  In this paper, the outage performance of relay-assisted millimeter wave
(mmWave) cellular systems employing joint spatial division and multiplexing
(JSDM) is investigated. It is assumed that the macro base station (BS) equipped
with a large number of antennas serves the single antenna pico BS (as a relay)
and users simultaneously, and that the pico BS is located at the edge of the
macro cell. Theoretical analysis of the signal-to-interference-plus-noise ratio
(SINR) outage probability of each user is first obtained. The cell SINR outage
probability is then derived. Under the noise-limited assumption, simplified
closed-form expressions of the outage probability are given as well. Simulation
results demonstrating the performance improvement due to the relay introduced
by the pico BS are provided. Overall, the impact of deploying pico BS as a
relay in the mmWave cellular systems is characterized.
