Ozlem Tugfe Demir

Optimum discrete single group multicast beamforming

In this paper, transmit beamformer design for single group multicast scenario is considered. The problem is solved in discrete form where the beamformer phase and amplitude values are selected from finite discrete sets. Original optimization problem is converted to a linear form by introducing new variables. The solution of the equivalent optimization problem is always feasible as long as the total power is above a certain value. The problem in its linear form is guaranteed to return optimum solution. Proposed approach is very effective and the number of bits can be increased to obtain close to optimum continuous phase and amplitude beamformers.

Antenna Selection and Hybrid Beamforming for Simultaneous Wireless Information and Power Transfer in Multi-Group Multicasting Systems

In this paper, low-cost alternatives to full digital beamforming, namely, antenna selection and hybrid beamforming, are proposed for simultaneous wireless information and power transfer in a multi-group multicasting scenario. The joint problem can be outlined as the design of beamformer weight vectors and power splitting ratios in order to satisfy both signal-to-interference-plus-noise-ratio and harvested power constraints at each user with minimum transmission power. We first develop an efficient algorithm for antenna selection by converting the original mixed integer programming problem into a continuous one and adapting feasible point pursuit-successive convex approximation. Second, a new hybrid beamforming structure is presented for multi-group multicasting. Both continuous and discrete-phase hybrid beamformers are considered in this content. Two algorithms for continuous-phase case are developed by employing two competing techniques. In addition, a special two-bit discrete-phase hybrid beamformer design is considered for practical systems. The integer constraints are converted into linear equality and inequalities for this specific structure and an efficient algorithm is designed. The proposed algorithms are compared for different scenarios revealing some interesting characteristics of each technique.

Max–Min Fair Resource Allocation for SWIPT in Multi-Group Multicast OFDM Systems

Simultaneous wireless information and power transfer is considered for multi-group multicasting OFDM systems. Each user has the energy harvesting capability through a power splitter (PS). The power and subcarrier allocation at the base station is done such that the minimum signal-to-noise ratio among the users for each subcarrier is maximized while user needs for harvested power are satisfied. The optimization of PS ratios in addition to resource allocation is realized in a joint manner. It is shown that the problem can be cast in a convex optimization form for the given subcarrier sets. In order to determine the subcarrier sets, an efficient subcarrier allocation algorithm is proposed. It is shown that the proposed method performs very close to the exhaustive search, which gives the optimum solution.

Multi-group multicast beamforming for simultaneous wireless information and power transfer

In this paper, simultaneous wireless information and power transfer (SWIPT) concept is introduced for multi group multicast beamforming. Each user has a single antenna and a power splitter which divides the radio frequency (RF) signal into two for both information decoding and energy harvesting. The aim is to minimize the total transmission power at the base station while satisfying both signal-to-interference-plus-noise-ratio (SINR) and harvested power constraints at each user. Unlike unicast and certain broadcast scenarios, semidefinite relaxation (SDR) is not tight and global optimum solution cannot be found for this problem. We propose an iterative algorithm where a convex optimization problem is solved at each iteration. Both perfect and imperfect channel state information (CSI) at the base station are considered. Simulation results show that the proposed solution is very close to the SDR lower bound and a few number of iterations are enough for the algorithm convergence.

Hybrid beamforming with two bit RF phase shifters in single group multicasting

In this paper, an efficient hybrid beamforming architecture combining analog and digital beamforming is proposed to reduce the number of radio frequency (RF) chains. It provides a good compromise between the higher degree of freedom of digital beamforming and hardware cost and complexity. In this hybrid system, two bit RF phase shifters are used as analog beamformers due to the fact that this enables the conversion of the combinatorial optimization problem to a continuous programming formulation. The overall optimization problem for the joint design of digital and analog beam-forming weights is formulated as a quadratic-cost problem which can be solved iteratively by exact penalty and semidefinite programming. Simulation results show that the proposed method designs hybrid beamformer effectively and it performs better than antenna selection for the given multicasting problem.

Optimum phase-only discrete broadcast beamforming with antenna and user selection in interference limited cognitive radio networks

Phase-only beamforming presents certain advantages in radar and communication systems. In cognitive radio, antenna and user selection are the two tools for increasing the quality of service (QoS) for the users. In this paper, discrete single group multicast transmit phase-only beamformer design is presented with antenna subset and user selection. The problem is converted into linear form and solved efficiently by using mixed integer linear programming to find the optimum subset of antennas and secondary users together with optimum beamformer phase coefficients. It is shown that significant power saving is possible compared to fixed antenna systems.

Transmit beamformer design with nonlinear optimization

Two methods for maxmin transmit beamformer design with multiple simultaneous beams are compared. Both methods involve convex optimization with semidefinite relaxation. In the proposed method, resulting beamformer is taken as an initial estimate for another optimization problem which is nonconvex and nonlinear. As a second method, a well-known procedure, randomization is used. Comparisons are made using total power and phase-only constrained beamformers. It is shown that the proposed approach performs better than randomization.

SWIPT for Max-Min Fair Multi-Group Multicast Beamforming through Power Splitting

In this paper, simultaneous wireless information and power transfer (SWIPT) is considered in the context of multi-group multicast beamforming. A base station transmits more than one multicast streams to the groups of users where each group targets its own information signal. Users have single antenna and the received radio frequency (RF) signals are split into two for both information decoding and energy harvesting through a power splitter device. The aim is to maximize the minimum signal-to-interference-plus-noise ratio (SINR) among all the users while satisfying the harvesting power demands and the maximum transmission power constraint at the base station. The max-min fair beamforming problem is not convex. Hence the problem is first reformulated in terms of new variables in an exact manner and successive convex approximation (SCA) is applied after some transformations. Two iterative algorithms are proposed the first of which relies on a previous convex approximation technique applied on the reformulated problem. The second algorithm introduces a new reformulation where less number of convex approximations are required. Simulation results show that this new approach is very effective and results better performance in comparison.

Distributed beamforming in relay networks for energy harvesting multi-group multicast systems

In this paper, simultaneous wireless information and power transfer (SWIPT) is considered for multi-group multicasting relay networks where there is no direct link between the source and destination nodes. Each source transmits its own multicast stream to a group of users with the help of single antenna relays which use amplify-and-forward relay protocol. Each user has energy harvesting capability. A part of the received signal is used for information decoding while the rest is used for energy harvesting. The design problem is to determine the complex relay weights and the power splitting ratios for the users. The resulting optimization problem is nonconvex and converted into a form suitable for quadratically constrained quadratic programming. In addition to the conventional relay beamforming, distributed phase-only beamformer design is also considered and both beamformer design problems are solved iteratively using feasible point pursuit-successive convex approximation algorithm. Several simulations are performed and the advantages of both approaches are outlined.

Optimum discrete phase-only multicast beamforming with joint antenna and user selection in cognitive radio networks

Spectrum underlay technique for secondary multicast network is an effective approach in cognitive radio. In such communication schemes, quality of service constraints are satisfied for the secondary users while interference to the primary users is kept under a threshold. Previous literature consists of suboptimum methods for the solution of the individual problems such as antenna and user selection. In this paper, the joint antenna and user selection problem is considered and solved in an optimum manner. Complete problem is formulated for a discrete beamformer which permits linear constraints and cost function resulting a form suitable for mixed integer 0-1 linear programming. It is shown that antenna selection is very effective to improve the user signal-to-noise ratio while user selection can be employed to increase feasibility. The proposed method performs significantly better compared to suboptimum only-antenna and only-user selection methods in the literature. The problem is further modified to design robust beamformers for any l p ball uncertainty region by using a linear approximation. Several simulations are done and it is shown that the proposed approach generates significant improvements in the transmit power and the number of serviced secondary users in a quality of service aware scenario.