Weiqiang Tan

Low Cost and High Efficiency Hybrid Architecture Massive MIMO Systems Based on DFT Processing

Low cost and high efficiency, defined as energy efficiency (EE) and spectral efficiency (SE), have raised more and more attention in the fifth generation (5G) communication systems due to steadily rising hardware cost, energy consumption, and mobile traffic. This paper studies the hybrid architecture of multiuser massive MIMO systems, where the digital domain utilizes the zero-forcing (ZF) precoding scheme and the analog domain uses discrete Fourier transform (DFT) processing that significantly reduces hardware cost and energy consumption. We derive analytical expressions on the total achievable SE and EE, as well as offering insight into some engineering parameters in the system performance. Our aim is to achieve low cost and high efficiency massive MIMO system, with constraints on the overall transmit power, the number of users, and the number of radio frequency (RF) chains. Results exhibit that the total achievable SE of the hybrid architectures with DFT precessing is inferior to the full digital architectures and hybrid architectures with the ideal phase shifters, but the performance attenuation can be compensated by providing the more input SNR and higher number of RF chains. Moreover, we find that the total achievable EE of hybrid architectures with DFT precessing outperforms other massive MIMO architectures that include a full digital implementation, ideal phase shifters, and a switched network.

Joint User and Relay Selection for Cooperative NOMA Networks

This paper investigates the joint user and relay selection algorithm for cooperative non-orthogonal multiple access networks, where multiple users transmit messages to two destinations by utilizing multiple amplify-and-forward relays. To improve the outage performance of system, an optimal selection criterion is proposed at the user-relay pairs. We derive the closed-form analytical expressions on the outage probability, as well as the asymptotic expressions for large transmission power. Based on the derived analytical expressions, the effect of the number of relays and the target data rate on the outage performance is revealed. Results indicate that when the target data rate becomes small enough, the diversity order of the outage probability equals to the number of relays, while the target data rate grows without bound, the outage probability achieves one.

Congestion-Optimal WiFi Offloading with User Mobility Management in Smart Communications

We study the WiFi offloading problem in smart communications and adaptively seek for the optimal offloading strategies with the consideration of the mobility management and the dynamical nature of network state. With users mobility management, we formulate the offloading ratio optimization problem based on Markov process. Then, we propose a novel Congestion-Optimal WiFi Offloading (COWO) algorithm based on subgradient method, which aims to obtain the optimal offloading ratio for each access point (AP) to maximize the throughput and minimize the network congestion. Due to the computational complexity of subgradient method, we further improve the COWO algorithm by the equivalent transformation. By viewing all the APs as one virtual WiFi network, we try to optimize the identical offloading ratio for virtual WiFi network and develop a Virtualized Congestion-Optimal WiFi Offloading (VCOWO) algorithm with lower complexity. Under the equivalent conditions, the performance of the VCOWO algorithm could well approximate the optimal results obtained by the COWO algorithm. It is found that the VCOWO algorithm could obtain the upper bound of multiple APs WiFi offloading performance. Moreover, we investigate the impacts of user mobility on the WiFi offloading performance. Simulation results show that the proposed algorithm could achieve higher throughput with lower network congestion compared with other current offloading schemes.

On the performance of three-dimensionalantenna arrays in millimetre wave propagation environments

In order to reap the full scale of benefits of millimetre wave (mmWave) massive multiple-input multiple-output (MIMO) systems, the design of antenna arrays at the transmitter or receiver becomes more critical due to the propagation characteristic at mm-frequencies. In this study, the authors investigate the performance of two types of antenna array, namely uniform rectangular planar array (URPA) and uniform cylindrical array (UCYA). The channel behaviour is presented in full-dimensional mmWave propagation conditions by considering both the azimuth and elevation dimensions. The squared inner product and singular value spread are studied for URPA and UCYA configurations, these properties reveal the effective interference and channels stability of antenna array. The authors also evaluate the achievable rate with the equal power allocation and water-pouring power allocation schemes. Simulation results show that under the same system configurations, the performance that includes the radiation pattern, the channel eigenvalue distribution, the effective interference, and the achievable rate, of UCYA configuration always outperforms that of URPA configuration. Therefore, it can be concluded that in three-dimensional propagation environments, the UCYA configuration is especially appealing for mmWave MIMO systems.

Low-Cost Massive MIMO:Pilot Length and ADC Resolution

When designing an energy efficient massive multiple-input multiple-output (MIMO) system where each receiver antenna is equipped with a low-resolution analog-to-digital converter (ADC), the number of base station (BS) antennas and quantization bits are generally two mutually conflicting system parameters. In this paper, we investigate the joint optimization of the number of BS antennas and ADC resolution in quantized massive MIMO systems, assuming imperfect channel state information (CSI). A tractable approximate expression for the uplink sum spectral efficiency (SE) using maximal ratio combining (MRC) receivers is derived, based on which the pilot length which maximizes the sum SE is put forward. Considering the effect of ADCs, a realistic model of total power consumption is given subsequently. Capitalizing on it, we formulate the optimization problem of selecting the number of BS antennas and ADC resolution to maximize the sum SE under a total power consumption constraint. Our results show that more pilot symbols should be assigned for massive MIMO systems with low-resolution ADCs, especially for the receivers with one-bit quantizers. Moreover, the results show the trade-off between the number of BS antennas and quantization bits. Numerical results suggest that there exists an optimal ADC resolution in massive MIMO systems, while lower quantization bits may cause a substantial degradation of the SE performance and higher one will consume more power.

Analysis of non-best user association scheme in K-tiers heterogeneous networks

In this paper, we propose a novel m-th best average biased received power (ABRP) user association (UA) scheme for K-tier heterogeneous networks (HetNets), where a user is associated with the base station (BS) having the m-th strongest ABRP. For this novel scheme, by using stochastic geometry and modeling the positions of network elements in each tier as a homogeneous Poisson point process (PPP), we study the UA probability and the corresponding statistical description of the distance from a typical user to its serving BS. The presented mathematical analysis and numerical results show that the impact of proposed m-th best ABRP UA on system performance greatly depends on where the layer stands in the ordered set of ABRPs and the choice of m. In addition, the observation is different from the one in conventionally best UA scheme.

Hybrid cache placement for improving physical layer security in cooperative networks

Employing traditional wireless two-hop relaying is very easy for eavesdropper to overhear message due to the broadcasting nature of transmission. However, relays, which are equipped with caching capabilities to store popular content, can serve the destination directly without fetching messages from source. In order to improve physical layer security, the design of hybrid cache placement becomes more and more critical. In this paper, we consider a cooperation network with caching relays and propose a novel method that by utilizing hybrid cache placement to improve the physical layer security. Based on the proposed method, the function of secrecy outage probability and average secrecy capacity are derived to optimize the hybrid cache placement. And the numerical and simulation results also demonstrate the proposed method outperforms the traditional transmission without caching.