What is a baseband unit?

Type of radio used in wireless telecommunications networks

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A remote radio head (RRH), also called a remote radio unit (RRU) in wireless networks, is a remote radio transceiver that connects to an operator radio control panel via electrical or wireless interface. When used to describe aircraft radio cockpit radio systems, the control panel is often called the radio head.

In wireless system technologies such as GSM, CDMA, UMTS, LTE, 5G NR this radio equipment is remote to baseband units[1] such as BTS/NodeB/eNodeB/gNodeB or gNB. This equipment is used to extend the coverage of a baseband unit in challenging environments such as rural areas or tunnels. RRHs are generally connected to the baseband unit or base station which can be an x86 server[2] on the ground near a cell tower, via a fiber optic cable using Common Public Radio Interface protocols.[3]

RRHs have become one of the most important subsystems of today's new distributed base stations. The RRH contains RF circuitry plus analog-to-digital/digital-to-analog converters and up/down converters, and connects to, and thus drives the cell site's antenna. RRHs also have operation and management processing capabilities and a standardized optical interface to connect to the rest of the base station/baseband unit. This will be increasingly true as LTE and WiMAX are deployed. Remote radio heads make MIMO operation easier; they increase a base station's efficiency and facilitate easier physical location for gap coverage problems. RRHs will use the latest RF component technology including gallium nitride (GaN) RF power devices and envelope tracking technology within the RRH RF power amplifier (RFPA).[4]

RRH protection in fiber to the antenna systems

Fourth generation (4G) and beyond infrastructure deployments will include the implementation of Fiber to the Antenna (FTTA) architecture. FTTA architecture has enabled lower power requirements, distributed antenna sites, and a reduced base station footprint than conventional tower sites. The use of FTTA will promote the separation of power and signal components from the base station and their relocation to the top of the tower mast in a Remote Radio Head (RRH).

According to the Telcordia industry standard that establishes generic requirements for Fiber to the Antenna (FTTA) protection GR-,[5] the RRH shifts the entire high-frequency and power electronic segments from the base station to a location adjacent to the antenna. The RRH will be served by optical fiber and DC power for the optical-to-electronic conversion at the RRH.

RRHs located on cell towers will require Surge Protective Devices (SPDs) to protect the system from lightning strikes and induced power surges. There is also a change in electrical overstress exposure due to the relocation of the equipment from the base station to the top of the mast.

Protection from lightning damage

RRHs can be installed in a low-profile arrangement along a rooftop, or can involve a much higher tower arrangement. When installed at the highest point on a structure (whether a building or a dedicated cell tower), they will be more vulnerable to receiving a direct lightning strike and higher induced lightning levels, compared with those installed in a lower profile manner below the upper edges of the building.

As noted in GR-, while surges can be induced into the RRH wiring for lightning striking the nearby rooftop or even the base station closure, the worst case will occur when a direct strike occurs to the antenna or its supporting structure. Designing the electrical protection to handle this situation will provide protection for less damaging scenarios... it can also be use in optical fiber communication but different type.

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See also

References

Baseband refers to the original frequency range of a transmission signal before it is modulated. Baseband can also refer to a type of data transmission in which digital or analog data is sent over a single non-multiplexed channel.

A baseband unit (BBU) is a unit that processes baseband in telecomm systems. A typical wireless telecom station consists of the baseband processing unit and the RF processing unit (remote radio unit - RRU). The baseband unit is placed in the equipment room and connected with RRU via optical fiber. The BBU is responsible for communication through the physical interface. A BBU has the following characteristics: modular design, small size, low power consumption and can be easily deployed.

A BBU in a cellular cell site is comprised of a digital signal processor to process forward voice signals for transmission to a mobile unit and to process reverse voice signals received from the mobile unit. The digital signal processor also serves to produce a first supervisory audio tone (SAT) for transmission to the mobile unit by generating successive digital SAT samples which are decoded into a continuous tone. Finally, the digital signal processor detects the presence of a second SAT generated by the mobile unit by sampling and processing successive samples of the second SAT and measuring the power.

Read our blost post to learn all about the importance of BBU emulation in the overall test process for tunring up cell sites.