[XGNews]: Semi active WDM technology innovation for 5g forward transmission

The following is the [XGNews]: Semi active WDM technology innovation for 5g forward transmission recommended by xgapn.com.

Zhanghui 1 limingli 1 liyuechen 2 Li Xin 2 Xu Rong 3

(1. China Mobile Communications Group Jilin Yanbian Prefecture Branch, Jilin Yanbian Prefecture 133000; 2. Chengdu Dingji Information Technology Co., Ltd.; 3. China Mobile Communications Corporation Research Institute, Beijing 100053;)

Abstract: 5g puts forward higher requirements for the forward transmission network, such as field deployment, high-speed transparent transmission, dense direct connection, cost sensitivity and high reliability. The core technology of low-cost WDM for 5g forward transmission is innovatively designed from five aspects: (1) point-to-point transparent WDM direct drive structure that can be installed in the field; (2) The low-cost medium and short range optical module is realized by direct detection; (3) The method of extending more cwdm/lwdm wavelengths in the o-band; (4) The mature cwdm/lwdm optical module stack is used to realize multi-directional convergence and multi-level cascade; (5) A new OLP protection mechanism for semi passive WDM. Based on these innovative technologies, a new semi-active WDM forward transmission innovation scheme with field installation, modular stacking and protection is proposed, which reduces the construction cost of 5g forward transmission network and meets the operation requirements of high reliability.

Keywords: semi active wavelength division multiplexing; Front transmission network; O-band wavelength division multiplexing; Direct pickup module; Semi passive optical line protection

CLC classification No. tp393/tn913 document identification code a

Innovative Half Active WDM Technologies for 5G FrontHaul

Zhang Hui1 , LiMingLi1 , LiYueChen2 , LiXin2 , XU Rong3

1. China Mobile Group Jilin Co., Ltd. Yanbian 133000, China

2. ChengDu DingJi Information Technology Co., Ltd, China

3. China mobile Research Institute, Beijing 100053, China

Abstract On-field installation, high speed transparency, dense connection, low cost, and high reliability are 5G FrontHaul networks’ new higher requirements. The innovation solution is based on five key technologies: (a) native passive point-to-point WDM with low cost. (b) low-cost transceiver devices based on direct detection for short-range transmission. (c)Extend more wavelength in O-band for CWDM/LWDM;(d) O-Band CWDM/LWDM Optical Module with Stacked for WDM cascaded system. (e) New half passive WDM with OLP protects solution. This solution will be effective on the fronthaul networks reliability and low Capex.

Keywords Half Active WDM, front Haul, O-band CWDM/LWDM, direct detection transceiver, half passive optical line protect

1 WDM is a required technology for 5g forward transmission

For the massive MIMO technology adopted by 5g new wireless, some physical layer functions can also be moved down to AAU (active antenna unit), which creates a new network connection requirement – the forward network. The forward is the transmission channel connecting AAU and Du (distributed unit), and the interface is ecpri (enhanced common public radio interface) [1,2], It is required to meet the transmission requirements of >25gbit/s with large bandwidth and low delay. In addition, compared with 4G, 5g[3] uses a higher frequency, and the coverage of single base station is smaller than that of 4G, which means that 5g pre transmission network needs more intensive networking to achieve more base station coverage.

The most essential connection needs of 5g pre transmission network are >25gbit/s high-speed, large granularity and direct and efficient transparent transmission. Therefore, if there are enough optical fibers available, optical fiber direct drive is the simplest solution. The optical fiber direct drive scheme is a point-to-point direct connection scheme between AAU and Du through the front optical transmission module and optical fiber. According to the general demand calculation, a common access point needs 36~48 optical fibers, a c-ran (centralized/collaborative/cloud radio access network) area needs more than 120 optical fiber cores, and the backbone optical cable needs more than 300 fiber cores.

In order to reduce the construction cost of optical cables and save optical fiber consumption, a single optical fiber can be used through WDM (wavelength division multiplexing) [4,5] technology to provide 18, 32, 40, or even 80 /96 waves, which greatly saves the use of access optical fibers and solves the pain point of lack of access optical fibers.

WDM wavelength division is a typical point-to-point topology. Independent transparent wavelength channels with different colors are used for uplink and downlink transmission. No special electric layer protocol processing is required. There is no dynamic allocation of bandwidth between channels. Therefore, the complexity of the system is greatly reduced and the transmission efficiency of transparent transmission is greatly improved. While providing a high-speed direct channel with higher bandwidth, the transmission delay is the lowest among all forward transmission schemes.

Therefore, WDM technology will become the best choice for 5g pre transmission network technology.

2 low cost, high reliability forward WDM innovation technology

2.1 WDM direct drive technology without power supply and field installation

In the past 20 years, WDM technology has been a necessary means to save optical fiber and solve long-distance transmission in backbone transmission network because it can perform dense wavelength division multiplexing (DWDM) and can be configured with relay amplification [4,5]. Nowadays, the huge demand for 5g prequel and Jike cloud dedicated line promotes DWDM technology to sink to the end of the network.

When WDM technology is applied on the network access side, long-distance transmission is not the first consideration, and low cost becomes the primary consideration. At this time, the simplest point-to-point direct connection structure without relay amplification, DCM (dispersion compensation), intermediate optical channel hopping and pure transparent multiplexing has become the inevitable choice of low-cost WDM. As shown in Figure 1 (a), this structure is end-to-end passive, and its working principle is similar to that of optical fiber direct drive. However, its biggest advantage is that it can provide a large number of virtual fibers (i.e. wavelength division channels) on one optical fiber for direct connection. Therefore, it is called wavelength division direct drive or point-to-point WDM direct drive.

In the passive wavelength division multiplexing (WDM) scheme [4], as shown in Figure 1 (b), the far end AAU directly adopts the color optical module, and the passive combiner / splitter adopted at the far end does not need power supply. The deployment location can be flexibly selected according to the power budget allocation of the line and the number of convergence directions. At the baseband station side, the passive combiner / demultiplexer performs wavelength multiplexing / demultiplexing to realize the connection between AAU and the corresponding wavelength of the baseband. The baseband side also adopts color light modules, which correspond to the working wavelength of the AAU side one by one. It can meet the networking requirements of point-to-point, ring network, star type, chain type and other networking scenarios with a short transmission distance (<10km).

Since the remote side of 5g forward transmission needs to be deployed outdoors, industrial grade (-40 ° c-85 ° C) optical modules that can be installed in the field are required. At present, the technical solutions to realize the working temperature work mainly include: (1) commercial 25gbit/s direct modulation (DML) chip + refrigeration packaging, which has the advantages of low requirements for laser chips and the disadvantages of increasing power consumption and cost. (2) The direct use of industrial 25gbit/s DML chips has the advantages of simple packaging and low power consumption cost, while the disadvantages are the difficulties in the process realization of industrial laser chips (such as the growth of aluminum doped quantum well materials).

The passive color light forward propagation wavelength division system can realize connection free, planning free and maintenance free. The simple and reliable low-cost passive system is used to solve the point-to-point transmission, which reduces a large number of active equipment and avoids the restriction of power supply required for remote service installation. Therefore, the complex operation and maintenance management can be avoided and the worry free service of maintenance and management can be truly realized. But it also leads to the biggest problem of traditional passive wavelength division that there is no line protection and management capability.

(a) A point-to-point WDM direct drive structure with pure transparent direct connection and low-cost no power layer convergence and multiplexing

(b) Field deployment mode without power supply

Figure 1 low cost passive WDM technology without power supply and field installation

2.2 regression of medium and short-range high-speed optical modules to low-cost direct detection

In addition to the multiplexer / demultiplexer, the most important part of the point-to-point WDM direct drive system with the simplest structure is the optical module technology representing the transceiver. In recent years, due to the needs of data center and mobile communication upgrading, the research focus of 25gbit/s optical module used in 5g forward transmission has gradually shifted to the short and medium distance transmission scenario based on direct detection and non coherent transmission below 40km.

In intensity modulation direct detection (IM-DD) optical fiber systems, physical damage affecting performance usually comes from the transmitter and fiber propagation process. The main damages are dispersion, laser phase noise, laser relative intensity noise and four wave mixing (FWM).

In IM-DD (intensity modulation direct detection) system, when dispersion exists, it will lead to power fading; When dispersion does not exist, four wave mixing will occur [4,5]; There is also the problem of channel nonlinear response when the symbol rate of the signal increases. Therefore, the purpose of the new modulation format, transceiver structure and DSP (digital signal processing) scheme proposed for low-cost IM-DD system is to resist the dispersion and nonlinear effects in the direct detection system.

The pulse broadening effect caused by dispersion is proportional to the square of the spectral width of the modulated light. In order to reduce the influence of dispersion, it is very effective to filter the unnecessary spectral part directly in the optical module. In the past, Fabry Perot (F-P) lasers were used to oscillate in multiple longitudinal modes, and the entire spectrum was intensity modulated, resulting in a very wide multi frequency spectrum. The pulse broadening caused by multi frequency components of laser can be eliminated by limiting the laser spectrum to single longitudinal mode, or using DFB single longitudinal mode laser to replace F-P multi longitudinal mode laser.

The use of more complex and higher-order modulation undoubtedly increases the processing complexity of DSP, and then requires PD (photodetector), TIA (transimpedance amplifier) and ADC (analog-to-digital conversion) with twice the bandwidth or even higher sampling rate.

At present, various mainstream device and optical module manufacturers are trying to realize low-cost 25gbit/s high-speed optical module by overclocking DML (direct modulation laser) temperature chip based on 10g electrical bandwidth. One is to realize 25gbit/s high-speed optical signal transmission and reception by frequency doubling using devices with 10g electrical chip bandwidth; Another idea is to use pam-4 (4-level pulse amplitude modulation) technology to transmit two bits of information in one cycle, as shown in Figure 2 (a), which is twice the frequency of transmitting one bit of information in one cycle of NRZ; Their basic idea is to use more complex electrical modulation and demodulation technology to reduce the physical bandwidth requirements of the transceiver module or reduce the number of lasers used to reduce the cost. For example, under the same rate, using pam-4 technology can save 50% of the use of optical devices.

(a) Original pam-4 Technology

(b) Comparison between PAM direct detection and coherent processing

Fig. 2 pam-4 technical principle and comparison between direct detection processing and coherent detection processing

Direct detection is often seen as a solution to achieve low-cost requirements. The basic principle of pam4 technology is to use more intensive levels to transmit more information. The signal processing of pam4 is also divided into two methods, one is the analog mode of CDR (clock and data recovery circuit) and the other is the digital mode of DSP (digital signal processing). As shown in Fig. 2 (b), considering the complexity, bandwidth requirements and digital processing capacity of some proposed direct detection schemes, whether a receiver using a single Pd or multiple PDS, it is not certain that direct detection is a correct method to realize a transceiver with low cost, low power consumption and small footprint. However, if we lower the expected transmission distance, for example, within 20km, do not need to use too complex modulation and coding technology, and the rate is limited to the order of 25gbit/s, the answer will be very positive.

Whether to use double signal bandwidth devices or double signal bandwidth devices requires comprehensive consideration of cost, power consumption and occupied space. At present, the intensity direct modulation direct detection scheme still has obvious cost and performance advantages below 20 km, while coherent detection plays a dominant role above 100 km.

2.3 technical innovation of extending more cwdm/lwdm wavelengths in the o-band

At present, the lwdm 8 optical module, which has been mature in the field of data communication, uses four wavelengths of 1274, 1278, 1282 and 1286nm using EML (electric absorption modulation laser) and 1295, 1300, 1305 and 1310nm using DML (direct modulation laser); Meanwhile, at present, the mature and very cheap CWDM operates at 1271, 1291, 1311, 1331, 1351 and 1371nm in the six DML lasers of the o-band.

Depending on the working mechanism of semiconductor devices, the operating wavelength of the laser usually changes with temperature. As shown in Figure 3, the temperature drift coefficient of DFB laser is about 0.08nm/ ° C. In the CWDM system with coarse wavelength division multiplexing (CWDM), because the wavelength interval is very wide (20nm), it is unnecessary to consider the drift of the central wavelength caused by temperature. Therefore, its laser often uses a non cooled laser. Because the wavelength interval is very narrow, lwdm/dwdm often uses a cooled laser, that is, the semiconductor thermoelectric cooler tec (thermo electric cooler), which can control the junction temperature change of the laser within ± 0.1 ° C. In view of this, we can stabilize the center wavelength of the laser or pull the center wavelength of the laser intentionally based on this Tec working mechanism, so that the same laser chip can output more WDM working wavelengths without adding new laser chip types.

Fig. 3 center wavelength values of DFB laser with a nominal value of 1310nm at different temperatures

According to the idea of accurately controlling different temperatures to stabilize different output wavelengths of the optical module, on the basis of 1271nm, 1291nm, 1311nm, 1331nm, 1351nm and 1371nm of the existing mature CWDM scheme with an o-band interval of 20nm, after each wavelength is controlled by Tec, The o-band WDM scheme with 12 wavelengths can be formed by shifting the wavelength of 3.5Nm to the left and right: 1267.5, 1274.5, 1287.5, 1294.5, 1307.5, 1314.5, 1327.5, 1334.5, 1347.5, 1354.5, 1367.5, 1374.5 (unit: nm), as shown in Figure 4.

It can be seen that the wavelength scheme based on TEC extended cwdm6 has unequal intervals of 7Nm and 13nm. Specifically, the first eight wavelengths are matched with dml+ pin (photodiode) + Tec, and the last four wavelengths need to be matched with dml+ APD (avalanche diode) + Tec due to large dispersion, so as to meet the budget requirements of 10km link.

Fig. 4 unequal interval 12 wave scheme based on TEC extending cwdm6 at +/-3.5nm

The function of Tec is to stabilize the wavelength. If the wavelength is adjusted by the temperature of Tec, TEC will work in the state of large current, so the power consumption of optical module will be much higher.

In order to obtain more wavelengths in the o-band WDM application system, we can also think of expanding the 12 wave scheme directly based on the mature lwdm, that is, adopting the wavelength allocation scheme of eight lwdm wavelengths + four CWDMs, all of which adopt equal intervals of 4.5nm, that is, 1269, 1273, 1277, 1282, 1286, 1291, 1295, 1300, 1305, 1309, 1313, 1317 (1331 can be used to replace this laser if it is not available). In this way, the laser chip partly inherits the EML laser used in lwdm8 and partly inherits the DML laser used in cwdm/lwdm, as shown in Figure 5. Because the wavelength interval of this scheme is narrow, each optical module must use Tec to stabilize the central wavelength.

This 25gbit/s color light module product based on the current lwdm expansion supports the sfp28 packaging form. The operating temperature is -40 to 85 ℃, the transmission distance is 10/20km, and its 12 wavelength schemes are 8 DMLS and 4 EMLs. It is realized through low-cost dml/eml to airtight packaging and built-in Tec to control wavelength drift.

Fig. 5 12 wave scheme for sharing DML of existing CWDM and EML of lwdm at an equal interval of 4.5nm

The working band of 12 wave WDM based on TEC extended CWDM is 1266~1377nm; The working band of 12 wave based on lwdm extension is in the range of 1269~1332nm. The main difference between the two is concentrated in the tens of nanometers of 133x~137x, because the dispersion of the optical fiber in this section is relatively large. In order to overcome the influence of dispersion, the solution given by cwdm12 is to use pin to receive the first eight waves and APD to improve the sensitivity of the last four waves, so as to compensate for the power cost caused by dispersion of 133x~137x. For lwdm12 of o-band, a narrower wavelength interval (4.5nm) can eliminate APD, but Tec cooler and EML laser are required.

Generally speaking, using EML means high cost compared with DML, and using Tec and APD will also increase the cost. In the design of 5g front-end optoelectronic devices, despite the great cost challenge, no matter what wavelength arrangement scheme is proposed, it must be based on the premise of ensuring the performance indicators of optical chips and optical modules, and the factors such as power consumption, life and long-term reliability need to be considered.

2.4 low cost, modular cwdm/lwdm o-band wavelength division technology

In order to reduce the influence of dispersion on WDM system, it is easy to think of a solution that works in the o-band region close to zero dispersion. Theoretically, the transmission distance can be increased. However, when the WDM operating point approaches the zero dispersion point, there are two shortcomings:

The first disadvantage is that there is a high fiber loss in this region. From the perspective of receiver sensitivity, this will reduce the transmission distance. At present, the attenuation coefficient of newer optical fibers at 1550nm is not higher than 0.18db/km, and the attenuation coefficient at 1310 nm is not higher than 0.32db/km. Assuming the same transmission power at both wavelengths, an increase in the attenuation coefficient will result in a 42.9% reduction in the transmission distance.

The second disadvantage is that in the spectral region near the zero dispersion wavelength, the ability of WDM to multiplex multiple carriers will be limited by the nonlinearity caused by Kerr effect, and they will increase with the increase of signal power. One of them is four wave mixing (FWM), which means that when multiple light waves of different wavelengths meet the phase matching conditions, they interact to produce mixing components at other wavelengths.

At present, CWDM (coarse wavelength division multiplexing) [6] and lwdm (LAN wavelength division multiplexing) [7] optical modules with various performance indicators in the market as shown in Table 1 have been mature and used in large scale.

Table 1 performance indexes of commercial o-band cwdm/lwdm optical modules

Based on the mature commercial CWDM and lwdm optical modules with more wavelengths innovated in the o-band, and in combination with the passive WDM direct drive technology discussed above, we have innovatively designed a 5g forward transmission oriented 25gbit/s WDM system scheme and products that are all passive, full-color optical, cwdm/lwdm modular, flexible stacking, multi-directional cascade convergence, and its working principle is shown in Figure 6.

This innovative scheme uses color light modules at both ends, multiplexes multiple wavelengths through a combiner and splitter to save optical fiber resources, adopts a waterproof and moisture-proof design, adapts to field multi scene installation, and provides various equipment forms such as outdoor wall hanging, outdoor pole holding, indoor rack, etc; In addition, based on the optical module of CWDM working in o-band or the optical module of lwdm working in o-band, multi module stacking is carried out. Through modular design, 5g forward passive WDM solutions such as 6-wave, 12 wave, 18 wave and 24 wave can be provided.

Figure 6 innovative scheme of stacked passive WDM based on cwdm/lwdm mature modules

This innovative system design idea, on the one hand, adopts a standardized and modular architecture, so that the equipment can be flexibly configured at a low cost; On the other hand, it shares the large-scale and mature optical module industry chain commercially available in the data center, which can be realized through pin/apd (photodiode / avalanche diode), dml/eml (direct modulation laser / electric absorption modulation laser), nrz/pam4 (non return to zero code / pulse amplitude 4 level modulation), cwdm/lwdm (coarse wavelength division multiplexing / LAN wavelength division multiplexing), wave plate /awg (array waveguide grating) Flexible selection and configuration of pic/plc (photonic integrated circuit / planar waveguide circuit), bidi/ non bidi (single fiber bidirectional), gray / color light, indoor / outdoor and other standard optical modules to meet the index requirements of various rates, transmission distances and line power budgets.

2.5 carrier level, highly reliable semi passive WDM OLP protection technology

In the traditional active WDM system, two optical cable routes are generally used, one for the working line and the other for the protection line. Under normal circumstances, the equipment works on the working line. In case of line accidents, such as fiber breakage or performance degradation of the working line (optical cable), the equipment will automatically switch to the protection line (optical cable) through the OLP (optical line protection) [4,5] board to ensure the interruption of the business department. In addition, the equipment has a real-time monitoring function for the protection line. When the protection line is broken or its performance is degraded, the equipment will also monitor it in time, so that it can be returned to the original state in time. Therefore, the protection object of WDM equipment is the transmission line on the optical layer. The optical line protection is realized through the OLP board, which improves the survivability of the network. This is the essential difference between OLP protection and OCP (optical channel protection) and omsp (optical multiplex section) protection.

(a) 1+1 OLP

(b)1:1 OLP

Figure 7 two OLP protection switching modes of active WDM system

OLP protection is mainly divided into two types: 1+1 protection mode and 1:1 protection mode. Their working principle is shown in Figure 7.

The OLP protection of 1+1 mainly adopts the protection mode of dual transmitter and selective receiver. The transmission optical power of TX port is distributed to T1 and T2 ports according to a certain optical splitting ratio (50:50), and transmitted to the opposite end along the main and standby optical fibers at the same time. The receiving end detects the R1 and R2 optical power, and selects the working path connected to RX according to the power status and the set switching conditions. The switching trigger does not require the transceiver to transmit the information of APS automatic protection switching protocol to each other, so the switching time is fast and the stability is good.

1: The OLP protection mode of 1 mainly adopts the protection mode of selective transmission and selective reception. Under this protection mode, the working service signals are transmitted along the working optical fiber, and the non working optical fiber can transmit other secondary service signals. The OLP equipment at both ends synchronously selects to work on the main optical fiber or switch to the standby optical fiber according to the status of the main optical fiber and the standby optical fiber. In order to ensure the effectiveness and reliability of switching at both ends, the equipment at both ends need to coordinate the protection switching action through the APS automatic protection switching protocol information, so the switching time is slightly slow.

It can be seen from the above description that both 1+1 OLP and 1:1 OLP protection devices must be in active working state. When such devices are used in the forward transmission scenario of c-ran, they must face the huge pressure of providing power to WDM devices at the outdoor antenna side, and cannot be deployed in the field.

Figure 8 new WDM OLP protection mechanism under semi passive / semi active operating mode

In order to improve the carrier level high reliability of the field installable, low-cost, modular passive WDM WDM forward transmission system, we have creatively designed a 1+1 OLP semi passive protection scheme that can be realized only by the near end active protection board with the remote end remaining passive. As shown in Figure 8, an active protection board is adopted at the local end side of the near end, and the far end side remains passive. At the office end, selective transmission and reception are used. One of the lines is selected as the main line and the other as the standby line. In the remote end, the parallel receiving mode is used. The transmitted optical signal is transmitted to the opposite end through the main and standby lines at the same time, while the parallel receiving mode selects to receive one signal according to the power of the two received signals. Once the optical fiber failure of the main line causes the communication quality to decline, the receiving end of the main line detects the signal power decline, and automatically switches the transmission signal from the main line to the standby line.

On the basis of keeping the remote passive, only add an active protection board to support OLP 1+1 protection. It is triggered based on Los alarm without signaling interaction. It supports the monitoring function of receiving and emitting power of each channel and is easy to locate and maintain faults; The OLP protection function can be selected according to the application scenario, and supports hot plug; The power supply mode of the protection board is flexible and diverse; Support SNMP, web and other graphical interface management, and provide carrier level network management and protection functions [8]; The semi passive protection scheme can realize low delay, pure physical transmission, and meet the requirement of 5g pre transmission network for small delay. The semi passive protection scheme has low cost, is conducive to the operation and maintenance requirements of operators, and can realize visual management for the whole network.

3 innovative semi-active WDM scheme with field installation, modular expansion and protection

In view of the problems that the traditional active WDM scheme cannot be installed in the field and needs power supply, and the traditional passive WDM scheme does not have any protection measures, we propose a semi-active WDM scheme with innovative technical capabilities such as field passive installation, cwdm/lwdm modular expansion and WDM OLP protection for 5g forward transmission. The working principle of this innovative forward transmission WDM system is shown in Figure 9.

Figure 9 working principle of semi active WDM innovation scheme

In order to reduce the cost, the innovative scheme first adopts the design idea of no relay amplification, no DCM and no intermediate OADM hop. The core architecture adopts the passive double star pure transparent WDM direct drive networking topology. The AAU side adopts passive equipment and cwdm/lwdm color light module. The AAU side passive combiner and splitter multiplexes multiple wavelengths for transmission to save optical fiber resources; The Du side adopts the full-color semi-active WDM equipment with only one active protection board to realize the semi-passive WDM OLP protection function described in Section 2.5. Since the remote side is a pure passive equipment, it takes into account the low-cost characteristics of passive equipment, such as no power supply and external deployment; There are various forms of active WDM devices at the office end. According to the specific application requirements in the network, a single active WDM service board can choose whether to integrate the optical protection function. The active protection board at the office end side can provide protection and automatic switching mechanism, and has basic OAM management functions such as real-time power monitoring, automatic switching of main and standby routes, working parameter setting, remote network management control, etc. it can monitor the receiving and emitting power of each channel and perform the optical layer protection function, and support the manageability and operation and maintenance of the forward network.

On the basis of keeping the end-to-end passive low cost, the active network management, protection and other auxiliary functions are added, which can protect the line of important base stations, avoid the disconnection of base stations caused by optical cable line failure, and ensure that the office end equipment will not affect the business operation in the case of power failure.

The remote deployment of passive WDM equipment requires no power supply and does not occupy the space of the computer room. The deployment location can be flexibly selected. It supports rack mounted, pole mounted and wall mounted installation. It can be installed near the AAU to solve single station services, and can also be installed at the secondary fiber distribution point to converge multi station convergence services. This application model is a typical application scenario faced by the operator in the construction of the front transmission network. It can effectively solve the problem of shortage of optical cable resources, quickly deploy 5g base stations, realize the rapid deployment of stations around the convergence machine room, and enable the existing 4G stations and the new 5g stations to share one core of optical cable resources.

The passive color optical double star structure is limited by the receiving sensitivity of the color optical module, and is suitable for the application scenario of dual fiber routing to the base station within 10km. The biggest advantage of the semi-active WDM scheme with remote passive and local active is that it maintains the passive characteristics of the remote WDM equipment, and solves the problem of line protection while obtaining the cost advantage of the passive system.

In addition to the active protection board, the two ends are all passive and full-color optical, which is very easy to deploy and maintain. At the same time, it meets the high reliability and greatly reduces the comprehensive cost of 5g construction. It can not only greatly relieve the pressure of optical fiber resources, but also take into account the advantages of cost, management and protection, and help operators to deploy 5g pre transmission network with low cost, high bandwidth and fast deployment.

The o-band semi-active WDM scheme can provide unified control of the front-end network equipment, which is conducive to the division of the maintenance interface management domain between wireless and transmission, as well as the decoupling of wireless equipment and optical modules.

4 Conclusion

On May 13, 2022, China Mobile released the announcement of candidates for winning the bid for centralized procurement of base station forward transmission equipment (CWDM base station forward transmission equipment) by China Mobile from 2022 to 2023. This should be the largest target of 5g prequel so far, with a demand of 180000 sets of CWDM equipment. As a result, 10 equipment manufacturers won the bid. As the largest bid of CWDM so far, the price hit a new low. The average quotation of 10 manufacturers was only 210million yuan, and the unit price of a single set of equipment was about 1220 yuan. Set a new low in price. When Chinatelecom group purchased in April, 2020, the unit price of a single set of equipment was about 2350 yuan. In two years, the price has halved. Although the price fight was fierce, from the result of winning the bid, several big 5g prequel manufacturers in the industry were basically shortlisted.

From the actual prequel procurement projects of major operators in recent years, it can be seen that low-cost o-band Wavelength Division products occupy the vast majority of the market share. Firstly, based on this mainstream forward WDM scheme, this paper proposes a low-cost o-band semi passive / semi-active forward WDM innovation scheme with new OLP protection, on the premise of maintaining the passivity at both ends, and according to the requirements of special scenarios for line protection functions. In many pilot applications, a pair of 12 wavelength in 1 CWDM passive devices, full-color optical 25gbit/s high-speed modules, and innovative semi passive 1+1 OLP protection boards are used, with a typical insertion loss of 2.0db, which simulates the protection switching operation in the case of fiber breakage, signal loss, power loss and other faults. From the application results, when a network fault occurs, it can completely not affect the 5g service performance, the test performance indicators meet the actual requirements, and the system performance is stable and reliable. In addition to the protection board, the end-to-end operation of this equipment scheme is passive full-color optical. Therefore, in the extreme case of power failure at the station, the business can not be affected at all. The customer has given this very high evaluation. The practical application shows that the innovative scheme not only makes full use of the existing optical fiber resources, but also greatly reduces the network deployment cost and operation and maintenance difficulty, and accelerates the construction speed of 5g network deployment.

The far end semi passive, near end semi active field installable, protected, modular, full-color optical WDM forward application scheme is innovative in the industry! It enables the remote end to be passive, and the office end only protects the board to be active. It has the OLP protection capability, realizes the network management and saves the electricity charge at the remote end, thus solving the problem that the remote end is difficult to supply power. Therefore, it is an innovative low-cost solution for 5g forward network construction.


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