Symbol and Bit Error Rates Analysis of Hybrid PIM-CDMA
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Symbol and Bit Error Rates Analysis of Hybrid PIM-CDMA Z. Ghassemlooy Optical Communications Research Group, School of Engineering & Technology, Northumbria University, Newcastle NE1 8ST, UK Email: [email protected]
C. K. See Optical Communications Research Group, School of Engineering & Technology, Northumbria University, Newcastle NE1 8ST, UK Email: [email protected] Received 7 April 2004; Revised 17 August 2004 A hybrid pulse interval modulation code-division multiple-access (hPIM-CDMA) scheme employing the strict optical orthogonal code (SOCC) with unity and auto- and cross-correlation constraints for indoor optical wireless communications is proposed. In this paper, we analyse the symbol error rate (SER) and bit error rate (BER) of hPIM-CDMA. In the analysis, we consider multiple access interference (MAI), self-interference, and the hybrid nature of the hPIM-CDMA signal detection, which is based on the matched filter (MF). It is shown that the BER/SER performance can only be evaluated if the bit resolution M conforms to the condition set by the number of consecutive false alarm pulses that might occur and be detected, so that one symbol being divided into two is unlikely to occur. Otherwise, the probability of SER and BER becomes extremely high and indeterminable. We show that for a large number of users, the BER improves when increasing the code weight w. The results presented are compared with other modulation schemes. Keywords and phrases: optical wireless, digital modulation, error rate, pulse modulation.
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INTRODUCTION
As in RF wireless systems, diffuse optical wireless systems can employ a multiple access scheme for channel reuse strategy. The direct-sequence (DS-) CDMA is one promising scheme, which operates in both time and wavelength (frequency) domains, and can enhance the channel capacity when the wavelength resources are constrained. However this is achieved at the cost of reduced data throughput. A DS-CDMA system employing on and off keying (OOK), which is known as OOK-CDMA, utilises OOCs to form a signature signal for the purpose of message separation, thus enabling the transmission of a large number of asynchronous users. In DSCDMA, encoding is carried out by “spreading” individual bits to form a signature sequence with a higher bandwidth. However, the bandwidth of a diffuse optical wireless system is limited by characteristics of the channel. Under such a bandwidth constraint, a DS-CDMA system’s throughput is reduced, which is inversely proportional to the length of the This is an open-access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
signature sequence. It has been reported that OOK-CDMA offers advantages over OOK with an increased number of channels, but at the cost of reduced data throughput under limited bandwidth [1]. To improve system’s power efficiency and the throughput without the need for bandwidth expansion and to enhance the da
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