Oregon State University

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PhD Final Oral Examination – Younghoon Whang

Tuesday, May 17, 2016 9:00 AM - 11:00 AM

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.

First, 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.

Second, in order to extend the analysis thoroughly 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.

Last, a low-complexity symbol-level combining (SLC) scheme is developed for Chase combining based 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 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.

Major Advisor: Huaping Liu
Committee: Ben Lee
Committee: Thinh Nguyen
Committee: Jinsub Kim
GCR: Leonard Coop

Kelley Engineering Center (campus map)
Nicole Thompson
1 541 737 3617
Nicole.Thompson at oregonstate.edu
Sch Elect Engr/Comp Sci
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