Transmitter OFF State

Description

The Transmitter OFF state is a normative operational state defined for network-controlled repeaters (NCRs) in 3GPP NR specifications 38.106 and 38.115. It represents a time period during which the repeater's downlink or uplink transmission path is explicitly not allowed to radiate energy in the corresponding direction. This state is distinct from simply having no signal to amplify: it is a controlled, gNB-scheduled condition in which the repeater must suppress transmission regardless of whether an input signal is present at its receive antenna. The concept is central to the operation of NCRs, which are introduced to extend NR coverage in a cost-effective, low-complexity manner without the full baseband processing capabilities of an integrated access and backhaul (IAB) node or a full gNB.

Architecturally, the Transmitter OFF state is managed by the network through side control information (SCI) delivered to the repeater. The gNB, or a controlling node, sends explicit on/off indications that tell the repeater when to transmit and when to remain silent. The repeater's control unit interprets these indications and configures its RF front-end accordingly. In the OFF state, the transmit chain is disabled or muted, preventing any amplified signal from reaching the transmit antenna. This applies independently to the downlink and uplink directions, meaning a repeater could be in Transmitter ON state for the downlink while simultaneously being in Transmitter OFF state for the uplink, or vice versa, depending on the scheduling decisions of the network.

The primary technical motivation for the OFF state is interference management. Because a repeater amplifies and forwards signals, uncontrolled amplification can create harmful interference to neighboring cells, especially in multi-cell deployments where the same frequency is reused. By forcing the repeater into the OFF state during specific time resources, the network can prevent the repeater from amplifying noise, unintended signals, or transmissions from user equipment that would otherwise cause cross-link interference. This is particularly important in dynamic TDD scenarios, where uplink and downlink allocations vary over time and adjacent cells may have opposite transmission directions.

Another important role of the Transmitter OFF state is energy efficiency. Repeaters are often deployed in remote or hard-to-reach locations where power may be limited. By turning off the transmit chain when no useful signal needs to be forwarded, the repeater reduces its power consumption and thermal load. This is especially relevant for repeaters powered by batteries or solar panels. The OFF state also reduces unnecessary electromagnetic emissions, which can be relevant for regulatory compliance and for coexistence with other systems.

The state is defined at the physical layer and is closely tied to the repeater's beamforming and amplification capabilities. In Rel-19 and Rel-20, the specifications define the behavior of the repeater in terms of transmitter on/off transitions, including timing requirements and the granularity at which the state can be changed. The repeater must be able to switch between ON and OFF states with sufficient speed to follow the network's scheduling, and the transition must not introduce spurious emissions that violate spectrum mask requirements. The Transmitter OFF state therefore represents a fundamental building block of NCR operation, enabling the network to treat repeaters as controllable, schedulable network elements rather than simple analog amplifiers.

Purpose & Motivation

The Transmitter OFF state exists to give the network fine-grained control over when a repeater radiates energy. Before network-controlled repeaters, traditional analog repeaters amplified whatever they received, which made interference management difficult and often forced operators to use conservative power settings or static on/off schedules. The OFF state solves this by allowing the gNB to explicitly silence the repeater during time resources where amplification would be harmful, such as during uplink transmissions in neighboring cells or during guard periods.

This capability is essential for deploying repeaters in dense or dynamic environments. In dynamic TDD, the direction of transmission can change rapidly, and a repeater that blindly amplifies could create severe cross-link interference. The OFF state allows the network to align repeater behavior with the instantaneous scheduling decisions, ensuring that the repeater only forwards signals when it is beneficial and remains silent otherwise. It also supports energy-saving objectives by deactivating the transmit chain when no useful signal is present.

The concept was introduced in Rel-19 as part of the broader work on network-controlled repeaters, which aimed to provide a low-cost coverage extension solution for NR. The OFF state, together with the ON state, forms the basic on/off control framework that makes repeaters manageable network nodes. In Rel-20, the definition carried over unchanged, indicating that the fundamental concept was stable and that subsequent work focused on other aspects of repeater operation.