Prof. DAI Lin (代琳)

Visiting address
YEUNG-G6504
Phone: +852 34427141

Author IDs

Biography

Dr. Lin Dai was born in Wuhan, China. At the age of 15, she excelled in the national college examination and was admitted into the "Special Class for Gifted Teenagers" at Huazhong University of Science and Technology. She received her BS degree in 1998, and the MS and PhD degrees from Tsinghua University, Beijing, China, in 2003, all in Electronic Engineering.

She was a postdoctoral fellow at The Hong Kong University of Science and Technology and University of Delaware. Since 2007, she has been with City University of Hong Kong, where she is currently an associate professor. She has broad interests in communications and networking theory, with special interests in wireless communications.

She was a co-recipient of the Best Paper Award at IEEE Wireless Communications and Networking Conference (WCNC) 2007, and the IEEE Guglielmo Marconi Prize Paper Award (the annual Best Paper Award of IEEE Transactions on Wireless Communications) in 2009. She served as a co-chair of PHY Track of IEEE WCNC 2013, and the leading co-chair of Wireless Communications Symposium of IEEE International Conference on Communications (ICC) 2015. She received The President's Award of City University of Hong Kong in 2017. Since 2014, she has been serving as an editor for IEEE Transactions on Wireless Communications, and an assistant dean of College of Science and Engineering of City University of Hong Kong.

Research Interests/Areas

Current Research Interests

A Coherent Theory of Random-Access Networks

Random access provides a simple and elegant solution for multiple users to share a common channel. Studies on random-access protocols date back to 1970s. After decades of extensive research, random access has found wide applications to Ethernet, IEEE 802.11 networks, cellular networks, and wireless ad-hoc networks. The minimum coordination and distributed control make it highly appealing for low-cost data networks.It has been long observed that a random-access network may suffer from low throughput, large delay jitter and even risks of collapse if the backoff parameters are improperly selected. Yet due to the difficulty in modeling and performance analysis, how to adaptively tune backoff parameters to optimize the network performance remains largely unknown. In many cases, the problem is complicated by the fact that the improvement in throughput/stability performance is obtained at the cost of sacrificing the delay performance. It is, therefore, highly desirable to develop a unified framework, within which the effect of key parameters on the network performance can be evaluated in a systematic manner.Such an analytical framework was recently proposed for two most representative random-access networks, Aloha [Dai'12] and Carrier Sense Multiple Access (CSMA) [Dai'13], further extended in [Li-Dai'16], [Sun-Dai'16], [Sun-Dai'17] and applied to IEEE 802.11 (WiFi) networks in [Dai-Sun'13, Gao-Sun-Dai'13, Gao-Dai'13, Gao-Sun-Dai'14, Sun-Dai'15]. Based on the proposed framework, the network steady-state points can be derived as explicit functions of key system parameters such as the network size, sensing capability and backoff parameters, which further enable the characterization of stable regions and performance optimization. The analysis sheds important light on the design and control of practical networks, and serves as a crucial step toward a unified theory of random access.

 

 

 

A Unified Analytical Framework of Aloha and CSMA

Performance Optimization of IEEE 802.11 DCF Networks

* A Unified Analysis: Stability, Throughput and Delay

* Backoff Design: Fundamental Tradeoff and Design Criterion

* IEEE 802.11e EDCA: Modeling, Differentiation and Optimization

 

Modeling and Performance Analysis of Large-Scale Distributed Antenna Systems (DASs)

The distributed antenna system (DAS) has become a promising candidate for next-generation (5G) mobile communication systems. In contrast to the conventional cellular structure where antennas are co-located at the tower-mounted base station (BS) in each cell, in a DAS, many low-power remote antenna ports are geographically distributed over a large area and connected to a central processor by fiber. The appealing features of distributed antennas have attracted considerable attention from both industry and academia, and been applied to the cutting-edge technologies such as small cells and the Cloud Radio Access Network (C-RAN).In the next-generation mobile communication systems, a large amount of BS antennas are expected to be deployed to meet the ever increasing demand of high data rate. Significant efforts have been made on the performance analysis of cellular systems with large antenna arrays at BSs (popularly known as “massive MIMO”). If the BS antennas are distributed, on the other hand, how the capacity scales with the number of BS antennas is less clear. In our recent work, a comprehensive comparison on the capacity scaling laws of MIMO cellular systems with co-located and distributed BS antennas is presented for both uplink [Dai'11, Dai'14] and downlink [Liu-Dai'14, Wang-Dai'15]. The asymptotic analysis shows that the scaling order is crucially dependent on the BS antenna layout. If the number of BS antennas and the number of users both go to infinity but their ratio is fixed, for instance, the uplink sum capacity [Dai'14] and the downlink sum rate with orthogonoal precoding schemes [Liu-Dai'14, Wang-Dai'15] converge to a constant with BS antennas co-located at the center of each cell. In contrast, with BS antennas uniformly distributed in each cell, the sum capacity/rate increases with the number of BS antennas unboundedly.The analysis also shows that despite better capacity/rate performance, the cell-edge problem could be exacerbated if distributed BS antennas are used in cellular systems. As pointed out in [Dai'14], the cell-edge problem has its roots in the cellular structure where cells are formed based on the coverage of each BS. Such a BS-centric structure, nevertheless, cannot be justified when both users and BS antennas are scattered around. Instead, the signal processing may be performed based on the unit of “virtual cells”. It is shown in [Dai'14] that a uniform inter-cell interference density can be achieved in a DAS if each user chooses a few surrounding BS antennas to form its virtual cell. By doing so, each BS antenna serves a declining number of users as the density of BS antennas increases, indicating good scalability that is much appreciated for a large-scale network.For virtual-cell based DASs, the virtual cell size, i.e., how many BS antennas should be included into each user's virtual cell, is a key system parameter. To study the effect of virtual cell size, [Wang-Dai'16] considered a large-scale downlink DAS with two representative linear precoding schemes: maximum ratio transmission (MRT) and zero-forcing beamforming (ZFBF). The analysis shows that if MRT is adopted in each user's virtual cell, a small virtual cell size should be chosen so as to avoid sharing BS antennas for different users which would otherwise cause strong interference. On the other hand, if users are grouped with joint ZFBF transmission from their virtual cells to eliminate the intra-group interference, the average user rate could be significantly improved by increasing the virtual cell size. A novel virtual-cell based user grouping algorithm is proposed, with which the rate difference among users is greatly reduced compared to the conventional BS-centric clustering.

 

Performance Comparison of Cellular Systems with Co-located BS Antennas and Distributed BS Antennas
* Uplink Sum Capacity
* Downlink Average User Rate with Linear Precoding
Performance Analysis and System Design for Virtual-cell based DASs

 

 

Previous Topics

 

Activities

RECENT TALKS

 

* On the Capacity of Distributed Antenna Systems (slides: 2014, 2016)

Abstract:

The distributed antenna system (DAS) has become a promising candidate for next-generation (5G) mobile communication systems. In DASs, many remote antenna ports are geographically distributed over a large area and connected to a central processor by fiber or coaxial cable. Although the idea of DAS was originally proposed to cover the dead spots in indoor wireless communication systems, research activities on cellular DASs have been intensified in the past few years owing to the fast growing demand for high data-rate services.

For cellular systems, the use of distributed base-station (BS) antennas enables efficient utilization of spatial resources, which, on the other hand, also significantly complicates the channel modeling and performance analysis. In this talk, I will introduce my recent work on the uplink capacity analysis of large-scale cellular DASs. I will start from the single-cell multi-user system, and address a series of fundamental questions such as how the uplink sum capacity varies with the BS antenna layout and key parameters including the availability of channel state information at the transmitter (CSIT) and the number of BS antennas. For the multi-cell case, I will show that with distributed BS antennas, despite substantial gains on the uplink sum capacity owing to the reduction of the inter-cell interference level, the cell-edge problem could be exacerbated. To demonstrate that the performance disparity originates from the cellular structure rather than the BS antenna layout, I will further introduce the concept of “virtual cell” and show that a uniform inter-cell interference density can be achieved in a DAS if each user chooses a few surrounding BS antennas to form its virtual cell. By doing so, each BS antenna serves a declining number of users as the density of BS antennas increases, indicating good scalability that is much appreciated in a large-scale network. I will conclude the talk by discussing the implications to cutting-edge cellular technologies such as small cells and pCell.

 

* Toward a Unified Theory of Random Access (slides: 2014, 2017)

Abstract:

Random access provides a simple and elegant solution for multiple users to share a common channel. Studies on random-access protocols date back to 1970s. After decades of extensive research, random access has found wide applications to Ethernet, WiFi networks and wireless ad-hoc networks. The minimum coordination and distributed control make it highly appealing for low-cost data networks.

In sharp contrast to the simplicity in concept, the performance analysis of random-access networks has long been known as notoriously difficult. Numerous protocols have been proposed, yet how to analyze them within a unified framework remains an open challenge. In this talk, I will introduce my recent work on modeling and performance optimization of two most representative random-access networks, Aloha and Carrier Sense Multiple Access (CSMA). I will show that the key to establishing a unified framework for throughput, delay and stability analysis lies in the proper modeling of head-of-line (HOL) packets’ behavior. Based on the proposed framework, the network steady-state points of both Aloha and CSMA can be derived as explicit functions of key system parameters such as the network size, sensing capability and backoff parameters, which further enable characterization of stable regions and performance optimization. The analysis sheds important light on the design and control of practical networks such as WiFi, and serves as a crucial step toward a unified theory of random access.

 

* Resource Allocation in Energy-constrained Cooperative Wireless Networks (slides)

Abstract:

Future wireless networks are expected to support a wide variety of communication services, such as voice, video, and multimedia. However, the wireless environment provides unique challenges to the reliable communication: time-varying nature of the channel and scarcity of the radio resources such as power and bandwidth. Therefore, it is of great interest to investigate how to efficiently allocate the limited radio resources to meet diverse quality-of-service (QoS) requirements of users and maximize the utilization of available bandwidth based on the channel states of users.

This talk will specifically focus on energy-constrained cooperative networks, where the traditional efficiency-fairness tradeoff does not work any more. A unified cross-layer framework will be introduced and it will be demonstrated that in energy-constrained cooperative ad-hoc networks, fairness can bring significant throughput gains.

URLs/Links

Teaching