Outphasing transmitter with over-the-air combining : mmWave prototype implementation and measurements
Glonti, Nikoloz (2024)
2024
Tekniikan ja luonnontieteiden tiedekunta - Faculty of Engineering and Natural Sciences
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Hyväksymispäivämäärä
2024-07-29
Julkaisun pysyvä osoite on
https://urn.fi/URN:NBN:fi:tuni-202406247368
https://urn.fi/URN:NBN:fi:tuni-202406247368
Tiivistelmä
Modern wireless communication systems in the vast majority use Orthogonal frequency-division multiplexing (OFDM) modulated signals, that allow for great flexibility in term of radio resource management. On the other hand, OFDM is known to have a high peak-to-average power ratio. As the signal has a high dynamic range, this leads to the need to back-off transmit power - to avoid non-linear distortions. This approach however results in lowering the RF power amplifier's energy efficiency. To achieve higher power efficiency, RF power amplifiers must work closer to their saturation point, but then, nonlinear distortions are created. Unwanted spectral products appear as well inside in-band and out-of-band (OOB) portions of the signal. Distortions reduce spectral efficiency partially by introducing higher Error Vector Magnitude (EVM). Due to the OOB distortion, some part of the adjacent frequency bands has to be treated as unusable. In the 5G specification, a portion of the frequency band that serves similar purpose is called guard band.
Nonlinear distortions could be reduced using outphasing. Instead of amplifying the signal with one PA, the initial signal would be transformed into two signals, so-called outphased signals, so that after independently amplifying both, and combining, an amplified initial signal is produced. Single-level outphasing allows the use of highly nonlinear power amplifiers, working close to their saturation, to amplify outphased signals. On the other hand, this approach suffers from low energy efficiency, due to RF PAs constantly having a signal with the same high power over time on its output, even when the initial signal's amplitude is low. This is a common scenario for signals with high PAPR.
An energy-efficient alternative to single-level outphasing relies on multiple levels of amplitude (at the signal components PA output). When input signal power has relatively small amplitude, outphased signals will have lower amplitude levels. ML-OP could be implemented with architecture similar to two Polar transmitters. It can be also implemented as an on-chip digital transmitter.
Regardless of the outphasing type, one known limitation is the RF combiner. It limits maximum transmitted power, operation bandwidth and introduces power losses. However, it is possible to bypass these limitations with over-the-air combining. Outphasing with over-the-air combining involves transmitting outphased signals using an antenna array with properly chosen antenna mapping. Moreover, this can be combined with beamforming, so the direction of propagation can be adapted.
This work presents single and multi-level outphasing with over-the-air combining and the prototype transmitter implementation. Moreover, the results of simulations, measurements of EVM around the desired angle, and TRP-ACLR on mmWave frequency band will be presented.
Nonlinear distortions could be reduced using outphasing. Instead of amplifying the signal with one PA, the initial signal would be transformed into two signals, so-called outphased signals, so that after independently amplifying both, and combining, an amplified initial signal is produced. Single-level outphasing allows the use of highly nonlinear power amplifiers, working close to their saturation, to amplify outphased signals. On the other hand, this approach suffers from low energy efficiency, due to RF PAs constantly having a signal with the same high power over time on its output, even when the initial signal's amplitude is low. This is a common scenario for signals with high PAPR.
An energy-efficient alternative to single-level outphasing relies on multiple levels of amplitude (at the signal components PA output). When input signal power has relatively small amplitude, outphased signals will have lower amplitude levels. ML-OP could be implemented with architecture similar to two Polar transmitters. It can be also implemented as an on-chip digital transmitter.
Regardless of the outphasing type, one known limitation is the RF combiner. It limits maximum transmitted power, operation bandwidth and introduces power losses. However, it is possible to bypass these limitations with over-the-air combining. Outphasing with over-the-air combining involves transmitting outphased signals using an antenna array with properly chosen antenna mapping. Moreover, this can be combined with beamforming, so the direction of propagation can be adapted.
This work presents single and multi-level outphasing with over-the-air combining and the prototype transmitter implementation. Moreover, the results of simulations, measurements of EVM around the desired angle, and TRP-ACLR on mmWave frequency band will be presented.