• ETRI Journal
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• v.26 no.1
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• pp.13-20
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• 2004

We propose a new optical code division multiple access (OCDMA) scheme for reducing multiple access interference (MAI) and enhancing performance for optical subscriber access networks using modified pseudorandom noise (PN)-coded fiber Bragg gratings with bipolar OCDMA decoders. Through the bipolar OCDMA decoder and the modified PN codes, MAI among users is effectively depressed. As the data are encoded either by a unipolar signature sequence of the modified PN code or its complement according to whether the data bit is 1 or 0, the bit error ratio (BER) can be more improved with the same signal to interference plus noise ratio over the conventional on-off shift keying-based OCDMA system. We prove by numerical analysis that the BER of the proposed bipolar OCDMA system is better than the conventional unipolar OCDMA system. We also analyze the spectral power distortion effects of the broadband light source.

• Journal of Satellite, Information and Communications
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• v.6 no.1
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• pp.32-36
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• 2011

In this paper, we propose a cancellation scheme of multiple access interference (MAI), which is caused by carrier frequency offset, for the receiver of an interleaved orthogonal frequency division multiple access (IFDMA) uplink system. In the IFDMA systems, carrier frequency offsets introduce adjacent channel interference from other users' subcarriers, which results in performance degradation. In the proposed method, we compensate the carrier frequency offset in time domain and then eliminate MAI by using parallel interference cancellation in frequency domain. Simulation results show that the proposed method is effective in removing the MAI especially when the number of users are large and the MAI severely degrades the system performance.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.34 no.12A
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• pp.998-1005
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• 2009

In the uplink of OFDMA (Orthogonal Frequency Division Multiple Access) based M-WiMAX(Mobile-Worldwide Interoperability for Microwave Access) system, linear phase shift is caused by signals transmitted from multiuser with different delay time and thus, MAI (Multiple Access Interference) occurs. MAI degrades the performance of ranging code detection and delay time estimation in the uplink. Therefore, in this paper, we propose ranging algorithm, applying SIC (Successive Interference Cancellation) to the conventional ranging algorithm, to minimize MAI and to improve ranging performance. The proposed ranging algorithm is verified through the Monte Carlo simulation, which shows the improved performance of ranging code detection and delay time estimation compared to the conventional algorithms. Through compared with random access of the 3GPP LTE, we can know limit of ranging performance.

• Journal of Communications and Networks
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• v.11 no.1
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• pp.1-10
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• 2009

In this paper, we propose a chip-level detection and a spectral-slice scheme for the tunable-transmitter/fixed-receiver (TTFR)-based asynchronous spectral phase-encoded optical codedivision multiple-access (CDMA) system combined with timeencoding. The chip-level detection can enhance the tolerance of multiple access interference (MAI) because the channel collision does not occur as long as there is at least one weighted position without MAI. Moreover, the spectral-slice scheme can reduce the interference probability because the MAI with the different frequency has no adverse effects on the channel collision rate. As a result, these techniques mitigate channel collisions. We analyze the channel collision rate theoretically, and show that the proposed system can achieve a lower channel collision rate in comparison to both conventional systems with and without the time-encoding method.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.9 no.6
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• pp.1174-1181
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• 2005

The exact bit error rate(BER) calculation of an UWB-TH PPM multiple access communication system, which is known to be suitable for the fast transmission of massive information data, is introduced. The statistic feature of the multiple access intereference (MAI) of the system is precisely modeled by the characteristic function technique. The concrete expression for the MAI allows the exact expression for BER to be derived. In addition, we propose the approximate expression for the BER which reveals superior accuracy to the expression from the previous Gaussian approximation of the MAI. The validity of the proposed expressions is confirmed from the comparison of proposed results with the results from Monte-Carlo simulation.

• Korean Journal of Optics and Photonics
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• v.15 no.3
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• pp.229-233
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• 2004

We propose a novel scheme to suppress the multiple-access interference(MAI) in coherent optical CDMA systems. This is based on a differential detection using the dual-control NOLM. For an experimental demonstration, two encoded channels we constructed and decoded. These decoded signals are sent to the dual-control NOLM and a high autocorrelation peak with suppressed MAI at the output of the NOLM is observed. Signal-to-interference ratio is improved by 7 ㏈.

• Journal of Advanced Navigation Technology
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• v.9 no.1
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• pp.56-60
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• 2005

To calculate the probability of bit error of UWB communication systems, the exact expression of multiple access interference is essential. So far, in many researches, MAI has been modeled by the Gaussian Approximation, which leads to the huge errors. And there are some tries to obtain the exact model fot the MAI but they have some problems such as long calculation time. We introduce the simple expression to calculate the probability of error of an UWB-TH system with BPSK. The multiple access interference is explained by the characteristic function method combined with the Gaussian approximation. It allows us to easily and fast calculate the bit error rate of an UWB-TH system.

• The Transactions of the Korean Institute of Electrical Engineers P
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• v.53 no.3
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• pp.129-133
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• 2004

As In DS/CDMA(direct sequence code division multiple access) system, the system capacity is limited by multiple access interference(MAI), and self-interference(SI) resulting from the multi-path propagation of the desired user signal. This paper, which the approximated analytic chip waveforms are nearly the same as the computer generated chip waveforms are shown. And then, the BER(Bit Error Rate) performances in CDMA system using the approximated analytic chip waveforms are shown.

• Journal of Communications and Networks
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• v.6 no.1
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• pp.26-34
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• 2004

A blind interference canceller in the presence of subchipspaced multipath channels for direct-sequence code division multiple access (DS-CDMA) down-link system is considered. This technique is based on combining the existing blind interference canceller with a technique that involves assigning subchip-tap spacing to the Rake receiver. The proposed receiver minimizes the receiver’s output energy subject to a constraint in order to mitigate the multiple access interference (MAI) along each multipath component, and then suboptimally combining all the multipath components. Moreover, it is able to mitigate the mismatch problem when subchip-spaced multipath components arrive at the blind interference canceller. It is known that optimal combining techniques perform a decorrelation operation before combining, which requires both knowledge and computational complexity. In the following, we have adopted a simpler but suboptimum approach in the combining of the suppressed signals at the output of our proposed receiver. Computer simulation results verify the effectiveness of the proposed receiver to handle subchip-spaced multipath components and still suppresses MAI significantly.

• Journal of Communications and Networks
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• v.13 no.2
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• pp.167-174
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• 2011

In this paper, we propose and simulate an efficient interference cancellation scheme with an optimal ordering successive interference cancellation (SIC) algorithm for ultra wideband (UWB)/multiple-input-multiple-output (MIMO) systems in a wireless body area network (WBAN). When there are several wireless communication devices on a human body, multiple access interference (MAI) usually occurs. To mitigate the effect of MAI and achieve additional diversity gain, we utilize SIC with an optimal ordering algorithm. A zero correlation duration (ZCD) code with robust MAI capability is employed as a spread code for UWB systems in a multi-device WBAN environment. The performance of the proposed scheme is evaluated in terms of the bit error rate (BER). Simulation results confirm that the BER performance can be improved significantly if the proposed interference cancellation scheme and the ZCD code are jointly employed.