In this paper, the CSK/SSMA ALOHA system with nonorthogonal sequences which combines the ALOHA system with Code Shift Keying (CSK) using nonorthogonal sequences is proposed. The throughput performance was evaluated by theoretical analysis. Moreover, the throughput performance of the system is compared with those of the DS/SSMA ALOHA and M-ary/SSMA ALOHA systems. It is found that the throughput performance of our system to be better than those of the other two systems.

M-ary/SS systems are compared with DS/SS systems applied the multicarrier systems and the RAKE receiver in the presence of AWGN, frequency selective Rayleigh fading and partial-band interference. In particular, the BER performance and SSMA capability are evaluated. Consequently, the M-ary/SSMA system using the multicarrier techniques is subject to the M-ary/SSMA system with the RAKE receiver in the presence of partial-band interference. The BER performance of the M-ary/SSMA system is better than that of the DS/SSMA system when the number of users is smaller than 20. And the spectral efficiency of the multicarrier M-ary/SSMA system is best in the four systems when JSR=20 [dB] and BER=10-3.

In this paper, a simple frame synchronization system for M-ary Spread Spectrum (M-ary/SS) communication system is analyzed. In particular, synchronization performance, bit error rate performance, and Spread Spectrum Multiple Access (SSMA) performance are analyzed. The frame synchronization system uses the racing counters. The transmitted signal contains framing chips that are added to spreading sequences. In the receiver, the framing chips are detected from several frames. The authors have proposed the simple frame synchronization system that detects framing chips from consecutive 2 frames. In this system, as the number of framing chips increases, synchronization performance improves and bit error rate performance degrades. In this paper a frame synchronization system that improves bit error rate performance is treated and analyzed. As the rusult, when the number of reference frames is 3, the bit error rate is much improved than the conventional system.

The performance of an M-ary spread-spectrum multiple-access (M-ary/SSMA) scheme in the presence of carrier frequency offset is discussed in this paper. The influence of carrier frequency offset on the non-coherent reception of M-ary/SSMA signals is examined and it is shown that the carrier frequency offset degrades the performance remarkably, yet. this influence has a distinctive property. Making use of this property, we propose a new M-ary/SSMA scheme that can mitigate the influence of the carrier frequency offset. The scheme is based on the assignment of two distinctive Hadamard codes to in-phase and quadrature components of the transmitted signal. The effect of simultaneous transmission is evaluated in terms of bit-error-rate performance with the carrier frequency offset. As the result, it is observed that the satisfactory bit-error-rate performance can be achieved in the presence of carrier frequendy offset.

This paper proposes M-ary/SSMA using co-channel interference cancellation techniques and presents comparisons with conventional DS/SSMA and other systems. First, ideal models of DS/SSMA and M-ary/SSMA using co-channel interference cancellation techniques are analyzed. In the cancellation circuit of DS/SSMA, when an error bit of other user's data arises, the received signal is degraded by "voltage addition" of the error sequence. While, in M-ary/SSMA, it is degraded by only "power addition" of the error code. Therefore, though the circuits are complicated, bit error rate of the proposed system can be improved considerably. Further, improvement of spectral efficiency in these systems are shown for several bit error rate and chip waveforms.

Spread spectrum (SS) communication systems have been studied and developed for commercial applications such as mobile communications, consumer communications and so on. The reason is that SS communication systems have various characteristic features, such as, robust immunity to interference and jamming, achieving privacy in communication and random access capability (spread spectrum multiple access: SSMA). The performance of the systems, mainly, depends on modulation method of spreading the spectrum. This paper introduces recent several methods of modulation in SS communication systems. First, direct-sequence (DS), which is a fundamental modulation in SS communication system, and M-ary/SSMA, which can increase the number of multiple access users, are described. For increasing the data rate per user, parallel SS, parallel combinatory SS and a code division multiplex using orthogonal Manchester coded M-sequences are introduced. Second, frequency hopping (FH), in which the spectrum of signal is spread by hopping carrier frequency, and related systems are shown. Finally, from a viewpoint of communication theory, spectral efficiency (i.e., data rate the number of multiple access users/bandwidth) of systems relating spreading codes are discussed. As a result, maximum spectral efficiency can be obtained by appropriate spreading factor for each spreading code.