Large-scale distributed-antenna system (L-DAS) with very large number of distributed antennas, possibly up to a few hundred antennas, is considered. A few major issues of the L-DAS, such as high latency, energy consumption, computational complexity, and large feedback (signaling) overhead, are identified. The potential capability of the L-DAS is illuminated in terms of an energy efficiency (EE) throughout the paper. We firstly and generally model the power consumption of an L-DAS, and formulate an EE maximization problem. To tackle two crucial issues, namely the huge computational complexity and large amount of feedback (signaling) information, we propose a channel-gain-based antenna selection (AS) method and an interference-based user clustering (UC) method. The original problem is then split into multiple subproblems by a cluster, and each cluster's precoding and power control are managed in parallel for high EE. Simulation results reveal that i) using all antennas for zero-forcing multiuser multiple-input multiple-output (MU-MIMO) is energy inefficient if there is nonnegligible overhead power consumption on MU-MIMO processing, and ii) increasing the number of antennas does not necessarily result in a high EE. Furthermore, the results validate and underpin the EE merit of the proposed L-DAS complied with the AS, UC, precoding, and power control by comparing with non-clustering L-DAS and colocated antenna systems.

Energy-Efficient, Large-Scale Distributed-Antenna System (L-DAS) for Multiple Users

Driven by the recent advancement in unmanned aerial vehicle (UAV) technology, this paper proposes a new wireless network architecture of coordinate multipoint (CoMP) in the sky to harness both the benefits of interference mitigation via CoMP and high mobility of UAVs. Specifically, we consider uplink communications in a multi-UAV enabled multi-user system, where each UAV forwards its received signals from all ground users to a central processor (CP) for joint decoding. Moreover, we consider the case where the users may move on the ground, thus the UAVs need to adjust their locations in accordance with the user locations over time to maximize the network throughput. Utilizing random matrix theory, we first characterize, in closed form, a set of approximated upper and lower bounds of the user’s achievable rate in each time epoch under the practical Rician fading channel model, which is shown to be very tight, both analytically and numerically. UAV placement and movement over different epochs are then optimized based on the derived bounds to maximize the minimum of user average achievable rates over all epochs for both cases of full information (of current and future epochs) and current information on the user’s movement. Interestingly, it is shown that the optimized location of each UAV at any particular epoch is the weighted average of the ground user locations at the current epoch as well as its own location at the previous and/or next epoch. Finally, simulation results are provided to validate and compare the performance of the proposed UAV placement and movement designs under different practical application scenarios.

CoMP in the Sky: UAV Placement and Movement Optimization for Multi-User Communications

Distributed multi-input multi-output (D-MIMO) system is a promising system to greatly improve the spectral efficiency and power efficiency of the cellular system. The performance analysis of the spectral efficiency of D-MIMO system is a fundamental problem for both theoretical study and technique evaluation. In this paper, the theoretical performances of spectral efficiency for D-MIMO system and traditional collocated MIMO (C-MIMO) system are studied and compared. First, a composite D-MIMO channel including path loss, shadow fading and multipath fading is given. Conditioned on the desired user position, by using the tight bounds and central limit theory, the analytical approximations of the mean and the cumulative distribution function (CDF) of the mutual information (MI) are derived for C-MIMO and D-MIMO channels at both high signal to noise ratio (SNR) and low SNR. Assuming that the users are randomly distributed in the cell, the CDFs of the spectral efficiency are also given for the C-MIMO and D-MIMO cellular system, and the closed-form expressions for the mean spectral efficiency, mean outage spectral efficiency are derived. Finally, the theoretical comparisons between C-MIMO and D-MIMO for large number of antennas are given, and simulation results are presented which validate the analytical results.

Spectral Efficiency of Distributed MIMO Systems

It has been long believed that the distributed antenna system (DAS) has great potentials for capacity improvement compared to the traditional cellular system with co-located base-station antennas. Intuitively, the distributed characteristic of antennas provides a much more efficient utilization of spatial resources. It, however, also significantly complicates the channel modeling and system analysis. Despite an increasing amount of academic attention and industrial interest, how to characterize the capacity advantages of the DAS in the multiuser scenario remains largely unknown. In this paper, we present a comparative study on the uplink ergodic sum capacity with multiple base-station antennas either co-located or uniformly distributed within a given area. We demonstrate that under the same consumption of transmission power, enormous gains can be achieved by the distributed antenna layout thanks to (1) reduced minimum access distance of each user; and (2) enhanced channel fluctuations which provide a significant boost to the sum capacity when the channel state information is available at both the transmitter and the receiver sides. We further apply the analysis to a cellular system with full cooperation among base stations. The comparison verifies that the DAS yields a much higher sum capacity, and capacity gains increase with the number of base-station antennas per cell.

A Comparative Study on Uplink Sum Capacity with Co-Located and Distributed Antennas

It has been long believed that the distributed an- tenna system (DAS) has great potentials for capacity improve- ment compared to the traditional cellular system with co-located base-station antennas. Intuitively, the distributed characteristic of antennas provides a much more efficient utilization of spatial resources. It, however, also significantly complicates the channel modeling and system analysis. Despite an increasing amount of academic attention and industrial interest, how to characterize the capacity advantages of the DAS in the multiuser scenario remains largely unknown. In this paper, we present a comparative study on the uplink ergodic sum capacity with multiple base-station antennas either co-located or uniformly distributed within a given area. We demonstrate that under the same consumption of transmission power, enormous gains can be achieved by the distributed antenna layout thanks to 1) reduced minimum access distance of each user; and 2) enhanced channel fluctuations which provide as ignificant boost to the sum capacity when the channel state information is available at both the transmitter and the receiver sides. We further apply the analysis to a cellular system with full cooperation among base stations. The comparison verifies that the DAS yields a much higher sum capacity, and capacity gains increase with the number of base-station antennas per cell.

http://ieeexplore.ieee.org/iel5/49/5770655/05770665.pdf

A Comparative Study on Uplink Sum Capacity with Co-Located and Distributed Antennas Lin Dai, Member, IEEE

A squared distance criterion for antenna location design in generalized distributed antenna systems (GDAS) is proposed to maximize the cell averaged ergodic capacity. The criterion requires the antenna port locations minimize the expectation of the squared distance between a randomly distributed user and the nearest antenna port. This is equivalent to codebook design in vector quantization. For uniform user distribution, we can easily derive analytical expressions for antenna locations. For more general user distribution, we can obtain numerical results using the codebook design algorithm. Applying the proposed criterion to circular-layout GDAS with uniform user distribution and linear cell with non-uniform user distribution, we achieve near optimal performance.

Antenna location design for generalized distributed antenna systems

This paper aims to find optimal antenna locations that maximize the cell averaged ergodic capacity for the distributed antenna system (DAS) with selective transmission (ST) in the downlink. We propose the squared distance criterion (SDC) and convert the antenna positioning problem to the codebook design problem. According to the SDC, we derive the suboptimal antenna placements for the linear and circular cells with uniform and non-uniform distribution of the mobile station. Simulation results show that the obtained antenna positions are near optimal and DAS with optimized antenna positions offers a large capacity gain over the traditional co-located antenna system (CAS).

Antenna location design for distributed antenna systems with selective transmission

Single carrier frequency division multiple access (SC-FDMA) is adopted as uplink multiple access scheme in 3G long-term evolution (LTE). Combining SC-FDMA with MIMO (multiple-input multiple-output) can increase capacity and throughput but detection technique is required at the base station (BS). Interference cancellation (IC) based on cyclic redundancy check (CRC) is a conventional detection algorithm. In this paper, we divide IC into useful IC and useless IC. Simulation results indicate that CRC-based IC algorithm performs too many useless ICs, which increases the complexity. Thresholded IC algorithm is proposed to reduce the complexity. In the proposed algorithm, IC is performed only when the probability that the IC is useful exceeds a threshold. We select 0.5 as the threshold. With the threshold, the proposed algorithm reduces the complexity significantly and keeps the performance loss negligible.

Thresholded Interference Cancellation Algorithm for the LTE Uplink Multiuser MIMO

The distributed antenna system (DAS) offers significant power savings but only if the antennas are properly located. In this letter, we convert antenna location optimization to the codebook design problem. For the widely studied circular-layout DAS with uniform user distribution, we derive closed-form expressions for antenna locations that yield near-optimal performance. For more general user distribution and antenna topology, the codebook design algorithms can provide numerical optimization results with acceptable performance and low complexity.

Antenna Location Optimization for Circular-Layout Distributed Antenna Systems

Threshold-based ordered successive interference cancellation (OSIC) detection algorithm is proposed for per-antenna-coded (PAC) two-input multiple-output (TIMO) orthogonal frequency division multiplexing (OFDM) systems. Successive interference cancellation (SIC) is performed selectively according to channel conditions. Compared with the conventional OSIC algorithm, the proposed algorithm reduces the complexity significantly with only a slight performance degradation.

Xinzheng Wang

Papers

Antenna location design for generalized distributed antenna systems

Antenna location design for distributed antenna systems with selective transmission

Thresholded Interference Cancellation Algorithm for the LTE Uplink Multiuser MIMO

Antenna Location Optimization for Circular-Layout Distributed Antenna Systems

Threshold-Based OSIC Detection Algorithm for Per-Antenna-Coded TIMO-OFDM Systems