RECENT TALKS

 

* Random Access: Packet-based or Connection-based? (slides)

Abstract:

Random access provides a simple and elegant solution for multiple users to share a common channel. With random access, each user determines when to access the network in a distributed manner. Due to its simplicity in concept and low implementation cost, random access has found wide applications to various wireless networks such as Long Term Evolution (LTE) cellular networks and WiFi networks, and is expected to play an increasingly important role in the next-generation Machine-to-Machine (M2M) communications networks to facilitate the massive access of machine-type devices.

In general, random access protocols can be divided into two categories: packet-based and connection-based. Different from the conventional packet-based random access where each data packet needs to contend for channel access, with connection-based random access, a connection is first established before the data packet transmission. As the length of a connection request is typically smaller than that of a data packet, the transmission failure time can be reduced, though at the cost of extra overhead caused by connection establishment. Intuitively, there exists a critical threshold of the data packet transmission time, only above which establishing a connection is beneficial. Characterization of such a threshold is of crucial importance to practical access protocol design, which, nevertheless, has long remained elusive. In this talk, I will introduce our recently proposed unified analytical framework for sensing-free (Aloha) and sensing-based (CSMA) random access, and show how to characterize the threshold of data packet transmission time for beneficial connection establishment based on it. The practical implications of the analysis to the optimal access design of M2M communications will also be discussed.

 

　

* Optimal Network Decomposition for Next-Generation Mobile Communication Systems (slides)

Abstract:

The fundamental idea of network decomposition is to break a large-scale network into smaller parts such that the subnetworks can operate in parallel, each with a much lower dimensionality. For large-scale wireless networks, the cellular structure is based on the idea of network decomposition, where the network is decomposed into multiple subnetworks, i.e., cells, according to the coverage of each base-station (BS). Such a decomposition scheme, nevertheless, leads to strong interference among subnetworks, which becomes increasingly significant as the density of BSs grows. For the next-generation cellular network where a massive amount of BSs need to be deployed to meet the ever-increasing demand of high data rate, it is of paramount importance to develop efficient network decomposition schemes to replace the current cellular structure. How to build such a decomposition framework, unfortunately, has remained largely unknown.

In this talk, I will introduce our recently proposed network decomposition theory for large-scale wireless networks. Specifically, starting from a novel bipartite graph representation of an infrastructure-based wireless network, I will show that in general the optimal network decomposition can be formulated as a graph partitioning problem. I will then focus on maximizing the number of decomposed subnetworks with bounded sum of interference-to-signal-power-ratio of nodes of each subnetwork, and demonstrate how to solve it by the proposed Binary Search based Spectral Relaxation (BSSR) algorithm. The performance of the proposed BSSR algorithm is further examined and compared to the current cellular structure and BS clustering in various scenarios. Significant gains are shown to be achieved by the proposed BSSR algorithm, which corroborates that the optimal network decomposition of next-generation cellular networks should be performed based on a bipartite graph where the geographical information of BSs and users are both included.

 

 

* Massive Random Access: Fundamental Limits, Optimal Design, and Applications to M2M Communications (slides)

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. With decades of extensive research, random access has found wide applications to various wireless networks such as Long Term Evolution (LTE) cellular networks and WiFi networks. Despite the simplicity, it has been long observed that with fixed access parameters, nodes in a random-access network may suffer from substantial performance degradation when the network size increases. Therefore, for the next-generation communication systems where a massive number of machine-type devices need to be connected, how to optimize their random access process is a significant challenge that needs to be addressed imminently.

In this talk, I will introduce my recent work on modeling and performance optimization of random-access networks. I will first show that a random-access network in general can be regarded as a multi-queue-single-server system, and the key to performance analysis lies in proper characterization of the service time distribution, which is difficult to obtain with either each node’s queue completely ignored or interactions among nodes’ queues taken into full consideration. I will then introduce our newly proposed node-centric model, and demonstrate how to establish a unified analytical framework to characterize the fundamental limits of random-access networks. I will conclude the talk by demonstrating how to apply the proposed theory to guide the optimal access design of M2M communications in LTE networks.

 

 

* Asymptotic Rate Analysis of Large-Scale Distributed Antenna System (DAS): From Cellular DAS to Virtual-Cell based DAS (slides)

Abstract:

The distributed antenna system (DAS) has attracted considerable attention from both industry and academia, and gained increasing momentum especially with the popularity of the Cloud Radio Access Network (C-RAN). In contrast to the conventional cellular systems 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 made it a promising candidate for next-generation (5G) mobile communication systems.

For 5G mobile communication systems, a large amount of BS antennas are expected to be deployed to meet the ever increasing demand of high data rate. Extensive studies have been focused on the capacity 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 understood. In this talk, I will introduce my recent work on the rate scaling laws of downlink large-scale cellular DASs. I will start from the single-cell case, and demonstrate that bounds are useful for analyzing the rate scaling behavior. A comparative study to the rate with co-located BS antennas reveals that although a higher scaling order can always be achieved with distributed BS antennas, the rate gains become more pronounced when an orthogonal precoding scheme is adopted. For the multi-cell case, I will further show that despite a higher average user rate, the cell-edge problem is indeed exacerbated if distributed BS antennas are used. To achieve more uniform rate performance among users, virtual cells should be adopted, which are formed in a user-centric manner. I will conclude the talk by discussing how to optimally choose the virtual cell size to maximize the average user rate of a virtual-cell based DAS.

 

* A Coherent Theory of Random-Access Networks (slides)

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. With decades of extensive research, random access has found wide applications to various wireless networks such as cellular networks, WiFi networks and sensor networks. The minimum coordination and distributed control make it especially appealing for low-cost data networks.

In sharp contrast to the simplicity in concept, performance analysis of random-access networks has long been known as notoriously difficult. Although it has been long observed that a random-access network may suffer from significant performance degradation when the network size and/or traffic input rates increase, how to properly choose key system parameters to optimize the network performance is little understood. In this talk, I will introduce my recent work on modeling and performance optimization of random-access networks. I will show that the key to performance analysis of a random-access network lies in proper characterization of the service time distribution of each node’s queue. Based on the proposed node-centric model, a unified analytical framework can be established to characterize the fundamental limits of random-access networks, and to evaluate effects of key parameters on a wide range of performance metrics in a systematic manner. I will conclude the talk by demonstrating how to apply the proposed theory to guide the optimal access design of WiFi networks.

 

* On the Capacity of Distributed Antenna Systems (slides)

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)

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.