Interference Randomization in Reconfigurable Intelligent Surface aided Communications

Reconfigurable Intelligent surface (RIS) offering controllable reflections of any target incident signal, is a technology that enables removal of blind-spots in millimeter wave (mmWave) wireless networks. Reflections can be controlled in a practical manner by imposing different quantized phase profiles on the RIS array. An undesirable by-product of quantization, however, is presence of one or more prominent side-lobes in the reflect beam patterns. These side-lobes represent leakage of target incident signal energy to unintended reflect directions. Furthermore, an RIS configured to assist a particular link can also reflect non-target incident signals prominently along a few directions, thereby hampering interference randomization in the network. In this paper we propose a novel approach for designing quantized phase-profiles, which trades off incorporating dither (i.e., introducing asymmetry) and optimizing reflect beam control. We show that this framework provides a designer with a range of choices for balancing interference randomization and gain maximization. It also does not require tight network coordination needed for obtaining knowledge of all significant interfering incident signal directions and target reflect directions. For cases when latter information is available, we also provide a systematic dither design method that maintains desired reflect direction gains while significantly improving sidelobe suppression (over 5 dB compared to pseudo-random dither design in one example), and is especially beneficial for RIS capable of amplification.