CPRI Frame Structure & Line Rates in RRH: An Overview

The CPRI frame structure is a fundamental element of the CPRI (Common Public Radio Interface) protocol, essential for the seamless operation of Remote Radio Heads (RRH). It organizes data into a structured format, ensuring efficient transmission between the RRH and the baseband unit (BBU).

This article delves into the details of the CPRI frame structure and its format, highlighting its components, CPRI line rates, and significance in wireless communication systems.

As we know, the deployment of wireless cellular systems is increasing due to increasing demand from users. This leads to an increase in the number of base station deployments supporting multiple wireless standards such as UMTS, CDMA, LTE, WiMAX, etc.

In order to avoid the installation of bulky base stations, a new concept of remote radio heads (RRH) has been developed. In this concept, the existing base station consisting of both baseband and radio functionalities has been divided into two components. These RRH components are referred to as REC (Radio Equipment Control) and RE (Radio Equipment). REC consists of baseband functionalities, and RE consists of RF and ADC/DAC functionalities.

In this RRH architecture, both RE and REC will have layer-1 and layer-2 protocol layers.

Below is the RRH frame structure used to carry signaling or information data.

The link between REC and one or multiple REs is referred to as a CPRI link/links. Various topologies such as chain, ring, or tree exist between them.

CPRI links in RRH technology carry IQ information data, synchronization data, L1 inband protocol, C&M data, and so on.

• 10ms - 150 Hyper frames
• 150 Hyper frames - 256 basic CPRI frame/Hyper frame
• 256 basic CPRI frame/Hyper frame - 16 words

The figure depicts the CPRI frame structure. The RRH frame has a duration of 10 ms with a total of 150 hyperframes. One hyperframe is composed of a total of 256 basic RRH frames. Each RRH basic frame is made up of 16 words with index W ranging from 0 to 15.

The length of a basic CPRI frame is 1 Tc, equal to a 260.4 ns time duration (Index X).

Tc = 1/Fc = 1/3.84e06 = 260.4 ns

In a CPRI frame structure, out of 16 words, 1 word is used for control information, and the remaining 15 words are used to carry user plane IQ data (SAP IQ).

CPRI Line Rates

The CPRI interface supports various line rates. The table below mentions CPRI line rates, including the length of words in bits and the total information data bits carried by them.

• CPRI line bit rate option 1: 614.4 Mbps
• CPRI line bit rate option 2: 1228.8 Mbps (2 x 614.4 Mbps)
• CPRI line bit rate option 3: 2457.6 Mbps (2 x 1228.8 Mbps) and so on.

Total no. of information bits in RRH frame = (Line rate/sample rate)(15/16)(8/10)

Here, the Line rate is as mentioned in the table, and the sample rate is 3.84 Mcps. The factor 15/16 accounts for 15 data words in a total of 16 words.

The factor 8/10 accounts for 8B10B encoding, which refers to 80% non-encoded data, and 20% redundant information is transmitted on the CPRI line interface.

Conclusion

The CPRI frame structure plays a vital role in optimizing the performance of Remote Radio Heads by providing an organized data transmission framework. Its efficient design supports the high-speed, low-latency requirements of modern communication networks, ensuring reliable connectivity and scalability for future advancements.