Exploiting multipath diversity in multiple antenna OFDM systems with spatially correlated channels

Multiple antennas are useful in orthogonal frequency division multiplexing (OFDM) systems for providing transmit and receive diversity to overcome fading. Typically, these designs require considerable separation between the antennas. Spatial correlation is introduced when antennas are not well separated, and it often leads to performance degradation in a flat fading environment. However, in frequency selective fading channels with rich multipath diversity, OFDM receivers can overcome this performance degradation due to antenna correlation. This is due to transformation of a highly spatially correlated channel impulse response to a less spatially correlated channel frequency response inherently by an OFDM system in the presence of rich multipath diversity. We illustrate this for a simple receive diversity OFDM system and hence introduce the concept of space sampling at the receiver where antennas are placed relatively close to each other. The minimum separation required between the antennas under such circumstances is derived analytically, and it is shown that even with a separation of only 0.44/spl lambda/, the required spatial correlation in the channel frequency response becomes sufficiently low. Simulated performance results with such spacing for various multiple antenna OFDM systems corroborate the analytical results.

I. Introduction

Multiple antennas with considerable separation are known to provide diversity in wireless systems to overcome fading. Such separations are practical only in the base stations and infeasible in the subscriber units due to size constraints. Hence, it is typical to use these base station antennas as multiple receive antennas to provide receiver diversity in the uplink, and as multiple transmit antennas to provide transmitter diversity in the downlink. However, if other forms of diversity like time, frequency, code, or Doppler are available, then it may be pragmatic to build a system with relatively lower spacing and still get comparable performance. In this paper, we deal with the use of multiple antennas which are closely spaced in the presence of rich multipath diversity. This opens up the possibility of using multiple antennas at the subscriber unit, and it enables one to take advantage of transmitter diversity in uplink and receive diversity in the downlink.