The susceptibility of millimeter waveform propagation to blockages limits thecoverage of millimeter-wave (mmWave) signals. To overcome blockages, we proposeto leverage two-hop device-to-device (D2D) relaying. Using stochastic geometry,we derive expressions for the downlink coverage probability of relay-assistedmmWave cellular networks when the D2D links are implemented in either uplinkmmWave or uplink microwave bands. We further investigate the spectralefficiency (SE) improvement in the cellular downlink, and the effect of D2Dtransmissions on the cellular uplink. For mmWave links, we derive the coverageprobability using dominant interferer analysis while accounting for bothblockages and beamforming gains. For microwave D2D links, we derive thecoverage probability considering both line-of-sight (LOS) and non-line-of-sight(NLOS) propagation. Numerical results show that downlink coverage and SE can beimproved using two-hop D2D relaying. Specifically, microwave D2D relays achievebetter coverage because D2D connections can be established under NLOSconditions. However, mmWave D2D relays achieve better coverage when the densityof interferers is large because blockages eliminate interference from NLOSinterferers. The SE on the downlink depends on the relay mode selectionstrategy, and mmWave D2D relays use a significantly smaller fraction of uplinkresources than microwave D2D relays.
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