外文翻译----无线红外通信(编辑修改稿)

外文翻译----无线红外通信(编辑修改稿)内容摘要：

or is normally much stronger than the signal light, we can neglect the dependency of N(t) on X(t) and consider the photon noise to be additive white Gaussian noise with twosided power spectral density where q is the electron charge, R is the responsivity, and Pn is the optical power of the noise (background light). Receiver noise is due to thermal effects in the receiver 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). Periodic noise is the result 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 several MHz. Mitigating the effect of periodic noise can be done using highpass filtering in bination with baseline restoration(6), or by careful selection of the modulation type, as discussed in Section . D. Safety There are two safety concerns when dealing with infrared munication systems. Eye safety is a concern 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 an。