Interface speed and quantity
4G network realizes the separation of BBU (baseband processing unit) and RRU (remote radio unit) based on the CPRI (Common Public Radio Interface) interface. When the LTE bandwidth is 20 MHz, the I/Q sampling bit width of 15 bits is used. The single-carrier I/Q data stream carried on the antenna is 921.6 Mbps, and the rate after adding control words and encoding is 1.2288 Gbps. If 2 antenna ports or 8 antenna ports are used, the CPRI bandwidth of 2.4576 Gbps or 9.8304 Gbps is required respectively, so the interface rate of the 4G CPRI optical module is 10G. The 5G spectrum bandwidth is increased from the original 20 MHz to 100 MHz, and the number of RF channels TR is increased to 64T64R, so the equivalent CPRI bandwidth is increased to 5×64=320 times on the basis of a single carrier 921.6 Mbps. The existing optical module technology is far from satisfying. For this reason, the eCPRI interface based on packet technology and supporting statistical multiplexing has been adopted, and the RAN architecture has been re-segmented, which effectively reduces the fronthaul bandwidth.
In terms of optical module rate selection, the optical modules used for 4G/5G fronthaul bearer are divided into 10Gbps optical modules and 25 Gbps optical modules. 10Gbps optical modules are mainly used for 4G fronthaul and 4TR, 2TR fronthaul in 5G; 25 Gbps optical modules are used for 64TR, 32TR, 16TR, 8TR fronthaul in 5G.
In terms of fiber core requirements, for a 5G single-macro station, 100 MHz bandwidth requires three 25Gbps eCPRI ports (six fiber cores); for the co-construction and sharing station of China Telecom and China Unicom, 200 MHz spectrum requires six 25Gbps eCPRI ports (12 cores). If the 5G co-location site is co-located with 4G, six 25Gbps eCPRI interfaces and three 9.8Gbps CPRI interfaces (18 cores) are required. If the 4G station has carrier aggregation at the same time, it will further increase the number of fronthaul cores required.
Latency and synchronization requirements
Latency is related to distance and the processing power of the wireless and bearer equipment. According to the eCPRI interface delay allocation, the fronthaul delay is about 100 µs. Without considering the node processing delay, for ultra-low delay services, the fronthaul distance should not exceed 20 km. Therefore, when introducing bearer equipment into the fronthaul network, the node delay should be reduced as much as possible. In actual C-RAN scenarios, the distance from AAU to BBU is usually no more than 10 km, and in a multi-station cascaded chain or ring scenario such as a subway, the distance will be greater than 10 km, but generally no more than 20 km.
The basic synchronization requirements of 5G are the same as those of 4G, and the time deviation between base stations should be less than 3 µs; 5G synergy enhancement puts forward high-precision requirements of 100 ns; some new services, including high-precision positioning services, high-speed mobile service coverage (such as car networking, remote medical, intelligent manufacturing), etc., require higher precision synchronization. Current synchronous transmission technologies such as Synchronous Ethernet (SyncE) and IEEE1588v2 can meet the synchronization accuracy requirements (1.5 µs) of 5G basic services; through 5G high-precision synchronization networking technology, it can meet the higher synchronization requirements of various services in the 5G system. In order to ensure the synchronization accuracy requirements of services, the fronthaul network link should eliminate asymmetric factors, including asymmetry caused by lines and optical modules.
Working environment requirements
The working environment of fronthaul equipment is complex and diverse, especially when working outdoors, it should be able to work stably and reliably for a long time in a temperature environment of -35°C to +55°C and a relative humidity of 5% to 98%. Outdoor front transmission equipment should also meet IP65 protection level and lightning protection requirements. In the selection of fronthaul optical modules, the outdoor requirements are industrial-grade temperature requirements (I-Temp, -40°C to +85°C).