2021 / 06 / 04
With the commercially available of 5G technology, the development of 5G mobile and wireless networks has progressed at a rapid pace. The significant increase in base station density, along with demands on latency, bandwidth and network flexibility, have placed higher requirements on the 5G network architecture and associated transmission solutions. Thus elevating the quality of the fiber optic transceivers applied in 5G networks. This article will focus on the fiber optic transceiver solution for 5G front-haul, mid-haul and back-haul networks.
5G bearer network can provide connections for 5G wireless access and core networks. Its network architecture and bandwidth have been changed greatly to adapt to larger bandwidth, lower latency and more connection services compared with 4G networks.
5G has moved part of the physical layer of the original BBU (baseband unit) in the 4G era to AAU (active antenna unit). Also, the interface has been changed from the original 100Gbit/s CPRI to 25Gbit/s. And the non-real-time functions of the BBU are moving up to the CU (centralized unit) to make preparations for the clouding network. In this way, the DU (distribution unit) is the only part of the BBU. In other words, the 5G access network has evolved from the two-tier architecture of the BBU and RRU to the three-tier architecture of CU, DU, and AAU. This not only ensures high bandwidth and low latency of the network but also contributes to flexible scheduling, network protection, and management control.
Figure 1: 5G Bearer Network Topology Architecture
As shown in the figure above, the 5G bearer network is divided into three parts by AAU, DU and CU, which are 5G front-haul, mid-haul and back-haul networks.
The applications of 5G front-haul, mid-haul and back-haul transmission are basically different, thus the requirements for transceivers and transmission distances are also varied.
The 5G front-haul transmission is strict about the bandwidth and latency (which is below 100µs), so 25Gbps eCPRI interface is considered as an optimal choice for the 5G front-haul network. Considering the convenience and efficiency of network construction, the initial 5G front-haul connection is based on fiber direct connection, which is supplemented by the passive WDM connection and the active WDM/OTN/SPN connection. Among them, fiber direct connection is easy to maintain but will consume more fiber resources. As a supplementary solution, WDM connection can save fiber resources and have a longer transmission distance than fiber direct connection, but the cost is expensive.
The fiber direct connection is used to link each AAU and DU, which can be deployed easily and save a great bunch of fiber resources. The 25G grey light modules will be the dominance of the direct connection in front-haul transmission. It can support dual-fiber and single-fiber bidirectional transmission to reach a transmission distance of 330m and 10km respectively.
Passive WDM connection can multiplex several wavelengths and transmit them on a pair of or a single fiber to connect multiple AAUs to DUs to save fiber. However, it brings difficulties for the network administrators to make daily maintenance because of the technical complexity. Generally, the 10G or 25G colored light transceivers (WDM modules) are applied for this connection with a 10km and 20km transmission distance.
Active WDM/OTN/SPN connection needs to deploy OTN devices (such as WDM Mux/Demux, OADM, EDFA, OEO, etc.) between the AAU site and DU equipment room. It also utilizes WDM technology and provides multiple AAU to DU connections by using a pair of optical fiber or single fiber. Usually, the 10G/25G grey light optical modules with a short transmission distance are applied for connecting AAU/DU with WDM/OTN/SPN. The connections between WDM/OTN/SPN network devices can be achieved by 10G/25G/50G/100G dual-fiber or single-fiber bidirectional fiber optic transceivers. Compared with the passive WDM solution, the active WDM connection is more flexible and more cost-effective, which can be deployed easily, it is likely to be gradually used by 5G.
Considering the budgets of the network, 10G fiber optical modules might be applied for network deployment in the 5G front-haul transmission by some network service suppliers. Nevertheless, the industry prefers to use 25G modules due to that the network granularity is 25Gbps. That is to say, 25G and 100G fiber optic transceivers may be the optimal choices for 5G front-haul network.
5G Mid-Haul & Back-Haul Technology
Since the requirements of bandwidth and networking flexibility are basically the same for 5G mid-haul and back-haul networks. They can utilize the same technology for transmission, like IPRAN (Internet Protocol Radio Access Network), PTN and OTN technology, etc. Two connecting methods for 5G mid-haul and back-haul network are presented below.
OTN & IPRAN Connection: the packet-enhanced OTN equipment with routing and forwarding functions are applied for the 5G mid-haul transmission, while 5G back-haul network continues to adopt BGP protocol for routing and forwarding between IPRAN and OTN devices. In order to meet the demand for large capacity and network slicing of 5G, IPRAN will introduce high-speed interface technologies such as 25Gbit/s, 50Gbit/s, and 100Gbit/s, or consider adopting new interface technologies such as FlexE (flexible Ethernet) to achieve physical isolation and offer better quality assurance for 5G transmission.
End-to-End OTN Connection: end to end packet-enhanced OTN network devices are applied for 5G mid-haul and back-haul transmission. Compared with the above solution, it has strong networking capabilities and end-to-end maintenance capabilities to avoid the issues of interoperability and cross-professional coordination between OTN and IPRAN.
Figure 2: 5G Mid-Haul & Back-Haul Topology Architecture
In addition, 5G mid-haul and back-haul networks cover the access layer, aggregation layer, and core layer of the MAN (metropolitan area network), and the optical transceivers used in its MAN are similar to the transceiver used in existing transmission networks and data centers. Among them, the 25G/50G/100G grey light or colored light modules will be mainly applied for the metro access layer network, and the metro convergence and core layer network will mainly use 100G/200G 400G DWDM color optical modules.
Currently, with the population of the 5G applications, there are multiple optical module technologies and solutions for the 5G bearer network, which poses huge potential and great challenges for the optical transceiver market. Since there is a large demand in the market for 5G optical transceivers, and the development of 5G optical modules is difficult at this moment, the prices might be a little expensive in the short term. It is believed that with the continuous maturity of 5G technology and applications, the price of 5G optical transceivers will gradually decrease in the future and the market is still promising.