XRAN-FH.WP.0-v01.00 White Paper

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1 White Paper xran Fronthaul Working Group White Paper The present document shall be handled under appropriate xran IPR rules. 0 xran.org All Rights Reserved

2 Revision History Date Revision Author Description S.Venkatraman Initial revision S.Venkatraman Revised first version based on received comments 0 xran.org All Rights Reserved

3 0 Contents Revision History... Overview... Architecture.... Selected Lower Layer Functional Split (-x)..... Fronthaul bandwidth.... M-Plane Architecture.... xran Fronthaul Capabilities... Enhancements & Roadmap...0. Specification enhancements Path to Interoperability PoCs... 0 References... 0 xran.org All Rights Reserved

4 0 0 Overview This whitepaper has been developed by the xran fronthaul working group in support of advancing an open, interoperable and efficient front haul interface. This document provides an update on the progress within the fronthaul working group since the publication of the original whitepaper in October 0 [] and summarizes key aspects of the xran fronthaul specification, XRAN-FH.CUS.0-v0.00 []. The publication of the xran fronthaul control, user and synchronization plane specification [] is a significant milestone in addressing key requirements & use cases identified in []. This was made possible due to extensive collaboration between an increasing number of operators and equipment manufacturers [] working in a collaborative fashion. The specification addresses the following key operator requirements for the fronthaul interface:. BBU RU interoperability based on well specified control, user and management plane interfaces.. Efficient bandwidth scaling as a function of user throughput and spatial layers to address increasing bandwidth needs and Massive MIMO deployments.. Support for LTE & NR with different RU product configurations (T-T & Massive MIMO antenna systems).. Support for advanced receivers and co-ordination functions.. Interface simplicity to enable interoperability & RU to support future standards enhancements.. Ethernet based transport layer solutions.. Extensible data models for management functions to simplify integration. The first version of specification provides a framework that address a broad range of use cases and includes support for LTE and NR. The interface supports an order of magnitude increase in fronthaul efficiency enabling advanced technologies like Massive MIMO & Virtualized RAN. It also fosters an ecosystem for development of a wide range of innovative RAN products that can be integrated easily using standardized interfaces & data models. Section of this document describes the chosen functional split, M-plane architecture & key capabilities of the fronthaul interface. Additional features like LAA, improvements to efficiency, dynamic delay management etc. are under consideration for the next version of specification, which will also address the M-plane specification. Section summarizes these enhancements and planned milestones to enable product interoperability. 0 xran.org All Rights Reserved

5 0 Architecture The architecture of enb or gnb with lls-cu and RUs is shown in Figure. LLS-C/U/S refers to C-plane, U-plane & S- plane communication over a lower layer split interface (LLS). LLS-M refers to M-plane communication over the LLS interface. In this architecture, lls-cu and RU can be defined as follows. Lower Layer Split Central Unit (lls-cu): a logical node that includes the enb/gnb functions (commonly also referred to as BBU), excepting those functions allocated exclusively to the RU. The lls-cu manages real-time control & user plane functions of the RUs. Radio Unit (RU): a logical node that includes a subset of the gnb functions as required by split option -x ( x here refers to xran). Management functions of the RU can be controlled over the LLS-M interface by the lls-cu or a management system. RRC PDCP-C gnb SDAP PDCP-U lls-cu Management System RLC MAC Phy-H XRAN C/U/S-Plane XRAN M-Plane LLS-C/U/S LLS-M 0 XRAN U/C/S-Plane Phy-L RF Chain XRAN M-Plane Figure. enb/gnb architecture with lls-cu and RUs. Selected Lower Layer Functional Split (-x) Figures & describe the functional split -x for DL and UL. In the DL, ifft, CP addition, and digital beamforming functions reside in the RU. The rest of the PHY functions including resource element mapping, precoding, layer mapping, modulation, scrambling, rate matching and coding reside in the lls-cu. Beamforming specific processing (expansion from layers/beams to digital transceivers) resides within the RU. In the UL, FFT, CP removal and digital beamforming functions reside in the RU. The rest of the PHY functions including resource element de-mapping, equalization, de-modulation, de-scrambling, rate de-matching and de-coding reside in the lls-cu. Beamforming specific processing (combining inputs from multiple digital transceivers to a set of beams/layers) resides within the RU. RU 0 xran.org All Rights Reserved

6 CRS, PSS SSS Signal gen. PDCCH PCFICH PHICH Scrambling Modulation Layer Mapping Precoding RE Mapping Antenna Port 0- TM-TM PDSCH Scrambling Modulation Layer Mapping Precoding Precoding (y=x) TM-TM0 P D S C H DMRS gen RE Mapping Broadcast Beam Beamforming ifft and CP addition ifft and CP addition DAC, RF chain processing, analog beamforming Physical Antennas Precoding per GPP TS. definition Scrambling Modulation Layer Mapping Antenna port, - = #of layers Figure. Lower layer DL split description CSI-RS gen Antenna Port - Beamforming lls-cu -x RU 0 0 Descrambling Demodulation idft Equalization P U S C H DMRS P U C C H SRS PRACH DMRS Format Proc. Channel Est Equalization Channel Est Channel Est Detection RE demapping RE demapping PUSCH Rx Beams PUCCH Rx Beams SRS Rx Beams PRACH Rx Beams Beamforming Beamforming Beamforming Beamforming FFT and CP removal Filtering analog beamforming, RF chain processing, ADC Physical Antennas lls-cu Figure. Lower layer UL split description -x split as described above was chosen for this initial version since it enables a simple interface specification whilst catering to many of the key operator requirements as described below. Additional splits or optimizations remain under consideration as candidates for future versions of specification. Benefits and Justification: - Interface simplicity: Transfer of user plane data is based on Resource Elements / Physical Resource Blocks, which simplifies the data mapping and limits the required associated control messages - Transport Bandwidth Scalability: Lower split options (e.g., splits - and ) scale based on number of antennas. In contrast, -x interface scales based on streams, which allows using high number of antennas without higher transport bandwidth. Further, user data transfer can be optimized to send only PRBs that contain user data for purpose of reducing transport bandwidth - Beamforming Support: The same interface design can support different beamforming techniques (digital, analog, hybrid) as well as different beamforming algorithms. Likewise, deployments using only analog beamforming are also possible with the same interface design. - Interoperability: Less user specific parameters are used at split -x (when compared to higher split options), which can simplify specification. - Advanced receivers and inter-cell coordination: this option allows implementation of advanced receivers and coordination features, which are also easier to implement and less restricted when most functions are placed at the lls-cu. For example, UL Comp is not possible when the UL upper-phy processing is in the RU. - Lower RU complexity: Less functions at RU (when compared to higher split options) allow reduction in compute and memory requirements. - Future proofness: Placing most functions at lls-cu will allow introduction of new features via software upgrades without inflicting HW changes at RU (e.g., spec changes due to URLLC or new modulation schemes). -x RU 0 xran.org All Rights Reserved

7 - Interface and functions symmetry: If the same interface and split point is used for DL and UL, specification effort can be reduced... Fronthaul bandwidth Improved fronthaul bandwidth efficiency is one of the key benefits of -x split, which offers significant fronthaul bandwidth savings and enables advanced massive MIMO deployments. Table below provides a summary of fronthaul bandwidth savings for use cases when using split -x relative to CPRI as an illustrative example. Use Case BW Reduction Factor Benefit T/T TM - x* - Simple & low-cost RU design - More carriers per fiber over C-RAN fronthaul - More efficient vran compute infrastructure 0 0 TRX FD-MIMO TM -0 - DL Layers - UL Streams x* - Significantly reduced fronthaul BW - Reduces fiber needs from bottom to top of tower - Reduces fronthaul fiber needs or enables re-use of existing fiber in C-RAN deployments * Bandwidth reduction estimates are in comparison with CPRI, assuming 0% RB loading with common bit-width. 0% overhead assumed for transport and signaling overheads. IQ compression is not considered.. M-Plane Architecture This interface is required to support configuration management (CM), performance management (PM) & fault management (FM) towards the RU. The xran fronthaul working group is currently developing the detailed M-plane specification & associated data models to complement the CUS plane specification. - A hybrid architecture as described in Figure is currently being considered for the M-plane interface. Hybrid architecture enables a direct logical interface between a management system and RU in addition to the lls-cu. This could simplify integration efforts to support multiple RU variants and reduce requirements on lls-cu. Functions like RU upgrades, performance management as well as fault reporting can be managed directly by a management system. The M-plane interface shall also support control of RU via lls-cu as employed in a traditional hierarchical deployment or for selective configuration management for time sensitive parameters. - Hybrid architecture as described above requiring direct logical communication between a RU and a management system can be enabled via RUs being assigned routable IPs or local private IPs resolved by a NAT function in the network (or implemented at the lls-cu) - NETCONF/Yang has been chosen as the network management protocol & data modeling language. NETCONF/Yang is already being used within operator networks for several network elements. Use of such a standardized framework & common modeling language simplifies integration between lls-cu and RU as well as operator network integration. The framework supports integration of products with differing capabilities or specifications enabled by well published data models. NETCONF also natively supports a hybrid architecture which enables multiple clients to subscribe and receive information originating at the RU. 0 xran.org All Rights Reserved

8 lls-cu OAM Management System Lls-CU OAM NETCONF Client lls-cu OAM lls-cu NETCONF Client LLS-M (CM, PM, FM) lls-cu NETCONF Client RU NETCONF Server RU NETCONF Server Hierarchical Hybrid lls-cu OAM interface is not in scope of this specification work. xran Fronthaul Capabilities Figure. M-Plane Architecture The interface has been designed to cater to various use cases, product implementations and provides a framework that can be extended or optimized in the future. Table below provides a summary of capabilities targeted to be supported by the LLS interface (based on v.0 C/U/S specification and supported by future M-Plane specification). Category Technologies Channel Bandwidth Subcarrier Spacing Transmission Modes RU Types (No. of Trx) Beamforming Fronthaul Bandwidth Saving Features Features LTE TDD, FDD NR TDD, FDD Normal and Extended CP LTE:.,,, 0,, 0 MHz NR: up to 00MHz Carrier Aggregation LTE: khz,.khz,.khz LTE PRACH:.kHz,.kHz NB-IoT PRACH:.kHz NR:, 0, 0, 0, 0 khz NR Multi Numerology NR PRACH:.,,, 0, 0, 0 khz DL: TM - TM0 UL: TM, TM T, T, T, T, T, T & beyond. Analog Beamforming Digital Beamforming: - Beam Indices - Real-time Weights - UE Channel Info Hybrid Beamforming Beam definition & combining weights per channel Variable Bitwidth IQ per channel IQ Compression BW Scaling based on Traffic 0 xran.org All Rights Reserved

9 Transport BW Scaling based on Layers Energy Saving (Semi static transmission blanking) L: Ethernet L: IPv, IPv UDP (optional) QoS over Fronthaul using DSCP or CoS Flow identification via VLAN, MAC address, IP address, UDP ports Routable & Local Private IP Address Support Table. xran Fronthaul Capabilities Summary Current specification effort targets to support physical layer channels as defined in GPP Release for LTE and NR. 0 xran.org All Rights Reserved

10 0 0 Enhancements & Roadmap. Specification enhancements Extensions to this first version of the xran C-Plane, U-Plane and S-Plane specification will be included in the next version. xran working group is currently developing the data models and M-plane specification required for configuration of system parameters & RU management. Future version of specification is also expected to identify mandatory and optional capabilities to aid in achieving interoperability over a wide range of capabilities. The following topics are to be considered for future versions of the specification.. Split Optimizations to further reduce fronthaul rates and/or RU complexity.. Inclusion of an option to carry time-domain PRACH data on the UL.. Support for IEEE... Compression method addition beyond the methods so far described.. Link latency measurement and Dynamic Delay Adjustment.. Enhancements to support features like LAA or feature specific improvements if required (embms, NB-IOT).. Considerations for antenna calibration.. Enhancements to support GPP evolution.. Path to Interoperability PoCs Figure. outlines the upcoming milestones towards reaching interoperability in product implementation, starting with interoperability PoCs between xran member companies. This will ensure any issues or defects in specification revealed during interoperability POCs can be addressed quickly by the vendor ecosystem. xran members intend to develop a test specification to assist OEMs & test equipment manufacturers to facilitate interoperability testing. Recently [], the xran forum has announced its intent to merge with the C-RAN alliance to form a world-wide carrier led ecosystem to combine and extend the work of both the C-RAN Alliance and the xran Forum. It is expected that the fronthaul working group specification effort will continue as planned and will be extended to cater to additional use cases of global operator interest. Q' Ver.0 C/U/S Plane M-plane Architecture Q' Interoperability PoCs Additional UseCases Q' C/U/S Enhancements Ver.0 M-Plane Figure. Upcoming xran Milestones 0 xran.org All Rights Reserved 0

11 0 References The following documents contain provisions which, through reference in this text, constitute provisions of the present document. - References are either specific (identified by date of publication, edition number, version number, etc.) or non-specific. - For a specific reference, subsequent revisions do not apply. - For a non-specific reference, the latest version applies. In the case of a reference to a GPP document (including a GSM document), a non-specific reference implicitly refers to the latest version of that document in Release. [] xran Fronthaul Whitepaper, Oct 0, WP-00.pdf [] xran Spec [] [] Alliance-Form-ORAN 0 xran.org All Rights Reserved