Maximum Likelihood Inter-Carrier Interference Suppression for Wireless OFDM with Null Subcarriers

Abstract

Orthogonal Frequency Division Multiplexing (OFDM) transmission is robust to frequency-selective channels but sensitive to time-selective channels. Time variations of channels destroy the orthogonality between subcarriers, resulting in a considerable performance loss due to intercarrier interference (ICI) between subcarriers. In this paper, we propose a Viterbi-type algorithm to effectively suppress the ICI, by exploiting the property of ICI terms and null subcarriers embedded in OFDM symbols for the reduction of interferences from/to adjacent bands. Simulations show that the proposed equalizer works well with affordable complexity and outperforms linear equalizers.

Pilot-Assisted Time-Varying Channel Estimation for OFDM Systems

“…We choose in (31). As a comparison, we also list the equalization performances, which are based on the estimated CE-BEM channel and the perfect CSI.…”

“…Equalization Performance Based on the Estimated Channel: For this test case, we will transmit quadrature phase-shift keying (QPSK)-modulated data symbols, and the channel will be equalized by the banded LMMSE equalizer proposed in [30], although other banded equalizers can be employed as well [29], [31]. In short, the equalizer can be described as (31) where is the banded approximation of the original channel matrix, with being an matrix whose main diagonal, subdiagonals, and super-diagonals are ones, and the remaining entries are zero. Thanks to the banded approximation and a simple banded decomposition of , the complexity of the banded LMMSE equalizer is .…”

“…If we then assume that the length of the cyclic prefix satisfies , both and are "pseudocirculant" with and Note that by means of an appropriate window , a banded approximation of is more accurate than the corresponding banded approximation of . Therefore, low-complexity equalization schemes that exploit the banded assumption of the channel matrix in the frequency domain [29]- [31] will have an improved performance when appropriate windowing is employed. In the sequel, we will refer to the windowed channel as "the channel," unless otherwise stated.…”

“…Recently, an increased interest has emerged in low-complexity equalization of TV channels for OFDM systems, many of which rely on the strictly banded assumption of the channel matrix in the frequency domain [29]- [31]. Undoubtedly, for the data model introduced in this paper, the out-of-band interference can compromise the performance of such equalizers.…”

Factor graph based detection approach for high-mobility OFDM systems with large FFT modes

“…Other research studies explore optimal or nearoptimal solutions, e.g., near-maximum likelihood (ML) approaches [20,21], sphere decoding (SD) [22], or maximum a posteriori (MAP) equalization employing Bahl Cocke Jelinek Raviv (BCJR) algorithm [23]. Apart from the frequency domain, time domain solutions have also been proposed [24][25][26].…”