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1、原文：Wireless Infrared CommunicationsI. IntroductionWireless 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 communication d
2、evice 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 conn
3、ections. 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 which
4、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-building c
6、onnections 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 communication s
7、ystem 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 receiver mus
8、t 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 be tem
9、porarily 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 reflec
10、ting 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 require
11、d. 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 communications sy
12、stems can be modeled as having an output signal Y (t) and an input signal X(t) which are related bywhere 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 by this
13、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 orientation of
14、 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 data sym
15、bol 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 standards
16、such as IrDA and 802.11.Finally, in Section 5, we consider how these systems can be improved in the future.II. Optical DesignA. Modulation and demodulationWhat characteristic of the transmitted wave will be modulated to carry information from the transmitter to the receiver? Most communication systems are based on phase, amplitude, or frequency modulation, or some combination of these techniques. However, it is difficult to detect such a signal followi
