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

The eficiency 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. INTRODUCTION The technique of Coded Orthogonal Frequency Division Multiplexing (COFDM) offers a robust method for digital radio transmission. Conventionally, COFDM employs a guard interval to combat delay spread of the radio signal that will otherwise result in inter-carrier interference (ICI) of the OFDM modulated data [l]. The use of a guard interval reduces transmission efficiency according to the ratio of un-extended and extended OFDM symbol periods DI. A combined COFDM-equalisation technique incorporating a pre-FFT Equaliser has been proposed previously [2][3]. The performance of this pre-FFI Equaliser has been analyzed in terms of its performance under additive noise in [4] and time variant channel conditions in [5]. This paper reviews the combined COFDM-Equalisation receiver and preFFI Equaliser designs and summarizes the implications of their use by comparison with the conventional COFDM method. As has been widely considered in the literature [6][7], the Decision Feedback Equaliser ( D E ) offers improved performance under additive noise conditions in comparison to the Linear Transverse Equaliser (LTE) due to reduced noise amplification. Thus, it is desirable to implement the pre-FIT Equaliser in the form of a DFE. In reality the best that can be achieved is a compromised form of the DFE [4]. The closeness of this approximation to an actual DFE that can be achieved is dependent upon the OFDM symbol period, channel delay spread and any delays that occur in the feedback loop of the combined COFDM-equalisation receiver. In those cases where a sufficiently close approximation to the DFE cannot be achieved, it is preferable to implement the pre-FFT Equaliser as an LTE. When the pre-FFT Equaliser is implemented as an LTE, the frequency response of the Equaliser defines the frequency spectrum of the noise output by the equaliser (assuming white noise). Thus, the noise spectrum can be determined and this information can be exploited in the Viterbi decoder. 0-7803-6465-5100 $10.00