The Research of WDM-PON Technology for 5G Fronthaul Applications
At present, the research on 5G network has formed the first wave, entering into the key stage of technical standard research and R&D(research and development)test. Compared with 4G technology, the performance of 5G network has been significantly improved in terms of handling capability, latency, the number of connection, etc. At the same time, 5G has new challenges for the fronthaul network, such as a lot of new demands on dense optical fiber deployment, higher transmission broadband and low latency. WDM-PON combines the characteristics of WDM technology and PON topology. It has the advantages such as high bandwidth, low latency, saving fiber, simple operation and management, low cost, etc. It has its unique advantages in the application of 5G fronthaul, paid great attention in recent years.
Limited by standard and technical maturity, the rate of WDM-PON commercial network or experimental prototype is relatively low, the wavelength of single wave usually not exceeding 10Gbps. In 2015, ITU-T defined the architecture and index of WDM-PON (PtP WDM option) in the NG-PON2 standard. Simultaneously, the line rate was defined to 10Gbps; wavelength assignment information and the Auxiliary Management and Control Channel (AMCC) of OAM data has also been defined in WDM-PON system. Out of consideration for demands on higher bandwidth of 5G fronthaul, ITU-T has recently started the the white paper research on single wave 25G WDM-PON technology used in the 5G fronthaul.
The Network Architecture of WDM-PON 5G Fronthaul
In the C-RAN architecture of 5G network fronthaul, the BBU function will be restructured as two functional entities, CU and DU. The CU device realizes the function of non-real-time, wireless and high-level protocol stack. It supports some core network function sinking and the deployment of edge application business; DU device realizes physical layer function and transmission layer function which has higher requirements for real time. In order to save the cost of transmission between RRU and DU, to reduce the transmission bandwidth between RRU and DU, some functions of physical layer are moved down to the RRU implementation. At present, the standardization of the interface between DU and RRU has not yet reached consensus, and the schemes in the industry mainly include NGFI, eCPRI and CPRI, etc.
The network architecture of WDM PON 5G fronthaul is shown in Figure 1, point-to-multipoint network topology connection between DU and RRU based on WDM PON, WDM PON OLT and ONU respectively connected with DU and RRU. The adoption of the wavelength pision multiplexing technology and AMCC technology to realize transparent business transmission between DU and RRU. OLT devices carry the midhaul service between DU and RRU at the same time that OLT device realizes the fronthaul service between DU and RRU.
The network architecture of above-mentioned WDM-PON 5G fronthaul is with technical features:
The adoption of PON network topology, for dense 5G fronthaul network, can save optic fiber deployment to a large extent, can share the existing space of fiber infrastructure setting and room space, and can save a lot of costs of network construction and maintenance.
The adoption of WDM and AMCC technology without frame processing and DBA scheduling, compared with active load devices and TDM-PON devices, has advantages such as low latency and low-frequency jitter, available to flexibly adapt to different types of fronthaul transmission interfaces.
Single-wavelength bandwidth exclusive, high transmission efficiency, rich bandwidth resources.
OLT can achieve xHaul (Fronthaul and Midhaul)sharing a device, OLT platform can be deployed in a computer room with DU pool, reducing the costs of associated equipment and computer room construction.
Colorless ONU technology, low cost, simple operation and management.
Unified optical access platform, shared OLT, and supporting business demands of mobile users and home users.
Colorless ONU Tech with Low Cost and High Speed
As the key technology of WDM-PON system, the colorless ONU technology generally uses tunable ONU technology based on tunable laser technology. The high-speed transmission can be easily realized via the integration of tunable laser and modulator. Currently, there have been various commercial tech solutions in the industry, such as Distributed Prague Reflector (DBR) laser, Digital Supermode Distributed Bragg Reflector (DS-DBR), External Cavity Laser (ECL), V-Cavity Laser. The tunable mechanism, integration mode, wavelength adjustment range and cost difference of the 4 commercial tunable laser tech schemes are shown in Table 1. It can be seen from table 1 that the 25G tunable laser is not fully mature at present, applied to 5G fronthaul network. It is still to be studied in terms of speed improvement and cost reduction.
The speed of tunable optical module can be improved by combining 10G tunable devices and higher-order modulation technology, such as 4-Level Pulse Amplitude Modulation (PAM4) and Optical Duobinary (ODB) technology. 25G tunable module can be achieved via high-order modulation tech. On the one hand, the mature industry chain of 10G optical components can be used to reduce the cost of optical components, to accelerate the commercial development of 25G tunable optical module; on the other hand, anti-dispersion performance of high-speed tunable lasers can be enhanced and the transmission distance that tunable module support can be increased.
To reduce the cost of tunable optical module, in addition to the adoption of low-cost tunable laser, it can also be achieved by reducing the cost of packaging. For example, combining AMCC technology with wavelength monitoring technology to reduce the requirement for wavelength stability of tunable laser module, to realize the simplified packaging process, so that the goal of reducing cost can be achieved.
The transparent transmission management function of WDM-PON 5G fronthaul adopts AMCC technology route. Its characteristic is to manage and control WDM-PON ONU without affecting data transmission, including wavelength assignment information and OAM data.
AMCC generally adopts the Transparent AMCC technical solution, which requires that the user data can not be encapsulated, encoded, and added any other operation such as FEC. There are two ways to implement Transparent AMCC. One is the baseband-over-modulation scheme, another is radio frequency top adjustment (RF Pilot Tone). The baseband-over-modulation technology is to stack a low-speed 0101 baseband data at the top of user data. The radio frequency top adjustment technology is to superimpose a low-speed ASK or FSK modulation data on the top of user data. The two schemes have their own advantages and disadvantages. The baseband-over-modulation reception scheme is simple, without the need of ADC and DSP chips, the transmission rate able to reach over 100Kbps, but with high requirements for the stability of transmission signal optical power in which the multi-channel AMCC signal can not be transmitted. The receiving side of the radio frequency adjustment scheme is more complex, and the transmission rate is limited in the signal rate, but it has the advantage of transmitting multiple signals simultaneously. The above two schemes have a certain effect on data transmission. Generally speaking, when 1dB optical power is introduced, the modulation degree of AMCC signal to data signal amplitude is not more than 10%.
In the 5G era, new network architecture, various new technologies and application characteristics have put forward new demands and challenges to the fronthaul network. The WDM-PON system is with the advantages such as low latency, high bandwidth, optical fiber saving, simple operation and maintenance, etc. It has unique advantages in the application of 5G fronthaul transmission. Via the breakthroughs in key technologies such as efficient and flexible networking structure, low cost, high-speed, colorless ONU technology and AMCC, the construction of low-cost and high-performance fronthaul solutions will definitely be the new impetus to 5G technology development and business.
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