Papers by Keyword: Multiple Access Interference (MAI)

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Authors: Chih Ta Yen, Wen Bin Chen
Abstract: This study analyzes and calculates unipolar and bipolar coded configurations of spectral-amplitude-coding optical division multiple access (SAC-OCDMA) systems by using analytic and simulated methods. The important feature of the SAC-OCDMA systems is that multiple access interference (MAI) can be eliminated by code sequences of a fixed in-phase cross-correlation value. This property can be effectively canceled by multiple access interference by using balance detection schemes. This study uses Walsh-Hadamard codes as signature codes for the unipolar and bipolar schemes. The coder and decoder structures are based on optical filters of fiber Bragg gratings (FBGs). The simulation results of unipolar/bipolar coding structures are first presented by commercial simulation obtained using OptiSystem software. The simulation results show that the bit error rate (BER) through use of the bipolar coding method is superior to the unipolar scheme, especially when the received effect power is large. The eye diagram also shows that the bipolar encoding structure exhibits a wider opening than the unipolar encoding structure.
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Authors: Chih Ta Yen, Chia Yu Liu
Abstract: With the development of the Internet, how to provide a high-speed and high-security comprehensive network has become urgent issue. In this project, we propose hybrid analog/digital transmissions format scheme which integrated optical code-division multiple-access (OCDMA) and polarization multiplexing technique in free space optics (FSO) transmission. Orthogonal frequency division multiplexing (OFDM) transmits as the analog format and (on-off keying) OOK transmits as digital format in the study, respectively. In the proposed hybrid OCDMA system, it has high-speed transmission, signal security and low cost...etc. we adopt quasi-orthogonal Walsh-Hadamard codes (WHC) that with fixed cross correlation value as the codeword for each base station. With the property, the multiple access interference (MAI) can be efficiently eliminated by using the balanced detection scheme at the receive end. The constellation and eye diagrams show that both OFDM signal as analog signal and OOK modulation as digital signal in the proposed hybrid OCDMA system perform good performance in FSO transmission.
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Authors: Nan Ping Liu, Xiang Bo Song, Ke Wen Xia, Yuan Shi
Abstract: Ultra-wideband(UWB) is currently one of three new technologies of wireless communication. Multi-user detection technique is mainly used to eliminate multiple access interference(MAI)in DS-CDMA UWB communication systems. Linear multi-user detection algorithm for multi-access interference suppression or even elimination has certain results. But for a fixed decision threshold is set to 0, the detection performance of linear multi-user detection algorithm will be decreased significantly. Therefore a kind of multi-user detection with pre-processing has been researched. Through the pre-processing, we set dynamic threshold to direct decision the user's data bit information so as to solve the problem the linear multi-user detector’s decision threshold which is fixed impact the detection performance. We may get the global optimal solution though pre-processing of using the thought of the branch and bound algorithm and forcing rules. Then the original problem changes into a smaller multi-user detection problem, in order to get the aim of reducing MAI, increasing the probability of a correct decision and reducing the bit error rate. From the simulation experiment results, it is shown that it has a significant effect in system detection, because after pre-processing the probability of a correct decision has increased, and the bit error rate has been reduced.
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Authors: Hong He, Dan Li, Tong Yang, Lin He
Abstract: Channel estimation of TD-SCDMA is inaccurate if Joint Detection is separately used in anti-interference progress, and when the number of users or channels increases, amount of calculation is large, real-time control is hard to implement too; if Smart Antenna is only adopted, the formed beam which points to the user is likely to interfere other users. Especially for high-speed mobile users, beam forming bias exists. To solve this problem, combination of Joint Detection and Smart Antenna is proposed in the paper. Simulation results show that the combination not only improves capacity of system remarkably but also eliminate the multiple access interference (MAI)
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