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Link Asymmetry in Virtual MISO-based Networks

Cooperative transmission (CT), in which spatially separated wireless nodes collaborate to form a virtual antenna array or virtual multiple-input-single-output (VMISO) link, is an effective technique to enhance the system performance by spatial diversity. In this paper, we study the link asymmetry issue of the VMISO links between two neighboring CT clusters, which can be used to design higher-layer CT algorithms such as routing and medium access control (MAC). While many authors simplify analysis on CT-based networks with co-located approximation that ignores disparate path losses in VMISO links by assuming co-located cooperators, this paper studies the impact of disparate path losses on link asymmetry by allowing the relay locations to be random. We identify two main factors of the VMISO link asymmetry, which do not appear in the conventional SISO-based networks: exclusive signal detection only at a single node and path-loss disparity. The link asymmetry level is evaluated by using the correlation and the ratio of two signal powers in the forward and reverse directions, which presents the distinct characteristics of the VMISO links compared to the conventional SISO links. Moreover, the simulation results show how the VMISO link asymmetry changes depending on various system parameters.