5G Bearer Network Topology
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.

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.

5G Front-Haul Technology
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. And according to the summary of the "5G Bearer Optical Module White Paper" released by the IMT-2020 (5G) Promotion Group,  25G and 100G fiber optic transceivers may be the optimal choices for 5G front-haul network. 
The following figure shows the specifications of 25G and 100G transceivers.

Optical Transceivers for 5G Front-Haul

Typical requirements for optical modules in 5G fronthaul application scenarios are as follows:

(1) Meet industrial temperature range and high reliability requirements: 
Considering the AAU full outdoor application environment, the front-haul optical module needs to meet working at industrial temperature range of -40℃~+85℃, and dustproof etc.
(2) Low cost: 
The total demand for 5G optical modules is expected to exceed 4G a lot, especially front-haul optical module, may be hundred millions. So low cost is one of the main requirements of the industry for optical modules. 

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. 

5G mid-haul/back-haul networks cover the access layer, aggregation layer, and core layer of the MAN (metropolitan area network), and the optical transceivers used in the core layer are similar to those used in existing transmission networks and data centers. Among them, the 25G/50G/100G grey light or colored light optical transceivers 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 colored light optical transceivers. 

Optical Transceivers for 5G Mid-Haul/Back-Haul

As a professional fiber optical transceiver innovator, 3Coptics transceiver module series for 5G networks include:

• 25G LR SFP28, supports 25G CPRI/eCPRI and 25GE links to transmit 10km in single-mode fiber;
• 25G BiDi SFP28 supports 25G CPRI/eCPRI and 25GE links in a single single-mode optical fiber for two-way transmission of 2km-40km;
• 25G LAN-WDM SFP28 contains six channels from 1287nm to 1309nm; supports 25G CPRI/eCPRI and 25GE links to transmit 20km in single-mode fiber;
• 50G LR QSFP28 supports 50GE links to transmit 10km in single-mode fiber;
• 50G ER QSFP28 supports 50GE links to transmit 40km in single-mode fiber;
• 50G LR BiDi QSFP28 supports 50GE link bidirectional transmission 10km in single-mode fiber;
• 50G ER BiDi QSFP28 supports 50GE link bidirectional transmission 40km in single-mode fiber;
• 100G LR4 QSFP28 supports 100GE links to transmit 10km in single-mode fiber;
• 100G ER4 Lite QSFP28 supports 100GE links to transmit 40km in single-mode fiber;
• 200G LR4 QSFP-DD supports 200GE links to transmit 10km in single-mode fiber;
• 200G ER4 QSFP-DD supports 200GE links to transmit 40km in single-mode fiber;
• 400G LR8 QSFP-DD supports 400GE links to transmit 10km in single-mode fiber;
• 400G ER8 QSFP-DD supports 400GE links to transmit 40km in single-mode fiber.