Novel PAPR Reduction Techniques for OFDM Signals
Date
2025-05-07
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
ORCID
0000-0003-4629-529X
Type
Thesis
Degree Level
Doctoral
Abstract
Orthogonal frequency division multiplexing (OFDM) has become an indispensable multiplexing technique in many modern high data rate communication systems over the last three decades. This is thanks to the excellent data rate, and the robustness that OFDM demonstrates against frequency-selective fading due to the multipath propagation environment. Furthermore, OFDM also offers efficiency and flexibility in implementation with the usage of the fast Fourier transform (FFT). With these benefits, OFDM finds applications in many communication systems that require high throughput, as well as providing multiple access options in multiuser networks.
However, OFDM has an inherent drawback that limits its scalability: the high peak-to-average power ratio (PAPR). By design, OFDM divides the communication bandwidth into multiple narrow flat fading subchannels (also known as subcarriers) to deal with the frequency-selective fading effect of multipath propagation. However, unless specifically addressed, the more subcarriers are used, the higher the PAPR of the OFDM signal becomes. The high PAPR problem introduces non-linear distortion to the OFDM signals, which severely degrades the reliability of communications.
In this thesis, the high PAPR problem in OFDM systems is addressed by introducing novel techniques based on two popular distortionless PAPR reduction methods: partial transmission sequences (PTS) and tone reservation (TR). In the PTS schemes, the OFDM signal is decomposed into sub-sequences, which are then phase-rotated and summed back together to obtain an alternate OFDM signal with lower PAPR. On the other hand, the TR schemes reduce the PAPR by reserving a number of subcarriers to transmit the so-called canceling signals, which are capable of suppressing the peak power of the original OFDM signal. Both methods have been widely studied in the literature, and have been showed to be able to effectively reduce the PAPR of OFDM signals without introducing any distortion to the system.
However, both PTS and TR result in data rate loss with their respective designs. For the PTS schemes, it is the data rate overhead to inform the receiver about the phase rotation performed at the transmitter, which is necessary to recover the original signal. Whereas the rate loss in the TR schemes comes from the reserved subcarriers for PAPR reduction purposes.
To deal with the data rate overhead problem in the PTS schemes, in the first part of the thesis, a blind-PTS design is developed, which is capable of recovering the OFDM signal without the need to inform the receiver about the phase rotation. The proposed method is based on convex optimization and phase quantization rather than the search-based approaches in the majority of works in the PTS literature. Not only does the proposed design not require any data rate overhead, but it also has lower complexity and better PAPR than the search-based PTS.
In the second part of the thesis, the work in the first part is extended to further reduce the PAPR of OFDM signals. This is done by utilizing the unused degrees of freedom in existing PTS schemes, which is the reference sub-sequence (also known as the first sub-block). By optimizing and adding an optimized canceling signal to this reference sub-sequence, the PAPR can be significantly further reduced. Similar to the work in part one, this design does not require any data rate overhead associated with informing the receiver about the PAPR reduction performed at the transmitter.
Finally, the third contribution of the thesis is compensating for the rate loss in the TR methods with the integration of index modulation (IM). Unlike conventional TR schemes, where the positions of the reserved tones (also known as TR tones) are predetermined and fixed, in the proposed design, the positions of the TR tones are treated as random, which forms an extra dimension to carry data bits. To help the receiver distinguish between TR tones and data tones, a novel TR tone quantization method is introduced, which not only differentiates the TR tones and the data tones but also enables each TR tone to carry two bits of data. Last but not least, due to the vulnerability to error when distinguishing between TR tones and data tones, a novel forward error correction (FEC) mechanism is developed to increase the reliability of the proposed OFDM-TR-IM systems. Overall, the proposed novel OFDM-TR-IM design does not require any data rate overhead, but can reduce the error probability when differentiating TR tones from data tones, and improve the data rate significantly.
Description
Keywords
OFDM, PAPR, Partial Transmission Sequences, Tone Reservations
Citation
Degree
Doctor of Philosophy (Ph.D.)
Department
Electrical and Computer Engineering
Program
Electrical Engineering