Design and Experimental Validation of MIMO Multiuser Detection for Downlink Packet Data
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Design and Experimental Validation of MIMO Multiuser Detection for Downlink Packet Data Dragan Samardzija Bell Laboratories, Lucent Technologies, Holmdel, NJ 07733, USA Email: [email protected]
Angel Lozano Bell Laboratories, Lucent Technologies, Holmdel, NJ 07733, USA Email: [email protected]
Constantinos B. Papadias Bell Laboratories, Lucent Technologies, Holmdel, NJ 07733, USA Email: [email protected] Received 8 March 2004; Revised 29 October 2004 In single-user MIMO communication, the first-order throughput scaling is determined by the smallest of the number of transmit and receive antennas. This typically renders terminals the constraining bottleneck. In a multiuser downlink, this bottleneck can be bypassed by having the base station communicate with multiple terminals simultaneously, in which case the receive antennas at those terminals are effectively pooled in terms of the capacity scaling. This, however, requires that the base have instantaneous channel information. Without such information, the structure and statistics of the channel can be exploited to form multiple simultaneous beams towards the various users, but these beams are in general mutually interfering. This paper proposes the use of multiuser detection to discriminate the signals conveyed over interfering beams. This approach is formulated and experimentally evaluated on an HSDPA MIMO testbed that involves a commercial base station, multiantenna terminals, and custom ASICs. Keywords and phrases: MIMO, HSDPA, UMTS, experimental validation.
1.
INTRODUCTION
MIMO (multiple-input multiple-output) schemes utilizing multiple transmit and receive antennas are posed to be a major ingredient in the evolutionary process of wireless communication. Widely recognized features associated with MIMO are spatial diversity, signal enhancements, interference mitigation, and spatial multiplexing. The latter, in particular, has driven a lot of the research over the last decade, ever since it was shown in [1, 2] that—in adequate channel conditions—the ergodic capacity (in bps/Hz) of a MIMO link as function of the average SNR (signal-to-noise ratio) behaves as
C(SNR) = min nT , nR log2 SNR +O(1),
(1)
where nT and nR denote the numbers of transmit and receive antennas, respectively. This linear scaling with the number of antennas is a powerful means to achieve high spectral utilization provided that antenna arrays can be effectively deployed. In actual wireless systems, of course, links do not operate in isolation: each base station must actively communicate
with a plurality of users and thus a number of MIMO links have to coexist. The behavior expressed by (1) can be immediately translated onto a multiuser environment by partitioning either time or frequency onto orthogonal sets, each of which is assigned to a particular user link. Focusing on the downlink, where nT indicates the number of transmit antennas at the base station while nR represents the number of receive antennas at the terminal, such orthogonal multiplexing incurs only a small loss in capacit
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