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Radio access network

A radio access network (RAN) is a fundamental component of wireless telecommunications systems that connects end-user devices, such as smartphones and computers, to the core network through radio links, enabling , , and services. It forms the outermost layer of a , managing radio resources and facilitating seamless communication across coverage areas divided into cells. The RAN architecture typically includes key elements such as base stations (e.g., in LTE or gNodeB in ), which house radio transceivers; remote radio heads (RRHs) for signal amplification; baseband units (BBUs) for ; and antennas that transmit and receive radio waves. These components work together to handle functions like , , error detection, and , ensuring efficient spectrum use and between cells as users move. In modern implementations, RAN supports advanced technologies such as multiple-input multiple-output () antennas for higher data rates and for targeted signal direction. Historically, RAN evolved from analog systems introduced in 1979 to digital networks in 1991, with subsequent generations like (2001) enabling mobile internet, (2009) providing IP-based all-packet connectivity, and New Radio (NR) from 2018 offering speeds exceeding 1 Gbps via sub-6 GHz and millimeter-wave bands. This progression has shifted RAN toward and openness, including cloud RAN (C-RAN) for centralized processing and open RAN (O-RAN) standards to promote among vendors. In contemporary networks, particularly , RAN plays a pivotal role in supporting diverse applications like enhanced mobile broadband, ultra-reliable low-latency communications for autonomous vehicles, and massive machine-type communications for devices, while integrating with core networks via fronthaul and backhaul links using fiber optics or . Its scalability and energy efficiency are crucial for meeting growing demands, with innovations like (SDN) and (NFV) enabling network slicing for customized services.

Overview

Definition

A radio access network (RAN) is the component of a mobile telecommunication system that connects (UE), such as smartphones and (IoT) devices, to the core network through (RAT). It implements protocols for radio transmission and reception, managing the allocation and release of specific radio resources to establish connections between the UE and the network. The RAN ensures efficient use of the by handling transmission and reception within a set of cells. Positioned between the and the core network (), the RAN manages the air interface for communication and performs initial , such as and error correction. In contrast, the CN focuses on non-radio functions like packet routing, session management, and billing. This separation allows the RAN to optimize radio performance independently from the broader network operations. Central to the RAN are radio access technologies (RATs), which define the specific methods for transmission; examples include for second-generation () cellular networks and NR (New Radio) for fifth-generation () systems. The term "radio access network" was formalized within the standards in 1998, as part of the development of third-generation () mobile systems based on evolved core networks and the Universal Terrestrial Radio Access (UTRA).

Role in Mobile Networks

The radio access network (RAN) serves as the essential in mobile networks, enabling wireless connectivity by bridging such as smartphones and devices to the core (CN). It facilitates the conversion of analog radio signals from user devices into digital data packets that can be routed through the CN for further processing and transmission. This integration occurs primarily via backhaul connections, such as fiber optic or microwave links, which transport the digitized traffic from base stations to the CN, ensuring seamless end-to-end communication. A key function of the RAN is supporting user through mechanisms, which allow active connections to transfer between adjacent cells or base stations without interruption, maintaining service continuity as users move across coverage areas. This is managed through direct coordination between base stations via interfaces like X2 (in ) or Xn (in ), based on signal strength and load conditions. In terms of performance, the RAN directly influences critical metrics including , throughput, and coverage; for instance, advanced features like massive enable up to 10 times higher downlink throughput compared to previous generations, while improves coverage in dense urban environments. Additionally, the RAN enforces initial (QoS) parameters, such as prioritizing voice traffic over data to minimize for applications and ensuring reliable packet delivery for services like video streaming. Within the broader mobile ecosystem, the RAN interacts closely with the to support essential functions like user authentication and session routing, where it forwards authentication requests and routing information to the core network via interfaces such as N2 (in ) or S1 (in ), and coordinates with other RAN nodes via X2/. It also accommodates multi-radio access technology (multi-RAT) scenarios, including dual connectivity in networks, where devices simultaneously connect to 4G and base stations to aggregate resources for enhanced reliability and capacity. Economically, the RAN represents a major portion of mobile operators' capital expenditures, typically accounting for 45-70% of total network capex due to the costs of deploying and upgrading base stations and antennas, which drive overall infrastructure investments.

Components

Base Stations and Radio Units

Base stations serve as the primary at cell sites in a radio access network (RAN), responsible for transmitting and receiving radio signals to and from . In networks, these are known as evolved Node Bs (eNodeBs), which integrate (RF) and functions to manage air interface communications. In New Radio (NR), they are termed gNodeBs (gNBs), supporting enhanced capabilities such as higher data rates and lower latency through advanced RF technologies. These base stations are classified by coverage area and transmit power according to specifications, including wide area base stations (macrocells) for broad coverage, medium range (microcells) for intermediate urban densities, local area (picocells) for indoor or scenarios, and home base stations (femtocells) for residential use. Macrocells provide wide-area coverage, typically deployed on tall towers or rooftops to serve large populations, with no upper limit on output power for wide area base stations in TS 38.104, allowing configurations up to several hundred watts total across sectors to achieve cell radii of several kilometers. Microcells and picocells target denser environments like urban streets or buildings, with maximum conducted output powers of 38 dBm (about 6.3 W) for medium range and 24 dBm ( mW) for local area classes, enabling smaller cell sizes of tens to hundreds of meters. Femtocells, limited to 20 dBm (100 mW), are designed for in-home deployment to offload traffic from networks while minimizing . Radio units (RUs) within base stations handle the analog RF signal processing, including up-conversion, amplification, filtering, and modulation of signals before transmission via antennas. In 5G, RUs often integrate with massive multiple-input multiple-output (MIMO) antenna arrays, supporting configurations like 64 transmit and 64 receive elements (64T64R) to enable beamforming, where directional beams focus energy toward users for improved spectral efficiency and reduced interference. These units operate across frequency ranges defined in 3GPP, including sub-6 GHz bands (FR1, 410 MHz to 7.125 GHz) for balanced coverage and capacity, and millimeter-wave (mmWave) bands (FR2, 24.25 GHz to 71 GHz) for ultra-high throughput in short-range, dense deployments. Typical power amplifiers in macro RUs deliver 20-60 W per sector to support these bands, with active antenna systems combining RF components directly with the antenna array for compact integration. Deployment of base stations involves site acquisition, where operators secure land or rooftop leases compliant with local and environmental regulations to ensure . For macrocells, tower mounting elevates antennas 30-100 meters to maximize coverage, using monopoles, lattice towers, or rooftops with sector frames to support multiple directional antennas. mitigation is achieved through sectorization, dividing the cell into 3-6 sectors with narrow-beam antennas (e.g., 65-120 degrees horizontal beamwidth), which reduces by limiting signal overlap between adjacent cells and allows frequency reuse factors as low as 1. This configuration enhances capacity in high-traffic areas while the connect to units via fronthaul for .

Baseband Units and Processing

The baseband unit (BBU) serves as the core processing element in a radio access network (RAN), handling the required to manage communication between and the network core. It performs critical functions such as and of signals, converting digital data streams into formats suitable for radio transmission and vice versa. Additionally, the BBU manages coding and decoding operations, including (FEC) techniques like or low-density parity-check (LDPC) codes, to ensure reliable data transmission by detecting and correcting errors introduced during propagation. In traditional RAN architectures, the BBU is typically integrated at the site, providing localized processing for one or more radio units. At the physical (PHY) layer, the BBU executes Layer 1 processing tasks essential for modern wireless standards, particularly those employing (OFDM). This includes (FFT) for demodulation in the uplink and inverse fast Fourier transform (IFFT) for in the downlink, along with cyclic prefix addition to mitigate inter-symbol . Resource allocation is managed through schedulers within the BBU, which dynamically assign time-frequency resources to users based on factors like channel quality and traffic demands, optimizing overall network efficiency. These functions enable the BBU to support high and low-latency communications in dense environments. Virtualization of the BBU, known as the virtual BBU (vBBU), leverages (NFV) principles to run baseband software on (COTS) hardware, such as general-purpose servers, rather than proprietary equipment. This approach enhances scalability by allowing dynamic resource pooling and orchestration across multiple sites, reducing capital expenditures and enabling faster deployment of new features through software updates. In cloud RAN (C-RAN) deployments, vBBUs are centralized in data centers, supporting functional splits like Option 7-2 where higher-layer is separated from lower-layer tasks. Regarding , a single BBU can handle thousands of users per sector while delivering aggregate throughput on the order of tens of Gbps, scaling to 100 Gbps or more in advanced configurations with wide bandwidths and . For instance, with 100 MHz carrier bandwidth and multiple antenna streams, BBUs achieve peak sector throughputs exceeding 10 Gbps, accommodating high-density scenarios like urban mobility or applications. This processing power is crucial for meeting performance targets, though it demands efficient for real-time operations.

Backhaul and Fronthaul Connections

In radio access networks (RAN), backhaul refers to the transport links connecting the baseband unit (BBU) or distributed unit (DU) to the core network (CN), aggregating and routing user data, control signaling, and management traffic from multiple radio sites. These links typically employ fiber optic infrastructure in dense urban environments, supporting Ethernet-based capacities ranging from 10 Gbps to 100 Gbps or higher to handle the surge in data throughput driven by mobile traffic growth. In rural or remote areas where fiber deployment is cost-prohibitive, microwave radio systems serve as an alternative, delivering up to 20 Gbps over line-of-sight paths of several kilometers, though with limitations in adverse weather conditions. Fronthaul, in contrast, provides the high-bandwidth, low-delay interconnection between the remote radio unit (RU) and the BBU/DU, transporting digitized in-phase and quadrature (IQ) samples or processed baseband signals essential for centralized radio processing. For 5G deployments, the Common Public Radio Interface (CPRI) and its enhanced version, eCPRI, standardize these links, with eCPRI enabling packet-based Ethernet transport to reduce bandwidth overhead compared to the circuit-switched CPRI. Depending on the functional split in the RAN architecture, fronthaul capacity requirements can exceed 25 Gbps per sector for lower-layer splits involving high-resolution signal sampling, such as those with multiple antennas and wide bandwidths. These connections integrate with baseband units to support disaggregated processing in split RAN designs. Key performance requirements for fronthaul emphasize ultra-low to ensure timely signal reconstruction, typically under 1 ms one-way (often targeting 100–250 µs), which is critical for maintaining air interface timing in coordinated multipoint operations. Backhaul latencies are more tolerant, generally below 10 ms, to accommodate aggregation without significantly impacting end-to-end . across these links is vital for phase alignment and timing accuracy in RAN, achieved primarily through the (PTP) as specified in IEEE , which distributes sub-microsecond precision over packet networks, often combined with Synchronous Ethernet for frequency stability. The evolution of backhaul and fronthaul has progressed from (TDM)-based systems in , which offered limited scalability, to IP/Multi-Protocol Label Switching (MPLS) packet transport in and , facilitating statistical multiplexing, , and cost efficiencies in shared . This shift, alongside the adoption of (WDM) on fiber, has addressed escalating capacity demands, but challenges persist, including fiber scarcity in underserved regions that drives up deployment costs and limits centralized RAN viability.

Architectures

Traditional RAN

The traditional radio access network (RAN) architecture is characterized by a monolithic design, where the baseband (BBU) and radio (RU) are tightly integrated into a single provided by a single vendor. This integration typically occurs at the , with standardized interfaces such as the (CPRI) facilitating communication between the components. Major vendors like and have historically supplied these end-to-end solutions, ensuring optimized performance but limiting interoperability with third-party equipment. In this architecture, exemplified by the NodeB in 3G Universal Terrestrial Radio Access Network (UTRAN), the combines transmission/reception and processing functions, connected to the radio network controller via the Iub . Key features include site-specific installations tailored to local conditions and a high degree of vendor-specific customization, which often results in for operators. This closed ecosystem prioritizes seamless integration over openness, making upgrades or expansions dependent on the original supplier. The advantages of traditional RAN lie in its proven reliability and stability, with mature hardware delivering consistent performance in established networks, alongside simplified deployment and dedicated vendor support for maintenance. However, disadvantages include high costs for scaling due to proprietary equipment and the need for full replacements during upgrades, as well as limited multi-vendor that hinders flexibility. Historically, this architecture dominated through deployments worldwide until the early , forming the backbone of global mobile infrastructure before shifts toward more disaggregated designs.

Virtualized RAN

Virtualized radio access network (vRAN) represents a in RAN architecture by leveraging (NFV) and (SDN) to execute RAN software functions on (COTS) general-purpose servers, rather than dedicated hardware appliances. This approach decouples software from proprietary hardware, enabling greater flexibility in deployment and management. As a precursor, cloud RAN (C-RAN) centralized baseband unit (BBU) processing in data centers to pool resources across multiple cell sites, reducing redundancy and improving efficiency in handling traffic variations. A key enabler of vRAN is the use of functional splits defined by the 3rd Generation Partnership Project (3GPP), which divide RAN processing between centralized and distributed units to optimize transport requirements and resource sharing. For instance, 3GPP Option 2 splits the packet data convergence protocol (PDCP) and radio link control (RLC) layers, allowing higher-layer functions to be virtualized and pooled in a central cloud while lower layers remain closer to the radio units. These splits facilitate efficient resource pooling for multiple sites, supporting dynamic allocation and scalability in response to varying network demands. The benefits of vRAN include significant cost reductions through the adoption of COTS hardware, with potential (TCO) savings of up to 25% for centralized RAN architectures compared to traditional deployments. Additionally, simplifies network upgrades and enables orchestration using container platforms like , allowing automated scaling, deployment, and management of RAN functions across cloud-native environments. This integration also supports by distributing virtualized functions closer to the network edge for low-latency applications. Major mobile network operators have adopted vRAN to modernize their infrastructures, with initiating virtualization efforts as early as 2013 and progressing to commercial Cloud RAN deployments in partnership with vendors like by 2024. Similarly, has incorporated vRAN in its expansions, leveraging centralized processing for enhanced performance and cost efficiency. These deployments demonstrate vRAN's role in enabling scalable, software-driven networks that align with evolving demands for and beyond.

Open RAN

Open RAN represents a disaggregated and interoperable approach to radio access networks, emphasizing open interfaces to foster vendor diversity and innovation in infrastructure. The O-RAN Alliance, established in 2018, defines Open RAN as an open and intelligent RAN architecture that promotes a broad industry ecosystem through standardized specifications for components and interfaces. This paradigm builds on principles by introducing openness, allowing multi-vendor integration while enabling intelligent control via the RAN Intelligent Controller (RIC), which incorporates and for real-time optimization and automation. Key components in Open RAN include the disaggregated Radio Unit (RU), Distributed Unit (DU), and Centralized Unit (CU), which separate hardware and software functions to enhance flexibility. The RU handles radio frequency transmission and reception, while the DU processes lower-layer baseband functions; the CU, often split into Control Plane (O-CU-CP) and User Plane (O-CU-UP) components, manages higher-layer protocols. Connectivity between these elements relies on open interfaces such as eCPRI for the fronthaul link between RU and DU, and the F1 interface for the split between CU and DU, ensuring interoperability across vendors. The RIC further integrates via interfaces like E2 for near-real-time decisions, supporting AI/ML-driven policies for traffic management and resource allocation. One primary advantage of Open RAN is the reduction of , enabling operators to select best-of-breed components from multiple suppliers, which lowers costs and accelerates through faster deployment cycles and participation. For instance, since 2020, has conducted extensive trials and deployments of multi-vendor Open RAN elements, including the first commercial multi-vendor O-RAN-based in 2024, demonstrating improved resilience and adaptability. Similarly, pioneered a large-scale Open RAN 5G buildout starting in 2020, integrating components from various vendors to create a cloud-native , though it faced financial difficulties, regulatory pressures, and scaling challenges, culminating in a spectrum sale to and the start of decommissioning in late 2025 (ongoing as of November 2025). Despite these benefits, Open RAN faces challenges, particularly in testing, where ensuring seamless performance across diverse vendor components requires rigorous validation of interfaces and protocols. As of 2025, Open RAN accounts for approximately 5-10% of the overall RAN market, with growing incorporation in new deployments driven by operators seeking enhanced flexibility.

Generations

2G and 3G RAN

The second-generation () radio access network (RAN), primarily based on the (GSM) and its enhancement (GPRS), was structured as the (BSS). The BSS comprised Base Transceiver Stations (BTS) for handling radio transmission and reception, and Base Station Controllers (BSC) for managing radio resource allocation, handover, and signaling across multiple BTS units. GSM employed (TDMA) combined with (FDMA) as its primary multiple access schemes, enabling efficient spectrum use for circuit-switched voice services. With the introduction of GPRS, packet-switched data capabilities were added, achieving a theoretical maximum data rate of approximately 171 kbit/s using eight time slots in the downlink. The third-generation (3G) RAN evolved to the UMTS Terrestrial Radio Access Network (UTRAN), featuring elements—analogous to —for radio frequency transmission and Radio Network Controllers (RNC) for centralized control of radio resources, , and interfacing with the core network. UTRAN utilized Wideband (WCDMA), a form of direct-sequence CDMA, to support higher capacity and better interference management compared to approaches. Basic UMTS provided circuit- and packet-switched services with a peak data rate of up to 2 Mbit/s, while enhancements like High-Speed Downlink Packet Access (HSDPA) improved packet data efficiency through adaptive modulation and faster scheduling, enabling downlink speeds beyond initial capabilities. Key evolutions from to shifted the RAN focus from predominantly voice-centric, circuit-switched operations in to integrated voice and higher-speed data services in , with GPRS marking the initial packet data transition and UTRAN enabling broadband-like internet access. A significant advancement in was the introduction of soft , where a maintains simultaneous connections to multiple Node Bs during transitions, reducing call drops and improving mobility in dense environments through macro-diversity combining. By 2025, and RAN deployments have been largely phased out in most urban and developed regions to reallocate for and , with 61 networks scheduled for shutdown that year alone as part of 131 total retirements by 2030. However, these legacy networks continue to support low-bandwidth (IoT) applications, such as remote metering and , in rural and underserved areas where advanced infrastructure rollout remains limited.

4G LTE RAN

The 4G Long-Term Evolution (LTE) Radio Access Network (RAN), formally known as the Evolved Universal Terrestrial Radio Access Network (E-UTRAN), marks a pivotal shift to an all-IP, packet-switched architecture optimized for high-speed mobile broadband, contrasting with the circuit-switched emphasis of prior generations. This design enables efficient data delivery for multimedia applications, with theoretical peak downlink speeds surpassing 100 Mbps under typical configurations. The E-UTRAN simplifies the network hierarchy by integrating functions traditionally handled by separate Base Station Controller (BSC) and Radio Network Controller (RNC) entities into a single logical node, the evolved Node B (eNodeB), which manages radio resource control, handover, and scheduling directly. This flattened structure reduces latency and enhances scalability, supporting seamless mobility across cells. At the physical layer, E-UTRAN employs (OFDM) for the downlink to combat multipath fading and achieve high , while Single-Carrier (SC-FDMA) is used for the uplink to minimize peak-to-average power ratio for battery efficiency. These schemes, combined with flexible options from 1.4 MHz to 20 MHz, facilitate downlink throughputs over 100 Mbps in 20 MHz channels. Key features include Multiple Input Multiple Output () technology, supporting up to 8x8 configurations in the downlink for enhanced capacity and reliability through and diversity. Carrier aggregation further boosts performance by combining multiple component carriers—up to five in LTE-Advanced—allowing effective s up to 100 MHz and proportional increases in data rates. Additionally, Self-Organizing Networks () automate through self-configuration of new eNodeBs, self-optimization of parameters like thresholds, and self-healing for fault recovery, reducing operational costs. LTE-Advanced, standardized in 3GPP Release 10, extends these capabilities with peak data rates approaching 1 Gbps in the downlink via advanced MIMO, carrier aggregation, and coordinated multipoint transmission, enabling gigabit-class mobile broadband. Voice services transitioned to Voice over LTE (VoLTE), which leverages IP Multimedia Subsystem (IMS) for high-definition voice delivery over the LTE packet core, supporting simultaneous voice and data without circuit-switched fallback. LTE dominated global mobile networks through the 2010s and into the 2020s, achieving approximately 90% population coverage worldwide by 2025 as the foundational technology for 4G broadband.

5G NR RAN

The 5G New Radio (NR) Radio Access Network (RAN), known as NG-RAN, represents a significant evolution in wireless infrastructure, designed to support a wide array of services through enhanced flexibility and efficiency. At its core, NG-RAN consists of gNodeB (gNB) base stations, which can be disaggregated into a Centralized (CU) and Distributed Unit (DU) to optimize resource allocation and scalability. The CU handles higher-layer functions such as (RRC) and (PDCP), while the DU manages lower-layer processing like (MAC) and (PHY), connected via the F1 interface. This split , standardized by in Release 15, enables and cloud-native deployments, improving operational efficiency in diverse environments. The NR air interface underpins NG-RAN's capabilities, featuring flexible numerology that allows subcarrier spacing to vary from 15 kHz to 240 kHz, accommodating different frequency bands and service requirements. This adaptability supports channel bandwidths up to 400 MHz in mmWave spectrum, enabling peak downlink data rates of up to 20 Gbps under ideal conditions. Key enablers include Massive MIMO, which deploys hundreds of antennas at the base station to serve multiple users simultaneously with spatial multiplexing; advanced beamforming techniques that direct signals precisely to improve coverage and reduce interference; and mmWave frequencies (above 24 GHz) for ultra-high throughput in dense urban areas. Additionally, network slicing allows the creation of isolated logical networks on shared infrastructure, each tailored with specific Quality of Service (QoS) parameters like latency and reliability, as defined in 3GPP TS 23.501. NG-RAN supports three primary use cases outlined by ITU-R M.2410: enhanced Mobile Broadband (eMBB) for high-data-rate applications like 4K/8K video streaming, achieving user-experienced speeds up to 100 Mbps; Ultra-Reliable Low-Latency Communications (URLLC) targeting end-to-end latency below 1 ms and reliability over 99.999% for mission-critical scenarios such as autonomous vehicles and industrial automation; and massive Machine-Type Communications (mMTC) enabling connectivity for up to 1 million devices per square kilometer in IoT deployments like smart cities. By November 2025, standalone (SA) 5G deployments have surpassed 60 commercial networks globally, with projections for over a dozen additional launches, marking significant progress toward full 5G core integration. Furthermore, NG-RAN facilitates seamless integration with non-3GPP accesses like Wi-Fi through the Non-3GPP Interworking Function (N3IWF), which establishes secure IPsec tunnels to the 5G core, enhancing coverage in hybrid environments.

Beyond 5G Developments

The vision for radio access networks (RANs) emphasizes (THz) frequencies to enable unprecedented data rates and connectivity densities, building on the foundational spectrum extensions explored in . THz bands, spanning 0.1 to 10 THz, offer vast bandwidths potentially exceeding hundreds of GHz, facilitating applications like holographic communications and immersive . Research indicates that sub-THz communications (100-300 GHz) could achieve peak rates of up to 1 Tbps over short distances, addressing the in data demands from AI-driven services and massive ecosystems. AI-native RAN architectures represent a core pillar of , where is embedded from the design phase to enable autonomous optimization of network resources, , and interference management. Unlike prior generations, AI-native designs integrate models directly into the RAN stack for real-time decision-making, such as predictive traffic routing and energy-efficient allocation, potentially improving by 20-50% in dynamic environments. Integrated sensing and communication (ISAC) further enhances this by merging radar-like sensing with data transmission, allowing RANs to simultaneously detect environmental changes and communicate, which supports applications in autonomous vehicles and smart cities while utilizing the same THz for dual purposes. Advanced antenna technologies like holographic and orbital angular momentum (OAM) are poised to revolutionize spatial . Holographic employs metasurface-based arrays to create dynamic, programmable radiation patterns, enabling ultra-massive with thousands of elements for precise beam control in THz bands. OAM, which exploits the helical phase structure of electromagnetic waves, provides an additional degree of freedom for orthogonal modes, potentially increasing by integrating with sub-THz channels without additional . Experimental demonstrations have shown OAM achieving multi-Gbps rates in near-field scenarios, with potential to Tbps in backhaul links. Ongoing research initiatives are steering RAN development toward commercialization by 2030. The 3GPP's Release 18 and subsequent releases (Rel-19 onward) lay the groundwork for by enhancing 5G-Advanced features like / integration in the air interface, with early studies focusing on THz feasibility and non-terrestrial networks (NTN) starting in Rel-20 around 2026. The European Union's Hexa-X project, a flagship initiative involving over 60 partners, targets a sustainable platform with key enablers like sub-THz transceivers and ISAC, aiming for deployment readiness by 2030 through collaborative trials on -optimized RAN fabrics. Emerging trends in 6G RAN include -driven optimization for self-healing networks and seamless satellite-terrestrial integration to achieve ubiquitous coverage. algorithms will enable and resource orchestration across hybrid architectures, reducing to microsecond levels for time-sensitive applications. Satellite-terrestrial , leveraging low-Earth orbit () constellations with terrestrial RANs, promises global coverage with integrated backhaul, where facilitates handovers and spectrum sharing between space and ground segments, targeting zero-coverage gaps in remote areas.

Protocols and Standards

Key Protocols

The radio access network (RAN) employs a layered to manage the air interface between (UE) and base stations, ensuring reliable data transmission, , and connection control. This stack, defined by the 3rd Generation Partnership Project (3GPP), is divided into physical (PHY), (MAC), (RLC), (PDCP), and (RRC) layers, with adaptations across generations for enhanced efficiency and performance. The PHY layer (Layer 1) handles the physical of data over the radio , including coding for , as well as schemes that map digital data to analog signals. In New Radio (NR), supported formats include quadrature (QPSK), 16-quadrature amplitude (16QAM), 64QAM, and 256QAM, enabling higher in favorable conditions while QPSK provides robustness in poor signal environments. (HARQ) operates at this layer to combine with retransmissions; upon detecting errors via cyclic redundancy checks, the receiver requests retransmissions of specific transport blocks, improving throughput in channels compared to pure ARQ. Layer 2 encompasses the MAC, RLC, and PDCP sublayers, which manage data flow, reliability, and security. The MAC sublayer performs scheduling to allocate radio resources dynamically based on UE needs and channel quality, multiplexes logical channels into transport channels, and handles priority queuing for diverse traffic types. The RLC sublayer provides segmentation and reassembly of data units, ensuring in-sequence delivery through acknowledged mode operations, while also supporting automatic repeat request (ARQ) for error recovery beyond PHY-level HARQ. The PDCP sublayer adds security via ciphering and integrity protection, performs header compression to reduce overhead (e.g., Robust Header Compression for IP packets), and enables robust header compression release for seamless mobility. The RRC layer, part of Layer 3, oversees , including establishment, reconfiguration, and release of radio bearers, as well as broadcast of system information and procedures. It coordinates UE states (e.g., idle, connected) and triggers measurements for decisions, ensuring efficient resource utilization across the network. Key air interface protocols facilitate initial access and . The channel (RACH) procedure allows to synchronize with the network and request resources; in , it involves a four-step contention-based process where the UE sends a , receives a response with , submits an identity, and resolves contention via a . procedures maintain during , involving measurement reporting by the UE, decision by the source , and execution through reconfiguration messages to minimize interruption (typically under 50 ms in ). For inter-base station coordination in , the X2 interface enables direct signaling between eNodeBs for preparation, resource status updates, and , reducing compared to core network routing. RAN protocols have evolved significantly from 3G Universal Mobile Telecommunications System (UMTS), where access stratum user plane protocols emphasized circuit-switched services with dedicated channels, to 5G NR's service-based architecture that prioritizes all-IP packet-switched transport, flexible numerology, and cloud-native integration for ultra-reliable low-latency communications. This progression, spanning 3GPP Releases 99 (3G) through 15+ (5G), has shifted from asynchronous transfer mode influences in 3G to OFDM-based waveforms and network slicing in 5G, enhancing scalability for diverse applications.

Standardization Bodies and Interfaces

The primary standardization body for radio access networks (RAN) is the 3rd Generation Partnership Project (3GPP), a collaborative organization comprising seven telecommunications standards development entities that has defined specifications for mobile technologies from GSM evolution to 6G. 3GPP's work ensures interoperability and global deployment of RAN architectures, starting with early enhancements to 2G systems and progressing through 3G, 4G LTE, and 5G NR. Complementing 3GPP, the International Telecommunication Union Radiocommunication Sector (ITU-R) manages global radio-frequency spectrum allocation and harmonization, defining International Mobile Telecommunications (IMT) requirements that guide RAN spectrum usage for international compatibility and efficient resource sharing. For open and disaggregated RAN, the O-RAN Alliance develops specifications that promote vendor-neutral interfaces and intelligent control, building on 3GPP standards to enable multi-vendor interoperability. Key interfaces standardized by these bodies facilitate RAN connectivity and . In 3GPP specifications, the S1 interface connects the RAN (e.g., eNB in ) to the core network (), supporting control and user plane signaling for and session . The Xn interface enables direct communication between RAN nodes (e.g., gNBs in ) for and coordination, enhancing inter-RAN efficiency without core network involvement. Within the O-RAN framework, the E2 links the near-real-time RAN Intelligent Controller () to RAN elements like distributed units () and central units (CUs), allowing dynamic policy enforcement and optimization. 3GPP's release evolution has shaped RAN standards since Release 99 (R99), which introduced initial UMTS features including the UTRAN architecture for wideband CDMA. Subsequent releases progressed through enhancements in Releases 4-8 for evolution, Releases 8-14 for LTE with OFDMA-based RAN, and Releases 15-17 for , incorporating massive , , and URLLC support. These releases include harmonization efforts, such as aligned spectrum bands and protocol profiles under IMT guidelines, to enable seamless global roaming and device compatibility across operators. As of 2025, Release 18 (Rel-18), branded as 5G-Advanced, was completed with specification freeze in mid-2024, focusing on enhancing RAN capabilities with / integration for resource optimization, support, and non-terrestrial network integration, while laying groundwork for through study items on new use cases. Ongoing work in 2025 centers on Release 19, the second phase of 5G-Advanced, which introduces further enhancements such as / at the for improved efficiency and spectrum use, enhanced positioning, and additional features for industrial and automotive applications, continuing to bridge 5G toward future IMT-2030 systems under .

Deployments and Variations

Global Deployments

By the end of 2025, radio access networks (RAN) have been deployed in 191 countries and territories worldwide, with 647 operators actively investing in the technology. Globally, the number of operational base stations exceeds 5 million, enabling coverage for approximately one-third of the world's population. leads this expansion, operating nearly 4.71 million base stations as of September 2025, which accounts for a significant portion of the global total and supports widespread urban and rural connectivity. A prominent in urban 5G deployment is Verizon's use of millimeter-wave (mmWave) , targeting dense city environments to deliver high-speed, low-latency services for applications like access and enterprise solutions. This approach has enhanced user experiences in major metropolitan areas by leveraging mmWave's capacity for ultra-high throughput, though it requires dense small-cell to mitigate challenges. In contrast, European deployments emphasize shared models to extend and coverage to rural areas, where population sparsity increases costs; neutral host networks, for instance, allow multiple operators to share towers and backhaul, reducing deployment expenses by 10-50% and improving access in underserved regions. Such models have been evaluated in countries like and , demonstrating higher profitability through collaborative site sharing. The global RAN vendor landscape remains concentrated, with and dominating approximately 60% of the in the first half of 2025, particularly in regions outside where geopolitical factors influence selections. holds the top position in three of five major geographical regions, while leads in business performance metrics. A notable shift toward multi-vendor environments is occurring through Open RAN adoption, which has stabilized after initial challenges, enabling operators to integrate equipment from diverse suppliers and reduce dependency on single vendors. Spectrum allocation via auctions has profoundly shaped 5G RAN deployments, with the C-band (3.7-4.2 GHz) emerging as a critical mid-band resource for balancing coverage and capacity. In the United States, the Federal Communications Commission's C-band auction generated over $81 billion, funding extensive mid-band 5G rollouts that enhanced nationwide performance. Globally, harmonized C-band allocations in and have promoted , though variations in auction designs—such as those in and —have led to differing efficiencies in spectrum utilization and operator investments.

Regional Variations

In the United States, the radio access network (RAN) market is characterized by a diverse vendor landscape dominated by , which holds over 50% as of 2025, alongside significant contributions from and . This mix reflects strategic operator choices amid geopolitical restrictions on certain foreign vendors, with and leading deployments for major carriers like and . A notable innovation is the Open RAN pilots led by DISH Wireless, which received a $50 million U.S. Department of Commerce grant in 2024 to establish an integration and deployment center, enabling multi-vendor, cloud-native networks that reduce dependency on proprietary hardware. The (FCC) has further supported high-capacity deployments through policies favoring millimeter wave (mmWave) spectrum, including a 2025 overhaul of rules for bands like 24 GHz, 28 GHz, and 37 GHz to facilitate sharing between federal and non-federal users, promoting rapid rollout in urban areas. Europe emphasizes collaborative and regulated RAN approaches, with the promoting shared infrastructure through host models to optimize costs and coverage. hosts, independent entities that deploy and manage shared RAN assets like indoor , have been pioneered in projects such as Ericsson's 2023 rollout with Proptivity, the world's first host-led shared indoor RAN, enabling multiple operators to access unified networks without ownership. These models can reduce densification costs by up to 47% in urban settings, aligning with goals for efficient spectrum use. Security in European RANs is heavily influenced by the General Data Protection Regulation (GDPR), which mandates robust processing safeguards for in , including and breach notifications that extend to network elements to protect user privacy across shared infrastructures. has emerged as a leader in Standalone (SA) deployments here, powering networks for operators like Telia across the Nordics and Baltics with cloud-native cores, and launching Europe's first private SA with Boldyn in 2025, supporting advanced services like low-latency . In , RAN variations are shaped by national priorities, with exemplifying state-led acceleration where maintains dominance, securing 52% of China Mobile's $1.1 billion contract from 2023 to 2024 through government-backed investments in domestic technology. This approach has enabled widespread Huawei-powered base stations, contributing to nearly 4.7 million sites nationwide as of September 2025. In contrast, India's deployments focus on affordability, led by Reliance Jio's homegrown stack using indigenous gear for low-cost rollout, serving 234 million subscribers with speeds up to 1 Gbps on the 700 MHz band for deep coverage. Jio's end-to-end in-house development reduces operational expenses by minimizing foreign vendor reliance, positioning it for to emerging markets. Beyond these major regions, Africa's RAN landscape relies heavily on microwave backhaul due to geographic and infrastructural challenges, with approximately 80% of base stations using it for cost-effective in rural areas where is uneconomical. This technology supports and early extensions across vast terrains, as seen in South Africa's 176,000 links generating R8.3 billion annually. In , economic factors have sustained dominance, with slower adoption driven by high deployment costs and modest GDP growth projected at 2.2% for 2025, limiting investments in auctions and upgrades. technologies still contribute 8.2% to regional GDP, but networks remain the backbone for the majority of connections amid fiscal constraints.

Challenges and Future Trends

Technical Challenges

One of the primary technical challenges in radio access network (RAN) design is achieving balanced coverage and capacity across diverse environments. In urban areas, densification through and mmWave frequencies enables high-capacity deployments but exacerbates due to reduced inter-cell distances and susceptibility to blockage by obstacles like buildings. This in dense mmWave setups requires advanced mitigation techniques, such as coordinated , to maintain signal quality amid overlapping transmissions. Conversely, rural areas face persistent coverage gaps stemming from sparse population densities and challenging backhaul requirements, where achieving 5-10 Gbps over distances of 20-60 km with limited channels proves difficult without costly fiber alternatives. Scalability poses another significant hurdle as mobile traffic surges, with global data volumes projected to increase fourfold by 2025 driven by adoption and new applications. The RAN, which consumes approximately 73-80% of a mobile network's total energy, struggles to handle this growth without proportional rises in power usage, as base stations alone account for 80% of RAN power draw. This inefficiency is compounded by the need for massive and proliferation, which, while boosting capacity, elevate operational costs and environmental impact unless offset by dynamic power-saving mechanisms. Integration challenges arise from interworking legacy systems, particularly in 5G non-standalone (NSA) deployments reliant on 4G LTE cores, which can lead to suboptimal performance and coverage inconsistencies due to mismatched . Enhancing spectrum efficiency through dynamic time division duplexing (TDD) in helps allocate uplink and downlink resources flexibly, but it demands precise to avoid cross-link between neighboring cells. Supply chain disruptions from the early , exacerbated by shortages and geopolitical tensions, led to significant contraction in the global RAN market in due to delayed equipment availability. However, as of mid-2025, the market has stabilized, with growth observed outside . Concurrently, preparations for quantum-safe encryption in RAN are underway, with and advancing standards for to protect against future quantum threats to protocols in architectures.

Security and Innovations

Radio access networks (RANs) face significant security threats from legacy protocols and modern architectures. In legacy systems, the SS7 signaling protocol, designed without or , enables attacks such as location tracking via HLR/VLR queries, call and interception, and through service abuse. These vulnerabilities persist in transitional networks, where elements may still interface with SS7-based core systems. In RAN, slicing introduces risks including failures, cross-slice attacks, and lateral movement due to errors or vulnerabilities, potentially compromising multiple virtual networks simultaneously. Supply chain risks further exacerbate these issues, as seen in international bans on equipment citing concerns over potential backdoors and in RAN hardware and software. Mitigation strategies have evolved through standardization and architectural shifts. The has enhanced security in Releases 15 and beyond with features like Subscription Concealed Identifier (SUCI) encryption, which conceals the Subscription Permanent Identifier (SUPI) using to prevent and identity mapping attacks during registration. These protections extend to and for user data in RAN interfaces. In Open RAN deployments, zero-trust architectures (ZTA) are increasingly implemented, enforcing continuous verification, micro-segmentation, and least-privilege access across disaggregated components to counter insider threats and supply chain compromises. ZTA maps controls to O-RAN functions like the and near-RT RIC, enabling incremental security adoption. Innovations in RAN leverage artificial intelligence (AI) and emerging technologies to enhance reliability and performance. AI and machine learning (ML) enable predictive maintenance by analyzing telemetry data from base stations to forecast failures, optimize resource allocation, and minimize outages, potentially reducing operational costs by detecting anomalies in real time. Edge AI integration within the RAN Intelligent Controller (RIC) supports near-real-time decisions, such as dynamic spectrum management and traffic steering, by processing data at the network edge for low-latency automation. Quantum computing integration, focused on post-quantum cryptography (PQC), is projected for widespread RAN adoption by 2030 to safeguard against quantum attacks on current encryption; 3GPP and GSMA recommend transitioning network equipment to quantum-resistant algorithms starting in 2026, with full compliance by 2030. This includes hybrid schemes combining classical and PQC keys for RAN air interfaces and backhaul. Emerging trends emphasize and expanded coverage in RAN evolution. Green RAN initiatives incorporate advanced sleep modes for base stations, activating during low-traffic periods to achieve power reductions of up to 30%, thereby lowering the of dense deployments. These modes, combined with AI-driven optimization, target overall gains in O-RAN architectures. The fusion of non-terrestrial networks (NTN) with terrestrial RAN, as standardized by Release 17, integrates and high-altitude platforms to extend seamless coverage, using O-RAN interfaces for hybrid topologies that support and broadband in remote areas. This convergence enables regenerative payloads for processing at orbital nodes, reducing in global networks.