Transmitter ON State

Description

The Transmitter ON 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 is actively transmitting downlink or uplink signals in the corresponding direction. In this state, the repeater's transmit chain is enabled, and the repeater amplifies and forwards the received signal toward the target coverage area. The ON state is the complement of the Transmitter OFF state and is the condition under which the repeater fulfills its primary function of extending network coverage.

Architecturally, the Transmitter ON state is controlled by the network through side control information (SCI). The gNB or a controlling node sends explicit on/off indications to the repeater, and the repeater's control unit configures its RF front-end to transmit when the ON state is indicated. The ON state can be applied independently to the downlink and uplink directions, allowing the repeater to forward downlink signals while remaining silent in the uplink, or vice versa. This per-direction control is essential for adapting to dynamic traffic patterns and interference conditions.

When in the Transmitter ON state, the repeater performs amplification and, in some cases, frequency translation. For a transparent payload repeater, the ON state involves receiving the signal on one frequency carrier, filtering and amplifying it, and retransmitting it on another frequency carrier. The repeater does not demodulate or decode the signal; it operates at the RF level. This makes the ON state a purely analog or RF-domain condition, but its timing and duration are dictated by the network's digital scheduling decisions. The repeater must be able to transition into and out of the ON state quickly and cleanly, without introducing spurious emissions or transient interference.

The ON state is closely tied to the repeater's beamforming and gain control. In advanced NCR deployments, the repeater may use multiple beams or antenna panels, and the ON state may be associated with specific beam configurations. The network can instruct the repeater to transmit with a certain beam direction or gain level while in the ON state, allowing spatial reuse and interference coordination. The ON state therefore is not merely a binary transmit/no-transmit condition; it is part of a richer control framework that includes beam selection, power control, and timing alignment.

From a system perspective, the Transmitter ON state is what makes the repeater useful. Without it, the repeater would be a passive element. By defining when the repeater is allowed to transmit, the specifications ensure that the repeater's transmissions are aligned with the network's scheduling and do not cause harmful interference. The ON state is also relevant for energy efficiency: the repeater consumes more power when transmitting, so the network aims to keep the repeater in the ON state only when necessary. The balance between ON and OFF states is a key optimization problem in NCR deployment, and the specifications provide the tools for the network to manage this balance dynamically.

Purpose & Motivation

The Transmitter ON state exists to define precisely when a network-controlled repeater is allowed to amplify and forward signals. Traditional repeaters were always on, which meant they amplified noise and interference even when no useful signal was present. The ON state, combined with the OFF state, allows the network to schedule repeater transmissions in a way that maximizes coverage benefit while minimizing interference and power consumption.

The ON state is essential for coverage extension in NR, particularly in millimeter-wave bands where propagation is challenging. By activating the repeater only when the network schedules transmissions that need forwarding, the ON state ensures that the repeater's energy is used efficiently and that its transmissions are coordinated with the rest of the network. This is especially important in multi-cell deployments where repeaters from different cells could interfere with each other if not properly controlled.

The concept was introduced in Rel-19 as part of the network-controlled repeater work item. The ON state, together with the OFF state, forms the basic control mechanism for NCRs. In Rel-20, the definition carried over unchanged, reflecting the stability of the fundamental concept. The ON state remains a cornerstone of repeater operation, enabling the network to treat repeaters as schedulable resources rather than uncontrollable amplifiers.