Analysis of link asymmetry in virtual multiple-input-single-output (VMISO) systems

Cooperative transmission (CT) is an effective technique to mitigate multi-path fading through spatial diversity. In CT, spatially separated wireless nodes collaborate to form a virtual multiple-input-single-output (VMISO) link, which provides signal-to-noise ratio advantage over the conventional single-input-single-output (SISO) communications. In this paper, we consider link asymmetry of the VMISO links between two neighboring CT clusters in wireless multi-hop networks. Link asymmetry is important because it impacts the performance of link and routing layer protocols. While most of the existing studies on CT simplify analysis on CT-based networks with co-located approximation that ignores disparate path losses in VMISO links by assuming co-located transmitting nodes, this paper presents the impact of disparate path losses on link asymmetry by allowing the relay locations to be random. Assuming exclusive signal detection by cluster heads, we identify two main causes of the VMISO link asymmetry, which do not appear in the conventional SISO-based networks: independent fading and path disparity. We evaluate the degree of link asymmetry in terms of three metrics: the power correlation, the power ratio, and the probability that the power gap is 3 dB or higher. Also, the simulation results reveal how system parameters and channel conditions impact the VMISO link asymmetry.