Relay Switching Aided Turbo Coded Hybrid-ARQ for Correlated Fading Channel

Hybrid-Automatic-Repeat-reQuest (HARQ) has become an indispensable technique in reliable communications systems. However, its performance is inevitably affected by the channel’s fading correlation. In this paper, we proposed a novel relay-switching aided HARQ scheme in order to mitigate the detrimental effects of correlated fading without unduly increasing the system’s complexity and delay. Our results show that the proposed relay-switching regime operates efficiently in correlated channels, hence significantly reduces the error floor of turbo-coded HARQ. Additionally, a HARQ scheme using Segment Selective Repeat (SSR) is incorporated in the relay-switching scheme for achieving further improvements. Quantitatively, the proposed relay-switching aided turbo-coded HARQ scheme using SSR may achieve an approximately 2 dB gain, compared to the conventional amplify-and-forward aided turbo coded HARQ arrangement using Chase Combining. Index Terms - Relay switching, correlated fading channel, Hybrid-ARQ, turbo codes, chase combining, incremental redundancy, selective segment repeat

Robust scheduling algorithm for Guaranteed Bit Rate services

This paper proposes a novel packet scheduling algorithm to overcome detrimental effects of channel impairments on the quality of service of delay-sensitive Guaranteed Bit Rate (GBR) services. The proposed algorithm prioritises the packets that require retransmission of Hybrid Automatic Repeat Request (HARQ) users compared to the packets of new users. The packets of new users are scheduled according to the Channel Quality Information (CQI), average throughput and packet delay information. Computer simulations have demonstrated that the proposed algorithm has 22.7% system capacity improvement over a well-known algorithm. It also tolerates for up to 200% delay of CQI and reduces the uplink signalling overhead by 150% compared to the well-known algorithm without compromising the quality of service requirements of the GBR services. Copyright © 2013 Inderscience Enterprises Ltd

Transmission and Combining for Hybrid Automatic Repeat Request in Multiple-Input Multiple-Output Systems

Hybrid automatic repeat request (HARQ) schemes combine packet retransmission with forward error correction to ensure a reliable communications. In multiple-input multiple output (MIMO) systems, interference cancellation (IC) detection is widely used where the detection and cancellation steps of the simultaneously transmitted data streams occur. In principle, the signal stream estimated at one IC stage is utilized to cancel the interference of other signal streams at the next IC stage. Thus, the detection probabilities of the transmitted data streams are mutually dependent. With HARQ, the detection performance of a packet also depends on how many times the packet has been retransmitted. The dissertation consists of three main contributions. Firstly, we develop a HARQ transmission state control algorithm for MIMO systems with IC detection to improve throughput. The HARQ transmission state is defined as the distribution of the initial packets and retransmission packets transmitted during a packet transmission time interval (PTTI). The proposed algorithm generates the transmission state in which initial packets and retransmission packets are sent together. The outcome is that it achieves a lower error probability for initial packets by exploiting the IC process and a significantly higher throughput than the conventional HARQ system, which is verified by simulation results. However, the maximum allowable number of retransmission is limited to one in this algorithm. Secondly, in order to extend the analysis for a more general case, we define the concept of the effective interference level (EIL) as the performance parameter to choose the set of packets during one PTTI and establish a relationship between EIL and the effective signal-to-interference-plus-noise ratio (SINR). We then show that choosing the set of packets that minimize the EIL successively from the lowest to the highest HARQ round leads to a lower packet error and higher throughput than conventional HARQ, which is verified by simulation. Also, the proposed EIL based scheme uses only the acknowledgement feedback messages like a conventional HARQ, because the number of HARQ rounds of each packet is the only required information to calculate the EIL. Simulation results highlight the superiority of the proposed scheme over the conventional scheme in terms of throughput with the signal-to-noise ratio gain of about 4.2 dB at maximum for MIMO systems with four transmit and four receive antennas. Thirdly, a low-complexity symbol-level combining (SLC) scheme is developed for Chase combining based HARQ (CC-HARQ) in MIMO systems, when the linear detection is considered at the receiver. In the proposed scheme, instead of using the entire channel matrix as in the existing SLC schemes, a subset of row vectors in the channel matrix is selected in the proposed scheme, and the selected row vectors are sequentially used during the estimation procedures of the retransmitted symbols, where the sequential utilization is enabled by using the Sherman-Morrison-Woodbury (SMW) lemma. Therefore, according to the number of the selected row vectors, this approach enables the proposed SLC scheme to have an advantage in complexity compared to the existing SLC schemes. In addition, we develop a row vector selection criterion for the proposed scheme to compute the amount of the SINR improvement by using a squared norm of each row vector with a significantly lower computational complexity. Simulation results show that compared to the existing SLC schemes, the proposed SLC scheme achieves similar or better error performance, while its computational complexity is lower or in the worst case similar