| Recently, infrared has attracted a considerable amount of interest as an alterna-tive medium to radio for wireless transmission. Its many advantages include a hugeunregulated bandwidth, high data rates, resistance to multipath fading relative securityagainst eavesdropping, and the absence of interference between links operating in adja-cent rooms for wireless indoor transmission. The work presented in this thesis focuseson modulation schemes, multiple access techniques and two prototype infrared wirelesssystems, one of which is diffuse link and the other of which is cellular link. One of the most important technical designs for an infrared wireless system is thechoice of modulation scheme. Concerns of eye safety and power consumption limitthe average transmitter power, which suggests that modulation techniques possessinga high peak-to-mean power ratio are favorable. This is generally achieved by tradingoff power ef?ciency against bandwidth ef?ciency. A novel dual-amplitude pulse in-terval modulation (DAPIM) for optical wireless communications is proposed in thispaper. The symbol structure, the spectral properties, and the expression for bandwidthrequirement, slot and packet error rates, and optical power requirement of DAPIM inthe absence of and in the presence of multipath dispersion is presented in this paper.In the non-distorting additive white Gaussian noise channel, DAPIM shows a bettercompromise between bandwidth and power requirements by increasing the number ofbits per symbol. In non-directed line of sight and diffuse link con?gurations, multipathpropagation causes intersymbol interference (ISI), which can greatly degrade link per-formance especially at high bit rate. From the analysis of the performance of DAPIMin the presence of ISI, it is found that the improved bandwidth ef?ciency of DAPIMresults in lower average optical power penalties than PPM and PIM when bit rate isgreater than 48Mbit/s with 4 bits per symbol. Code division multiple access has been investigated as an attractive candidate foroptical wireless systems because it allows multiple users to access the same channelasynchronously with no time synchronization or frequency management as time di-vision multiple access or wavelength division multiple access. In this thesis, a newDAPIM-CDMA for optical wireless communications is proposed. From the analysisof the upper band and lower bound of slot and packet error rates in the presence of – III –Abstractmultiple access interference, it is shown that DAPIM-CDMA offers improvement inthe error performance compared with OOK-CDMA. This thesis also introduces a diffuse link with slot rate 10MHz. The diffuse link iscon?gured in a house of 7.4 × 4.9× 2.8m, and the transmitter and receiver point straightup. Its error performances employing OOK, PPM, PIM and DAPIM are measuredrespectively, and theoretical results with little ISI are con?rmed. A channel with severemultipath ISI is also built, experiments in which validate that DAPIM achieves thebetter power ef?ciency than PPM and PIM. Finally, details are given of an infrared communication system applying for Elec-tronic Toll Collection System. Its communication speed is 4.096Mbit/s, its commu-nication distance is 30m, and the width of one lane is 4m. The bit error rate in thecommunication area is less than 10? . This system has been checked and accepted by 6our copartner MHI.
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