Secondary Serving Enhanced Dedicated Channel cell

Description

The Secondary Serving E-DCH cell is a concept introduced in 3GPP Release 9 as part of the Dual-Carrier High-Speed Uplink Packet Access (DC-HSUPA) feature. In DC-HSUPA, a UE can transmit on two uplink carriers simultaneously, and the network can schedule resources on both. The Secondary Serving E-DCH cell is the cell on the secondary uplink frequency that provides scheduling grants (Absolute Grants) to the UE via the secondary downlink. This allows the Node-B to control the UE's transmit power and data rate on the secondary uplink carrier, ensuring efficient resource utilization and interference management.

In the architecture of DC-HSUPA, each uplink frequency has an associated Serving E-DCH cell. The primary Serving E-DCH cell is on the primary uplink frequency, and the secondary Serving E-DCH cell is on the secondary uplink frequency. The UE receives Absolute Grants from both cells, which indicate the maximum allowed transmit power or data rate. The grants are sent on the E-DCH Absolute Grant Channel (E-AGCH) on the respective downlink frequencies. The Secondary Serving E-DCH cell is responsible for sending grants on the secondary downlink, which may be on the same frequency as the secondary uplink or a different one, depending on the configuration.

The concept of a Serving E-DCH cell is fundamental to HSUPA operation. In single-carrier HSUPA, there is only one Serving E-DCH cell, which is the cell that the UE is connected to for uplink data transmission. With DC-HSUPA, the UE can be connected to two cells, one for each carrier. The Secondary Serving E-DCH cell is typically the same as the primary serving cell if the secondary carrier is in the same cell, but it can be a different cell if the carriers are in different cells (e.g., in a multi-cell scenario). The UE must be able to receive grants from both cells and adjust its transmission accordingly.

The introduction of the Secondary Serving E-DCH cell in Release 9 was motivated by the need to increase uplink throughput and improve user experience. By allowing the UE to transmit on two carriers, DC-HSUPA doubles the peak uplink data rate compared to single-carrier HSUPA. The Secondary Serving E-DCH cell enables the network to schedule resources on the secondary carrier independently, providing flexibility and efficiency. This is particularly beneficial in scenarios where the primary carrier is congested or where the UE has a good channel condition on the secondary carrier.

The specifications 25.319 and 25.321 define the procedures for E-DCH operation, including the handling of Serving E-DCH cells and grants. These specifications ensure that the UE and Node-B can coordinate the scheduling of uplink resources across multiple carriers. The Secondary Serving E-DCH cell is a key enabler for DC-HSUPA and has been maintained in subsequent releases, although its relevance has diminished with the advent of LTE and 5G. Nevertheless, it remains an important concept for understanding the evolution of uplink scheduling in 3GPP.

Purpose & Motivation

The Secondary Serving E-DCH cell was created to support the introduction of Dual-Carrier HSUPA (DC-HSUPA) in Release 9. Before DC-HSUPA, UEs could only transmit on a single uplink carrier, limiting the peak data rate and capacity. The demand for higher uplink speeds was driven by applications like video sharing, video calling, and large file uploads. To address this, 3GPP introduced DC-HSUPA, which allows simultaneous transmission on two carriers. However, this required a mechanism to schedule resources on both carriers independently, as the channel conditions and interference levels could differ between them.

The Secondary Serving E-DCH cell solves this problem by providing a separate scheduling entity for the secondary uplink carrier. It allows the Node-B to send Absolute Grants on the secondary downlink, giving the UE permission to transmit on the secondary uplink. This separation ensures that the network can control the UE's transmission on each carrier separately, optimizing resource allocation and minimizing interference. Without this concept, the network would not be able to efficiently manage dual-carrier transmissions, leading to potential inefficiencies and reduced performance.

Furthermore, the Secondary Serving E-DCH cell enables the UE to adapt to changing conditions on the secondary carrier. For example, if the secondary carrier experiences high interference, the Node-B can reduce the grant, and the UE will lower its transmit power or data rate accordingly. This dynamic scheduling is essential for maintaining a stable and efficient uplink. The concept also supports mobility, as the UE can handover the secondary Serving E-DCH cell to another cell if needed, ensuring continuity of dual-carrier operation.

The specifications 25.319 and 25.321 define the detailed procedures for the Secondary Serving E-DCH cell, including the signaling of grants, the UE's behavior, and the interaction with the primary Serving E-DCH cell. These specifications were developed to ensure interoperability and consistent performance across different vendors. Although DC-HSUPA has been largely superseded by LTE and 5G, the concept of a Secondary Serving E-DCH cell remains a testament to the evolution of uplink scheduling in 3GPP.