Macro diversity handover

Description

Macro diversity handover (MDHO) is a mobility and link reliability mechanism defined in 3GPP TS 21.905 for UTRAN (UMTS Terrestrial Radio Access Network). In this operation state, a User Equipment (UE) simultaneously establishes and maintains radio links with two or more UTRAN access points (typically Node Bs) for the sole purpose of improving the quality of the radio connection or providing seamless handover. This is a form of soft handover, where the UE combines signals from multiple cells rather than performing a hard switch from one cell to another. The concept is rooted in CDMA-based radio access, where the same information is transmitted on the downlink from multiple cells and received on the uplink by multiple cells, allowing the RNC (Radio Network Controller) to perform macro diversity combining.

Architecturally, macro diversity handover operates within the UTRAN framework, which consists of the Core Network (CN), the UTRAN (comprising Node Bs and RNCs), and the UE. The RNC is the key controlling node that manages the active set of radio links. When a UE is in soft handover, the RNC coordinates the addition and removal of radio links as the UE moves through the network. The UE continuously measures signal strengths from neighboring cells and reports them to the RNC via RRC (Radio Resource Control) measurement reports. Based on these reports, the RNC decides when to add or remove a radio link from the active set, using thresholds and hysteresis to avoid ping-pong effects. The active set is the list of cells with which the UE has established radio links.

The mechanism works in both the downlink and uplink directions. In the downlink, multiple Node Bs transmit the same data to the UE, and the UE uses a RAKE receiver to combine the signals, thereby improving the received signal quality through diversity gain. In the uplink, the UE transmits to multiple Node Bs, and each Node B forwards the received data to the RNC. The RNC then performs macro diversity combining, selecting the best frame or combining frames from different Node Bs to produce a single reliable data stream. This combining can be done at the frame level (selection combining) or at the symbol level (maximum ratio combining), depending on the transport channel type and configuration.

The benefits of macro diversity handover include improved coverage and capacity, reduced interference, and seamless mobility without perceptible interruption. It is particularly effective at cell edges where signal strength is weak and interference is high. By maintaining multiple links, the UE can achieve a diversity gain that combats fading and shadowing. Additionally, soft handover reduces the chance of call drops during handover and improves the overall quality of service. However, it also increases network resource usage because multiple cells are simultaneously serving the same UE, consuming additional downlink power and uplink capacity. Therefore, network operators must balance the benefits against the cost of resource consumption.

Macro diversity handover is applicable to both circuit-switched and packet-switched services in UMTS. It is a key feature for real-time services like voice and video telephony, where seamless handover is critical. The term has been present in 3GPP specifications since Release 4 and continues to be defined in TS 21.905 through Release 19, although its practical relevance has diminished with the evolution to LTE and 5G, which use different handover mechanisms (e.g., hard handover in LTE, and dual connectivity in 5G). Nevertheless, it remains a fundamental concept in the study of cellular network mobility.

Purpose & Motivation

Macro diversity handover was created to address the inherent challenges of mobility in CDMA-based cellular networks, particularly in UMTS. In early cellular systems, hard handover (break-before-make) was used, which could cause momentary interruptions and call drops, especially at cell boundaries. With the advent of CDMA, it became possible to have multiple simultaneous connections, enabling soft handover (make-before-break). Macro diversity handover extends this concept by allowing the UE to communicate with multiple access points simultaneously, thereby improving link reliability and seamless service continuity.

The primary motivation was to enhance coverage and capacity at cell edges and to provide seamless mobility for real-time services. In CDMA networks, the interference-limited nature means that maintaining multiple links can actually reduce overall interference by lowering the required transmit power from each link due to diversity combining. This improves both the user experience and the network efficiency. Furthermore, macro diversity handover ensures that ongoing sessions, such as voice calls, are not disrupted when the UE moves from one cell to another, which is crucial for meeting quality of service requirements.

Historically, macro diversity handover was a key differentiator of UMTS compared to GSM, which used hard handover. It allowed UMTS to provide superior voice quality and data services. However, as networks evolved towards OFDMA-based systems like LTE and 5G, the need for macro diversity handover decreased because those systems employ different techniques such as frequency reuse and advanced scheduling. Nonetheless, the concept remains relevant in the context of multi-connectivity in 5G, where a UE can connect to multiple access points simultaneously for enhanced reliability and throughput.