Ebrahim Saberinia

Robustness of star graph network under link failure

The star graph is an attractive underlying topology for distributed systems. Robustness of the star graph under link failure model is addressed. Specifically, the minimum number of faulty links, f(n,k), that make every (n-k)-dimensional substar Sn-k faulty in an n-dimensional star network Sn, is studied. It is shown that f(n,1)=n+2. Furthermore, an upper bound is given for f(n,2) with complexity of O(n^3) which is an improvement over the straightforward upper bound of O(n^4) derived in this paper.

Pulsed-OFDM Modulation for Ultrawideband Communications

In this paper, we describe a novel approach for reducing the power consumption and complexity of a multiband orthogonal frequency-division multiplexing (MB-OFDM) ultrawideband (UWB) system by applying ideas from pulsed UWB systems. The approach is quite general and applicable to many other systems. Unlike the MB-OFDM system, the enhancement that we propose uses pulses with duty cycles of less than 1 as the amplitude shaping pulse of orthogonal frequency-division multiplexing (OFDM) modulation. Pulsating OFDM symbols spread the spectrum of the modulated signals in the frequency domain, leading to a spreading gain that is equal to the inverse of the duty cycle of the pulsed subcarriers. We study the spectral characteristics of pulsed OFDM and the added degrees of diversity that it provides. We show that pulsed-OFDM signals can easily be generated by either upsampling an equivalent OFDM baseband signal with a reduced number of carriers or by replacing the digital-to-analog converter (DAC) of a normal MB-OFDM transmitter with a low-duty-cycle DAC. We establish that a pulsed-OFDM receiver can fully exploit the added diversity without using Rake receivers. It is shown that, while pulsed OFDM has superior or comparable performance to MB-OFDM in multipath fading channels, it also has intrinsic low-complexity and power consumption advantages compared with MB-OFDM. To establish this fact, we describe an example design for the IEEE 802.15.3a Standard and present full simulation results for the UWB indoor propagation channels provided by the IEEE 802.15.3a Standard activity committee.

A Practical Multibit Data Combining Strategy for Cooperative Spectrum Sensing

In this paper, we study a cooperative spectrum sensing scheme for cognitive radio systems where each sensor transmits multibit quantized information to a fusion center where the decision about the availability or occupancy of the channel is made. In particular, we introduce a linear-quantization scheme that is based on a single parameter Δ and derive the optimal detector at the fusion center for this scheme. We also derive the performance of this detector as a function of Δ and use it as a cost function in an optimization problem to find the Δ that provides minimum error. Furthermore, we propose a suboptimal detector with much lower complexity and compare its performance with the optimal detector. Finally, we compare the performance of our multibit combining scheme with the hard and soft combining schemes and show that, with transmission of a few bits of information from each sensor, the system can achieve an error rate very close to the optimal soft combining scheme.

Orthogonal frequency division multiplexing and channel models for payload communications of unmanned aerial systems

Orthogonal Frequency Division Multiplexing (OFDM) can be a good candidate for wideband communications to transmit payload data from an Unmanned Aerial Vehicle (UAV) to the ground station in an Unmanned Aerial System (UAS). However, OFDM systems are prone to inter-channel interference caused by the Doppler spread. Furthermore, because of possible high speed of UAVs, the Doppler spread can be large. In order to design a proper OFDM system for a UAS, it is essential to have an appropriate air-to-ground channel model that accurately models the multipath and Doppler properties of the wideband channel from the UAV to the ground station. Six different channel models are proposed based on various scenarios of the altitude of the UAV (very low, low, and high) and the type of the environment that they are flying over (low-density suburban areas and high-density urban areas). Since no measurement data has been published for wideband signaling from UAVs to a ground station, these models are created by combining parameters of narrowband aeronautical channel models with downlink channel models of wideband terrestrial systems, including HiperLAN, LTE and IEEE 802.16 systems. These channel models were used to evaluate the performance of an OFDM for UAV-to-ground communications. Simulation results show that for high-speed UAVs, the number of sub-channels in an OFDM should be relatively small in order to have reliable communications.

Implementation of a Multi-band Pulsed-OFDM Transceiver

A multi-band orthogonal frequency division multiplexing (OFDM) ultra wideband (UWB) system is being considered for the IEEE 802.15.3a wireless personal area networks. An enhancement to this system, named pulsed-OFDM, has been proposed to reduce the complexity and power consumption of the transceiver without sacrificing performance. In this paper, we describe a detailed implementation of a pulsed-OFDM transceiver. The main focus of the paper is designing each section with maximum power saving and minimum complexity. Specially we design each section such that each part of the pulsed-OFDM transceiver has less or equal complexity and power consumption than the corresponding part in the original multi-band OFDM transceiver. Different options to implement encoder and decoder as well as modulator and demodulator (Inverse Fast Fourier Transform and Fast Fourier Transform) are examined. We also present the simulation results to choose appropriate resolution for analog-to-digital and digital-to-analog converters (ADC and DAC). Finally we investigate the effect of fixed point arithmetic in calculating FFTs and required resolution using simulation results.

A low complexity multi-threshold centralized detection strategy for cooperative spectrum sensing

In this paper, we propose a low complexity multi-bit multi-user combining strategy for cooperative spectrum sensing in cognitive radio networks. We consider the optimal detector where the fusion center receives multi-bit sensing information from several nodes and combine them to make a final decision on the status of the channel. Each node employs a linear quantization technique to generate its multi-bit decision before sending it to the fusion center. Our goal is to optimize this quantization to have the minimum error in sensing. To do so, we use the central limit theorem to get an analytical expression to approximate the performance of the optimal detector at the fusion center. Then, we use this expression to find optimal parameter which is used for quantizing the information in each node. Simulation results are provided to show that the performance evaluation expression is very close to the actual values. Furthermore, we show that using the optimized quantization in each node, the sensing accuracy improves significantly by using 2 or 3 quantization bits compared to the single bit hard decision.

Optimal Transmission Time of Secondary User in an Overlay Cognitive Radio System

Optimal Opportunistic channel access of the unlicensed users has been a major problem in a cognitive radio system. In this paper, we consider an overlay cognitive radio system, where the secondary user senses the channel for an empty slot and transmits a constant power for a time period without sensing the channel again. We obtain the optimal transmission time of the secondary user to achieve the maximum data rate while keeping the interference on the primary user under a given threshold. We derive closed-form expressions for the interference at the primary receiver side and the achievable data rate of the secondary user. Our analysis is based on a Markov model for the primary user statuses and we consider the probability of error in sensing by the secondary user. Computer simulation results of the system show the validity of our analysis.

Channel estimation, equalisation, and evaluation for high-mobility airborne hyperspectral data transmission

In the past few years, unmanned aerial vehicles (UAVs) have become a primary airborne platform for hyperspectral imager for studies on precision agriculture, defence, and the environment. The ‘push-broom’ type of hyperspectral sensors require moving vehicle, and transmission and analysis of hyperspectral data by means of a UAVs high-mobility channel is challenging. While high bandwidth of hyperspectral imaging justify using orthogonal frequency division multiplexing (OFDM) for data transmission, the high speed of UAVs imposes intercarrier interference (ICI) on the transmitted OFDM signal because of the Doppler shift. This study proposes a technique for channel estimation and equalisation in order to compensate the ICI. This technique uses a complete channel matrix estimation in the frequency domain in contrast to conventional methods that only use diagonal elements when recovering the data. In order to evaluate the received data using this technique, a classification framework was designed that took into consideration both spectral and spatial information. In order to verify the robustness of the proposed model, the system was analysed using a Pavia Center hyperspectral dataset, and evaluated against speeds of 50 and 500 m/s. By using this method, improvement in both data transmission and the analysis was achieved.

ICI Mitigation for High-Speed OFDM Communications in High-Mobility Vehicular Channels

The performance of an Orthogonal Frequency Division Multiplexing (OFDM) system to transmit high bandwidth data from a vehicle to a base station can suffer from Inter-Carrier Interference (ICI) created by high Doppler shifts. In current communication systems, high Doppler shifts can happen because of the high speed of the vehicles such as fixed wings unmanned aircraft vehicles (UAVs) and high speed trains (HST). In next generation wireless systems with high carrier frequency, such as 5G cellular data systems at center frequency between 27.5–71 GHz, even a vehicle moving with moderate speed can cause Doppler shift of several kilohertz. To cancel the ICI, the time variant channel matrix should be estimated in the frequency domain. In this paper, a new channel estimation scheme is presented suitable for high Doppler scenarios. To estimate the channel in the frequency domain, a training sequence in the time domain is transmitted, and both channel amplitudes and Doppler shifts are estimated in time domain. Then, the complete frequency domain channel matrix is constructed from the estimated parameters and used for ICI mitigation. In contrast to conventional methods that only estimate diagonal elements of the frequency domain channel matrix or other partial section of the matrix to reduce the complexity, this new method estimate the complete matrix. Simulation results show significant gain in performance for the complete channel estimation as compared to conventional methods using least square and minimum mean square diagonal elements of the channel estimators in high Doppler scenarios.

Performance Enhancement of OMP Algorithm for Compressed Sensing Based Sparse Channel Estimation in OFDM Systems

Long duration of the channel impulse response along with limited number of actual paths in orthogonal frequency division multiplexing (OFDM) vehicular wireless communication systems results in a sparse discrete equivalent channel. Implementing different compressed sensing (CS) algorithms enables channel estimation with lower number of pilot subcarriers compared to conventional channel estimation. In this paper, new methods to enhance the performance of the orthogonal matching pursuit (OMP) for CS channel estimation method is proposed. In particular, in a new algorithm dubbed as linear minimum mean square error-OMP (LMMSE-OMP), the OMP is implemented twice: first using the noisy received pilot data as the input and then using a modified received pilot data processed by the outcome of the first estimator. Simulation results show that LMMSE-OMP improves the performance of the channel estimation using the same number of pilot subcarrier. The added computational complexity is studied and several methods are suggested to keep it minimal while still achieving the performance gain provided by the LMMSE-OMP including using compressive sampling matching pursuit (CoSaMP) CS algorithm for the second round and also changing the way the residue is calculated within the algorithm.