0-7803-7016-3/01/$10.00 ©2001 IEEE Transmission Scheduling for Efficient Wireless Utilization Xin Liu, Edwin K. P. Chong, and Ness B. Shroff School of Electrical and Computer Engineering Purdue University, West Lafayette, IN 47907-1285 xinliu, echong, shroff @ecn.purdue.edu Abstract— We present an “opportunistic” transmission scheduling pol- icy that exploits time-varying channel conditions and maximizes the system performance stochastically under a certain resource allocation fairness con- straint. We establish the optimality of the scheduling scheme and also de- scribe a practical scheduling procedure to implement our scheme. Through simulation results, we show that the scheme also works well for nonsta- tionary scenarios and results in performance improvements of – compared with a scheduling scheme that does not take into account chan- nel conditions. Furthermore, we note that in wireless networks, an impor- tant role of resource allocation is to balance the system performance and fairness among “good” and “bad” users. We propose three heuristic time- fraction assignment schemes, which approach the problem from different viewpoints. Keywords—Scheduling, fairness, wireless, high-rate-data. I. I NTRODUCTION UTURE wireless networks are designed to support wide- band data communications as well as voice communica- tions. Such a network will be the basis for a wireless information society where access to information and information services such as electronic commerce is available anytime, anywhere, and to anybody. Throughout the world, the demand for wire- less communication systems has increased exponentially in the last few years. Because wireless frequency is a scarce resource, efficient frequency utilization is becoming increasingly impor- tant. Resource allocation schemes and scheduling policies are critical to achieving these goals. In wireline networks, resource allocation schemes and scheduling policies play important roles in providing service performance guarantees, such as throughput, delay, delay-jitter, fairness, and loss rate [1]. However, resource allocation and scheduling schemes from the wireline domain do not carry over to wireless systems because wireless channels have unique char- acteristics not found in wireline channels, such as limited band- width, time-varying and location-dependent channel conditions, and channel-condition-dependent throughput. In wireless networks, the channel conditions of mobile users are time-varying. Radio propagation can be roughly char- acterized by three nearly independent phenomena: path-loss variation with distance, slow log-normal shadowing, and fast multipath-fading. Path losses vary with the movement of mobile stations. Slow log-normal shadowing and fast multipath-fading are time-varying with different time scales. Thus, users per- ceive time-varying service quality and/or quantity because chan- nel conditions are time-varying. For voice users, better channel conditions may result in better voice quality. For packet data service, better channel conditions (or larger signal to interfer- ence plus noise ratio (SINR)) can be used to provide higher data rates by reducing coding or spreading and/or increasing the con- stellation density. Previous research shows that cellular spectral efficiency (in terms of b/s/Hz/sector) can be increased by a fac- tor of two or more if users with better links are served at higher data rates [2]. Procedures to exploit this are already in place for all the major cellular standards: adaptive modulation and coding schemes are implemented in the 3G TDMA standards, and vari- able spreading and coding are implemented in the 3G CDMA standards. In general, a user is served with better quality and/or at a higher bit rate when the channel condition is better. On one hand, good scheduling schemes should be able to ex- ploit the time-varying channel conditions of users to achieve higher utilization of wireless resources. On the other hand, the potential to exploit higher data throughputs in an opportunis- tic way, when channel conditions permit, introduces the trade- off problem between wireless resource efficiency and fairness among users. Because wireless spectrum is a scarce resource, improving the efficiency of spectrum utilization is important, especially to provide high-rate-data service. Hence, we cannot expect the same throughput for all users because the users in general can have very different channel conditions. However, a scheme designed only to maximize the overall throughput could be very unfair among users, especially users with widely dis- parate distances from the base station. For example, allowing only users close to the base station to transmit with high trans- mission power may result in very high throughput, but is un- fair to other users. This basic dilemma motivates our work: to improve wireless resource efficiency by exploiting time-varying channel conditions while at the same time control the level of fairness among users. In this paper, we consider a time-slotted system in which time is the resource to be shared among the users. Associated with each user is a number between and representing the long- term fraction of time to be assigned to the user. This “time- fraction assignment” to users represents the fairness constraint. Given this fairness constraint, the problem is to determine which user should be scheduled to transmit at each time slot so that network performance is optimized. To solve this problem, we present an opportunistic scheduling scheme that maximizes the wireless resource utilization by exploiting time-varying chan- 776 IEEE INFOCOM 2001