Shahriar Emami

Pulse spectrum optimization for ultra-wideband communication

Ultra-wideband (UWB) communications employ very short pulses with different modulation that result in extremely wideband spectra for high data-rate links. These spectra are a function of both the spreading operation as well as the pulse shapes. We propose a pulse spectrum optimization technique based on classical filters. Non-ideal impulse characteristics are then considered and the optimum pulse derived for either minimal energy (highest spectral bandwidth). The motivation is to show that filtered pulses can achieve significant spectral control in direct baseband UWB signals for high data-rate communications.

On the suitability of 802.11a/RA for high-mobility DSRC

The 802.11a standard has been considered and applied to DSRC (dedicated short-range communication) in the 5.9 GHz band. Among the parameters of the DSRC are support for data transfers during high-mobility applications. Given that the 802.11a standard was designed for relatively low speed (3 MPH) wireless LAN applications, it is necessary to analyze the suitability of the standard in outdoor high-speed mobility applications. We present an analysis of the error rate of 802.11a in high-speed mobile applications. It is shown that 802.11a/RA communication exhibits high packet error rates and consequently lower channel capacity. An example is submitted and discussed.

DPCM picture transmission over noisy channels with the aid of a Markov model

The problem of DPCM picture transmission over noisy channels is considered. It is well known that DPCM systems are very sensitive to channel errors. The goal in this work is to build robustness against channel errors. Three methods are proposed in this paper and are obtained by modeling the encoded signal as a Markov sequence. First, an optimum method for decoding correlated sequences is derived, and it is shown to require Viterbi decoding. Then, a modified MAP method (MMAP) for Markov sequences is described. A maximal signal-to-noise (MSNR) receiver for DPCM systems is also developed that minimizes the distortion power due to channel errors. The appropriate cost matrix for this receiver is computed. These methods are applied to DPCM picture transmission over noisy channels and are compared with a another method. The SNR graphs, as well as subjective examination of the received pictures, demonstrate that the proposed procedures are quite effective and superior to that method. Among the proposed methods, the MSNR receiver was found to be more effective than the others for a given order of the Markov model. It is observed that the proposed methods are most beneficial for low detail pictures.

Pseudo-pilot OFDM scheme for 802.11a and R/A in DSRC applications

A pseudo-pilot scheme is proposed for 802.11a and 802.11R/A (roadside applications version) in DSRC (dedicated short range communication) applications. The scheme overcomes the shortcomings of the original IEEE 802.11a standard by substituting pilots in selected data slots for channel equalization; at the receiver, the pilots are removed and random data inserted. It is shown that the technique is very effective in addressing the high mobility DSRC environment with minor changes to the 802.11a and R/A physical layer.

Composite antenna pattern for realistic ray tracing simulations

The widespread adoption of wireless local area networks (WLAN) and the emerging wireless personal area networks (WPAN) has required a deeper understanding of the indoor propagation channel. As a consequence, ray analysis techniques based on geometrical optics have emerged as the tools of choice for modeling indoor and outdoor environments. In realistic scenarios, antennas will be installed within proximity to wall structures or embedded in complex environments such as vehicle interiors for telematics applications. Hence. we no longer have free space antenna patterns but composite antenna patterns. We consider here the application of the finite-difference time-domain (FDTD) technique for obtaining 3D antenna patterns in complex structures with different material properties. Simulation results are submitted and discussed.

New methods for computing interpolation and decimation using polyphase decomposition

There are many applications in communication systems where the sampling rate must be changed. Interpolators and decimators are utilized to increase or decrease the sampling rate. Computationally efficient implementation of interpolators and decimators employ polyphase decomposition. In this work, the authors present new methods for performing decimation or interpolations similar to the way numbers are multiplied on paper. Their proposed methods are easy to learn and compute and can be incorporated into digital signal processing (DSP) courses that deal with multirate filters.

Nonsymmetric sources and optimum signal selection

There are some applications where the information source is nonsymmetric. Here, it is shown that differential pulse code modulation (DPCM) of video signals results in a nonsymmetric source. Previous works on 2-D signal design have been focused around equally likely sources. In this paper, an iterative algorithm for minimum error signal design subject to average power (or peak power) constraint is presented. Then, design of minimum cost signal sets is addressed, and application to a DPCM system for picture transmission is developed. Comparisons are made between the nonsymmetric and symmetric signal design in one case and between minimum error signal design and minimum cost signal selection in the other case, for the transmission of DPCM-coded signals. It is seen that nonsymmetric signal design gains significant improvements over the equally likely signal selection when the channel is noisy. Minimum cost signal design also demonstrated some improvement relative to minimum error signal selection over noisy channels.

Signal selection for non-symmetric sources in two dimensions

An attempt is made to determine which two-dimensional signal constellation minimizes the probability of error under an average power constraint for nonsymmetric sources. Two different approaches to the problem are presented. First, a numerical method with the help of Lagrange multipliers is described. Then, a normalized conjugate gradient method and a normalized gradient method based on the approach given by G.J. Foschini et al. (1974) are provided. The gradient-based methods are powerful tools that can be applied to signal sets of any size and of any statistics. The normalized conjugate gradient method is a faster than the normalized gradient method.<<ETX>>

An error resilient JPEG 2000 for wireless applications

An efficient and versatile wavelet based picture compression standard, known as JPEG 2000, is under development. Since wireless multimedia is among the intended applications of this standard, JPEG 2000 has to be robust to channel errors. Our results indicate that the quality of received imagery using JPEG 2000 even with error resilient provisions could be very poor. Worst case PSNR serves as an important bench mark for comparing image transmission schemes and a larger worst case peak signal-to-noise ratio (PSNR) is preferred. We present a scheme for increasing worst case PSNR, known as transmission diversity for JPEG 2000. In TD-JPEG 2000, instead of transmitting an image at full rate, the image is transmitted twice at half rate. The image encoder and decoder structures are given and a detailed operation of the decoder based on a set of rules is provided. It is observed that the worst case PSNR improves significantly compared to conventional JPEG 2000 when our method is applied to the transmission of images over noisy channels. Additionally, subjective evaluations of the received images verify the improvements in picture quality.

A Synergistic Text Compression Method-STCM

The method introduced in this article is a new approach to text compression that shows merit in a higher compression ratio, its ability to be integrated with existing utilities to augment their performance, and its resiliency to error introduced through bit corruption by limiting the error effect to only that word where the error occurred. The results introduced, based on a comprehensive evaluation of the proposed method with respect to existing utilities, demonstrate such merits and are provided in support of these assertions.