Zhikun Xu

Energy-efficient wireless communications: tutorial, survey, and open issues

With explosive growth of high-data-rate applications, more and more energy is consumed in wireless networks to guarantee quality of service. Therefore, energy-efficient communications have been paid increasing attention under the background of limited energy resource and environmental- friendly transmission behaviors. In this article, basic concepts of energy-efficient communications are first introduced and then existing fundamental works and advanced techniques for energy efficiency are summarized, including information-theoretic analysis, OFDMA networks, MIMO techniques, relay transmission, and resource allocation for signaling. Some valuable topics in energy-efficient design are also identified for future research.

Throughput and Optimal Threshold for FFR Schemes in OFDMA Cellular Networks

Fractional frequency reuse (FFR) is an efficient way to mitigate inter-cell interference (ICI) in multi-cell orthogonal frequency division multiple access (OFDMA) networks. In this paper, we investigate the throughput and the optimal threshold for the FFR scheme. The average cell throughputs are derived for both round robin (RR) and maximum SINR (MSINR) scheduling strategies when users are uniformly distributed in the cell region. It is shown from the analysis and simulation results that the throughput increases and the optimal distance threshold decreases with the number of users for both scheduling strategies. The optimal distance threshold approaches the minimum distance that users can be away from the base station when the number of users goes to infinity. The optimal distance threshold increases with the frequency reuse factor of the cell-edge region when the MSINR scheduling is used. The impact of the RR scheduling strategy on the optimal threshold of the FFR scheme is negligible. Simulation also demonstrates that the FFR scheme with the optimal threshold significantly outperforms that with the existing fixed threshold.

Energy-Efficient Configuration of Spatial and Frequency Resources in MIMO-OFDMA Systems

In this paper, we investigate adaptive configuration of spatial and frequency resources to maximize energy efficiency (EE) and reveal the relationship between the EE and the spectral efficiency (SE) in downlink multiple-input-multiple-output (MIMO) orthogonal frequency division multiple access (OFDMA) systems. We formulate the problem as minimizing the total power consumed at the base station under constraints on the ergodic capacities from multiple users, the total number of subcarriers, and the number of radio frequency (RF) chains. A three-step searching algorithm is developed to solve this problem. We then analyze the impact of spatial-frequency resources, overall SE requirement and user fairness on the SE-EE relationship. Analytical and simulation results show that increasing frequency resource is more efficient than increasing spatial resource to improve the SE-EE relationship as a whole. The EE increases with the SE when the frequency resource is not constrained to the maximum value, otherwise a tradeoff between the SE and the EE exists. Sacrificing the fairness among users in terms of ergodic capacities can enhance the SE-EE relationship. In general, the adaptive configuration of spatial and frequency resources outperforms the adaptive configuration of only spatial or frequency resource.

Energy-Efficient CoMP Precoding in Heterogeneous Networks

Heterogeneous networks (HetNets) consisting of macro- and pico-cells can well tackle the contradictory requirements on large coverage of the network and high data rate at the hot spots. In this paper, we study energy-efficient precoding for coordinated multi-point (CoMP) transmission for HetNets. We formulate a problem that maximizes the energy efficiency under the constraints of individual date rate requirement from each user, maximal transmit power of each base station (BS), and zero-forcing (ZF) for a HetNet where the macro-BS cooperates with multiple pico-BSs. We then simplify the problem with the help of the practical topology of HetNets where the macro-BS cooperates with each of the pico-BSs. By introducing the subspace decomposition method and exploiting the feature of HetNets, we obtain the structure of the optimal ZF CoMP precoder. To reduce the computational complexity and hence the circuit power consumed for signal processing, we propose a suboptimal ZF precoder with closed-form expression for the HetNets with isolated pico-cells. We demonstrate the performance of the proposed ZF precoders by comparing with several existing optimal linear precoders.

New Paradigm of 5G Wireless Internet

5G network is anticipated to meet the challenging requirements of mobile traffic in the 2020s, which are characterized by super high data rate, low latency, high mobility, high energy efficiency, and high traffic density. This paper provides an overview of China Mobiles 5G vision and potential solutions. Targeting a paradigm shift to user-centric network operation from the traditional cell-centric operation, 5G radio access network (RAN) design considerations are presented, including RAN restructure, Turbo charged edge, core network (CN) and RAN function repartition, and network slice as a service. Adaptive multiple connections in the user-centric operation is further investigated, where the decoupled downlink and uplink, decoupled control and data, and adaptive multiple connections provide sufficient means to achieve a 5G network with “no more cells.” Software-defined air interface (SDAI) is presented under a unified framework, in which the frame structure, waveform, multiple access, duplex mode, and antenna configuration can be adaptively configured. New paradigm of 5G network featuring user-centric network (UCN) and SDAI is needed to meet the diverse yet extremely stringent requirements across the broad scope of 5G scenarios.

Sum rate optimization for MIMO non-orthogonal multiple access systems

Non-orthogonal multiple access (NOMA) is expected to be a promising technique for future wireless networks due to its superior spectral efficiency. In this paper, the sum rate optimization problem for multiple-input multiple-output (MIMO) NOMA systems is studied with the total transmit power constraint and the minimum rate constraint of weak user. We first derive a channel state information (CSI) condition in which MIMO NOMA systems can achieve full rate transmission, i.e. the transmission rate of the weak user equals to the channel capacity of weak user. Based on the CSI condition, we propose an optimal power allocation scheme for MIMO NOMA systems, which can achieve the capacity region of MIMO Broadcast channel as dirty paper coding. A low complexity suboptimal scheme is proposed as well for all CSI channel conditions. Numerical results show that the proposed NOMA schemes significantly outperform the traditional time division based single user MIMO scheme and the multi-user MIMO scheme.

Reference Signals Design for Hybrid Analog and Digital Beamforming

There are many issues regarding the implementation of digital beamforming (BF) structures in massive MIMO: calibration, complexity, and cost. In a practical massive MIMO deployment, hybrid digital and analog BF structures with active antennas can be an alternative choice. In this letter, an N (the number of transceivers) by M (the number of active antennas per transceiver) hybrid BF structure is investigated, where the analog BF (ABF) is performed per transceiver and digital BF (DBF) is performed across N transceivers. A hybrid BF scheme is proposed, where the same ABF is applied to each transceiver, on top of which a DBF is designed to maximize the gain in a certain direction around the ABFs main beam direction. The proposed hybrid BF design establishes an inherent connection between the DBF and the beam direction of hybrid BF. Therefore, beam domain reference signals (RSs) can be conveniently implemented via applying properly designed DBF on subcarriers within the coherent bandwidth. Simulation results are given to show the validity of the proposed scheme.

Large scale antenna system with hybrid digital and analog beamforming structure

Large scale antenna systems (LSAS) are expected to significantly enhance the energy efficiency (EE) and spectrum efficiency (SE) of wireless communication systems. However, there are many open issues regarding the implementation of digital beamforming (BF) structures: calibration, complexity, and cost. In a practical LSAS deployment, hybrid digital and analog BF structures with active antennas can be an alternative choice. In this paper, an N (the number of transceivers) by M (the number of active antennas per transceiver) hybrid BF structure is investigated, where the analog BF is performed per transceiver and digital BF is performed across N transceivers. Analysis of the N by M BF structure includes: the optimal analog and digital BF design, EE-SE relationship at the green point (i.e. the point with highest EE) in EE-SE curve, impact of N on EE performance at a given SE value, and impact of N on the green point EE. Numerical simulations are provided to support the analysis.

Energy-Efficient Power Allocation for Pilots in Training-Based Downlink OFDMA Systems

In this paper, power allocation between pilots and data symbols is investigated to maximize energy efficiency (EE) for downlink orthogonal frequency division multiple access (OFDMA) networks. We first derive an EE function considering channel estimation error, which depends on large-scale channel gains of multiple users, allocated power to pilots and data symbols, and circuit power consumption. Then an optimization problem is formulated to maximize the EE under overall transmit power constraint. Exploiting the quasiconcavity property of the EE function, we propose an alternating optimization method in the low transmit power region and reformulate a joint quasiconcave problem in the high transmit power region. Analysis and simulation results show that the power ratio for pilots decreases with the circuit power. When the circuit power is small, the optimal overall transmit power increases with the circuit power. Otherwise, the optimal transmit power does not depend on it. Transmitting more data symbols to the users with higher channel gains improves the EE but at a cost of sacrificing the fairness among multiple users. Simulation results also demonstrate that compared with spectral efficiency (SE)-oriented design, the EE-oriented design can improve the EE performance significantly with a relatively small SE loss.

Optimal Threshold Design for FFR Schemes in Multi-Cell OFDMA Networks

Fractional frequency reuse (FFR) is an efficient method to suppress inter-cell interference (ICI) in multi-cell OFDMA networks. In this paper, the optimal threshold is designed for FFR schemes to identify cell central and edge users. We first derive throughput of the FFR scheme considering small-scale fading channels and different scheduling strategies. Then we conclude from the analysis and simulation results that the throughput of maximum normalized SINR (MNSINR) scheduling is larger than that of round robin scheduling, and the optimal thresholds for both scheduling strategies decrease with the increase of cell user number. Simulation results also show that the performance of FFR scheme with the optimal threshold outperforms that with the conventional predefined threshold.