Frequency scaled time domain equalization for OFDM in broadband fixed wireless access channels

In this paper, the use of orthogonal frequency division multiplexing (OFDM), combined with a time domain equalizer (TEQ), is investigated for broadband fixed wireless access (BFWA) systems. OFDM systems use a cyclic prefix (CP) that is inserted at the beginning of each symbol to convert the linear convolution of data and channel into a circular one. If the CP is longer than the channel length, intersymbol interference (ISI) is avoided. However the use of the CP reduces the efficiency of the system. A TEQ is often used to reduce the channel length, enabling a shorter CP to be used. The TEQ schemes that have been proposed to date result in the effective channel impulse response (EIR), (i.e. including the TEQ) having spectral nulls. This prevents some of subchannels; from being used for data transmission. We present the frequency scaled time domain equalization (FSTEQ) algorithm which optimises the TEQ coefficients in both the time and frequency domains resulting in a flatter spectral response. We show that the algorithm when applied to Stanford University Interim (SUI)-2 channels for the MMDS band yields an improvement of up to 100 fold in the BER at an SNR of 20 dB.

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