High Capacity Downlink Transmission with MIMO Interference Subspace Rejection in Multicellular CDMA Networks
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High Capacity Downlink Transmission with MIMO Interference Subspace Rejection in Multicellular CDMA Networks Henrik Hansen INRS-T´el´ecommunications, Universit´e du Qu´ebec, Place Bonaventure, 800 de la Gaucheti`ere Ouest, Suite 6900, Montr´eal, Qu´ebec, Canada H5A 1K6 Email: [email protected]
` Affes Sofiene INRS-T´el´ecommunications, Universit´e du Qu´ebec, Place Bonaventure, 800 de la Gaucheti`ere Ouest, Suite 6900, Montr´eal, Qu´ebec, Canada H5A 1K6 Email: aff[email protected]
Paul Mermelstein INRS-T´el´ecommunications, Universit´e du Qu´ebec, Place Bonaventure, 800 de la Gaucheti`ere Ouest, Suite 6900, Montr´eal, Qu´ebec, Canada H5A 1K6 Email: [email protected] Received 31 December 2002; Revised 18 September 2003 We proposed recently a new technique for multiuser detection in CDMA networks, denoted by interference subspace rejection (ISR), and evaluated its performance on the uplink. This paper extends its application to the downlink (DL). On the DL, the information about the interference is sparse, for example, spreading factor (SF) and modulation of interferers may not be known, which makes the task much more challenging. We present three new ISR variants which require no prior knowledge of interfering users. The new solutions are applicable to MIMO systems and can accommodate any modulation, coding, SF, and connection type. We propose a new code allocation scheme denoted by DACCA which significantly reduces the complexity of our solution at the receiving mobile. We present estimates of user capacities and data rates attainable under practically reasonable conditions regarding interferences identified and suppressed in a multicellular interference-limited system. We show that the system capacity increases linearly with the number of antennas despite the existence of interference. Our new DL multiuser receiver consistently provides an Erlang capacity gain of at least 3 dB over the single-user detector. Keywords and phrases: CDMA, downlink multiuser detection, interference rejection, space-time processing, code allocation, MIMO.
1.
INTRODUCTION
Third generation wireless systems will deploy wideband CDMA (W-CDMA) [1, 2] access technology to achieve data transmission at variable rates. Standards [1] call for transmission rates up to 384 Kbps for mobile users and 2 Mbps for portable terminals. On the downlink (DL), high-speed DL packet access (HSDPA) [3, 4] allows for transmission rates up to about 10 Mbps in the conventional single-input singleoutput (SISO) channel and about 20 Mbps in the multipleinput multiple-output (MIMO) channel. It is expected that most of the traffic will be DL due to asymmetrical services like FTP and web browsing. The DL will therefore become
the limiting link, and only high DL performance can give the network operator maximal revenue from advanced radionetwork technologies. MIMO [5] and multiuser detection (MUD) [6, 7, 8] are both very promising techniques for high capacity on the DL in wireless systems. In a noise-limited MIMO system, Shannon capacities incre
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