On-Off Keying

Description

On-Off Keying (OOK) is a fundamental amplitude-shift keying (ASK) modulation technique where the presence or absence of a carrier signal directly encodes binary data. In the context of 3GPP, particularly for NR-Light (RedCap) and IoT enhancements, OOK is leveraged for its extreme simplicity and energy efficiency. The transmitter architecture is straightforward: for a binary '1', a radio frequency (RF) carrier is generated and transmitted for the duration of the symbol period; for a binary '0', the transmitter is effectively silenced, consuming minimal power. This on/off switching is controlled by the baseband digital signal.

At the receiver, demodulation typically involves an envelope detector or a simple energy detection circuit. The receiver measures the received signal strength or power over a symbol period. A signal power above a certain threshold is decoded as a '1', while power below the threshold (often just noise) is decoded as a '0'. This non-coherent detection does not require complex carrier phase recovery, further simplifying receiver design. The key parameters governing OOK performance in 3GPP systems include the symbol rate, which dictates the data throughput, and the decision threshold at the receiver, which impacts the bit error rate (BER) in the presence of noise and interference.

OOK's role in modern 3GPP networks, especially from Release 18 onwards, is primarily in enabling ultra-low-power wake-up radios (WURs) and for simple sensor communications. It is specified within the framework of NR sidelink and potentially for direct device-to-device communication in IoT scenarios. Its spectral efficiency is low compared to more advanced modulation schemes like QPSK or 16QAM, as it only transmits one bit per symbol and uses a relatively wide bandwidth due to the sharp transitions between on and off states. However, this trade-off is acceptable for applications where minimizing device cost, complexity, and energy consumption are paramount over achieving high data rates.

Purpose & Motivation

OOK was introduced into 3GPP standards to address the need for extreme energy efficiency in specific Internet of Things (IoT) and machine-type communication (MTC) use cases. Prior to its formal inclusion, more complex modulation schemes were used even for simple status reporting or wake-up signals, which consumed more power than necessary for the required functionality. The historical motivation stems from the proliferation of battery-powered sensors and wearables that require years of operation on a single battery charge.

The limitations of previous approaches, such as using a full OFDM-based NR physical layer for infrequent, tiny data transmissions, were excessive overhead and power consumption. OOK provides a minimalist alternative. Its creation within 3GPP Rel-18 standardizes a common, interoperable method for implementing these ultra-low-power links, ensuring they can coexist efficiently with wider bandwidth 5G NR carriers without causing harmful interference. It solves the problem of maintaining a radio link for control purposes (like waking up a main radio) or transmitting minimal sensor data while drawing minuscule amounts of power from the device battery.