frequency division multiplexing
Frequency Division Multiplexing (FDM) is to divide the total bandwidth used for a transmission channel into several sub-bands (or sub-channels), and each sub-channel transmits one signal. Frequency division multiplexing requires that the total frequency width is greater than the sum of the frequencies of each sub-channel. At the same time, in order to ensure that the signals transmitted in each sub-channel do not interfere with each other, an isolation band should be set up between each sub-channel, which ensures that the signals of each channel are mutually exclusive. Do not interfere (one of the conditions). The characteristic of the frequency division multiplexing technology is that the signals transmitted by all sub-channels work in parallel, and the transmission delay can be ignored when each signal is transmitted, so the frequency division multiplexing technology has been widely used. Besides the frequency division multiplexing (FDM) in the traditional sense, there is also an orthogonal frequency division multiplexing (OFDM).
1.1 Traditional Frequency Division Multiplexing
The typical application of traditional frequency division multiplexing is the transmission of radio and television HFC network TV signals, whether it is analog TV signals or digital TV signals, because for digital TV signals, although each channel (8 MHz) The channels are transmitted by time division multiplexing, but the channels are still transmitted by frequency division multiplexing.
1.2 Orthogonal Frequency Division Multiplexing
OFDM (Orthogonal Frequency Division Multiplexing) is actually a multi-carrier digital modulation technology. All carrier frequencies of OFDM have equal frequency intervals, which are integer multiples of a fundamental oscillation frequency. Orthogonal means that the signal spectrum of each carrier is orthogonal.
OFDM systems require much less bandwidth than FDM systems. Since OFDM uses a non-interfering orthogonal carrier technology, there is no need for guard bands between individual carriers, which makes the use of the available spectrum more efficient. In addition, OFDM technology can dynamically allocate data in sub-channels. In order to obtain maximum data throughput, multi-carrier modulators can intelligently allocate more data to sub-channels with less noise. At present, OFDM technology has been widely used in broadcast audio and video fields and civil communication systems. The main applications include: Asymmetric Digital Subscriber Loop Line (ADSL), Digital Video Broadcasting (DVB), High Definition Television (HDTV) , wireless local area network (WLAN) and 4th generation (4G) mobile communication systems.
time division multiplexing
Time Division Multiplexing (TDM, Time Division Multiplexing) is to divide the time provided for the entire channel transmission information into several time slices (referred to as time slots), and assign these time slots to each signal source for use. The exclusive channel in the time slot is used for data transmission. The feature of time division multiplexing technology is that time slots are planned and allocated in advance and are fixed, so it is sometimes called synchronous time division multiplexing. The advantage is that the time slot allocation is fixed, which is easy to adjust and control, and is suitable for the transmission of digital information; the disadvantage is that when a signal source has no data transmission, its corresponding channel will be idle, and other busy channels cannot occupy this idle channel. , thus reducing the utilization of the line. Time division multiplexing technology, like frequency division multiplexing technology, has a very wide range of applications. Telephone is the most classic example. In addition, time division multiplexing technology has also been widely used in radio and television, such as SDH, ATM, IP and HFC networks. The communication between the CM and the CMTS uses the technology of time division multiplexing.
wavelength division multiplexing
Communication is the way in which light is used to carry signals for transmission. In the field of optical communication, people are used to naming by wavelength rather than frequency. Therefore, the so-called wavelength division multiplexing (WDM, Wavelength Division Multiplexing) is essentially frequency division multiplexing. WDM is a system that carries multiple wavelengths (channels) on one optical fiber, and converts one optical fiber into multiple "virtual" fibers. Of course, each virtual fiber works independently on different wavelengths, which greatly improves the transmission capacity of the optical fiber. . Because of the economy and effectiveness of the WDM system technology, it has become the main means of expanding the capacity of the current optical fiber communication network. As a system concept, wavelength division multiplexing technology usually has three multiplexing methods, namely wavelength division multiplexing of 1 310 nm and 1 550 nm wavelength, coarse wavelength division multiplexing (CWDM, Coarse Wavelength Division Multiplexing) and dense wave Division Multiplexing (DWDM, Dense Wavelength Division Multiplexing).
(1) Wavelength division multiplexing of 1 310 nm and 1 550 nm wavelengths
This multiplexing technology only used two wavelengths in the early 1970s: one wavelength in the 1310 nm window, and one wavelength in the 1550 nm window, using WDM technology to achieve single-fiber double-window transmission, which is the original wavelength division multiplexing. usage.
(2) Coarse wavelength division multiplexing
Following the application in the backbone network and long-distance network, the wavelength division multiplexing technology has also begun to be used in the metropolitan area network, mainly referring to the coarse wavelength division multiplexing technology. CWDM uses a wide window of 1 200 to 1 700 nm, and is currently mainly used in systems with a wavelength of 1 550 nm. Of course, a wavelength division multiplexer with a wavelength of 1 310 nm is also under development. The spacing between adjacent channels of a coarse wavelength division multiplexer (large wavelength spacer) is generally ≥20 nm, and the number of wavelengths is generally 4 or 8 waves, with a maximum of 16 waves. When the number of multiplexed channels is 16 or less, since the DFB laser used in the CWDM system does not require cooling, the CWDM system has more advantages than the DWDM system in terms of cost, power consumption requirements and equipment size, and CWDM is more and more widely used. accepted by the industry. CWDM does not need to choose expensive dense wave decomposition multiplexer and "optical amplifier" EDFA, but only needs to use cheap multi-channel laser transceivers as relays, so the cost is greatly reduced. Today, many manufacturers have been able to provide commercial CWDM systems with 2 to 8 wavelengths, which are suitable for use in cities where the geographic scope is not particularly large and the data service development is not very fast.
(3) Dense wavelength division multiplexing
Dense Wavelength Division Multiplexing (DWDM) can carry 8 to 160 wavelengths, and with the continuous development of DWDM technology, the upper limit of the number of demultiplexed waves is still increasing, and the interval is generally ≤1.6 nm. distance transmission system. Dispersion compensation techniques (overcoming nonlinear distortion in multi-wavelength systems - the phenomenon of four-wave mixing) are required in all DWDM systems. In the 16-wave DWDM system, the conventional dispersion compensation fiber is generally used for compensation, while in the 40-wave DWDM system, the dispersion slope compensation fiber must be used for compensation. DWDM can combine and transmit different wavelengths in the same fiber at the same time. In order to ensure effective transmission, one fiber is converted into multiple virtual fibers. At present, with DWDM technology, a single fiber can transmit data traffic up to 400 Gbit/s, and as manufacturers add more channels to each fiber, the transmission speed of terabits per second is just around the corner.
code division multiplexing
Code Division Multiplexing (CDM, Code Division Multiplexing) is a multiplexing method that distinguishes various original signals by different codes. It is mainly combined with various multiple access technologies to generate various access technologies, including wireless and wired access. enter. For example, in the multiple-access cellular system, the communication objects are distinguished by channels. One channel only accommodates one user to talk, and many users who are talking at the same time are distinguished from each other by the channel, which is multiple access. The mobile communication system is a system with multiple channels working simultaneously, and has the characteristics of broadcasting and large-area coverage. In the radio wave coverage area of the mobile communication environment, establishing a wireless channel connection between users is a wireless multiple access method, which belongs to the multiple access technology. Unicom CDMA (Code Division Multiple Access) is a way of code division multiplexing, called code division multiple access, in addition to frequency division multiple access (FDMA), time division multiple access (TDMA) and synchronous code division multiple access (SCDMA) ).
（1）FDMA
FDMA frequency division multiple access adopts frequency modulation multiple access technology, and the traffic channels are allocated to different users in different frequency bands. FDMA is suitable for accessing a large amount of continuous non-burst data, and it is rare to simply use FDMA as a multiple access method. At present, the GSM mobile phone network used by China Unicom and China Mobile is a combination of FDMA and TDMA.
(2) TDMA time division multiple access
TDMA time division multiple access adopts the time division multiple access technology to allocate the traffic channel to different users in different time periods. The advantage of TDMA is that the spectrum utilization rate is high, and it is suitable for supporting the access of multiple bursty or low-rate data users. In addition to the combination of FDMA and TDMA in the GSM mobile phone network used by China Unicom and China Mobile, the communication between CM and CMTS in the radio and television HFC network also uses time division multiple access (based on DOCSIS 1.0 or 1.1 and Eruo DOCSIS 1.0 or 1.1).
(3) CDMA Code Division Multiple Access
CDMA is a new and mature wireless communication technology developed by the branch of digital technology - spread spectrum communication technology. It is developed on the basis of FDM and TDM. The characteristic of FDM is that the channel is not monopolized, but the time resources are shared, and the frequency bands used by each sub-channel do not overlap with each other; The characteristic is that all sub-channels can use the entire channel for data transmission at the same time, and it shares both channel and time resources. Therefore, the efficiency of the channel is high and the capacity of the system is large. The technical principle of CDMA is based on spread spectrum technology, that is, the information data with a certain signal bandwidth to be transmitted is modulated with a high-speed pseudo-random code (PN) whose bandwidth is much larger than the signal bandwidth, so that the bandwidth of the original data signal is expanded, and then the The carrier is modulated and sent out; the receiving end uses the exact same pseudo-random code to perform correlation processing with the received wideband signal, and replace the wideband signal with the narrowband signal of the original information data, that is, despread, to realize information communication. CDMA code division multiple access technology is fully suitable for the large capacity, high quality, comprehensive services, soft handover, etc. required by modern mobile communication networks, and is being favored by more and more operators and users.
(4) Synchronous code division multiple access technology
Synchronous Code Division Multiple Access (SCDMA, Synchronous Code Division Multiplexing Access) means that the pseudo-random codes are synchronous and orthogonal, which can be accessed wirelessly or by wire, and is widely used. This technology is used in the communication between CM and CMTS in the radio and television HFC network, such as American Terayon and Beijing Kai Shitong cable TV broadband access, combined with ATDM (Advanced Time Division Multiple Access) and SCDMA uplink channels Communication (based on DOCSIS2.0 or Eruo DOCSIS2.0).