732 IEEE TRANSACTIONS ON INFORMATION THEORY, VOL. 53, NO. 2, FEBRUARY 2007 Achieving Wireline Random Access Throughput in Wireless Networking Via User Cooperation Alejandro Ribeiro, Student Member, IEEE, Nikolaos D. Sidiropoulos, Senior Member, IEEE, Georgios B. Giannakis, Fellow, IEEE, and Yingqun Yu, Student Member, IEEE Abstract—Well appreciated at the physical layer, user coopera- tion is introduced here as a diversity enabler for wireless random access (RA) at the medium access control sublayer. This is accom- plished through a two-phase protocol in which active users start with a low power transmission attempting to reach nearby users and follow up with a high power transmission in cooperation with the users recruited in the first phase. We show that such a coop- erative protocol yields a significant increase in throughput. Specif- ically, we prove that for networks with a large number of users, the throughput of a cooperative wireless RA network operating over Rayleigh-fading links approaches the throughput of an RA network operating over additive white Gaussian noise links—thus justifying the title of the paper. The message borne out of this re- sult is that user cooperation offers a viable choice for migrating diversity benefits to the wireless RA regime, thus bridging the gap to wireline RA networks, without incurring a bandwidth or energy penalty. Index Terms—Fading channels, random access, user coopera- tion. I. INTRODUCTION O FFERING well-documented countermeasures against fading, diversity techniques find widespread applications in modern wireless systems. Such techniques capitalize on natural phenomena, e.g., the exploitation of multipath diversity in direct-sequence (DS) spread-spectrum (SS) channels [7], to receive/transmit antenna arrays [3], which require expen- sive additional radio frequency (RF) components (separate transmit/receive RF chains). User cooperation is a recently introduced diversity technique in which many single-antenna Manuscript received May 2, 2005; revised September 18, 2006. The mate- rial in this paper was prepared through collaborative participation in the Com- munications and Networks Consortium sponsored by the U.S. Army Research Laboratory under the Collaborative Technology Alliance Program, Cooperative Agreement DAAD19-01-2-0011. The U.S. Government is authorized to repro- duce and distribute reprints for Government purposes notwithstanding any copy- right notation thereon. The work of N. D. Sidiropoulos was supported in part by a bilateral cooperative research grant of the Greek Secretariat for Research and Technology. A. Ribeiro, G. B. Giannakis, and Y. Yu are with the Department of Electrical and Computer Engineering, University of Minnesota, Minneapolis, MN 55455 USA (e-mail: aribeiro@ece.umn.edu; georgios@ece.umn.edu; yyu@ece.umn. edu). N. D. Sidiropoulos is with the Department of Electrical and Computer Engi- neering, Technical University of Crete, Chania—Crete 731 00, Greece (e-mail: nikos@telecom.tuc.gr). Communicated by E. Modiano, Associate Editor for Communication Net- works. Color versions of Figs. 4, 5, 11, and 12 are available online at http://ieeex- plore.ieee.org. Digital Object Identifier 10.1109/TIT.2006.889718 users share their information to construct a distributed virtual antenna array—an idea that has gained rapid acceptance as a sensible compromise between dependability and deployment cost [21], [22]. User-collaborative diversity in fixed access point to point links is by now well understood (see, e.g., [9]). Recent works have also pursued user cooperation in multiple access channels [17], [20]. Particularly relevant to the present work is the notion that spatial separation in multiple-access channels allows the use of a shared channel for peer-to-peer communications whereby “good reception” opportunities of nearby idle users are exploited [20]. In the present paper, we introduce user cooperation in random access (RA) channels by drawing from two different sources. On the one hand, we draw from well-established spread-spectrum random-access (SSRA) protocols; see, e.g., [2], [8], [13] and references therein. And on the other hand, we draw from the observation that user cooperation can be viewed as a form of multipath, a type of diversity for which SS with long pseudonoise (PN) sequences used as spreading codes is particularly well suited [17]. An intuitive notion underlying the main results of this paper is that user cooperation is a form of diversity well matched to the very nature of RA networks. Indeed, the random nature of RA dictates that at any given time only a fraction of potential users is active, the others having either empty queues or their transmissions deferred. Accordingly, given that only a few out of the total number of transmitters are active at any given time, transmission hardware resources are inherently underutilized in wireless RA networks. As we will show, user cooperation can exploit these resources to gain a diversity advantage, without draining additional energy from the network and without band- width expansion. Reinforcing this intuitively reasonable notion, the number of temporarily idle users increases with the size of the network, indicating that user cooperation is available when most needed; for instance, in congested heavily populated net- works. While intuitive notions not always turn out to be true, this one will; the main purpose of this paper being precisely to es- tablish that as the network size increases, there is an increasing diversity advantage to be exploited leading to a limiting scenario in which the throughput of cooperative RA over wireless fading channels approaches that of an equivalent system operating over an additive white Gaussian noise (AWGN) channel. Building on an existing network diversity multi-access (NDMA) protocol [23], cooperative RA has been also con- sidered in [10], [27], [11], where retransmitting cooperators aid the separation of multiple collided packets. However, NDMA-based schemes are known to be challenged by channel 0018-9448/$25.00 © 2007 IEEE