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Abstract-The literature of up-link SDMA systems is rich, but at the time of writing there is a paucity of information on the employment of SDMA techniques in the down-link. Hence, in this paper a Space Division Multiple Access (SDMA) down-link (DL) multi-user communication system invoking a novel low-complexity Maximum Likelihood (ML) space-time detection technique is proposed, which can be regarded as an advanced extension of the Complex Sphere Decoder (CSD). We demonstrate that as opposed to the previously published variants of the CSD, the proposed technique may be employed for obtaining a high effective throughput in the so-called "over-loaded" scenario, where the number of transmit antennas exceeds that of the receive antennas. The proposed method achieves the optimum performance of the ML detector even in heavily over-loaded scenarios, while the associated computational complexity is only moderately increased. As an illustrative example, the required Eb/N0 increased from 2 dB to 9 dB, when increasing the normalized system load from unity, representing the fully loaded system, to a normalized load of 1.556.

Device-to-device (D2D) communication, an emerging form of wireless communication, has attracted considerable attention, but an efficient synchronization protocol has not yet been developed. This has crucial implications for public safety applications that lack sufficient network infrastructure. In the public safety applications, the cellular network may not be available or fully functional when the base stations are malfunctioned or destroyed due to disasters, such as an earthquake, a tsunami, or an attack. One of the major features of D2D is to provide a self-organized communication network for emergency use. In this paper, we develop a synchronization protocol to assist mobile devices in a target area in establishing a synchronized network for public safety applications. The signal-to-interference-plus-noise ratio-based synchronization protocol proposed accounts for the dynamic physical layer reception effect, and its algorithm can efficiently designate D2D devices to assist in forwarding timing signals, thereby enhancing the coverage of the synchronous network. More importantly, our synchronization protocol and its algorithm may assist D2D devices in dynamically changing their state to adapt to the variant wireless channel conditions and dynamic topology of the network. In the simulation results, we show that the proposed synchronization protocol enabled more than 90% of the D2D devices to successfully synchronize with the network, whereas only 75% of D2D devices successfully synchronized with the network through the legacy synchronization protocol.

We analyze a precoded and iteratively detected downlink multiuser system employing imperfect Spatio-Temporal Channel Impulse Response at the Transmitters (ST-CIRT) with the aid of Extrinsic Information Transfer (EXIT) charts. A Precoded Iterative DownLink Space Division Multiple Access (PI-DL-SDMA) system proposed in our previous research is further improved and shown to be capable of maintaining an infinitesimally low BER, despite using imperfect ST-CIRT. A further novel feature of the PI-DL-SDMA system is that we design an IrRegular Convolutional Code (IRCC) with the aid of EXIT chart analysis for creating an open EXIT tunnel between the inner decoder's and outer decoder's EXIT curve at a reduced E b /N 0 value and hence maintain an infinitesimally low BER.

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