Virtual RAN (vRAN) Over DOCSIS, DOCSIS as a Midhaul (2023)

By Ramneek Bali, Charter Communications

The objective of this paper is to present real-world results using commercial solutions that integrate Midhaul interface (F1) over DOCSIS and performance under varying traffic demands and user profiles in RAN and DOCSIS networks. The interface between virtualized distributed unit (vDU) and virtualized central unit (vCU) is called F1 or midhaul. Virtual Radio Access Network (vRAN) enables the move to more disaggregated and virtualized architecture for deploying 5G networks. Three important elements, which are fully disaggregated in RAN, are the Centralized Unit (CU), the Distributed Unit (DU) and the Radio Unit (RU). Building elements of RAN have further been virtualized, vDU and vCU, and can be deployed on over-the-shelf hardware. To see the benefits of the disaggregated vDU and vCU deployment model, vDU with RU can be deployed at a site and vCU can be centralized at a data center or far-edge location. Virtual Centralized Unit (vCU) provides non-real-time processing and access control. It manages higher layer protocols including Radio Resource Control (RRC) from the control plane, and Service Data Adaptation Protocol (SDAP) and Packed Data Convergence Protocol (PDCP) from the user plane. The vCU is connected between the 5G core network and the vDU. One vCU can be connected to multiple vDUs. Virtual Distributed Unit (vDU) provides real-time processing and coordinates lower layer protocols including Physical Layer (PHY), Radio Link Control (RLC) and Media Access Control (MAC). Virtualization shifts the vCU and vDU from dedicated hardware to software components, allowing for flexible scaling, as well as rapid and continuous evolution. This virtualization allows the networks to easily meet the evolving demands of new and existing services with minimal impact on the deployment and operation costs. With vDU, all the baseband functions of real-time RLC/MAC/PHY layers are executed over the commercial-off-the-shelf (COTS) server. Remote Radio Unit (RRU) provides the physical layer transmission and reception, supporting technologies such as Multiple Input Multiple Output (MIMO). vRAN offers the flexibility of varied deployment options based on the latency requirements. The lowest latency is achieved by deploying the vDU and vCU at the edge cloud, along with the core in the mobile edge compute (MEC)server. Consequently, the user plane is now located nearest to the service available cell and with the core user plane terminated at the edge cloud itself. A key asset in the cable industry is the hybrid fiber coax (HFC) network. The vRAN architecture split depends on the throughput and latency performance of the HFC network. These requirements will decide which interface (fronthaul, mid (haul or backhaul) is best suited to deploy over DOCSIS. Fronthaul has the most stringent bitrate and latency constraints and backhaul has the least. Studies and vendor requirements have shown that the enhanced Common Public Radio Interface (eCPRI) based fronthaul transport needs significantly more bandwidth, overhead and one-way latency in the neighborhood of 100 microseconds between the RU and vDU. Hence, front haul is preferred to be on fiber exclusively, while mid-haul and backhaul can be supported by a transport network with 10-100ms latency. These limitations and cost considerations of deploying fiber makes it easier to deploy RU and vDU at cell site location and vCU/core at centralized location.

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