Equaliser techniques for QAM transmissions over dispersive mobile radio channels

Channel equalisation of 32 Mbit/s data transmissed via 16-level QAM at 1.9 GHz over dispersive fading channels is addressed using computer simulations. The multipath propagation channels used had three independent Rayleigh fading paths. The conventional equaliser systems were found to be unsatisfactory and an alternative system was conceived based on a RAKE-type equaliser controlling an iterative decision feedback equaliser. Although complex, simulations showed this equaliser system to yield an order of magnitude reduction in BER compared to that for a linear equaliser at SNRs in excess of 15 dB. Advantages are presented of using error correction coding in the conventional way and in an iterative manner to aid convergence of the equaliser. Gains of 2 to 3 dB were achieved by iterative decoding but at the expense of considerable increase in complexity. When second order spatial diversity was used we achieved improvements of about 5 to 7 dB when both systems were uncoded despite the inherent diversity that can be gained from equalising a wideband channel. By operating at SNRs greater than 25 dB, we were able to simulate transmission at 32 Mbit/s with a BER below 10−3.