74 IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS, VOL. 3, NO. 1, JANUARY 2004 Joint Scheduling and Power Control for Wireless Ad Hoc Networks Tamer ElBatt, Member, IEEE, and Anthony Ephremides, Fellow, IEEE Abstract—In this paper, we introduce a cross-layer design framework to the multiple access problem in contention-based wireless ad hoc networks. The motivation for this study is twofold, limiting multiuser interference to increase single-hop throughput and reducing power consumption to prolong battery life. We focus on next neighbor transmissions where nodes are required to send information packets to their respective receivers subject to a constraint on the signal-to-interference-and-noise ratio. The multiple access problem is solved via two alternating phases, namely scheduling and power control. The scheduling algorithm is essential to coordinate the transmissions of independent users in order to eliminate strong levels of interference (e.g., self-in- terference) that cannot be overcome by power control. On the other hand, power control is executed in a distributed fashion to determine the admissible power vector, if one exists, that can be used by the scheduled users to satisfy their single-hop transmission requirements. This is done for two types of networks, namely time-division multiple-access (TDMA) and TDMA/code-division multiple-access wireless ad hoc networks. Index Terms—Code-division multiple-access (CDMA), cross- layer protocol design, multiple access, power control, scheduling, time-division multiple-access (TDMA), wireless ad hoc networks. I. INTRODUCTION I T is well known that power is a precious resource in wire- less networks due to the limited battery life. This is further aggravated in ad hoc networks since all nodes are mobile ter- minals of limited weight and size. In addition, power control is of paramount importance to limit multiuser interference and, hence, maximize the spatial reuse of resources [1]. Power control has been studied extensively in the context of channelized cellular systems [2], [4], code-division multiple-ac- cess (CDMA)-based systems [7], and in a general framework [8]. Distributed iterative power control algorithms have been introduced for cellular systems and convergence results have been established [2], [4], [8]. More recently, there has been some focus on formulating the distributed power control problem as a noncooperative game [9]–[12]. In [12], the authors modified Manuscript received August 1, 2000; revised December 1, 2001; accepted February 1, 2003. The editor coordinating the review of this paper and approving it for publication is S. Tekinav. This work was supported by the Center for Satel- lite & Hybrid Communication Networks, a NASA Commercial Space Center (CSC) at the University of Maryland, under a NASA Cooperative Agreement NCC3-528. T. ElBatt is with the Information Sciences Lab, HRL Laboratories, LLC, Malibu, CA 90265 USA (e-mail: telbatt@hrl.com). A. Ephremides is with the Electrical and Computer Engineering De- partment, University of Maryland, College Park, MD 20742 USA (e-mail: tony@eng.umd.edu). Digital Object Identifier 10.1109/TWC.2003.819032 the power control problem formulation to incorporate the no- tions of utility and cost which are shown to improve the con- vergence characteristics of the algorithm. Our main objective in this paper is to develop a power control-based multiple access algorithm for contention-based wireless ad hoc networks. This is done via investigating the similarities and differences of this problem from the problem solved earlier for cellular networks. The concept of controlling the transmission radii in multihop packet radio networks was first introduced in [15]. They determined the optimal transmission radius (that maximizes the packet forward progress toward destination) under the constraint that the transmission powers for all nodes are the same. In [16], the authors developed a model for analyzing the throughput and forward progress where each mobile node may have a variable and different transmission range. Recently, the work in [17] employed transmission power as the link metric for shortest path routing algorithms in an attempt to realize the minimum-power routing algorithm discussed in [13]. However, the congestion caused by multiuser interference was not represented in the link metric. In [18], the authors employed transmission power adjustment in order to control the topology of wireless ad hoc networks. Unlike these studies, our work employs power control as part of the multiple access algorithm. Although the authors in [19] introduced a power control-based multiple access protocol, it was limited only to the class of carrier sense multiple access with collision avoidance (CSMA/CA) protocols. In this study, we introduce the notion of power control as part of a contention-based multiple access protocol that characterizes successful transmissions depending on a set of signal-to-interference-and-noise ratio (SINR) con- straints (which directly translates to quality of service (QoS) constraints on the bit-error rate (BER) at individual receivers). Moreover, there are no guarantees in [19] that the computed powers are minimum, while in our study we determine the minimum power vector subject to SINR constraints. On the other hand, the problem of scheduling nonconflicting transmissions, in order to achieve efficient spatial reuse, in TDMA multihop packet radio networks has received consider- able attention in the literature [20]–[27]. Two versions of this problem have been addressed, namely, broadcast scheduling [20], [23], [25], [26] and unicast scheduling [21], [23], [24]. However, the common limitation among these studies is the limited transmission range assumption where no interference is caused beyond that range. Accordingly, nodes that are more than two hops away are assumed to be conflict-free (i.e., do not cause interference to each other at their respective receivers). This, in turn, led to the strong connection between the trans- mission scheduling problem and graph-theoretic problems, 1536-1276/04$20.00 © 2004 IEEE