The so-called two-way transmission means that the system can send information to users from the front end, that isdown-load, users can also send messages to the front end, that isUplink transmission。
SDM is a communication mode in which a fiber is used in each direction of two-way communication, which is called simplex mode.In SDM mode, signals in two directions are transmitted in two completely independent optical fibers, which do not affect each other. The transmission performance is the best, and the system design is also the simplest. However, a pair of optical fibers is required to complete the task of two-way transmission, which is not economical when the transmission distance is long.For applications where 0LT is close to ONU, SDM is still a two-way transmission scheme that can be considered due to the declining price of optical fiber.Finally, since the signal transmission paths in the two directions are independent of each other, there is no special requirement for the wavelength of the light source, as long as it is in the 1310 nm wavelength region, it does not matter whether it is the same or not.
TCM mode is one of the effective means to solve two-way transmission.This method only uses one optical fiber, but constantly changes the transmission direction alternately, so that signals in both directions can be transmitted on the same optical fiber in turn, just like playing table tennis, so it is also called "table tennis".There are two ways to realize TCM transmission. The first way is to use a laser as both a light source and a detector. It is very simple, as long as there is a transceiver control switch to accurately control its transceiver time so that it does not conflict.However, the sensitivity of this method is poor, and the available optical budget of the optical channel is very small when the rate is high.The second method is to use two sets of independent transceivers, and one optical coupler is set at both ends to separate the uplink and downlink signals. The signal transmission in the two directions is separated in time and takes different time slots to transmit in turn.Since only one direction is allowed to transmit signals at the same time, it is called half duplex mode, which is different from full duplex mode of WDM and SCM.When TCM mode is used, signals in both directions are allowed to work at the same wavelength, but it is specified that they must be in the 1310 nm wavelength zone.
Note that in the access network environment, PON mainly works in the point to multipoint mode, so the processing methods of uplink and downlink signals are different.The signals in the downlink direction are sent in a continuous arrangement and broadcast to each ONU. Each ONU receives all the signals but can only take out its own signal in its own time slot.The uplink direction is different. Each ONU sends signals in a burst mode and can only send signals in its own time slot. Therefore, the signals from each ONU are discontinuous burst blocks with different amplitudes.
TCM mode can use one optical fiber to complete two-way transmission tasks, which saves optical fiber, splitter and movable connector, and network management system can easily judge faults, so it is widely used.The disadvantage of this system is that the couplers at both ends have 3 dB power loss, and the circuits of OLT and ONU are relatively complex.
3. Wavelength division multiplexing (WDM)
When the transmission power of the light source does not exceed a certain threshold, the optical fiber works in a linear transmission state.At this time, signals of different wavelengths can be transmitted independently on the same fiber with a certain interval without mutual interference, which is the basic principle of wavelength division multiplexing.For two-way transmission, the purpose of single fiber two-way transmission can be achieved by adjusting the signals in both directions to different wavelengths, which is called different wavelength duplex mode.This method has great potential for future upgrade and expansion, and it is easy to expand to dozens of wavelengths. However, the cost of WDM devices is too high, so it is not economical when the transmission distance is not long.
The principle of using SCM to realize two-way transmission is very simple. Just arrange the signals in both directions in different frequency bands to achieve the purpose of two-way transmission of single fiber with the same wavelength.In the actual oAN transmission system, TDM baseband transmission is often used in the downlink direction, so the frequency component is concentrated at the low frequency end, while the subcarrier multiple access (SCMA) mode is used in the uplink direction, that is, the frequency of each user is adjusted at a higher frequency band, separated from the spectrum of the downlink signal.Since the uplink and downlink signals occupy different frequency bands, the system is not sensitive to reflection, and does not need the complex delay adjustment circuit necessary for TDMA mode. The transmission delay is small and the circuit is simple.[1]
Frequency division
Announce
edit
Frequency division bidirectional transmission mode uses different carrier frequencies to transmit uplink and downlink signals respectively.According to different uplink and downlink transmission contents, different uplink and downlink frequency bands can be used, but there must be a protection frequency band (free frequency band that neither transmits uplink signals nor downlink signals) in the middle to reduce the cross band impact caused by the non steep filtering characteristics.
According to the provisions of GY/T106-1999, the uplink signal is transmitted in the 5-65 MHz band, the 110-1000 MHz band is used to transmit analog TV, digital TV and digital services, and the 65-87 MHz band is its transition band.
Since the uplink signal and downlink signal are arranged in different frequency bands, the signal can be transmitted through one transmission line, and the signal separation can be solved by using different filters.This method is characterized by simple and mature technology. As long as a general filter is used, the uplink and downlink signals can be easily separated.
According to different applications, frequency segmentation can be divided into:
Low division: the division frequency is 30-40MHz;
Middle division: the division frequency is about 100MHz;
High division: the division frequency is about 200MHz.
The selection of high, medium and low segmentation modes mainly depends on the amount of information transmitted.In general, the application system with small node scale and less uplink information (such as TV on demand, Internet access, data retrieval, etc.) can adopt the low segmentation mode.For applications with large contact scale and more uplink information (such as video phone, conference television, etc.), the medium and high segmentation mode is adopted.
Space partition
Announce
edit
Space division uses different lines to transmit uplink and downlink signals respectively.In the optical fiber transmission system, this mode is mostly used, which uses two core fibers to transmit the uplink and downlink signals respectively.But in the cable transmission system, this method is usually not used, because the cost is too high.The advantage of this method is that the technology is simple, and there is no interference between the uplink signal and the downlink signal. However, the space division transmission actually uses two sets of one-way transmission systems, which is a combination of two one-way transmission systems, and cannot really be called a two-way system.
Time division
Announce
edit
The time division method uses time division multiplexing technology to separate uplink and downlink signals, that is, divide the system transmission signal into several time periods, and transmit uplink and downlink signals alternately in time division.
In the digital communication system, the time of transmitting the uplink or downlink signal is controlled by a pulse switch, the downlink signal is transmitted in one pulse cycle, and the uplink signal is transmitted in the next pulse cycle.The advantage of this method is that the uplink and downlink signals are transmitted alternately, without mutual interference. However, the switch of the sending end and the receiving end of the signal is required to be synchronized accurately, and the signal processing is relatively complex.
