【正文】 ipment has photoelectric conversion module and digital signal processing unit is posed of two parts. Satellite munications: Distance munications, transmission capacity, coverage, and not subject to geographical constraints and high reliability. At present, the use of sophisticated techniques Analog modulation, frequency division multiplexing and frequency division multiple access. Digital satellite munication using digital modulation, time division multiple road in time division multiple access. Mobile Communications: GSM, CDMA. Number of key technologies for mobile munications: modulation techniques, error correction coding and digital voice encoding. Data Communication Systems. Five basic types of data munication system: （ 1） Offline data transmission is simply the use of a telephone or similar link to transmit data without involving a puter equipment used at both ends of such a link is not part of a puter, or at least does not immediately make the data available for puter process, that is, the data when sent and / or received are 39。offline39。. 4 This type of data munication is relatively cheap and simple. （ 2） Remote batch is the term used for the way in which data munication technology is used geographically to separate the input and / or output of data from the puter on which they are processed in batch mode. （ 3） Online data collection is the method of using munications technology to provide input data to a puter as such input arisesthe data are then stored in the puter (say on a magic disk) and processed either at predetermined intervals or as required. （ 4） Enquiryresponse systems provide, as the term suggests, the facility for a user to extract information from a enquiry facility is passive, that is, does not modify the information interrogation may be simple, for example, 39。RETRIEVE THE RECORD FOR EMPLOYEE NUMBER 1234 39。or systems may use terminals producing hard copy and / or visual displays. （ 5） Realtime systems are those in which information is made available to and processed by a puter system in a dynamic manner so that either the puter may cause action to be taken to influence events as they occur (for example as in a process control application) or human operators may be influenced by the accurate and uptodate information stored in the puter, for example as in reservation systems. 2 Signal spectrum with bandwidth Electromagic data signals are encoded, the signal to be included in the data transmission. Signal in time for the general argument to show the message (or data) as a parameter (amplitude, frequency or phase) as the dependent variable. Signal of their value since the time variables are or not continuous, can be divided into continuous signals and discrete signals。 according to whether the values of the dependent variable continuous, can be divided into analog signals and digital Signal. Signals with timedomain and frequency domain performance of the two most basic forms and features. Timedomain signal over time to reflect changing circumstances. Frequency domain characteristics of signals not only contain the same information domain, and the spectrum of signal analysis, can also be a clear understanding of the distribution of 5 the signal spectrum and share the bandwidth. In order to receive the signal transmission and receiving equipment on the request channel, Only know the timedomain characteristics of the signal is not enough, it is also necessary to know the distribution of the signal spectrum. Timedomain characteristics of signals to show the letter .It’s changes over time. Because most of the signal energy is concentrated in a relatively narrow band, so most of our energy focused on the signal that Paragraph referred to as the effective band Bandwidth, or bandwidth. Have any signal bandwidth. In general, the greater the bandwidth of the signal using this signal to send data Rate on the higher bandwidth requirements of transmission medium greater. We will introduce the following simple mon signal and bandwidth of the spectrum. More or less the voice signal spectrum at 20 Hz ~ 2021 kHz range (below 20 Hz infrasound signals for higher than 2021 KHz. For the ultrasonic signal), but with a much narrower bandwidth of the voice can produce an acceptable return, and the standard voicefrequency signal gnal 0 ~ 4 MHz, so the bandwidth of 4 MHz. As a special example of the monostable pulse infinite bandwidth. As for the binary signal, the bandwidth depends on the generalThe exact shape of the signal waveform, as well as the order of 0,1. The greater the bandwidth of the signal, it more faithfully express the number of sequences. 3 The cutoff frequency channel with bandwidth According to Fourier series we know that if a signal for all frequency ponents can be pletely the same through the transmission channel to the receiving end, then at the receiving frequency ponents of these formed by stacking up the signal and send the signal side are exactly the same, That is fully recovered from the receiving end of the sendside signals. But on the real world, there is no channel to no wear and tear thr