MIMO OFDM for broadband fixed wireless access

A broadband multiple-input multiple-output (MIMO) OFDM system has been designed and evaluated for a fixed wireless link between tall buildings in an urban area. Channel measurements for a typical installation location are analysed to determine the relevant spatial, temporal and frequency characteristics. The small number of multipath components leads to a high spatial correlation and limited potential for spatial diversity to provide spectral efficiency gains. However, the slowly-varying characteristics support the use of a closed-loop diversity scheme, such as spatial multiplexing along the eigenmodes of the channel matrix, to maximise the achievable throughput. This scheme is evaluated using the measured channel data, and it is shown that the self-interference caused by channel estimation errors is the limiting factor on the system performance. Taking into account the estimation errors and feedback delay, it is demonstrated that a three-fold increase in spectral efficiency relative to a single-element antenna system is achievable using eight-element antennas, even in this highly correlated environment.