原文：
Wireless Infrared Communications
I. Introduction
Wireless infrared communications refers to theu of free-space propagation of light waves in thenear infrared band as a transmission medium forcommunication(1-3), as shown in Figure 1. The communication can be between one portable communication device and another or between a portable device and a tethered device, called an access pointor ba station. Typical portable devices includelaptop computers, personal digital assistants, andportable telephones, while the ba stations are usually connected to a computer with other networkedconnections. Although infrared light is usually udother regions of the optical spectrum can be ud (sothe term wireless optical communications" insteadof wireless infrared communications" is sometimesud).
创新英语大赛Wireless infrared communication systems can becharacterized by the application for which they aredesigned or by the link type, as described below.
A.Applications
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The primary commercial applications are as follows:
怎样蒸馒头²short-term cable-less connectivity for informationexchange (business cards, schedules, file sharing) between two urs. The primary example is IrDA systems (e Section 4).
²wireless local area networks (WLANs) provide network connectivity inside buildings. This can eitherbe an extension of existing LANs to facilitate mobility, or to establish “ad hoc”networks where there isno LAN. The primary example is the IEEE 802.11standard (e Section 4).
²building-to-building connections for high-speednetwork access or metropolitan- or campus-area net-works.
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²wireless input and control devices, such as wirelessmice, remote controls, wireless game controllers, andremote electronic keys.
B. Link Type
Another important way to characterize a wirelessinfrared communication system is by the “link type”which means the typical or required arrangement ofreceiver and transmitter. Figure 2 depicts the twomost common configurations: the point-to-point system and the diffu system.
The simplest link type is the point-to-pointsystem. There, the transmitter and receiver must bepointed at each other to establish a link. The line-of-sight (LOS) path from the transmitter to the receivermust 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 temporarily created for adata exchange
ssion between two urs, or established more permanently by aiming a mobile unit ata ba station unit in the LAN replacement application.
In diffu systems, the link is always maintainedbetween any transmitter and any receiver in the samevicinity by reflecting or |“bouncing”the transmittedinformation-bearing light off reflecting surfaces suchas ceilings, walls, and furniture. Here, the transmitter and receiver are non-directed; the transmitteremploys a wide transmit beam and the receiver hasa wide field-of-view. Also, the LOS path is not required. Hence, diffu systems are also called non-directed non-LOS systems. The systems are wellsuited to the wirelessLAN application, freeing theur from knowing and aligning with the locations ofthe other communicating devices.
C. Fundamentals and Outline
Most wireless infrared communications systemscan be modeled as having an output signal Y (t) andan input signal X(t) which are related by
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where denotes convolution, C(t) is the impul respon of the channel and N(t) is additive noi.This article is organized around answering key questions concerning the system as reprented by thismodel.
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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 ofthe receiver and transmitter? How does C(t) dependon the location, size, and orientation of the receiverand transmitter? How do X(t) and Y (t) relate to optical process? What wavelength is ud for X(t)?What devices produce X(t) and Y (t)? What is thesource of N(t)? Are there any safety considerations?In Section 3, we consider questions of communications design. How should a data symbol quence bemodulated onto the input signal X(t)? What detection mechanism is best for extracting the informationabout the data from the received signal Y (t)? Howcan one measure and improve the performance of thesystem? In Section 4, we consider the design choices
made by existing standards such as IrDA and 802.11.Finally, in Section 5, we consider how the systemscan be improved in the future.
II. Optical Design
A. Modulation and demodulation
二月用英语怎么读What characteristic of the transmitted wave willbe modulated to carry information from the transmitter to the receiver? Most communication systemsare bad on pha, amplitude, or frequency modulation, or some combination of the techniques.However, it is difficult to detect such a signal following nondirected propagation, and more expensivenarrow-linewidth sources are required(2). An effective solution is to u intensity modulation, wherethe transmitted signal's 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 signalwith a carrierlight wave. This coherent detection technique is bestwhen the signal pha can be maintained. However,this can be difficult to implement and additionally, innondirected propagation, it is difficult to achieve therequired mixing efficiency. Instead, one can u directdetection using a photodetector. The photodetectorcurrent is proportional to the re
ceived optical signalintensity, which for intensity modulation, is also theoriginal modulating signal. Hence, most systems uintensity modulation with direct detection (IM/DD)to achieve optical modulation and demodulation.
In a free-space optical communication system, thedetector is illuminated by sources of light energyother than the source. The can include ambientlighting sources, such as natural sunlight, fluorescent lamp light, and incandescent lamp light. Thesources cau variation in the received photocurrentthat is unrelated to the transmitted signal, resultingin an additive noi 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 impul responc(t) is simply R times the optical impul responh(t). Depending on the situation, some authors u(t) and some u h(t) as the impul respon.
B. Receivers and Transmitters
A transmitter or source converts an electrical signal to an optical signal. The two most appropriatetypes of device are the light-emitting diode (LED)and miconductor lar diode (LD).LEDs havea naturally wide transmission pattern, and so aresuited to nondirected links. Eye safety is much simpler to achieve for an LED than for a lar diode,which usually have very narrow transmit beams.The principal advantages of lar diodes are theirhigh energy-conversion efficiency, their high modulation bandwidth, and their relatively narrow spectral width. Although lar diodes offer veral advantages over LEDs that could be exploited, mostshort-range commercial systems currently u LEDs.
午睡起来头疼A receiver or detector converts optical power intoelectrical current by detecting the photon flux incident on the detector surface. Silicon p-i-n photodiodes are ideal for wireless infrared communications asthey have good quantum efficiency in this band andare inexpensive(4). Avalanche photodiodes are notud here since the dominant noi source is back-ground light-induced shot noi rather than thermalcircuit noi.
C. Transmission W avelength and Noi
The most important factor to consider whenchoosing a transmission wavelength is the availability of
effective, low-cost sources and detectors. Theavailability of LEDs and silicon photodiodes operating in the 800 nm to 1000 nm range is the primaryreason for the u of this band. Another importantconsideration is the spectral distribution of the dominant noi source: background lighting.
The noi N(t) can be broken into four components: photon noi or shot noi, gain
noi, receiver circuit or thermal noi, and periodic noi.Gain noi is only prent in avalanche-type devices,so we will not consider it here.
Photon noi is the result of the discreteness ofphoton arrivals. It is due to background lightsources, such as sun light, fluorescent lamplight,and incandescent lamp light, as well as the signaldependent source X(t) - c(t). Since the backgroundlight striking the photodetector is normally muchstronger than the signal light, we can neglect the dependency of N(t) on X(t) and consider the photonnoi to be additive white Gaussian noi with two-sided power spectral density where qis the electron charge, R is the responsivity, and Pnis the optical power of the noi (background light).
Receiver noi is due to thermal effects in the receiver circuitry, and is particularly dependent on thetype of preamplifier ud. With careful circuit design, it can be made insignificant relative tothe photon noi(5).
Periodic noi is the result of the variation of fluorescent lighting due to the method of driving thelamp using the ballast. This generates an extraneous periodic signal with a fundamental frequency of44 kHz with significant harmonics to veral MHz.Mitigating the effect of periodic noi can be doneusing high-pass filtering in combination with balinerestoration(6), or by careful lection of the modulation type, as discusd in Section 3.1.
D. Safety
There are two safety concerns when dealing withinfrared co mmunication systems. Eye safety is a concern becau of a combination of two effects: thecornea is transparent from the near violet to the nearIR. Hence, the retina is nsitive to damage from light sources transmitting in the bands. However,the near IR is outside the visible range of light, andso the eye does not protect itlf from damage byclosing the iris or closing the eyelid. Eye safety canbe ensured by restricting the transmit beam strengthaccording to IEC or ANSI standards(7,8).
Skin safety is also a possible concern. Possibleshortterm effects such as heating of the skin are accounted for by eye safety regulations (since the eyerequires lower power levels than the skin). Longtermexposure to IR light is not a concern, as the ambientlight sources are constantly submitting our bodies tomuch higher radiation levels than the communication systems do.
III. Communications Design
Equally important for achieving the design goals ofwireless infrared systems are communications issues.In particular, the modulation signal format togetherwith appropriate error control coding is critical toachieving power efficiency. Channel characterizationis also important for understanding performance limits.
A. Modulation Techniques
To understand modulation in IM/DD systems, wemust 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 theconstraint X(t). The average transmitted optical power PT is the time average of X(t). Our goalis to minimize the transmitted power required to attain a certain probability of bit error Pe, also knownas a bit error rate (BER).It is uful to define the signal-to-noi ratio SNRas
where H(0) is gain of the channel, i.e. itis the Fourier transform of h(t) evaluated at zerofrequency, so
The transmitted signal can be reprented as
The quence reprents the digital informationbeing transmitted, where
is one of L possible datasymbols from 0 to L-1. The function Si(t) reprentsone of L pul shapes with duration Ts, the symboltime. The data rate (or bit rate) Rb, bit time T,symbol rate Rs, and symbol time Ts are related asfollows:
.
There are three commonly ud types of modulation schemes: on-off keying (OOK) with non-return-to-zero puls, OOK with return-to-zero puls ofnormalized width and pul position modulation with L puls (L-PPM). OOK and aresimpler to implement at both the transmitter and receiver than L-PPM. The pul shapes for the modulation techniques are shown in Figure 3. Reprentative examples of the resulting transmitted signalX(t) for a short data quence areshown in Figure4.
We compare modulation schemes in Table 1 bylooking at measures of power efficiency and bandwidth efficiency. Bandwidth efficiency is measuredby dividing the zero-crossing bandwidth by the datarate. Bandwidth efficient schemes have veraladvantages—the receiver and transmitter electronicsare cheaper, and the modulation scheme is less likelyto be affected by multipath distortion. Power efficiency is measured by comparing the required transmit power to achieve a target probability of error Pefor different modulation techniques. Both andPPM are more power efficient than OOK, but at thecost of reduced bandwidth efficiency. However, for agiven bandwidth efficiency, PPM is more power efficient than,
