Abstract
Spread spectrum (SS) and multicarrier modulation (MCM) techniques are recognized as potential candidates for the design of underlay and interweave cognitive radio (CR) systems, respectively. Direct Sequence Code Division Multiple Access (DS-CDMA) is a spread spectrum technique generally used in underlay CR systems. Orthogonal Frequency Division Multiplexing (OFDM) is the basic MCM technique, primarily used in interweave CR systems. There are other MCM schemes derived from OFDM technique, like Non-Contiguous OFDM, Spread OFDM, and OFDM-OQAM, which are more suitable for CR systems. Multicarrier Spread Spectrum Modulation (MCSSM) schemes like MC-CDMA, MC-DS-CDMA and SS-MC-CDMA, combine DS-CDMA and OFDM techniques in order to improve the CR system performance and adaptability. This article gives a detailed survey of the various spread spectrum and multicarrier modulation schemes proposed in the literature. Fast Fourier Transform (FFT) plays a vital role in all the multicarrier modulation techniques. The FFT part of the modem can be used for spectrum sensing. The performance of the FFT operator plays a crucial role in the overall performance of the system. Since the cognitive radio is an adaptive system, the FFT operator must also be adaptive for various input/output values, in order to save energy and time taken for execution. This article also includes the various efficient FFT algorithms proposed in the literature, which are suitable for CR systems.
Highlights
The spectrum requirements for wireless multimedia applications have increased exponentially, leading to spectrum scarcity
In [57], Multi-user detection (MUD) schemes based on the zero forcing (ZF), minimum mean square error (MMSE) and interference cancellation (IC) techniques were proposed for MC-Direct Sequence Code Division Multiple Access (DS-CDMA) based cognitive radio (CR) systems
We observe that the BER performance is significantly improved by increasing the number of transmit antennas T from 1 to 2
Summary
The spectrum requirements for wireless multimedia applications have increased exponentially, leading to spectrum scarcity. Multicarrier modulation and spread spectrum modulation are the most common types of modulation schemes used in wireless communication systems. DS-CDMA is a spread spectrum modulation technique that is commonly used in underlay cognitive radio systems. In DS-CDMA, data rate and power level are controlled based on the spectrum availability and interference constraint on the primary user. In DS-CDMA based underlay cognitive radio systems, the secondary CDMA users spread their data using a signature sequence and the primary users can be CDMA or non-CDMA users. A secondary user finds the unused spectral holes which can be used for its transmission without causing interference to the primary users.
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