Seung Hwan Won

Non-coherent and differentially coherent code acquisition in MIMO assisted DS-CDMA multi-path downlink scenarios

In this contribution we investigate both Differentially Coherent (DC) and Non-Coherent (NC) code acquisition schemes in the Multiple Input Multiple Output (MIMO)-aided Direct Sequence-Code Division Multiple Access (DS-CDMA) downlink, when communicating over uncorrelated Rayleigh channels. It is demonstrated that the employment of multiple transmit antennas has a detrimental impact on the achievable diversity gain at typical operational Signal-to-Interference plus Noise Ratios (SINR), as the number of transmit antennas is increased, regardless whether single-path or multi-path scenarios are considered. Our findings suggest that increasing the number of transmit antennas in a MIMO-aided CDMA system results in increasing the Mean Acquisition Time (MAT) by as much as an order of magnitude, when the SINR per chip value is relatively low. The main reasons for the performance trends are plausible, since we have to reduce each individual MIMO elements signal power for maintaining the same total power as in a single-antenna system and this will be further justified by information theoretic considerations in the NC MIMO-aided scenarios considered.

Layered Multi-Group Steered Space-Time Shift-Keying for Millimeter-Wave Communications

We propose a novel multi-user multiple-input multiple-output (MIMO) technique termed the layered multi-group steered space-time shift keying (LMG-SSTSK) for the downlink of millimeter wave (mmWave) communications, which combines the concepts of multi-user MIMO, space-time shift keying, beamforming and orthogonal frequency-division multiplexing to simultaneously convey information to multiple users. The LMG-SSTSK tackles the propagation challenges of the high-attenuation mmWave frequencies by sub-dividing the users into multiple groups. The proposed system allows more users to be served simultaneously in the downlink over the same time- and frequency-resources than a system dispensing with the proposed grouping technique.

Multi-Set Space-Time Shift-Keying With Reduced Detection Complexity

In this paper, we propose a novel multi-set space-time shift keying (MS-STSK) technique, where the source bits are conveyed over two components, namely, by activating one out of MQ dispersion matrices of STSK and M antenna combination of the Nt available antennas, in a similar fashion to spatial modulation but activating multiple antennas rather than a single antenna. This system requires a smaller number of RF chains than antenna elements (AEs) to achieve an improved throughput. We opt for STSK as the main building block of our proposed MS-STSK as a benefit of its design flexibility, where STSK is capable of providing the system with both diversity and multiplexing gains. The proposed MS-STSK system achieves both higher data rates and a lower bit error rate than the conventional STSK scheme, which is attained at the cost of increased number of AEs and a high-speed antenna switch. Furthermore, as the symbol rate of the MS-STSK system increases, its performance compared with the STSK scheme is significantly improved because of the enhanced diversity introduced by the additional AEs. Furthermore, we propose a reduced complexity detector for Quadrature Amplitude Modulation (QAM)/Phase Shift Keying (PSK) constellations, which-despite its reduced complexity-is capable of achieving the same performance as the optimal maximum likelihood detector.

Initial Synchronisation of Wideband and UWB Direct Sequence Systems: Single- and Multiple-Antenna Aided Solutions

This survey guides the reader through the open literature on the principle of initial synchronisation in single-antenna-assisted single- and multi-carrier Code Division Multiple Access (CDMA) as well as Direct Sequence-Ultra WideBand (DS-UWB) systems, with special emphasis on the DownLink (DL). There is a paucity of up-to-date surveys and review articles on initial synchronization solutions for MIMO-aided and cooperative systems - even though there is a plethora of papers on both MIMOs and on cooperative systems, which assume perfect synchronization. Hence this paper aims to fill the related gap in the literature.

Initial Code Acquisition in the Cooperative Noncoherent MIMO DS-CDMA Downlink

In this paper, we investigate a realistic code-acquisition-assisted cooperative noncoherent (NC) multiple-input-multiple-output (MIMO) direct-sequence code-division multiple-access (DS-CDMA) downlink scenario, when communicating over uncorrelated single- and multipath Rayleigh channels. The probabilities of correct detection and false alarm have analytically been derived. Furthermore, a mean acquisition time (MAT) formula is provided for the cooperative transmission scenario considered. The associated MAT performance trends are characterized as a function of both the number of relay stations (RSs) and the number of receive antennas and link imbalance. As opposed to the classic scenario of having colocated MIMO elements, our findings suggest that employing distributed MIMO elements acting as RSs combined with multiple receive antennas leads to an improved MAT performance.

Analysis of Serial-Search-Based Code Acquisition in the Multiple-Transmit/Multiple-Receive-Antenna-Aided DS-CDMA Downlink

In this paper, we investigate the serial-search-based initial code-acquisition performance of direct-sequence code division multiple access (DS-CDMA) employing multiple transmit/multiple receive antennas when communicating over uncorrelated Rayleigh channels. We characterize the associated performance trends as a function of the number of antennas. It is demonstrated that, in contrast to our expectation, the achievable correct-detection probability degrades in our typical target operational range as the number of transmit antennas is increased. When maintaining a given total transmit power, our findings suggest that increasing the number of transmit antennas results in the combination of the low-energy noise-contaminated signals of the transmit antennas, which ultimately increases the mean acquisition time (MAT). However, it is extremely undesirable to increase the MAT when the system is capable of attaining its target bit-error-ratio performance at reduced signal-power levels, as a benefit of employing multiple transmit antennas.

Initial and Post-Initial Acquisition in the Serial Search Based Noncoherent Multiple Transmit/Receive Antenna Aided DS-CDMA Downlink

In this paper we investigate the issues of both initial and post-initial acquisition schemes in the multiple transmit/receive antenna aided DS-CDMA downlink, when communicating over uncorrelated Rayleigh channels. The associated mean acquisition time (MAT) performance trends are characterised as a function of the number of transmit/receive antennas. Furthermore, we characterise both the initial and post-initial acquisition performance as a function of the relevant system parameters. It is demonstrated that in contrast to our expectations, the achievable MAT degrades at low Ec/Io values, except for the case of P = 2 transmit antennas operating in conjunction with R = 1 receive antenna over the specific signal-to-interference plus noise ratio (SINR) per chip (Ec/I o) range considered, as the number of transmit antennas is increased. Ironically, our findings suggest that increasing the number of transmit antennas in a MIMO-aided CDMA system results in combining the low-energy, noise-contaminated signals of the transmit antennas, which ultimately increases the MAT by an order of magnitude, when the SINR is relatively low. This phenomenon has a detrimental effect on the performance of Rake receiver based synchronisation schemes, when the perfectly synchronised system is capable of attaining its target bit error rate performance at reduced SINR values, as a benefit of employing multiple transmit antennas. Therefore our future research will be focused on specifically designing acquisition schemes for MIMO systems

Initial and Post-Initial Code Acquisition in the Noncoherent Multiple-Input/Multiple-Output-Aided DS-CDMA Downlink

In this paper, we investigate the issues of both initial and post-initial acquisition schemes in the multiple-input/multiple-output (MIMO)-aided direct-sequence code-division multiple-access (DS-CDMA) downlink when communicating over spatially uncorrelated Rayleigh channels. The associated mean acquisition time (MAT) performance trends are characterized as a function of the number of MIMO elements. Furthermore, we characterize both the initial and post-initial acquisition performance as a function of the relevant system parameters. Our findings suggest that increasing the number of transmit antennas in a MIMO-aided CDMA system results in combining the low-energy noise-contaminated signals of the transmit antennas, which ultimately increases the MAT by an order of magnitude when the signal-to-interference-plus-noise ratio (SINR) is relatively low, regardless of whether single- or multipath scenarios are considered. This phenomenon has a detrimental effect on the performance of Rake-receiver-based synchronization schemes when the perfectly synchronized system is capable of attaining its target bit-error-rate performance at reduced SINR values, as a benefit of employing multiple transmit antennas. Based on our analysis justified by information-theoretic considerations, our acquisition design guidelines are applicable to diverse noncoherent (NC) MIMO-assisted scenarios.

Multiuser Steered Multiset Space-Time Shift Keying for Millimeter-Wave Communications

The recently proposed concept of multiset space-time shift keying (MS-STSK) is intrinsically amalgamated with the multiple-input multiple-output (MIMO) philosophy for the sake of enhancing the attainable system throughput. Explicitly, we propose a multiuser steered MS-STSK (MU-SMS-STSK) scheme for the downlink of millimeter-wave (mmWave) communications, which is combined with analogue beamforming (BF) that relies on phase shifters and power amplifiers to overcome the high attenuation of mmWaves. Hence, our MU-SMS-STSK system combines the concepts of MU-MIMO, MS-STSK, BF, and orthogonal frequency-division multiplexing for communicating with multiple users relying on the same time and frequency resources.

Synchronization issues in relay-aided cooperative MIMO networks

In this article, we address some of the key design issues of relay-aided cooperative noncoherent MIMO downlink synchronization schemes conceived for communicating over multi-path fading channels. We demonstrate that an explicit relationship exists between the achievable timing acquisition performance and the schemes employed by a multihop aided initial synchronization procedure, which is capable of improving the cellular coverage. We conclude by discussing the relationships between the factors either beneficially or detrimentally affecting the achievable performance as well as providing guidelines for designing relay-aided cooperative non-coherent MIMO aided schemes.