In this letter, we propose and evaluate a novel optical wireless configuration that employs a multibeam transmitter in conjunction with a narrow field-of-view direction diversity receiver. Such a configuration overcomes the drawbacks and combines the advantages of both types of optical wireless links, including line-of-sight and diffuse transmission. A multibeam transmitter placed on the communication floor was adopted to produce multiple diffusing spots on the middle of the ceiling in the form of a line strip. The design goal is to reduce the effect of intersymbol interference and to improve the signal-to-noise ratio (SNR) when the system operates under the constraints of background noise and multipath dispersion. Simulation results show that our line strip multibeam transmitter (LSMT) with only three branches diversity gives about 23 dB SNR improvement over the conventional system. The results also show that the multipath dispersion, which induces pulse spread, is significantly reduced when the LSMT with diversity detection is used.

Line strip spot-diffusing transmitter configuration for optical wireless systems influenced by background noise and multipath dispersion

The demand for wireless broadband communications has been growing steadily for last several years. The congestion and the limitations on bandwidths of the radio spectrum have inhibited unrestricted growth of radio wireless systems. Wireless optical, however, holds the promise of delivering data rates that can meet the broadband requirements. As a result, wireless optical is believed to be a viable long term option for many applications of wireless communications. Nevertheless, the advantages of optical wireless have not yet been fully exploited. Basic and applied research is needed at the laboratory and commercial levels to bring the performance of real-life wireless optical systems into higher levels. The Gb/s level data rate has been validated in laboratory, however, available indoor systems only realize a data rate of 155Mb/s. Current research in wireless optical concentrates on increasing the communication capacity and improving the performance. In this paper, we survey the wireless optical discipline emphasizing major design, performance, and safety issues. Research directions that have the potential to close the gap between theory and practice, in wireless optical communications, are also presented.

Wireless optical communications: a survey

The main challenges facing high data rate visible light communication (VLC) are the low-modulation bandwidth of the current transmitters (i.e., light emitting diodes), the intersymbol interference (ISI) caused by the multipath propagation and cochannel interference (CCI) due to multiple transmitters. In this paper, for the first time, to the best of our knowledge, we propose, design, and evaluate the use of laser diodes (LDs) for communication as well as illumination. In addition, we propose an imaging receiver for a mobile VLC system to mitigate ISI. A novel delay adaptation technique is proposed to mitigate CCI, maximize the signal to noise ratio, and reduce the impact of multipath dispersion under user mobility. The proposed imaging system is able to provide data rates of 5 Gb/s in the worst-case scenario. The combination of a delay adaptation approach with an imaging receiver (DAT imaging LD-VLC system) adds a degree of freedom to the link design, which results in a VLC system that has the ability to provide higher data rates (i.e., 10 Gb/s) in the considered harsh indoor environment. The proposed technique (delay adaptation) achieves significant improvements in the VLC channel bandwidth (more than 16 GHz) over an imaging system in the worst-case scenario. The VLC channel characteristics and links were evaluated under diverse situations including an empty room and a room with very strong shadowing effects resulting from minicubicle offices.

Mobile Multi-Gigabit Visible Light Communication System in Realistic Indoor Environment

In this letter, the performance of an indoor optical wireless spot-diffusing system using various multibeam transmitter configurations, in association with direction diversity and combining techniques, is assessed and compared under the impact of multipath dispersion and ambient light noise through theoretical analysis and computer simulation. Computer simulation for three different multibeam transmitter configurations and a conventional diffuse transmitter is carried out. Diversity receiver and wide field-of-view (FOV) receiver configurations are evaluated in conjunction with the proposed configurations. For the diversity-detection case, a receiver comprising an array of narrow FOV detectors (three and seven segments) oriented in different directions is used to maximize the collected signals and minimize noise. A novel line-strip multibeam system (LSMS) is investigated with single and diversity receiver configurations, and is compared with other spot-diffusing methods. Combining schemes, including selection combining, maximum ratio combining, and equal gain combining are employed for the presented configurations. Our results indicate that the performance improvement obtained through the use of LSMS with a three-direction diversity receiver is about 20 dB signal-to-noise ratio enhancement over the conventional diffuse system, and 26 dB when combining techniques are used. Root mean square delay-spread performance for the proposed configurations, at different positions on the communication floor, are also evaluated and compared.

Analysis of diffuse optical wireless channels employing spot-diffusing techniques, diversity receivers, and combining schemes

The spot-diffusing geometry is one of the attractive configurations considered in the literature. It provides a better signal-to-noise ratio (SNR) than the conventional diffuse system (CDS), but its SNR can be degraded due to shadowing, signal blockage and mobility. Three methods: imaging reception, beam angle and beam power adaptation are introduced to the design of spot-diffusing OW systems to effectively mitigate the degradation due to mobility in the presence of ambient light noise, multipath propagation, and shadowing. The performance of our systems was evaluated through channel and noise modeling. The CDS SNR performance improves by more than 20 dB when an imaging receiver with maximum ratio combining (MRC) replaces a non-imaging receiver. A 24 dB SNR gain can be achieved when spot-diffusing is employed with an imaging MRC receiver instead of the imaging MRC CDS. In an imaging spot-diffusing system, the SNR is independent of the transmitter position and can be maximized at all receiver locations when our new methods (beam angle and beam power adaptation) are implemented. Regardless of the transmitter position, beam angle adaptation can target the spots at the optimum location that yields the best SNR at the receiver. A significant SNR improvement of 36 dB in the imaging spot-diffusing performance can be achieved when angle adaptation is introduced. Further SNR improvement of 4 dB can be obtained if the power is adaptively distributed among the spots. Furthermore, an increase in the channel bandwidth from 43 MHz (non-imaging CDS) to 8.19 GHz can be achieved through the combination of these methods (imaging reception, spot-diffusing, beam angle and beam power adaptation). The increase in channel bandwidth and SNR can enable the OW system to achieve higher data rates and 2.5 Gbit/s and 5 Gbit/s mobile OW systems are shown to be feasible. The results also prove that the influence of shadowing and signal blockage can be sufficiently combated through the use of these methods.

High-Speed Spot Diffusing Mobile Optical Wireless System Employing Beam Angle and Power Adaptation and Imaging Receivers

A new hybrid optical wireless system based on pulse position modulation (PPM) and PIN-BJT receivers is proposed and its performance is evaluated. The system is operated at a bit rate of 10 Mbit/s and h=0.85 /spl mu/m. The receiver achieves a low noise current of /spl sim/2.7 pA//spl radic/Hz. Original results are presented demonstrating that the system can achieve a sensitivity of -50.5 dBm at an error rate of 10/sup -9/. This represents a sensitivity improvement of 9.8 dB over a comparable on-off keying (OOK) PIN-BJT system. The sensitivity enhancement has a significant impact on the optical wireless network, where the maximum allowed transmitted power is limited by safety issues. Moreover, the use of PPM offers other advantages, such as the low average power which is suited for battery operation and a high peak power that aids detection.

Sensitivity evaluation of optical wireless PPM systems utilising PIN-BJT receivers

An investigation into the sensitivity performance of both a PPM and an OOK optical wireless communication system has been carried out in the presence of both ambient noise and multipath dispersion. It is shown that the sensitivity of a PPM scheme is more affected by multipath dispersion than a similar OOK format. Under the constraint of ambient noise, the findings were opposite. Both systems operate at 10 Mb/s and a wavelength of 850 nm. In a line-of-sight configuration, PPM offers an 8.77 dB sensitivity improvement over a comparable OOK system. In a semi-diffuse system impaired by multipath dispersion, the sensitivity improvement is reduced to 7.13 dB. When corrupted by ambient noise, the improvement was found to be 8.8 dB, which is slightly more.

Performance of optical wireless OOK and PPM systems under the constraints of ambient noise and multipath dispersion

A hybrid based on pulse position modulation code division multiple access (PPM CDMA) is studied in the context of the indoor infrared LAN. The utilisation of optical orthogonal codes (OOC) as signature sequences is investigated. Original results are presented illustrating the improvement in bit rate achieved by utilising PPM CDMA rather than on-off keying (OOK) CDMA. It is demonstrated that there exists an optimum PPM order, at a given number of users. Original results are presented that evaluate the dispersion in the optical diffuse infrared channel and its effects on optical orthogonal codes (OOC). A new equalisation technique is introduced and analysed in the context of this channel. The method is shown to be effective in reducing the resultant intersymbol interference (ISI). Illustrative examples are given on an isolated pulse and a typical OOC. The results indicate that good performance can be achieved with minimum hardware overheads.

Performance analysis of indoor infrared wireless networks utilising PPM CDMA

A hybrid pulse position modulation (PPM) code division multiple access (CDMA) optical communication system is proposed and its performance is analyzed using three different orthogonal coding schemes: optical orthogonal codes, prime codes and extended prime codes. The hybrid can usefully be exploited in optical wireless and optical fiber networks. Two types of PPM CDMA structures have been considered and analyzed. The implications of the structures and the coding schemes on the system bit rate and number of users are assessed. Original results are presented for the system error probability against the number of users using the three different orthogonal codes.

Optical Wireless PPM CDMA Networks Employing OC and PC Signature Codes

Original results are presented that evaluate the dispersion in the optical diffuse infrared channel and its effects on optical orthogonal codes (OOCs). A new equalization technique is introduced and analyzed in the context of this channel. The method is shown to be effective in reducing the resultant intersymbol interference (ISI). Illustrative examples are given on an isolated pulse and on a typical OOC. The results indicate that good performance can be achieved with minimum hardware overheads. © 1995 John Wiley & Sons, Inc.

H.H. Chan

Papers

Sensitivity evaluation of optical wireless PPM systems utilising PIN-BJT receivers

Performance of optical wireless OOK and PPM systems under the constraints of ambient noise and multipath dispersion

Performance analysis of indoor infrared wireless networks utilising PPM CDMA

Optical Wireless PPM CDMA Networks Employing OC and PC Signature Codes

An equalization technique for indoor IR wireless LANs