Ping Liang

Small Cell In-Band Wireless Backhaul in Massive MIMO Systems: A Cooperation of Next-Generation Techniques

Massive multiple-input multiple-output (MIMO) systems, dense small-cells (SCs), and full duplex are three candidate techniques for next-generation communication systems. The cooperation of next-generation techniques could offer more benefits, e.g., SC in-band wireless backhaul in massive MIMO systems. In this paper, three strategies of SC in-band wireless backhaul in massive MIMO systems are introduced and compared, i.e., complete time-division duplex (CTDD), zero-division duplex (ZDD), and ZDD with interference rejection (ZDD-IR). Simulation results demonstrate that SC in-band wireless backhaul has the potential to improve the throughput for massive MIMO systems. Specifically, among the three strategies, CTDD is the simplest one and could achieve decent throughput improvement. Depending on conditions, with the self-interference cancellation capability at SCs, ZDD could achieve better throughput than CTDD, even with residual self-interference. Moreover, ZDD-IR requires the additional interference rejection process at the BS compared to ZDD, but it could generally achieve better throughput than CTDD and ZDD.

On the matrix inversion approximation based on neumann series in massive MIMO systems

Zero-Forcing (ZF) has been considered as one of the potential practical precoding and detection method for massive MIMO systems. One of the most important advantages of massive MIMO is the capability of supporting a large number of users in the same time-frequency resource, which requires much larger dimensions of matrix inversion for ZF than conventional multi-user MIMO systems. In this case, Neumann Series (NS) has been considered for the Matrix Inversion Approximation (MIA), because of its suitability for massive MIMO systems and its advantages in hardware implementation. The performance-complexity trade-off and the hardware implementation of NS-based MIA in massive MIMO systems have been discussed. In this paper, we analyze the effects of the ratio of the number of massive MIMO antennas to the number of users on the performance of NS-based MIA. In addition, we derive the approximation error estimation formulas for different practical numbers of terms of NS-based MIA. These results could offer useful guidelines for practical massive MIMO systems.

Small cell in-band wireless backhaul in massive Multiple-Input Multiple-Output systems

Massive Multiple-Input Multiple-Output (MIMO) systems, dense Small-Cells (SCs), and full duplexing are three candidates for next-generation wireless systems. The cooperation of the three techniques could offer more benefits, e.g., SC in-band wireless backhaul in massive MIMO systems. In this paper, three strategies of SC in-band wireless backhaul in massive MIMO systems are introduced and compared. Simulation results show that SC in-band wireless backhaul has the potential to improve the throughput for massive MIMO systems, and applying full-duplexing techniques at SCs could provide greater gain.

A Novel Hybrid Beamforming Algorithm With Unified Analog Beamforming by Subspace Construction Based on Partial CSI for Massive MIMO-OFDM Systems

Hybrid beamforming (HB) has been widely studied for reducing the number of costly radio frequency (RF) chains in massive multiple-input multiple-output (MIMO) systems. However, previous works on HB are limited to a single user equipment (UE) or a single group of UEs, employing the frequency-flat first-level analog beamforming (AB) that cannot be applied to multiple groups of UEs served in different frequency resources in an orthogonal frequency-division multiplexing (OFDM) system. In this paper, a novel HB algorithm with unified AB based on the spatial covariance matrix (SCM) knowledge of all UEs is proposed for a massive MIMO-OFDM system in order to support multiple groups of UEs. The proposed HB method with a much smaller number of RF chains can achieve more than 95% performance of full digital beamforming. In addition, a novel practical subspace construction (SC) algorithm based on partial channel state information is proposed to estimate the required SCM. The proposed SC method can offer more than 97% performance of the perfect SCM case. With the proposed methods, significant cost and power savings can be achieved without large loss in performance. Furthermore, the proposed methods can be applied to massive MIMO-OFDM systems in both time-division duplex and frequency-division duplex.

Repeater-enhanced millimeter-wave systems in multi-path environments

The millimeter Wave (mmWave) is considered as a promising technology of the future fifth-generation wireless systems due to the currently underutilized multi-GHz spectrum surrounding the 60GHz carrier frequency, particularly with the recent advances in low cost sub-terahertz semiconductor circuitry. Since the mmWave suffers high attenuation during transmission, the repeater is a key technique to enable mmWave systems with seamless coverage. This paper considers the following practical design issues of Amplified-and-Forward Repeater (AFR) enhanced mmWave systems. First, the distribution of excess delay through multiple AFR hops is derived, which helps the design of cyclic-prefix length to avoid inter-symbol interference in orthogonal frequency division multiplexing systems. In addition, we show that the AFR decreases the channel coherence bandwidth and then propose a novel AFR design with a finite impulse response filter based channel equalizer to address this problem. Simulation results show that the proposed method is able to significantly increase channel coherence bandwidth and effectively reduce bit error rate.

Channel estimation error and beamforming performance in repeater-enhanced massive MIMO systems

The demand for data service is increasing dramatically and wireless systems with high throughput and the capability to serve a large number of User Equipments (UEs) are desired. The massive MIMO system is considered as one of the most promising systems for the fifth generation of mobile telecommunication technology. Compared to conventional wireless systems, the spectrum efficiency of massive MIMO can be increased by an order of magnitude as tens of UEs can be served on the same time-frequency resource by exploring spatial multiplexing. Though extensively studied, massive MIMO still faces tough practical issues, such as excessive cost for fronthaul, channel estimation, beamforming matrix calculation within the latency requirement, signal quality at the deep fading zones or cell edge, etc. This paper presents a repeater-enhanced massive MIMO system with improved channel quality and reduced fronthaul cost, where Amplify-and-Forward Repeaters (AFRs) are employed without additional baseband processing at the base station or UEs. The disadvantages of AFRs, such as extra delay, increased delay spread and compromised channel reciprocity, are discussed and the corresponding solutions are suggested. Performances of different beamforming methods are analyzed and verified by simulations.