What is a Baseband Unit?
A baseband unit (BBU) is a telecommunication network device used to process baseband signals. Baseband is the term used to describe the original frequency of a transmission prior to modulation. Traditional Radio Access Network (RAN) consist of a BBU connected to one or more remote radio units (RRUs) positioned near the antenna(s).
The baseband unit is responsible for communication through the physical interface to the core network, while the remote radio unit performs the transmit and receive RF functions. The two elements are typically linked together via optical fiber.
Centralized functionality and fixed positioning at the foot of the cell tower are aspects of the traditional BBU meaning being redefined by 5G. Next-generation RAN architecture divides BBU functionality between a Distributed Unit (DU) for real time functions and a Centralized Unit (CU) for non-real time functions such as Radio Resource Control (RRC). Virtualization and disaggregation of 5G baseband units increases capacity and reduces latency.
What Does a Baseband Unit Do?
RAN consists of the baseband processing unit and the RF processing unit. The baseband unit acts as the centralized “hub” of the base station, processing uplink and downlink data traffic and controlling RRU functionality. A conventional BBU contains a digital signal processor (DSP) used to convert signals from analog to digital or vice versa.
Additional processes baseband units complete include alarm monitoring and system clock operation for synchronization. The BBU also controls the transfer of user data, session management, and mobility functions that define the communication link between users.
Base station architecture originally positioned the BBU and RRU on digital and radio shelves co-located within a cabinet or enclosure at the base of the cell tower. The base station was connected to the antenna through RF cabling which was prone to loss and interference. The release of the Common Public Radio Interface (CPRI) protocol in 2003 defined the transport and connectivity between the RRU and BBU, allowing for greater physical separation of these elements through a fronthaul link.
Since the first cellular networks were introduced over forty years ago, basic RAN elements and the interfaces between them have remained in continuous flux. The functional split that originated with the LTE baseband unit continues to evolve with the disaggregated BBU architecture defined by 3GPP TR 38.801.
4G Baseband Architecture
4G LTE networks represented a significant milestone for BBU development, with the introduction of the remote radio unit RRU, fronthaul, and MIMO antennas to increase capacity. Distributed RAN (D-RAN) architecture for the BBU RRH combination led to the familiar baseband unit deployment at the macro cell tower base. This concept was expanded through the introduction of centralized RAN (C-RAN) which further liberated the BBU full form from physical constraints.
5G Baseband Architecture
5G RAN moves the baseband unit architecture of telecom systems to the next level by dividing the layers of BBU functionality. The DU is positioned closest to the RRU (RU) and connects to it via the fronthaul link. The new packet-based optical fiber link connecting the DU and CU is known as the “midhaul”. With processor intensive, non-real time functions shifted to more remote, centrally located CUs, resource utilization is optimized, and traffic is routed more efficiently.
RAN virtualization also allows the functional split between the DU and CU to be customized based on the 5G use case. Open RAN (O-RAN) is moving split architecture towards greater interoperability by defining transport conventions between open distributed units (O-DU), open centralized units (O-CU), and open radio units (O-RU) from different vendors. At the same time, the open, packet-based eCPRI interface streamlines data transfer between the DU and RU, replacing the less efficient CPRI serial transport mode.
What is the Function of a BBU?
In a split RAN architectural model, the functions assigned to the CU and DU are defined by the division of OSI layers. This model compliments 5G by providing more flexible load management, use case optimization, and scalability. CU/DU functions can also be moved close to the radio unit (RU) for use cases with high bandwidth demands and fixed user locations.
The VIAVI TeraVM O-CU Test DU Sim brings the benefits of O-DU emulation to the lab or production floor, allowing equipment developers and manufacturers to accurately simulate a wide range of Open RU (O-RU) and user equipment (UE) profiles with a scalable traffic load. The TeraVM also includes the industry’s first O-CU wraparound test that is compliant with the latest 3GPP and O-RAN standards.
Importance of Emulating When Installing Cell Sites
As capacity demands and device propagation increase, mobile operators are challenged to efficiently deploy new cell sites. Installation, commissioning, and site tuning phases each require a breadth of technical resources and expertise for construction, verification, and troubleshooting. New radios and antennas are mounted to the tower during the installation phase, but their functional testing cannot be completed until a BBU or DU is in place.
Baseband unit emulation provides a more efficient, parallel approach to cell site deployment by simulating the BBU transmission and RRU communication using advanced test equipment and without a commissioned BBU in place. Risky tower climbs can be minimized as the remote radio head (RRH) configuration, signal quality, link status, and antenna tilt are verified before entering the commissioning phase of site deployment.
This approach to RRU testing reduces OpEx by minimizing truck rolls and unforeseen issues prior to turn-up. With tower installation equipment and personnel still on site for any adjustment or troubleshooting, mean time to repair (MTTR) is reduced and cell site installation can be certified with confidence. The site tuning phase is minimized or eliminated with “diagnosis and repair” replaced by “final confirmation” of hardware and performance.
Challenges with Emulating
BBU emulation must continue to evolve and keep pace with new baseband features and capabilities accompanying 5G adoption. Each baseband unit configuration introduces unique variables to be emulated. The breadth of existing BBU form factors and the trends towards virtualization and disaggregation create challenges for test equipment and technicians performing BBU emulation.
Benefits of BBU Emulation
The benefits of BBU emulation that begin with increased efficiency and reduced overall deployment time extend to improved visibility and troubleshooting of active networks throughout their lifecycle. By simplifying the commissioning phase and virtually eliminating the tuning phase, BBU emulation dramatically reduces OpEx and time-to-market. Emulation has also become an important tool for base station maintenance and troubleshooting.
What is the Difference Between BBU and RRU?
Also known as the Remote Radio Head (RRH), the RRU is intrinsically linked to the BBU, despite the physical split of the wireless base station. Each RRU includes separate transmit and receive circuitry. When a signal is received from a nearby antenna, it is filtered, amplified, and converted to a digital format before being routed via fiber to the BBU. Conversely, digital signals from the BBU are converted to RF before being amplified and sent to the antenna for transmission.
Unlike the baseband unit, positioning at the top of the tower is essential for RRUs, since their function is more closely tied to antenna performance than the processing functions of the BBU or CU/DU combination. What began with 4G as a limited separation between the BBU cabinet and RRU atop the cell tower has now been expanded. BBUs are now being disaggregated, with their requisite elements positioned in remote locations or consolidated, often many kilometers away from the RRUs.
The Future of BBU Emulation
The deconstruction of the traditional base station is coinciding with increased schedule pressure for cell site deployment. New RAN test solutions incorporating DU or O-DU emulation must be highly responsive to evolving 5G standards and infrastructure. This includes the integration of digital I/O in radio units, eCPRI adoption, and the widespread deployment of beam forming and Massive MIMO.
Even as Open RAN drives much needed standardization for RU, CU, and DU suppliers, interoperability concerns will inevitably complicate emulation practices. This new reality highlights the value of baseband unit emulation. Cost and time saving opportunities abound during 5G RAN hardware and software development, as well as site installation and turn-up.
Emulating all Open RAN elements throughout the development cycle can head off interoperability issues and minimize surprises during site deployment. More testing and emulation than ever before will be necessary to incorporate multi-vendor O-RU, O-DU, and O-CU test vectors within a growing matrix of simulated real-world conditions. With industry leading BBU emulation capabilities for field, laboratory, and manufacturing applications, VIAVI is leading the way towards optimized and efficient cell site deployment.
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