【正文】 給出 計(jì)算 模型詳細(xì)空間特性參數(shù)的方法， 使系統(tǒng)設(shè)計(jì)人員可以快速地估計(jì) 出 各種的信道特性。 其基本假設(shè)是，大多數(shù)室內(nèi)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(qiáng)度都與曲面法線的映像的余弦角度成正比 ） 的映像 。一個(gè)特別的研究可能會(huì)為 i 產(chǎn)生成百上千的例子脈沖響應(yīng) Ci（ t） 。 dB 具有相同的帶寬已經(jīng)公布了 2 / 3編碼的 8PPM 編碼效益比未編碼編碼 16 PPM 高 （ 11）。信源編碼和 ARQ 編碼是不列入考慮。 需要的 數(shù)據(jù)速率非常高 ， 例如 100 Mb / s 或 更大時(shí) OOK 最有用。 帶寬效率 方案 有幾個(gè)優(yōu)勢(shì)，接收器和發(fā)送器電子產(chǎn)品便宜，調(diào) 制方案不太可能被多徑失真的影響。圖 3 介紹了這種脈沖調(diào)制技術(shù)。這對(duì)定義信噪比 SNR 是非常有用的 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(shí)的傅里葉變換，因此 所傳輸?shù)男盘?hào)可以表示為 序列 代表傳輸?shù)臄?shù)字信息，其中 是 L 的一個(gè)可能的數(shù)據(jù)符號(hào)其范圍從 0到 L 1。 A．調(diào)制技術(shù) 為了了解 IM/ DD 調(diào)制系 統(tǒng)，我們必須 分析 以下通道模型并考慮其具體特點(diǎn)。長(zhǎng)期暴露在紅外光 下 不是一個(gè) 值得擔(dān)心的事 ，隨著環(huán)境光源不斷提 高 我們的身體輻射水平 會(huì) 比這些通信系統(tǒng)更高。 然而，近紅外 光 超出了可見(jiàn)光范 圍，所以閉眼不會(huì)使眼睛免受損壞。 可以通過(guò)使用高通過(guò)濾與基線恢復(fù)組合或通過(guò)仔細(xì)挑選調(diào)制類型減輕周期性噪聲的影響（ 6），如 節(jié)討論。 接收機(jī)的噪聲起因是接收器電路的熱效應(yīng)，尤 其是對(duì)前置放大器的使用。增益噪聲 來(lái)源 是唯一 的， 來(lái)源于 典型的雪崩式設(shè)備，因此這里我們不會(huì)考慮。 C．傳輸波長(zhǎng)和噪音 如何去選擇一個(gè)傳輸波長(zhǎng)是有效的，低成本的供應(yīng)源和探測(cè)器是最重要的。雖然激光二極管比發(fā)光二極管適合開(kāi)發(fā)，但目前大多數(shù)短期投資 的 商業(yè)系統(tǒng)仍使用 LED。典型的電光轉(zhuǎn)換器件是發(fā)光二極管（ LED）和半導(dǎo)體激光二極管（ LD）。 我們可以寫(xiě)出接收端的光 其中 R 是接收光電二極管（ A/ W）的響應(yīng)。因此，大多數(shù)系統(tǒng)使用直接檢測(cè)（ IM/ DD） 中 的光強(qiáng)度調(diào)制來(lái)實(shí)現(xiàn)調(diào)制和解調(diào)。 當(dāng)有連續(xù)相位的信號(hào)時(shí)，使用相干解調(diào)是最好的 。 A．調(diào)制解調(diào) 發(fā)射器發(fā)出的調(diào)制波攜帶有怎樣的特征信息？大多數(shù)通信系統(tǒng)是基于相位，振幅或頻率調(diào)制，或這些技術(shù)的結(jié)合。 在第 2 節(jié)中，我們考慮 下面幾個(gè) 問(wèn)題。因此， 擴(kuò)散 性 系統(tǒng)也稱為非定向非視距系統(tǒng)。該鏈接可以暫時(shí)創(chuàng)建一個(gè)數(shù)據(jù) 在 兩個(gè)用戶之間 進(jìn)行 交換，或 在 移動(dòng)基站單元建立局域網(wǎng) 對(duì)這一單元進(jìn)行校準(zhǔn)使之長(zhǎng)久保持連接 。 最簡(jiǎn)單的鏈接 類型是點(diǎn)至點(diǎn)系統(tǒng)。 3．為城域網(wǎng)或校園區(qū)網(wǎng)絡(luò) 內(nèi)的 建筑物 提供 高速網(wǎng)絡(luò)連接。主要的例子是紅外線系統(tǒng)（見(jiàn)第 4 節(jié)）。典型的便攜設(shè)備包括筆記本電腦，個(gè)人數(shù)字助理，便攜式電話，而基站通常與網(wǎng)絡(luò)內(nèi)的其他計(jì)算機(jī)相連來(lái)工作。 the transmitter employs a wide transmit beam and the receiver has a wide fieldofview. Also, the LOS path is not required. Hence, diffuse systems are also called nondirected nonLOS systems. These systems are well suited to the wireless LAN application, freeing the user from knowing and aligning with the locations of the other municating devices. C. Fundamentals and Outline Most wireless infrared munications 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 by this model. In Section 2, we consider questions of optical design. What range of wireless infrared munications 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 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 munications design. How should a 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 standards such as IrDA and , 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 munication systems are based on phase, amplitude, or frequency modulation, or some bination of these techniques. However, it is difficult to detect such a signal following nondirected propagation, and more expensive narrowlinewidth sources are required(2). An effective solution is to use intensity modulation, where the transmitted signal39。 shortterm cableless connectivity for information exchange (business cards, schedules, file sharing) between two users. The primary example is IrDA systems (see Section 4). 178。 wireless local area works (WLANs) provide work connectivity inside buildings. This can either be an extension of existing LANs to facilitate mobility, or to establish “ ad hoc” works where there is no LAN. The primary example is the IEEE (see Section 4). 178。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 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 an