Reconfigurable Intelligent Surface (RIS) consists of mostly-passive elements capable of electronically steering the impinging signal with configured phase shifts. However, achieving infinite phase resolution is infeasible and the phases needs to be quantized for practical implementation. In this paper, we propose an unsupervised learning-based method to estimate the optimal discrete RIS phase configuration by utilizing key eigen vectors of the available channel state information (CSI) as the input. The direct link between a base station (BS) and user equipment (UE) is not considered in the proposed method and only the CSI of the RIS-aided link is utilized. Further, the proposed network design is independent of the number of transmit and receive antennas at the BS and UE respectively, making the solution generalized and adaptable towards different BSs and UEs in the communication. Due to the unsupervised nature of the proposed method, the solution does not rely on any labeled data, obtaining which is otherwise an exhaustive process. The performance of the proposed solution is analyzed through rigorous simulations and shows the effectiveness of directly estimating discrete RIS configurations as compared to obtaining infinite resolution phase-shifts and then quantizing them. The performance analysis further demonstrates the efficacy of the proposed system with reduced computational complexity and generalized solution with respect to number of BS and UE antennas.

In this paper, a novel distributed precoding (DiP) algorithm for ultra-dense small cell (SC) networks is developed, where the SCs cooperate to perform a joint transmission to users (UEs) with limited and individual transmit powers. Different to most state of the art (SotA) DiP algorithms, the proposed precoder design is based on the assumption that each SC has only local channel state information (CSI) available. Additionally, there are no constraints on the number of antennas for each SC, but only one constraint on the sum of all transmit antennas. A solution for the considered problem based on the Lagrangian method of multipliers (MoM) is presented, by formulating the precoder design as a constrained convex optimization problem. The obtained solution can be implemented in a fully distributed way among the SCs by using the preconditioned Richardson (PR) iteration. In numerical simulations, the convergence of the proposed DiP algorithm is verified and it is shown that the sum rate significantly increases, if the SCs cooperate with each other.

Herein, we treat Space Division Multiple Access (SDMA) based on partial channel state information and limited feedback. We propose a novel framework utilizing subspace packings, where beamforming, feedback, and scheduling are integrated. Advantages of the proposed framework are that the fed back supportable rates are based on the post-scheduling SINR and that the feedback implicitly contains information about the spatial compatibility of the users. The feasibility region of packings of different dimensions is indicated by the allocation outage probability which is derived. Grassmannian subspace packings, DFT-based packings, and non-orthogonal Grassmannian packings are formulated and studied as candidates. Numerical simulations show better performance for the proposed scheme compared to conventional channel quantization at the receiver with zero-forcing transmission in i.i.d. Rayleigh fading. Finally, we propose and evaluate a beam-graph method to further reduce the feedback load, that can be used in the context of tracking quantized beamformers.

In this work, we analyze opportunistic beamforming with finite number of single-antenna users under the constraint that the feedback overhead from the mobiles to the base is constant. First, we characterize the impact of the fading variances of the users and the spatial correlation on the sum rate of TDMA based opportunistic beamforming using majorization theory. Further, we describe quantitatively the high-SNR behavior in terms of throughput slope and power offset. Next, the impact of the fading variances of the users on an upper bound of the sum rate for space division multiple access (SDMA) based opportunistic beamforming is derived which is tight for high SNR. We propose to adapt the number of active beams to the SNR and the number of active users in a cell and illustrate the corresponding optimization problem by simulations.

Orthogonal frequency-division multiple access (OFDMA) is an attractive technique for exploiting multiuser diversity in the downlink of a cellular system. This paper addresses three problems in multiuser diversity for OFDMA systems. First, we propose a way to significantly reduce the amount of channel state information (CSI) feedback without sacrificing performance too much, by selective and adaptive feedback. Second, we propose a way to increase the cell throughput and fairness by applying an opportunistic beamforming scheme to orthogonal frequency-division multiplexing. This beamforming scheme increases the frequency fading rate, which increases the multiuser diversity effect. Thirdly, we deal with the issue of fairness and quality-of-service (QoS) in opportunistic systems by proposing a modified proportional fair (PF) scheduler for OFDMA. Key features in the scheduler are that it incorporates QoS classes into the PF scheduler and that it has a tunable fairness level. Extensive simulation results are presented to evaluate the performance of the proposed schemes. The opportunistic beamforming scheme performed well in comparison with several other schemes. The modified PF scheduler was able to give users different QoS, based on their requirements, while still exploiting multiuser diversity

In the above titled paper (ibid., vol. 55, no. 5, pp. 941-952, May 07), a change to equation (7), that was requested by the author, was omitted in the final version. The paragraph leading up to, and including the equation, is repeated here.

In opportunistic SDMA, the base station forms several random beams simultaneously and relies on fast but limited feedback for scheduling users on the beams. In this paper, opportunistic SDMA is applied to an OFDMA downlink by having independent beamformers run in parallel on different frequency bands (SD-OFDMA). A method for improving the performance of completely random beamforming is proposed and evaluated. It exploits the temporal channel correlation and the partial channel state information available at the transmitter to keep "successful" beams over longer periods of time than the default block duration. Extensive simulation results show a significant improvement in cell throughput compared to a completely random scheme. The results also show that in order to gain more from the multiple simultaneous beams than what is lost in additional interference, the system load must be high.

In this paper, we consider the unclaimed sub-carriers that no user feeds back in an opportunistic OFDMA system with a per sub-carrier power constraint. Unclaimed sub- carriers appear in OFDMA systems with reduced feedback, where the users concentrate the feedback on the sub-carriers with good channel quality. Here we propose and evaluate two ways to use these sub-carriers to improve the system performance. One approach is to transmit pilots symbols on the unclaimed sub- carriers. A second approach is to schedule users based on their feedback about adjacent sub-carriers. Simulation results show that for low to moderate channel RMS delay spread, it is more advantageous to transmit data on unclaimed sub-carriers, even at a low rate, than to use them for additional pilot symbols. For high delay spreads however, transmitting pilots on the unclaimed sub-carriers gives higher system throughput.

This paper describes the implementation and performance of three communication schemes on a DSP-based MIMO test-bed. A multicell scenario with two basestations and two users in the same room is evaluated. Eigenbeamforming with and without interference pre-whitening at both transmitter and receiver is compared with SISO multicell communication. Experimental results show that the gain from using interference pre-whitening is significant. The BER performance of beamforming without intercell interference suppression is only marginally better than single antenna communication. The paper also presents the implementation of synchronization, equalization, frequency offset estimation and decision directed beamforming mismatch compensation.