Abstract
Orthogonal frequency division-multiplexing (OFDM) [1, 2] has received considerable attention as a method to efficiently utilize channels with non-flat frequency responses and/or non-white noise. In its most common form, a high rate data stream is divided up among the many carriers in the system in a manner which optimizes the capacity of the overall channel. OFDM can also be used as a spread spectrum modulation (OFDM-SS) [3, 4, 5] wherein spectral spreading is accomplished by putting the same data on all the carriers, producing a spreading factor equal to the number of carriers. At the receiver, the energy from all the carriers is coherently combined to produce the decision variable. Multiple users can be supported in the same channel through code division-multiple access (CDMA) [6]. In this case each user has a unique signature sequence which determines the set of carrier phases. To receive a particular signal, the receiver needs to know the signature sequence for that user in order to align the carrier phases for the coherent combining operation. Figure 2.1 is a block diagram of an OFDM-SS transmitter/receiver pair. As shown in the figure, carrier generation is usually performed efficiently using an inverse fast Fourier transform (FFT) while demodulation is performed using a forward FFT.
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Hassan, A.A., Hershey, J.E., Saulnier, G.J. (1998). OFDM Spread Spectrum Communications. In: Perspectives in Spread Spectrum. The Springer International Series in Engineering and Computer Science, vol 459. Springer, Boston, MA. https://doi.org/10.1007/978-1-4615-5531-5_2
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DOI: https://doi.org/10.1007/978-1-4615-5531-5_2
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