1、原文 ： Wireless Infrared Communications I. Introduction Wireless infrared communications refers to the use of free-space propagation of light waves in the near infrared band as a transmission medium for communication(1-3), as shown in Figure 1. The communication can be between one portable communicati
2、on device and another or between a portable device and a tethered device, called an access point or base station. Typical portable devices include laptop computers, personal digital assistants, and portable telephones, while the base stations are usually connected to a computer with other networked
3、connections. Although infrared light is usually used other regions of the optical spectrum can be used (so the term wireless optical communications instead of wireless infrared communications is sometimes used). Wireless infrared communication systems can be characterized by the application for whic
4、h they are designed or by the link type, as described below. A. Applications The primary commercial applications are as follows: short-term cable-less connectivity for information exchange (business cards, schedules, file sharing) between two users. The primary example is IrDA systems (see Section 4
5、). wireless local area networks (WLANs) provide network connectivity inside buildings. This can either be an extension of existing LANs to facilitate mobility, or to establish “ ad hoc” networks where there is no LAN. The primary example is the IEEE 802.11standard (see Section 4). building-to-buildi
6、ng connections for high-speed network access or metropolitan- or campus-area net-works. wireless input and control devices, such as wireless mice, remote controls, wireless game controllers, and remote electronic keys. B. Link Type Another important way to characterize a wireless infrared communicat
7、ion system is by the “ link type” which means the typical or required arrangement of receiver and transmitter. Figure 2 depicts the two most common configurations: the point-to-point system and the diffuse system. The simplest link type is the point-to-point system. There, the transmitter and receiv
8、er must be pointed at each other to establish a link. The line-of-sight (LOS) path from the transmitter to the receiver must be clear of obstructions, and most of the transmitted light is directed toward the receiver. Hence, point-to-point systems are also called directed LOS systems. The links can
9、be temporarily created for a data exchange session between two users, or established more permanently by aiming a mobile unit at a base station unit in the LAN replacement application. In diffuse systems, the link is always maintained between any transmitter and any receiver in the same vicinity by
10、reflecting or |“ bouncing” the transmitted information-bearing light off reflecting surfaces such as ceilings, walls, and furniture. Here, the transmitter and receiver are non-directed； the transmitter employs a wide transmit beam and the receiver has a wide field-of-view. Also, the LOS path is not
11、required. Hence, diffuse systems are also called non-directed non-LOS systems. These systems are well suited to the wireless LAN application, freeing the user from knowing and aligning with the locations of the other communicating devices. C. Fundamentals and Outline Most wireless infrared communica
12、tions systems can be modeled as having an output signal Y (t) and an input signal X(t) which are related by where denotes convolution, C(t) is the impulse response of the channel and N(t) is additive noise. This article is organized around answering key questions concerning the system as represented
13、 by this model. In Section 2, we consider questions of optical design. What range of wireless infrared communications systems does this model apply to? How does C(t) depend on the electrical and optical properties of the receiver and transmitter? How does C(t) depend on the location, size, and orien
14、tation of the receiver and transmitter? How do X(t) and Y (t) relate to optical processes? What wavelength is used for X(t)?What devices produce X(t) and Y (t)? What is the source of N(t)? Are there any safety considerations? In Section 3, we consider questions of communications design. How should a
15、 data symbol sequence be modulated onto the input signal X(t)? What detection mechanism is best for extracting the information about the data from the received signal Y (t)? How can one measure and improve the performance of the system? In Section 4, we consider the design choices made by existing s
16、tandards such as IrDA and 802.11.Finally, in Section 5, we consider how these systems can be improved in the future. II. Optical Design A. Modulation and demodulation What characteristic of the transmitted wave will be modulated to carry information from the transmitter to the receiver? Most communi
17、cation systems are based on phase, amplitude, or frequency modulation, or some combination of these techniques. However, it is difficult to detect such a signal following nondirected propagation, and more expensive narrow-linewidth sources are required(2). An effective solution is to use intensity m
18、odulation, where the transmitted signals intensity or power is proportional to the modulating signal. At the demodulator (usually referred to as a detector in optical systems) the modulation can be extracted by mixing the received signal with a carrier light wave. This coherent detection technique i
19、s best when the signal phase can be maintained. However, this can be difficult to implement and additionally, in non directed propagation, it is difficult to achieve the required mixing efficiency. Instead, one can use direct detection using a photodetector. The photodetector current is proportional
20、 to the received optical signal intensity, which for intensity modulation, is also the original modulating signal. Hence, most systems use intensity modulation with direct detection (IM/DD)to achieve optical modulation and demodulation. In a free-space optical communication system, the detector is i
21、lluminated by sources of light energy other than the source. These can include ambient lighting sources, such as natural sunlight, fluorescent lamp light, and incandescent lamp light. These sources cause variation in the received photocurrent that is unrelated to the transmitted signal, resulting in
22、 an additive noise component at the receiver. We can write the photocurrent at the receiver as where R is the responsivity of the receiving photodiode (A/W). Note that the electrical impulse response c(t) is simply R times the optical impulse response h(t). Depending on the situation, some authors u
23、se (t) and some use h(t) as the impulse response. B. Receivers and Transmitters A transmitter or source converts an electrical signal to an optical signal. The two most appropriate types of device are the light-emitting diode (LED)and semiconductor laser diode (LD).LEDs have a naturally wide transmi
24、ssion pattern, and so are suited to non directed links. Eye safety 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 energy-conversion efficiency, their high modulation bandwidth, and t
25、heir relatively narrow spectral width. Although laser diodes offer several advantages over LEDs that could be exploited, most short-range commercial systems currently use LEDs. A receiver or detector converts optical power into electrical current by detecting the photon flux incident on the detector
26、 surface. Silicon p-i-n photodiodes are ideal for wireless infrared communications 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 back-ground light-induced shot noise rather than thermal circuit noi
27、se. C. Transmission Wavelength and Noise The most important factor to consider when choosing a transmission wavelength is the availability of effective, low-cost 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 components: photon noise or shot noise, gain
