A Joint MMSE Channel and Noise Variance Estimation for OFDM/OQAM Modulation

This paper deals with the minimum mean square error (MMSE)-based multipath channel and noise variance estimation in the case of a pilot-aided orthogonal frequency division multiplexing/offset quadrature amplitude modulation (OFDM/OQAM) system. The theoretical expression of the LMMSE channel estimation is formulated and a simpler closed-form is derived. The MSE analysis shows that LMMSE inevitably reaches an error floor due to the interference inherent to OFDM/OQAM modulation. Furthermore, an algorithm for the joint MMSE estimation of both the channel and the noise variance (J-MCNE) is proposed and features a reduced complexity thanks to the low-rank approximation method. The noise level estimation represents a challenge as it is concealed in the intrinsic interferences generated by the OFDM/OQAM. Simulations reveal the capability of the proposed J-MCNE method to estimate the noise variance and show that the achieved bit error rate (BER) is close to that of the perfect estimation.

I. Introduction

Orthogonal frequency division multiplexing/offset quadrature amplitude modulation (OFDM/OQAM) is an attractive modulation scheme in the current context of spectrum scarcity. In particular, it is known to be a good alternative to cyclic prefix- (CP-) OFDM, since it does not utilize rectangular time-window shaping nor any CP. It can therefore achieve a higher spectral efficiency and a better spectral containment [1]. However, OFDM is very robust against multipath channels thanks to the CP and the complex orthogonality that exists between subcarriers. Unlike OFDM, the OFDM/OQAM subcarriers are only orthogonal in the real field. As a consequence, the presence of the multipath channel induces complex interferences from all neighboring subcarriers and symbols of the time-frequency grid [2], [3]. For this reason, the channel estimation process, which is simple in OFDM systems, becomes a challenge in OFDM/OQAM.