OFDM pilot allocation for sparse channel estimation

Abstract

In communication systems, efficient use of the spectrum is an indispensable concern. Recently the use of compressed sensing for the purpose of estimating orthogonal frequency division multiplexing (OFDM) sparse multipath channels has been proposed to decrease the transmitted overhead in form of the pilot subcarriers which are essential for channel estimation. In this article, we investigate the problem of deterministic pilot allocation in OFDM systems. The method is based on minimizing the coherence of the submatrix of the unitary discrete fourier transform (DFT) matrix associated with the pilot subcarriers. Unlike the usual case of equidistant pilot subcarriers, we show that non-uniform patterns based on cyclic difference sets are optimal. In cases where there are no difference sets, we perform a greedy method for finding a suboptimal solution. We also investigate the performance of the recovery methods such as orthogonal matching pursuit (OMP) and iterative method with adaptive thresholding (IMAT) for estimation of the channel taps.

On the Pilot Carrier Placement in Multicarrier-Based Systems

“…In [1], the authors maximize the distance between adjacent pilot carriers; this distribution generalizes the quasi-uniform distribution from [10] for the case when guard bands are present. In [11], the authors minimize the coherence in partial DFT matrices to obtain the pilot carrier positions. In [12], the pilot carrier placement is selected based on the minimization of a function of the Cramer-Rao lower bound (CRB).…”

A low complexity DFT-matrix based pilot allocation algorithm for sparse channel estimation in OFDM systems

“…Using the pilots, different conventional channel estimation methods such as least square (LS) or minimum mean square error (MMSE) can be employed to estimate OFDM channel. However, in high-speed wideband wireless communication, due to the sparse structure of the scattering objects, the number of non-zero paths is normally small [2]. Therefore, the channel can be modeled as a sparse channel.…”

“…A criterion to select CS algorithms based on the MMSE has also been proposed in the same paper. The modified method of [8] was presented in [2,9] with difference in the coherence minimization of measurement matrix (which is introduced in Section 3) is replaced by using channel data in order to minimize the MSE of the estimated channels. Therefore, the applicability is increased without prior knowledge of the channel.…”

How to select the pilot carrier positions in CP-OFDM?

“…In [1] and [11], ad hoc solutions to the pilot positions problem were proposed for N/M not integer; the maximum distance distribution from [1] reduces to the ad hoc distribution from [11] for the special case of no guard bands or null carriers, where it is not allowed to place pilot carriers. Other researchers have tried to find a good pilot carrier placement by optimizing some cost function [12]- [15]. Common to all these techniques is that none of the proposed techniques achieves the minimum MSE of the channel estimate.…”