Le Liang

Low-Complexity Hybrid Precoding in Massive Multiuser MIMO Systems

Massive multiple-input multiple-output (MIMO) is envisioned to offer considerable capacity improvement, but at the cost of high complexity of the hardware. In this paper, we propose a low-complexity hybrid precoding scheme to approach the performance of the traditional baseband zero-forcing (ZF) precoding (referred to as full-complexity ZF), which is considered a virtually optimal linear precoding scheme in massive MIMO systems. The proposed hybrid precoding scheme, named phased-ZF (PZF), essentially applies phase-only control at the RF domain and then performs a low-dimensional baseband ZF precoding based on the effective channel seen from baseband. Heavily quantized RF phase control up to 2 bits of precision is also considered and shown to incur very limited degradation. The proposed scheme is simulated in both ideal Rayleigh fading channels and sparsely scattered millimeter wave (mmWave) channels, both achieving highly desirable performance.

Performance enhanced transmission in device-to-device communications: Beamforming or interference cancellation?

This paper considers device-to-device (D2D) communications underlaying cellular networks with a multi-antenna base station (BS). The BS serves its own cellular users while letting another remote terminal directly transmit signals to its nearby receiver via a D2D link. Two transmit strategies including beamforming (BF) and interference cancellation (IC) are considered at the BS for performance evaluation in terms of achievable channel capacity. The capacity performance of two different cases with perfect and quantized channel knowledge at the transmitter is derived with closed-form expressions. Based on these results, an adaptive transmission scheme to switch between BF and IC is proposed. Numerical results verify the accuracy of the derived expressions and draw the operating regions of BF/IC strategies.

Resource Allocation for D2D-Enabled Vehicular Communications

The widely deployed cellular network, assisted with device-to-device (D2D) communications, can provide a promising solution to support efficient and reliable vehicular communications. Fast channel variations caused by high mobility in a vehicular environment need to be properly accounted for when designing resource allocation schemes for the D2D-enabled vehicular networks. In this paper, we perform spectrum sharing and power allocation based only on slowly varying large-scale fading information of wireless channels. Pursuant to differing requirements for different types of links, i.e., high capacity for vehicle-to-infrastructure (V2I) links and ultrareliability for vehicle-to-vehicle (V2V) links, we attempt to maximize the ergodic capacity of the V2I connections while ensuring reliability guarantee for each V2V link. Sum ergodic capacity of all V2I links is first taken as the optimization objective to maximize the overall V2I link throughput. Minimum ergodic capacity maximization is then considered to provide a more uniform capacity performance across all V2I links. Novel algorithms that yield optimal resource allocation and are robust to channel variations are proposed. Their desirable performance is confirmed by computer simulation.

Limited Feedback-Based Multi-Antenna Relay Broadcast Channels with Block Diagonalization

The relay technology is effective in extending radio coverage and improving the performance of cell edge users. In multi-antenna relay channels, good knowledge of the channel state information (CSI) at the transmitter is important to achieve multiplexing gains of the multiple-input multiple-output technique. In this paper, we study the multi-antenna relay downlink channel with limited feedback CSI from both two-hop links. Data streams from the base station (BS) are first transmitted to a relay station (RS) with singular value decomposition-based precoding and receiver pulse shaping at the BS and RS, respectively. The block diagonalization precoding is then applied at the RS to forward the received signals to the remote multi-antenna users. We derive an upper bound for the system throughput loss due to CSI quantization error, and then propose a feedback quality control strategy to maintain a bounded rate loss relative to the perfect CSI case. It reveals that the feedback size B1 from the RS to BS needs to scale in proportion to both transmit power at the BS and RS while the feedback size B2 from each user to the RS only needs to scale linearly with the transmit power at the RS.

Vehicular Communications: A Physical Layer Perspective

Vehicular communications have attracted more and more attention recently from both industry and academia due to their strong potential to enhance road safety, improve traffic efficiency, and provide rich on-board information and entertainment services. In this paper, we discuss fundamental physical layer issues that enable efficient vehicular communications and present a comprehensive overview of the state-of-the-art research. We first introduce vehicular channel characteristics and modeling, which are the key underlying features differentiating vehicular communications from other types of wireless systems. We then present schemes to estimate the time-varying vehicular channels and various modulation techniques to deal with high-mobility channels. After reviewing resource allocation for vehicular communications, we discuss the potential to enable vehicular communications over the millimeter wave bands. Finally, we identify the challenges and opportunities associated with vehicular communications.

Spectrum and Power Allocation for Vehicular Communications With Delayed CSI Feedback

Channel state information (CSI) at the base station (BS) is critical to resource allocation design for wireless networks, but it is hard to obtain accurate CSI in a high mobility vehicular environment. In this letter, we study the spectrum and power allocation problem in device-to-device-enabled vehicular networks, where CSI of vehicular links is only reported to the BS periodically. We maximize the sum throughput of all vehicle-to-infrastructure (V2I) links while guaranteeing the reliability of each vehicle-to-vehicle (V2V) link with the delayed CSI feedback. We propose a low-complexity algorithm to find the optimal spectrum sharing strategy among V2I and V2V links and properly adjust their transmit powers. Its desirable performance is confirmed by computer simulation.

On Coordinated Multi-point Transmission with Partial Channel State Information Via Delayed Feedback

Coordinated multi-point transmission (CoMP) is considered as a strategic approach to handle inter-cell interference, hence increasing spectral efficiency in interference-limited cellular networks. This paper studies a CoMP downlink transmission with delayed and quantized channel state information feedback through a capacity-limited uplink channel. The delay effects and quantization error as well as large scale path loss potentially degrade entire network performance. Under this consideration, we first characterize the system performance in terms of achievable rate for two typical CoMP techniques, namely joint processing (JP) and coordinated beamforming (CBF), in a multi-cell network. Accurate closed-form expressions are derived for evaluating the CoMP performance. Based on the derived results, an adaptive transmission strategy of switching between JP and CBF is proposed to maximize the system throughput and the operating regions of the two schemes are provided under different system setup parameters. Simulation verifies that the superiority of JP or CBF depends on system parameters including feedback delay, codebook size as well as CoMP user locations.

Meeting different QoS requirements of vehicular networks: A D2D-based approach

The widely deployed cellular network, assisted with device-to-device (D2D) communications, can provide a promising solution to support efficient and reliable vehicular communications. In this paper, we identify differentiated requirements for different types of vehicular links, i.e., high capacity for vehicle-to-infrastructure (V2I) links and ultra reliability for vehicle-to-vehicle (V2V) links, and attempt to maximize the ergodic capacity of V2I connections while ensuring reliability guarantee for each V2V link. To account for fast channel variations caused by high mobility, we propose to perform spectrum sharing and power allocation based only on slowly varying large-scale fading information of wireless channels. A novel algorithm that yields optimal resource allocation and is robust to channel variations is proposed. Their desirable performance is confirmed by computer simulation.

Adaptive coordinated multi-point transmission based on delayed limited feedback

This paper studies the capacity performance of coordinated multi-point (CoMP) downlink transmissions based on limited feedback. We consider both path loss effects and channel imperfections including feedback delay and quantization error. Closed-form expressions are derived to characterize ergodic achievable rates for joint processing (JP) and coordinated beamforming (CBF) techniques, respectively. According to the derived expressions, an adaptive transmission strategy to switch between JP and CBF is proposed to maximize cell throughput. Simulation shows the CBF scheme is preferred at medium SNR with varying switch points jointly determined by the feedback size, delay, and locations of CoMP users.

Approaching downlink capacity of multiplexing relay channels with imperfect channel information

This paper studies the multi-antenna relay downlink channel with quantized channel information feedback from both two-hop links. By characterizing the system capacity loss due to channel quantization, we propose a strategy of controlling feedback sizes to help the system approach the channel capacity with respect to full channel information case within an arbitrary predetermined capacity loss. It reveals that the feedback load B1 from the relay station (RS) to the base station (BS) needs to scale in proportion to both the transmit power at the BS and RS while the feedback size B2 from each user to the RS only needs to scale linearly with the transmit power at the RS. Simulation results illustrate the effectiveness of the proposed design strategy.