Impact of Pilot Allocation Strategies on Outage in Wireless Energy Transfer Using Massive Antenna Arrays

We investigate the viability of wireless energy transfer (WET) to multiple sensors using massive number of base station (BS) antennas based on estimates obtained through different uplink pilot signaling strategies, namely, orthogonal and shared. For the aforementioned strategies, we derive novel upper bounds on probability of outage in WET referred to as the probability that any sensor node fails to harvest a certain minimum amount of energy ${E_{u}}$ that it needs to send uplink pilots plus the energy $E_{p}$ that it needs to process its main tasks. We show how number of BS antennas scales with the number and position of sensor nodes, array transmit energy, channel estimation errors and nature of pilot signaling strategy employed. We prove when the sensor nodes are all equidistant relative to the BS, shared strategy gives identical outage probability as obtained through orthogonal strategy. However, when they are placed at different distances, shared strategy gives poorer outage performance than orthogonal strategy. To address this, we propose a simple location-dependent clustering and hybrid pilot assignment algorithm and also derive the corresponding probability of outage in WET. The proposed strategy elucidates an interesting trade-off between the outage performance that can be obtained and resources that must be spent on channel estimation.