Absolute Grant

Description

The Absolute Grant (AG) is a critical component of the Enhanced Uplink (EUL), also known as High Speed Uplink Packet Access (HSUPA), within the 3GPP UMTS framework. It operates as part of the fast Node B-controlled scheduling mechanism for the Enhanced Dedicated Channel (E-DCH). The primary function of the AG is to provide the User Equipment (UE) with an explicit, absolute value for its maximum allowed uplink transmission power ratio, specifically for E-DCH data transmission. This power ratio is defined relative to the UE's Dedicated Physical Control Channel (DPCCH) power. The AG is transmitted from the serving Node B to the UE over the E-DCH Absolute Grant Channel (E-AGCH), a shared downlink physical channel. The grant value itself is quantized and represented by a 5-bit field, allowing the Node B to command a specific power headroom for the UE's uplink transmissions.

Architecturally, the AG mechanism is central to the fast scheduling loop in the Node B, as opposed to the slower Radio Network Controller (RNC)-based scheduling used in earlier UMTS releases. When the Node B decides to adjust a UE's uplink resource allocation—for instance, to manage cell load, prioritize traffic, or respond to a scheduling request—it sends an AG command. This command directly sets the UE's Serving Grant, which dictates the maximum E-DPDCH/DPCCH power ratio the UE can use for its next transmission. The AG can command a wide range of values, from a very low grant (effectively silencing the UE) to the maximum grant allowed for that UE's category.

The AG works in conjunction with the Relative Grant (RG). While the AG provides coarse, absolute power level settings, the RG (sent on the E-RGCH) provides fine, incremental adjustments (up, down, or hold) to the current grant. The AG is typically used for significant changes in resource allocation, such as when a UE starts a new data session or when the Node B needs to drastically reduce interference. The UE continuously monitors the E-AGCH from its serving cell and applies the new grant value at a defined activation time, enabling sub-2ms reaction times for scheduling decisions. This fast and direct control allows the Node B to tightly manage uplink interference and optimize cell throughput and latency.

Key components involved in the AG process include the Node B scheduler, which generates the grant value based on uplink load measurements and QoS requirements; the E-AGCH physical channel, which carries the grant command; and the UE's E-DCH medium access control (MAC-e/es) entity, which interprets the grant and applies it to select the appropriate Transport Format Combination (TFC) for uplink transmission. The AG's role is pivotal in transforming the UMTS uplink from a capacity-limited, RNC-controlled system to a high-speed, low-latency, and efficiently scheduled channel, forming the backbone of HSPA's uplink performance.

Purpose & Motivation

The Absolute Grant (AG) was introduced to solve fundamental limitations in the original UMTS Release 99 uplink. In the initial architecture, uplink resource scheduling was performed solely by the Radio Network Controller (RNC), a centralized entity with relatively slow reaction times (on the order of hundreds of milliseconds). This slow, cell-wide scheduling could not efficiently handle bursty packet data traffic, leading to poor uplink latency, low throughput, and inefficient use of the uplink power resource—the primary interference-limited commodity in WCDMA systems.

The creation of Enhanced Uplink (EUL) in Release 6 was motivated by the need for faster, more granular, and distributed uplink control. The AG mechanism was developed to enable this by putting scheduling decisions directly in the Node B, the network element closest to the radio interface. This allows the Node B to react in milliseconds to instantaneous changes in uplink interference and UE buffer status. The AG provides a direct and unambiguous command to control a UE's transmission power, which directly translates to its data rate and generated interference. This solves the problem of slow resource re-allocation and enables efficient multiplexing of multiple users' bursty traffic.

Historically, the AG addressed the key challenge of uplink interference management in a CDMA system. By giving the Node B a fast, explicit tool to set a UE's maximum power, the network could prevent a single UE from causing excessive interference and degrading service for all other users in the cell. It also allowed for rapid granting of resources when data arrived at a dormant UE, significantly improving user-perceived latency for interactive services. The AG, as part of the Node B scheduling framework, was a cornerstone innovation that made high-speed, low-latency uplink packet access a reality in 3G networks.

Key Features

• Explicitly sets the maximum allowed E-DPDCH/DPCCH power ratio for a UE's uplink transmission.
• Transmitted on the dedicated E-DCH Absolute Grant Channel (E-AGCH) from the Node B.
• Uses a 5-bit quantized value to command a specific absolute grant level.
• Enables very fast Node B-controlled scheduling with reaction times under 2ms.
• Provides coarse power control for significant changes in UE resource allocation.
• Works in tandem with Relative Grants (RG) for fine-tuning the granted power level.

Evolution Across Releases

Defining Specifications