Near-capacity co-located and distributed MIMO systems.
University of Southampton, School of Electronics and Computer Science,
Space-time transmission based colocated and distributed Multiple-InputMultiple-Output (MIMO) systems are investigated. Generally speaking, there are two types of fundamental gains, when using multiple antennas in wireless communications systems: the multiplexing gain and the diversity gain. Spatial multiplexing techniques such as the Vertical Bell-labs LAyered Space-Time (V-BLAST) scheme exploit the associated multiplexing gain in terms of an increased bit rate, whereas spatial diversity techniques such as Space-Time Coding (STC) aim for achieving a diversity gain, which results in a reduced error rate. Firstly, we concentrate our attention on a novel space-time transmission scheme, namely on Generalized Multi-Layer Space-Time Codes (GMLST), which may be viewed as a composite of V-BLAST and STC, hence they provide both multiplexing and diversity gains. The basic decoding procedure conceived for our GMLST arrangement is a certain ordered successive decoding scheme, which combines group interference nulling and interference cancellation. We apply a specifically designed power allocation scheme, in order to avoid the overall system performance degradation in the case of equal power allocation. Furthermore, the optimal decoding order is found, in order to enhance the system’s performance with the aid of the channel state information (CSI) at the receiver. However, our decoding scheme relying on power allocation or on the optimal decoding order does not take full advantage of the attainable receive antenna diversity. In order to make the most of this source of diversity, an iterative multistage Successive Interference Cancellation (SIC) detected GMLST scheme was proposed, which may achieve the full receive diversity after a number of iterations, while imposing only a fraction of the computational complexity of Maximum Likelihood (ML)-style joint detection.
Furthermore, for the sake of taking full advantage of the available colocated MIMO channel capacity, we present a low-complexity iteratively detected space-time transmission architecture based on GMLST codes and IRregular onvolutional Codes (IRCCs). The GMLST arrangement is serially concatenated with a Unity-Rate Code (URC) and an IRCC, which are used to facilitate near-capacity operation with the aid of an EXtrinsic Information Transfer (EXIT) chart based design. Reduced-complexity iterative multistage SIC is employed in the GMLST decoder instead of the significantly more complex ML detection. For the sake of approaching the maximum attainable rate, iterative decoding is invoked to achieve decoding convergence by exchanging extrinsic information across the three serially concatenated component decoders. Finally, it is shown that the iteratively detected IRCC-URC-GMLST scheme using SIC strikes an attractive trade-off between the complexity imposed and the effective throughput attained, while achieving a near-capacity performance.
The above-mentioned advances were also exploited in the context of near-capacity communications in distributed MIMO systems. Specifically, we proposed an Irregular Cooperative Space-Time Coding (Ir-CSTC) scheme, which combines the benefits of Distributed Turbo Codes (DTC) and serially concatenated schemes. Firstly, a serial concatenated scheme comprising an IRCC, a recursive URC and a STC was designed for the conventional single-relay-aided network for employment at the source node. The IRCC is optimized with the aid of EXIT charts for the sake of achieving a near-error-free decoding at the relay node at a minimum source transmit power. During the relay’s transmit period, another IRCC is amalgamated with a further STC, where the IRCC employed at the relay is further improved with the aid of a joint source-and-relay mode design procedure for the sake of approaching the relay channel’s capacity. At the destination node, a novel three-stage iterative decoding scheme is constructed in order to achieve decoding convergence to an infinitesimally low Bit Error Ratio (BER) at channel Signal-to-Noise Ratios (SNRs) close to the relay channel’s capacity. As a further contribution, an extended Ir-CSTC scheme is studied in the context of a twin-relay aided network, where a successive relaying protocol is employed. As a benefit, the factor two multiplexing loss of the single-relay-aided network - which is imposed by the creation of two-phase cooperation - is recovered by the successive relaying protocol with the aid of an additional relay. This technique is more practical than the creation of a full-duplex system, which is capable of transmitting and receiving at the same time. The eneralized joint sourceand-relay mode design procedure advocated relies on the proposed procedure of finding the optimal cooperative coding scheme, which performs close to the twin-relay-aided network’s capacity. The corresponding simulation results verify that our proposed Ir-CSTC schemes are capable of near-capacity communications in both the single-relay-aided and the twin-relay-aided networks.
Having studied diverse noise-limited single-user systems, we finally investigate a multiuser space divisionmultiple access (SDMA) uplink system designed for an unterferencelimited scenario, where the multiple access interference (MAI) significantly degrades the overall system performance. For the sake of supporting rank-deficient overloaded systems, a maximumsignal-to-interference-plus-noise ratio (MaxSINR) based SIC multiuser detection (MUD) algorithm is proposed for the multiple-antenna aided multi-user SDMA system, which is capable of striking a trade-off between the interference suppression and noise enhancement. Furthermore, the multiuser SDMA system is combined with channel codes, which assist us in eliminating the typical error floors of rank-deficient systems. Referring to the Ir-CSTC scheme designed for the single-user scenario, relaying techniques are invoked in our channel-coded SDMA systems, which benefit from extra spatial diversity gains. In contrast to the single-user Ir-CSTC schemes, interference suppression is required at both the base station (BS) and the relaying mobile station (MS). Finally, a more practical scenario is considered where the MSs have spatially correlated transmit antennas. In contrast to the conventional views, our simulation results suggest that the spatial correlation experienced at the transmitter is potentially beneficial in multiuser SDMA uplink systems, provided that efficient MUDs are invoked.
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