MIMO-OFDM under nonlinear channel distortions

Abstract

In recent years, multiple-input multiple-output (MIMO) communication systems employing multiple antennas have received much attention owing to their capabilities to exploit the spatial diversity. Among various space-time (ST) codes proposed for MIMO systems, the Alamouti scheme for systems with two transmit antennas is particularly attractive and widely adopted because of its simplicity and outstanding performance. On the other hand, the multicarrier modulation known as orthogonal frequency-division multiplexing (OFDM) has emerged as a dominant technology for broadband communications and has been employed in many modern wireless/wired communication standards. The advantage of OFDM stems from the process of transforming frequency-selective fading channel into a set of parallel frequency-flat fading channels, thus greatly simplifying the equalization solution at the receiver.Since integrating MIMO and OFDM, it is natural to introduce ST coding to each subcarrier to enable spatial diversity in OFDM systems. This leads to ST coded MIMO-OFDM systems. Unfortunately, an OFDM signal typically has a very high dynamic range resulting in high peak-to-average power ratio (PAPR) because of the large number of subcarriers in use. For this reason, OFDM signals are more susceptible to nonlinear distortions due to saturation of transceiver components such as high-power amplifiers (HPA), digital-to-analog (D/A) converters, and analog-to-digital (A/D) converters. Intentional clipping may be used to control the dynamic range of OFDM signals. However, clipping is a nonlinear process and also introduces nonlinear distortions. It has been shown that channel nonlinearity can significantly degrade OFDM system performance and an efficient receiver with iterative distortion noise cancellation has been proposed in. MIMO-OFDM employs multiple transmit antennas to send multiple OFDM signals simultaneously. This further increases the PAPR of the signal at the receiving antenna. Hence nonlinear distortions are almost unavoidable in MIMO-OFDM.This talk will investigate the practical effect of nonlinear channel distortion on space-time blockcoded MIMO-OFDM signals. In particular, we consider a slow fading environment under which the performance of an iterative nonlinear distortion cancellation algorithm is studied.We prove analytically that nonlinear noise due to the two input OFDM symbols involved in Alamouti coding is completely separated at the receiver. This result is also valid for any other orthogonal space-time block codes whose codeword entries are selected from the original input complex data symbols and their negates, conjugates, and multiples by the imaginary unit. It is shown through simulations that MIMO-OFDM with iterative noise cancellation is more robust against nonlinear distortion than OFDM. This allows MIMO-OFDM to clip its signal aggressively without incurring heavy penalty in bit error rate (BER) performance.