Background

Although Common Public Radio Interface (CPRI) has been the main Fronthaul interface standard, many operators started to question its suitability to high bandwidth 5G use cases. Improvements to efficiency and link capacity utilization were requested. Also advanced networking and OAM features of mainstream packet transport standards were requested.

What is CPRI?

A digital public interface to transport antenna samples between a Radio Equipment (RE) and a
Radio Equipment Control (REC) performing the digital processing of these signals
• Antennas signals are interleaved in a TDM-like fashion supported by a Constant Bit Rate
transport solution
• CPRI v7.0 bit rates range from 614 Mbit/s (Rate 1) up to 24330 Mbit/s (Rate 10)
• Mix of Radio Access Technologies is supported
• Provide time and synchronization information for the Radio Air Interface
• Originally specified for point-to-point topology
• Maximum latency assuming no intermediate nodes
• Multipoint topologies supported but networking management left to the application layer
• Interoperability limited to the low layers covered by the specification
• CPRI define how to exchange the radio signal data, not the data content itself, nor the
associated management plane

eCPRI introduction

• eCPRI is packet based fronthaul interface developed by CPRI forum
• Same level of interoperability as CPRI
• Ethernet/IP networking, synchronization and security relying on existing standards

eCPRI Specifications

• Throughput: 3/1.5 Gbps (DL/UL, end-user data rate, transport block from/to MAC)
• Air bandwidth: 100 MHz (5 * LTE20) -> 500 PRB
• Number of downlink MIMO-layers: 8
• Number of uplink MIMO-layers: 4 (with 2 diversity streams per uplink MIMO layer)
• MU-MIMO: No
• TTI length: 1 ms
• Digital beamforming where BF-coefficients calculation is performed in eREC
• Rate matching assumptions: Code rate: ~0.80
• Modulation scheme (Downlink & Uplink): 256 QAM
• Number of antennas: 64
• Subcarrier spacing: 15 kHz
• IQ sampling frequency: 122.88 Msps (3.84*32)
• IQ-format: 30 bits per IQ Samples
• No IQ compression

eCPRI Transport Network

• eCPRI does not restrict the Transport Network to be Ethernet or IP based
• In case eCPRI User Plane over Ethernet directly: eCPRI Ethertype (AEFE 16 ) shall be used
• eCPRI defines a protocol for the transfer of user plane information between eREC and eRE via a packet based fronthaul transport network.
• For C&M and synchronization information flows, existing protocols and standards are referenced as proposals.

eCPRI protocol stack over IP / Ethernet