Discussion on various reuse technologies

In data communication, multiplexing technology greatly improves the channel transmission efficiency and has been widely used. Multiplexing technology is to combine multiple signals at the sending end (such as the mixer used at the front-end of broadcasting and television), and then implement transmission on a dedicated physical channel. The receiving end then separates the compound signal. Multiplexing technology is mainly divided into two categories: Frequency Division Multiplexing (Frequency Division Multiplexing) and time division multiplexing (time division multiplexing ), wavelength Division Multiplexing and Statistical Multiplexing are essentially two Multiplexing technologies. There are also some other Multiplexing technologies, such as code division multiplexing, Polarization Wave multiplexing, and space division multiplexing.

1-Frequency Multiplexing

OFDM (Frequency Division Multiplexing) divides the total bandwidth used for the 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 be greater than the sum of the frequencies of each sub-channel. To ensure that the signals transmitted in each sub-channel do not interfere with each other, an isolation band should be set up between the sub-channels, this ensures that each signal does not interfere with each other (one of the conditions ). The feature of frequency division multiplexing technology is that signals transmitted by all sub-channels work in parallel. transmission latency is not considered during each channel of signal transmission, therefore, the frequency division multiplexing technology has been widely used. In addition to traditional frequency division multiplexing (OFDM), the frequency division multiplexing technology also involves orthogonal frequency division multiplexing (OFDM ).

1.1 traditional frequency division multiplexing

The typical application of traditional frequency-division multiplexing is the transmission of TV signals on the radio/TV or on the TV network of the TV. both analog and digital TV signals are the same, because for digital TV signals, although each channel (8 MHz) is time-division multiplexing transmission, the channels are still transmitted in frequency-division multiplexing mode.

1.2 orthogonal frequency division multiplexing

Orthogonal Frequency Division Multiplexing is a multi-carrier digital modulation technique. All OFDM carrier frequencies have equal frequency intervals, which are an integer multiple of the basic oscillation frequencies. Orthogonal means that the signal spectrum of each carrier is orthogonal.

The OFDM system requires much less bandwidth than the OFDM system. Because OFDM uses non-interference orthogonal carrier technology, there is no need to protect the frequency band between individual carriers, which makes the use of available spectrum more efficient. In addition, the OFDM technology can dynamically allocate data in sub-channels. To achieve the maximum data throughput, the multi-carrier modulation can intelligently allocate more data to sub-channels with low noise. At present, OFDM technology has been widely used in broadcast audio and video fields and civil communication systems. Its main applications include Asymmetric Digital user loop (ADSL) and digital video broadcast (DVB) high Definition TV (HDTV), wireless LAN (WLAN), 4th generation (4G) mobile communication system, etc.

2. time division multiplexing

TDM (Time Division Multiplexing) is to divide the Time that is provided to the whole channel for transmission information into several Time slices (Time slots), and assign these Time slots to each signal source for use, each signal is transmitted through an exclusive Channel in its own time slot. The time-division multiplexing technology is characterized by the time-division allocated in advance and fixed, so it is sometimes also called synchronous time-division multiplexing. The advantage is that the time slot allocation is fixed, which facilitates adjustment and control and is suitable for the transmission of digital information. The disadvantage is that when a signal source does not transmit data, the corresponding channel will be idle, other busy channels cannot occupy this idle channel, which reduces the utilization of the line. The time division multiplexing technology, like the frequency division multiplexing technology, is widely used. telephones are the most classic examples. In addition, the time division multiplexing technology has also been widely used in radio and television, for example, SDH, ATM, IP, and the communication between CM and cmts in the hfcnetwork is based on the time-division multiplexing technology.

3. Wavelength Division Multiplexing

Optical Communication is the transmission of signals carried by light. In the field of optical communication, people are used to naming by wavelength rather than by 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. It converts one optical fiber into multiple "virtual" fibers. Of course, each virtual fiber operates independently at different wavelengths, this greatly improves the optical fiber transmission capacity. Due to the economics and effectiveness of the WDM system technology, it has become the main means of resizing the current optical fiber communication network. As a system concept, Wavelength Division Multiplexing technology usually has three kinds of Multiplexing methods, namely, Wavelength Division Multiplexing and Coarse Division Multiplexing (CWDM, Coarse Wavelength Division Multiplexing) at 1 310 nm and 1 550 nm wavelengths) and Dense Wavelength Division Multiplexing (DWDM, Dense Wavelength Division Multiplexing ).

(1) wavelength division multiplexing at 1 310 nm and 1 550 nm

In the early 1970s S, this multiplexing technology only used two wavelengths: 1 310 nm window with one wavelength and 1 550 nm window with one wavelength. The WDM technology was used to achieve single-fiber dual-window transmission, this is the initial use of WDM multiplexing.

(2) coarse Wavelength Division Multiplexing

After its application in backbone networks and Long-Distance Networks, wavelength division multiplexing technology has also begun to be used in man. It mainly refers to coarse wavelength division multiplexing technology. CWDM 1 200 ~ 1 700 nm wide window. At present, the main application wavelength is 1 550 nm. Of course, 1 310 nm wavelength division multiplexing is also being developed. The spacing between adjacent channels of coarse Wavelength Division Multiplexing (large wavelength interval) is generally greater than or equal to 20 nm, and the number of wavelengths is generally 4 or 8 waves, up to 16 waves. When the number of multiplexing channels is 16 or less, the CWDM system has more advantages than the DWDM System in terms of cost, power consumption requirements, and device size because the DFB Laser Used by the CWDM system does not require cooling, CWDM is increasingly widely accepted by the industry. CWDM eliminates the need to choose expensive intensive wave decomposition musters and "Optical release" EDFA, and uses a cheap multi-channel laser transceiver as a relay, greatly reducing costs. Today, many manufacturers have been able to provide ~ The eight-wavelength commercial CWDM system is suitable for use in cities where the geographical range is not particularly large and the data business development is not very fast.

(3) Dense Wavelength Division Multiplexing

Dense Wavelength Division Multiplexing (DWDM) technology can carry 8 ~ With the continuous development of DWDM technology, the upper limit of the number of split waves is still increasing. The interval is generally less than or equal to 160 nm, which is mainly used in long-distance transmission systems. Dispersion Compensation Technology is required in all DWDM systems to overcome nonlinear distortion in multi-wavelength systems-four-wave mixing ). In a 16-wave DWDM System, the conventional Dispersion Compensation Optical Fiber is used for compensation. In a 40-wave DWDM System, the Dispersion Slope Compensation Optical Fiber must be used. DWDM can combine and transmit different wavelengths in the same optical fiber. To ensure effective transmission, a single optical fiber is converted into multiple virtual optical fibers. Currently, using DWDM technology, a single optical fiber can transmit up to 400 Gbit/s of data traffic. As the manufacturer adds more channels to each optical fiber, the transmission speed of the second is just around the corner.

4-bit multiplexing

CDM (Code Division Multiplexing) is a method of Multiplexing that uses different codes to distinguish original signals from each other. It is mainly used in combination with multiple access technologies to generate various access technologies, including wireless and wired access. For example, in a multi-access cellular system, communication objects are differentiated by channels. A channel can only accommodate one user for calls. Many simultaneous users are differentiated by channels. This is multi-access. The mobile communication system is a multi-channel system that operates simultaneously and features broadcast and wide coverage. In the radio wave coverage area of the mobile communication environment, the establishment of wireless channel connections between users is a wireless multi-access method, which is a multi-access technology. China Unicom CDMA (Code Division Multiple Access) is a method of Code Division multiplexing, called Code Division Multiple Access. In addition, there are also FDMA and TDMA) and Synchronous Code Division Multiple Access (TD-SCDMA ).

(1) FDMA

FDMA frequency division multiplexing uses the frequency modulation multiplexing technology, and business channels are allocated to different users in different frequencies. FDMA is suitable for accessing a large amount of continuous non-burstable data. It is rare to simply use FDMA as a multi-access method. Currently, China Unicom and China Mobile use the combination of FDMA and TDMA.

(2) TDMA

TDMA uses the time division multiplexing technology to allocate business channels to different users in different time periods. The advantage of TDMA is its high spectrum utilization, which is suitable for supporting access to multiple users with sudden or low-rate data. In addition to the combination of FDMA and TDMA, the communication between CM and CMTS in the radio and television network also adopts the Time Division Multiple Access Method (based on DOCSIS1.0, 1.1, and Eruo DOCSIS1.0 or 1.1 ).

(3) CDMA code division multiple access

CDMA is a new and mature wireless communication technology developed by the spread spectrum communication technology, which is developed on the basis of OFDM and TDM. The channel is not exclusive, and time resources are shared. Each sub-channel uses different bandwidths. TDM features exclusive time slots, while channel resources are shared, the time slots used by each sub-channel do not overlap. CDMA features that all sub-channels can use the entire channel for data transmission at the same time. It shares the data between the channel and time resources. Therefore, high channel efficiency and large system capacity. The technical principle of CDMA is based on the Spread Spectrum Technology. Information data that requires certain signal bandwidth will be modulated using a high-speed Pseudo-Random Code (PN) with a bandwidth far greater than the signal bandwidth, the bandwidth of the original data signal is extended and then modulated and sent by the carrier. The receiving end uses identical pseudo-random codes for processing, the bandwidth signal is replaced with the narrowband signal of the original information data, that is, the signal is unexpanded to achieve information communication. The CDMA code division multiple access technology is perfect for the large capacity, high quality, comprehensive services, and soft switching required by modern mobile communication networks. It is favored by more and more operators and users.

(4) Synchronous Code Division Multiple Access Technology

Synchronous Code Division Multiple Access (TD-SCDMA, Synchrnous Code Division Multiplexing Access) refers to the orthogonal synchronization between pseudo-random codes, which can be wireless Access or wired Access, and is widely used. This technology is used in the communication between CM and CMTS in the radio and television network. For example, Terayon and Beijing kaishi cable TV broadband access, combined with ATDM (advanced time division multiplexing) communication with the TD-SCDMA uplink channel (based on DOCSIS2.0 or Eruo DOCSIS2.0 ).

China's 3rd-generation mobile communication system also uses the Synchronous Code Division Multiple Access Technology, which means that the pseudo-random codes representing all users are synchronized when arriving at the base station. Due to the synchronization orthogonal between the pseudo-random codes, the Inter-code interference can be effectively eliminated, and the system capacity will be greatly improved. Its system capacity is the same as that of the other 3rd-generation mobile communication standards ~ 5 times.
 
5. Space Division Multiplexing

Space Division Multiplexing (SDM) is a Multiplexing method that shares one pair of wires or optical fiber cables. For example, four pairs of twisted pair wires share one cable and dozens of local cable lines. The precondition for achieving space division multiplexing is that the diameter of the optical fiber or wire is very small, and multiple optical fiber or multiple pairs of wires can be made in one cable, which saves both the material of the outer sheath and is easy to use.

6. Statistics Reuse

Statistical Multiplexing (SDM, Statistical Division Multiplexing) is also known as tag Multiplexing, Statistical time Division Multiplexing, or intelligent time Division Multiplexing. In fact, it is called Bandwidth Dynamic Allocation. In essence, statistical multiplexing is asynchronous time division multiplexing. It dynamically allocates time slots on demand, instead of the fixed time slot allocation used by time division multiplexing, allocates time slots based on whether the signal source needs to send data signals and the bandwidth requirements of the signal itself. The main applications include digital TV programs.