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. This paper first reviews and compares the conventional COFDM and combined COFDMequalisation 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 signifcant 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 in this paper. 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 amplcjication. 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.
Sponsorship: The authors wish to acknowledge the financial
support of the UK government Electronics and
Physical Sciences Research Council (EPSRC).
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Name of Conference: 11th IEEE International Symposium on Personal, Indoor and Mobile Radio Communications, 2000 (PIMRC 2000)