A rapid synchronization method is presented for an orthogonal frequency-division multiplexing (OFDM) system using either a continuous transmission or a burst operation over a frequency-selective channel. The presence of a signal can be detected upon the receipt of just one training sequence of two symbols. The start of the frame and the beginning of the symbol can be found, and carrier frequency offsets of many subchannels spacings can be corrected. The algorithms operate near the Cramer-Rao lower bound for the variance of the frequency offset estimate, and the inherent averaging over many subcarriers allows acquisition at very low signal-to-noise ratios (SNRs).

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Robust frequency and timing synchronization for OFDM

We present the joint maximum likelihood (ML) symbol-time and carrier-frequency offset estimator in orthogonal frequency-division multiplexing (OFDM) systems. Redundant information contained within the cyclic prefix enables this estimation without additional pilots. Simulations show that the frequency estimator may be used in a tracking mode and the time estimator in an acquisition mode.

/pdf/ml-estimation-of-time-and-frequency-offset-in-ofdm-systems-22x9rvpv87.pdf

ML estimation of time and frequency offset in OFDM systems

We present and analyze low-rank channel estimators for orthogonal frequency-division multiplexing (OFDM) systems using the frequency correlation of the channel. Low-rank approximations based on the discrete Fourier transform (DFT) have been proposed, but these suffer from poor performance when the channel is not sample spaced. We apply the theory of optimal rank-reduction to linear minimum mean-squared error (LMMSE) estimators and show that these estimators, when using a fixed design, are robust to changes in channel correlation and signal-to-noise ratio (SNR). The performance is presented in terms of uncoded symbol-error rate (SER) for a system using 16-quadrature amplitude modulation (QAM).

/pdf/ofdm-channel-estimation-by-singular-value-decomposition-2q9twaj1cb.pdf

OFDM channel estimation by singular value decomposition

Channel estimation techniques for OFDM systems based on a pilot arrangement are investigated. Channel estimation based on a comb type pilot arrangement is studied through different algorithms for both estimating the channel at pilot frequencies and interpolating the channel. Channel estimation at pilot frequencies is based on LS and LMS methods while channel interpolation is done using linear interpolation, second order interpolation, low-pass interpolation, spline cubic interpolation, and time domain interpolation. Time-domain interpolation is obtained by passing to the time domain by means of IDFT (inverse discrete Fourier transform), zero padding and going back to the frequency domain by DFT (discrete Fourier transform). In addition, channel estimation based on a block type pilot arrangement is performed by sending pilots in every sub-channel and using this estimation for a specific number of following symbols. We have also implemented a decision feedback equalizer for all sub-channels followed by periodic block-type pilots. We have compared the performances of all schemes by measuring bit error rates with 16QAM, QPSK, DQPSK and BPSK as modulation schemes, and multipath Rayleigh fading and AR based fading channels as channel models.

/pdf/channel-estimation-techniques-based-on-pilot-arrangement-in-usn3udu2w0.pdf

Channel estimation techniques based on pilot arrangement in OFDM systems

Orthogonal frequency-division multiplexing (OFDM) modulation is a promising technique for achieving the high bit rates required for a wireless multimedia service. Without channel estimation and tracking, OFDM systems have to use differential phase-shift keying (DPSK), which has a 3-dB signal-to-noise ratio (SNR) loss compared with coherent phase-shift keying (PSK). To improve the performance of OFDM systems by using coherent PSK, we investigate robust channel estimation for OFDM systems. We derive a minimum mean-square-error (MMSE) channel estimator, which makes full use of the time- and frequency-domain correlations of the frequency response of time-varying dispersive fading channels. Since the channel statistics are usually unknown, we also analyze the mismatch of the estimator-to-channel statistics and propose a robust channel estimator that is insensitive to the channel statistics. The robust channel estimator can significantly improve the performance of OFDM systems in a rapid dispersive fading channel.

Robust channel estimation for OFDM systems with rapid dispersive fading channels

PROBLEM TO BE SOLVED: To provide a device for satisfying the quality of respective different target services by changing the number of times of encoding step repetition for each user. SOLUTION: Concerning the device composed of a signal processing unit 40 and a decoding resource for repeating a process for repeatedly decoding a signal at least once so as to repeatedly decode the signal, the said repeated decoding resource is arranged inside a central pool 41 and when the repeated decoding process is required, this resource is allocated to the signal processing unit 40. COPYRIGHT: (C)1999,JPO

Device and method for repeatedly decoding signal

Industrial control applications are deployed in environments characterized by high signal attenuation and severe multipath fading, which is a potential cause for significant packet losses. This is in conflict with achieving low latency and highly reliable wireless communication, a requirement of closed-loop control applications. We propose SoLiCiT, a wireless networking protocol that employs concurrent transmissions with specifically designed erasure coding to meet these demands without the need for retransmissions. Synchronous transmissions help to realize a simple protocol design while using helper nodes to transmit redundancy allows for lost packets to be recovered within fixed and tight latency bounds. We provide a range of coding solutions that can be tailored to the application and also present a case study with a battery storage management application in which battery cells report their status to a central management unit. Using a realistic ray tracing model of the actual battery storage container, we evaluate the radio environment and show the efficacy of SoLiCiT compared to benchmarking schemes, such as a low-power wireless bus. 

Magnus Sandell

Papers

Device and method for repeatedly decoding signal

SoLiCiT: Synchronous Listen, Code, and Transmit Protocol for Wireless Control Applications