Secondary uplink frequency

Description

In 3GPP UMTS/HSPA networks, the secondary uplink frequency is a key feature introduced in Release 9 to support dual-carrier HSUPA (DC-HSUPA). It allows a User Equipment (UE) to transmit on two uplink frequencies simultaneously, effectively doubling the uplink data rate. The secondary uplink frequency carries an E-DCH (Enhanced Dedicated Channel) that corresponds to a serving E-DCH cell. This serving E-DCH cell is associated with a secondary serving HS-DSCH cell, which is the downlink carrier that provides the corresponding control and feedback information. The association between the uplink and downlink frequency pair is signaled by higher layers (RRC), ensuring that the UE and network are synchronized on which carriers to use.

The architecture builds on the existing single-carrier HSUPA framework. In DC-HSUPA, the UE maintains two uplink carriers: the primary uplink frequency (which carries the primary E-DCH) and the secondary uplink frequency (which carries the secondary E-DCH). Each uplink carrier has its own E-DCH transport channel, and the UE can transmit data on both simultaneously. The NodeB schedules both carriers independently but may coordinate them to manage interference. The secondary uplink frequency is only configured when the UE is in a dual-carrier capable cell and has the necessary capability. The network controls activation and deactivation of the secondary uplink frequency via HS-SCCH orders or RRC signaling, allowing dynamic adaptation to traffic and radio conditions.

Key components include the E-DCH transport channel, which supports HARQ and scheduling, and the associated control channels: E-DPCCH (for control information) and E-DPDCH (for data). On the secondary uplink frequency, these channels are replicated. The UE also needs to monitor the downlink control channels from the secondary serving HS-DSCH cell to receive scheduling grants and feedback. The timing and power control for the secondary uplink are aligned with the primary uplink to simplify implementation. The secondary uplink frequency can be on a different band or the same band as the primary, depending on the UE's capabilities and network deployment.

The main benefit is increased uplink throughput and improved user experience for uplink-intensive applications such as video uploading and cloud services. It also improves coverage and load balancing by allowing the network to distribute uplink traffic across multiple carriers. However, it introduces additional complexity in terms of UE power consumption, inter-carrier interference, and scheduling. The feature was further enhanced in later releases with improvements like uplink MIMO and multi-carrier aggregation, but the fundamental concept of a secondary uplink frequency remains as defined in Release 9.

Purpose & Motivation

The secondary uplink frequency was introduced to address the growing demand for higher uplink data rates in UMTS/HSPA networks. Before Release 9, HSUPA operated on a single uplink carrier, limiting peak uplink speeds to 5.76 Mbps (later 11.5 Mbps with 16QAM). As uplink-heavy applications like video sharing and social media emerged, operators needed a way to boost uplink capacity without acquiring new spectrum. Dual-carrier HSUPA, enabled by the secondary uplink frequency, allows the UE to use two adjacent or non-adjacent carriers simultaneously, effectively doubling the uplink throughput to up to 23 Mbps. This approach leverages existing spectrum assets and provides a smooth evolution path for operators, while also improving spectral efficiency and user experience.