Multiple ARQ Processes for MIMO Systems
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Multiple ARQ Processes for MIMO Systems Haitao Zheng Wireless Research Laboratory, Lucent Technologies, 791 Holmdel-Keyport Road, Holmdel, NJ 07733, USA Email: [email protected]
Angel Lozano Wireless Research Laboratory, Lucent Technologies, 791 Holmdel-Keyport Road, Holmdel, NJ 07733, USA Email: [email protected]
Mohamed Haleem Wireless Research Laboratory, Lucent Technologies, 791 Holmdel-Keyport Road, Holmdel, NJ 07733, USA Email: [email protected] Received 4 December 2002; Revised 19 August 2003 We propose a new automatic repeat request (ARQ) scheme for MIMO systems with multiple transmit and receive antennas. The substreams emitted from various transmit antennas encounter distinct propagation channels and thus have different error statistics. When per-antenna encoders are used, separating ARQ processes among the substreams results in a throughput improvement. Moreover, it facilitates the interference cancellation in certain MIMO techniques. Quantitative results from UMTS simulations demonstrate that the proposed multiple ARQ structure yields more than 30% gain in link throughput. Keywords and phrases: MIMO systems, automatic repeat request, throughput, wireless communication, UMTS.
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INTRODUCTION
Third-generation cellular systems are being designed to support high-speed packet data services. In the downlink, which has more stringent requirements in many of such services, high-speed packet access is provided through a shared channel where time-division multiplexing is used. Time slots are assigned to users at specific data rates through a scheduling algorithm based on the user data backlog and on channel quality indication (CQI) received via a feedback channel.1 Such a transmission scheme allows multiple users to share the system resources efficiently by adapting to traffic and channel variations and it also avoids possible resource limitations that might occur if each user were allocated a dedicated code-multiplexed channel. Therefore, it has the potential to improve the capacity for delay-tolerant bursty services. Examples where this scheme will be implemented include the CDMA 1x EV-DO and 1x EV-DV and the UMTS high-speed downlink packet access (HSDPA) [1, 2]. Several advanced technologies are employed in high-speed downlink transmission to improve link throughput or reduce packet delay by adapting to the time-varying channel conditions, 1 Each terminal measures its channel condition and translates it into a metric to be fed back to the serving base station.
traffic statistics, and quality-of-service requirements. Some of these adaptive techniques, relevant to this paper, are summarized below. Multiple transmit and receive antennas. The use of multiple antennas at each base station sector is already part of every third-generation standard. In the downlink, specifically, these antennas can be used to provide transmit diversity and/or to direct a beam towards the intended terminal. The deployment of multiple receive antennas at data terminals is also being considered. The combination of multiple transmit and recei
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