【正文】 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). The average transmitted optical power PT is the time average 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 signaltonoise ratio SNR as where H(0) is the . gain of the channel, . it is the Fourier transform of h(t) evaluated at zero frequency, so The transmitted signal can be represented as The sequence represents the digital information being transmitted, where is one of L possible data symbols from 0 to L1. The function Si(t) represents one of L pulse shapes with duration Ts, the symbol time. The data rate (or bit rate) Rb, bit time T, symbol rate Rs, and symbol time Ts are related as follows: . There are three monly used types of modulation schemes: onoff keying (OOK) with nonreturntozero pulses, OOK with returntozero pulses of normalized width and pulse position modulation with L pulses (LPPM). OOK and are simpler to implement at both the transmitter and receiver than LPPM. The pulse shapes for these 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. We pare modulation schemes in Table 1 by looking at measures of power efficiency and bandwidth efficiency. Bandwidth efficiency is measured by dividing the zerocrossing bandwidth by the data rate. Bandwidth efficient schemes have several advantages— the receiver and transmitter electronics are cheaper, and the modulation scheme is less likely to be affected by multipath distortion. Power efficiency is measured by paring the required transmit power to achieve a target probability of error Pe for different modulation techniques. Both and PPM are more power efficient than OOK, but at the cost of reduced bandwidth efficiency. However, for a given bandwidth efficiency, PPM is more power efficient than , and so PPM is most monly used. OOK is most useful at very high data rates, say 100 Mb/s or greater. Then, the effect of multipath distortion is the most significant effect and bandwidth efficiency bees of paramount importance(9). B. Error Control Coding Error control coding is an important technique for improving the quality of any digital munication system. We concentrate here on forward error correction channel coding, as this specifically relates to wireless infrared munications。雖然通常采用紅外光，光頻譜的其他地區(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）。 2．無線局域網(wǎng)（ WLAN）提供建筑物內(nèi)的網(wǎng)絡(luò)連接工作。 4．無線輸入和控制設(shè)備，如無線鼠標(biāo)，遙控器，無線游戲控制器，遠(yuǎn)程電子鑰匙。在那里，發(fā)射器和接收器必須指定 對方 并建立鏈接。 在 擴(kuò)散 系統(tǒng) 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(jī)（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(jī) 接收到的 反射光 。它非常適合于無線局域網(wǎng)應(yīng)用， 從認(rèn)知 上說， 與其他通信設(shè)備的 按用戶位置調(diào)整 鏈接不同 。 紅外在什么樣的無線通信系統(tǒng)中適用 ？接收器和發(fā)送器的電 學(xué) 和光學(xué)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(lián)系 ？ 波長 X（ t） 的范圍是什么 ？ 需要怎樣的設(shè)備才能實(shí)現(xiàn) ？ 為什么會產(chǎn)生 N（ t）？是否