On spread spectrum multiple access using continuous phase modulation
基於連續相位調制 (CPM) 的擴頻多址技術研究
Student thesis: Master's Thesis
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Award date  2 Oct 2003 
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Permanent Link  https://scholars.cityu.edu.hk/en/theses/theses(249252222ea24165896193f32787a15d).html 

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Abstract
Most researches on spread spectrum techniques are investigated on direct sequence spread spectrum (DSSS) whose waveforms generally have large envelope fluctuations. Such fluctuations will cause performance degradation due to the nonlinearity of powerefficient amplifiers. It is desirable that modulated signals have constant envelope which is particularly useful for mobile handsets. In this thesis, continuous phase modulation (CPM) is applied to spread spectrum in order to generate spread spectrum signals having constant envelope. There are several advantages of using CPM in addition to the property of constant envelope. Under the constraint of constant envelope, continuous phase property has good spectral efficiency. Also it gives a coding gain due to the memory property. Unlike the direct sequence spread spectrum in which the spreading sequence is directly to modulate the carrier, the spread spectrum based on the CPM uses spreading sequence to vary the phase of the carrier. Therefore, the design for spreading sequences for CPMbased spread spectrum systems (CPMSS) is very different from that of the direct sequence. The orthogonal criterion for eliminating multiple access interferences is derived for synchronous spread spectrum systems. The properties of orthogonal spreading sequences are derived from the criterion. A systematic procedure for generating orthogonal spreading sequences is developed from the properties. The power spectrum of CPMSS signal with two binary spreading sequences is formulated and analyzed. Similar to conventional CPM systems, it is shown that the power spectrum of CPMSS signal may have discrete spectral components when the spectral constant is not less than one. Discrete spectral components are undesirable because their presence is a waste of transmitted power and a disturbance to adjacent channels. In order to avoid discrete spectral components, proper choice of spreading sequence is needed. Near orthogonal phase spreading sequences (NOPSS) are proposed that can avoid discrete components in the power spectrum. To compafi the spectral efficiency of different systems, the bandwidth defined by the percentage power containment is used. The ratio of power containment bandwidth (PCB) of DSSS to CPMSS is found varied from 2 for 90% containment to more than 10 for 99%. For detection, the conventional quadrature demodulation scheme is applied together with Viterbi detection. Two different correlators are employed. One is nonpunched (conventional) correlator while the other is punched correlator. The difference between the two correlators is on the integral period. When the near orthogonal phase spreading sequences are used, the punched correlator is preferred because it can remove the multiple access interference (MAI) while the nonpunched one cannot. The bit error rate of CPMSS is shown to outperform DSSS using BPSK. In case of downlink communication, all users' signals are combined before modulating the carrier for transmission. The peaktoaverage power ratio (PAR) of the resultant signal becomes large that may incur intermodulation and outofband interference caused by the nonlinearity of powerefficient amplifier. By simulation, the starting phase of each user's CPMSS signal can be set arbitrarily such that the PAR of the combined CPMSSMA signal is reduced. The results show that the PAR of CPMSSMA is smaller than that of CDMA2000 and IS95. The bit error rate (BER) of generalized minimum shift key (GENMSK) for spread 9 spectrum systems using quadrature structure is analyzed for asynchronous transmission. This analysis is applicable to binary CPMSS and binary DSSS with arbitrary chip waveform as well as synchronous CDMA systems. The biterrorrate is carried out by simplified improved Gaussian approximation (SIGA) method. The formulae of the analysis in timedomain and frequency domain are useful for timelimited or bandlimited chip waveform, respectively. The performance analysis shows that the BER of MSWSSMA is better than BPSWSSMA with rectangular chip waveform but is comparable to that of OQPSWSSMA and BPSWSSMA with raised cosine.
 Code division multiple access, Phase modulation