Multiuser detection of overloaded systems employing HARQ

Abstract

We propose two schemes to perform multiuser detection (MUD) for multiuser multiple-input multiple-output (MIMO) systems under overload. Typically linear MUD algo rithms fail to perform under overload and maximum likelihood detection is often prohibitively complex. In this paper, we use hybrid automatic repeat request (HARQ) retransmissions to transform an overloaded system into a critically or underloaded system based on the concept of virtual receive antennas. Hence, low complexity linear MUD algorithms can be used and achieve diversity gains. Simulation results clearly highlight the advan tages obtained by the proposed schemes.Index Terms-MIMO, HARQ, Multiuser detection. I. IN TRODUCTIONThe performance and reliability of wireless communication systems can be improved by employing multiple antennas at both ends, thereby creating multiple-input multiple-output (MIMO) channels. However, once multiple co-channel users are added to the system it can be difficult to provide as many receive antennas, Nn as transmit antennas, N t , resulting in an overloaded (Nr < N t ) condition. Multiuser detection (MUD) algorithms are used to detect the data transmitted from all co-channel users. Co-channel inter ference (CCI) introduced by multiple users severely degrades the performance of linear MUD algorithms under overloaded conditions compared to critically loaded (Nr = N t ) and underloaded (Nr > N t ) conditions [1], [2]. Joint maximum likelihood (JML) [3] detection works well under overloaded conditions. However, there is an exponential growth in com plexity with the number of transmit antennas, making it less feasible for practical applications. To overcome this limitation, reduced-complexity MUD algorithms have been developed for overloaded scenarios [4], [5], [6]. These algorithms demand sophisticated implementation and still have quite high com putational complexity. Furthermore, these algorithms exhibit error floors at high signal-to-noise ratio (SNR). The primary focus of this paper is to combine hybrid automatic repeat request (HARQ) [7] with the use of virtual receive antennas [8] and a simple MUD algorithm resulting in a simple and novel multiuser transmission approach that works well under overloaded conditions. Most previous research on MIMO HARQ systems [9] -[10] has focused on critically or underloaded conditions. In [9], a symbol-level packet combining scheme was proposed, based on uncertainty processing to improve the decoding performance in MIMO HARQ systems. In [10], a linear MIMO HARQ precoder was designed to optimize the mutual information, and joint HARQ detection was performed by stacking the received vectors from all (re-)transmissions. The same signal vector was transmitted from all antennas during all (re-)transmissions. To the best of our knowledge, research to date has not considered MIMO HARQ in overloaded conditions. Here we consider MUD and HARQ techniques for overloaded systems. We describe two new schemes which stack the received vector from all (re-)transmissions. Unlike the work of [10], sta...

Multiuser Detection of Co-Channel Systems Using Combination of Basic Network Coding and HARQ

“…The main theme behind this scheme is to treat NC based HARQ retransmission as if it was arriving at additional N r receive antennas [7] and avoiding multiple retransmissions with the help of NC. Equation (1) can be redefined as for the g th re-transmission Y(g) = H(g)x(g) + v(g), g = 1, 2, ..., G…”

“…Applying the above equations on the multi-user scenario, the same generality holds allowing us to remove notions and we can write the packet drop rate as P AB = P BC = P rate . This packet drop rate can be defined as [7] P rate = (Number o f packets dropped) (Total Number o f transmitted packets (N avg )) (11) And it can be combined with the spectral efficiency to define the throughput (η) of the above system model as [7] …”

Multiuser Detection Using Hybrid ARQ with Incremental Redundancy in Overloaded MIMO Systems (Workshop Paper)

“…Zahid et al [12] in his work used the concept of HARQ CC and combined all the vectors at receiver end by considering them as a single virtual receive antenna [16]. We proposed a different approach by modifying his method and used the concept of HARQ IR in overloaded MIMO system.…”

“…The proposed scheme is evaluated in this section and compared with HARQ CC [12] in order to confirm its effectiveness using bit error rate (BER) and throughput as performance parameters in various simulations scenarios. In simulations, four users are considered.…”

“…Moreover, reliability is increased by retransmitting incremental redundancy packets on requirement basis only. Similarly, Zahid et al [12] focused on an overloaded MIMO scenario and presented schemes of converting an overloaded MIMO systems (N t > N r ) into critically loaded MIMO systems (N t = N r ) or under loaded systems (N t < N r ) by combining all retransmissions at receiver end using HARQ [13] and HARQ Chase Combining (CC) [10] techniques.…”