TR Tone Reservation

Tone Reservation (TR) is a technique used in communication systems, particularly in orthogonal frequency-division multiplexing (OFDM) systems, to mitigate the effects of nonlinear distortion caused by power amplifiers. Nonlinear distortion can introduce unwanted spectral regrowth and intermodulation distortion, leading to a degradation of the communication system's performance.

In an OFDM system, the transmitted data is divided into multiple subcarriers, each carrying a part of the information. These subcarriers are modulated using different phases and amplitudes to encode the data. When the signal passes through a power amplifier, the amplifier's nonlinear characteristics can cause intermodulation products and spectral regrowth, leading to adjacent channel interference and reduced signal quality.

Tone Reservation is designed to mitigate these nonlinear effects by reserving specific frequencies, known as "tone reservation tones," to inject additional signals into the transmitted OFDM signal. These reserved tones are carefully chosen and constructed to minimize the distortion introduced by the power amplifier. The main steps involved in the Tone Reservation technique are as follows:

1. Power Amplifier Characterization: Before applying the Tone Reservation technique, the power amplifier's nonlinear characteristics need to be characterized. This involves measuring the amplifier's input-output transfer function to understand its distortion behavior.
2. Reserved Tone Generation: Based on the power amplifier's nonlinear characteristics, specific tones are selected as "tone reservation tones." These tones are usually located in regions where the amplifier exhibits significant distortion. The amplitude and phase of these tones are carefully calculated to create a signal that would counteract the distortion effects introduced by the power amplifier.
3. Signal Combination: The reserved tones are combined with the original data-carrying OFDM signal to form the composite signal that will be transmitted. The reserved tones are superimposed onto the OFDM signal, and the resulting composite signal is amplified using the power amplifier.
4. Distortion Compensation: When the composite signal passes through the power amplifier, the nonlinear distortion is introduced. However, because of the carefully designed tone reservation tones, the amplifier's distortion will also produce cancellation effects at these reserved frequencies, reducing the overall distortion of the transmitted signal.
5. Signal Reconstruction: At the receiver end, the transmitted signal is demodulated, and the reserved tones are extracted. These extracted tones are then used to estimate the distortion introduced by the power amplifier during transmission.
6. Distortion Removal: The estimated distortion is subtracted from the received signal, effectively removing the power amplifier's nonlinear effects. The resulting signal contains the original data without the distortion caused by the power amplifier.

By employing the Tone Reservation technique, communication systems can significantly improve their performance in the presence of nonlinear power amplifiers. This method effectively mitigates the distortion effects, leading to better signal quality, reduced adjacent channel interference, and enhanced overall system performance. Tone Reservation is widely used in practical communication systems to enhance their efficiency and reliability in real-world operating conditions.