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Handover

Handover, also known as handoff, is the process in mobile telecommunications networks by which an ongoing call, data session, or connection is seamlessly transferred from one radio channel, , or to another as a moves between coverage areas, ensuring uninterrupted service continuity. This mechanism is fundamental to cellular systems, enabling user mobility without service disruption in technologies ranging from to modern networks, where it supports high-speed data transfer and low-latency applications. Handovers are triggered by factors such as signal strength degradation, , or requirements, with the network evaluating metrics like received signal code power (RSCP) or (SIR) to initiate the switch. Handovers are broadly classified into hard handover, where the connection to the source is broken before establishing the target link (common in and for inter-frequency transfers), and soft handover, which allows simultaneous connections to multiple cells during the transition for smoother operation (prevalent in CDMA-based systems like ). Additional variants include softer handover within the same in and advanced features like conditional handover, where the device pre-configures multiple targets to reduce failure rates in high-mobility scenarios. Further categorizations encompass network-controlled, mobile-assisted, and mobile-controlled handovers, depending on the degree of involvement from the () or core network elements as defined in standards.

Fundamentals

Definition

In , particularly within cellular networks, handover refers to the process of transferring an ongoing call, data session, or other from one radio channel or to another, ensuring the maintenance of service continuity and defined (QoS). This transfer occurs as a moves between coverage areas, preventing service interruption by seamlessly reassigning the to a more suitable serving point. The manages this change in radio transmitters, access modes, or systems while preserving the bearer service characteristics. The primary components involved in a handover include the source (the current serving cell), the target (the new serving cell), the —often termed () or —and core network elements such as the () or equivalent, which orchestrate signaling and resource allocation across the network. The source monitors the connection, while the target prepares resources; the reports measurements or receives instructions; and the core network facilitates the handover decision and execution to ensure compatibility and minimal latency. Handover was first commercially implemented in analog cellular systems with the launch of the world's first cellular network by (NTT) in , , in 1979. The (AMPS), developed by and commercially launched in the United States in 1983, marked a pivotal development in by introducing automated handoff mechanisms to address the limitations of fixed-location , laying the groundwork for modern mobile networks. The basic handover process flow generally comprises four stages: detection of the need, where signal strength or quality from the source degrades below a , prompting by the or device; selection of the target, involving reports from the and evaluation of candidate cells based on criteria like received signal strength; execution of the transfer, during which the core signals the target to allocate resources and instructs the to switch connections; and completion, verifying the new link's stability and releasing old resources with negligible disruption, often under 100-200 milliseconds in early systems like .

Terminology

In mobile telecommunications, the terms "" and "handoff" both refer to the process of seamlessly transferring an active connection from one serving or to another as a moves across coverage areas, ensuring continuity of service. The term "" is the standard nomenclature in specifications and European-centric standards, reflecting its origins in systems like , while "handoff" is more prevalent in IEEE standards and North American contexts, such as in early and documentation. Related concepts include cell reselection, which occurs in idle mode where the (UE) autonomously evaluates and camps on a suitable based on signal quality criteria like signal strength (S) exceeding a minimum threshold (S > 0), without involving active data transfer or network-directed control. This contrasts with , which is a network-initiated procedure in connected mode to maintain ongoing sessions. Another term, redirection, describes a network command to instruct the UE to move to a different , frequency, or (), typically resulting in a temporary service interruption rather than seamless continuity. Key acronyms in handover discussions include (User Equipment), denoting the mobile device accessing the network; BS (Base Station), a general term for the radio access node providing coverage; and gNB (next generation Node B), the 5G-specific base station entity responsible for radio resource management and handover execution in NG-RAN architectures. The terminology has evolved alongside cellular generations: "handoff" dominated in 1G analog systems like AMPS for basic voice continuity, transitioning to "handover" in 2G digital networks like GSM for automated cell transfers, and expanding in 3G and beyond to terms like "inter-RAT handover" to address mobility across heterogeneous technologies such as UMTS to LTE or 5G NR.

Purpose

The primary goals of handover in cellular networks are to maintain connection quality as user equipment (UE) moves beyond the coverage of the serving cell, to balance load across cells by redistributing traffic, and to enable seamless mobility throughout the network. By transferring an active session from one base station to another, handover ensures the UE remains connected to the cell providing the strongest signal, preventing degradation in voice or data services during movement. This process is essential for supporting continuous communication in dynamic environments, such as urban areas or highways, where signal strength fluctuates due to distance, interference, or obstacles. Key benefits of handover include reducing call drops and data interruptions by proactively switching connections before service loss occurs, supporting high-speed data transmission for users in moving vehicles, and optimizing across the network. For instance, effective handover mechanisms minimize and spikes, which is critical for applications like video streaming or vehicular communications. In load-heavy scenarios, handover facilitates traffic offloading to underutilized cells, enhancing overall spectrum efficiency and without requiring additional . Handover integrates with core network mobility management entities, such as the Mobile Switching Center () in 2G systems for circuit-switched operations and the Access and Mobility Management Function (AMF) in 5G for handling registration updates and session continuity during transfers. This integration allows the network to coordinate signaling and resource reservation, ensuring uninterrupted service across generations. Quantitatively, handover extends effective coverage beyond single-cell limits, from approximately 1 km radii in early deployments to dynamic ranges in modern mmWave systems, where cells often span only 100-200 meters but rely on frequent handovers for broader mobility support.

Types

Hard Handover

Hard handover, also known as break-before-make handover, is a process in mobile networks where the connection to the serving cell is terminated before establishing a new connection to the target cell, resulting in a brief interruption of service. This approach contrasts with make-before-break methods and is particularly suited to (FDMA) and (TDMA) systems, where simultaneous connections to multiple cells are not feasible due to hardware limitations. The hard process in networks typically involves the following steps, controlled primarily by the base station controller (BSC) or (MSC). First, the (MS) monitors signal strength from neighboring cells and reports measurements to the serving (BTS) and BSC when thresholds are met. The BSC then decides on the handover, allocates resources in the target cell, and sends a handover command to the MS via the serving BTS. The MS breaks the current connection, tunes to the target cell's , accesses the new using a handover reference, and sends a handover complete message upon successful . Finally, the network releases resources from the old cell and confirms the handover. One key advantage of hard handover is its simpler and lower , as it requires only a single active channel at any time, reducing the need for advanced capabilities in the . This makes it cost-effective for deployment in systems. However, disadvantages include potential service disruptions lasting approximately 200-500 milliseconds, increased risk of call drops due to the interruption, and higher , which can affect applications. Hard handover is predominant in networks, where it supports seamless mobility across cells using TDMA/FDMA air interfaces, ensuring efficient spectrum use despite the brief outage.

Soft and Softer Handover

Soft handover is a handover mechanism in (CDMA)-based systems where the () maintains simultaneous connections to multiple cells operating on the same , with signals from these cells combined at side to enhance reliability. This approach, known as make-before-break, allows the addition of a new radio link before the removal of the existing one, ensuring continuous without interruption. In the uplink direction, the transmits a single waveform that is received and processed by multiple base stations, while in the downlink, transmits the same data over multiple paths, which the combines using a to exploit multipath diversity. Softer handover represents a specialized variant of soft handover, occurring between sectors of the same ( in terminology), where is performed more simply within the base station itself rather than at a higher network level like the controller (RNC). This intra- process leverages the closer of sectors under a single site, reducing the complexity of frame alignment compared to inter- soft handover. The in softer handover similarly despreads and combines multipath components from different sectors, but the control is managed locally by the , enabling faster execution. The primary advantages of soft and softer handover stem from diversity gains, which mitigate and by selecting or combining the strongest signal paths, thereby reducing bit error rates and improving overall link quality. In ideal conditions, such as synchronized cells with low , these mechanisms can achieve very low handover failure rates. These handovers are primarily applied within the same (), such as intra-system scenarios in UTRAN. Soft and softer handover are foundational to third-generation () systems, serving as core features in Universal Mobile Telecommunications System () and CDMA2000 networks to support seamless mobility in macrodiversity environments. In UMTS, they enable efficient resource utilization in overlapping cell coverage areas, while in CDMA2000, they similarly enhance coverage and capacity through soft handoff procedures.

Vertical Handover

Vertical handover refers to the process by which a mobile station transfers its active connection between disparate radio access technologies (s), such as from cellular to WLAN, necessitating reconfiguration at multiple protocol layers to maintain service continuity. Unlike horizontal handovers within the same , vertical handover decisions are typically policy-driven, incorporating factors like received signal strength, user mobility patterns, application requirements, and network load to select the optimal . The vertical handover process encompasses several key stages: network discovery, where the mobile device scans for available heterogeneous networks and gathers metrics such as signal quality and available ; domain-specific , which involves , re-authorization, and re-registration to comply with the target network's security protocols; and session continuity management, often achieved through IP-layer mobility protocols like (MIP), which registers a new care-of address with a home agent to tunnel ongoing packets seamlessly to the device's updated location. This layered approach ensures minimal disruption but requires coordination between link-layer and higher-layer functions. Vertical handover offers significant advantages in heterogeneous environments, including expanded coverage by leveraging complementary network strengths—for instance, switching to in dense indoor areas where cellular signals weaken; improved capacity through traffic offloading to high-throughput alternatives like WLAN; and cost optimization by prioritizing economical access options based on policies or user subscriptions. However, vertical handover faces notable challenges stemming from RAT heterogeneity, such as increased due to extensive parameter reconfiguration and adaptations compared to intra-RAT handovers; and vulnerabilities during cross-domain transitions, where differing mechanisms can expose sessions to unless mitigated by trusted interworking frameworks. Practical implementations include 3GPP-Wi-Fi interworking in systems, where the Evolved Packet Core () enables policy-controlled handovers between 3GPP and non-3GPP accesses to support seamless offloading and mobility. This interworking is further refined in architectures with enhanced non-3GPP access integration for broader heterogeneity support.

Mechanisms

Intra-System Handover

Intra-system handover involves the transfer of a (UE) connection from one to another while remaining within the same (RAT), such as between two GSM base stations or two LTE eNodeBs. This process ensures seamless mobility without altering the underlying air interface or core network elements associated with the RAT. The primary triggers for intra-system handover include degradation in signal quality, such as rising bit error rates or decreasing signal-to-interference ratios, which indicate potential service interruption if the remains connected to the serving cell. Additionally, network-initiated handovers occur for load balancing, redistributing traffic across cells to optimize resource utilization and prevent congestion in heavily loaded sectors. The process commences with the performing radio measurements on neighboring cells as configured by the network, followed by the transmission of measurement reports via uplink signaling. Based on these reports and internal network criteria, the serving decides to initiate the handover and coordinates with the target through backhaul interfaces. Execution then proceeds over dedicated channels, where the synchronizes to the target cell and switches its connection, minimizing disruption to the ongoing session. Intra-system handovers are subdivided into intra-frequency and inter-frequency types. Intra-frequency handovers occur between cells operating on the same carrier frequency, allowing continuous without interrupting the UE's primary . In contrast, inter-frequency handovers involve cells on different carrier frequencies within the same , requiring the UE to utilize measurement gaps or periods to assess cells while maintaining the source . Central to the intra-system handover mechanism is Radio Resource Control (RRC) signaling, which handles measurement configuration, report transmission, and reconfiguration commands in standards like (TS 25.331) and (TS 36.331). For instance, in , the RRCConnectionReconfiguration message conveys handover commands to the , enabling context transfer and at the target cell. Soft handover, as an example in , exemplifies intra-system continuity by allowing simultaneous connections to multiple cells during the transition.

Inter-System Handover

Inter-system handover, also referred to as inter-radio access technology (inter-RAT) handover, is the process by which an active user equipment (UE) connection is transferred between different radio access technologies within 3GPP-managed networks, such as from Universal Mobile Telecommunications System (UMTS) to Global System for Mobile Communications (GSM). This mechanism ensures service continuity across RAT boundaries while remaining within the same public land mobile network (PLMN), distinguishing it from broader vertical handovers that may involve non-3GPP accesses like Wi-Fi. Triggers for inter-system handover typically arise from coverage limitations in the serving , the need for enhanced service availability in an alternative , or unmet (QoS) thresholds, such as when packet-switched data rates degrade or circuit-switched services become unavailable. For example, radio condition changes or can prompt the source (RAN) to evaluate neighboring RATs via measurement reports from the . In service-specific cases, initiation occurs when the current RAT lacks support for required features, like voice over packet-switched domains. The handover process requires coordination through the core network, often involving entities like the in and the Serving GPRS Support Node (SGSN) in legacy systems, with potential relocation of the 's core network anchor points. The source or NodeB initiates the procedure by sending a handover request to the core network, which forwards it to the target 's controller (e.g., controller for ); this includes reselection of radio resources and validation of capabilities via information transfer containers. Data forwarding may occur indirectly to minimize , and the target confirms before the is redirected or handed over, completing the switch with path update in the core network. Key challenges in inter-system handover include compatibility mismatches between RATs, which can lead to rejection if the target network detects unsupported behaviors via specific information elements, and elevated signaling overhead from core network traversals, potentially causing delays up to several hundred milliseconds. These issues may result in buffered data loss if the source network is not notified of the successful handover, exacerbating service interruptions in high-mobility scenarios. A representative example is circuit-switched fallback (CSFB) in early networks, where UEs fallback from E-UTRAN to / RATs for voice or services unsupported in pure packet-switched . Triggered by mobile-originated or -terminated circuit-switched events, the coordinates with the mobile switching center () over the SGs interface to release resources and establish a in the legacy RAT, often using redirection rather than full to reduce complexity.

Conditional Handover

Conditional Handover (CHO) is a mobility robustness enhancement introduced in 3GPP Release 16 for both 5G New Radio (NR) and Long-Term Evolution (LTE) networks, extending principles from dual connectivity to enable more reliable cell transitions. In this mechanism, the network proactively prepares handover configurations for multiple candidate target cells, allowing the user equipment (UE) to autonomously evaluate and execute a handover to the most suitable target based on predefined execution conditions, such as signal thresholds for Reference Signal Received Power (RSRP), Reference Signal Received Quality (RSRQ), or Signal-to-Interference-plus-Noise Ratio (SINR). This UE-centric approach decouples the handover preparation from execution, minimizing dependency on real-time network signaling and improving performance in dynamic radio environments. The CHO process unfolds in two distinct phases: preparation and execution. In the preparation phase, the source gNB triggers the procedure upon receiving a UE measurement report indicating potential mobility needs; it then coordinates with one or more candidate target gNBs by sending handover requests, performing admission control, and receiving acknowledgments containing dedicated configurations for up to eight candidate cells. The source gNB compiles these into an RRCReconfiguration message sent to the UE, which includes the list of candidate cells, their associated execution conditions (typically based on measurement events like A3 for neighbor becoming offset better than serving or A5 for serving below threshold), and optional sidelink resource grants. During the execution phase, the UE continuously monitors radio conditions against the configured thresholds; upon fulfillment of a condition for a specific candidate—without awaiting further commands—it detaches from the source cell, synchronizes with the target cell, initiates random access, and transmits an RRCReconfigurationComplete message to finalize the handover, enabling the target gNB to notify the source for resource release. CHO provides key advantages, particularly in reducing handover failure rates through early preparation of multiple targets, which allows the UE to select the optimal cell at execution time even if conditions degrade post-measurement. This results in significantly faster and more robust mobility in high-speed scenarios, with studies demonstrating a 3-4 times reduction in mobility failure rates at 60 km/h in Frequency Range 2 (FR2) deployments compared to baseline handovers. By avoiding late-stage signaling overhead, CHO also minimizes service interruptions, supporting ultra-reliable low-latency communication requirements. Primary use cases encompass high-speed rail environments, where rapid cell changes demand high reliability to prevent outages, and dense urban areas with millimeter-wave small cells, benefiting from CHO's ability to handle frequent, beam-sensitive handovers.

Implementations

In 2G and 3G Networks

In 2G Global System for Mobile Communications (GSM) networks, handover is exclusively hard handover, involving a break-before-make transition where the mobile station (MS) disconnects from the serving cell before connecting to the target cell. The Base Station Controller (BSC) plays a central role in handover decisions, evaluating measurement reports sent by the MS on the Slow Associated Control Channel (SACCH), which is part of the Dedicated Control Channel (DCCH). These reports include signal strength and quality metrics from the serving and neighboring cells, enabling the BSC to select the target cell based on criteria such as received signal level and bit error rate. Upon decision, the BSC issues a Handover Command message via the main DCCH (typically the Standalone Dedicated Control Channel (SDCCH) or Fast Associated Control Channel (FACCH)), specifying parameters like the handover reference, channel description, and timing advance for synchronization. The MS then accesses the target channel with a Handover Access burst and confirms completion with a Handover Complete message on the DCCH, after which the BSC releases resources from the old channel. The introduction of General Packet Radio Service (GPRS) and Enhanced Data rates for GSM Evolution (EDGE), often termed 2.5G enhancements, evolved handovers to support packet-switched data while maintaining the hard handover paradigm and circuit-switched focus for voice. In these extensions, the BSC coordinates dual transfer mode (DTM) handovers, allowing simultaneous circuit- and packet-switched operations, with signaling on the Packet Associated Control Channel (PACCH) for packet data aspects. However, handovers remain BSC-driven and DCCH-based, without macrodiversity support. In Universal Mobile Telecommunications System () networks, handover protocols advanced significantly with the introduction of soft and softer handover, leveraging (CDMA) to enable make-before-break transitions and macrodiversity. Soft handover occurs between cells of different Node Bs on the same frequency, where the () maintains simultaneous connections to multiple cells in the active set, with the Radio Network Controller (RNC) combining signals via macrodiversity to improve reliability. The RNC oversees handover decisions and active set management, processing measurement reports from the (e.g., strength via events 1A, 1B, 1C) and instructing Node Bs to add or remove radio links through procedures like Radio Link Addition or Removal. Node Bs execute these by allocating dedicated transport channels and handling combining. Softer handover, a subset for cells within the same Node B, performs combining at the Node B level using site selection diversity transmission (SSDT), reducing RNC signaling overhead. Key features in include tight integration with , where inner-loop adjustments via Transmit Power Control (TPC) commands ensure balanced transmission across active set cells during , minimizing . initiation uses margins such as relative thresholds of 3-6 dB (e.g., active set threshold with 6 dB ) to trigger additions or removals, preventing frequent "ping-pong" effects. For inter-frequency handovers, compressed mode creates transmission gaps (e.g., 3-14 slots long) to allow measurements on other carriers without dedicated hardware, supporting transitions like frequency-division duplex (FDD) to time-division duplex (TDD). Inter-RAT handovers to are facilitated via RNC coordination, but native support is limited to systems, lacking direct integration with non-3GPP accesses like without later enhancements. UMTS handover completion times typically range from 200-500 ms or more, influenced by signaling delays, measurement reporting, and compressed mode gaps for inter-frequency cases, which can degrade packet in data sessions; however, soft handovers achieve near-zero interruption through simultaneous connections. This contrasts with 's simpler BSC-centric process. The shift from primarily circuit-switched voice in core to packet-oriented in UMTS Packet Switched () domains marked an evolutionary step, with enhanced support for via (HSPA) precursors in later releases.

In 4G LTE Networks

In 4G LTE networks, handover procedures are facilitated by the E-UTRAN architecture, which employs the X2 interface for direct communication between eNodeBs (eNBs) to enable efficient intra-LTE mobility without involving the Evolved Packet Core (EPC) in every case. The X2 interface supports both control plane signaling (via X2-AP over SCTP/IP) and user plane data forwarding (via GTP-U over UDP/IP), allowing the source eNB to prepare and execute handovers with the target eNB. When the X2 interface is unavailable or for scenarios requiring EPC involvement, such as changes in Mobility Management Entity (MME) or Serving Gateway (S-GW), the S1 interface is used, connecting eNBs to the EPC via S1-MME (for signaling over S1-AP) and S1-U (for user plane over GTP-U). This dual-interface approach, defined in Release 8 and refined through subsequent releases, optimizes handover speed and reduces latency compared to circuit-switched systems in prior generations. The handover process in LTE begins with UE measurements of Reference Signal Received Power (RSRP) and Reference Signal Received Quality (RSRQ) for the serving and neighboring s, configured by the source eNB via RRC signaling. These measurements are filtered at Layer 3 and reported periodically or upon event triggers, such as Event A3 (neighbor signal exceeds serving by an offset, e.g., 3 dB, for a time-to-trigger duration like 320 ms), enabling the source eNB to select a . Preparation involves the source eNB sending a HANDOVER REQUEST to the eNB over X2 (including UE context like E-RAB and information), followed by admission control and a HANDOVER REQUEST ACKNOWLEDGE from the . Execution occurs when the source eNB forwards the RRCConnectionReconfiguration message (containing the handover command) to the UE, which then performs to the and completes attachment. Completion includes the UE sending RRCConnectionReconfigurationComplete, the eNB initiating a SWITCH REQUEST to the MME over S1 to update bearer paths, and the source eNB releasing resources after receiving UE CONTEXT RELEASE, with data forwarding over X2 to minimize . This process supports intra- handovers across frequencies and ensures seamless mobility. LTE handover features encompass intra-LTE procedures for mobility within the E-UTRAN, including intra-frequency and inter-frequency handovers, as well as inter-Radio Access Technology (RAT) handovers to legacy 3G (UMTS) or 2G (GSM/EDGE) networks when coverage overlaps or signal quality degrades. Intra-LTE handovers leverage X2 for direct eNB coordination, while inter-RAT handovers involve S1 signaling to the MME, which forwards a HANDOVER REQUIRED message to the target RAT's controller (e.g., RNC in UMTS), adapting UE context and bearers accordingly. Carrier aggregation (CA), introduced in Release 10, is supported during handover through RRC connection reconfiguration, allowing secondary cells (SCells) to be added or changed without full handover interruption, thereby maintaining aggregated bandwidth for higher throughput. These features enable robust mobility in all-IP networks, contrasting with the circuit-oriented approaches of earlier generations. Performance in handovers is characterized by a typical interruption time of 30-60 ms, during which the cannot transmit or receive data, achieved through rapid synchronization and data buffering. To ensure (RLC) continuity, particularly for acknowledged mode (RLC-AM) bearers, the source eNB sends an SN STATUS TRANSFER message over X2 to the target eNB, conveying uplink PDCP sequence number (SN) receiver status, downlink PDCP SN transmitter status, and Hyper Frame Number (HFN) for each E-RAB. This mechanism supports in-sequence delivery and minimizes reordering delays at the target, enhancing reliability for real-time applications. Enhancements from Release 8 to 15 have improved handover efficiency, with notable additions in Release 12 introducing dual connectivity (), allowing a to maintain simultaneous connections to a master eNB (MeNB) and secondary eNB (SeNB) over non-ideal backhaul. In , SeNB changes—such as addition, modification, or removal—do not trigger a full handover; instead, they use lightweight RRC reconfiguration procedures anchored at the MeNB, reducing interruption and supporting bearer split for increased throughput. Further refinements in Releases 13-15 extended to multi-RAT scenarios and optimized robustness, building on the foundational all-IP architecture for self-organizing networks.

In 5G NR Networks

In networks, handover procedures are designed to support high-mobility scenarios, ultra-reliable low-latency communication (URLLC), and enhanced mobile broadband (eMBB), leveraging the NG-RAN architecture for seamless connectivity transitions. The process involves preparation, execution, and completion phases, with optimizations for beam-based operations and network slicing to minimize service disruptions. Key advancements focus on reducing interruption times and improving robustness through features introduced across Releases 15 to 18. The handover architecture in utilizes the interface for direct inter-gNB communication during intra-NG-RAN handovers, enabling efficient context transfer and data forwarding without core network involvement. The Application () handles signaling, such as the REQUEST from the source gNB, which includes context, QoS parameters, and slice information, followed by the target gNB's REQUEST ACKNOWLEDGE with RRC reconfiguration details. For handovers requiring core involvement, such as inter-AMF scenarios, the NG interface connects gNBs to the Access and Mobility (AMF) via NGAP signaling, where the source gNB sends a REQUIRED , and the AMF coordinates path switching with the target gNB using PATH SWITCH REQUEST and ACKNOWLEDGE procedures. Additionally, E-UTRA-NR Dual Connectivity (EN-DC) supports handover in hybrid LTE-NR deployments, with the eNB as the coordinating via the X2 interface and the gNB as the secondary , preserving keys and PDCP configurations during transitions. Handover processes in 5G NR incorporate beam-level measurements to address mmWave challenges, where the UE reports Layer 1 (L1) metrics like RSRP, RSRQ, and SINR for specific beams via RRC signaling, enabling precise cell and beam selection during preparation. Conditional handover (), introduced in Release 16, allows the UE to evaluate execution conditions (e.g., signal thresholds) and autonomously switch to a pre-configured target cell upon meeting criteria, reducing failure rates in poor coverage areas. Dual Active Protocol Stack (DAPS) handover further enhances this by maintaining simultaneous source and target connections post-RRC reconfiguration, using separate PDCP entities for each stack to forward data and transfer sequence numbers via EARLY STATUS TRANSFER messages, achieving near-zero interruption for URLLC applications. Features of 5G NR handover include intra-NR (inter-gNB) procedures for seamless transitions within the NR domain, supporting multicast/broadcast services continuity via MBS session IDs in signaling. Inter-RAT handover to enables fallback from NR standalone () to , with the AMF facilitating NAS signaling and PDU session relocation over the interface for voice and data continuity. Slicing-aware handover ensures alignment with network slices by including Single Network Slice Selection Assistance Information (S-NSSAI) in XnAP and NGAP messages, allowing the target gNB to admit or reject sessions based on slice-specific QoS and resource availability during preparation. Performance targets for handover emphasize low interruption times, with DAPS achieving 0 ms data disruption by sustaining source links until target activation, meeting URLLC requirements for industrial applications. Mobility Robustness Optimization (MRO) enhances reliability by analyzing handover reports (e.g., too-early/late failures) via XnAP and adjusting parameters like cell individual offsets, increasingly incorporating models to predict and mitigate issues based on historical data patterns. Advancements across releases build progressively: Release 15 established baseline L3 handover with beam management for intra-gNB mobility; Release 16 introduced CHO and DAPS for conditional execution and zero interruption; Release 17 refined these with enhanced reporting and aerial UE support; and Release 18 added L1/L2-triggered mobility, using MAC CE signaling based on L1 measurements for pre-synchronized cell switches, reducing overall interruption to 20-30 ms in high-speed scenarios.

Challenges and Optimizations

Criteria for Initiation

The initiation of a handover in mobile networks is primarily driven by measurements of radio signal conditions to ensure seamless connectivity as a () moves. Key primary criteria include signal strength indicators such as (RSSI) in / systems and (RSRP) in and networks, where a threshold drop in the serving cell's signal prompts evaluation of neighboring cells. Signal quality metrics, including (BER) or RXQUAL in earlier generations and (SINR) or Reference Signal Received Quality (RSRQ) in modern systems, further trigger handovers when degradation risks service interruption. Distance estimates, derived from measurements that gauge propagation delay between the UE and , also serve as a criterion, particularly in and to detect when the UE approaches cell boundaries. Secondary factors refine these triggers to optimize and . Cell load, representing , influences load-balancing handovers to redistribute traffic from congested cells to underutilized ones, often quantified by metrics like frequency load percentage or available . UE velocity, estimated via Doppler shift in signal measurements or historical handover patterns, adjusts thresholds to accommodate high-speed , reducing unnecessary handovers in fast-moving scenarios. Service type considerations prioritize applications like data services, applying stricter quality thresholds to minimize for delay-sensitive traffic. To avoid frequent oscillations known as ping-pong handovers, thresholds incorporate margins, typically around 3 dB, ensuring a neighbor signal must exceed the serving signal by this amount before triggering. Additionally, a time-to-trigger (TTT) period, often ranging from 100 to 500 ms, requires the condition to persist before initiation, promoting stability. Handover decisions are typically network-based, with the evaluating reports from the . A representative algorithm is the A3 event in , triggered when a neighbor cell's RSRP surpasses the serving cell's by a configured offset, as defined in specifications. Similar mechanisms apply in , adapting to advanced and multi-connectivity scenarios. If these criteria are not met, handovers may fail, leading to connection drops as detailed in related analyses.

Reasons for Failure

Handover failures in cellular networks can arise from several primary causes, including a too-weak target cell signal, where the reference signal received power (RSRP) or quality (RSRQ) from the target base station falls below the threshold required for stable connection, often due to path loss, shadowing, or distance despite initial measurement reports indicating viability. Synchronization loss occurs when the user equipment (UE) cannot properly align its timing advance or frequency with the target cell during the handover execution phase, leading to failed access. Resource unavailability at the target cell, such as insufficient radio resources or admission control rejection due to high load, results in the target node denying the handover request. Interference spikes, caused by sudden environmental changes or neighboring cell activity, can degrade signal-to-interference-plus-noise ratio (SINR) abruptly, preventing successful attachment to the target cell. These failures manifest in distinct types during the handover process. Radio link failure (RLF) during execution is a common type, where the connection to the source is lost before the successfully synchronizes and completes to the target , often triggered by timers like T304 expiring. Handover rejection by the target occurs when the target or gNodeB responds with a failure message, citing causes such as "no radio resources available" or "handover desirable for radio reasons not supported," as defined in signaling protocols. Revert to source failure happens if the handover aborts and the cannot re-establish with the original source , exacerbating the outage. These types are detected through reports or network counters in standards like TS 36.331 for and TS 38.331 for . The impacts of handover failures significantly undermine network reliability, leading to call drops in voice services, abrupt throughput degradation in data sessions, and increased end-to-end latency as the UE enters re-establishment or idle mode. In operational networks, handover success rates are targeted above 98% to maintain quality of service, with failure rates below 2% considered acceptable for intra-frequency handovers in LTE environments under typical mobility speeds up to 120 km/h. Despite meeting initiation criteria such as signal strength thresholds, these failures highlight the gap between trigger detection and execution robustness. In modern 5G deployments using millimeter-wave (mmWave) bands, failure rates are notably higher due to beam blockage from obstacles like buildings or human bodies, which cause rapid signal attenuation and disrupt beamformed connections. Network operators detect specific failure patterns, such as too-late (where RLF occurs just after initiation due to delayed execution), too-early handovers (leading to premature detachment from a viable source), or ping-pong effects (repeated between cells), through mobility robustness optimization (MRO) mechanisms that analyze RLF reports and handover traces. These insights inform parameter tuning without delving into full corrective strategies.

Prioritization and Optimization Techniques

In mobile networks, handover prioritization involves ranking candidate target cells using multi-attribute (MADM) approaches that evaluate factors such as signal strength, network load, and (QoS) requirements. These methods apply weighted scoring to assign scores to potential cells based on parameters like (RSSI), throughput, delay, , and Signal-to-Noise-plus-Interference Ratio (SNIR), enabling the selection of the most suitable target for seamless connectivity. For example, the (TOPSIS) integrates user preferences (e.g., cost or QoS) with predicted to prioritize cells likely to maintain connections longer than 2 seconds, reducing unnecessary handovers in heterogeneous environments. Standard measurement events in and further support coverage-based prioritization during candidate selection. Event A4 triggers when a neighboring 's reference signal received power (RSRP) or quality (RSRQ) exceeds a predefined , identifying viable alternatives without immediate serving degradation. In contrast, Event A5 activates when the serving 's metric falls below one while a neighbor surpasses another, allowing prioritized ranking of s offering superior coverage and quality. These events, defined in specifications, facilitate efficient scanning and reporting to minimize processing overhead while focusing on high-potential targets. Optimization techniques leverage (ML) for predictive handover decisions, particularly through trajectory analysis of user patterns. (LSTM) networks process time-series data on RF conditions (e.g., RSRP, SINR), UE velocity, and to forecast optimal handover triggers and destinations, enabling proactive adjustments in ultra-dense deployments. Complementing this, Self-Organizing Networks (SON) automate parameter tuning via Mobility Robustness Optimization (MRO), which refines and time-to-trigger thresholds to curb ping-pong effects, and Mobility Load Balancing (MLB), which prioritizes offloading to underutilized cells based on load and QoS metrics. In heterogeneous networks, advanced strategies dynamically adjust handover margins (0-10 dB) and employ blacklists to exclude repeatedly failing cells, preventing frequent re-handovers. As of 2025, -Advanced ( Release 18) introduces enhancements like AI-assisted predictive handovers for ultra-reliable low-latency communications (URLLC) and support for non-terrestrial networks (NTN), improving success rates in high- scenarios. These techniques yield measurable benefits, including reductions in outage probability and ping-pong occurrences, alongside a 25-35% decrease in unnecessary handovers that lowers overall signaling load and enhances throughput by approximately 5%. By focusing on proactive and auto-tuning, they ensure robust without excessive network resource consumption.

Comparisons

Performance Metrics

Performance metrics for handover processes in cellular networks quantify the reliability, efficiency, and seamlessness of transitions between base stations, ensuring minimal disruption to user connectivity. These indicators are essential for network operators to monitor and optimize across generations of mobile systems. A primary metric is the success rate (HSR), defined as the ratio of successful handover completions to the total number of handover attempts. In networks, HSR is measured through operations and maintenance (OAM) systems, with operators typically targeting values exceeding 99% to maintain high . In , features like Dual Active Protocol Stack (DAPS) and Conditional Handover () can achieve interruption times approaching 0 ms, improving HSR to over 99.5% in optimized deployments as of 2025. Interruption time, representing the duration of radio link downtime during , is another key measure, often captured as the average execution time from resource release to path switch completion. , specifically the round-trip time for handover-related messages between network elements, further assesses the responsiveness of the procedure. Supplementary metrics include the ping-pong rate, which tracks the frequency of unnecessary oscillatory s between adjacent cells, potentially wasting resources. The failure rate, broken down by causes such as too-early or too-late triggers, helps identify specific vulnerabilities in the process. Additionally, the robustness index evaluates overall stability, derived from counters in robustness optimization (MRO) mechanisms. All these key performance indicators (KPIs) are collected and analyzed via OAM frameworks in systems, enabling proactive adjustments. Performance varies with factors such as () speed, which increases frequency and potential failures at higher velocities, and environmental conditions like versus rural sparsity, influencing signal and overlap.

Hard vs. Soft Handover

Hard handover, also known as break-before-make, involves disconnecting from the serving cell before connecting to the target cell, utilizing a single radio link throughout the process. In contrast, soft handover, or make-before-break, establishes a new connection while retaining the existing one, allowing simultaneous links to multiple cells before releasing the old. This fundamental distinction arises from the underlying multiple access technologies: hard handover aligns with / systems, while soft handover leverages 's ability to support concurrent codes. Regarding complexity, hard handover is simpler, as it requires minimal coordination beyond signaling for link release and reconfiguration, avoiding the need for multi-link management. Soft handover introduces greater complexity through at the radio network controller, where signals from up to six cells in the active set are merged to select the best quality, demanding advanced and . A key trade-off is service disruption: hard handover causes a temporary gap of approximately 200–500 ms in and 20-50 ms in , during which no data transmission occurs. Soft handover minimizes this to near-zero interruption by overlapping connections, ensuring continuous service, though at the expense of added latency in combining operations. Resource utilization further highlights the differences, with hard handover being efficient in FDMA/TDMA environments like , as it allocates resources to only one cell at a time, reducing overhead and . Soft handover in CDMA-based , however, is more resource-intensive, requiring duplicated transmission paths and increased power consumption at the to maintain multiple uplink links, which elevates overall system . These characteristics influence suitability: hard handover performs well for delay-tolerant voice services in , where short interruptions do not severely impact call quality. Soft handover excels in for both voice and data, particularly at cell edges, by improving signal reliability through macro-diversity. In evolutionary terms, soft handover's resource demands led to its phase-out in and , where hard handover predominates but incorporates hybrid optimizations like coordinated multipoint (CoMP) transmission and conditional handover to approach soft handover's seamlessness while preserving efficiency. These approaches reduce interruption times below traditional hard handover levels without the full complexity of multi-link maintenance. Such trade-offs directly affect metrics like handover interruption time, balancing continuity against overhead in modern networks.

Intra- vs. Inter-System Handover

Intra-system handovers occur within the same (RAT), such as from one 5G New Radio (NR) cell to another or one Long-Term Evolution () cell to another, allowing the (UE) to maintain connectivity using the same core network elements without requiring a change in the access stratum. In contrast, inter-system handovers involve transitions between different RATs, like from to or vice versa, which necessitate core network relocation, such as shifting from the Access and Mobility Management Function (AMF) in 5G Core (5GC) to the Mobility Management Entity (MME) in Evolved Packet Core (EPC). This relocation in inter-system cases, exemplified by MME changes during -to-3G handovers, increases complexity due to additional signaling for context transfer and security procedures. Latency in intra-system handovers is typically lower, around 30-60 ms for interruption time in and similar or reduced in via optimizations like Dual Active Protocol Stack (DAPS), as the procedure relies on direct (RAN) interfaces without extensive core involvement. Inter-system handovers, however, exhibit higher latency, often exceeding 100 ms, owing to mandatory , bearer reconfiguration, and interworking signaling between disparate core networks. Reliability is generally higher for intra-system handovers, with success rates approaching 99% in optimized deployments, as they avoid compatibility issues between . Inter-system handovers are more prone to failure, with risks from () mismatches, such as unsupported frequency bands or incompatibilities, leading to higher failure rates compared to intra-system handovers, particularly in early 5G-LTE interworking scenarios. Intra-system handovers are primarily applied for seamless coverage extension within a single type, ensuring continuous as users traverse cells in or mobility patterns. Inter-system handovers support fallback mechanisms during coverage gaps in the preferred or traffic offloading to networks for load balancing. In networks, trends show a reduction in inter-system and through the unified 5GC , which enables optimized interworking with via the interface, minimizing core relocations and promoting standalone deployments over dual-connectivity modes.

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