外文翻譯----無線紅外通信

1、<p><b>　　原文：</b></p><p>　　Wireless Infrared Communications</p><p>　　I. Introduction</p><p>　　Wireless infrared communications refers to the use of free-space propag

2、ation 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 device and another or between a portable d

3、evice 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</p>

4、<p>　　Wireless infrared communication systems can be characterized by the application for which they are designed or by the link type, as described below. </p><p>　　Applications</p><p>　　T

5、he primary commercial applications are as follows:</p><p>　　² short-term cable-less connectivity for information exchange (business cards, schedules, file sharing) between two users. The primary example

6、 is IrDA systems (see Section 4).</p><p>　　² wireless local area networks (WLANs) provide network connectivity inside buildings. This can either be an extension of existing LANs to facilitate mobility,

7、or to establish “ad hoc”networks where there is no LAN. The primary example is the IEEE 802.11standard (see Section 4).</p><p>　　² building-to-building connections for high-speed network access or metro

8、politan- or campus-area net-works.</p><p>　　² wireless input and control devices, such as wireless mice, remote controls, wireless game controllers, and remote electronic keys.</p><p>　　B.

9、Link Type</p><p>　　Another important way to characterize a wireless infrared communication system is by the “l(fā)ink type” which means the typical or required arrangement of receiver and transmitter. Figure 2 d

10、epicts the two most common configurations: the point-to-point system and the diffuse system.</p><p>　　The simplest link type is the point-to-point system. There, the transmitter and receiver must be pointed

11、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

12、 systems are also called directed LOS systems. The links can be temporarily created for a data exchange session between two users, or established more permanently by aiming a </p><p>　　In diffuse systems, th

13、e link is always maintained between any transmitter and any receiver in the same vicinity by reflecting or |“bouncing” the transmitted information-bearing light off reflecting surfaces such as ceilings, walls, and furnit

14、ure. 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 required. Hence, diffuse systems are also called

15、 non-directed non-LOS systems. These s</p><p>　　C. Fundamentals and Outline</p><p>　　Most wireless infrared communications systems can be modeled as having an output signal Y (t) and an input si

16、gnal X(t) which are related by</p><p>　　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

17、the system as represented by this model.</p><p>　　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

18、on the electrical and optical properties of the receiver and transmitter? How does C(t) depend on the location, size, and orientation of the receiver and transmitter? How do X(t) and Y (t) relate to optical processes? Wh

19、at 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 consid</p><p>　　II. Optical Design</p><p>　　A. Modulation and demodulation<

20、;/p><p>　　What 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 modulati

21、on, 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 us

22、e intensity modulation, where the transmitted signal's intensity or power is pro</p><p>　　At the demodulator (usually referred to as a detector in optical systems) the modulation can be extracted by mixi

23、ng the received signal with a carrier light wave. This coherent detection technique is best when the signal phase can be maintained. However, this can be difficult to implement and additionally, in non directed propagati

24、on, it is difficult to achieve the required mixing efficiency. Instead, one can use direct detection using a photodetector. The photodetector current is proportional t</p><p>　　In a free-space optical commun

25、ication system, the detector is illuminated 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. The

26、se sources cause variation in the received photocurrent that is unrelated to the transmitted signal, resulting in an additive noise component at the receiver.</p><p>　　We can write the photocurrent at the re

27、ceiver as</p><p>　　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 situatio

28、n, some authors use (t) and some use h(t) as the impulse response.</p><p>　　B. Receivers and Transmitters</p><p>　　A transmitter or source converts an electrical signal to an optical signal. The

29、 two most appropriate types of device are the light-emitting diode (LED)and semiconductor laser diode (LD).LEDs have a naturally wide transmission pattern, and so are suited to non directed links. Eye safety is much simp

30、ler 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 modulat</p><p&g

31、t;　　A receiver or detector converts optical power into electrical current by detecting the photon flux incident on the detector surface. Silicon p-i-n photodiodes are ideal for wireless infrared communications as they ha

32、ve 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 noise.</p>

33、<p>　　C. Transmission Wavelength and Noise</p><p>　　The most important factor to consider when choosing a transmission wavelength is the availability of effective, low-cost sources and detectors. The

34、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 sou

35、rce: background lighting.</p><p>　　The noise N(t) can be broken into four components: photon noise or shot noise, gain noise, receiver circuit or thermal noise, and periodic noise. Gain noise is only present

36、 in avalanche-type devices, so we will not consider it here.</p><p>　　Photon noise is the result of the discreteness of photon arrivals. It is due to background light sources, such as sun light, fluorescent

37、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

38、dependency of N(t) on X(t) and consider the photon noise to be additive white Gaussian noise with two-sided power spectral density where q is the e</p><p>　　Receiver noise is due to thermal effects in the r

39、eceiver 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).</p><p>　　Periodic noise is the re

40、sult 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 sev

41、eral MHz. Mitigating the effect of periodic noise can be done using high-pass filtering in combination with baseline restoration(6), or by careful selection of the modulation type, as discussed in Section 3.1.</p>

42、<p><b>　　D. Safety</b></p><p>　　There are two safety concerns when dealing with infrared communication systems. Eye safety is a concern because of a combination of two effects: the cornea i

43、s 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

44、not protect itself from damage by closing the iris or closing the eyelid. Eye safety can be ensured by restricting the tran</p><p>　　Skin safety is also a possible concern. Possible shortterm effects such as

45、 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 subm

46、itting our bodies to much higher radiation levels than these communication systems do. </p><p>　　III. Communications Design</p><p>　　Equally important for achieving the design goals of wireless

47、infrared systems are communications issues. In particular, the modulation signal format together with appropriate error control coding is critical to achieving power efficiency. Channel characterization is also important

48、 for understanding performance limits.</p><p>　　A. Modulation Techniques</p><p>　　To understand modulation in IM/DD systems, we must look again at the channel model</p><p>　　and con

49、sider 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). The average transmitted optical power PT is the time aver

50、age of X(t). Our goal is to minimize the transmitted power required to attain a certain probability of bit error Pe, also known as a bit error rate (BER).It is useful to define the signal-to-noise ratio SNR as</p>

51、<p>　　where H(0) is the d.c. gain of the channel, i.e. it is the Fourier transform of h(t) evaluated at zero frequency, so</p><p>　　The transmitted signal can be represented as</p><p>　　Th

52、e sequence represents the digital information being transmitted, where is one of L possible data symbols from 0 to L-1. The function Si(t) represents one of L pulse shapes with duration Ts, the symbol time. The data ra

53、te (or bit rate) Rb, bit time T, symbol rate Rs, and symbol time Ts are related as follows: .</p><p>　　There are three commonly used types of modulation schemes: on-off keying (OOK) with non-return-to-zero

54、pulses, OOK with return-to-zero pulses of normalized width and pulse position modulation with L pulses (L-PPM). OOK and are simpler to implement at both the transmitter and receiver than L-PPM. The pulse shapes for t

55、hese modulation techniques are shown in Figure 3. Representative examples of the resulting transmitted signal X(t) for a short data sequence are shown in Figure4.</p><p>　　We compare modulation schemes in Ta

56、ble 1 by looking at measures of power efficiency and bandwidth efficiency. Bandwidth efficiency is measured by dividing the zero-crossing bandwidth by the data rate. Bandwidth efficient schemes have several advantages—th

57、e receiver and transmitter electronics are cheaper, and the modulation scheme is less likely to be affected by multipath distortion. Power efficiency is measured by comparing the required transmit power to achieve a targ

58、et probability of error P</p><p>　　B. Error Control Coding</p><p>　　Error control coding is an important technique for improving the quality of any digital communication system. We concentrate h

59、ere on forward error correction channel coding, as this specifically relates to wireless infrared communications; source coding and ARQ coding are not considered here.</p><p>　　Trellis-coded PPM has been fou

60、nd to be an effective scheme for multipath infrared channels(10,11).The key technique is to recognize that although on a distortion-free channel, all symbols are orthogonal and equidistant in signal space, this is not tr

61、ue on a distorting channel. Hence, trellis-coding using set partitioning designed to separate the pulse positions of neighboring symbols is an effective coding method. Coding gains of 5.0 dB electrical have been reported

62、 for rate 2/3-coded 8-PPM over</p><p>　　C. Channel impulse response characterization</p><p>　　Impulse response characterization refers to the problem of understanding how the impulse response C(

63、t) in Equation (1) depends on the location, size, and orientation of the receiver and transmitter. There are basically three classes of techniques for accomplishing this: measurement, simulation, and modeling. Channel me

64、asurements have been described in several studies(9,12,2), and these form the fundamental basis for understanding the channel properties. A particular study might generate a collec</p><p>　　Simulation method

65、s have been used to allow direct calculation of a particular impulse response based on a site-specific characterization of the propagation environment(13,14). The transmitter, receiver, and the reflecting surfaces are de

66、scribed and used to generate an impulse response. The basic assumption is that most interior surfaces reflect light diffusely in a Lambertian pattern, i.e. all incident light, regardless of incident angle, is reflected i

67、n all directions with an intensity proporti</p><p>　　A third technique attempts to extract knowledge gained from experimental and simulation-based channel estimations into a simple-to-use model. In (15), for

68、 example, a model using two parameters (one for path loss, one for delay spread) is used to provide a general characterization of all diffuse IR channels. Methods for relating the parameters of the model to particular ro

69、om characteristics are given, so that system designers can quickly estimate the channel characteristics in a wide range of sit</p><p><b>　　翻譯:</b></p><p><b>　　無線紅外通信</b>&

70、lt;/p><p><b>　　I.導言</b></p><p>　　無線紅外通信指的是使用近紅外波段的光作為通信傳輸介質的一種通訊方式（1-3），如圖1所示。通信可于一個便攜式設備和另一個便攜式設備或系留設備之間進行，這樣的設備稱之為接入點或基站。典型的便攜設備包括筆記本電腦，個人數字助理，便攜式電話，而基站通常與網絡內的其他計算機相連來工作。雖然通常采用紅外光，光頻譜

71、的其他地區(qū)可以使用（所以說，我們一般稱為無線光通信，而不是無線紅外通信）。</p><p>　　無線紅外通信系統可以以他們所設計的應用程序或由鏈路類型為特征，特點如下所述。</p><p><b>　　應用</b></p><p>　　主要的商業(yè)應用如下：</p><p>　　1．短期無電纜連接的兩個用戶之間的信息交流（

72、名片，日程安排，文件共享）。主要的例子是紅外線系統（見第4節(jié)）。</p><p>　　2．無線局域網（WLAN）提供建筑物內的網絡連接工作。這可以是對現有的局域網的延伸，或在沒有網絡的地方建立網絡。主要的例子是IEEE802.11標準（見第4節(jié)）。</p><p>　　3．為城域網或校園區(qū)網絡內的建筑物提供高速網絡連接。</p><p>　　4．無線輸入和控制設備，

73、如無線鼠標，遙控器，無線游戲控制器，遠程電子鑰匙。</p><p><b>　　B．鏈接類型</b></p><p>　　鏈接類型是描述無線紅外通信系統的另一個重要途徑，這意味著典型或有要求的接收和發(fā)射。兩種最常見的通信制度是：點至點制度和擴散制度。</p><p>　　最簡單的鏈接類型是點至點系統。在那里，發(fā)射器和接收器必須指定對方并建立鏈接

74、。該系列產品的視距（LOS）路徑從發(fā)射器到接收器必須沒有障礙物，最好是光直接射向接收機。因此，點至點系統也稱為定向LOS系統。該鏈接可以暫時創(chuàng)建一個數據在兩個用戶之間進行交換，或在移動基站單元建立局域網對這一單元進行校準使之長久保持連接。</p><p>　　在擴散系統中，在發(fā)射器發(fā)光后，需始終保持在同一地區(qū)中的任何反射機（如天花板，墻壁和家具等傳輸資料的軸承）表面有能讓接收機接收到的反射光。在這里，發(fā)射器和接收

75、器非定向：發(fā)射器采用了廣泛的發(fā)射波束，而接收器有一個寬視角的鏡頭。此外，LOS的路徑是不需要的。因此，擴散性系統也稱為非定向非視距系統。它非常適合于無線局域網應用，從認知上說，與其他通信設備的按用戶位置調整鏈接不同。</p><p><b>　　C．基礎和大綱</b></p><p>　　大多數無線紅外通信系統，可以模擬其輸出信號Y（t）和輸入信號X（t），并得出其表

76、達式</p><p>　　其中 表示卷積，c（t）是在通道N（t）為加性噪聲時的脈沖響應。這篇文章圍繞該模型系統中的關鍵問題進行討論。</p><p>　　在第2節(jié)中，我們考慮下面幾個問題。紅外在什么樣的無線通信系統中適用？接收器和發(fā)送器的電學和光學特性決定怎樣的c（t）？接收器和發(fā)送器的大小和方向如何確定c（t）的位置？光路中X（t）和Y（t）有什么聯系？波長 X（t）的

77、范圍是什么？需要怎樣的設備才能實現？為什么會產生N（t）？是否有任何安全方面的考慮？在第3節(jié)中，我們考慮的通信設計問題。應該如何對數據符號序列進行調制得到輸入信號X（t）？提取接收信號Y（t）的有關數據資料的最好的檢測機制是什么？如何才能衡量和提高系統性能？在第4節(jié)，我們會從現有的標準進行設計制造，如紅外線和802.11的選擇。最后，在第5節(jié)，我們考慮這些系統在今后將如何改進。</p><p><b>

78、　　II.光學設計</b></p><p><b>　　A．調制解調</b></p><p>　　發(fā)射器發(fā)出的調制波攜帶有怎樣的特征信息？大多數通信系統是基于相位，振幅或頻率調制，或這些技術的結合。但是，對一個信號進行調制需要占用更大的帶寬。一個有效的解決方法是使用強度調制，在調制信號中傳輸信號的強度和功率成正比。</p><p>

79、　　解調器（通常指的是光學系統探測器）可以提取運載信息的混合光波中的信號。當有連續(xù)相位的信號時，使用相干解調是最好的。然而，這可能很難實現，在非定向傳播中，它很難達到規(guī)定的混合效率。此外，人們可以使用光電探測器直接檢測。該探測器的電流與接收到的光信號強度成正比，這便于強度調制，也就是原始信號調制。因此，大多數系統使用直接檢測（IM/ DD）中的光強度調制來實現調制和解調。</p><p>　　在自由空間光通信系統

80、中，探測器接收到的光可能有其它來源。這些包括環(huán)境照明如自然光，日光燈光，白熾燈光等。這些與光傳輸無關的信號的變化，會在接收器中產生噪聲分量。</p><p>　　我們可以寫出接收端的光</p><p>　　其中R是接收光電二極管（A/ W）的響應。請注意，電脈沖響應的C（t）是簡單的r次的光脈沖響應h（t）。有些人使用（t）而另一些使用h（t）作為脈沖響應，我們計算時要視情況而定。<

81、/p><p><b>　　B．接收機和發(fā)射機</b></p><p>　　發(fā)射器將原始電信號轉化為光信號。典型的電光轉換器件是發(fā)光二極管（LED）和半導體激光二極管（LD）。LED擁有自然寬傳輸模式，因此它適合于非直接傳輸。對人眼來說發(fā)光二極管比激光二極管更安全，激光二極管通常具有很窄的光束。激光二極管的主要優(yōu)點是其高能量轉換效率，其高調制帶寬，和其相對窄的光譜寬度。雖然

82、激光二極管比發(fā)光二極管適合開發(fā)，但目前大多數短期投資的商業(yè)系統仍使用LED。</p><p>　　一個接收器或檢測器通過把檢測到得探測器表面的光子通量轉換成電流來計算入射光功率。硅PIN光電二極管完美的應用于紅外無線通信，是因為他們在這一過程中良好的量子效率和其便宜的價格（4）。這里不使用雪崩光電二極管，是因為主要的噪音源是回到地面的光致熱散粒噪聲，而不是電路的噪聲。</p><p>&l

83、t;b>　　C．傳輸波長和噪音</b></p><p>　　如何去選擇一個傳輸波長是有效的，低成本的供應源和探測器是最重要的。對在800納米至1000納米范圍內工作的LED和硅光電二極管其可用性是選擇它們在這個頻段使用的主要原因。另一個重要的考慮因素是主要的噪音源—背景照明的光譜分布。</p><p>　　噪聲N（t）可以分為四個部分：光子噪聲或散粒噪聲，增益噪聲，接收電

84、路或熱噪聲，周期性噪聲。增益噪聲來源是唯一的， 來源于典型的雪崩式設備，因此這里我們不會考慮。</p><p>　　光子噪聲是光子到達的離散的結果。其起因是背景光源，如陽光，日光燈燈光和白熾燈的光，以及依賴信號源的X（t）-c（t）。由于打在光檢測器上的背景光通常比信號光強，我們可以忽略N（t）對X（t）的影響，并考慮光子噪聲是加性白高斯噪聲，其具有雙面電譜密度，其中q為電子電荷，R為響應，PN是噪聲（背景光）的

85、光功率。</p><p>　　接收機的噪聲起因是接收器電路的熱效應，尤其是對前置放大器的使用。通過精心的電路設計，相對于光子噪聲它可以忽略（5）。 周期性的噪音是變化的熒光燈中驅動燈管的鎮(zhèn)流器使用的結果。這將生成一個有著44千赫根本頻率和幾個兆赫諧波的不相干的周期信號?？梢酝ㄟ^使用高通過濾與基線恢復組合或通過仔細挑選調制類型減輕周期性噪聲的影響（6），如3.1節(jié)討論。</p><p&g

86、t;<b>　　D．安全</b></p><p>　　處理紅外線通信系統時有兩個安全問題值得關注。眼睛安全值得關注是因為兩個效果組合：角膜是從近紫外到近紅外變得透明。因此，視網膜對光源發(fā)出的這些頻段的光的損害很敏感。然而，近紅外光超出了可見光范圍，所以閉眼不會使眼睛免受損壞。按照IEC或ANSI標準（7,8）限制的強度發(fā)射波束可以保證人眼安全。</p><p>　　皮

87、膚安全也需要引起關注。相對人眼安全法規(guī)皮膚加熱可能造成短期的影響是（因為眼睛要比皮膚功率水平低）。長期暴露在紅外光下不是一個值得擔心的事，隨著環(huán)境光源不斷提高我們的身體輻射水平會比這些通信系統更高。</p><p><b>　　III.通訊設計</b></p><p>　　實現無線紅外系統設計目標最重要的解決是通信問題。調制信號編碼與相應的錯誤控制，對于實現功率效率是

88、關鍵的。通道特性對其性能理解來說也是重要的。</p><p><b>　　A．調制技術</b></p><p>　　為了了解IM/ DD調制系統，我們必須分析以下通道模型并考慮其具體特點。</p><p>　　首先，由于我們使用的是強度調制， X（t）是信道輸入的光的強度，我們用X（t）表示。平均傳輸光功率PT是平均時間內的X（t）。我們的目標

89、是盡量減少所需的發(fā)射功率達到了一定的概率誤碼Pe，也稱為誤碼率（BER）。這對定義信噪比SNR是非常有用的</p><p>　　其中H（0）為直流電流通道的增益，即它是h（t）的頻率在零時的傅里葉變換，因此</p><p>　　所傳輸的信號可以表示為</p><p>　　序列代表傳輸的數字信息，其中是L的一個可能的數據符號其范圍從0到L- 1。函數Si（t）代表符號

90、 L在時間符號Ts時間時的脈沖形狀。數據速率（或比特率）Rb，位時間T，符號率R和符號時間Ts相關如下：</p><p>　　常用的有三種類型的調制方案：通斷鍵控（OOK）和不歸零的脈沖，OOK和標準寬度歸零脈沖，脈沖與L脈沖位置調制（L型的PPM）。 OOK和比L- PPM更簡單的實現發(fā)射和的接收。圖3介紹了這種脈沖調制技術。典型短期數據序列所產生的傳輸信號X（t）顯示在圖4。</p><p

91、>　　我們比較了功率效率和帶寬效率的調制方案如表1。帶寬效率是由數據速率除以過零帶寬求得的。帶寬效率方案有幾個優(yōu)勢，接收器和發(fā)送器電子產品便宜，調制方案不太可能被多徑失真的影響。電源效率是通過比較不同調制技術發(fā)射功率和誤碼率之后眼得出的。和PPM比OOK更有效率，但它們占用更大的帶寬。但是，對于一個給定的帶寬效率，PPM比更有效，所以PPM是最常用的。需要的數據速率非常高，例如100 Mb / s或更大時OOK最有用。然后，多徑失

92、真效果變得更為顯著，帶寬效率變得極為重要（9）。</p><p><b>　　B.錯誤編碼控制</b></p><p>　　錯誤編碼控制是提高任何數字通信系統質量的重要技術。我們集中在這里糾錯信道編碼，因為這特別涉及到無線紅外通訊;信源編碼和ARQ編碼是不列入考慮。</p><p>　　格狀編碼的PPM已被認為是一個有效的多路徑紅外通道的方案（

93、10,11）。關鍵技術是要認識到雖然在無失真通道，所有的符號和信號是正交的，空間距離相等，但它不是真正的通道上的扭曲。因此，用網格分割編碼設計分隔相鄰符號的脈沖位置是一種有效的編碼方法。5.0 dB具有相同的帶寬已經公布了2 / 3編碼的8PPM編碼效益比未編碼編碼16 PPM高（11）。</p><p><b>　　C.沖擊響應特性</b></p><p>　　脈沖

94、響應特性指的是方程（1）中的C（t）如何，取決于它在脈沖系統中的位置，大小和接收器與發(fā)送器的方向。有三種基本技術類實現這個目標：測量，仿真和建模。幾項研究描述了通道測量（9,12,2），這些構成了理解通道屬性的基本依據。一個特別的研究可能會為i產生成百上千的例子脈沖響應Ci（t）。通過看點散圖路徑損耗對距離，延誤散點圖與傳播距離，發(fā)射器和接收器的方向，魯棒性效果陰影等的影響可以測量收集脈沖響應Ci（t）。</p><

95、p>　　模擬方法已被用來允許計算一個特定的傳播環(huán)境站點上的脈沖響應（13,14）。發(fā)射器，接收器和反射面用于描述和產生一個脈沖響應。其基本假設是，大多數室內表面反射光在彌漫朗伯模式（即所有入射光，不管入射角，方向和強度都與曲面法線的映像的余弦角度成正比）的映像?，F有方法的困難在于，需要準確模型和大量計算。</p><p>　　第三種方法是從實驗和簡單信道仿真模型去提取所需知識。例如（15），一個模型使用兩個