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PhD Oral Preliminary Examination – Younghoon Whang


Thursday, March 19, 2015 12:30 PM - 2:30 PM

Transmission and Combining for Hybrid Automatic Repeat Request
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.

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.

Last, the pre-combining scheme outperforms the post-combining scheme for Chase combining based hybrid automatic repeat request (CC-HARQ) in MIMO systems at the price of the complicated detection structure. It is shown that the pre-combining scheme achieves the identical performance to the brute-force combining scheme with a reduced computational complexity. To reduce the complexity, the QR decomposition based symbol-level combining (SLC) scheme had been proposed, which has the same performance as the pre-combining scheme with a reduced computational complexity for the maximum-likelihood (ML) detection. However, if a linear detection is considered at the receiver, it still requires an additional computational complexity compared to other existing SLC schemes. Therefore, we propose a low-complexity SLC scheme for CC-HARQ in MIMO systems with linear detection. In the proposed SLC scheme, the Sherman-Morrison-Woodbury matrix inversion lemma is used for operations of matrix about a part of the receive symbols instead of direct inversion of the matrix during the detection and combining processes. We show the complexity order of existing and proposed SLC schemes in MIMO systems with CC-HARQ and linear detection briefly as one of further investigations.

Major Advisor: Huaping Liu
Committee: Ben Lee
Committee: Raviv Raich
Committee: Jinsub Kim
GCR: Oksana Ostroverkhova


Kelley Engineering Center (campus map)
3114
Nicole Thompson
1 541 737 3617
Nicole.Thompson at oregonstate.edu
Sch Elect Engr/Comp Sci
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