Distributed antenna systems and linear relaying for gigabit MIMO wireless

Spatial multiplexing is mandatory to achieve the extreme bandwidth efficiency of future gigabit/sec WLAN. Both distributed antenna systems (DAS) at the access point and cooperative relaying (the infrastructureless counterpart) have been recognized as means to meet coverage/range requirements and to enable spatial multiplexing in a low scattering environment. In this paper we evaluate three candidate schemes under a two-hop (relay) traffic pattern: (i) DAS with decode and forward in the access point (DDAS), (ii) DAS with linear processing in the access point (LDAS) and (iii) linear relaying without any information exchange between the relay nodes. We give lower bounds on the capacity of LDAS and DDAS. A main contribution of this paper is a systematic derivation of local gain allocation strategies for linear relaying with multi-antenna source and destination nodes, which are based on large system analysis and do not require global channel knowledge at the relays. We derive approximate expression for the ergodic capacity. We show for a source and destination with M antennas, that asymptotically (large number of relays) linear relaying with MN support nodes performs similar to LDAS with M distributed antenna elements. Finally we propose a zero forcing gain allocation, which enables spatial multiplexing of multiple single antenna source/destination pairs based on a small number of autonomous relays. The theory is supported by comprehensive performance results.