Implicit Grant handling

Description

Implicit Grant handling is a Medium Access Control (MAC) scheduling mechanism introduced in 3GPP Release 12 for High Speed Packet Access (HSPA) uplink, specifically for the Dual-Cell HSUPA (DC-HSUPA) and later Multi-Cell HSUPA features. In HSPA uplink, the E-DCH (Enhanced Dedicated Channel) scheduling is controlled by the NodeB through two types of grants: Absolute Grants (AG), which set an absolute value of the serving grant, and Relative Grants (RG), which adjust the grant up or down relative to its current value. Each UE has a Serving Grant that determines the maximum E-DPDCH power it may use. With the introduction of a Secondary Serving E-DCH cell (the second uplink carrier in DC-HSUPA), the scheduler must manage grants on two uplink carriers simultaneously, and the control signalling budget on the downlink becomes a limiting factor.

Implicit Grant handling addresses this by allowing the network to revoke a UE's Scheduling Grant on the Secondary Serving E-DCH cell indirectly, i.e. implicitly. Instead of sending a dedicated Absolute Grant to the UE whose grant is to be revoked, the NodeB sends an Absolute Grant addressed to a different UE. The addressed UE interprets the AG normally, but the non-addressed UE that detects the AG intended for another UE understands that its own grant on the Secondary Serving E-DCH cell is implicitly revoked (set to zero or a minimum value). This is possible because E-AGCH (E-DCH Absolute Grant Channel) transmissions are broadcast-like: all UEs in the cell monitor the E-AGCH, and each transmission carries a UE identity (E-RNTI). A UE that decodes an AG whose identity does not match its own can still use the presence of that transmission as an implicit signal.

The mechanism relies on the MAC-e/es and MAC-i/is entities in the UE and the MAC-e scheduler in the NodeB. In the UE, the E-DCH MAC entity monitors the E-AGCH for its own E-RNTI and for other E-RNTIs. When it detects an Absolute Grant for another UE while it holds a non-zero grant on the Secondary Serving E-DCH cell, it applies the implicit revocation rule defined in 3GPP TS 25.321 (MAC protocol specification) and TS 25.331 (RRC protocol specification). The exact conditions, timers and the value to which the grant is reduced are configured by RRC signalling, as described in TS 25.319 (Enhanced uplink; Overall description).

The primary benefit is downlink control channel overhead reduction. In DC-HSUPA, each UE would otherwise require explicit AG signalling on both carriers to manage its grants, consuming E-AGCH resources and UE-specific signalling. By allowing one AG transmission to serve a dual purpose — granting resources to one UE and implicitly revoking another UE's secondary-carrier grant — the scheduler can control a larger number of UEs with the same E-AGCH capacity. This improves cell throughput and scheduling fairness, particularly in scenarios with many UEs and bursty traffic.

Implicit Grant handling is tightly coupled to the concept of the Secondary Serving E-DCH cell and the E-DCH Serving Grant update procedures. It is a Release 12 feature that remains part of the HSPA specifications through Release 19, although HSPA itself is in maintenance mode as networks migrate to LTE and NR. The term is defined consistently across TS 25.319, TS 25.321 and TS 25.331, and its definition text has remained essentially unchanged since introduction, reflecting a stable, mature mechanism.

Purpose & Motivation

Implicit Grant handling was created to solve a specific downlink signalling bottleneck in Dual-Cell HSUPA (DC-HSUPA). When a UE transmits on two uplink carriers, the NodeB scheduler must control the UE's transmit power on both the Primary and Secondary Serving E-DCH cells. The straightforward approach is to send explicit Absolute Grants for each carrier, but E-AGCH capacity is limited and each AG consumes downlink power and code resources. With many UEs active, explicit per-carrier grant management becomes inefficient and can limit the number of UEs that can be scheduled simultaneously.

The mechanism exploits an existing property of HSPA: E-AGCH transmissions are monitored by all UEs in the cell, and each carries an E-RNTI. By defining a rule that an AG addressed to one UE implicitly revokes another UE's grant on the Secondary Serving E-DCH cell, the network can manage secondary-carrier grants with fewer transmissions. This reduces control overhead, frees E-AGCH capacity for actual grant assignments, and improves uplink scheduling efficiency.

The feature also reflects the general HSPA design philosophy of reusing existing physical channels and MAC procedures rather than introducing new ones. Instead of defining a new downlink control channel for secondary-carrier revocation, 3GPP reused the E-AGCH and added an interpretation rule in the UE MAC. This keeps UE complexity low and preserves backward compatibility with earlier HSPA releases.