CPRI & eCPRI with Open RAN Split Option 8 and 7-2x

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Importance of Simplified RU

A simplified RU because only the RF functions remain at the remote site. This small RU costs less, uses less power, and more easily fits on a tower. Operators can perform most network upgrades at the CU, requiring fewer site visits. The simplified RU can handle multiple Radio Access Technologies (RAT) further reducing the footprint of the remote mast, which must support multiple cellular generations

There are 2 types of protocol used for fronthaul between RE/eRE to REC/eRCE these protocols are CPRI and eCPRI

The Purpose of development of CPRI and eCPRI is as follows. 

Radio BSs should offer flexibility during deployment to MNOs( mobile network operators). This is achieved by simplifying BS architecture by dividing radio BS functionality into two modules viz. eREC and eRE. Both parts may be physically separated where in eRE is kept close to RF antenna where as eREC kept at a distant end. Both are connected via a transport network. The eREC contains part of PHY layer functions and upper layer functions of the air interface whereas eRE contains the other part of the PHY layer functions and the analog radio frequency (ARF) functions. The different functions can be located either in the eREC or in the eRE



CPRI: Common Public Radio Interface

CPRI defines key interface specification between REC (Radio Equipment Control) and RE (Radio Equipment) of radio base stations used for cellular wireless networks. CPRI is the short form of Common Public Radio Interface. CPRI is popular standard for transporting baseband I/Q signals to the radio unit in traditional BS (Base Station). CPRI allows efficient and flexible I/Q data interface for various standards e.g. GSM, WCDMA, LTE etc. 

1. A digital interface standard to transport antenna samples between a Radio Equipment (RE) and a Radio Equipment Control (REC) performing the digital processing of these signals 

2. Antennas signals are interleaved in a TDM-like fashion supported by a Constant Bit Rate transport solution 

3. CPRI v7.0 bit rates range from 614 Mbit/s (Rate 1) up to 24330 Mbit/s (Rate 10) 

4.Mix of Radio Access Technologies is supported 

5.Provide time and synchronization information for the Radio Air Interface 

6.Originally specified for point-to-point topology 

7.Maximum latency assuming no intermediate nodes 

8.Multipoint topologies supported but networking management left to the application layer 

9. Interoperability limited to the low layers covered by the specification 

10.CPRI define how to exchange the radio signal data not the data content itself nor the associated management plane


eCPRI: Enhanced Common Public Radio Interface

eCPRI was created and published after CPRI. The eCPRI standard defines specification which connects eREC and eRE via fronthaul transport network. It is used for 5G systems, LTE-Advanced and LTE-Advanced Pro

1. eCPRI is packet based fronthaul interface developed by CPRI forum 

2. Same level of interoperability as CPRI 

3. Ethernet/IP networking, synchronization and security relying on existing standards

4. Throughput: 3/1.5 Gbps (DL/UL, end-user data rate, transport block from/to MAC) 

5. Air bandwidth: 100 MHz (5 * LTE20) -> 500 PRB 

6. Number of downlink MIMO-layers: 8 

7.Number of uplink MIMO-layers: 4 (with 2 diversity streams per uplink MIMO layer) 

8.MU-MIMO: No 

9.TTI length: 1 ms 

10. Digital beamforming where BF-coefficients calculation is performed in eREC. 

11.Rate matching assumptions: Code rate: ~0.80 

12.Modulation scheme (Downlink & Uplink): 256 QAM 

13.Number of antennas: 64 • Sub-carrier spacing: 15 kHz 

14.IQ sampling frequency: 122.88 Msps (3.84*32) 

15.IQ-format: 30 bits per IQ-sample 

16.No IQ compression




Propietary CPRI interface (left) versus open eCPRI interface being able to support low latency at much lower cost

RAN Split Option 8 and 7-2x

• Split points D, ID , IID , IU and E are examples covered in the eCPRI specification 
• Split points Option 6, 7-1, 7-2, 7-3 and 8 are 3GPP split options and sub-options



Split 7.2x is the O-RAN Alliance fronthaul specification between O-DU to O-RRU. It has two variants: 7.2a and 7.2b based on where precoding occurs. If precoding and resource element mapper are part of the O-DU and the O-RRU handles beamforming onwards then it is 7.2a split. If precoding happens in the O-RRU then this is 7.2b split.

7.2a splits supporting RRU are much simpler and lower in cost as compared to 7.2b splits supporting O-RRU due to how much functionality has been pushed to the O-RRU or remains in the O-DU. An important aspect of this split is the fronthaul bandwidth requirement gets smaller while using O-RRU. E.g., DL precoding in O-RRU prevents further increased demand in fronthaul bandwidth when the number of MIMO streams are greater than MIMO layers. However, processing and memory requirements are increased in the O-RRU

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