Spread Spectrum, CDMA, and Ultra-Wideband Communications

The subject of this chapter is devoted to spread spectrum (SS), CDMA, and ultra-wideband (UWB) communication systems. After the introduction of SS systems, both direct sequence-spread spectrum (DS-SS) and frequency hopping-spread spectrum (FH-SS) systems are described in detail. Regarding the DS-SS systems, after typical transmitter and receiver configurations are described, we describe in detail the synchronization process including acquisition and tracking stages. To deal with the multipath fading effects, the use of RAKE receiver is described. The section on DS-SS systems concludes with the spreading sequences description, in particular pseudo-noise (PN) sequences. In this section, after the introduction of the basic randomness criteria, we introduce maximal-length sequences (m-sequences) and describe their basic properties. In the same section, the concept of nonlinear PN sequences is introduced as well. Regarding the FH-SS systems, both slow-frequency hopping (SFH) and fast-frequency hopping (FFH) systems are introduced. We provide the detailed description of FH-SS systems, including both transmitter and demodulator as well as various time acquisition schemes. In the same section, we describe how to generate the FH sequences. After the introduction of CDMA systems, we describe various signature waveforms suitable for use in DS-CDMA systems including Gold, Kasami, and Walsh sequences. We then describe relevant synchronous and asynchronous CMDA models, with special attention devoted to discrete-time CDMA models. In section on multiuser detection (MUD), we describe how to deal with interference introduced by various CDMA users. The conventional single-user detector is used as the reference case. The various MUD schemes are described then, with special attention being paid to the jointly optimum MUD. The complexity of jointly optimum MUD might be prohibitively high for certain applications, and for such applications, we provide the description of the decorrelating receiver, which can compensate for multiuser interference, but enhances the noise effect. To solve for this problem, the linear minimum mean-square error (MMSE) receiver is introduced. The section on MUD concludes with a description of the nonlinear MUD schemes employing decisions on bits from other users (be preliminary or final), with description of successive cancelation and multistage detection schemes. In section on optical CDMA (OCDMA), we describe both incoherent and coherent optical detection-based OCDMA schemes. Regarding the incoherent optical detection-based schemes, we first describe how to design various unipolar signature sequences, known as optical orthogonal codes (OOC), followed by the description of basic incoherent OCDMA schemes including time-spreading, spectral amplitude coding (SAC), and wavelength hopping (WH) schemes. Related to coherent optical detection-based OCDMA systems, we describe both pulse laser-based and CW laser-based OCDMA systems. Further, the hybrid OFDM-CDMA systems are described, taking the advantages of both OFDM and CDMA concepts into account, in particular in dealing with frequency-selective fading and narrowband interference, while at the same time supporting multiple users. The following broad classes of hybrid OFDM-CDMA systems are described: multicarrier CDMA (MC-CDMA), OFDM-CDMA, and OFDM-CDMA-FH systems. In section on UWB communications, we describe the concepts and requirements as well as transmitter and receiver configurations. Further, various modulation formats, suitable for UWB communications, are described and categorized into time-domain category (pulse-position nodulation and pulse-duration modulation) and shape-based category [on-off keying, pulse-amplitude modulation, bi-phase modulation (BPM) or BPSK, and orthogonal pulse modulation]. Regarding the pulse shapes suitable for orthogonal pulse modulation (OPM), the modified Hermite polynomials, Legendre polynomials, and orthogonal prolate spheroidal wave functions (OPSWF) are described. Regarding the UWB channel models suitable for indoor applications, the model due to Saleh and Valenzuela is described. Both types of UWB systems are described: the impulse radio UWB (IR-UWB), typically DS-CDMA based, and multiband OFDM (MB-OFDM).