Madan Kumar Lakshmanan

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.

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.

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.

Maximally Frequency Selective Wavelet Packets Based Multi-Carrier Modulation Scheme for Cognitive Radio Systems

In this paper we demonstrate the operation of a wavelet packet based multi-carrier modulation (WP-MCM) scheme in the context of cognitive radio. The wavelet packets are derived from multistage tree-structured paraunitary filter banks. The emphasis is on the design and development of maximally frequency selective wavelets derived from a modified Remez exchange algorithm. To enable the WP-MCM cognitive radio system to co-exist with other licensed users a common spectrum pool is maintained and the WP-MCM transmission waveform characteristics are shaped to communicate in the idle time-frequency gaps of the licensed user. This is achieved by dynamically deactivating wavelet packet carriers in and near the region of the licensed user spectrum. Through simulation results, we demonstrate the efficacy of the proposed wavelet packet based mechanism in seamlessly cohabiting with licensed users. The bit error rate (BER) performance is shown to be comparable, and even at times better, to conventional Fourier based OFDM system.

A study on the application of wavelet packet transforms to Cognitive Radio spectrum estimation

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, variance and high side lobes/leakages. In this article, the possibility of employing wavelet packet decomposition as a basis for new spectrum sensing approach is investigated. Three types of sources, namely partial band, single tone and swept tone, are used to investigate the performance of the proposed wavelet based approach. Swept tone source is a special kind of source devised to simulate the temporal dynamics in cognitive radio. The results of the experiments show that the wavelet based approach is quite promising for spectral estimation and can be a suitable technology for CR applications.

Wavelet Packet Multi-Carrier Modulation MIMO Based Cognitive Radio Systems with VBLAST Receiver Architecture

This article presents a cognitive radio system built from a wavelet packet multi-carrier modulation MIMO scheme with a VBLAST receiver architecture. The objective is to combine the promise of optimum utilization of radio resources by cognitive radio, the high data throughput without additional bandwidth/power requirements offered by MIMO and the flexibility of wavelets, to develop intelligent wireless systems. Taking the example of effective spectrum utilization, we demonstrate the feasibility and efficiency of the proposed system through simulation studies.

Wavelet Packet based Strategy to Mitigate Wideband Interference on Impulse Radio

In this contribution we present a novel wavelet packet based method to mitigate interference from wide band sources such as the WLAN standard IEEE 802.11a on impulse radio. The scheme works by linearly decomposing the UWB transmission pulse into an expansion set and selectively removing sub-components in a way that interference zones are avoided. This way the transmission signal spectrum is dynamically sculpted by a proper choice of pulse waveforms and wavelet packet codes, to mitigate interference. The focus of the work is on the design and application of maximally frequency selective filter banks derived from a modified Remez exchange algorithm. Through simulation results the proposed scheme is shown to be effective for mitigation of IEEE 802.11a interference with a much improved bit error rate performance.

Construction of Optimum Wavelet Packets for Multi-Carrier Based Spectrum Pooling Systems

The growing popularity of wireless applications has placed enormous burden on valuable resources such as spectral bandwidth. This has brought about a major revamp of traditional resource allocation policies culminating in an explosion of research activity in the field of cognitive radio (CR) (Mitola, Cognitive radio: an integrated agent architecture for software defined radio, Doctoral dissertation, 2000; Haykin, J Sel Areas Commun 23:201–220, 2005). In this paper we demonstrate the operation of a Wavelet Packet based multi-carrier modulation (WP-MCM) scheme in the context of cognitive radio. The wavelet packet (WP) bases are derived from multistage tree-structured paraunitary filter banks. To enable the WP-MCM cognitive radio system to co-exist with other licensed users a common spectrum pool is maintained and the WP-MCM transmission waveform characteristics are shaped to communicate in the idle time-frequency gaps of the licensed user. This is achieved by dynamically deactivating wavelet packet carriers in and near the region of the licensed user spectrum. Various wavelets including the well-known families Daubechies, Coiflet, Symlet are applied and studied. The emphasis is on the design and development of optimal WP carriers that have narrow and well confined spectral footprints. To this end filter banks that are maximally frequency selective are derived through a modified Remez exchange algorithm. Through simulation results the ability of the proposed wavelet packet based mechanism in seamlessly cohabiting with licensed users is demonstrated. The Bit Error rate (BER) performance is shown to be comparable, and even at times better, to the conventional Fourier based OFDM system.

Cognitive Radio transmission and spectrum sensing using a wavelet packet transceiver

In this paper we demonstrate the operation of a Wavelet Packet based multi-carrier modulation (WPMCM) transceiver for Dynamic Spectrum Access. The wavelet packet (WP) bases are derived from multistage tree-structured paraunitary filter banks. To enable the WPMCM Cognitive Radio (CR) system to co-exist with other Licensed Users (LU) a common spectrum pool is maintained and the WPMCM transmission waveform characteristics are shaped to communicate in the idle time-frequency gaps of the licensed user. This is achieved by dynamically vacating WP carriers in and near the region of the licensed user spectrum. Furthermore, a spectrum estimation unit is tagged to the WPMCM transceiver structure by exploiting the filter bank infrastructure used for Discrete Wavelet Packet Transform implementation. Thus spectrum analysis is done at no additional cost. In the studies, two types of LUs are employed, namely, partial band and multiple tone. The simulation results show that in the presence of LU, the proposed spectrum adaptation method offers significant BER improvements allowing the CR to operate invisibly to the LU.