PhantomNet: Exploring Optimal Multicellular Multiple Antenna Systems
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PhantomNet: Exploring Optimal Multicellular Multiple Antenna Systems Syed A. Jafar Electrical Engineering and Computer Science, University of California, Irvine, Irvine, CA 92697-2625, USA Email: [email protected]
Gerard J. Foschini Bell Laboratories, Lucent Technologies, 791 Holmdel-Keyport Road, Holmdel, NJ 07733, USA Email: [email protected]
Andrea J. Goldsmith Wireless Systems Laboratory, Stanford University, Stanford, CA 94305-9505, USA Email: [email protected] Received 20 December 2002; Revised 11 August 2003 We present a network framework for evaluating the theoretical performance limits of wireless data communication. We address the problem of providing the best possible service to new users joining the system without affecting existing users. Since, interferencewise, new users are required to be invisible to existing users, the network is dubbed PhantomNet. The novelty is the generality obtained in this context. Namely, we can deal with multiple users, multiple antennas, and multiple cells on both the uplink and the downlink. The solution for the uplink is effectively the same as for a single cell system since all the base stations (BSs) simply amount to one composite BS with centralized processing. The optimum strategy, following directly from known results, is successive decoding (SD), where the new user is decoded before the existing users so that the new users’ signal can be subtracted out to meet its invisibility requirement. Only the BS needs to modify its decoding scheme in the handling of new users, since existing users continue to transmit their data exactly as they did before the new arrivals. The downlink, even with the BSs operating as one composite BS, is more problematic. With multiple antennas at each BS site, the optimal coding scheme and the capacity region for this channel are unsolved problems. SD and dirty paper (DP) are two schemes previously reported to achieve capacity in special cases. For PhantomNet, we show that DP coding at the BS is equal to or better than SD. The new user is encoded before the existing users so that the interference caused by his signal to existing users is known to the transmitter. Thus the BS modifies its encoding scheme to accommodate new users so that existing users continue to operate as before: they achieve the same rates as before and they decode their signal in precisely the same way as before. The solutions for the uplink and the downlink are particularly interesting in the way they exhibit a remarkable simplicity and an unmistakable, near-perfect, up-down symmetry. Keywords and phrases: channel capacity, dirty paper coding, duality, broadcast channel, successive decoding, multiple-input multiple-output systems.
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INTRODUCTION
The rapid growth of cellular networks and the anticipation of ever increasing demand for higher data rates have expanded the scope of wireless research from single user, and single cell, and single antenna systems to multiuser multicellular systems employing multiple antennas. A traditional way of handling the multiantenna, multiu
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