Relay selection for mmWave communications

Due to high propagation loss and directivity, mmWave links are very susceptible to obstacles blocking the direct line-of-sight path for communication. In this case, indirect communication via a relay may help to circumvent the blockage. In this paper, we propose a two-hop relay selection algorithm for mmWave communications. For the relay selection, we analyze the probability that an indirect path is available given that the direct path is blocked trough geometric analysis. We then choose the most promising node among neighbors as relay. The analysis shows that the probability of an indirect path is a function of the obstacle density as well as the location of relay nodes. When the density is low, the correlation between the direct path and an indirect path is dominant, i.e., the angle between the direct path and the path to relay should be large, whereas the blockage probability of an indirect path becomes more dominant as the density increases, i.e., relay links should not be too long. The probability analysis also allows to decide an initial antenna angle for beam-training in mobile mmWave environments. Through numerical studies, we verify our analytical results.