This is part of our ‘5G Anyhaul’ blog series providing expert insights and best practices to help you prepare your transport network for 5G and support innovative services that create new revenue opportunities.
Mobile data traffic continues to grow exponentially and operators need to keep up with demand. While early LTE deployments concentrated on providing coverage, operators are now focusing on adding capacity through spectral efficiency improvements, cell densification and more spectrum. In doing so, they must upgrade their fronthaul transport networks while also preparing for 5G.
In many cases, operators are looking to centralized Cloud RAN (C-RAN) architectures with capacity demands and 5G in mind. According to Rethink Research, operators will deploy centralized and virtualized macrocells and microcells at a compound annual growth rate (CAGR) of 23 percent between 2017 and 2025. These deployments will overtake new deployments of conventional cells by 2022.
Using centralized RAN to spur network transformation
The deployment of RAN network functions virtualization (NFV) will quickly bring many more C-RAN sites into the network. Early centralized RAN deployments for 4G provide a solid foundation and stepping stone for moving to C-RAN. In many cases, the central offices or hub sites that house the central baseband processing will coexist with Multi-access Edge Computing (MEC) data centers required for 5G networks.
The coexistence of 4G and 5G will mean that operators need to serve a mix of RAN architectures with converged mobile transport to avoid costly overlays. This includes supporting existing fronthaul protocols (CPRI, OBSAI) and new 5G-specific protocols (eCPRI, NGFI).
Figure 1: RAN evolution with different architectures in the mix
By centralizing RAN functions, operators can reduce CAPEX and OPEX through cell site simplification while improving overall system performance. The separation of the radios from the baseband units (BBUs) also creates performance efficiencies. By supporting locally centralized baseband clusters, operators can improve coordination among the radios in the pool. This enhances the performance of LTE-A features such as inter-cell carrier aggregation (CA) and coordinated multi-point (CoMP), which rely on strict coordination among cells.
Embracing C-RAN to capitalize on 5G functional splits
Operators are also keen to take advantage of NFV and software-defined networking (SDN) by moving to C-RAN architectures. New 5G functional splits that functionally decompose the RAN will relax transport bandwidth and latency requirements and lead to greater adoption of C-RAN.
These functional splits will also enable flexible placement of key RAN functionality across the radio unit (RU), distributed unit (DU) and centralized unit (CU). For example, operators can offer latency below 1 ms for real-time applications such as autonomous driving and remote surgery by directing traffic to a DU that resides at a MEC data center within a few kilometers of the RU. As a result, the central office that serves as the 4G centralized RAN hub can become the new MEC site.
For non-real-time applications with a latency budget of approximately 5 ms, operators could forward traffic to a data center in the core cloud to take advantage of packet aggregation. In these cases, operators could use a networked interface to provide connectivity to the CU, which could be located over 100 km away.
Evolving CPRI to support 5G
4G centralized RAN networks have traditionally relied on the transport of CPRI traffic between the radio and baseband units. But CPRI does not scale cost effectively for 5G, which uses much larger swaths of spectrum and massive MIMO. The CPRI cooperation group has released an eCPRI Transport Network requirements document that defines an enhanced protocol for low-layer intra-PHY functional splits (FS-LL) supporting connectivity between the RU and DU. This document outlines delay budget and timing accuracy requirements and recommends the use of Ethernet or IP packet fronthaul networks for better scale and aggregation.
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