary phase coded spread spectrum systems.

A spread spectrum system is one in which the transmitted signal is spread over a frequency band which is much wider than the minimum bandwidth required to transmit the information content of the signal. Such systems are comprehensively discussed in 'Spread Spectrum Systems' by R.C. Dixon, published by John Wiley a & Sos, 1984, 2nd Edition. There are several types of modulation applicable to spread spectrum systems, one of which is biphase-shift keying (BPSK) modulation. In BPSK modulation the carrier frequency, which may be modulated by a digital pseudo random code sequence whose bit rate is much higher than the information signal bandwidth, is phased shifted by + 1800 for the binary coded information.In addition, the actual transmitted signal may be one in which the carrier component is itself suppressed or held to a level below the modulation components by virtue of there being no dc level in the modulating sequence. In order to optimise recovery of the transmitted signal it is necessary to recover or synthesise the carrier in the receiver.

The optimum demodulation method for BPSK suppressed carrier signals utilises a so-called Costas loop to reconstruct a coherent carrier from the received signal.

The Costas loop is described in $ynchronous Communications', by John P. Costas, ?roc. IRE, December 1956, pp.1713-1718. The reconstructed carrier and its 900 phase shifted replica are mixed with the input signal to obtain in-phase and quadrature-phase signals. These signals are multiplied in the loop multiplier and result in a sin (20) tracking curve. Once initial phase/frequency lock is attained, a steady state condition exists and the loop will automatically make adjustments to maintain lock by tracking any phase changes in the incoming carrier.

Because of the sin (20) tracking curve, the phase tracking limit is + 45 , i.e. one cycle of the loop tracking curve is 1800. A Costas loop is therefore transparent to phase changes of 0 or 1800 (BPSK) i.e. a 1800 phase ambiguity exists, though this can be resolved by differentially encoding the data.

A basic Costas loop has limitations for use on spread spectrum systems. For a spread-spectrum coded bi-phase modulated carrier the basic Costas loop would demodulate the code and not the data. The carrier recovery process would therefore not take advantage of the available processing gain, hence allowing the system to be jammed, i.e. preventing information recovery and eventually the loop would lose lock. In the absence of jamming the loop would suffer a noise penalty resulting from the wide pre-detection bandwidth required to accommodate the spread-spectrum signal. To reduce the noise penalty and to give the carrier recovery process protection from jamming the loop error voltage must be updated inside the correlation bandwidth. Also the loop has a limited frequency tracking range.It is not capable of coping with a large initial frequency offset between transmitter and receiver oscillators at switch on. The loop must also be capable of coping with possible frequency drift due to the tolerance of frequency sources and with Doppler shifts in some applications.

In accordance with the present invention there is provided a radio frequency (r.f.) tracking loop including a Costas loop carrier recovery circuit wherein the in-phase (I) and quadrature (Q) channels each include analogue-to-digital conversion means to which the low pass filtered signals are applied, digital correlation means to which the digital signals are applied and peak tracking means to which the correlation means output is applied, the outputs of the peak tracking means of each channel being the signals applied-to the Costas loop multiplier to form the loop error voltage.

Embodiments of the invention will now be described with reference to the accompanying drawing which illustrates an r.f. tracking loop.

The basis of the r.f. tracking loop is a Costas loop comprising a pre-detection bandpass filter 10, to which the incoming signal is applied; in-phase (I) and quadrature (Q) mixers 11,12 wherein the filtered input signal is mixed with the output of a VCO 13; I and Q low pass filters 14,15; a Costas multiplier 16; and a frequency control feedback path from the multiplier 16 to the VCO 13.

The output of the VCO is fed direct to the I mixer 11 and via a 90" phase shift network 17 to the Q mixer 12. The bandwidth of filter 10 is twice the baseband spread spectrum bandwidth, whereas filters 14 & 15 have bandwidths equal to the baseband spread spectrum bandwidth. The feedback loop requires a bandwidth sufficient only to pass carrier frequency changes.

To allow the received spread spectrum signal to be despread and carrier recovery performed on the despread signal the basic loop is augmented by the addition of correlators in each of the I & Q arms and a peak tracking circuit. Since the correlators 18,19 are essentially digital code correlators, e.g. multi-bit parallel correlators, they are preceded by analogue-to-digital converters 20,21. The correlators will each produce a peak output which indicates the degree of correlation between the input pseudo-random code (the data is coded at the transmitter) and a replica pseudo-random code held in a reference register. The peak tracking circuit 22 holds the correlator peak values and updates at the end of each epoch. By correct interpretation of the peak tracker output on the I arm of the loop the de-spread information can be obtained.

At the end of each code transmission period the peak tracker circuit will hold the value of the last correlation peak on each of I & Q arms by means of latches 23,24. These peaks are multiplied together by the Costas multiplier 16 and the resultant acts as a static feedback control signal until such time as dynamic feedback is returned.

The feedback loop includes a filter 16a to which the error voltage output of multiplier 16 is applied. The kind of loop filter selected depends upon the dynamic acquisition and tracking ability required for a given application. For example a proportional plus integral filter allows the loop to track phase and frequency steps in the carrier with no error in the steady-state and frequency ramps (Doppler shifts) with a constant steady-state error.

When the loop is locked in phase and in frequency the quadrature component will be zero and the in-phase component will provide the demodulated data.

By sampling the code at twice the code rate and doubling the length of the correlators no code synchronisation is required. This allows synchronisation to the correlation peaks, where the signal-to-noise ratio is higher by a factor equal to the system process gain, i.e. the carrier recovery process being then transparent to the spread spectrum process.

The frequency tracking range of the Costas loop is limited by the loop gain and therefore to provide an acceptable noise performance/acquisition range trade-off for a given bandwidth a frequency acquisition aid 25 is employed. This generates a sweep voltage which is summed with the loop filter output until it is inhibited by the output of a "lock" detector 26 which operates according to: < 12 ~ Q2 > using a look-up table held in a PROM. When the loop is out of lock the output of the PROM look-up table (followed by a sequential averager) will be zero. When lock occurs the output will be a constant value: (I - Q > = A 2 where A is the input signal amplitude. A comparator is used to detect when this condition results and generates a pulse to stop the frequency acquisition aid.

Automatic gain control (AGC) 27 required to hold the signal level at the I and Q channel ADC inputs constant, allowing their full dynamic range to always be available. This is essential if a jamming signal is present, as saturation of the ADC's would otherwise occur, preventing correlation taking place. The AGC maintains wanted and unwanted signal powers in the correct proportions. This allows any jamming signal to be decorrelated and its power spread over a wide bandwidth, whilst the wanted signal is correlated correctly.

As the tracking curve of the Costas loop is proportional to the power of the input carrier, the AGC also aids the fast feed-forward gain control employed within the Costas loop for amplitude normalisation of the phase detector characteristic.

The AGC is implemented as a secondary loop developing control signals from I and Q and using these to control a variable gain amplifier (VGA) 28 at the Costas loop input.

I and Q are two multi-bit words representing the value of each correlation peak. The nominal value of I or Q midway between minimum and maximum values corresponds to zero correlation. The minimum and maximum values of I & Q correspond to negative and positive peaks respectively.

To obtain a measure of the instantaneous signal power the AGC must remove any offset corresponding to zero correlation, form I2 + Q2 and compare it to a reference level. This can be done digitally by using I & Q to address a PROM look-up table. The power tracking loop is then closed using an analogue RC integrator to feed the amplifier 28. A digital-to-analogue converter will be required to convert the PROM output data to an analogue voltage to feed the amplifier.

Claims (6)

CLAIMS:

1. A radio frequency (r.f.) tracking loop including a Costas loop carrier recovery circuit wherein the in-phase (I) and quadrature (Q) channels each include analogue-to-digital conversion means to which the low pass filtered signals are applied, digital correlation means to which the digital signals are applied and peak tracking means to which the correlation means output is applied, the outputs of the peak tracking means of each channel being the signals applied to the Costas loop multiplier to form the loop error voltage.

2. An r.f. tracking loop according to claim 1 wherein means are provided for holding the outputs of the I and Q peak tracking means for the duration of each input signal code transmission period.

3. An r.f. tracking loop according to claim 1 or 2 wherein means are provided for generating a sweep voltage signal, means for summing the sweep signal with the error voltage in the loop and means for inhibiting the sweep signal generator when the correlation means outputs assume predetermined values.

4. An r.f. tracking loop according to claim 1, 2 or 3 wherein automatic gain control means responsive to the values of the correlation means outputs is provided to control the power level of the input signals to the loop.

5. A radio frequency tracking loop substantially as described with reference to the accompanying drawing.

6. A coherent detection circuit for BPSK signals comprising a carrier recovery loop having a phase detector, the operation of which is transparent to the modulation process, including a Costas loop with the phase detector such as to make the detection process independent of signal amplitude, wherein the processing required to generate the loop error voltage is performed at zero intermediate frequency, the loop I & Q channels each ~iScorporating digitising means and parallel correlators.