Hlaing Minn

A robust timing and frequency synchronization for OFDM systems

A robust symbol-timing and carrier-frequency synchronization scheme applicable to orthogonal frequency-division-multiplexing systems is presented. The proposed method is based on a training symbol specifically designed to have a steep rolloff timing metric. The proposed timing metric also provides a robust sync detection capability. Both time domain training and frequency domain (FD) training are investigated. For FD training, maintaining a low peak-to-average power ratio of the training symbol was taken into consideration. The channel estimation scheme based on the designed training symbol was also incorporated in the system in order to give both fine-timing and frequency-offset estimates. For fine frequency estimation, two approaches are presented. The first one is based on the suppression of the interference introduced in the frequency estimation process by the training symbol pattern in the context of multipath dispersive channels. The second one is based on the maximum likelihood principle and does not suffer from any interference. A new performance measure is introduced for timing estimation, which is based on the plot of signal to timing-error-induced average interference power ratio against the timing estimate shift. A simple approach for finding the optimal setting of the timing estimator is presented. Finally, the sync detection, timing estimation, frequency estimation, and bit-error-rate performance of the proposed method are presented in a multipath Rayleigh fading channel.

On timing offset estimation for OFDM systems

Two timing offset estimation methods for orthogonal frequency division multiplexing (OFDM) systems as modifications to Schmidl and Coxs method (see IEEE Trans. Commun., vol.45, p.1613-21, 1997) are presented. The performances of the timing offset estimators in additive white Gaussian noise channel and intersymbol interference channel are compared in terms of estimator variance obtained by simulation. Both proposed methods have significantly smaller estimator variance in both channel conditions.

An investigation into time-domain approach for OFDM channel estimation

A time-domain based channel estimation for OFDM system with pilot-data multiplexed scheme is investigated. As an approximation to linear minimum mean square estimator (LMMSE), a time-domain based channel estimation is proposed where intra-symbol time-averaging and most significant channel taps selection are applied. The relation and differences of the proposed method to DFT-based LMMSE methods are discussed. The performances of the proposed method, DFT-based LMMSE method and the methods of Chini, Wu, El-Tanany and Mahmoud (see IEEE Trans. on Broadcasting, vol.44, no.1, p.2-11, 1998) and of Yeh and Lin (see IEEE Trans. on Broadcasting, vol.45, no.4, p.400-409, 1999) are evaluated in multipath fading channels. The simulation results show that proposed method achieves almost the same performance as DFT-based LMMSE method and better BER performance than the other methods while keeping less complexity.

Apnea MedAssist: Real-time Sleep Apnea Monitor Using Single-Lead ECG

We have developed a low-cost, real-time sleep apnea monitoring system ``Apnea MedAssist” for recognizing obstructive sleep apnea episodes with a high degree of accuracy for both home and clinical care applications. The fully automated system uses patients single channel nocturnal ECG to extract feature sets, and uses the support vector classifier (SVC) to detect apnea episodes. “Apnea MedAssist” is implemented on Android operating system (OS) based smartphones, uses either the general adult subject-independent SVC model or subject-dependent SVC model, and achieves a classification F-measure of 90% and a sensitivity of 96% for the subject-independent SVC. The real-time capability comes from the use of 1-min segments of ECG epochs for feature extraction and classification. The reduced complexity of “Apnea MedAssist” comes from efficient optimization of the ECG processing, and use of techniques to reduce SVC model complexity by reducing the dimension of feature set from ECG and ECG-derived respiration signals and by reducing the number of support vectors.

Optimal training signals for MIMO OFDM channel estimation

This work presents general classes of optimal training signals for estimation of frequency-selective channels in MIMO OFDM systems. Basic properties of the discrete Fourier transform are used to derive the optimal training signals which minimize the channel estimation mean square error. Both single and multiple OFDM training symbols are considered. Several optimal pilot tone allocations among the transmit antennas are presented and classified as frequency division multiplexing, time division multiplexing, code division multiplexing in frequency-domain, code division multiplexing in time-domain, and combinations thereof. All existing optimal training signals in the literature are special cases of the presented optimal training signals and our designs can be applied to pilot-only schemes as well as pilot-data multiplexed schemes.

A reduced complexity channel estimation for OFDM systems with transmit diversity in mobile wireless channels

A reduced complexity channel estimation for OFDM systems with transmit diversity is proposed by exploiting the correlation of the adjacent subchannel responses. The sizes of the matrix inverse and the FFTs required in the channel estimation at every OFDM data symbol are reduced by half of the existing method for OFDM systems with nonconstant modulus subcarrier symbols or constant modulus subcarrier symbols with some guard tones. The complexity reduction of half FFTs size and some matrix multiplications is still achieved for constant modulus subcarrier symbols with no guard tones. The price for the complexity reduction is a slight BER degradation and for the channels with small relative delay spreads, the BER performance of the reduced complexity method becomes quite comparable to the existing method. An alternative approach for the number of significant taps required in the channel estimation is described which achieves a comparable performance to the case with the known suitable number of significant taps. A simple modification which reduces the lost leakage of the nonsample-spaced channel paths is also proposed. This modification achieves a substantial performance improvement over the existing method without any added complexity.

A combined timing and frequency synchronization and channel estimation for OFDM

This letter addresses training-signal-based combined timing and frequency synchronization and channel estimation for orthogonal frequency-division multiplexing systems. The proposed scheme consists of two stages. At the first stage, coarse timing and frequency-offset estimates are obtained. Based on these estimates, a (coarse) channel response estimate is obtained. The timing and frequency-offset estimates at the second stage are obtained by maximum-likelihood (ML) realization based on a sliding observation vector. Then ML channel estimation is performed. A means of complexity reduction by an adaptive scheme is also presented. The simulation results show that the proposed combined approach performs quite well, and circumvents the problem of mismatch among individual synchronization tasks.

Optimal periodic training signal for frequency offset estimation in frequency-selective fading channels

This paper addresses an optimal periodic training signal design for frequency offset estimation in frequency-selective multipath Rayleigh fading channels. For a fixed transmitted training signal energy within a fixed-length block, the optimal periodic training signal structure (the optimal locations of identical training subblocks) and the optimal training subblock signal are presented. The optimality is based on the minimum Cramer-Rao bound (CRB) criterion. Based on the CRB for joint estimation of frequency offset and channel, the optimal periodic training structure (optimality only in frequency offset estimation, not necessarily in joint frequency offset and channel estimation) is derived. The optimal training subblock signal is obtained by using the average CRB (averaged over the channel fading) and the received training signal statistics. A robust training structure design is also presented in order to reduce the occurrence of outliers at low signal-to-noise ratio values. The proposed training structures and subblock signals achieve substantial performance improvement

Differential modulation for two-user cooperative diversity systems

This paper introduces a novel differential modulation scheme for a two-user cooperative diversity system which does not require channel state information at either the users or the destination. The performance of fixed decode-and-forward and selection relaying protocols is evaluated in both symmetric and asymmetric interuser-channel cases. The lower bound on the performance of the decode-and-forward protocol is given, while the exact bit-error probability of the selection relaying protocol is thoroughly derived. The decode-and-forward relaying protocol achieves a performance gain when the signal-to-noise ratios in the interuser channels are symmetric and sufficiently high. The selection relaying protocol shows a larger performance gain and does not exhibit an error floor like in the case of the decode-and-forward protocol. In addition, it is robust to the asymmetric interuser channels.

A new ranging method for OFDMA systems

We present a new initial ranging method for OFDMA systems such as IEEE 802.16-2004. First, a new orthogonal ranging signal design is proposed by which an efficient, low-complexity multi-user ranging signal detection is developed. Based on the ranging signal detector results, power estimation for the detected ranging subscriber stations is performed. Then, by utilizing the proposed orthogonal ranging signal design together with the redundancy introduced by cyclic prefixes, a new iterative multi-user timing offset estimator is presented. Compared with the existing methods utilizing the CDMA-type ranging codes (in frequency-domain) defined in IEEE 802.16-2004, the proposed method achieves a better performance and greater robustness against multi-user interference and multi-path fading channels