Performance Analysis of MIMO-HARQ Assisted V2V Communications With Keyhole Effect

TL;DR

This paper analyzes the impact of keyhole effects on MIMO-HARQ V2V communications in urban environments, deriving outage probabilities and revealing diversity limitations and rate behaviors.

Contribution

It provides closed-form outage probability expressions for various HARQ schemes under keyhole fading and offers insights into diversity and rate performance limitations.

Findings

Full time diversity can be achieved, but full spatial diversity is limited.

Asymptotic outage probability increases monotonically and convexly with rate.

HARQ-IR's advantage over HARQ-CC diminishes in large-scale array regimes.

Abstract

Vehicle-to-vehicle (V2V) communications under dense urban environments usually experience severe keyhole fading effect especially for multi-input multi-output (MIMO) channels, which degrades the capacity and outage performance due to the rank deficiency. To avoid these, the integration of MIMO and hybrid automatic repeat request (HARQ) is proposed to assist V2V communications in this paper. By using the methods of integral transforms, the outage probabilities are derived in closed-form for different HARQ-assisted schemes, including Type I-HARQ, HARQ with chase combining (HARQ-CC), and HARQ with incremental redundancy (HARQ-IR). With the results, meaningful insights are gained by conducting the asymptotic outage analysis. Specifically, it is revealed that full time diversity order can be achieved, while full spatial diversity order is unreachable as compared to MIMO-HARQ systems without keyhole effect. Moreover, we prove that the asymptotic outage probability is a monotonically increasing and convex function of the transmission rate. More importantly, although HARQ-IR performs better than HARQ-CC owing to its higher coding complexity, this advantage becomes negligible in the large-scale array regime. Finally, the numerical results are verified by Monte-Carlo simulations along with some in-depth discussions.