  The susceptibility of millimeter waveform propagation to blockages limits the
coverage of millimeter-wave (mmWave) signals. To overcome blockages, we propose
to leverage two-hop device-to-device (D2D) relaying. Using stochastic geometry,
we derive expressions for the downlink coverage probability of relay-assisted
mmWave cellular networks when the D2D links are implemented in either uplink
mmWave or uplink microwave bands. We further investigate the spectral
efficiency (SE) improvement in the cellular downlink, and the effect of D2D
transmissions on the cellular uplink. For mmWave links, we derive the coverage
probability using dominant interferer analysis while accounting for both
blockages and beamforming gains. For microwave D2D links, we derive the
coverage probability considering both line-of-sight (LOS) and non-line-of-sight
(NLOS) propagation. Numerical results show that downlink coverage and SE can be
improved using two-hop D2D relaying. Specifically, microwave D2D relays achieve
better coverage because D2D connections can be established under NLOS
conditions. However, mmWave D2D relays achieve better coverage when the density
of interferers is large because blockages eliminate interference from NLOS
interferers. The SE on the downlink depends on the relay mode selection
strategy, and mmWave D2D relays use a significantly smaller fraction of uplink
resources than microwave D2D relays.
