Orthogonal Random Beamforming with Beam Selection for MISO Wiretap Broadcast Channels

This paper studies the downlink of a wireless system with an M-antenna base station, K single-antenna legitimate users and a single-antenna eavesdropper. A limited feedback-based scheduling and precoding scenario is considered that employs the orthogonal random beamforming (ORBF). The sum-rate of ORBF exhibits an identical growth rate as that of dirty paper coding for a large number of users. Unfortunately, for more realistic (low to moderate) K values, ORBF yields degraded performance. It is shown that having as many active beams as the number of transmit antennas is not always optimal for sparse network, thus scheduling strategies for ORBF with beam selection is investigated. Several user-beam selection algorithms featuring different complexity levels are proposed and their performance in terms of secrecy sum-rate is assessed. Numerical results are provided to illustrate the impact of the number of active beams on the secrecy performance.