Junsu Kim

Mimicking Full-Duplex Relaying Using Half-Duplex Relays With Buffers

We propose a new relaying scheme referred to as space full-duplex max-max relay selection (SFD-MMRS), which uses relay selection and half-duplex (HD) relays with buffers to mimic full-duplex (FD) relaying. SFD-MMRS allows the selection of different relays for reception and transmission, which, in turn, enables the relays selected for reception and transmission to simultaneously receive and transmit. With SFD-MMRS, the prelog factor 1/2 is removed from the capacity expression, and better performance in terms of both throughput and outage probability is achieved. We provide a comprehensive analysis of the capacity and outage probability of the proposed scheme for a decode-and-forward (DF) protocol in Rayleigh fading. This analysis reveals that the proposed scheme provides better performance, compared with HD MMRS and HD best relay selection (BRS). Moreover, our simulation results show that the capacity of the proposed scheme with HD relays exceeds twice the capacity of BRS with HD relays for any number of relays. Furthermore, the proposed scheme provides full diversity and large signal-to-noise ratio (SNR) gains, compared with competing schemes in the literature.

Combined relay selection and cooperative beamforming for physical layer security

In this paper, we propose combined relay selection and cooperative beamforming schemes for physical layer security. Generally, high operational complexity is required for cooperative beamforming with multiple relays because of the required information exchange and synchronization among the relays. On the other hand, while it is desirable to reduce the number of relays participating in cooperative beamforming because of the associated complexity problem, doing so may degrade the coding gain of cooperative beamforming. Hence, we propose combined relay selection and cooperative beamforming schemes, where only two of the available relays are selected for beamforming and data transmission. The proposed schemes introduce a selection gain which partially compensates for the decrease in coding gain due to limiting the number of participating relays to two. Both the cases where full and only partial channel state information are available for relay selection and cooperative beamforming are considered. Analytical and simulation results for the proposed schemes show improved secrecy capacities compared to existing physical layer security schemes employing cooperative relays.

Diversity Analysis of Multi-User Multi-Relay Networks

In this paper, we analyze the diversity order of opportunistic scheduling networks with arbitrary numbers of relays and users. We show that the opportunistic selection of the relay-user pair with the best end-to-end signal-to-noise ratio (SNR) among M relays and N users achieves a diversity order in the range of [M + N, MN + N] for amplify-and-forward (AF) relays and in the range of [N, MN + N] for decode-and-forward (DF) relays. Our analysis reveals that the achievable diversity order with AF relays depends on the relative strength of the source-relay (SR) and relay-destination (RD) links, while the achievable diversity order with DF relays depends on the SR link SNR. Based on our analysis, which is verified by simulation results, we show that, for AF relays, the maximum diversity order of MN + N is achieved if the SR link quality is better than the RD link quality, and, for DF relays, if the SR link is sufficiently strong such that the relays always succeed in decoding.

Optimization of Wireless Multicast Systems Employing Hybrid-ARQ with Chase Combining

The throughput of conventional wireless multicast systems is limited by the multicast user with the lowest channel quality, which leads to a low throughput, particularly if the number of multicast users is large. In this paper, we show that hybrid-automatic repeat request with Chase combining (HARQ-CC) is a promising technique to overcome this problem and optimize the corresponding transmission rate. We analyze the throughput of wireless multicast systems with HARQ-CC under various channel conditions and derive a closed-form approximation for the optimal rate. The numerical evaluation of our analytical expressions reveals that HARQ-aided multicast outperforms conventional multicast in most channel environments. To further improve performance, we propose a dynamic rate-allocation scheme that combines the advantages of conventional multicast and HARQ-aided multicast.

Hybrid Scheduling Algorithm for Guaranteeing QoS of Real-Time Traffic in High Speed Uplink Packet Access (HSUPA)

As a demand for high speed uplink packet services increases, the WCDMA enhanced uplink, also known as high speed uplink packet access (HSUPA), has been specified in release 6 by 3GPR The performance of the HSUPA has been improved using fast scheduling, hybrid ARQ, and shorter TTI. This HSUPA will provide various types of multimedia services, such as realtime video streaming, gaming, VoIP, and FTP. Generally, the performance of HSUPA is dominated by scheduling policy. Therefore, it is required to design a scheduling algorithm considering the traffic characteristics to provide QoS guaranteed services in various traffic environments. In this paper, we propose a scheduling algorithm considering the traffic characteristics to guarantee QoS in a mixed traffic environment. Finally, the performance of the proposed scheduling algorithm is evaluated in terms of average packet delay, packet delay jitter, and system throughput using a system level simulator.

Performance Analysis of Orthogonal Frequency and Code Hopping Multiplexing

Beyond-3G wireless communication systems should support packet traffic with both low/medium rate and high rate services. Scheduling-based resource allocation schemes may cause a problem in accommodating low/medium rate services due to a rate mismatch and a large signaling overhead. Dedicated resource allocation/multiplexing schemes are also inadequate for accommodating packet-type traffic that is characterized by low channel activity and high burstiness. Therefore, we propose a new orthogonal frequency and code hopping multiplexing (OFCHM) scheme for downlink multiplexing to provide efficient support of packet traffic with low/medium data rates in a spread OFDM environment. The performance of the proposed scheme is evaluated in terms of the statistical multiplexing gain, the block error rate, and the overall capacity by using both an analytical approach and simulation. The proposed scheme yields a larger overall downlink capacity than a dedicated resource allocation/multiplexing scheme. A comparison study with the variable spreading factor-orthogonal frequency and code division multiplexing (VSF-OFCDM) system through system-level simulations is also performed. The comparison results show that the OFCHM system yields better performance than the VSF-OFCDM system in packet delay, packet delay variance, and fairness, and has a system throughput advantage with low/medium rate services.

Combined antenna selection and beamforming for secure transmission in distributed antenna systems

In this paper, we propose two combined antenna selection and beamforming schemes: eavesdropper channel nulling (ECN) and distributed phase alignment (DPA), for physical layer security in distributed antenna systems. Two out of all available distributed antennas are selected for secure transmission which is the minimum setting for implementing beamforming. Among all possible two-antenna combinations, the best one that shows the maximum secrecy rate is selected and, therefore, provides a selection gain. ECN cancels the received signal at an eavesdropper when the distributed antennas are subject to a total power constraint. As the distributed antennas may have individual power constraints in practice, we also propose DPA which aligns the phases of the received beams at the eavesdropper to minimize the received signal strength. For ECN with an average total power constraint, we propose an analysis model to track the achievable secrecy rate. Numerical results show that ECN exhibits a closed performance as the optimal beamforming with all distributed antennas.

Multi-QoS scheduling algorithm for class fairness in high speed downlink packet access

Since there are various packet-type services with different QoSs , a scheduling algorithm that can support multiple QoS classes is needed to efficiently provide data services in the high speed downlink packet access (HSDPA). In this paper, we investigate the capability of HSDPA to support multiple QoS classes and propose an improved scheduling algorithm to serve multiple QoS classes

Multi-user diversity gain in uplink cognitive radio systems with spectrum sensing reliability

In this paper, we investigate the performance of opportunistic scheduling in uplink cognitive radio (CR) systems taking into account the spectrum sensing reliability of each CR user. In uplink CR systems, each secondary transmitter (CR user) has a spectrum sensing responsibility to protect the primary system, and the secondary receiver (base station) schedules the transmission opportunity for each CR user. Unlike non-CR multi-user systems, the spectrum sensing reliability may degrade the achievable multi-user diversity (MUD) gain. We propose novel scheduling schemes taking into account the spectrum sensing reliability and data channel quality simultaneously to improve the MUD gain in uplink CR systems.

Optimal rate allocation for wireless multicast systems employing hybrid-ARQ with Chase combining

The throughput of conventional wireless multicast systems is limited by the multicast user with the lowest channel quality, which leads to a very low throughput especially if the number of users is large. In this paper, we show that hybrid-automatic repeat request with Chase combining (HARQ-CC) is a promising technique to overcome this problem and optimize the transmission rate of wireless multicast systems with HARQ-CC. We analyze the throughput of wireless multicast systems with HARQ-CC and derive a closed-form approximation for the optimal rate. To further improve performance, we propose a dynamic rate allocation scheme which combines the advantages of conventional multicast and HARQ-aided multicast.