Multiple-input multiple-output (MIMO) technology is a key candidate for enhancing the capabilities of modem wireless communication systems. We consider the MIMO channel of fixed length arrays, accounting for both spatial correlation and mutual coupling at the transmitter and receiver....[ Read more ]

Multiple-input multiple-output (MIMO) technology is a key candidate for enhancing the capabilities of modem wireless communication systems. We consider the MIMO channel of fixed length arrays, accounting for both spatial correlation and mutual coupling at the transmitter and receiver.

We investigate the important question of how many antennas to place in the arrays to maximize the capacity. It is shown that there is an optimal antenna configuration yielding the highest capacity which depends strongly on the array length and the transmission wavelength, but does not depend strongly on the signal to noise ratio (SNR). Moreover, we show that ignoring the effect of mutual coupling gives misleading results, yielding unbounded capacity growth. As another key finding, we demonstrate the surprising result that if the optimal antenna configuration is employed, then further optimizing the transmission based on the channel statistics gives very little benefit over simple equal-power spatial multiplexing. On the other hand, with perfect channel knowledge optimizing the transmission (waterfilling) gives large benefit over other transmission schemes.

We derive a linear approximation for the capacity at low SNR. The performance of different transmit and receive methods are investigated at low SNR regime. We also provide a straightforward method for estimating the optimal number of antennas.