Secondary Enhanced Dedicated Physical Control Channel

Description

The Secondary Enhanced Dedicated Physical Control Channel (S-E-DPCCH) is an uplink physical channel in the UMTS/HSPA radio interface, specifically for the Enhanced Uplink (E-DCH or HSUPA). It is used when the User Equipment (UE) is configured to transmit on two or four Enhanced Dedicated Channel (E-DCH) transport channels simultaneously, a feature known as multi-stream transmission introduced in later HSPA+ releases. The primary Enhanced Dedicated Physical Control Channel (E-DPCCH) carries control information for the primary E-DCH transport channel, while the S-E-DPCCH carries control information for the secondary E-DCH transport channel(s).

The S-E-DPCCH transmits crucial information for the Node B's scheduler and Hybrid Automatic Repeat Request (HARQ) processes related to the secondary stream. This includes the Retransmission Sequence Number (RSN), which indicates whether the current transmission is a new packet or a retransmission, and the Happy Bit. The Happy Bit is a 1-bit indicator that informs the network if the UE would be 'happy' with its current granted uplink resources or desires a higher data rate grant. This feedback is essential for the Node B to perform efficient packet scheduling and link adaptation.

From a physical layer perspective, the S-E-DPCCH is transmitted using a specific channelization code and is time-aligned with its associated Secondary E-DCH Dedicated Physical Data Channel (S-E-DPDCH). Its presence is conditional, based on the UE's E-DCH configuration. The channel allows for more granular control and feedback for multiple parallel uplink transmissions, increasing the efficiency of the HSUPA system by enabling better utilization of the UE's power and the cell's uplink capacity when the UE has multiple packets to send.

Purpose & Motivation

The S-E-DPCCH was created to support multi-stream transmission in HSUPA (Enhanced Uplink). Initial HSUPA (Rel-6) supported a single E-DCH transport channel. To increase peak uplink data rates and improve throughput, especially for bursty traffic, later releases introduced the ability to use multiple E-DCH transport channels in parallel. However, each independent transport channel requires its own control signaling for HARQ and scheduling feedback.

The primary E-DPCCH could not support this multi-stream operation. The S-E-DPCCH was introduced to solve this problem by providing a dedicated control channel for the secondary stream(s). This enabled features like 2ms TTI with multiple HARQ processes per stream, allowing for higher uplink data rates, reduced latency, and more efficient use of the UE's transmit power by enabling parallel packet transmissions. It was a key enhancement to keep HSUPA competitive and increase uplink spectral efficiency.