Low-Complexity Banded Equalizers for OFDM Systems in Doppler Spread Channels

Abstract

Recently, several approaches have been proposed for the equalization of orthogonal frequency-division multiplexing (OFDM) signals in challenging high-mobility scenarios. Among them, a minimum mean-squared error (MMSE) block linear equalizer (BLE), based on a band LDL factorization, is particularly attractive for its good tradeoff between performance and complexity. This paper extends this approach towards two directions. First, we boost the BER performance of the BLE by designing a receiver window specially tailored to the band LDL factorization. Second, we design an MMSE block decision-feedback equalizer (BDFE) that can be modified to support receiver windowing. All the proposed banded equalizers share a similar computational complexity, which is linear in the number of subcarriers. Simulation results show that the proposed receiver architectures are effective in reducing the BER performance degradation caused by the intercarrier interference (ICI) generated by time-varying channels. We also consider a basis expansion model (BEM) channel estimation approach, to establish its impact on the BER performance of the proposed banded equalizers.

Iterative channel estimation and turbo equalization for time-varying OFDM systems

“…The time-domain receiver window [4] as well as the equalizer are designed for a bandwidth parameter B c = 3. A rate 1/2 convolutional code with generator matrix [1 0 1;1 1 1] and a block length of 8192 is used.…”

“…Fortunately, with a proper window design, H f is almost banded, with the most significant elements around the main diagonal [2,4].…”

“…In order to calculateŝ i , we need A −1 (y f − Hm) and t i . First, we exploit the banded structure of the approximated frequency-domain channel matrix H to reduce the complexity of A −1 (y f − Hm) by applying a band LDL H factorization of A [4]. To reduce the complexity of the t i calculations, we exploit the fact that the vector h i is characterized by 2B c + 1 non-zero entries.…”

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.…”

“…6.5(c).) The resulting scheme has approximately the same complexity as the fast MMSE-DFE from [RBL06], yet results in performance that is almost indistinguishable from MLSD.…”