Description
The Special Burst Scheduling Period (SBSP) is a concept defined within the 3GPP UMTS specifications, specifically related to the physical layer operation of the Wideband Code Division Multiple Access (WCDMA) air interface. It is a configurable network parameter that establishes a periodic time frame during which the network can schedule 'Special Bursts' on the uplink Dedicated Physical Control Channel (DPCCH). The DPCCH is a control channel that accompanies the Dedicated Physical Data Channel (DPDCH) in a WCDMA connection, carrying pilot bits for channel estimation, Transport Format Combination Indicator (TFCI), Feedback Information (FBI), and Transmit Power Control (TPC) commands.
During normal operation, the uplink DPCCH transmits continuously. However, the SBSP mechanism allows the Radio Network Controller (RNC) to instruct the User Equipment (UE) to temporarily transmit a predefined 'Special Burst' pattern on the DPCCH during specific, scheduled gaps. The primary technical purpose of these Special Bursts is to facilitate compressed mode operation. Compressed mode is a technique used in UMTS to create transmission gaps in the normal uplink/downlink frames, allowing the UE's receiver to tune to other frequencies (e.g., for inter-frequency or inter-RAT measurements like to GSM carriers) without needing a dual receiver.
The SBSP defines the periodicity of these opportunities for Special Burst transmission. When the network configures compressed mode, it uses the SBSP to align the transmission gaps. During the scheduled gap, the UE stops its normal DPDCH/DPCCH transmission and may send a Special Burst, which is a short, known signal. This Special Burst helps maintain the uplink power control loop and allows the Node B to estimate the channel condition even during the gap, ensuring a faster and more stable recovery of the connection once the gap ends. The configuration of the SBSP, including its period and length, is signaled to the UE via Radio Resource Control (RRC) messages from the RNC as part of the compressed mode parameters. This parameter is crucial for balancing the need for measurement gaps with the need to maintain link quality and synchronization.
Purpose & Motivation
The SBSP was introduced to support the critical mobility function of handover between different frequencies or different radio access technologies (inter-RAT), specifically in UMTS (Release 99 and beyond). In a WCDMA system, a UE typically transmits and receives continuously on its assigned frequency. To prepare for a handover to another UMTS frequency band or to a GSM network, the UE must measure the signal strength and quality of those potential target cells. Without a specific mechanism, this would require expensive and power-inefficient dual receivers in the UE.
Compressed mode was developed as a solution, temporarily compressing data into shorter transmission frames to create idle periods for measurements. The SBSP is an integral part of this solution, providing a structured and predictable scheduling framework for these compressed mode gaps. It solves the problem of how to periodically create these measurement opportunities in the uplink without causing the uplink control loop (especially power control) to fail. The Special Burst transmitted during the SBSP gap maintains a minimal link, allowing the Node B to keep track of the UE's signal.
Its creation was motivated by the requirement for seamless mobility in multi-frequency and multi-RAT deployment scenarios, which are essential for network load balancing, coverage optimization, and the graceful evolution from 2G to 3G. The SBSP parameter gave network operators control over the timing and frequency of these measurement gaps, allowing them to trade off between measurement performance (more/frequent gaps) and data transmission efficiency (fewer gaps).
Key Features
- Defines periodic scheduling for uplink Special Burst transmission
- Enables and structures compressed mode operation in UMTS
- Facilitates inter-frequency and inter-RAT measurements by the UE
- Helps maintain uplink power control and synchronization during transmission gaps
- Configurable parameter signaled via RRC from the network
- Integral to mobility and handover procedures in WCDMA
Evolution Across Releases
Introduced with the specification of compressed mode procedures for UMTS. The initial definition established the SBSP as a parameter to schedule gaps in uplink DPCCH transmission for the purpose of allowing the UE to perform measurements on other frequencies or RATs, supporting mobility from UMTS to GSM.
Defining Specifications
| Specification | Title |
|---|---|
| TS 25.224 | 3GPP TS 25.224 |