This work examines the user-pair selection problem for distributed-input distributed-output (DIDO) wireless systems. A DIDO system refers to a network of densely deployed transmitter and receiver pairs, where the transmitters are connected to and coordinated by a DIDO server. The system sum rate is known to grow without bound as the number of transmitter-receiver pairs increases. However, when zero-forcing (ZF) beamforming is adopted across the transmitters (as assumed in most existing works on DIDO), the effect of power amplification due to ill-conditioned channel matrices may significantly reduce the system sum rate. In this work, a decremental user-pair selection (DUPS) algorithm is proposed to determine the set of transmitter-receiver pairs that should be simultaneously active in order to reduce the impact of power amplification and increase the system sum rate. A low-complexity variant of DUPS is also proposed and an asymptotic lower bound of its sum rate is derived using extreme value theory. Moreover, inspired by the semi-orthogonal user selection (SUS) algorithm, often adopted in the conventional multiple-input multiple-output (MIMO) literature, a semi-orthogonal DUPS algorithm is also proposed by taking into consideration the orthogonality of the users' channel vectors in the selection process. Simulations are provided to demonstrate the effectiveness of the proposed schemes.
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