Multiflow mode

Description

Multiflow mode is a feature introduced in 3GPP Release 11 for High-Speed Packet Access (HSPA) networks, specifically for the downlink. It allows a User Equipment (UE) to receive High-Speed Downlink Shared Channel (HS-DSCH) transmissions simultaneously from two or more cells. The term 'Multiflow' refers to the ability to have multiple data flows to a single UE. The definition states that the UE is configured in Multiflow mode when it is configured with an assisting serving HS-DSCH cell. This means that in addition to the primary serving HS-DSCH cell, the network configures one or more assisting serving HS-DSCH cells. These cells can be from the same NodeB (intra-node) or different NodeBs (inter-node), and they coordinate to transmit data to the UE.

The architecture of Multiflow mode builds upon the existing HSPA framework. The primary serving HS-DSCH cell is responsible for the main control and data transmission, while the assisting serving HS-DSCH cell(s) provide additional data streams. The UE must be capable of receiving and combining these multiple streams. The network configures the UE via Radio Resource Control (RRC) signalling, indicating the assisting cells and the necessary parameters. The data is split at the Radio Link Control (RLC) layer or at the Medium Access Control (MAC) layer, depending on the specific configuration. The MAC layer handles the scheduling and multiplexing of data from different cells. The physical layer (Layer 1) must support the reception of multiple HS-DSCH streams, which may be on different frequencies (inter-frequency) or the same frequency (intra-frequency).

Key components of Multiflow mode include the serving HS-DSCH cell set, which comprises the primary and assisting cells. The UE measures the channel quality of these cells and reports to the network, which then decides on the scheduling and data split. The network can use Coordinated Multi-Point (CoMP) techniques to manage interference and improve spectral efficiency. The UE's receiver must be able to handle multiple input streams, often requiring advanced equalization and interference cancellation. The feedback channel (HS-DPCCH) is enhanced to carry Channel Quality Indicator (CQI) and Precoding Control Information (PCI) for multiple cells, as well as HARQ acknowledgements.

In terms of operation, Multiflow mode can be configured in different modes: single-frequency (where both cells operate on the same carrier) and dual-frequency (where cells operate on different carriers). The latter is also known as Dual-Cell HSDPA with Multiflow. The network can dynamically switch between these modes based on load and channel conditions. The scheduling algorithm in the NodeB must coordinate transmissions from multiple cells to avoid excessive interference and to maximize throughput. The UE combines the received signals using techniques such as soft combining or interference cancellation. The HARQ process is also coordinated across cells to ensure reliable delivery.

The role of Multiflow mode in the network is to enhance user experience by providing higher peak data rates, improved cell-edge performance, and better mobility robustness. It is particularly beneficial in heterogeneous networks where small cells are deployed within the coverage of macro cells. By allowing the UE to receive from multiple cells, it can exploit diversity and load balancing. Multiflow mode is part of the continuous evolution of HSPA, which remains relevant in many networks as a complement to LTE and 5G. It is specified in 3GPP TS 25.101 (UE radio transmission and reception), TS 25.211 (physical channels), TS 25.212 (multiplexing and channel coding), and TS 25.214 (physical layer procedures).

Purpose & Motivation

Multiflow mode was created to address the need for higher data rates and improved reliability in HSPA networks, especially at cell edges where signal quality is poor. Prior to Rel-11, HSPA UEs could only receive data from a single serving cell at a time, which limited throughput and made cell-edge performance susceptible to interference and fading. While features like Dual-Cell HSDPA (DC-HSDPA) allowed aggregation of two carriers from the same NodeB, they did not support simultaneous transmission from multiple cells, particularly from different NodeBs. This limitation meant that UEs could not benefit from coordinated transmission across cells, which is crucial in heterogeneous networks with macro and small cells.

Multiflow mode solves these problems by enabling the UE to receive data from multiple cells, thereby increasing throughput through spatial diversity and load balancing. It also improves cell-edge performance by allowing the UE to combine signals from multiple cells, effectively turning interference into useful signal. This is similar to CoMP in LTE but adapted for HSPA. The feature was motivated by the widespread deployment of HSPA and the need to remain competitive with emerging LTE networks. It provides a smooth evolution path for operators to enhance their existing HSPA investments without immediate migration to LTE.

Furthermore, Multiflow mode addresses the challenge of mobility and load balancing. By allowing the UE to connect to multiple cells, the network can better manage resources and offload traffic from congested cells. It also enhances the user experience during handover by maintaining connectivity to multiple cells. The introduction of Multiflow mode in Rel-11 was a significant step in HSPA evolution, and it continued to be refined in subsequent releases, ensuring its relevance in the 5G era as part of multi-RAT networks.