Channel estimation for multiuser MIMO-OFDM systems based on Kalman algorithms

Abstract

I. IintroductionIt is now widely accepted that multiple input multiple output (MIMO) systems increase the link reliability and/or spectral efficiency of multiuser wireless communications [1]. Moreover, when channel state information (CSI) is available at the transmitter, linear precoding can be used to further improve system performance by tailoring the transmission to the instantaneous channel conditions [2]-[5] while retaining the benefits of alllinear processing. CSI at the transmitter is mandatory in the multiuser downlink, where a base station attempts to communicate simultaneously with multiple users. On a different front, orthogonal frequency division ultiplexing (OFDM) is a simple, and now well-accepted, technique to mitigate the effects of intersymbol interference in frequency selective channels [6]. OFDM converts a broadband frequency selective channel to a series of narrowband channels by transmitting data in parallel over many subcarriers. Combining OFDM with MIMO, producing so called MIMO-OFDM, significantly reduces receiver complexity in wireless multiuser broadband systems [7], thus making it a competitive choice for future broadband wireless communication systems. Since OFDM uses multiple subcarriers, optimal linear precoding for MIMO-OFDM can be implemented by deriving linear precoders for each subcarrier independently.However, due to the generally large number of subcarriers, the computational load is excessive, and this approach is probably impossible to implement in practice. Furthermore, this approach is computationally inefficient since the MIMO channels associated with adjacent subcarriers are highly correlated; the precoder and decoders are correlated as well. II. MU-MIMO-OFDM system overviewThe MU-MIMO-OFDM system under consideration is shown in Fig. 1. It consists of K single-antenna users, transmitting to a receiver (the base-station) equipped with N antennas. The users transmit blocks of S OFDM symbols,each containing M sub-carriers. The first Sp OFDM symbols are reserved forpilot symbols, which are known to the receiver. The following S d = S -Sp OFDM symbols contain coded data. The total number of information bearing signal constellation points per block, transmitted from each user, then becomes L = S d M. A forward error correcting code (FEC) with rate R is used to generate codewords, which after interleaving, are mapped onto the L signal constellation points. We restrict our investigations to the case of QPSK. An extension to other constellations is conceptually straightforward, but in general non-trivial [8]. After OFDM modulation and pilot insertion, the users transmit their signals over a frequency selective block fading channel, where the different multiantenna links are independent and identically distributed (IID). The block fading assumption holds if the transmitted data blocks are much shorter than the channel coherence time. Thus, a system with short data blocks transmitted over a channel with moderate Doppler spread is considered. Furthermore, to allow for correct OFDM d...