Adaptive adjustment of receiver for distorted digital signals
This report describes the investigation which has led to the development of a novel technique for the adaptive adjustment of the receiver in a digital data-transmission system, where the latter operates with additive noise and severe intersymbol interference in the received signal. This technique is suitable both for the adjustment of a conventional non-linear (decision-feedback) equaliser, and also for the adjustment of a linear feedforward transversal filter that is employed ahead of a near-maximum-likelihood detector. In the latter case, the technique provides, in addition, an estimate of the sampled impulse-response of the channel and filter, to give the information on the received signal needed by the detector. The adaptive system requires an estimate of the sampled impulse-response of the channel and it involves finding the roots (zeros) of the z-transform of the sampled impulse-response that lie outside the unit circle. The aim of the investigation has been to develop a root-finding algorithm which would be used to find some or all. of the roots, with moduli greater than unity, of a high-degree (>20) complex polynomial. The first part of the report describes the study of an on-line system, where the receiver operates on a continuous stream of received samples subject to both intersymbol interference and noise, and 'it presents the results of a large number. of tests using computer simulation which show the root-finding capability and vulnerability of the system to white Gaussian noise. .The results of the tests point to the potential advantages of an off-line system, which is much simpler to implement and'requires less computation. The modified technique operates solely and directly on the estimate of sampled impulse-response of the channel, which must,again be,provided at the receiver. The report gives the results of computer-simulation tests measuring the speed and accuracy of two different arrangements of the system, for different channels, and it also studies the effects of double roots and roots just outside the unit circle. The off-line system is suitable for a 16-point QAM system operating at 9600 bit/s over the British public switched telephone network, and with slight modification, it can also be used for a 64-point QAM system operating at 19200 bit/s.