In this paper, the design and experimental characterization of a wireless optical transceiver for indoor applications, based on Frequency-Hopping Spread-Spectrum techniques, is presented. Using these techniques reduce the narrowband interference produced by optical sources and the intersymbol interference induced by multipath propagation. It also makes possible using the CDMA capabilities associated with Spread Spectrum, in order to improve the performances when several emitters and receivers are considered. The main drawback of these kind of systems lies on the high complexity of the synchronization system of the receiver, typically consisting on two cascaded structures: acquisition and tracking. We propose using a dual-pilot signal, transmitted by a master emitter, for reducing both complexity and cost of the synchronization stage of the receiver.
KEYWORDS: Modulation, Infrared radiation, Wireless communications, Optical filters, Receivers, Local area networks, Gaussian filters, Telecommunications, Signal to noise ratio, Short wave infrared radiation
A carrier-based modem for an IR Wireless LAN (IR-WLAN) design is presented. It is based on GMSK modulation schemes. This modem is capable of operating at 2 Mb/s, but it can easily modify to work up to 4.8 Mb/s. GMSK Schemes are filtered modulation schemes with higher spectral efficiency and robustness against jitter than basic FSK. The emitter uses a pulse-conformation stage for full-digital gaussian pulse conformation. Transitions are codified and the resulting waveform is similar to the output of a gaussian filter. The main applications of the proposed GMSK modem are in the area of a full-duplex IR link. And can be applied either in point-to-point links or into a local area network. It can work without interference wither with IrDA systems (even with the proposed PPM 4.4 Mb/s link) or with IEEE 802.11-baseband link (1 or 2 Mb/s). It is also more spectral efficient and jitter resistant than PPM. Carrier-based systems can be used either in point-to-point links or in diffuse systems. For diffuse systems, this modem assumes a high level of use of the available IR spectrum, supporting at least 4 channels in a 30 MHz bandwidth. We also compare the performances of GMSK with other schemes (as can be OQPSK or FQPSK-2).
A low complexity system of optical links using indoor unguided infrared channels and direct-sequence code division multiple access is presented. Direct sequence spread spectrum techniques improve performance of optical unguided links with background illumination noise, and multipath propagation. It also allows several users to use code division multiple access, to share the same infrared channel. In the system designed, optical infrared carrier is intensity modulation by a direct-sequence spread spectrum electrical signal, driving the optical emitter. Infrared radiation is directly detected by receiver photodiode.
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