Haitao Zheng

Utilization and fairness in spectrum assignment for opportunistic spectrum access

The Open Spectrum approach to spectrum access can achieve near-optimal utilization by allowing devices to sense and utilize available spectrum opportunistically. However, a naive distributed spectrum assignment can lead to significant interference between devices. In this paper, we define a general framework that defines the spectrum access problem for several definitions of overall system utility. By reducing the allocation problem to a variant of the graph coloring problem, we show that the global optimization problem is NP-hard, and provide a general approximation methodology through vertex labeling. We examine both a centralized strategy, where a central server calculates an allocation assignment based on global knowledge, and a distributed approach, where devices collaborate to negotiate local channel assignments towards global optimization. Our experimental results show that our allocation algorithms can dramatically reduce interference and improve throughput (as much as 12-fold). Further simulations show that our distributed algorithms generate allocation assignments similar in quality to our centralized algorithms using global knowledge, while incurring substantially less computational complexity in the process.

Distributed coordination in dynamic spectrum allocation networks

Device coordination in open spectrum systems is a challenging problem, particularly since users experience varying spectrum availability over time and location. We propose a distributed coordination approach that handles spectrum heterogeneity without relying on the existence of a preassigned common control channel. Our approach carries potential to provide robust operation under network dynamics. While this approach can be implemented by upgrading the legacy protocol stack without modifying the MAC protocol, we also describe modifications to the MAC protocol that address spectrum heterogeneity and significantly improve system performance. Experimental results show that the proposed distributed coordination scheme outperforms the existing coordination schemes by 25-35% in throughput and provides 50% of delay reduction

Distributed spectrum allocation via local bargaining

In this paper, we present an adaptive and distributed approach to spectrum allocation in mobile ad-hoc networks. We propose a local bargaining approach where users affected by the mobility event self-organize into bargaining groups and adapt their spectrum assignment to approximate a new optimal assignment. The number of computations required to adapt to topology changes can be significantly reduced compared to that of the conventional topology-based optimizations that ignore the prior assignment. In particular, we propose a Fairness Bargaining with Feed Poverty to improve fairness in spectrum assignment and derive a theoretical lower bound on the minimum assignment each user can get from bargaining for certain network configurations. Such bound can be utilized to guide the bargaining process. We also show that the difference between the proposed bargaining approach and the true optimal approach is upper-bounded. Experimental results demonstrate that the proposed bargaining approach provides similar performance as the topology-based optimization but with more than 50% of reduction in complexity.

Collaboration and fairness in opportunistic spectrum access

The open spectrum approach to spectrum access can achieve near-optimal spectrum utilization by letting users sense and utilize available spectrum opportunistically. However, naive spectrum assignment can lead to significant interference. We propose a network controlled spectrum access scheme where users behave collaboratively to optimize spectrum allocation for the entire network. We develop a graph-theoretical model to characterize the spectrum access problem under a number of different optimization functions, and devise rules for users to utilize available spectrum while avoiding interference with their neighbors. Experimental results confirm that user collaboration yields significant benefits (as much as 50% improvement) in opportunistic spectrum access.

A General Framework for Wireless Spectrum Auctions

We propose a real-time spectrum auction framework to distribute spectrum among a large number wireless users under interference constraints. Our approach achieves conflict-free spectrum allocations that maximize auction revenue and spectrum utilization. Our design includes a compact and yet highly expressive bidding language, various pricing models to control tradeoffs between revenue and fairness, and fast auction clearing algorithms to compute revenue-maximizing prices and allocations. Both analytical and experimental results verify the efficiency of the proposed approach. We conclude that bidding behaviors and pricing models have significant impact on auction outcomes. A spectrum auction system must consider local demand and spectrum availability in order to maximize revenue and utilization.

Full length article: Proactive channel access in dynamic spectrum networks

Open Spectrum systems allow fast deployment of wireless technologies by reusing under-utilized pre-allocated spectrum channels, all with minimal impact on existing primary users. However, existing proposals take a reactive sense-and-avoid approach to impulsively reconfigure spectrum usage based solely on the latest observations. This can result in frequent disruptions to operations of both primary and secondary users. In this paper, we propose a proactive spectrum access approach where secondary users utilize past channel histories to make predictions on future spectrum availability, and intelligently schedule channel usage in advance. We propose two channel selection and switching techniques to minimize disruptions to primary users and maintain reliable communication at secondary users. Both simulation and testbed results show that the proactive approach effectively reduces the interferences to primary users by up to 30%, and significantly decreases throughput jitters at secondary users.

Route and spectrum selection in dynamic spectrum networks

Efficient spectrum allocation in dynamic spectrum systems is a challenging problem, particularly for multi-hops transmissions. The inter-dependence between route selection and spectrum management makes it important to examine interaction between the two and the corresponding performance and com- plexity tradeoffs. In this paper, we explore two design method- ologies: a decoupled design where these tasks are carried out independently by different protocol layers, and a collaborative design that integrates them into a single task. Experimental results show that the collaborative design, if well-provisioned, offers significant performance improvement compared to the decoupled design. I. INTRODUCTION

Device-centric spectrum management

Efficient spectrum allocation in open spectrum systems is a challenging problem, particularly for devices with constrained communication resources such as sensor and mobile ad hoc networks. We propose a device-centric spectrum management scheme with low communication costs, where users observe local interference patterns and act independently according to preset spectrum rules. We propose five rules that tradeoff performance with implementation complexity and communication costs, and derive a lower bound on each users allocation based on these rules. Experimental results show that our proposed rule-based approach reduces communication costs from efficient collaborative approaches by a factor of 3-4 while providing good performance

Mirror mirror on the ceiling: flexible wireless links for data centers

Modern data centers are massive, and support a range of distributed applications across potentially hundreds of server racks. As their utilization and bandwidth needs continue to grow, traditional methods of augmenting bandwidth have proven complex and costly in time and resources. Recent measurements show that data center traffic is often limited by congestion loss caused by short traffic bursts. Thus an attractive alternative to adding physical bandwidth is to augment wired links with wireless links in the 60 GHz band. We address two limitations with current 60 GHz wireless proposals. First, 60 GHz wireless links are limited by line-of-sight, and can be blocked by even small obstacles. Second, even beamforming links leak power, and potential interference will severely limit concurrent transmissions in dense data centers. We propose and evaluate a new wireless primitive for data centers, 3D beamforming, where 60 GHz signals bounce off data center ceilings, thus establishing indirect line-of-sight between any two racks in a data center. We build a small 3D beamforming testbed to demonstrate its ability to address both link blockage and link interference, thus improving link range and number of concurrent transmissions in the data center. In addition, we propose a simple link scheduler and use traffic simulations to show that these 3D links significantly expand wireless capacity compared to their 2D counterparts.

Joint channel-aware and queue-aware data scheduling in multiple shared wireless channels

This paper investigates multiuser downlink data scheduling with quality-of-service (QoS) provisioning over multiple shared fading channels, which, for a network point of view, provides line flexibility and granularity for resource allocation. A user-centric metric-a utility function with respect to mean waiting time-which is able to maintain fairness among users while providing delay QoS to individual users is used. This paper proposes scheduling algorithms that are aware of both channel and queue state information to achieve the maximum aggregate utility in the network. Simulation results confirm the significant performance and stability improvement provided by the utility-based scheduling scheme balancing multiuser diversity and queueing delay.