Research On Key Issues About M-ary Spread Spectrum Technique In Satellite Overlap Communication Systems

So-called satellite overlap communication (abbreviated as SOC) is such a communication mode that covertly shares a channel provided by a transparent transponder with other communication systems, whose communication signal is overlapped on the signals of the others but much weaker than them so that it does not affect their normal running. Therefore, SOC system can be employed as an emergency communication instrument and runs by using, without the owner's permission, the links of any communication or broadcast satellite as long as they are located in the space area where they can be used by the user. The SOC system is a covert communication system so that it can easily keep away from destroy, jamming or intercept. Since its signal is extreme weak, it is necessary to use some special techniques, where M-ary spread spectrum technique is a very useful candidate. This dissertation researches on several key issues about M-ary spread spectrum system. The author's main contributions are as follows.The short time spectra of single-chip waveform coded DSSS (Direct Sequence Spread Spectrum) signals and their despreaded signals are analyzed with time-frequency analysis method. A computer searching method of designing M-ary spreading codes is proposed. A wavelet packet construction method of spreading codes is also proposed, where a group of wavelet packets is employed as the radices of the spreading codes to construct a set of orthogonal codes.After thoroughly analyzing the effect of frequency-offset on zero-IF noncoherent despread/demodulator (abbreviated as NDD), a modified NDD is proposed and analyzed. Through comparing the BER performance of the proposed NDD with the conventional NDD, the result suggests that the conventional zero-IF NDD is very sensitive to frequency-offset while the proposed NDD is able to tolerate greater frequency-offset. Meanwhile, based on the equivalent property theory of vector norm, a simplified algorithm for calculating the energy of NDD signals is proposed with 1-norm method.An M-ary spread spectrum system is based on many orthogonal spreading codes, so that its despreading/demodulation appears to be quite complex computationally, which limits its wider applications. A novel M-ary spread spectrum system based on cyclic spreading codes is presented, where only one prototype spreading code is assigned to each user and the codes for different users are orthogonal or quasi-orthogonal with each other. The M spreading codes for each user to represent K bits data are generated by circularly shifting the assigned code and/or reversing its polarity. The spreading codes generated as above are called as Orthogonal Cyclic Codes (OCCs) or BiOrthogonal Cyclic Codes (BOCCs). At the receiver of the system, a transform domain matched-filter implemented by means of Fast Fourier Transform (FFT) is employed to despread and demodulate the received signals.A novel M-ary MC-CDMA system applied to the uplink of TDD (Time Division Duplication) mode is proposed, which is based on OCCs and pre-equalization. At the transmitter, every mobile user employs the OCCs as M-ary spreading codes, which can reduce the spreading codes selection difficulty of M-ary MC-CDMA system. In order to keep good orthogonal property between every user's signals, a pre-equalizer is employed to process the M-ary MC-CDMA signal before transmitted. The received signal is multiplied by the local sequence, and then despreaded & demodulated by base-station OFDM demodulator at the same time. The proposed scheme has better performances than traditional M-ary MC-CDMA system and MC-CDMA system on BER performance, bandwidth efficiency, convenience in the spreading codes selection and anti-fading potential.A scheme of M-ary spread spectrum receiver based on OCCs and single chip SAWTDL device is proposed, which can bypass the difficulty in high consistency requirement for multiple SAWTDL devices. At the same time, a quasi-optimal M-ary spread spectrum receiver scheme is also proposed, which can despread long M-ary spreading codes with one chip short-delay-time SAWTDL device. Meanwhile, the influence of combination-correlation interfere can be reduced by employing the presented MOCCs for an M-ary spread spectrum system. The analysis of the system performance in the case of carrier frequency with offset shows that quasi-optimal M-ary spread spectrum receiver has much stronger ability against frequency offset than the optimal receiver.