Channel Tracking in SDMA-Based Multi-User MIMO-OFDM Communications Systems

Multiple-input multiple-output (MIMO) and orthogonal frequency division multiplexing (OFDM) stand as promising technologies to resolve bottlenecks in the traffic capacity of current and future high data rate wireless systems such as longterm evolution (LTE), Wi-Fi, and worldwide interoperability for microwave access (WiMAX). Recently, the expansion of MIMO systems to employ multiple users (multi-user MIMO) has been a key topic of interest, mainly motivated by the need to recognise the network capacity enhancements resulting from the use of MIMO technology. In Australia, the Commonwealth Scientific and Industrial Research Organisation (CSIRO) has proposed and implemented a novel and feasible system called the “Ngara wireless broadband access” system using space division multiple access (SDMA) to allow multiple users to employ the same frequency at the same time in MIMO-OFDM systems. In SDMA-based multi-user MIMO-OFDM, wireless broadband services with higher spectral efficiency can be enabled by employing multiple antennas at an access point to serve each mobile stations equipped with a single antenna. Over the last fifteen years, multi-user MIMO-OFDM wireless communications have been the subject of extensive interest from both the academic and industrial communities. However, a realistic multi-user MIMO-OFDM transceiver design, with the ability to achieve channel capacity boundaries in practical channel conditions remains a primary problem. Specifically, the performance of multi-user MIMO-OFDM depends heavily on the accuracy of the estimation of channel state information (CSI) which is defined by the propagation environment of wireless systems. Practically, wireless channels experience multi-path propagation and time-variations which should be tracked recursively to accurately detect transmitted data at the receiver. Typically, CSI can be estimated by sending known symbols. However, this increases overheads if the channel changes significantly over time. Therefore, the design and implementation of an efficient SDMA-based multi-user MIMO-OFDM system still faces major challenges, in particular, the development of an efficient and accurate channel tracking method. Moreover, channel compensation has been identified as a key issue from the beginning of wireless communications, and the latest growth in this dominion has made the old issue more challenging. The exploitation of an equalizer which can track the changing channel with increased accuracy with the introduction of SDMA-based multi-user, multiple-antenna systems with OFDM transmission is more demanding.