【正文】 y is much simpler to achieve for an LED than for a laser diode, which usually have very narrow transmit beams. The principal advantages of laser diodes are their high energyconversion efficiency, their high modulation bandwidth, and their relatively narrow spectral width. Although laser diodes offer several advantages over LEDs that could be exploited, most shortrange mercial systems currently use LEDs. A receiver or detector converts optical power into electrical current by detecting the photon flux incident on the detector surface. Silicon pin photodiodes are ideal for wireless infrared munications as they have good quantum efficiency in this band and are inexpensive(4). Avalanche photodiodes are not used here since the dominant noise source is background lightinduced shot noise rather than thermal circuit noise. C. Transmission Wavelength and Noise The most important factor to consider when choosing a transmission wavelength is the availability of effective, lowcost sources and detectors. The availability of LEDs and silicon photodiodes operating in the 800 nm to 1000 nm range is the primary reason for the use of this band. Another important consideration is the spectral distribution of the dominant noise source: background lighting. The noise N(t) can be broken into four ponents: photon noise or shot noise, gain noise, receiver circuit or thermal noise, and periodic noise. Gain noise is only present in avalanchetype devices, so we will not consider it here. Photon noise is the result of the discreteness of photon arrivals. It is due to background light sources, such as sun light, fluorescent lamplight, and incandescent lamp light, as well as the signal dependent source X(t) c(t). Since the background light striking the photo detector is normally much stronger than the signal light, we can neglect the dependency of N(t) on X(t) and consider the photon noise to be additive white Gaussian noise with twosided power spectral density where q is the electron charge, R is the responsivity, and Pn is the optical power of the noise (background light). Receiver noise is due to thermal effects in the receiver circuitry, and is particularly dependent on the type of preamplifier used. With careful circuit design, it can be made insignificant relative to the photon noise(5). Periodic noise is the result of the variation of fluorescent lighting due to the method of driving the lamp using the ballast. This generates an extraneous periodic signal with a fundamental frequency of 44 kHz with significant harmonics to several MHz. Mitigating the effect of periodic noise can be done using highpass filtering in bination with baseline restoration(6), or by careful selection of the modulation type, as discussed in Section . D. Safety There are two safety concerns when dealing with infrared munication systems. Eye safety is a concern because of a bination of two effects: the cornea is transparent from the near violet to the near IR. Hence, the retina is sensitive to damage from light sources transmitting in these bands. However, the near IR is outside the visible range of light, and so the eye does not protect itself from damage by closing the iris or closing the eyelid. Eye safety can be ensured by restricting the transmit beam strength according to IEC or ANSI standards(7,8). Skin safety is also a possible concern. Possible shortterm effects such as heating of the skin are accounted for by eye safety regulations (since the eye requires lower power levels than the skin). Longterm exposure to IR light is not a concern, as the ambient light sources are constantly submitting our bodies to much higher radiation levels than these munication systems do. III. Communications Design Equally important for achieving the design goals of wireless infrared systems are munications issues. In particular, the modulation signal format together with appropriate error control coding is critical to achieving power efficiency. Channel characterization is also important for understanding performance limits. A. Modulation Techniques To understand modulation in IM/DD systems, we must look again at the channel model and consider its particular characteristics. First, since we are using intensity modulation, the channel input X(t) is optical intensity and we have the constraint X(t).