arXiv subjects

Mohammad Noshad

Publications and source records attributed to Mohammad Noshad.

17 recordsLinked to original sources

arXiv2018

Comparison of DCO-OFDM and M-PAM for LED-Based Communication Systems

Light-emitting diode (LED)-based communications, such as visible light communications (VLC) and infrared (IR) communications, are candidate techniques to provide short-range and high-speed data transmission. In this paper, M-ary pulse amplitude modulation (M-PAM), used as a high bandwidth efficiency scheme, is compared with an optimized DC-biased optical orthogonal frequency division multiplexing (DCO-OFDM) scheme. Considering the bandwidth limit and constrained peak transmitted power characteristics of LEDs, a bit loading algorithm with an optimized modulation index is used for the DCO-OFDM. To reduce the inter-symbol interference caused by LEDs, a waveform design algorithm with a minimum mean squared error (MMSE) equalizer is applied to the M-PAM system. From numerical results, M-PAM with the optimized signal processing can provide a substantially higher data rate than the optimally designed DCO-OFDM for the same performance.

cs.IT↗

arXiv2017

Positioning for Visible Light Communication System Exploiting Multipath Reflections

In this paper, we introduce a new uplink visible light indoor positioning system that estimates the position of the users in the network-side of a visible light communications (VLC) system. This technique takes advantage of the diffuse components of the uplink channel impulse response for positioning, which has been considered as a destructive noise in existing visible light communication positioning literature. Exploiting the line of sight (LOS) component, the most significant diffusive component of the channel (the second power peak (SPP)), and the delay time between LOS and SPP, we present a proof of concept analysis for positioning using fixed reference points, i.e. uplink photodetectors (PDs). Simulation results show the root mean square (RMS) positioning accuracy of 25 cm and 5 cm for one and 4 PDs scenarios, respectively.

cs.IT↗

arXiv2017

On Sequential Elimination Algorithms for Best-Arm Identification in Multi-Armed Bandits

We consider the best-arm identification problem in multi-armed bandits, which focuses purely on exploration. A player is given a fixed budget to explore a finite set of arms, and the rewards of each arm are drawn independently from a fixed, unknown distribution. The player aims to identify the arm with the largest expected reward. We propose a general framework to unify sequential elimination algorithms, where the arms are dismissed iteratively until a unique arm is left. Our analysis reveals a novel performance measure expressed in terms of the sampling mechanism and number of eliminated arms at each round. Based on this result, we develop an algorithm that divides the budget according to a nonlinear function of remaining arms at each round. We provide theoretical guarantees for the algorithm, characterizing the suitable nonlinearity for different problem environments described by the number of competitive arms. Matching the theoretical results, our experiments show that the nonlinear algorithm outperforms the state-of-the-art. We finally study the side-observation model, where pulling an arm reveals the rewards of its related arms, and we establish improved theoretical guarantees in the pure-exploration setting.

stat.ML↗

arXiv2017

The Number of Independent Sets in Hexagonal Graphs

A new method is proposed to derive rigorous bounds on η, the growth rate of the logarithm of the number of independent sets on a hexagonal lattice. Specifically, we prove that 1.546440708536001 <= η <= 1.5513, which improves upon the best known 1.5463 <= η <= 1.5527 due to Nagy and Zeger. Our lower bound matches the numerical estimate of Baxter up to 9 digits after the decimal point.

cs.IT↗

arXiv2017

Capacity of Hexagonal Checkerboard Codes

In this paper, we propose a new method to bound the capacity of checkerboard codes on the hexagonal lattice. This produces rigorous bounds that are tighter than those commonly known.

cs.IT↗

arXiv2015

Data-Driven Learning of the Number of States in Multi-State Autoregressive Models

In this work, we consider the class of multi-state autoregressive processes that can be used to model non-stationary time-series of interest. In order to capture different autoregressive (AR) states underlying an observed time series, it is crucial to select the appropriate number of states. We propose a new model selection technique based on the Gap statistics, which uses a null reference distribution on the stable AR filters to check whether adding a new AR state significantly improves the performance of the model. To that end, we define a new distance measure between AR filters based on mean squared prediction error (MSPE), and propose an efficient method to generate random stable filters that are uniformly distributed in the coefficient space. Numerical results are provided to evaluate the performance of the proposed approach.

stat.ML↗

arXiv2015

Learning the Number of Autoregressive Mixtures in Time Series Using the Gap Statistics

Using a proper model to characterize a time series is crucial in making accurate predictions. In this work we use time-varying autoregressive process (TVAR) to describe non-stationary time series and model it as a mixture of multiple stable autoregressive (AR) processes. We introduce a new model selection technique based on Gap statistics to learn the appropriate number of AR filters needed to model a time series. We define a new distance measure between stable AR filters and draw a reference curve that is used to measure how much adding a new AR filter improves the performance of the model, and then choose the number of AR filters that has the maximum gap with the reference curve. To that end, we propose a new method in order to generate uniform random stable AR filters in root domain. Numerical results are provided demonstrating the performance of the proposed approach.

stat.ML↗

arXiv2015

Complementary Lattice Arrays for Coded Aperture Imaging

In this work, we consider complementary lattice arrays in order to enable a broader range of designs for coded aperture imaging systems. We provide a general framework and methods that generate richer and more flexible designs than existing ones. Besides this, we review and interpret the state-of-the-art uniformly redundant arrays (URA) designs, broaden the related concepts, and further propose some new design methods.

cs.IT↗

arXiv2015

Hadamard Coded Modulation for Visible Light Communications

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.

cs.IT↗

arXiv2014

Hadamard Coded Modulation: An Alternative to OFDM for Optical Wireless Communications

Orthogonal frequency division multiplexing (OFDM) is a modulation technique susceptible to source, channel and amplifier nonlinearities because of its high peak-to-average ratio (PAPR). The distortion gets worse by increasing the average power of the OFDM signals since larger portion of the signals are affected by nonlinearity. In this paper we introduce Hadamard coded modulation (HCM) that uses the fast Walsh-Hadamard transform (FWHT) to modulate data as an alternative technique to OFDM in direct-detection wireless optical systems. This technique is shown to have a better performance for high average optical power scenarios because of its small PAPR, and can be used instead of OFDM in two scenarios: 1) in optical systems that require high average optical powers such as visible light communications (VLC), and 2) in optical wireless systems unconstrained by average power, for which HCM achieves lower bit error rate (BER) compared to OFDM. The power efficiency of HCM can be improved by removing a part of the signal's DC bias without losing any information. In this way, the amplitude of the transmitted signal is decreased and the signals become less susceptible to nonlinearity. Interleaving can be applied on HCM to make the resulting signals resistent against inter-symbol interference (ISI) effects in dispersive channels by uniformly distributing the interference over all symbols.

cs.IT↗

arXiv2013

Application of Expurgated PPM to Indoor Visible Light Communications - Part I: Single-User Systems

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.

cs.IT↗

arXiv2013

Application of Expurgated PPM to Indoor Visible Light Communications - Part II: Access Networks

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 (MEPPM). Using these multiple-access schemes we are able to control the optical peak to average power ratio (PAPR) in the system, and hereby control the dimming level. In the first technique, the M-ary data of each user is first encoded using an optical orthogonal code (OOC) assigned to the user, and the result is fed into a EPPM encoder to generate a multilevel signal. The second multiple access method uses sub-sets 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.

cs.IT↗

arXiv2013

Can Visible Light Communications Provide Gb/s Service?

Visible light communications (VLC) that use the infrastructure of the indoor illumination system have been envisioned as a compact, safe, and green alternative to WiFi for the downlink of an indoor wireless mobile communication system. Although the optical spectrum is typically well-suited to high throughput applications, combining communications with indoor lighting in a commercially viable system imposes severe limitations both in bandwidth and received power. Clever techniques are needed to achieve Gb/s transmission, and to do it in a cost effective manner so as to successfully compete with other high-capacity alternatives for indoor access, such as millimeter-wave radio-frequency (RF). This article presents modulation schemes that have the potential to overcome the many challenges faced by VLC in providing multi Gb/s indoor wireless connectivity.

cs.IT↗

arXiv2013

High-Speed Visible Light Indoor Networks Based on Optical Orthogonal Codes and Combinatorial Designs

Interconnecting devices in an indoor environment using the illumination system and white light emitting diodes (LED) requires adaptive networking techniques that can provide network access for multiple users. Two techniques based on multilevel signaling and optical orthogonal codes (OOC) are explored in this paper in order to provide simultaneous multiple access in an indoor multiuser network. Balanced incomplete block designs (BIBD) are used to construct multilevel symbols for M-ary signaling. Using these multilevel symbols we are able to control the optical peak to average power ratio (PAPR) in the system, and hereby control the dimming level. In the first technique, the M-ary data of each user is first encoded using the OOC codeword that is assigned to that user, and then it is fed into a BIBD encoder to generate a multilevel signal. The second multiple access method uses sub-sets of a BIBD code to apply multilevel expurgated pulse-position modulation (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 with bandwidth-limited LEDs.

cs.IT↗

arXiv2012

NLOS UV Channel Modeling Using Numerical Integration and an Approximate Closed-Form Path Loss Model

In this paper we propose a simulation method using numerical integration, and develop a closed-form link loss model for physical layer channel characterization for non-line of sight (NLOS) ultraviolet (UV) communication systems. The impulse response of the channel is calculated by assuming both uniform and Gaussian profiles for transmitted beams and different geometries. The results are compared with previously published results. The accuracy of the integration approach is compared to the Monte Carlo simulation. Then the path loss using the simulation method and the suggested closed-form expression are presented for different link geometries. The accuracies are evaluated and compared to the results obtained using other methods.

physics.optics↗

arXiv2012

Expurgated PPM Using Symmetric Balanced Incomplete Block Designs

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 equidistance 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).

cs.IT↗

arXiv2010

Novel Codes Family for Modified Spectral-Amplitude-Coding OCDMA Systems and Performance Analysis

In this paper 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 number of processing gains comparing to the previously reported codes. 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.

cs.IT↗