The third assumption—that the background noise is Gaussian and uncorrelated with the transmissions—is especially suspect in a cellular MIMO system. All well-designed cellular systems are by nature interference limited: If they were not, it would be possible to increase the spectral efficiency by lowering the frequency reuse or increasing the average loading per cell.

In the downlink of a cellular system, where MIMO is expected to be the most profitable and viable, there will be an effective number of interfering signals, the number of nonnegligible interfering neighboring base stations is .

Figure  illustrates the impact of other cell interference in cellular MIMO systems. It is extremely difficult for a MIMO receiver at the MS to cope simultaneously with both the spatial interference, due to the transmit antennas, and a high-level of other-cell interference. Although most  researchers have neglected this problem, owing to its lack of tractability, it has been shown, using both information and communication theory, that the capacity of a MIMO cellular system can decrease as the number of transmit antennas increases if the spatial interference is not suitably addressed.

In summary, most theoretical MIMO results are for high-SNR environments with idealized (ML) decoding; in practice, MIMO must function in low-SINR environments with low-complexity receivers. The other-cell interference problem is perhaps the most pressing problem confronting the use of spatial multiplexing in WiMAX systems.

Various solutions for dealing with the other-cell interference have been suggested, including interference-aware receivers, multicell power control, distributed antennas, and multicell coordination. None of these techniques are explicitly supported by the WiMAX standard as of press time of this book, although the deployment of interference-aware receivers is certainly not precluded by the standard.

We predict that creative approaches to the other-cell interference problem will be needed in order to make spatial multiplexing viable for users other than those very near the base station and hence experiencing a very low level of interference. Therefore, the requirement for rich scattering in MIMO systems may compete with the use of directional / sectorized antennas to reduce other-cell interference.