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We consider the inter-antenna separation in a distributed antenna array. We introduce a new separation constraint with respect to the signalling wavelength, which equals the chip wavelength in DS-CDMA systems and the symbol wavelength in non-spread systems. This constraint ensures the phases of the frequency responses are exercised over 2π in the baseband channel matrix. The diverse phases provide a better channel matrix for signal detection. The multiuser performance improvement due to this constraint has been named as the signalling wavelength antenna placement (SWAP) gain.