Asymmetric PHY Split for Cloud RAN Fronthaul Latency
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Solution Overview
Problem
Conventional Radio Access Networks (RANs) face challenges in scalability, cost, and ease of upgrade due to their integrated design, making it difficult to adapt to evolving network demands and new wireless technologies, particularly in supporting features like massive densification and unlicensed spectrum operations.
Innovation Solution
The implementation of a cloud-based Radio Access Network (CRAN) architecture, where the RAN layer processing is split between a central unit (CU) and a distributed unit (DU), with a fronthaul interface that allows for flexible functional splits, such as asymmetric splits between downlink and uplink at the physical layer, enabling efficient transport and virtualization while optimizing system performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the entire RAN is processed at the base station using application specific hardware, then processing performance is improved, but scalability and ease of upgrade deteriorate
Solution Approach 1:
The RAN base station is segmented into two functional units: a Remote Radio Unit (RRU) for real-time RF processing and a Baseband Unit (BBU) for non-real-time baseband processing. This segmentation allows the RRU to handle time-critical operations with dedicated hardware while the BBU can be virtualized and upgraded via software, thus resolving the contradiction between processing performance and adaptability.
Solution Approach 2:
The real-time critical RF processing functions are extracted from the baseband processing and placed in the RRU, while the BBU handles higher-layer protocols and baseband processing. This extraction enables independent optimization of each unit - the RRU maintains high-performance real-time processing while the BBU can be virtualized for improved scalability and ease of upgrade.
2Adaptability or versatility
If RAN functions are split between RRU and BBU, then scalability and flexibility are improved, but transport latency and bandwidth requirements increase
Solution Approach 1:
The functional split separates real-time RF processing in the RRU from baseband processing in the BBU, with the fronthaul interface optimized for low-latency communication. The RRU processes time-critical operations locally while only exchanging essential data with the BBU, minimizing transport latency while maintaining scalability and flexibility.
Solution Approach 2:
The RRU is equipped with dedicated real-time processing capabilities for local quality optimization, handling time-critical RF operations without requiring constant communication with the BBU. This local processing quality ensures low latency while the distributed architecture maintains scalability and flexibility.
3Device complexity
If conventional integrated RAN architecture is used, then device complexity is reduced, but capital and operating costs increase
Solution Approach 1:
The RAN is segmented into RRU and BBU components, allowing the BBU to be virtualized and deployed on commercial off-the-shelf servers. This segmentation reduces capital costs by eliminating specialized hardware for baseband processing while maintaining manageable device complexity through standardized interfaces and protocols.
Solution Approach 2:
The baseband processing functions are copied from proprietary hardware implementations to software virtualization on standard servers. This copying approach reduces capital and operating costs by utilizing commodity hardware while maintaining functional equivalence, and the modular RRU design keeps overall system complexity manageable.
Data Source
AI summary
A cloud radio access network (CRAN) system includes a baseband unit (BBU) and a radio unit (RU) remote from the BBU. The fronthaul interface between the RU and the BBU includes a radio frequency interface (RF) functionality implemented in the RU, and implementation of asymmetrical physical layer (PHY) functionality split between the BBU and RU. The asymmetrical physical layer (PHY) functionality split includes: downlink (DL) antenna port mapping and DL precoding implemented in the RU; and the split of the PHY functionality for uplink (UL) at the antenna port mapping in the BBU. For the DL, precoding and resource element (RE) mapping to frequency resources is implemented in BBU, and RE mapping for antenna ports is implemented in the RU|[WA1]. The split also provides support for license-assisted access (LAA) in the CRAN system.