Description
Dynamic Resource Allocation Control (DRAC) is a fundamental Radio Resource Management (RRM) function defined for the UMTS Terrestrial Radio Access Network (UTRAN) in 3GPP specifications. It operates within the Radio Network Controller (RNC) to manage the allocation of the critical, shared radio resources on the air interface: transmission power and channelization codes (OVSF codes). DRAC is responsible for the setup, modification, and release of these resources for dedicated channels (DCH) assigned to user equipment (UE) for carrying circuit-switched (CS) and packet-switched (PS) data. Its operation is continuous and dynamic, responding to changes in several key inputs.
The algorithm works by evaluating requests for resources (e.g., from a Radio Bearer Setup or Radio Bearer Reconfiguration procedure) against the current state of the cell. Key inputs include the measured uplink interference (Rise Over Thermal - RoT), downlink transmission power, available channelization code tree space, and the QoS parameters of the requested service (e.g., guaranteed bit rate, traffic class). For uplink, DRAC primarily controls the maximum allowed uplink power for a UE, which directly limits its data rate. For downlink, it manages the allocation of OVSF codes and the power budget for each connection. The core decision logic involves admission control and congestion control. Admission control determines if a new request can be granted without violating the stability or QoS of existing connections. Congestion control (or load control) actively monitors the cell and can trigger a reduction of resources for existing users if the load (interference or power) exceeds predefined thresholds.
DRAC's implementation is closely tied to other RRM functions like Packet Scheduler (PS), which handles shared channel (HSDPA/HSUPA) resources, and Handover Control. For services on dedicated channels, DRAC may work in conjunction with the Transport Format Combination (TFC) selection in the UE, where the UE selects a data rate (transport format) that does not exceed the resource grant provided by the RNC via DRAC. This dynamic and reactive control loop is essential for managing the interference-limited nature of WCDMA, ensuring system stability, maximizing spectral efficiency, and delivering the negotiated QoS to end users across varying network conditions.
Purpose & Motivation
DRAC was created to address the core challenge of resource management in WCDMA-based UMTS networks, which are inherently interference-limited and use shared, variable-rate dedicated channels. Unlike GSM's time-slot based allocation, WCDMA resources (power and codes) are continuous and shared by all users, making their management complex. The primary problem DRAC solves is the efficient and stable utilization of these shared resources while guaranteeing Quality of Service (QoS) for different traffic classes (conversational, streaming, interactive, background).
Before such dynamic control, simpler static or semi-static allocation methods would be highly inefficient, either wasting capacity by over-provisioning or causing service degradation and dropped calls during congestion. DRAC's dynamic nature was motivated by the need to support bursty packet data traffic efficiently. It allows the network to quickly grant high resources when a user is active (e.g., downloading a file) and rapidly scale them back or release them during inactivity periods, freeing capacity for other users. This is crucial for statistical multiplexing gain. Furthermore, by continuously monitoring uplink interference (RoT), DRAC protects the cell from instability—a situation where rising interference causes all UEs to increase power, leading to a runaway "cell breathing" effect or collapse. It provides the necessary real-time feedback control to maintain the network within its planned operational envelope.
Key Features
- Real-time allocation and de-allocation of uplink power and downlink OVSF codes
- Admission Control to accept or reject new radio bearer requests based on cell load
- Congestion/Load Control to reduce resource grants during high interference or power conditions
- QoS-aware resource management for different traffic classes
- Interaction with Transport Format Combination (TFC) control in the UE
- Fundamental for managing interference and stability in WCDMA networks
Evolution Across Releases
Introduced as a core Radio Resource Management (RRM) function for UTRAN. Initial architecture defined its role in the RNC for managing dedicated channel (DCH) resources, including admission control and dynamic channel reconfiguration based on traffic activity and interference measurements.
Defining Specifications
| Specification | Title |
|---|---|
| TS 21.905 | 3GPP TS 21.905 |
| TS 25.331 | 3GPP TS 25.331 |