“WDM, wavelength division multiplexing, the full English name is Wavelength Division Multiplexing, WDM technology is a relatively advanced optical fiber communication technology, called optical wavelength division multiplexing technology. It is a technology that combines a variety of optical signals with different wavelengths and different rates in different optical channels through a multiplexer and couples them into the same fiber for data transmission. The digital signals carried by these optical signals of different wavelengths may be of the same rate and format, or may be of different rates and different data formats.
WDM, wavelength division multiplexing, the full English name is Wavelength Division Multiplexing, WDM technology is a relatively advanced optical fiber communication technology, called optical wavelength division multiplexing technology. It is a technology that combines a variety of optical signals with different wavelengths and different rates in different optical channels through a multiplexer and couples them into the same fiber for data transmission. The digital signals carried by these optical signals of different wavelengths may be of the same rate and format, or may be of different rates and different data formats.
How WDM works
Wavelength × frequency = speed of light (constant value), so wavelength division multiplexing is actually frequency division multiplexing. Optical wavelength division multiplexing includes frequency division multiplexing and wavelength division multiplexing. There is no obvious difference between optical frequency division multiplexing (FDM) technology and optical wavelength division multiplexing (WDM) technology, because light waves are part of electromagnetic waves, and the frequency of light has a single correspondence with wavelength.
At the receiving end, the combined signals of different wavelengths are separated and further processed, and the original signals are recovered and sent to different terminals. Therefore, this technology is called optical wavelength division multiplexing, or optical wavelength division multiplexing technology for short.
Here, an optical fiber can be regarded as a “multi-lane” public road. The traditional TDM system only uses one lane of this road. Increasing the bit rate is equivalent to speeding up the driving speed in this lane to increase the unit time. volume of transportation. Using DWDM technology is similar to using unused lanes on public roads to obtain huge untapped transmission capabilities in optical fibers. In this way, the transmission capacity of the optical fiber is improved, and the utilization efficiency of the optical fiber resource is improved.
For a WDM system, it is obviously necessary to control the wavelength (frequency) of each optical signal in order to make it work properly. If the wavelength interval is too short, it is easy to “crash”. If the wavelength interval is too long, the utilization rate is very low.
WDM technology is of great significance to the expansion and upgrading of the network, the development of broadband services, the exploitation of optical fiber bandwidth capabilities, and the realization of ultra-high-speed communications.
The basic structure of WDM system
The basic structure of the WDM system is mainly divided into two modes: dual-fiber unidirectional transmission and single-fiber bidirectional transmission. One-way WDM means that all optical paths are transmitted in the same direction on one optical fiber at the same time. At the transmitting end, the modulated optical signals with different wavelengths carrying various information are combined together by an optical extension device, and are transmitted on one optical fiber. In the unidirectional transmission in the optical fiber, since each signal is carried by light of different wavelengths, it will not be confused with each other. At the receiving end, the optical signals of different wavelengths are separated by the optical multiplexer to complete the transmission of multiple optical signals. The opposite direction is transmitted through another fiber.
Two-way WDM means that the optical channel transmits in two different directions at the same time on one optical fiber, and the wavelengths used are separated from each other to realize full-duplex communication between each other.
A WDM system generally consists of four parts: optical transmitter, optical relay amplifier, optical receiver, and optical monitoring channel.
Optical transmitter: As the core equipment of the WDM system, at the transmitting end, the optical signal output from the terminal equipment is first converted into a signal with a stable specific wavelength by using an optical repeater, and then synthesized by a combiner. The multi-channel optical signal is amplified and output by the optical power amplifier.
Optical relay amplifier: After long-distance (80-120km) optical fiber transmission, the optical signal needs to be optically relayed and amplified. In the WDM system, the gain flat technology must be adopted, so that the EDFA has the same amplification gain for the optical signals of different wavelengths, and ensures that the gain competition of the optical channel does not affect the transmission performance.
Optical receiver: At the receiving end, the optical preamplifier amplifies the main channel signal that has been attenuated by transmission, and uses a demultiplexer to separate the optical channel of a specific wavelength from the main channel optical signal. The requirements of parameters such as overload power, but also a signal that can withstand a certain amount of optical noise.
Optical supervisory channel: The optical supervisory channel is established for the monitoring of the WDM optical transmission system. ITU-T recommends that the wavelength of 1510nm is preferably used, and the capacity is 2Mbit/s. Relying on the high receiving sensitivity at low speed (better than -48dBm), it can still work normally. But the light path must be off before the EDFA, and the light path must be added after the EDFA.
In the whole WDM system, the optical wavelength division multiplexer and the demultiplexer are the key components in the WDM technology, and their performance plays a decisive role in the transmission quality of the system. A device that combines signals of different light source wavelengths together and output through a transmission fiber is called a multiplexer;
Conversely, a device that decomposes the multi-wavelength signal sent by the same transmission fiber into individual wavelengths and outputs them separately is called a demultiplexer. In principle, the device is bidirectionally reversible, that is, as long as the output and input of the demultiplexer are used in reverse, it is a multiplexer. The performance indicators of the optical wavelength division multiplexer mainly include access loss and crosstalk. The loss and frequency offset are required to be small, the access loss should be less than 1.0 ~ 2.5db, the crosstalk between channels is small, the isolation is large, and the influence between signals of different wavelengths is small.
Advantages of WDM System
1. Ultra-large capacity, ultra-long distance transmission:
The bandwidth that can be transmitted by ordinary optical fibers currently used is very wide, but its utilization rate is still very low. Using DWDM technology can increase the transmission capacity of an optical fiber several times, tens of times or even hundreds of times compared to the transmission capacity of a single wavelength. Now the highest capacity optical fiber transmission system is 3.2Tbit/s.
2. Transparent transmission of data:
Since the DWDM system performs multiplexing and demultiplexing according to the different optical wavelengths, it has nothing to do with the rate of the signal and the electrical modulation mode, that is, it is “transparent” to the data. What the WDM system completes is transparent transmission. For the “service” layer signal, each optical wavelength channel in the WDM system is like a “virtual” optical fiber.
3. High networking flexibility, economy and reliability:
The new communication network formed by using WDM technology is simpler than the network formed by the traditional electrical time division multiplexing technology, and the network layer is distinct, and the scheduling of various services can be realized only by adjusting the wavelength of the corresponding optical signal. The resulting flexibility, economy and reliability of the network are obvious.
Classification of WDM
1. CWDM (Coarse WDM) sparse wavelength division multiplexing
Due to the limited technical conditions in the early stage, the wavelength interval will be controlled at several tens of nm, which is a relatively scattered wavelength division multiplexing.
The wavelength interval of CWDM is 20nm, the wavelength range is from 1270nm to 1610nm, and there are 18 bands.
At the beginning, the wavelength range specified by the International Telecommunication Union ITU for CWDM (ITU-T G.694.2) was 1271 to 1611 nm.
However, because the 1270-1470nm band has a significant increase in attenuation, many old optical fibers cannot be used normally, so CWDM generally prefers the 8 bands of 1470-1610nm.
2. DWDM (Dense WDM) dense wavelength division multiplexing
The wavelength interval is getting shorter and shorter, and when it reaches the level of several nm, it becomes a tight WDM-DWDM.
The wavelength interval of DWDM can be 1.6nm, 0.8nm, 0.4nm, 0.2nm, and can accommodate 40, 80, 160 waves (maximum 192 waves). The wavelength range of DWDM is 1525nm to 1565nm (C-band) and 1570nm to 1610nm (L-band).
DWDM commonly used C-band, wavelength interval 0.4nm, channel frequency interval 50GHz
MWDM Medium Wavelength Division Multiplexing, Metro Wave Division Multiplexing
This was proposed by China Mobile in June 2019 with the source-forward transmission scheme (also known as Open WDM).
MWDM reuses the low-cost DML optical chips of the first 6-wave industry chain of 25G CWDM, rapidly promotes the 12-wavelength WDM system, greatly saves valuable fronthaul fiber resources, and adapts to the urgency of current 5G commercial use.
The current network pilot of the MWDM semi-active 5G fronthaul solution strongly supports the maturity of MWDM technology and will accelerate the commercial process of the semi-active 5G fronthaul solution.
5G fronthaul requires at least 12 wavelength channels, so the plans of the three major operators are all aimed at realizing 12 wavelengths.
This solution not only reuses the CWDM industry chain, but also meets China Mobile’s own 10km fronthaul distance requirement, and saves a lot of fiber resources at the same time.
LWDM is based on Ethernet channel wavelength division multiplexing (LAN WDM), also known as fine wavelength division multiplexing.
It is extended from the existing 8 waves to 12 waves according to the channel spacing of 800GHz.
DML refers to the Directly Modulated Laser (Directly Modulated Laser) at the TOSA sending end of the optical module, which corresponds to EML (Electlro-absorption Modulated Laser, electro-absorption modulated laser). EML costs more. PIN refers to the diode of the ROSA receiving end of the optical module.
Application scenarios under 5G prequel
The 5G fronthaul is mainly based on 25G gray light. China Mobile’s statement at the September 2019 Optical Expo: We believe that in the CRAN scenario, 25GBiDi is mainly used where there are fiber resources, and wavelength division solutions are mainly used where there are no fiber resources.
In the case of semi-active, there are also 12 optical modules in a single station: we believe that the cost of the semi-active type A (24 optical modules) is high, and it has not been applied in the existing network. type, only 12 optical modules are required.
Mobile spread spectrum and telecom sharing (China Telecom and China Unicom jointly build and share a 5G network) bring the demand for 12 optical modules per station, and CWDM needs to be expanded to 12 waves.
The mobile 2.6GHz spectrum is extended to 160MHz, and the ITU sharing is extended to 200MHz. Therefore, for the 64TRX station type, a single station needs 12 optical modules under the 64TRX station type, and the 12-wave scheme is expected to become the mainstream. Look is expected to account for 50%.
The cost of the MWDM solution is higher. Because it is supported by China Mobile, it will receive support from the industry chain. LWDM has more advantages than MWDM in terms of industry chain maturity, cost, and power consumption, and may become the main reason for the subsequent 12 waves of construction. plan.