Closed-Form Blockage Probability for Relays-Assisted mmWave D2D Communications: Benefits of Using Relays

One of the important challenges of millimeter wave (mmWave) device-to-device (D2D) communications is its vulnerability to obstacles. While relaying is a possible solution when the direct mmWave D2D link is blocked by obstacles, the selected relay link may also be blocked by dynamic blockers that are difficult to know in advance. This paper focuses on investigating the dynamic blockage effects in relays-assisted mmWave D2D communication systems, where we assume that D2D users can switch to other available relays when the current link is blocked. A closed-form expression for the probability of all available relays being simultaneously blocked for a pair of D2D users is derived, which reveals the effects of the density and position of relays on the blockage characteristics and system reliability. Additionally, a closed-form expression for the minimum relay density required to satisfy certain quality of service requirements is obtained accordingly, and the feasible positions for a relay to effectively reduce the blockage probability is found based on particle swarm optimization. To the best of our knowledge, this is the first work to theoretically analyze the dynamic blockage in relays-assisted mmWave D2D communications, which provides valuable insights for designing network parameters.