The adoption of a reconfigurable intelligent surface (RIS) for downlink multi-user communication from a multi-antenna base station is investigated in this paper. We develop energy-efficient designs for both the transmit power allocation and the phase shifts of the surface reflecting elements subject to individual link budget guarantees for the mobile users. This leads to non-convex design optimization problems for which to tackle we propose two computationally affordable approaches, capitalizing on alternating maximization, gradient descent search, and sequential fractional programming. Specifically, one algorithm employs gradient descent for obtaining the RIS phase coefficients, and fractional programming for optimal transmit power allocation. Instead, the second algorithm employs sequential fractional programming for the optimization of the RIS phase shifts. In addition, a realistic power consumption model for RIS-based systems is presented, and the performance of the proposed methods is analyzed in a realistic outdoor environment. In particular, our results show that the proposed RIS-based resource allocation methods are able to provide up to 300% higher energy efficiency in comparison with the use of regular multi-antenna amplify-and-forward relaying.

Reconfigurable Intelligent Surfaces for Energy Efficiency in Wireless Communication

The adoption of a Reconfigurable Intelligent Surface (RIS) for downlink multi-user communication from a multi-antenna base station is investigated in this paper. We develop energy-efficient designs for both the transmit power allocation and the phase shifts of the surface reflecting elements, subject to individual link budget guarantees for the mobile users. This leads to non-convex design optimization problems for which to tackle we propose two computationally affordable approaches, capitalizing on alternating maximization, gradient descent search, and sequential fractional programming. Specifically, one algorithm employs gradient descent for obtaining the RIS phase coefficients, and fractional programming for optimal transmit power allocation. Instead, the second algorithm employs sequential fractional programming for the optimization of the RIS phase shifts. In addition, a realistic power consumption model for RIS-based systems is presented, and the performance of the proposed methods is analyzed in a realistic outdoor environment. In particular, our results show that the proposed RIS-based resource allocation methods are able to provide up to $300\%$ higher energy efficiency, in comparison with the use of regular multi-antenna amplify-and-forward relaying.

We consider a multi-user Multiple-Input Single-Output (MISO) communication system comprising of a multiantenna base station communicating in the downlink simultaneously with multiple single-antenna mobile users. This communication is assumed to be assisted by a Large Intelligent Surface (LIS) that consists of many nearly passive antenna elements, whose parameters can be tuned according to desired objectives. The latest design advances on these surfaces suggest cheap elements effectively acting as low resolution (even 1-bit resolution) phase shifters, whose joint configuration affects the electromagnetic behavior of the wireless propagation channel. In this paper, we investigate the suitability of LIS for green communications in terms of Energy Efficiency (EE), which is expressed as the number of bits per Joule. In particular, for the considered multi-user MISO system, we design the transmit powers per user and the values for the surface elements that jointly maximize the system's EE performance. Our representative simulation results show that LIS-assisted communication, even with nearly passive 1-bit resolution antenna elements, provides significant EE gains compared to conventional relay-assisted communication.

Energy Efficient Multi-User MISO Communication Using Low Resolution Large Intelligent Surfaces

The main limitations of employing large-scale antenna (LSA) architectures for broadband frequency-selective channels include, but are not limited to their complexity, power consumption, and the high cost of multiple radio frequency (RF) chains. Promising solutions can be found in the recently proposed family of single-carrier (SC) spatial modulation (SM) transmission techniques. Since the SM scheme’s transmit antenna (TA) activation process is carried out in the context of a SC-SM architecture, the benefits of a low-complexity and low-cost single-RF transmitter are maintained, while a high MIMO multiplexing gain can be attained. Moreover, owing to its inherent SC structure, the transmit signals of SC-SM have attractive peak power characteristics and a high robustness to RF hardware impairments, such as the RF carrier frequency offset (CFO) and phase noise. In this paper, we present a comprehensive overview of the latest research achievements of SC-SM, which has recently attracted considerable attention. We outline the associated transceiver design, the benefits and potential tradeoffs, the LSA aided multiuser (MU) transmission developments, the relevant open research issues as well as the potential solutions of this appealing transmission technique.

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Single-Carrier SM-MIMO: A Promising Design for Broadband Large-Scale Antenna Systems

We consider a multi-user Multiple-Input Single-Output (MISO) communication system comprising of a multi-antenna base station communicating in the downlink simultaneously with multiple single-antenna mobile users. This communication is assumed to be assisted by a Large Intelligent Surface (LIS) that consists of many nearly passive antenna elements, whose parameters can be tuned according to desired objectives. The latest design advances on these surfaces suggest cheap elements effectively acting as low resolution (even $1$-bit resolution) phase shifters, whose joint configuration affects the electromagnetic behavior of the wireless propagation channel. In this paper, we investigate the suitability of LIS for green communications in terms of Energy Efficiency (EE), which is expressed as the number of bits per Joule. In particular, for the considered multi-user MISO system, we design the transmit powers per user and the values for the surface elements that jointly maximize the system's EE performance. Our representative simulation results show that LIS-assisted communication, even with nearly passive $1$-bit resolution antenna elements, provides significant EE gains compared to conventional relay-assisted communication.

Energy Efficient Multi-User MISO Communication using Low Resolution Large Intelligent Surfaces

A novel single-RF MIMO transmitter is presented. It comprises only a single power amplifier and does not require any mixer. The voltage source is a fixed sinusoidal oscillator. For each antenna element, data modulation is performed using adjustable two-port networks that control the currents on the individual elements. It is shown that the proposed transmitter can support arbitrary types of modulation at low signal distortion and that the source can be matched perfectly for a large number of antennas. The power amplifier generates a fixed amount of power. A massive MIMO system with OFDM-modulation can be operated at very high amplifier efficiency. To limit the maximum radiated power, two clipping methods are suggested. It is shown that the total power efficiency of the proposed transmitter in the massive MIMO regime using the proposed clipping techniques is significantly higher than the total power efficiency in classical design of MIMO transmitters. Utilizing the aforementioned advantages of the proposed MIMO transmitter makes massive MIMO systems more realistic in terms of cost and size for future generations of wireless networks.

A Novel Single-RF Transmitter for Massive MIMO

Some recent progresses in Load-Modulated Single- RF (LMSRF) multiple-antenna transmitters are presented including the circuit analysis in the case of closely spaced antennas. Mutual coupling effect is considered and it is shown that there is no need for any decoupling network since the mutual coupling effect can be considered in digital baseband domain. Furthermore, discrete LMSRF is introduced and an implementation method for it using PIN diodes and microstrip lines is presented. Some simulation results for Orthogonal Frequency Division Multiplexing (OFDM) signals with Quadrature Amplitude Modulation (QAM) and Quadrature Phase Shift Keying (QPSK) inputs are presented, e.g., it is shown that using 8 switches in every load modulator results in a signal to distortion ratio of 17.8dB at the transmitter.

Discrete Load-Modulated Single-RF MIMO Transmitters

A new modulation technique called Multi-Dimensional Continuous Phase Modulation (MCPM) is proposed for user devices with a single-RF MIMO transmitter in cellular communications. MCPM has a Peak to Average Power Ratio (PAPR) equal to 0dB, thus the power amplifier in user terminal works with the highest power efficiency. In fact, MCPM is a generalization of classical CPM for multiple antennas systems. It is shown that using a few number of antennas, MCPM reaches to the capacity of Gaussian input approximately. Utilizing MCPM modulation in user device enables us to implement very cheap power amplifier while there is negligible loss in information capacity. Discrete constellation MCPM is also proposed and its mutual information is presented using some numerical techniques.

A novel modulation scheme for user devices equipped with a single-RF MIMO transmitter

A novel single-RF multiple-input multiple-output transmitter architecture based on the outphasing principle is presented. An efficient switching amplifier feeds a tunable analogue network with the unmodulated carrier. The carrier is then modulated utilising the outphasing principle and distributed to the antennas. Featuring only a small set of standard RF components - namely power splitters, phase shifters, and hybrid couplers - the structure can be readily implemented in a costefficient, passive integration manner. Various modulation schemes for the architecture are investigated including the particular power efficient phase shift keying on the hypersphere. Our numerical results show that the proposed architecture is a power- and cost-efficient solution for multi-antenna transmitters on energy-limited user devices.

A Novel Single-RF Outphasing MIMO Architecture

A partially connected hybrid analogue-digital beamformer architecture with adjustable amplitude and phase weights in both the digital and the analogue part is presented. Amplitude control in the analogue beamformer is enabled by a lossless passive outphasing network. The architecture is designed to be implementation-friendly as the only required RF components are low-resolution phase shifters, power dividers, and hybrid couplers. If phase shifters of arbitrary resolution are assumed, any beamforming weights can be generated, whereas finite resolution phase shifters naturally lead to quantisation in amplitude and phase. In that case, in order to find the optimum phase shifter states for approximating a given beamforming vector, a full search over all possible states is necessary. To avoid the associated computational costs, a suboptimal but faster and less memory consuming algorithm is presented. Furthermore, an antenna arrangement for sub-connected hybrid beamforming utilising low-redundancy sub-arrays is proposed. For sub-array based processing, the performance in the presence of a sparse channel can be improved compared against conventional antenna arrangements.

Ralf R. Mueller

Papers

A Novel Single-RF Transmitter for Massive MIMO

Discrete Load-Modulated Single-RF MIMO Transmitters

A novel modulation scheme for user devices equipped with a single-RF MIMO transmitter

A Novel Single-RF Outphasing MIMO Architecture

Hybrid Analogue Digital Outphasing Beamforming Architecture with Amplitude and Phase Control.