Receiver system for processing signals received on diversity channels

A receiver system processes signals transmitted via a multipath dispersive transmission medium. It includes a device for digitizing the received signals to supply corresponding digital signals. An adaptive digital equalizer responsive to the digital signals supplies information symbols. A symbol-clock signal controls the digitizer and the equalizer. A timing signal recovery device responsive to the digital signals and to the information symbols derives a phase control signal for controlling the phase of the symbol-clock signal.

BACKGROUND OF THE INVENTION 
1. Field of the invention 
The field of the invention is that of signal transmission systems, in 
particular systems transmitting digital signals modulated by a form of 
modulation such as Quadrature Phase Shift Keying (QPSK) in which the 
signals are transmitted over a multipath dispersive medium. 
The invention is more particularly concerned with a receiver system for 
processing signals transmitted over a multipath dispersive medium and 
received on a plurality of diversity channels. 
2. Description of the prior art 
It is standard practice to use adaptive digital equalizers in receiver 
systems to minimize the disturbing effects of the transmission medium. In 
particular, multipath propagation of signals generates inter-symbol 
interference. This is the case with transmission by a line of sight or 
tropospheric microwave beam, for example. The signal received by the 
receiver system is then the time-varying weighted sum of differently 
delayed different replicas of the transmitted signal. 
Digital equalizers are usually implemented in the form of digital 
tranversal filters with multiple coefficients and corresponding symbol 
taps. Adaptive digital equalizers may incorporate a backward decision 
filter which offers an improvement in performance, especially on media 
subject to severe distortion. 
The performance of an adaptive digital equalizer is critically dependent on 
the phase of the symbol-clock signal regenerated locally in the receiver 
to digitize the demodulated received signals and to clock the digital 
equalizer at the same rate as the digitized complex samples. 
Known adaptive digital equalizers include oversampling adaptive digital 
equalizers in which the demodulated received signal is sampled at a 
frequency which is a multiple of the symbol-clock signal frequency. 
Oversampling adaptive digital equalizers comprise digital filters adapted 
to vary their group propagation time according to the phase of the 
symbol-clock signal using algorithms such as the stochastic gradient 
algorithm, for example, to calculate coefficients of the digital 
transversal filters which automatically compensate phase errors of the 
symbol-clock signal. 
In practice, however, it is found that this advantageous feature of 
oversampling adaptive digital equalizers is not sufficient to compensate 
for serious phase errors in the regenerated symbol-clock signal because 
the number of coefficients of the digital filters is necessarily limited. 
Consequently it is necessary to provide a symbol-clock signal control 
device to monitor and correct the phase of the symbol-clock signal to 
ensure correct operation of the digital equalizer. 
The document IEEE TRANSACTIONS ON COMMUNICATION, vol.COM-24, N.degree. 8, 
AUGUST 1976, pages 856-863 discloses a receiver system comprising an 
oversampling adaptive digital equalizer and a timing recovery device 
controlling means for generating a symbol-clock signal. The timing 
recovery device derives a phase control signal from the coefficients of 
the transversal filter of the adaptive digital equalizer to control the 
phase of the generated symbol-clock signal. The control signal is 
representative of the weighted sum of the coefficients of the equalizer 
filter. This weighted sum is expressed by the equation: 
##EQU1## 
in which M has a selected value slightly greater than N/2. Thus movement 
of the coefficients of the filter causes the value D to vary either side 
of the null value. The value D is filtered to form the phase control 
signal which controls the phase of the symbol-clock signal. 
However, it has been found that the performance of the clock recovery 
device remains mediocre because the range of adjustment, in terms of 
bandwidth, of the phase of the symbol-clock signal is essentially limited 
by the loop bandwidth of the digital equalizer, which is a few hertz. 
What is more, the performance of the equalizer is degraded because the 
phase control signal is generated from the movement of the coefficients of 
the equalizer filter, which must adapt to random variations in the phase 
of the symbol-clock signal, whereas the transmission medium may remain 
stable. This leads to the selection of costly highly stable clocks. 
Finally, this timing recovery device is not suitable for a digital 
equalizer with multiple diversity channels comprising multiple digital 
transversal filters. The above equation caters for only one diversity 
channel and it would not seem possible to expand this equation to cover 
multiple diversity channels. 
An object of the invention is therefore to alleviate the aforementioned 
drawbacks and in particular to solve the problem of symbol-timing recovery 
in a receiver system comprising an adaptive digital equalizer with 
multiple diversity channels. 
SUMMARY OF THE INVENTION 
The present invention consists in a receiver system for processing signals 
transmitted via a multipath dispersive transmission medium, said receiving 
system including means for digitizing the received signals to supply 
corresponding digital signals, adaptive digital equalization means 
responsive to said digital signals for supplying information symbols, 
means for producing a symbol-clock signal controlling said digitizer means 
and said equalizer means and means for recovering a timing signal 
responsive to said digital signals and to said information symbols in 
order to derive a phase control signal for controlling the phase of said 
symbol-clock signal. 
Other features and advantages of the invention will emerge more clearly 
from the following description of one example of the invention given with 
reference to the appended drawings.

DETAILED DESCRIPTION OF THE INVENTION 
The receiver system in accordance with the invention is designed to process 
digital signals modulated by QPSK modulation, for example, transmitted 
over a multipath dispersive medium. A signal transmitted by a transmitter 
is received by the receiver in the form of a plurality of replicas of the 
transmitted signal, these replicas being received on diversity channels of 
the receiver system. 
Referring to FIG. 1, in a conventional manner the receiver system comprises 
demodulators (not shown) to demodulate the replicas of the transmitted 
signal and to supply demodulated received signals, converters 30.sub.1 
through 30.sub.n receiving at their input the demodulated signals received 
via the diversity channels 20.sub.1 through 20.sub.n and adapted to 
digitize the demodulated received signals to form complex samples A, an 
adaptive digital equalizer 40 receiving the digitized signals on diversity 
channels at its input and supplying complex information symbols B such as 
decided symbols or reference symbols, timing recovery means 50 and 
generator means 70 for generating a controlled phase symbol-clock signal 
H, the symbol-clock signal H controlling the analog-digital converter 
means and the digital equalizer. In what follows A represents a signal 
sampled at intervals of k.T/2 in which T represents the symbol time. 
The symbol-clock signal generator means 70 (receiver clock) is 
advantageously a voltage/frequency or like controlled oscillator having a 
control input controlling the phase of the symbol-clock signal generated. 
The timing recovery device 50 derives a phase control signal via a matched 
shaping filter 60 by processing the digital signals (samples A) and the 
information symbols B. The phase control signal is applied to the control 
input of the clock generator 70 to control the phase of the symbol-clock 
signal and to correct symbol-clock signal phase errors. 
The timing recovery device in accordance with the invention is shown in 
more detail in FIG. 2. It comprises a medium estimator 150 responsive to 
the complex samples A and to the complex information symbols B (decided or 
referenced) to supply complex estimate samples G of the impulse response 
of the medium for each diversity channel, a buffer RAM 110 for storing the 
complex estimate samples for a predetermined period and a microprocessor 
or like calculation means 120 for operating on the stored estimate 
samples. 
It must be understood that the impulse response of the medium must be 
estimated over a certain number of information symbols to be sufficiently 
representative of the quality of the medium. Also, the impulse response of 
the medium is estimated at regular intervals under the control of control 
means 130 adapted to synchronize the medium estimator 150, the buffer 110 
and the calculation means 120. 
The impulse response of the medium is the result of convolution of the 
transmit filter, the transmission medium and the receive filter. 
Consequently, the medium estimator means 150 supplies estimate samples for 
each diversity channel at times which are multiples of a symbol half-time 
(k.T/2). FIG. 3 is a diagram showing one embodiment of a medium estimator 
for QPSK modulation which comprises a battery of correlators 160 each 
assigned to one diversity channel 20 (four are shown in the figure). Each 
correlator 160 receives the information symbols B at times which are 
multiples of the symbol time (kT) and samples A from the corresponding 
diversity channel at times which are multiples of a symbol half-time 
(k.T/2) and supplies the estimate samples G1, G2, G5, G4. If it is assumed 
that the estimate samples G are obtained for a large number of symbols B, 
the output of the correlators 160 may be used directly as a medium 
estimate without deteriorating the performance of the timing recovery 
loop. The estimate timing requirement for controlling the symbol-clock 
signal is very small in relation to the information symbol timing 
requirement. 
FIG. 4 shows a simplified correlator 160 suitable for QPSK modulation. The 
correlator 160 is constructed in a standard manner around a series of 
registers 170 for the samples A, a series of multipliers 180 and a series 
of integrators 190 whose operation will not be described hereinafter as it 
is well known to one skilled in the art. The figure also shows time-delay 
means 100 associated with a diversity channel 20 to delay the complex 
samples A by a time which is adjusted so that the correlator 160 supplies 
the highest energy of the impulse response of the medium for the channel 
in question. Consequently, the time-delay means 100 are adapted to 
position the complex samples A in the analysis window of the correlators 
160, that is to say to adjust the complex samples A substantially into 
corresponding temporal relationship with the information symbols B. 
The correlator 160 receives at its input the delayed samples A divided into 
even rank samples A(2k) and odd rank samples A(2k+1) and the decided or 
reference symbols corresponding to the delayed samples. It supplies a set 
of medium estimate samples for the channel in question. The correlator 
shown in FIG. 4 comprises eight integrators 190 which each deliver medium 
estimate components G(k) for values of k running from -4 to 3, k 
representing the rank of the sample A tap. 
At the beginning of the medium estimation, the control means 160 reset the 
integrators 190 of the correlators 160 by means of a RESET command. During 
a sufficiently long sequence of information symbols B the integrators 190 
accumulate the result of multiplying the complex samples A(2k+1) or A(2k) 
by the complex information symbols B. In the case shown in FIGS. 3 and 4 
the medium estimator 150 delivers 32 estimate samples G(k), eight for each 
correlator. The estimate samples G(k) are stored in the buffer 110. 
Hereinafter G1(k), G2(k), G3(k), etc designate the estimate sample located 
at k.T/2 from the respective impulse response estimate for channel 1, 
channel 2, channel 3, etc. 
The calculation means 120 is adapted to extract the value of the estimate 
samples from the buffer 110 and to calculate the expression: 
##EQU2## 
Values of D to either side of the null value represent a symbol-clock 
signal phase error. Of course, this expression is given for four diversity 
channels but it could readily be generalized to suit a greater number of 
diversity channels. Also, tests have shown that the above expression for D 
can be simplified by eliminating the denominator without making it 
unreliable. It may also be remarked that the phase error value is a linear 
combination of the moduli of the complex medium estimate values taken for 
a plurality of information symbols. 
The error value D calculated by the calculation means 120 is filtered by a 
filter 60 to provide the control signal which controls the phase of the 
symbol-clock signal. The timing recovery system in accordance with the 
invention enables receiver clock acquisition and synchronization even if 
the transmitted signal is highly disrupted by the medium or there is 
strong fading on one of the diversity channels. What is more, the timing 
recovery device in accordance with the invention adds very little 
complexity to the receiver system given that it is implemented around a 
device identical to that used to control the filter coefficients of the 
adaptive digital equalizer.