PEmORMANCE ANALYSIS OF A PRE-FFT EQUALIZER DESIGN FOR DVB-T

Conventional OFDM employs a guard interval to combat delay spread distortion oftransmitted data. This reduces the efliciency of the OFDM transmission. The combined OFDM-Equalization strategy described here employs an Adaptive Equalizer to combat delay spread distortion and thereby facilitates the use of very short guard intervals and thus a more eflcient OFDM modulation scheme. The OFDM receiver and the equalizer are described and performance is simulated for transmission in a multipath radio environment. It is shown that the combined OFDMEqualization method offers improved bandwidth eflciency over the conventional method but performance is sensitive to additive noise. INTRODUCTION OFDM is the specified modulation method for the ETSI Terrestrial Digital Television Broadcast (DVB-T) standard. Conventionally, OFDM employs a cyclic extension of transmitted OFDM symbols to combat delay spread distortion. In this paper, a novel equalizer design is proposed which allows for a considerable reduction in the length of the guard interval and thus offers a significant improvement in bandwidth efficiency. Inter-Cell Interference in Single Frequency Networks (SFNs) manifests itself as very long delay spreads of the broadcast signal. This requires particularly long guard intervals for effective OFDM transmission. These long guard intervals can only be achieved by systems operating with an 8192 point FFT ('8k' mode). The pre-FFT Equalizer design described here obviates this limitation. OFDM MODULATION A conventional OFDM modulation process [I] is employed to produce cyclically extended OFDM symbols Y(n , I ) from an input data vector X ( k , l ) . k indexes the OFDM subband, n indexes the transmission symbol and I indexes the OFDM symbol. The ' indicates a cyclically extended sequence. x'(n,l) is up-sampled, D-A converted and RF modulated to produce the transmittable signal . ;(I) . The transmitted signal is distorted by delay spread and additive noise to produce a received signal y,'(t). THE COMBINED OFDM-EQUALIZATION RECEIVER The structure of an OFDM receiver employing a pre-FFT Equalizer is shown in Figure 1. Its function can be seen to be that of a conventional OFDM receiver [I] with the addition of the adaptive equalizing filter and a feedback path which generates w'(n,I) an estimate of the transmitted OFDM symbol based on the post decision output data, W ( k 1 ) . THE PRE-FFT EQUALIZER The structure of the pre-FFT equalizer is shown in figure 2. This design is similar to that of a Decision Feedback Equalizer (DFE) that might be used in a single carrier system. An adaptation strategy combining the use of training sequences and decision directed adaptation is employed. The equalizer output is defined as: z'(n,/)= c c b ) y ' ( ( n j ( N + M ) k / + I ) + i d J ) y ' ( ( n i l l) + c , , , ( n ) ~ d j ) z ' ( ( n j l / ) + C c ( j ) w ' ( ( n + ( ~ + ~ ) j k / ~ ) -is*"