Use of linear transverse equalisers and channel state information in combined OFDM-equalisation

The efficiency of a coded orthogonal frequency division multiplexing (COFDM) receiver can be improved by use of a pre-FFT equaliser (PFE). This technique is known as combined OFDM-equalisation. This paper considers the noise amplification effect of the PFE and its effect on the performance of a combined COFDM-equalisation modem and proposes a method to improve this performance. It first reviews and compares the conventional COFDM and combined COFDM-equalisation techniques. The PFE is then described and the conditions required to allow it to take the form of a decision feedback equaliser (DFE) are discussed. It is shown that these conditions often cannot be met. In these cases the PFE must take the form of a linear transversal equaliser (LTE). The performance of the LTE is inferior to that of the DFE due to significant noise amplification. Hence, if the performance of combined COFDM-equalisation is to match that of conventional COFDM, a method for mitigating the noise amplifying effect of an LTE type PFE is required. One such method is proposed. This technique exploits channel state information (CSI) available in a COFDM receiver in order to enhance the performance of a Viterbi convolutional decoder. This 'CSI modified' Viterbi algorithm is capable of mitigating the noise amplification occurring in the equaliser. To demonstrate this, the performance of conventional COFDM and combined COFDM-equalisation are compared by means of software simulation using both standard and CSI modified Viterbi decoding. The results for the standard Viterbi decoding illustrate the performance penalty due to noise amplification. The results for the CSI modified Viterbi algorithm demonstrate the effective mitigation of noise amplification and the comparable performance of conventional COFDM and combined COFDM-equalisation.