Homayoun Nikookar

A Review of Wavelets for Digital Wireless Communication

Wavelets have been favorably applied in almost all aspects of digital wireless communication systems including data compression, source and channel coding, signal denoising, channel modeling and design of transceivers. The main property of wavelets in these applications is in their flexibility and ability to characterize signals accurately. In this paper recent trends and developments in the use of wavelets in wireless communications are reviewed. Major applications of wavelets in wireless channel modeling, interference mitigation, denoising, OFDM modulation, multiple access, Ultra Wideband communications, cognitive radio and wireless networks are surveyed. The confluence of information and communication technologies and the possibility of ubiquitous connectivity have posed a challenge to developing technologies and architectures capable of handling large volumes of data under severe resource constraints such as power and bandwidth. Wavelets are uniquely qualified to address this challenge. The flexibility and adaptation provided by wavelets have made wavelet technology a strong candidate for future wireless communication.

A survey on spectrum sensing techniques for cognitive radio

Spectrum sensing is an important functionality of cognitive radio (CR). Accuracy and speed of estimation are the key indicators to select the appropriate spectrum sensing technique. Conventional spectrum estimation techniques which are based on short time Fourier transform (STFT) suffer from familiar problems such as low frequency resolution, high variance of estimated power spectrum and high side lobes/leakages. Methods such as multitaper spectrum estimation successfully alleviate these infarctions but exact a high price in terms of complexity. On these accounts, it appears that the filter bank spectrum estimation formulated by F. Boroujeny and wavelet based spectrum estimates are the most promising and pragmatic approaches for CR applications. This article surveys and appraises available literature on various spectrum sensing techniques and discusses spectrum sensing as a key element of CR system design.

An overview of ultra wide band indoor channel measurements and modeling

In this paper, an overview of reported measurements and modeling of the ultra wide band (UWB) indoor wireless channel is presented. An introduction to UWB technology and UWB channels is provided. Different UWB channel sounding techniques are discussed and approaches for the modeling of the UWB channel are reviewed. The available indoor UWB channel measurement results are consulted and accordingly, the major UWB channel parameters are presented and compared to those of narrowband systems. The novelty of this work is the gathering of different UWB channel parameters, analysis, and comparison. Added with the influence of UWB antenna in channel-modeling as well as the frequency-dependency of the channel parameters, leading to a conclusion on the UWB radio channel modeling.

Random phase updating algorithm for OFDM transmission with low PAPR

A novel random phase updating algorithm for reducing the peak-to-average power ratio (PAPR) of OFDM signals is addressed. The phase of each subcarrier is updated by a random increment until the PAPR goes below a certain threshold level. We investigate the influence of different distributions for the phase increments and the variance of distributions on the mean and variance of PAPR as well as the number of iterations to reach the threshold. Further, the random phase updating algorithm has been extended by dynamically reducing the threshold level. After successful updating of the phase shifts, the threshold level is reduced and the variance of the phase increments is changed. Simulation results of the algorithm are provided. It is shown that the random phase updating algorithm with dynamic threshold gives the best results and can reduce the mean power variance of an 8-carrier OFDM signal with BPSK modulation by a factor of 7 dB. In order to reduce the complexity, the random phase updating algorithm is investigated with quantization and grouping of the phase shifts. Results show that for a 16-carrier OFDM system, 2-level quantization of phase shifts in 8 groups of 2 carriers give no significant increase in the power variance while reducing complexity. Further, the impact of phasing on the bit error rate performance of the OFDM system is studied.

Cognitive radio modulation techniques

The growing demand on wireless communication systems to provide high data rates has brought with it the need for a flexible and efficient use of the spectrum resource, which is a scarce commodity. The regional spectrum allocation policy counteracts the free mobility of radio communication equipment. The vast majority of the available spectral resources have already been licensed, so it appears that there is little or no room to add any new services, unless some of the existing licenses are discontinued. Furthermore, recent studies and measurements have shown that vast portions of the licensed spectra are rarely used due to the inflexible spectrum regulations. The whole idea behind cognitive radio (CR) use is that it should prompt effective spectrum use, since intelligence and learning processes aid the radio system to access the spectrum effectively. The CR system has learning and understanding capabilities so that the stated goals may be achieved. In this article, we shall limit the scope of cognition to reduce mutual interference between CR-based rental (unlicensed) users (RUs) and licensed users (LUs) and in providing coexistence between them. It is expected that the rental users will be allowed to transmit and receive data over portions of spectra when primary (i.e., licensed) users are inactive. In this article, we will introduce the modulation strategies employed to realize a coexistence between the CR-based rental system and the licensed system. This is done in such a way that the RUs are invisible to the LUs. We consider the rental user accesses the unoccupied LU band in overlay fashion.

Cognitive Radio Dynamic Access Techniques

The ever growing demand for wireless services has placed enormous burden on valuable resources such as spectral bandwidth. This has resulted in a major rethinking in resource allocation policies culminating in an explosion of research activity in the field of Cognitive Radio (CR) towards optimum resource usage. In this tutorial paper the physical layer design and transmission techniques for CR in the context of efficient spectrum utilization are discussed. Spectrum sensing as the key element of CR awareness is described. Orthogonal frequency division multiplexing (OFDM) as a spectrally efficient modulation scheme is discussed and the rationale for its use in the CR system is explained. Spectrum pooling for efficient use of spectrum is studied and the role of adaptive OFDM techniques in this method is highlighted. Wavelet basis function as a replacement of Fourier transform in OFDM is evaluated. MIMO system as an added value to the CR performance is described. Adaptive Waveform and beamforming as alternative techniques in CR are reviewed.

On the Capability of a Radar Network to Support Communications

Recent achievements in wireless technology could promote a new generation of radar networks, where the communication message for instance the reports on the detected targets, is embedded into the OFDM radar waveform. In this paper, the performance of a four-station network is assessed at the physical layer, in terms of bit error rate (BER) and throughput rate. On the one hand, we analyse the system performance w.r.t. the antenna pattern on reception and on the other hand we study the effects of different network or antenna set-ups. We consider 2 different design cases: (i) same directional radar antenna for transmission and reception and (ii) an additional omni-directional antenna at reception to support communications. Hints for an intelligent data fusion are provided. None of the traditional radar specific issues have been addressed.

Comparison of sensitivity of OFDM and Wavelet Packet Modulation to time synchronization error

Wavelet Packet Modulation (WPM) is a novel multicarrier modulation (MCM) technique and a promising alternative to the well established OFDM. However a few key research questions remain to be addressed before WPM can become practically viable. One of them is their sensitivity and vulnerability to time synchronization errors. This is particularly interesting because WPM symbols overlap in the time domain. In this paper we investigate the BER performance degradation of WPM systems in the presence of timing offset and compare it with OFDM. Several well-known wavelets such as Daubechies, Symlets, Coiflets, discrete Meyer and biorthogonal wavelets are selected and studied. The result of this research show how these wavelet-based systems cope with time synchronization errors and how their performances compare with OFDM.

UWB Channel Measurements and Results for Office and Industrial Environments

This paper presents the results of ultra wide band (UWB) channel measurements carried out at the campus of Delft University of Technology. The measurements were conducted in an indoor office and industrial environment using a time domain setup which allows measurements from 3.1 to 10.6 GHz. Results on large scale path-loss exponent, shadowing, small scale fading and rms delay spread (RDS) for indoor office and industrial area propagation are presented

UWB channel measurements and results for wireless personal area networks applications

This paper presents the results of ultra wide band (UWB) channel measurements for wireless personal area networks (WPAN). A time domain measurement setup has been developed covering the frequency band from 3.1 GHz to 10.6 GHz. Relevant scenarios for WPAN applications have been proposed and measurements have been conducted in an office at very short distances including both LOS and NLOS cases. Additional NLOS measurements with a human body obstructing the direct path have also been performed to determine the effects of the body on the UWB-WPAN channel characteristics. In addition to this, the near field effect between transmit and receive antenna has been examined and channel parameters such as root-mean-square (RMS) delay spread, Ricean K-factor and path-loss exponent are obtained. These results can be used for analysis and design of UWB systems for WPAN applications