【正文】 below. A. Applications The primary mercial applications are as follows: 178。通信可于一個(gè)便攜式設(shè)備和另一個(gè)便攜式設(shè)備或系留設(shè)備之間進(jìn)行，這樣的設(shè)備稱(chēng)之為接入點(diǎn)或基站。 主要的例子是 標(biāo)準(zhǔn)（見(jiàn)第 4 節(jié)） 。因此，點(diǎn)至點(diǎn)系統(tǒng)也稱(chēng)為定向 LOS 系統(tǒng)。這篇文章圍繞該模型系統(tǒng)中的關(guān)鍵問(wèn)題 進(jìn)行討論 。 解調(diào)器（通常指的是光學(xué)系統(tǒng)探測(cè)器）可以提取 運(yùn)載信息的混合光波中的信號(hào) 。 這些與 光傳輸無(wú)關(guān)的信號(hào)的變化， 會(huì)在接收器中產(chǎn)生噪聲分量。 激光二極管的主要優(yōu)點(diǎn)是其高能量轉(zhuǎn)換效率，其高調(diào)制帶寬，和其相對(duì)窄的光譜寬度。 噪聲 N（ t）可以分為四個(gè)部分：光子噪聲或散粒噪聲，增益噪聲，接收電路或熱噪聲，周期性噪聲。 這將生成一個(gè)有著 44 千赫根本頻率和幾個(gè)兆赫諧波的不相干的周期信號(hào)。 相對(duì) 人眼安全法規(guī)皮膚加熱可能 造成 短期的影響是（因?yàn)檠劬σ绕つw功率水平低）。我們的目標(biāo)是盡量減少所需的發(fā)射功率達(dá)到了一定的概率誤碼 Pe，也稱(chēng)為誤碼率（ BER）。帶寬效率是由數(shù)據(jù)速率除以過(guò)零帶寬求得的。我們集中在這里糾錯(cuò)信道編碼，因?yàn)檫@特別涉及到無(wú)線(xiàn)紅外通訊 。 幾項(xiàng)研究描述 了 通道測(cè)量（ 9,12,2），這些構(gòu)成了理解通道屬性的基本依據(jù) 。例如（ 15），一個(gè)模型使用兩個(gè)參數(shù)（一個(gè)用于路徑損耗，一個(gè)用于時(shí)延擴(kuò)展）去提供所有彌漫紅外通道的一般特征。 現(xiàn)有方法的困難在于，需要準(zhǔn)確模型和大量計(jì)算。 脈沖響應(yīng)特性指的是 方程 （ 1） 中的 C（ t） 如何 ， 取決于 它在脈沖系統(tǒng)中的位置 ， 大小和接收器與發(fā)送器的方向 。 然后，多徑失真效果 變得更為 顯著，帶寬效率變得極為重要（ 9）。 典型短期數(shù)據(jù)序列所產(chǎn)生的 傳輸信號(hào) X（ t）顯示在圖 4。 首先，由于我們使用的是強(qiáng)度調(diào)制， X（ t）是信道輸入 的 光的強(qiáng)度，我們用 X（ t）表示。按照 IEC 或 ANSI 標(biāo)準(zhǔn)（ 7,8）限制的強(qiáng)度發(fā)射波束可以保證人眼安全。通過(guò)精心的電路設(shè)計(jì)，相對(duì)于光子噪聲它可以忽略（ 5）。對(duì)在 800 納米至 1000 納米范圍內(nèi)工作的 LED 和硅光電二極管其可用性是選擇它們?cè)谶@個(gè)頻段使用的主要原因。 LED 擁有 自然寬傳輸模式，因此 它 適合于非直接 傳輸 。 在自由空間光通信系統(tǒng)中，探測(cè)器接收到的光可能有其它來(lái)源 。但是，對(duì)一個(gè)信號(hào)進(jìn)行調(diào)制需要占用更大的帶寬 。它非常適合于無(wú)線(xiàn)局域網(wǎng)應(yīng)用， 從認(rèn)知 上說(shuō)， 與其他通信設(shè)備的 按用戶(hù)位置調(diào)整 鏈接不同 。在那里，發(fā)射器和接收器必須指定 對(duì)方 并建立鏈接。 2．無(wú)線(xiàn)局域網(wǎng)（ WLAN）提供建筑物內(nèi)的網(wǎng)絡(luò)連接工作。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 and additionally, in non directed propagation, it is difficult to achieve the required mixing efficiency. Instead, one can use direct detection using a photodetector. The photodetector current is proportional to the received optical signal intensity, which for intensity modulation, is also the original modulating signal. Hence, most systems use intensity modulation with direct detection (IM/DD)to achieve optical modulation and demodulation. In a freespace optical munication 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. These sources cause variation in the received photocurrent that is unrelated to the transmitted signal, resulting in an additive noise ponent 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 impulse response c(t) is simply R times the optical impulse response h(t). Depending on the situation, some authors use (t) and some use h(t) as the impulse response. B. Receivers and Transmitters A transmitter or source converts an electrical signal to an optical signal. The two most appropriate types of device are the lightemitting 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 simpler 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 energyconversion efficiency, their high modulation bandwidth, and their relatively narrow spectral width. Although laser diodes offer several advantages over LEDs that could be exploited, most shortrange mercial systems currently use LEDs. A receiver or detector converts optical power into electrical current by detecting the photon flux incident on the detector surface. Silicon pin photodiodes are ideal for wireless infrared munications as they have good quantum efficiency in this band and are inexpensive(4). Avalanche photodiodes are not used here since the dominant noise source is background lightinduced shot noise rather than thermal circuit noise. C. Transmission Wavelength and Noise The most important factor to consider when choosing a transmission wavelength is the availability of effective, lowcost sources and detectors. The 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 source: background lighting. The noise N(t) can be broken into four ponents: photon noise or shot noise,