Wooram Shin

Efficient 2-D mesh election algorithms for OFDMA-based wireless mesh networks

Recently, wireless mesh networks (WMNs) have received enormous attention for wireless backbone network as well as sensor network due to its flexibility and cost-effectiveness in deployment. Particularly, IEEE 802.16-2004 based on time division multiple access (TDMA) with orthogonal frequency division multiplexing (OFDM) air interface supports mesh mode operation for last-mile connection to backhaul networks by means of distributed control functionalities. For the distributed operation, network and scheduling information is conveyed via control channels under a channel access protocol, named mesh election (ME). Meanwhile, in order to provide high data rate multimedia services, orthogonal frequency division multiple access (OFDMA) has been widely accepted by many proponents in the standardization parties such as 3GPP and IEEE 802.16. In these respects, we propose efficient two-dimensional ME protocols for OFDMA-based WMNs. The proposed protocols simply work by separating its procedure into two steps, subframe-and subchannel-level elections, which satisfy collision-free criteria for broadcast transmission and reception. Also, we suggest summation and product metrics for subchannel-level election, which have better performance in terms of transmission interval and resource utilization than that of independence metric by increasing the probability that a node succeeds in subframe- and subchannel-level elections subsequently.

LMMSE channel estimation with soft statistics for turbo-MIMO receivers

Channel estimation techniques for turbo multiple-input multiple-output receivers are addressed, which exploit a priori information to refine the poor performance of initial channel estimates solely using training symbols. We study linear minimum mean squared error channel estimation (LMMSE-CE) using second-order soft statistics to incorporate the degree of decoding reliability. Utilizing this additional information effectively minimizes the error propagation which the maximum likelihood based expectation maximization channel estimation (ML-CE) suffers. The LMMSE-CE provides a smaller mean square error of channel estimation and block error rate than the ML-CE.

Iterative LMMSE Channel Estimation, Detection, and Decoding with A Priori Information for ST-BICM Systems over Block Fading Channels

Recently, researches on space-time codes deploying multiple transmit and receive antennas have abruptly emerged due to its potential to greatly improve spectral efficiency. Partic- ularly, space-time bit-interleaved coded modulation (ST-BICM) gives rise to very reliable bit error rate (BER) performance with quite flexible rate compatibility. In this paper, we propose an iterative linear minimum-mean-squared-error (LMMSE) channel estimation, detection, and decoding for ST-BICM systems over block fading channels in order to resolve more practical problems concerned with the performance degradation due to imperfect channel state information (CSI). The LMMSE channel estimation and detection are performed using ap rioriinformation fed back from the soft-input / soft-output (SISO) channel decoder in a computational efficient manner. The simulation results demonstrate that the performance of our proposed algorithm approaches to the matched filter bound (MFB) with an SNR loss of less than 1 dB at BER of 10 −6 , and its convergence of behavior is presented with the extrinsic information transfer (EXIT) charts.

Distributed data scheduling for OFDMA-based wireless mesh networks

In this paper, we consider the distributed data scheduling for wireless mesh networks (WMNs) using orthogonal frequency division multiple access (OFDMA). With the collision-free control channel by OFDMA-based mesh election, we present the data scheduling comprised of the resource allocation and link adaptation based on the 3-way handshaking procedure. Using the scheduling information of neighbor nodes obtained from the previous control channels, request and grant nodes schedule data transmission without making any interference to neighbors. Request nodes determine its available resource for each grant node, and grant nodes schedule the competing requests within its available resource where the equal sharing policy for the resource allocation is used. The link adaptation based on the outer-loop power control is also used to meet the target packet error rate (PER) and to compensate the channel variation due to the large scheduling interval. We evaluate the system level performance in the full buffer traffic model to verity the proposed data scheduling in the fixed and moving scenarios.

Partial Linear and SQRD-based DF Detections for DSTTD-OFDM Systems in Severely Time-Varying Multipath Channels

This paper proposed partial linear and sorted QR decomposition (SQRD)-based decision feedback detection (DFD) schemes for double space time transmit diversity (DSTTD)-orthogonal frequency division multiplexing (OFDM) systems to overcome severely time-varying multipath channels. The rapid channel variation deteriorates BER performance due to the introduction of intra-STTD-unit interference and intercarrier interference (ICI). The partial linear approaches mitigate the ICI effects with banded submatrices, instead of full channel frequency response (CFR) matrix. For further improvement of BER performance, the partial SQRD-based DFD with a modified ordering criterion is developed, which effectively removes above two types of interferences including inter-STTD-unit interference simultaneously. Computer simulation results show that the performance of the partial SQRD-based DFD is superior to those of the other linear approaches, with comparable complexity.

Iterative Biased MMSE Detections for DSTTD-OFDM Systems over Time-Varying Multipath Channels

In this paper, we propose iterative biased minimum-mean-squared-error (BMMSE) detection schemes for double space-time transmit diversity (DSTTD)-orthogonal frequency division multiplexing (OFDM) systems over time-varying multipath channels. The time-varying effects cause intercarrier interference (ICI) and destroy the orthogonality of each STTD unit, considerably degrading system performance. To mitigate these impairments, we first estimate all transmitted symbols temporarily using sorted-QR-decomposition (SQRD)-based decision-feedback detection (DFD) considering the partial ICIs. Next, the significantly dominant ICI terms are eliminated with the help of the estimated symbols in the previous step. Finally, the SQRD-based DFD with the only desired subcarriers is performed. In each detection steps, we employ a biased MMSE criterion, which considers ICIs as well as noise. Numerical results demonstrate that the system performance of the proposed algorithms is greatly enhanced in comparison to the conventional schemes.