Browsing by Author "Lin, Hsin-De"

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A Histogram-based Symbol Timing Synchronization Algorithm for OFDM Systems
(2006)
Lin, Hsin-De
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Sang, Tzu-Hsien
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Hsu, Sheng-Yi
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電子工程學系及電子研究所
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Department of Electronics Engineering and Institute of Electronics
A novel data-aided OFDM symbol timing synchronization method was proposed to reduce the computational cost and achieve more robust performance [1]. The algorithm uses histograms to process the received signal's phase information for estimating symbol timing delay. In this paper, the histogram-based algorithm is extended further to cover MIMO-OFDM systems. In addition, detailed analysis is carried out to unveil the unique characteristics of the algorithm, in contrast to popular synchronization methods based on time-domain correlations. In particular, the algorithm converges on the mass center of a channel's power profile as the channel delay estimation. Good balance between performance and cost makes the algorithm a very attractive algorithm of choice for symbol timing synchronization.
An MAP ICI Equalizer with Variable-Width Trellis for Fast-Fading Channels
(2015)
Sang, Tzu-Hsien
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Lai, Yun-Kai
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Lin, Hsin-De
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電子工程學系及電子研究所
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Department of Electronics Engineering and Institute of Electronics
For fast-moving vehicles, radio channels can be severely fast-fading. Under such circumstances, the performance of multi-carrier communication systems, such as OFDM-based ones, are often limited by the inter-carrier interference (ICI) caused by time variation of the channel within one transmit symbol. The conventional way to mitigate the effect of ICI is to increase the frequency spacing between neighboring sub-carriers; yet it leads to the decrease of spectral efficiency. Many types of ICI cancellation methods have been developed. Most of them are only effective for mild to medium ICI, and the ones that are effective for severe ICI either suffer from heavy computational cost or need transmitter-side modifications and thus may not be standard compatible. In this paper, we report a maximum a posteriori (MAP) ICI equalizer that are effective for severe ICI and with acceptable computational cost. Two advantages make it stand out as a very promising technical choice. First, it utilizes an ICI indicator that we introduced in [1] and [2] to build a variable-width trellis such that the computational cost can be dramatically reduced. Second, it is possible to loop the MAP equalizer and an outer channel decoder to form an iterative receiver to further enhance the system performance. This receiver-side technique can improve the reliability of communication systems under high mobility or increase spectral efficiency under mild to medium mobility.
An SFBC-OFDM Receiver to Combat Multiple Carrier Frequency Offsets in Cooperative Communications
(2010)
Lu, Tsung-Ta
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Lin, Hsin-De
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Sang, Tzu-Hsien
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電子工程學系及電子研究所
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Department of Electronics Engineering and Institute of Electronics
In this paper, a new space-frequency combination technique is proposed for Alamouti coded Orthogonal Frequency Division Multiplexing (OFDM) in the context of cooperative communications. Since cooperative antennas are distributed, there may exist multiple carrier frequency offsets (MCFOs) which cause problems for conventional space-frequency decoding. The proposed algorithm, taking cues from existing MCFO-compensating algorithms [13][19], combines two sets of separately synchronized signal to mitigate inter-carrier interference (ICI). Iterative interference cancellation and a maximum-ratio-combining-like technique are also deployed to further improve performance with low computational complexity. It is observed that the proposed method achieves better bits error rate (BER) performance and has a superior tolerance of multiple CFOs, compared to existing methods.
An SFBC-OFDM Receiver with MLSE Equalizer to Combat Multiple Frequency Offsets
(2012)
Lee, Jyun-Yu
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Lin, Hsin-De
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Sang, Tzu-Hsien
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電子工程學系及電子研究所
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Department of Electronics Engineering and Institute of Electronics
Synchronization is an important issue arises when distributed multiple transmitters are deployed. Unlike conventional MIMO systems, cooperative transmitters may distribute geographically instead of co-locating at the same spot. Therefore, it is hard for the destination node to compensate multiple carrier frequency offsets (MCFOs) even if the cooperative transmitters use the same clock, and the conventional MIMO decoding strategy may not be suitable. In this paper, we consider cooperative SFBC-OFDM transmission with MCFOs. The proposed strategy carefully combines two sets of separately synchronized signal to mitigate inter-carrier interference (ICI). Maximum-likelihood sequence estimation (MLSE) and iterative interference cancellation is further deployed to improve performance. Simulation results show the diversity gain is maintained and an improved tolerance range of MCFOs is achieved.
Estimation of Channel Variation and Its Applications in OFDM-Based Systems
(2012-01-01)
Lin, Hsin-De
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Lin, Tzu-Chieh
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Sang, Tzu-Hsien
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電子工程學系及電子研究所
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Department of Electronics Engineering and Institute of Electronics
High mobility in vehicle-centric networks can impose serious challenges to the design of communication systems based on orthogonal frequency-division multiplexing (OFDM). A timely estimation of channel variation due to fast fading can be very useful in terms of receiver adaptation and quality of service (QoS) control, etc. In this paper, several algorithms for OFDM-based systems to estimate channel variation are studied and compared, while possible applications are also described. Simulation results show that, with little extra overhead, OFDM-based systems may benefit much from utilizing the information of channel variation obtained by simple estimation algorithms.
Low-Cost Perturbation-Based ICI Equalizers for OFDMA Mobile Systems
(2012-11-01)
Lin, Hsin-De
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Sang, Tzu-Hsien
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Chen, Jiunn-Tsair
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電子工程學系及電子研究所
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Department of Electronics Engineering and Institute of Electronics
For advanced mobile communication systems that adopt orthogonal frequency-division multiple access (OFDMA) technologies, intercarrier interference (ICI) significantly degrades performance when mobility is high. Standard specifications and concerns about complexity demand low-cost methods with deployment readiness and decent performance. In this paper, novel zero forcing (ZF) and minimum mean-square error (MMSE) equalizers based on per-subcarrier adaptive (PSA) processing and perturbation-based (PB) approximation are introduced. The proposed equalizers strike a good balance between implementation cost and performance; therefore they are especially suitable for OFDMA downlink receivers. Theoretical analysis and simulations are provided to verify our claims.
On the Diversity Order of BICM-OFDM in Doubly Selective Fading Channels
(2013-05-01)
Lin, Hsin-De
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Sang, Tzu-Hsien
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電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
Bit-interleaved coded modulation with orthogonal frequency-division multiplexing (BICM-OFDM) is an attractive approach to achieve time and frequency diversity. Remarkable diversity gain can be obtained when the channel is doubly selective fading. In this paper, the asymptotic diversity orders of BICM-OFDM systems in doubly selective fading channels for both single-input-single-output (SISO) and multiple-input-multiple-output (MIMO) cases are derived. In addition, the system bit-error-rate (BER) behavior in practical situations with moderate signal-to-noise ratios (SNRs) is also investigated. In the SISO case, the diversity order depends on the rank of the channel correlation matrix. Therefore, the channel variations induced by fast fading contribute to improving diversity. In the MIMO case, the diversity order can be further increased when factors such as cyclic delays or phase rolls are introduced. Simulations are provided to verify the analysis.