Simple equalization of time-varying channels for OFDM

Abstract

Abstract-We present a block minimum mean-squared error (MMSE) equalizer for orthogonal frequency-division multiplexing (OFDM) systems over time-varying multipath channels. The equalization algorithm exploits the band structure of the frequency-domain channel matrix by means of a band LDL H factorization. The complexity of the proposed algorithm is linear in the number of subcarriers and turns out to be smaller with respect to a serial MMSE equalizer characterized by a similar performance.

Enhanced MMSE-FDE method for TS-OFDM under higher mobile environments

“…To solve the above ICI problem, various frequency domain equalization (FDE) methods have been proposed both for a cyclic prefix OFDM (CP-OFDM) and time domain training sequence OFDM (TS-OFDM) [2,3,4]. In [2], a low-complexity minimum mean square error-frequency domain equalization (MMSE-FDE) method was proposed for CP-OFDM.…”

“…In [2], a low-complexity minimum mean square error-frequency domain equalization (MMSE-FDE) method was proposed for CP-OFDM. In this method, the full elements of channel frequency response (CFR) matrix which is converted from the time domain channel impulse response (CIR) is approximated by a banded matrix so as to employ a fast algorithm for inverse matrix calculation [5].…”

Equalization of Time-Varying Channels

“…More recently, Rugini, Banelli and Leus proposed O(KD 2 ) exact LMMSE [RBL05] and MMSE-DFE [RBL06] schemes for narrowly banded 16 Q based on fast LDU decomposition. Furthermore, they showed how to design a receiver window to ensure that the noise covariance C z is quasi-banded, making the observation covariance C y = σ 2 a Q H Q + C z banded as well.…”

“…In this case, the LMMSE estimation step consumes only O(K) operations (because the matrix to invert is diagonal), and FDE consumes O(K log 2 K) operations in total. Note that, for large M, FDE would be significantly cheaper than LMMSE estimation of a from y via fast LDU [RBL05] (as discussed in Section 6.3.3), which consumes O(KM 2 ) operations, where typically M ≈ K/4.…”

“…6.2(b), in which case any of the fast linear equalization techniques described in Section 6.3.3 can be used to estimate a from y. For example, in [TL08], Tang and Leus proposed a method to equalize a single carrier system using the OFDM fast LMMSE technique [RBL05].…”