Intelligent reflecting surface (IRS) has recently been emerging as an enabler for smart radio environment in which passive antenna arrays can be used to actively tailor/control the radio propagation (e.g., to support users under adverse channel conditions). With multiple IRSs being launched (e.g., coated on various buildings) to support various group of users, it is critical to jointly optimize the phase-shifts of all IRSs to mitigate the interference as well as to leverage the secondary reflections amongst IRSs. This work takes the first step by considering the uplink of multiple users that are grouped and supported by multiple IRSs to a multi-antenna base station. Each IRS with multiple controllable phase-shift elements is intended to serve a group of near-by users. We first formulate the minimum achievable rate (from all users) maximization problem by jointly optimizing phase-shifts of elements from all IRSs and the received beamformers at the MIMO base station. The problem turns out to be non-convex. We then derive its solution using the alternating optimization mechanism. Our simulations show that by properly managing interference and leveraging the secondary reflections amongst IRSs, there is a great benefit of deploying more IRSs to support different groups of users to achieve a higher rate per user. In contrast, without properly managing the secondary reflections, increasing the number of RSs can adversely impact the achievable rate per user, especially for higher transmit power.

We consider a wireless relay network with a single source, a single destination, and a multiple relay. The relays are half-duplex and use the decode-and-forward protocol. The transmit source is a layered video bitstream, which can be partitioned into two layers, a base layer (BL) and an enhancement layer (EL), where the BL is more important than the EL in terms of the source distortion. The source broadcasts both layers to the relays and the destination using hierarchical 16-QAM. Each relay detects and transmits successfully decoded layers to the destination using either hierarchical 16-QAM or QPSK. The destination can thus receive multiple signals, each of which can include either only the BL or both the BL and the EL. We derive the optimal linear combining method at the destination, where the uncoded bit error rate is minimized. We also present a suboptimal combining method with a closed-form solution, which performs very close to the optimal. We use the proposed double-layer transmission scheme with our combining methods for transmitting layered video bitstreams. Numerical results show that the double-layer scheme can gain 2-2.5 dB in channel signal-to-noise ratio or 5-7 dB in video peak signal-to-noise ratio, compared with the classical single-layer scheme using conventional modulation.

In this paper, a 60-GHz channel model that incorporates the presence of human activity, and which is an extension of the standard channel model proposed by the IEEE 802.15.3c group, is presented. In the presence of human activity, various measurement results show that the attenuation due to human blockage can be 20 dB or more, and thus is non-negligible. Various measurement results in the literature are used to determine the distribution of the shadowing attenuation caused by human blockage. A multiuser single-carrier system with a single receiver is considered, and system performance in the presence of blockage because of human activity is evaluated. Numerical results show that the 60-GHz system can incur a large degradation in bit error rate due to human blockage.