5G Remote Radio Head (RRH)

5G Band n77 Remote Radio Head (RRH)

Component parts of a 5G Remote Radio Head:

What’s in a 5G Remote Radio Head (RRH) ?

A 5G Remote Radio Head (RRH) comprises several key component parts. Here we look at some of the key technologies involved

5G Band n77 Remote Radio Head (RRH)
5G Band n77 Remote Radio Head (RRH)

Remote Radio Head (RRH) Transceiver, (TRX)

In a modern RRH The Transceiver module may be a single high density PCB containing high speed logic, RF, slow speed analogue and management functions.

The RF processing stages will include complex technologies such as Digital Predistortion (DPD) and Crest Factor Reduction (CFR) which are key to ensure high fidelity of transmitted signals across the airside interface.

The high speed logic may be implemented in a Field Programmable Gate Array (FPGA).

High speed interfaces may include CPRI and/or 10Gbps Ethernet (IP) eCPRI interfaces. More modern Open RAN (ORAN) and Virtualised RAN (VRAN) networks require Ethernet interfaces and more complexity inside the RRH.

Lower speed interfaces may be used to control antenna arrays and other functions.

Front End Module (FEM)

An FEM is used in TDD variants as a highly integrated module. The Front End Module (FEM) contains the following key elements:

  • Power Amplifier (PA)
  • Low Noise Amplifier (LNA)
  • Transmit-Receive switch

FEM Model variants used in Remote Radio Heads to cover different frequency bands and power levels. Please note this is only a partial list of examples:

EWA1057: 2496 – 2690 MHz, 2x20W (Watts output power)

EWA1054: 2300 – 2400 MHz 2x5W (Watts output power)

EWA1056: 2496 – 2690 MHz, 2x5W (Watts output power)

EWA1055: 2300 – 2400 MHz, 2x20W ((Watts output power)

EWA1053: 3400 – 3700 MHz, 2x5W (Watts output power)

EWA1051: 3800 – 4200 MHz, 2x1W (Watts output power)

Power Amplifier (PA)

The Power Amplifier takes signals from the Transceiver (TRX) and boosts to high power levels needed for over-the-air transmission. The output power may vary from 100mW up to 80W per chain (channel) depending on the specific model and application.

Low Noise Amplifier (LNA)

The Low Noise Amplifier (LNA) takes low level input signals from the antenna and boosts them to suitable levels to feed to the Transceiver (TRX) module. The LNA must be suitable shielded to prevent unwanted signals entering, including from other items within the RRH.

RF Duplexer or Filter

Depending whether the RRH is being used in FDD or TDD mode, an RF Duplexer or Filter is used. TDD units feature a filter, and FDD units feature a Diplexer

This RF stage is key to remove unwanted harmonics from the transmission, and also block any unwanted signals from adjacent bands entering the sensitive RF receiver

DC-DC Power Supply (PSU)

The DC-DC Power Supply (PSU) takes input power (typically 36-72V, nominally 48V DC) and converts to smooth, stable power voltage rails to power the key items within the Remote Radio Head.

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CPRI and eCPRI Interfaces for 5G

CPRI, or Common Public Radio Interface, defines key interface specification between REC (Radio Equipment Control) and RE (Radio Equipment) of radio base stations used for cellular wireless networks. It is a protocol of choice for fronthaul communications between towers and base stations (BSs) through several generations of wireless networks. CPRI has efficient and flexible I/Q data interfaces for various standards, such as GSM, WCDMA, LTE, etc.

CPRI for 5G: 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. 

eCPRI for 5G: 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. 

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.

Interfaces used for Fronthaul in 4G and 5G Wireless Networks

Both CPRI and eCPRI can be used in 5G fronthaul. While, eCPRI is more suitable in 5G fronthaul. Here are the network line rates:

CPRI Line RateLine Bit Rate (Gbps)Line CodingBits/WordTransport Capacity (#WCDMA AxC)Transport Capacity (#20 MHz LTE AxC)
CPRI line rates used in 4G and 5G

This table shows the different transport capacity at different line rates. Since 5G requires tremendous bandwidth expansion, line rate option 10, the newest standard, can meet latest 5G requirements. However, this processing capacity has reached the upper limit of the standard, causing doubt about whether 5G can be used with CPRI for future expansion.

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CPRI and eCPRI in 5G and open vRAN

5G Band n77 Remote Radio Head (RRH)

CPRI, eCPRI and open vRAN: What’s happening?

Common Public Radio Interface (CPRI) has been around for quite some time. But now, enhanced CPRI (eCPRI) is becoming an important technology to understand for 5G.
Before looking in detail at eCPRI, it’s helpful to understand some of the basic topology of the cellular network, which currently uses CPRI.

CPRI, Remote Radio Heads (RRH) in 4G and 5G

5G Band n77 Remote Radio Head (RRH)
5G Remote Radio Head (RRH) with CPRI or eCPRI fibre optic interface

At the outer circle of a cellular network topology, remote radio units (RRUs) are distributed every few miles in cities and suburban areas. These RRUs comprise antennas and also some compute functionality.
Fiber runs from a cluster of these RRUs to connect to a more centralized baseband unit. The baseband unit is also sometimes called the “central processing unit.” Baseband units are typically distributed within approximately 10-mile circles for good coverage in populated areas. The connection between the RRUs and the base station is often referred to as “fronthaul.”


CPRI is an interface that sends data from the RRUs to the baseband unit: CPRI is a serial interface, which is a very high-speed connection, a way to translate all those radio signals back to the computing function.

As we go to 5G, the fiber between the RRUs and the baseband unit is going to carry much more traffic, and that makes it more difficult to do a serial interface. Extreme 5G requirements are stretching the limits of fiber bandwidth.

Enter eCPRI, which is a way of splitting up the baseband functions and putting some of that functionality in the RRU to reduce the burden on the fiber.

AT&T is among many carriers that are working on eCPRI. AT&T has made “the world’s first” eCPRI connection for mmWave at its 5G Labs in Redmond, Washington. AT&T made calls testing eCPRI, using systems from both Nokia and Samsung Electronics America.

This opens the door for higher network throughput with less fiber, which will create more efficient mmWave deployments, among other benefits, This is also a significant step in creating an open architecture within the radio access network (RAN).

Vendor lock-in

Another problem with the CPRI interface today is that it has become a proprietary technology.
In addition to supporting more bandwidth across fewer fibers, the enhanced CPRI also addresses the proprietary concerns. eCPRI will be an open interface, making it easier for carriers to mix and match vendor equipment for their RRUs and their baseband units.
Historically, because each vendor would have its own implementation of CPRI, it became proprietary. This forces carriers to buy both their RRUs and their baseband unit from the same vendor in order for the interface to work. With O-RAN this interface will be open. Carriers such as AT&T who implement eCPRI will be able to run equipment from different vendors, or even generic off-the-shelf equipment.
However, existing networks with CPRI installed will most likely remain in place for years to come.

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