Diversity Handover

Description

Diversity Handover (DHO), specifically defined for UMTS/WCDMA systems, is a form of soft handover where the User Equipment (UE) is in simultaneous communication with two or more NodeBs (base stations) belonging to different cells during the handover process. The UE maintains multiple active radio links within its Active Set, which is managed by the Radio Network Controller (RNC). The core principle is macro-diversity: the same uplink data transmitted by the UE is received by all NodeBs in the Active Set, and the downlink data transmitted to the UE is sent from all these NodeBs as well.

From an architectural perspective, DHO involves the UE, the involved NodeBs, and the RNC. The RNC is the central controlling entity that executes the handover algorithm, manages the Active Set (adding and removing radio links based on UE measurement reports), and performs selection combining in the uplink and transmission in the downlink. In the uplink, each NodeB receives the signal, decodes it, and forwards the resulting frame to the RNC. The RNC selects the best frame from among those received (selection combining) and forwards it to the core network. In the downlink, the RNC sends the same data stream to all NodeBs in the Active Set, which then transmit it synchronously to the UE. The UE uses a RAKE receiver to combine these multiple received signals, improving signal quality and mitigating fading.

This process works continuously as the UE moves. The UE performs measurements on neighboring cells (intra-frequency measurements) and reports them to the RNC via measurement reports. When the signal from a new cell becomes strong enough, the RNC initiates a radio link addition procedure. When the signal from an existing cell becomes too weak, the RNC initiates a radio link deletion procedure. The UE is always connected to at least one cell, ensuring uninterrupted service. DHO's role is to provide seamless mobility, enhance coverage at cell edges, increase capacity through interference reduction, and significantly improve call reliability in a CDMA network where all cells use the same frequency.

Purpose & Motivation

DHO was created to solve critical challenges inherent in CDMA-based cellular systems like UMTS, specifically the 'near-far' problem and the need for seamless mobility on a single frequency. In a WCDMA network, all cells operate on the same frequency, making hard handovers (break-before-make) risky and prone to call drops, especially at cell boundaries where signal levels from multiple cells are comparable. The purpose of DHO is to enable a 'make-before-break' handover, ensuring the UE always has at least one stable link.

Historically, it evolved from the soft handover concepts in IS-95 (cdmaOne). For 3GPP's UMTS (R99), it was a fundamental mobility feature to guarantee the quality of service for real-time applications like voice and video calls over a wide area. It addresses the limitations of GSM's hard handover by eliminating the brief interruption during cell switch and by providing diversity gain to combat fading and shadowing. This results in fewer dropped calls, better voice quality at cell edges, and more efficient use of the radio spectrum through reduced interference, as a UE in DHO transmits with lower power due to the combined reception at multiple NodeBs.