  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.
