Soft Handover

Description

Soft handover is a mobility procedure defined in 3GPP TS 21.905 for UMTS (WCDMA) networks. Unlike hard handover, where the radio link to the source cell is broken before a new link to the target cell is established (break-before-make), soft handover follows a make-before-break principle: the UE establishes a new radio link to a target cell before releasing the link to the source cell. At all times during the procedure, the UE keeps at least one radio link to the UTRAN, ensuring service continuity and eliminating the interruption gap inherent in hard handover.

In the WCDMA radio access network, soft handover is implemented through the concept of the Active Set — the set of cells with which the UE simultaneously maintains radio links. The network (specifically the RNC, Radio Network Controller) manages the Active Set based on measurement reports from the UE. The UE continuously measures the received signal quality of neighbouring cells and reports events such as 'a primary CPICH enters the reporting range' (Event 1A) or 'a primary CPICH leaves the reporting range' (Event 1C). Based on these reports, the RNC decides to add or remove radio links, sending Active Set Update messages to the UE.

When multiple radio links are active, the UE combines the received signals using maximum ratio combining (MRC) or similar techniques at the physical layer. This combining provides a macro-diversity gain that improves both uplink and downlink performance: the probability of a deep fade affecting all links simultaneously is much lower than for a single link, so the effective link reliability is significantly enhanced. On the uplink, the serving RNC combines the data received from multiple NodeBs via the Iub interface, again achieving diversity gain.

Soft handover is closely related to softer handover, which occurs when the radio links belong to different sectors of the same NodeB. In softer handover, combining can be performed within the NodeB itself, which is more efficient than the RNC-level combining required for soft handover across different NodeBs. Both procedures are managed by the RNC and are transparent to the core network.

The procedure is particularly important in WCDMA because the system is interference-limited: all users share the same frequency carrier. Soft handover reduces the transmit power needed to maintain a given quality of service, thereby lowering the overall interference level in the network and increasing capacity. This is a fundamental advantage over hard handover in CDMA-based systems. However, soft handover consumes additional network resources (multiple radio links per UE, additional Iub transport capacity, and RNC processing) and is therefore subject to careful radio resource management.

With the introduction of LTE and 5G NR, which use OFDMA and do not support soft handover in the same form, the concept remains relevant primarily for UMTS/HSPA networks. Nevertheless, the term is still maintained in the 3GPP vocabulary (TS 21.905) across all releases from Rel-4 to Rel-19, reflecting its historical and architectural importance in the 3GPP ecosystem.

Purpose & Motivation

Soft handover was created to solve a fundamental problem in CDMA-based cellular systems such as UMTS/WCDMA: the need for seamless mobility without service interruption while simultaneously managing interference. In a CDMA system, all users share the same frequency band, so the system is interference-limited. A hard handover, where the connection is briefly interrupted, not only causes a perceptible gap in service but also requires the UE to transmit at higher power to re-establish the link, increasing interference for all other users. Soft handover addresses both issues by allowing the UE to communicate with multiple cells simultaneously.

The macro-diversity gain provided by soft handover means that the combined signal from multiple cells is more robust against fading and shadowing than any single link. This reduces the required transmit power on both uplink and downlink, which directly translates into lower overall interference and higher system capacity. In the early days of UMTS deployment, this capacity gain was essential for making WCDMA commercially viable, as it allowed operators to support more users per carrier.

Before soft handover, hard handover was the standard approach in GSM and other TDMA/FDMA systems. In those systems, the break-before-make approach was acceptable because the system was not interference-limited in the same way, and the interruption was short enough for voice services. However, for UMTS, which aimed to provide higher data rates and better spectral efficiency, a more sophisticated mobility mechanism was required. Soft handover, together with its variant softer handover, became a cornerstone of UMTS radio resource management, and its principles influenced the design of later multi-connectivity features in LTE and 5G.