Papers by Mohammad Noshad
Visible light communication (VLC) systems using the indoor lighting system to also provide downli... more Visible light communication (VLC) systems using the indoor lighting system to also provide downlink communications require high average optical powers in order to satisfy the illumination needs. This can cause high amplitude signals common in higher-order modulation schemes to be clipped by the peak power constraint of the light emitting diode (LED) and lead to high signal distortion. In this paper we introduce Hadamard coded modulation (HCM) to achieve low error probabilities in LED-based VLC systems needing high average optical powers. This technique uses a fast Walsh-Hadamard transform (FWHT) to modulate the data as an alternative modulation technique to orthogonal frequency division multiplexing (OFDM). HCM achieves a better performance for high illumination levels because of its small peak to average power ratio (PAPR). The power efficiency of HCM can be improved by reducing the DC part of the transmitted signals without losing any information. The resulting so-called DC-reduced HCM (DCR-HCM) is well suited to environments requiring dimmer lighting as it transmits signals with lower peak amplitudes compared to HCM, which are thus subject to less nonlinear distortion. Interleaving can be applied to HCM to make the resulting signals more resistant against inter-symbol interference (ISI) in dispersive VLC links.
Visible light communications (VLC) in indoor environments suffer from the limited bandwidth of LE... more Visible light communications (VLC) in indoor environments suffer from the limited bandwidth of LEDs as well as from the inter-symbol interference (ISI) imposed by multipath. In this work, transmission schemes to improve the performance of indoor optical wireless communication (OWC) systems are introduced. Expurgated pulse-position modulation (EPPM) is proposed for this application since it can provide a wide range of peak to average power ratios (PAPR) needed for dimming of the indoor illumination. A correlation decoder used at the receiver is shown to be optimal for indoor VLC systems, which are shot noise and background-light limited. Interleaving applied on EPPM in order to decrease the ISI effect in dispersive VLC channels can significantly decrease the error probability. The proposed interleaving technique makes EPPM a better modulation option compared to PPM for VLC systems or any other dispersive OWC system. An overlapped EPPM pulse technique is proposed to increase the transmission rate when bandwidth-limited white LEDs are used as sources.
Providing network access for multiple users in a visible light communication (VLC) system that ut... more Providing network access for multiple users in a visible light communication (VLC) system that utilizes white light emitting diodes (LED) as sources requires new networking techniques adapted to the lighting features. In this paper we introduce two multiple access techniques using expurgated PPM (EPPM) that can be implemented using LEDs and support lighting features such as dimming. Multilevel symbols are used to provide M -ary signaling for multiple users using multilevel EPPM. In the first technique, the M -ary data of each user is first encoded using an optical orthogonal code assigned to the user, and the result is fed into a EPPM encoder to generate a multilevel signal. The second multiple access method uses subsets of the EPPM constellation to apply MEPPM to the data of each user. While the first approach has a larger Hamming distance between the symbols of each user, the latter can provide higher bit-rates for users in VLC systems using bandwidth-limited LEDs. Both techniques are able to support up to 15 simultaneous users transmitting 200 Mb/s with a bit error rate of 10 ā3 under normal lighting conditions.
In this letter, we propose a new pulse position modulation (PPM) scheme, called expurgated PPM (E... more In this letter, we propose a new pulse position modulation (PPM) scheme, called expurgated PPM (EPPM), for application in peak power limited communication systems, such as impulse radio (IR) ultra wide band (UWB) systems and free space optical (FSO) communications. Using the proposed scheme, the constellation size and the bit-rate can be increased significantly in these systems. The symbols are obtained using symmetric balanced incomplete block designs (BIBD), forming a set of pair-wise equidistant symbols. The performance of Q-ary EPPM is better than any Q-ary pulse position-based modulation scheme with the same symbol length. Since the code is cyclic, the receiver for EPPM is simpler compared to multipulse PPM (MPPM).
In this paper, two new modulation schemes using multilevel pulse-position modulation (PPM) for ap... more In this paper, two new modulation schemes using multilevel pulse-position modulation (PPM) for application in unipolar optical wireless systems are presented. Balanced incomplete block designs (BIBD) are used for constructing the symbol alphabets. Each symbol is obtained by combining multiple codewords of a BIBD code. In one scheme the symbols have equal energies, and therefore, no threshold is needed to make a decision on the received signal. The other modulation has better performance yet higher complexity. Since cyclic BIBDs are used for constructing the symbols, the transmitters and receivers have simple structures, and can be implemented using shift registers. These schemes can achieve high spectral-efficiencies, and are therefore suitable for systems with bandlimited sources or highly dispersive channels, where intersymbol interference (ISI) has a significant impact on the performance. We also show that using the same receiver structure, the constellation size can be increased by including the complements of the codewords. The performance of the proposed schemes are compared to other modulation schemes for both LED-based non-dispersive and dispersive freespace optical (FSO) systems.
We present an M -ary spectral amplitude code (SAC) modulation technique to improve the performanc... more We present an M -ary spectral amplitude code (SAC) modulation technique to improve the performance of free-space optical (FSO) communication systems. Although this approach can be used in any dispersive FSO system, in this paper we focus on non-line of sight (NLOS) ultraviolet (UV) systems relying on atmospheric scattering. Spectral amplitude encoding is applied on a broadband UV source using the same code families for the M -ary alphabet as used previously in SAC optical code division multiple access (OCDMA) systems. A differential structure using two photomultiplier tubes is utilized in conjunction with various demodulation algorithms to decode the received signal. Intersymbol interference (ISI), received beam divergence and shot noise are considered as the main factors limiting the system performance. An upper bound on the bit error probability is presented and compared with simulation results for various geometries and for different code parameters. The maximum bit rate for a fixed bit error probability is calculated in terms of the link length, and results for different alphabet sizes are shown. By sacrificing spectral efficiency without becoming more susceptible to ISI, the proposed system can support higher rates and longer distances for the same performance compared with on-off keying systems.
IEEE/OSA Journal of Lightwave Technology, 2011
In this paper, two lower and upper bounds for the bit error rate of codes with fixed cross correl... more In this paper, two lower and upper bounds for the bit error rate of codes with fixed cross correlation in spectral-amplitude-coding optical code-division multiple access systems are offered. In this study, we consider only the phase-induced intensity noise and neglect other noises such as shot and thermal noises, in the performance analysis. Also, it is assumed that users are synchronous. This bound is obtained using combinatorics approach and utilizing simple inequalities. In numerical results, the comparison between the upper bound, lower bound, and simulation results for MQC, MFH, BIBD, and Hadamard codes is presented, and the tightness of the bounds is evaluated.
Journal of Optical Communications and Networking, 2010
A novel family of codes for modified spectral-amplitude-coding optical code division multiple acc... more A novel family of codes for modified spectral-amplitude-coding optical code division multiple access (SAC-OCDMA) is introduced. The proposed codes exist for more processing gains than the previously reported codes do. In the network using these codes, the number of users can be extended without any essential changes in the previous transmitters. In this study, we propose a construction method for these codes and compare their performance with previously reported codes.
Fiber and Integrated Optics, 2011
The impact of the compensator's location along the optical fiber on a variation of four-wave mixi... more The impact of the compensator's location along the optical fiber on a variation of four-wave mixing power penalty is investigated theoretically and numerically for dispersion-managed fibers in wavelength division multiplexing systems. The power penalty is analyzed for fibers with different attenuation coefficients and lengths, and the optimum positions for the compensator along the fiber is discussed in cases of both dispersion compensation of each fiber section between two amplifiers and whole-compensation.
In this paper a novel design for improving the performance of non-line of sight (NLOS) ultraviole... more In this paper a novel design for improving the performance of non-line of sight (NLOS) ultraviolet (UV) communication systems using spectral amplitude coding (SAC) is proposed. The same technique as in SAC-OCDMA systems is used as a high-order modulation instead of as a code. Intersymbol interference (ISI) and receiver noise are considered as the main limiting factors. The NLOS channel is modeled to obtain the ISI coefficients. Two bounds are presented and compared with simulation results for different number of symbols and for various geometries. The proposed system can support longer distances for the performance compared with on-off keying (OOK) systems.
In this paper, theoretical and experimental investigation of a novel compact and high-precision (... more In this paper, theoretical and experimental investigation of a novel compact and high-precision (0.2 nm) optical displacement sensor is presented. The sensor operation is based on optical intensity modulation due to diffraction. A circular disk, including a central transmitter and circumferential receivers (8 and 12 configurations), is considered as high precision displacement sensor. Effects of the number of receivers and the reflection coefficient of the receiver's surface on sensitivity of the proposed sensor are investigated. It is shown that with increase of the reflection coefficient of the surface, the sensitivity will increase up to five times compared with the traditional case. It is also demonstrated that there is an excellent agreement between theoretical and experimental results.
Optik
Four wave mixing (FWM) induced power penalty is investigated theoretically for asymmetrical dispe... more Four wave mixing (FWM) induced power penalty is investigated theoretically for asymmetrical dispersion-managed fibers in wavelength division multiplexing (WDM) systems. The power penalty in these fibers is analyzed for various values of channel spacing, maximum dispersion and number of sections in different channels. Using numerical simulations, it is illustrated that the FWM induced power penalty is minimized for the case of two fiber sections with unequally special lengths and symmetrical dispersion values.
Impulse-radio UWB (IR-UWB) transceivers are quite interesting since they can achieve high data ra... more Impulse-radio UWB (IR-UWB) transceivers are quite interesting since they can achieve high data rate while consuming low power. But UNII Band interferers are a serious problem for using the whole spectrum in IR-UWB. Also the short range of UWB is a serious limitation to this technology. Here we have proposed a solution to overcome both of the above mentioned problems. Matched filter concept shapes the output noise according to its spectrum so it can act as a noise filter. Matched filter is implemented in analog domain by designing an all optical UWB pulse featuring notches that can be tuned to a certain frequency. Therefore, any narrowband interferer can be nulled out passing through the matched filter. Also no transmission is done in the spectrum of the narrow band system so no interference is caused by UWB system. On the other hand since the pulse is implemented fully optically, by using UWB over fiber, the UWB range can be greatly extended. Modified Hermite polynomials have unique properties which are used for designing an optical UWB pulse with frequency nulls in its spectrum. For optical implementation of the proposed UWB pulse, Gaussian laser beams are combined to provide a signal with similar properties of Modified Hermite polynomials. The proposed system consists of a single wave length laser source, optical delay, power splitter/combiner, a length of single mode fiber (SMF) and a photo detector (APD). The simulation results shows that for short range communication the proposed architecture can tolerate large in band interferers up to 30 dBm. For long range the tolerance is degraded to 0dBm interferer power which is still quite high.
In this paper an interesting high-precision optical displacement sensor based on fiber optics and... more In this paper an interesting high-precision optical displacement sensor based on fiber optics and intensity modulation is presented. The proposed sensor has below nm precision and so excellent for high precision measurement and instrumentation systems design and implementation. The proposed method is so easy for implementation and based on diffractive optics. With variation of the barrier the reflected intensity is varied and this is a measure for distance measurement.
Journal of Physics: Conference Series, 2012
We have investigated the dependence of resonance frequency of micro-cavity on its geometry for T-... more We have investigated the dependence of resonance frequency of micro-cavity on its geometry for T-shaped beam splitter using finite difference time domain (FDTD) method. We derived resonance frequencies and output powers for elliptical cavity with various radii in both vertical and horizontal directions (X and Y directions). The results showed that resonance frequency is highly related to the cavity radii and broad range of frequencies is accessible. We also discussed T-beam splitter performance for a rotated cavity by different degrees. We observed that rotation of the elliptical cavity make changes only in power transmission and interestingly resonance frequencies become fixed. By setting the cavity angle at 45 degree we get the equivalent division of power with the transmission efficiency of 40% per each output port. Results showed that rotation of the elliptical cavity is an appropriate and easy way to modify the division of input power into two output waveguides.
Balanced incomplete block design (BIBD) is an area of combinatorial mathematics that was first in... more Balanced incomplete block design (BIBD) is an area of combinatorial mathematics that was first introduced in 1936 and developed very fast. Because of their unique fixed cross-correlation property, BIBDs have attracted the attention of researchers in communication systems. BIBDs have been used in various areas, such as error control coding, cryptography, code division multiple access networks, key distribution in sensor networks and watermarking. Among different BIBDs, symmetric designs are often used as codes; in order to simplify the signal processing, cyclic structures are of greatest interest.
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Papers by Mohammad Noshad