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In the last few years, there has been a growing interest in optical wireless communication for indoor and outdoor application. The high cost of reconfiguring and maintaining wired network makes wireless an economical and flexible alternative to wired system. Lately, two major transmission technologies have been used to achieve indoor wireless communication. Which are radio frequency (RF) and infrared (IR). Interest in infrared technology as a medium to convey information has grown considerably. The popularity of IR is based on the advantages it has over radio including unregulated bandwidth, immunity to radio interference and internet security. Wireless optical are expected to be dominant mode of access technology in next century.
Usually, one may see that computer terminals are clustered within office environment, labs, conference room, educations institutions, libraries, hospitals or production floor. In all these environment, the inconvenience and high cost of maintaining and reconfiguring wired systems has lead to alternative used of wireless communication. Wireless offer flexibility in the placement of terminals in work environment and avoids the waste of time and cost that reconfiguring a wired system imply. A way to achieved high speed indoor wireless communication is by using infrared radiation. The idea of using infrared as a medium to communicate in house environment was proposed about two decades ago, but it has been in the last few years that the interest in optical wireless communication has growth. As optical system operates in the near-infrared part of the spectrum, they make use of very low cost optoelectronic component available today. These components are generally small and consume little power, which is very important when manufacturing mobile terminal for telecommunication in large quantities.
Now days, wireless optical communication has been widely use in Malaysia. Gigantic Communication company such as Celcom., Maxis, Digi and Telekom Malaysia implement wireless optical communication iin their product. Broadband is the best examples that use indoor wireless optical communication. Wireless is a term used to describe telecommunication in which electromagnetic waves carry the signal over parts of all of communication path. Optical communication is defined as the transmission or the reception information using optic signal. Optical communication may use guides of free space transmission to transfer optic signal. In Malaysia, we have used four types of indoor wireless which are fixed wireless, mobile wireless, portable wireless and IR wireless. Fixed wireless is the operation of wireless devices or systems in home and in particular equipment connected to the internet via specialized modem. Mobile wireless is the use of indoor wireless devices or system in moving vehicle, example include the automotive cell phone and PCS (personal communication services). Portable wireless is the operation of battery powered wireless devices or system outside the office, home and vehicle. The example of portable wireless includes handheld, cell phone and PCS units. The fourth type of indoor wireless is IR(Infrared Radiation) wireless that use of device that convey data via IR, employed in certain limited range communication and control system.
Optical wirelesses have the potential to provide bandwidth far in excess of those available with current or planned RF network. There are several approaches to implement optical wireless system but these usually involve the integration of optical wireless system but these usually involve the integration of optical, optoelectronic and electronic component in order to create transceiver such systems are necessarily complex and the wide spread use of optical wireless is likely to be dependent on the ability to fabricate they require transceiver component of low cost.
The main reasons why indoor wireless communication has be widely used in Malaysia is because of advantage of optical communication are threefold. First, the high frequency of the optical carrier (typically of the order 300 000GHz) permit much more information to be transmitted over a single channel that is possible with a conventional radio or microwave system. Second, the very short wavelength of the optical carrier( typically of the order i micrometer) permit the realization of very small compact component. Third, the highest the highest transparency for electromagnetic radiation yet achieved in any solid material that silica glass in the wavelength region 1- 1.5Î¼m. This transparency is order of magnitude higher that of any other solid material in other part of the spectrum.
In indoor wireless communication environment, reflection from wall, the floor or the ceiling cause many signal propagation paths and delays, consequently degrading the received signal quality and receiver performance. One of the possible solution is a beam forming technique to direct antenna main beam toward a transmitter and to direct null towards interference or multipath signal direction, such that incoming signal from reflection path are suppressed while increasing the antenna gain for a desired signal direction.
The vehicular cellular phone system initiated a rapid growth of wireless communication. However with the growth of these system cell sizes are made smaller to increase user capacity. Meanwhile the interest in indoor systems for telephony (cordless phone) and data services (e.g. Wireless LANâ€™S) also started. Currently, more research is being conducted on indoor propagation and outdoor propagation.
Infrared radiation appears to be viable alternative to radio for indoor wireless optical communication in Malaysia. This is because for indoor short range communication applications, infrared present certain advantage when compared with radio frequency. The infrared region of the spectrum on the other hand offers the large bandwidth potential that is unregulated all over the world.
Table 1 show a comparison of the infrared red radio medium characteristic for indoor application. Infrared radiation, just like visible light is considered to the room in which it is generated. Hence it cannot be detected outside, securing transmission against eavesdropping. Besides that, infrared radiation does not interference with system of the same nature operating in neighbouring room and does not interference with the radio frequency either. Another advantage is that the infrared components is inexpensive small and consume little power and it very important for mobile terminal system. In spite of advantage presented by infrared over medium for different application, it has some draws back as well. Infrared may suffer from blocking a person and objects, resulting in problems on the communication link. In infrared system the transmitted power level is limited due to eye safety considerations and this is implies that the range of the system is restricted as well. It is possible to conclude that radio and infrared technologies can operate in complementary way, but one may be preferred over the other depending on the application. Radio is the most convenient medium when transmission over long ranged and high mobility are necessary and infrared media is favoured in short range application when highâ€perlinkâ€ bit rate is required.
The indoor channel can less easily be captured in rough path loss exponent. While delay spread are often much smaller than outdoor, the indoor system often have to carry very high data rate to support wireless multimedia computing. These are several cause of signal corruption in wireless channel. The primary cause of attenuation are distance, penetrate loss through wall and floor and multipath propagation. The main different between indoor and outdoor propagation is that in an outdoor macro cellular network propagation is fairly predictable. A topographical database can be used to determine what will be shape of a cell if we put a base station somewhere. Signal attenuation over distance is observed when the mean is attenuated as a function of the distance. In addition to free space loss effect, the signal experienced decay due to ground wave loss although this typically only comes into play for very large distance (on order kilometre). For indoor propagation this mechanism is relevant, but effect on wave guidance through outdoor can occurs.
The different kind of links for indoor optical wireless communication has been classified depends on the existence of a line of sight (LOS) pat between the transmitter and receiver. It also depend on the degree of directionality (directed, non directed or hybrid). The six configurations are shown in figure 1.
The figure shown, NON LOS link increase link robustness as they allow to operate the system when obstacle is placed between transmitter and receiver while LOS link system improve power efficiency and minimise multipath distortion. On the other hand directed LOS link improve power efficiency because the transmitted power is concentrated into narrow optical beam making impossible to use narrow field of view (FOV) receiver and improve links budget. A special case of this topology is tacked system. This configuration of infrared links presents the advantage of maximum power efficiency and high coverage.
Hybrid NON LOS system does not present the blocking problem but suffer from multipath distortion that increases as the area is increase. System that working under this configuration do not require direct line of sight or alignment between the optical transmitter and receiver because the optical wave are spread uniformly as possible in the room by making use reflection properties of the wall and ceiling. Beside that this kind of link has a advantaged that it can be operate even when barrier are placed between the transmitter and receiver. This makes this configuration are the most robust and flexible. In spite of advantage of diffuse configuration this kind of system suffers from multipath dispersion and higher optical losses than LOS and hybrid LOS.
The application of indoor wireless equipment that has been used in Malaysia are cordless telephone set, cordless computer peripheral, satellite television, broadband and etc. Beside voice, new ranges of service such as multimedia and high speed data are being offered for delivery over wireless network. Mobility will be shameless realizing the concept of persons being rapidly involving and is playing an increasing role in the lives of people throughout the world. In addition, very large number of people relying on the technology directly or indirectly.
The latest technology of indoor wireless optical communication is WI-FI at the speed f light. The research shows wireless optical network could provide gigabit per second data transfer. A wireless network that uses the reflected infrared instead of radio waves has transmitted data through the air at the speed one gigabit per second and six to 14 times faster that the WI-FI network. This optical network could provide faster and more secure communication would be especially suitable for use in hospital, aircraft and factories when radio frequency transmission can interference with navigation equipment, medical devices and control system. Besides that, another possible application is wireless networking for home theatre, a system that transmit data at the 1.6 gigabit per second could broadcast two separate high destination TV channel across rooms, a capacity that exceed the band width or any existing radio frequency.
Figure 2 shows the model of Bright Light. This experimental system can transfer data at one gigabit per second. An infrared laser (the black devices on the right) is used to transmit the data. Setup of the model sent a data across a room by modulating abeam of infrared light that was focussed on the ceiling and picking up the reflection using a special modified photo detector. The measurement show the system could support data rates well beyond the one gigabit per second.
As a conclusion, in spite of the advances achieved so far there is still a lot of work to be done to exploit completely the advantages and the potential offered by the optical medium. For indoor wireless system application, the use of optical communication offers an important alternative for the growing area of mobile computers and communications. Optical wireless network also could offer less interference and greater security than radio frequency network. While radio signal passes through wall and door, light does not passes and making it easier to reuse frequency and more difficult to intercept transmission. Unlike radio frequency spectral region for all light infrared, visible, and ultraviolet is unregulated worldwide. This could make easier to commercialize optical wireless network.