Eigen-Coherence and Link Performance of Closed-Loop 4G Wireless in Measured Outdoor MIMO Channels

Employing feedback in multiple-input-multiple-output (MIMO) systems offers the potential to significantly increase data rates over open-loop schemes. However, the impact the propagation channel has on the behavior and performance of the scheme must be considered and, where appropriate, the signaling designed to suit. In this paper, measured outdoor MIMO propagation data is used to evaluate initially the distributions and statistics of the coherence of the eigenmodes of the channel, for a wide range of mobility scenarios and user devices. Then, the impact on link performance of the differing coherences in time and frequency is evaluated in terms of the feedback delay and spectral spacing of pilots in a practical feedback scheme used in mobile WiMAX. It is found that not only mobility and physical environment, but also the directivity of the antenna array affect the eigen-coherence and also, therefore, the performance of a feedback scheme. Bit-error rate and information capacity are both shown to be sensitive to increased feedback delay and pilot spacing, however, the closed-loop system always outperforms its open-loop counterpart.