A MULTI-STAGE IIR TIME DOMAIN EQUALIZER FOR OFDM SYSTEMS WITH ISI

Abstract

In an orthogonal frequency division multiplexing (OFDM) system, it is known that when the delay spread of the channel is larger than the cyclic prefix (CP) size, intersymbol interference will occur. The time-domain equalizer (TEQ), designed to shorten the channel impulse response (CIR), is a common device to solve this problem. Conventionally, the TEQ is treated as a finite-impulse-response (FIR) filter, and many TEQ design methods have been proposed. However, a wireless channel typically has multi-path responses, exhibiting FIR characteristics. Thus, the corresponding TEQ will have an infinite impulse response (IIR), and the FIR modeling of the TEQ is inefficient, i.e., the required order for the TEQ will be high. The conventional approach will then suffer from the high computation complexity problem, both in the derivation of TEQ and in the operation of channel shortening. In this paper, we propose a new scheme to overcome these problems. In the derivation of the TEQ, we propose to use a multistage structure, replacing a high-order TEQ with a cascade of several low-order TEQs. In the shortening operation, we propose to use an IIR TEQ approximating a high-order FIR TEQ. Since the ideal TEQ exhibits low-order IIR characteristics, the order of the IIR TEQ can be much lower than the FIR TEQ. Simulations show that while the proposed method can reduce computational complexity significantly, its performance is almost as good as existing methods.