Optical (infrared) wireless communications links offer an attractive solution for indoor applications. To enable terminal mobility and reduce temporal dispersion, a configuration known as multispot-diffusing (MSD) is used, which is a multiinput, multioutput (MIMO) architecture. In this configuration, a transmitter generates multiple narrow beams that get uniformly spread over a reflection surface accessible to a service area. A multibranch receiver with each branch having access to a diffusing spot (generated by transmitter) combines signals on its branches. This scheme creates multiple virtually ideal communications channels between a base station and terminals. This paper considers issues involved in the design of a transmitter-based holographic spot array generator. The generator produces equally-spaced diffusing spots on the room ceiling and/or walls. To overcome power limitation set by eye safety requirements, a receiver optical concentrator is proposed. Furthermore, to improve receiver signal-to-noise ratio, an optical filter that rejects optical noise is needed. Thus, functionally receiver branch optical front-end consists of an optical concentrator and an optical band-pass filter. A single holographic optical element, capable of performing both functions is proposed. Link performance is investigated by providing equivalent link model and comparing probability of error for a bare and a holographic receiver. From performance evaluations, our results show that it is possible to achieve an increase of 11 dB in the SNR and improve power budget by reducing the path loss by over 6 dB.

In this paper, we investigate vulnerability of a so called "cellular" and that of a multi-spot diffusing (MSD) multi-input-multi-output (MIMO) architecture to shadowing and its effect on communications cell size and the required transmit power. Because signal path obstruction by furniture is easier to predict and avoid, we are mainly concerned with shadowing and blockage caused by people either sitting or standing. To avoid blockage of communication link by a user of portable units, maximum radiation angle at the transmitter in both configurations must not exceed 45/spl deg/. This restricts communication cell size. We show the probability of blockage of a cellular link depends almost linearly on the distance of the portable unit from the communication cell center. Unlike cellular links, MSD-MIMO links can be designed to be robust against blockage, though still vulnerable to shadowing. In a typical office having a height of 3 m, probability of shadowing is less than 2% and in majority of cases shadowing causes less than 50% reduction in the received signal power. Power penalty due to shadowing is insignificant (less than 0.1 dB) with a 1% outage.

While use of power-efficient signaling schemes appears to be effective at compensating for the inherent high path-loss associated with pure diffuse infrared links, it begins to lose its effectiveness as the data rate is increased. At very high data rates, intersymbol interference (ISI) can result in a very high and sometimes irreducible power penalty, preventing the system from operating at a low bit-error probability. We use a link design employing a multibeam transmitter in conjunction with a narrow field-of-view (FOV) direction diversity receiver. The design goal is to eliminate the effect of ISI so that power-efficient signaling schemes such as pulse-position modulation (PPM) can be employed at very high data rates. We also use high-rate Reed-Solomon codes to further increase the power efficiency of PPM signals. The proposed system can be made rate-adaptive through varying modulation level L and/or code rate R without increasing the complexity significantly. This provides a dynamic range large enough to allow efficient utilization of available bandwidth, i.e., to allow portable terminals to communicate at their highest permitted data rate, without sacrificing the quality of service. It is shown that a bit error rate not exceeding 10/sup -9/ can be achieved within the link coverage area with 99% probability at bit rates up to a few hundreds of megabits per second, using transmitted power levels well below 1 W.

For pt.I see ibid., vol.50, no.7, p.1098-1109 (2002). We examine an infrared link composed of a multibeam transmitter and a direction-diversity receiver, employing code combining. The latter represents an added dimension to the conventional diversity concepts, which are limited to combining the individual received symbols. Rate-compatible punctured convolutional codes are used to encode intensity-modulated on-off keying (OOK) optical power, to create an adaptive environment for efficient utilization of channel spectral bandwidth, to provide a means for accurate channel estimation, and to maintain a guaranteed bit error rate (BER) performance at all receiver positions. It is shown that a BER not exceeding 10/sup -9/ with 99% probability can be achieved at bit rates up to a few hundreds of megabits per second, at very low transmitted power levels.

This paper addresses one of the most promising candidates for high-speed in-house wireless communications, namely, the multi-spot diffusing configuration (MSDC). Since it uses the optical infrared medium for data transmission, it has the inherent potential for achieving very high capacities. The channel characteristics in MSDC are simulated and the causes for channel distortion are analyzed. Then, conditions for creation of a virtually ideal channel are derived. It is shown that the 3-dB channel bandwidth can be extended up to beyond 2 GHz. This bandwidth comes at the cost of a poor power efficiency. In order to compensate for that, a novel receiver optical front-end design is proposed and its performance is analyzed. Taking advantage of the unique properties of the holographic optical elements, the conventional optical front-end, consisting of a concentrator and a filter, is replaced by a single holographic curved mirror. The utilization of such a holographic optical element improves the signal-to-shot noise ratio by up to 18.5 dB.

While use of a power efficient signaling scheme appears to be effective in compensating for inherent high path-loss associated with pure diffuse infrared links, it begins to lose its effectiveness as data rate is increased. At very high data rates, intersymbol interference (ISI) can result in a very high and sometimes irreducible power penalty, preventing the system from operating at a low bit-error probability. In this paper, we investigate a link design employing a multi-beam transmitter in conjunction with a narrow field-of-view (FOV) direction diversity receiver. The design goal is to eliminate the effect of ISI, so that power efficient signaling schemes such as pulse-position modulation (PPM) can be employed at very high data rates. It is shown that an average bit-error rate (BER) not exceeding 10/sup -9/ can be achieved within the link coverage area with more than 99% probability at bit rates up to a few hundreds of megabits per second, using transmitted power levels well below one Watt.

In order to combine the advantages and to overcome the drawbacks of the direct line-of-sight and diffuse configurations for indoor wireless optical LANs, a multi-spot diffusing concept utilizing holographic spot array generator is suggested. Simulation results for the link characteristics are presented and comparison is made with a pure diffuse configuration.