Ibrahim Budiarjo

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.

Efficient Pilot Pattern for OFDM-based Cognitive Radio Channel Estimation - Part 1

This paper is the extension of the first paper (Rashad et al., 2007) that described the modification of rectangular pilot pattern applied in OFDM-based cognitive radio system. In this paper, a new concept for pilot symbols arrangement in OFDM transmission system is introduced. The new idea is the virtual pilot symbols concept which is based on the interpolation of the received pilot symbols. Further in the paper virtual pilot symbols concept together with the modified hexagonal pilot pattern are proposed as a new pilot symbols arrangement technique for OFDM-based cognitive radio system. Using the proposed pilot patterns, the channel estimation based on two cascaded one dimensional (2times1-D) Wiener filter is investigated by simulation. Both narrowband and wideband Licensed Users are considered. The results have shown more than 15 dB SNR gain is achieved by the hexagonal pilot pattern with virtual pilot symbol at channel estimate mean square error (MSE) 2 times 10-3 compared to the regular rectangular pilot pattern.

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.

Combined Spectrum Pooling and Adaptive Bit Loading for Cognitive Radio OFDM Based System

This paper presents the combination of adaptive bit loading and spectrum pooling to be applied in a cognitive radio orthogonal frequency division multiplexing (OFDM) based system. Adaptive bit loading is a method to adapt the constellation size of the transmitted signal according to the channel variations by exclusion of heavily faded carriers from transmission and as a compensation higher modulation modes are allocated to subcarriers with good SNR. Windowing of OFDM signal is a spectrum pooling method for shaping the signal spectrum in order to reduce interference to neighbouring system (legacy users). In this paper different OFDM adaptive bit loading algorithms are combined with several window functions, and the bit error rate performance of the system is simulated for a wide sense stationary uncorrelated scattering (WSSUS) mobile multipath channel. The new results of this paper show how promising is the combination of bit loading algorithm and window form

On the Use of Virtual Pilots with Decision Directed Method in OFDM Based Cognitive Radio Channel Estimation Using 2x1-D Wiener Filter

In this paper we propose the novel concept of virtual pilots to aid the channel estimation In OFDM based Cognitive Radio system with the optimum Hexagonal pilot pattern. The virtual pilots are derived from the simple linear Interpolation and extrapolation combined with decision directed method in reducing the effect of noise to the initial channel estimates. The results have shown that the use of virtual pilots has almost reached the mean square error performance of two times oversampling pilot allocation without adding additional pilots.

Cognitive radio with single carrier TDCS and multicarrier OFDM approach with V-BLAST receiver in Rayleigh fading channel

This article presents the performance comparison of TDCS and OFDM based cognitive radio for MIMO system using VBLAST receiver architecture to reconstruct the transmitted data. The interference avoidance performance in terms of BER and bitrate are improved by adding multiple antennas to the system and the use of V-BLAST technique at the receiver. The results show the most promising interference avoidance technique combined with MIMO V-BLAST architecture to be applied in the CR system.

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.

Cognitive Radio with Single Carrier TDCS and Multicarrier OFDM Approach with V-BLAST Receiver in Rayleigh Fading Channel

This paper presents the performance comparison of Cognitive Radio TDCS and OFDM based for MIMO system using VBLAST receiver architecture to reconstruct the transmitted data. The interference avoidance performance in terms of BER and bitrate are improved by adding multiple antennas to the system and the use of V-BLAST technique at the receiver. The results show the most promising interference avoidance technique combined with MIMO V-BLAST architecture to be applied in the CR system.

MIMO TDCS with Extra Embedded Symbol for Higher Data Rates in Overlay Spectrum Sharing System

In this paper an extension to our work on embedded symbol in TDCS-CCSK system [1] is provided by adding MIMO and V-BLAST receiver architecture to the system in order to counteract the fading of the wireless channel. The embedded symbol can be derived from PAM, QAM or PSK mapping. In the context of overlay spectrum sharing PSK is considered as the most appropriate mapping for the embedded symbol since it has no impact on the overall TDCS-CCSK power spectrum density. As each transmit antenna transmits different data, the bit rate of the proposed TDCS system becomes higher. The simulation results show that for the ratio between the number of the transmit antennas and the receive antennas of 1/2 or lower, the bit error rate (BER) performance improves significantly. The limit of the embedded symbol constellation size to have a better overall BER compared to the conventional TDCS-CCSK in MIMO VBLAST system is found to be 16.