K Giridhar

Multiple receive antennas are known to provide receiver diversity but typically require considerable separation between them. We introduce the concept of space sampling at the receiver where antennas are placed relatively close to each other. Since the antennas are close, the samples are highly spatially correlated and does not help in performance improvement in a typical wireless system with flat fading. However, this concept is very useful particularly in an OFDM system with frequency selective fading which has an inherent multipath diversity. Under these circumstances, the required space diversity for performance improvement is obtained by the transformation of multipath diversity to useful space diversity in frequency domain inherently by an OFDM system (G. V. Rangaraj et al., June 2003). The minimum separation required between the antennas under such circumstances is derived analytically and we have show that even with a separation of only 0.44λ, the required spatial correlation in the frequency domain becomes sufficiently low.

Spatial correlation is introduced when antennas are not well separated, which typically leads to performance degradation in space diversity systems for flat fading wireless channels. However, in a frequency-selective environment with orthogonal frequency division multiplexing (OFDM), multipath diversity can help in overcoming this performance degradation. This is due to transformation of a highly spatially correlated channel impulse response to less spatially correlated channel frequency response inherently by ah OFDM system in the presence of multipath diversity [6]. In this letter, we numerically evaluate the minimum antenna separation required for a target spatial correlation in the frequency domain in situations with low multipath diversity. Performance results for two highly spatially correlated receive antennas with such spacing are provided for low multipath diversity channels and it is found to be within 1 dB (at BER = 10 -3) of spatially uncorrelated reception for QPSK modulation. © 2004 IEEE.

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 0.44λ, 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. © 2005 IEEE.