García-Palencia, Oscar

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    A novel fft pruning algorithm: improving efficiency for a nc-ofdm cognitive radio
    (2012) García-Palencia, Oscar; Morales-Tirado, Lizdabel; College of Engineering; Rodríguez, Domingo; Lu, Kejie; Department of Electrical and Computer Engineering; Walker, Uroyoán R.
    Cognitive Radio (CR) is a promising candidate to deploy future wireless networks because CR can enable an efficient use of the resources involved in wireless communications networks, specifically the Electromagnetic (EM) spectrum. Recent research shows that there is an overuse of some spectrum bands such as cell phone or IEEE 802.11 bands; meanwhile, other bands such as the regional analog television are abandoned. This current allocation of the EM spectrum yields an underutilization of this resource. Furthermore, this misallocation has generated a false notion of a spectrum scarcity. Thus, trying to solve the current underutilization of the channel, an interdisciplinary research is addressing all aspects related to this problem, namely, technical, scientific, and regulatory. Specifically, from a technical-scientific point of view, a transmission model where a secondary non-licensed user could use the bands assigned to incumbent (licensed) users without causing any interference has been proposed. This approach is known as opportunistic use of the spectrum and offers great challenges in terms of implementation. In the physical layer, many modulation schemes have been proposed, but Non-Contiguous OFDM appears to be a suitable candidate to build practical CR systems (NC-OFDMCR). According to the availability of the spectrum, the cognitive engine of a NC-OFDMCR selects the transmission pattern activating or deactivating specific bands of sub-carriers. In this work, the main goal is to contribute to the implementation of NC-OFDMCR in the physical layer performing a research on the implementation issues. Among these is- sues, the most important one is the computation of the Discrete Fourier Transform (DFT) for sparse signals. This work shows the mathematical and computational (implementation) aspects related to the DFT. Then, the work proposes an algorithm based on Cooley-Tukey Radix-2 Fast Fourier Transform (FFT) to calculate the DFT using a hybrid dynamic structure to represent sparse signals. When the dynamic structure is traversed, irrelevant computations can be pruned. The algorithm is evaluated for signal lengths greater than 216, and it shows efficient results. These lengths are important in the sense that they allow to perform NC-OFDM wide-transmissions. Besides, the proposed data structure enables the representation of sparse signals in other algorithms that compute the DFT such as those based on Polynomial Algebras. In addition, the algorithm is coded in C-language allowing its implementation in other platforms.