Transmission Methods for Wireless Infrared
Contributed by J.M. Kahn
Figure: Classification of IR transmission methods.
Traditionally, wireless infrared
links have been classified according to whether they employ directional or non-directional transmitters and receivers, and whether or not they rely upon the existence of an uninterrupted line-of-sight path between the transmitter and the receiver. Non-directed, non-line-of-sight, or diffuse infrared links behave much like radio links, and are used in several commercially available in-building wireless LANs.
Infrared Channels with Intensity Modulation and Direct Detection
Intensity modulation with direct detection (IM/DD) represents the only viable means of modulating and demodulating the optical carrier, because of spatially incoherent propagation.
Due to multipath propagation, the envelope of the received field is subject to fluctuations on the sub wavelength scale, as in a multipath radio system. However, the large square-law photodetector provides high-order spatial diversity, eliminating multipath fading, i.e., the drastic changes in signal magnitude and phase that accompany movement of the antenna by a fraction of a wavelength. The existence of multiple paths between the transmitter and receiver does lead to multipath distortion, which can be modeled accurately as a linear, time-invariant system, in which the powers of the different paths add incoherently.
Multipath distortion is observed in non-directed links, and typical channels exhibit r.m.s. delay spreads up to a few tens of nanoseconds, depending on the room size.
Achieving a High Signal-to-Noise Ratio
The electrical signal-to-noise ratio (SNR) of IM/DD links is limited by noise from ambient light sources. Intense, steady infrared sources, such as sunlight and skylight, lead to high-intensity shot noise, which forms the limiting noise source in a well-designed receiver under conditions of bright ambient illumination. This noise can be modeled to very high accuracy as white, Gaussian-distributed, and independent of the received intensity of the weak desired signal. Fluorescent lamps, especially those driven by electronic ballasts, emit modulated infrared signals that can interfere strongly with infrared links.
Both steady and modulated ambient light noises are minimized by the use of proper optical filtering. LED-based systems often employ long-pass optical filters, while LD-based systems can employ narrow bandpass optical filters. The passband of a multilayer bandpass filter shifts with angle of incidence, perhaps seeming to preclude the simultaneous achievement of a narrow passband and a wide field of view (FOV).
Since a photodetector produces a current proportional to the received optical power, the SNR of IM/DD links is proportional to the square of the received optical power. As a result, the effective decrease in SNR due to propagation is twice the path loss (in decibels), which limits the transmission range of infrared links using non-directional transmitters.
Achievement of a high SNR requires a large effective light-collection area. It is desirable to minimize the actual detector area, because the capacitance associated with a large-area detector can limit the receiver bandwidth and increase its noise. Fortunately, the receivers effective area can be increased by using an optical concentrator. A concentrator having refractive index n and acceptance half-angle theta can achieve an optical gain as high as
Thus, the optical gain can be increased dramatically by reducing the acceptance angle. Non-imaging hemispherical lenses provide an omnidirectional gain of n2, making them suitable for non-directed links. If a bandpass filter is placed on the surface of a hemispherical lens, all rays striking the detector hit the filter near normal incidence, making it possible to achieve high gain, wide FOV and narrow passband simultaneously. A hemispherical lens-bandpass filter combination has been used in a prototype 50-Mb/s diffuse infrared link.
Bandwidth Reuse in Multi-User Systems
The fact that infrared is blocked by walls makes it possible to reuse the same bandwidth in different rooms without any limit. It is less widely known that because IM/DD doubles the effect path loss, there is far less interference between cells using the same bandwidth within a single room than would occur in a similar radio system. Evaluations of the performance of time-, subcarrier frequency- and code-division multiple-access (TDMA, FDMA, and CDMA) schemes for bandwidth reuse, demonstrating this advantage of infrared over radio.It appears also possible to employ space-division multiple-access (SDMA) with infrared, leading to an even greater increase in network capacity.