The present invention relates to systems for transmitting data, in the form of successive quantified symbols, between two distant locations, using a communication channel and, more specifically, to a correction device constructed to be located at the output of the communication channel in a modulator-demodulator or modem, and to correct the distortions of the signal, whereby increasing the reliability of the system.
Before the invention is described, it may be useful to summarize some features of such data transmission systems and to list the notations which will be used in the following.
The systems to which the invention relates are for transmission of a sequence of data which will be referred to as {. . . , a.sub.j, . . . } which may have anyone of a plurality of quantified data levels (typically levels of .+-.1). The sub j indicates the jth data symbol.
Usually, the jth data a.sub.j is transmitted as a proportional impulse designated as: EQU a.sub.j .multidot.d(t-j.DELTA.), t.epsilon.[j.DELTA., (j+1).DELTA.](1)
in which:
a.sub.j is the quantified level of a data,
.DELTA. is the time interval between two successive data, i.e. the time interval which is allowed for transmission of data a.sub.j,
d is a narrow pulse.
The transmission uses a carrier at frequency .nu..sub.c. Its initial phase will be designated as .phi..sub.o. Two types of modulation are currently used.
1. In quadrature amplitude modulation, which will be referred to in the following as QAM, two sequences of data (a'.sub.j, a".sub.j) are transmitted on the communication channel each of them modulating one of two quadrature carriers; then the transmitted signal can be represented by the real part of the complex signal: EQU D(t)=.SIGMA..sub.j a.sub.j d(t-j.DELTA.) exp i(2.pi..nu..sub.c t-.phi..sub.o) (1)
where: EQU a.sub.j =a'.sub.j +i a".sub.j ( 2)
and d(t) is a real function.
2. In single side band modulation, currently referred to as SSB, the data a.sub.j is real and it is passed through two quadrature filters. Then, the transmitted signal takes also the form indicated by the formula (1) above, but d(t) is then a complex analytical function.
Last, a mention should be made of doubleside band modulation or DSB which uses a single carrier. Formula (1) is still valid, but then a".sub.j =0 and d(t) is a real function.
The communication channel acts as a filter causing distortion in the transmitted signal and generates intersymbol interferences, designated as ISI. ISI is all the more significant as the bit rate is higher; at the currently used 9600 bts rate, systems suffer very much from it and may provide inaccurate results.
In such systems, demodulation by the modem at the output of the channel implies generating two quadrature signals, at least for QAM systems. This is done using any one of several methods.
1. In a first approach, which is quite conventional, demodulation is achieved by multiplication with two quadrature carriers and low pass filtering, as will be shown later. In SSB and DSB systems, demodulation is not always performed with two quadrature carriers, but sometimes with only one carrier in order to simplify the receiver as shown in FIG. 1 for DSB. However, this simplification adversely affects the performances of the system, if ISI is severe.
After demodulation, the complex received signal EQU x(t)=x'(t)+i.x"(t) (3)
can be written as: EQU x(t)=.SIGMA..sub.j a.sub.j s(t-j.DELTA.) exp i.phi.+b(t) (4)
where:
.phi. is the phase error .phi..sub.1 -.phi..sub.o between the received carrier and the demodulating reference wave;
b(t) is the complex noise on the two quadrature channels, collected by the transmission channel;
s(t) is the complex impulse response of the channel, including the modulating and demodulating filters;
When s(t) is complex, which is usually the case, the two quadrature signals interfere between each other. Attempts have already been made to suppress said intersymbol interference by providing a filter known as an "equalizer", which has a transfer function as close as possible to the inverse of the channel filter. This equalizer usually comprises a sampler and a transversal digital filter. The sampling phase .tau..sub.o is selected at a value close to the time where s(t), i.e. the response of the channel, is maximum in an attempt to have it as close as possible to .tau. (delay due to the channel). The sampled signal is then: EQU . . , x.sub.o =x(.tau..sub.o), . . . , x.sub.j =x(.tau..sub.o +j.DELTA.), . . . (5)
and the equalized signal is: ##EQU1## and H.sub.j.sup.T is the tap vector of the equalizer at time j.DELTA.: ##EQU2##
H.sub.j.sup.T is a complex vector; N+M+1 is the number of taps of the equalizer.
Demodulation of SSB or DSB may be performed only on one carrier. Then: EQU x".sub.j =0, H".sub.j.sup.T =0. (9)
2. A different approach, known as "phase splitting" has been described in a paper by R. D. Gitlin et al entitled "Passband equalization for differentially phase modulated data signals" in Bell System Technical Journal, No. 2, February 1973, pp. 219-238.
According to the phase splitting method, the received signal is processed in a 90.degree. phase shift filter which provides a second signal in quadrature with the received signal at the input of a demodulator. A sampler and a complex equalizer are located downstream of the modulator and operate along the same line as the equalizer of the first method for delivering a complex output. Then, demodulation is done digitally by complex multiplication of this output with EQU e.sup.-i(2.pi..nu..sbsp.c.sup.j.DELTA.-.phi..sbsp.1.sup.).
Such a scheme is generally referred to as pass-band equalization. A corresponding network will be described in more detail hereinafter with reference to FIG. 9a. In FIG. 9b, the equalization is performed after a complex multiplication by e.sup.-(2i.pi..nu..DELTA..sbsp.c.sup.j.DELTA.-i.phi..sbsp.1.sup.). The systems of FIGS. 9a and 9b deliver equivalent outputs provided the complex coefficients h.sub.k.sup.1 and h.sub.k.sup.2 of the passband and base-band equalizers are related by ##EQU3##
The latter system has a base band equalizer as in the conventional system (first method) which will be more completely described with reference to FIG. 8.
The above-described equalizers perform satisfactorily with an adaptive or learning algorithm which is of the type: EQU H.sub.k+1 =H.sub.k -.mu.x.sub.k (y.sub.j -a.sub.j) (11)
in the system of FIG. 8, or an equivalent algorithm in the phase splitting method, if, and only if, variations in the channel characteristics (and the variations of the carrier phase) are slow. In equation (11), .mu. is a step size which is a real quantity. X.sub.k * is the conventional representation for the complex quantity conjugate X.sub.k. As a practical rule, satisfactory results may be achieved when the signal/noise ratio is high (20 dB or more) and the phase offset is low and slowly variable.
However, in most telephone networks, the phase error .phi. which appears in equation (4) exhibits rapid changes, due to phase jitter or possibly to frequency offset. Now, the speed of adaptation or learning according to algorithm (11) is a function of the number of taps, and it is slow for an equalizer having a large number of taps. Since such a large number of taps is necessary to achieve satisfactory equalization, conventional equalizers cannot remove phase jitter which may have a frequency higher than 100 cps and a large amplitude (30.degree. peak to peak or more).
Numerous attempts have been made to solve the problem. An approach has been described by D. D. Falconer in "Analysis of a gradient algorithm for simultaneous passband equalization and carrier-phase recovery" (The Bell System Technical Journal, Vol. 55, No. 4, April 1976, pp. 409-428). It consists in evaluating the phase error .phi. and introducing the estimated value in the demodulation process. That approach may compensate for phase errors, but is unable to correct for amplitude distortions due to variable attenuation.
It is also old in the art (U.S. Pat. No. 3,935,535 to Motley et al) to provide a correction device having an equalizer for removing ISI and a phase correction network having a phase lock circuit for error evaluation. The device relies for operation on the periodic transmission of a special sequence of data and this constitutes a severe limitation on the estimation of the phase error.
Still another prior art correction device (French Pat. No. 2,283,606 to Compagnie IBM France) comprises an equalizer in series relation with a phase detector and a phase filter without adaptation features. Other systems (see for instance IEEE International Conference on Communications, June 11-13, 1973, pp. 2-31 to 2-38) are similar to that of Pat. No. 3,935,535.
It is an object of the invention to provide an improved device for correction of distortions due to the communication channels in data transmission systems.
It is another object of the invention to provide a device which makes it possible to dispense with phase evaluation and which is carried out by the use of linear processing only.
It is a more specific object of the invention to provide a device permitting accurate data transmission in presence of simultaneous fast phase jitter and ISI.
According to an aspect of the invention, there is provided a correction device for a system for transmitting data between remote locations and the like, using a communication channel whose transfer function includes a main component which does not vary or varies slowly and at least another component whose time constant is substantially lower and whose effect is lesser. The device is located between the output of the channel and detection circuits. It comprises a cascade arrangement of an equalizing filter and a digital self-adapting network associated with a control system for operating according to a learning algorithm. The equalizing filter is designed for removing the effects of said main component. The digital network constitutes a transversal adaptive filter having a small number of taps and a short time constant as compared to the speed of variation of the other component. The control system for the digital network can use a linear algorithm of the "gradient" type for adapting the tap coefficients.
When the digital network is for overcoming the effects of phase jitter on telephone lines, it is generally sufficient to provide it with only one (possibly complex) coefficient. In other cases, it may be preferable to design the filter with two or three taps, or even to use two network stages or even more in cascade; then, the number of taps of the stages will usually be decreasing as they are farther along the data path. The invention is not limited to phase jitter on telephone lines. It applies whenever a communication channel introduces at least two types of linear distortion having distinct ranges in the variation speed, the prevalent one varying more slowly.
It may be that the major or main effect is invariant or constant: then ISI may be suppressed by the use of an equalizing filter which needs not to be digital and adaptive. In case of very slow variation, periodic adjustment by an operator may be sufficient: such is the case for communication on cables. Then, the adaptive network is adapted to track or follow any fast variation of the transfer function of the channel, for instance for correcting timing jitter on cables with repeaters.