A radio communication system consists of two main components - the base transceiver station (BTS) and the remote radio unit (RRU). The BTS traditionally housed the radio equipment like power amplifiers, frequency mixers, and antennas required for radio signal transmission and reception. However, with the advent of technologies like software-defined radio and network virtualization, the functions of a BTS have been split between centralized and distributed units.

The centralized unit (CU) now handles the core digital signal processing functions like coding, modulation, scheduling etc. while the distributed unit (DU) handles radios tasks like up/down conversion between radio frequency and baseband. The Remote Radio Unit  is a sub-component of the DU that is installed remotely and handles the analog radio functions. It is connected to the DU via common public radio interface (CPRI) or open base station architecture initiative (OBSAI) protocols.

Key Functions and Components of an RRU

An RRU consists primarily of radio frequency (RF) components needed for radio signal transmission and reception. The key functions performed by an RRU include:

- Frequency Up/Down Conversion: The RRU converts the received baseband IQ samples from the DU to radio frequency for transmission and vice versa for received radio signals. This up/down conversion is done using components like mixers and local oscillators.

- Power Amplification: The power amplifier in the RRU boosts the power of the transmitted radio signal to the required output power level before transmission through the antenna.

- Filtration: Bandpass filters are used in the RRU to select the desired channel frequency during up/down conversion and remove adjacent channel interference.

- Antenna Interface: RRU is connected to one or multiple antennas for transmission and reception of radio signals over the air interface.

- Digitization: The received radio signal from the antenna is digitized using an analog-to-digital converter before transmission to the DU over CPRI/OBSAI.

Benefits of Remote Radio Deployment

Deploying RRUs remotely from the centralized baseband processing units provides several technological and operational benefits:

- Flexibility: RRUs allow flexible deployment of radio resources at the network edge close to users. This enhances coverage and capacity where needed.

- Scaling: RRU deployment decouples radio resources from baseband resources, enabling independent scaling of both domains as per traffic demand.

- Maintenance: With RRUs installed at cell sites, maintenance activities like firmware upgrades or unit replacement can be done remotely without affecting the baseband processing units.

- Power Savings: RRUs consume significantly less power than traditional BTS equipment since the power-hungry baseband functions are centralized. This reduces energy costs, battery backup requirements and cooling needs.

- Technology Evolution: The network core can evolve independent of radio access by upgrading baseband hardware and software without requiring changes to remote radio units.

- Improved Performance: Distributing radio functions close to antennas improves radio frequency performance parameters like reference signal received quality.

Implementing Remote Radio Networks

Successful RRU deployments require careful network planning and design. Mobile network operators need to consider factors like:

- CPRI/OBSAI Transport: High-capacity transmission links are needed between RRUs and centralized baseband units to transport digitized IQ samples without loss. Both fiber and microwave backhaul can be used.

- Synchronization: Precise timing and phase synchronization between RRUs and centralized hardware is critical for maintaining waveform coherence across sites.

- Fronthaul Latency: End-to-end delay constraints over the fronthaul network must be met to avoid degradation of Quality of Experience for real-time services.

- RRU Configurations: Appropriate RRU configurations need to be selected based on coverage area, supported technologies and frequency bands, antenna configurations etc.

- Integration with Network Elements: RRU deployments require tight integration with existing core network elements and OSS/BSS systems for effective lifecycle management.

Thus, while remote radio unit address technology and business drivers for mobile operators, their deployments need to be systematically planned to fully realize gains in performance, economics, flexibility and scalability. With 5G on the horizon, RRUs will play an increasingly important role in enabling new capabilities for enhanced mobile broadband and ultra-reliable low-latency services.

Get more insights, On Remote Radio Unit