Chantri Polprasert

Capacity of OFDM Systems Over Fading Underwater Acoustic Channels

In this paper, we derive bounds to the channel capacity of orthogonal frequency division multiplexing (OFDM) systems over the underwater (UW) acoustic fading channel as a function of the distance between the transmitter and the receiver. The upper bound is obtained under perfect channel state information (CSI) at the receiver. The lower bound is obtained assuming the input is drawn from phase-shift keying (PSK) constellation which results in non-Gaussian distribution of the output signal and no CSI. The reduction from the upper bound is due to limited mutual information that can be conveyed by PSK constellation and the linear minimum mean square prediction error. Our UW channel deviates from the wide sense stationary and uncorrelated scattering (WSSUS) model commonly used for small bandwidths. We incorporate frequency-dependent path loss due to the acoustic propagation into each arrival path between the transmitter and the receiver. This leads the UW channel to be modeled as a frequency-dependent doubly spread fading channel characterized by the wide sense stationary and correlated scattering (WSS-non-US) fading assumption. Both Rayleigh and Ricean fading assumptions are investigated in our model. Results from the model show a gap between the upper and lower bounds which depends not only on the ranges and shape of the scattering function of the UW channel but also on the distance between the transmitter and the receiver. Our model for the scattering function was suggested by Rescheduled Acoustic Communications Experiment (RACE08) experimental data, leading to a multilag autoregressive (AR- q) model for the fading.

Underwater Acoustic Communication in a Highly Refractive Environment Using SC–FDE

Single-carrier frequency-domain equalization (SC-FDE) is a promising technique for coherent underwater acoustic communications. The method has a lower peak-to-average power ratio than orthogonal frequency-division multiplexing (OFDM) and is more resistant to frequency offset. In this work, SC-FDE performance is evaluated using data collected during the 2009 Cooperative Array Performance Experiment (CAPEx09) that took place near Seattle, WA, USA. CAPEx09 had two densely populated vertical receiving arrays and a highly refractive environment, features that make it suitable for examining issues related to sensor positioning in range and depth. Two-channel equalization results show only a weak dependence on the spacing between the two sensors. Communications performance is interpreted in terms of relevant propagation physics, including acoustic ducting and rough surface scattering. The implications for undersea acoustic networks are discussed.

A nakagami fading phase difference distribution and its impact on BER performance

We propose a new model for investigating the impact of complex imperfect channel state information (CSI) over Nakagami-m fading channels. The phase difference between two correlated complex Nakagami random variables (RVs) is proposed and its statistical properties are investigated. A closed-form expression for the distribution is obtained using an approach from [1] and depends on the Nakagami-m parameter and the power correlation coefficient (p) between two correlated complex RVs. A simple approximation is given when p is close to unity. The proposed phase difference is shown to accurately approximate that of the Ricean. We derive the joint density function of this phase difference and its associated correlated envelopes. This is given in a closed form in terms of a generalized hypergeometric function, but the RVs can be easily generated for simulation. We analyze the impact of imperfect CSI on the bit error rate (BER) of uncoded and coded modulation. For uncoded modulation, the average BER and a closed-form expression of the asymptotic BER or error floor are obtained. For coded modulation, we determine the error free feedback bound of bit-interleaved coded modulation with iterative decoding for both perfect and imperfect CSI. We show that imperfect phase estimation is the major cause of the error floor.

Performance of the bit-interleaved frequency domain turbo equalization over experimental underwater acoustic channels

We investigate the performance of the bit-interleaved frequency-domain turbo equalization (Bi-FDTE) with spatial diversity over experimental underwater acoustic (UWA) channels. Our receiver combines the frequency domain minimum mean square error (MMSE) equalization with the soft interference cancellation with the bit-interleaved turbo equalization. Pilot symbols are periodically inserted and multiplexed with data symbols similar to that of the orthogonal frequency division multiplexing (OFDM). We test our transmission system over a real UWA channel environment conducted at Narragansett Bay, RI. We develop three different transmission scenarios to investigate performance/data rate tradeoff. Results from the experiment show that without spatial diversity the bit error rate (BER) reaches 10-4 at the 5th iteration when signal-to-noise ratio is equal to 18.3 dB at 3.71 kbps data rate. The BER performance is improved by exploiting the spatial diversity of the UWA channels by combining and equalizing data from multiple receive channels.

New stimulation pattern design to improve P300-based matrix speller performance at high flash rate.

OBJECTIVE We propose a new stimulation pattern design for the P300-based matrix speller aimed at increasing the minimum target-to-target interval (TTI). APPROACH Inspired by the simplicity and strong performance of the conventional row-column (RC) stimulation, the proposed stimulation is obtained by modifying the RC stimulation through alternating row and column flashes which are selected based on the proposed design rules. The second flash of the double-flash components is then delayed for a number of flashing instants to increase the minimum TTI. The trade-off inherited in this approach is the reduced randomness within the stimulation pattern. MAIN RESULTS We test the proposed stimulation pattern and compare its performance in terms of selection accuracy, raw and practical bit rates with the conventional RC flashing paradigm over several flash rates. By increasing the minimum TTI within the stimulation sequence, the proposed stimulation has more event-related potentials that can be identified compared to that of the conventional RC stimulations, as the flash rate increases. This leads to significant performance improvement in terms of the letter selection accuracy, the raw and practical bit rates over the conventional RC stimulation. SIGNIFICANCE These studies demonstrate that significant performance improvement over the RC stimulation is obtained without additional testing or training samples to compensate for low P300 amplitude at high flash rate. We show that our proposed stimulation is more robust to reduced signal strength due to the increased flash rate than the RC stimulation.

Performance analysis of the bit-interleaved coded modulation using turbo equalization with single carrier frequency domain equalization over fast fading channels

The performance of the bit-interleaved coded modulation using turbo equalization with single carrier frequency domain equalization (BICM TE-SC-FDE) over frequency-selective fading channels with the residual inter-symbol interference (ISI) is investigated. We focus on the impact of the residual ISI on the convergence behavior using the extrinsic information transfer (EXIT) chart. We first exhibit that the residual ISI impacts not only the error floor but also the convergence behavior of the BICM TE-SC-FDE. Then, we show that the Gaussian assumption of the soft information within the TE-SC-FDE, with or without the residual ISI, can accurately track the flow of the extrinsic information in the iterative procedure. We exhibit through simulation the accuracy of the EXIT chart in predicting the transfer function of the equalizer. We show that the product of the transmit block length and Doppler spread is a key parameter affecting the convergence behavior of the TE-SC-FDE with the residual ISI. The accuracy of our EXIT chart prediction is verified by simulation.

Phase difference statistics for Nakagami-m fading channels

We propose a new channel model for investigating the impact of imperfect channel state information (CSI) over Nakagami-m fading channels. A new definition of the phase difference between two correlated Nakagami random variables is proposed and its probability density function, which depends on the fading parameter m and the power correlation coefficients, is developed. Its first order statistical properties are investigated and simplified. This model looks to be a major contender for investigating the impact of imperfect CSI on Nakagami fading channels.

Sensor Networks for Acoustic Source Localization Using Acoustic Fingerprint in Urban Environments and Construction Sites

We investigate the feasibility and performance of the acoustic localization system using the acoustic fingerprint for asynchronous wireless sensor network (WSN) in urban environments and construction sites. The location estimation is calculated by comparing the acoustic fingerprint obtained from multiple sensors with those pre-computed in the database. To calculate the fingerprint, we avoid expensive measurement process by using a 2-dimensional finitedifference time-domain (FDTD) to approximate acoustic propagation in urban area. The implementation cost for constructing the acoustic fingerprint map using FDTD is small compared to that of the exhaustive data measurements over an entire test site. We select the direction-of-arrival (DoA) of the first arrival path of the acoustic signal as a location fingerprint to avoid complexity from synchronization. The fingerprint from each node is weighted by the measured amplitude of the received acoustic waveform to take into account the decrease in the amplitude due to the distance between the source and the sensor node. We test our proposed localization algorithm at the 140x80 m artificial village area used for military drills. With proper node placement, our proposed algorithm can achieve strong localization performance using small number of sensor nodes. In particular, the root-mean-square-error (RMSE) between the estimated and accurate source position is 6.30 meters using observations from only 3 sensor nodes. Our proposed algorithm exhibits robustness to DoA estimation error representing the cumulative effect of uncertainties in urban environments and construction sites.

A novel stochastic model and fast generation method for Nakagami fading channels

Nakagami fading channels are widely accepted to model a variety of wireless channels. They are parameterized by the m-factor, which can take on any value m > 0. The case m = 1 subsumes Rayleigh fading, while 0 < m < 1 models channels that exhibit a larger percentage of deep fades. The Nakagami amplitude distribution is closely related to the gamma distribution, which describes the instantaneous intensity. In this paper, we address generation of random samples from the univariate Nakagami, or equivalently, the gamma distribution. When m takes on integral values, a simple stochastic description in terms of exponential variates is commonly employed. Recently acceptance-rejection techniques have been described that hold for non-integral m. We describe a little known stochastic description as the product of uniform variates, and provide a simple approximate description which allows for rapid generation of Nakagami variates with very few random degrees of freedom. The stochastic model is also of interest in that it suggests new interpretations of Nakagami fading as a random product model.

Effect of imperfect CSI on iteratively decoded BICM over Nakagami fading channels

We investigate the impact of imperfect channel state information (CSI) on the bit error rate (BER) performance of uncoded and coded modulation schemes over Nakagami fading channels using the model proposed in C. Polprasert and JA Ritcey (2005). The analytical and asymptotic BER performance of uncoded M-PSK modulation is evaluated and verified by the simulation. For coded modulation, the error free feedback bound (EF bound) of the bit-interleaved coded modulation with iterative decoding (BICM-ID) for both perfect and imperfect CSI is developed. We show that asymptotically, the error floor generated due to imperfect CSI depends on the fading parameter and the power correlation coefficient between the fading coefficient and its estimate.