Jawad A. Salehi

Bio: Jawad A. Salehi is an academic researcher from Sharif University of Technology. The author has contributed to research in topics: Code division multiple access & Bit error rate. The author has an hindex of 41, co-authored 213 publications receiving 8770 citations. Previous affiliations of Jawad A. Salehi include University of Tehran & Shahid Sadoughi University of Medical Sciences and Health Services.

Code division multiple-access techniques in optical fiber networks. I. Fundamental principles

Abstract: An examination is made of fiber-optic code-division multiple-access (FO-CDMA), a technique in which low information data rates are mapped into very-high-rate address codes (signature sequences) for the purpose of achieving random, asynchronous communications free of network control, among many users. The need for a special class of signature sequences to achieve the multiple-access capability using fiber-optic signal processing techniques is discussed. A class of signature sequences called optical orthogonal codes (OOCs) that provide the auto- and cross-correlation properties required for FO-CDMA is introduced and used in an experiment to show the principles of FO-CDMA. The experiment demonstrates the auto- and cross-correlation properties of the codes. The concept of optical disk patterns, an equivalent way of representing the OOCs, is introduced. The patterns are used to derive the probability density functions associated with any two interfering OOCs. A detailed study of different interference patterns is presented, and the strongest and the weakest interference patterns are determined. >

Optical orthogonal codes: design, analysis and applications

Abstract: An optical orthogonal code (OOC) is a family of (0,1) sequences with good auto- and cross-correlation properties, i.e the autocorrelation of each sequence exhibits the `thumbtack' shape and the cross correlation between any two sequences remains low throughout. The use of optical orthogonal codes enables a large number of asynchronous users to transmit information efficiently and reliably. The thumbtack-shaped autocorrelation facilitates the detection of the desired signal, and low-profile cross correlation reduces interference from unwanted signals. Theoretical upper and lower bounds on the maximum possible size of OOCs are derived. Methods for the design and analysis of OOCs, using tools from projective geometry, the greedy algorithm, iterative constructions, algebraic coding theory, block design, and various other combinational disciplines, are discussed

Optical orthogonal codes: design, analysis, and applications

Abstract: An optical orthogonal code (OOC) is a family of (0,1) sequences with good auto- and cross-correlation properties, i.e the autocorrelation of each sequence exhibits the `thumbtack' shape and the cross correlation between any two sequences remains low throughout. The use of optical orthogonal codes enables a large number of asynchronous users to transmit information efficiently and reliably. The thumbtack-shaped autocorrelation facilitates the detection of the desired signal, and low-profile cross correlation reduces interference from unwanted signals. Theoretical upper and lower bounds on the maximum possible size of OOCs are derived. Methods for the design and analysis of OOCs, using tools from projective geometry, the greedy algorithm, iterative constructions, algebraic coding theory, block design, and various other combinational disciplines, are discussed

Coherent ultrashort light pulse code-division multiple access communication systems

Abstract: A new technique for encoding and decoding of coherent ultrashort light pulses is analyzed. In particular, the temporal and statistical behavior of pseudonoise bursts generated by spectral phase coding of ultrashort optical pulses is discussed. the analysis is motivated by recent experiments that demonstrate high-resolution spectral phase coding of picosecond and femtosecond pulses and suggest the possibility of ultrahigh speed code-division multiple-access (CDMA) communications using this technique. The evolution of coherent ultrashort pulses into low intensity pseudonoise bursts as a function of the degree of phase coding is traced. The results are utilized to analyze the performance of a proposed CDMA optical communications system based upon encoding and decoding of ultrashort light pulses. The bit error rate (BER) is derived as a function of data rate, number of users, and receiver threshold, and the performance characteristics are discussed for a variety of system parameters. It is found that performance improves greatly with increasing code length. >

Encoding and decoding of femtosecond pulses

Abstract: We demonstrate the spreading of femtosecond optical pulses into picosecond-duration pseudonoise bursts. Spreading is accomplished by encoding pseudorandom binary phase codes onto the optical frequency spectrum. Subsequent decoding of the spectral phases restores the original pulse. We propose that frequency-domain encoding and decoding of coherent ultrashort pulses could form the basis for a rapidly reconfigurable, code-division multiple-access optical telecommunications network.

Wireless Infrared Communications

Abstract: The use of infrared radiation as a medium for high-speed short-range wireless digital communication is discussed. Available infrared links and local-area networks are described. Advantages and drawbacks of the infrared medium are compared to those of radio and microwave media. The physical characteristics of infrared channels using intensity modulation with direct detection (IM/DD) are presented including path losses and multipath responses. Natural and artificial ambient infrared noise sources are characterized. Strategies for designs of transmitter and receivers that maximize link signal-to-noise ratio (SNR) are described. Several modification formats are discussed in detail, including on-off keying (OOK) pulse-position modulation (PPM), and subcarrier modulation. The performance of these techniques in the presence of multipath distortion is quantified. Techniques for multiplexing the transmissions of different users are reviewed. The performance of an experimental 50-Mb/s on-off-keyed diffuse infrared link is described.

On the capacity of a cellular CDMA system

Abstract: It is shown that, particularly for terrestrial cellular telephony, the interference-suppression feature of CDMA (code division multiple access) can result in a many-fold increase in capacity over analog and even over competing digital techniques. A single-cell system, such as a hubbed satellite network, is addressed, and the basic expression for capacity is developed. The corresponding expressions for a multiple-cell system are derived. and the distribution on the number of users supportable per cell is determined. It is concluded that properly augmented and power-controlled multiple-cell CDMA promises a quantum increase in current cellular capacity. >

Localization via ultra-wideband radios: a look at positioning aspects for future sensor networks

Abstract: UWB technology provides an excellent means for wireless positioning due to its high resolution capability in the time domain. Its ability to resolve multipath components makes it possible to obtain accurate location estimates without the need for complex estimation algorithms. In this article, theoretical limits for TOA estimation and TOA-based location estimation for UWB systems have been considered. Due to the complexity of the optimal schemes, suboptimal but practical alternatives have been emphasized. Performance limits for hybrid TOA/SS and TDOA/SS schemes have also been considered. Although the fundamental mechanisms for localization, including AOA-, TOA-, TDOA-, and SS-based methods, apply to all radio air interface, some positioning techniques are favored by UWB-based systems using ultrawide bandwidths.

Femtosecond pulse shaping using spatial light modulators

Abstract: We review the field of femtosecond pulse shaping, in which Fourier synthesis methods are used to generate nearly arbitrarily shaped ultrafast optical wave forms according to user specification. An emphasis is placed on programmable pulse shaping methods based on the use of spatial light modulators. After outlining the fundamental principles of pulse shaping, we then present a detailed discussion of pulse shaping using several different types of spatial light modulators. Finally, new research directions in pulse shaping, and applications of pulse shaping to optical communications, biomedical optical imaging, high power laser amplifiers, quantum control, and laser-electron beam interactions are reviewed.

The Quantum Theory of Light

Abstract: (b) Write out the equations for the time dependence of ρ11, ρ22, ρ12 and ρ21 assuming that a light field interaction V = -μ12Ee iωt + c.c. couples only levels |1> and |2>, and that the excited levels exhibit spontaneous decay. (8 marks) (c) Under steady-state conditions, find the ratio of populations in states |2> and |3>. (3 marks) (d) Find the slowly varying amplitude ̃ ρ 12 of the polarization ρ12 = ̃ ρ 12e iωt . (6 marks) (e) In the limiting case that no decay is possible from intermediate level |3>, what is the ground state population ρ11(∞)? (2 marks) 2. (15 marks total) In a 2-level atom system subjected to a strong field, dressed states are created in the form |D1(n)> = sin θ |1,n> + cos θ |2,n-1> |D2(n)> = cos θ |1,n> sin θ |2,n-1>