Understanding and Mitigating PAPR in OFDM Systems
- Venkateshu
- Jun 16
- 5 min read
Orthogonal Frequency Division Multiplexing (OFDM) is a cornerstone of modern wireless communication systems, offering robustness against multipath fading and high spectral efficiency. However, one of its key limitations is the high Peak-to-Average Power Ratio (PAPR), which negatively impacts system efficiency and hardware design. This article explores the origins and implications of PAPR in OFDM systems, followed by a deep dive into classical and emerging mitigation techniques that balance performance with practical implementation complexity.
1. Introduction
Orthogonal Frequency Division Multiplexing (OFDM) is a digital modulation technique that divides the total available bandwidth into many narrowband subcarriers. Each subcarrier is modulated with a low data rate stream using conventional modulation schemes like QAM or PSK.
OFDM is based on the well-known technique of Frequency Division Multiplexing (FDM). In FDM different streams of information are mapped onto separate parallel frequency channels. Each FDM channel is separated from the others by a frequency guard band to reduce interference between adjacent channels.
The OFDM scheme differs from traditional FDM in the following interrelated ways:
1. Multiple carriers (called subcarriers) carry the information stream,
2. The subcarriers are orthogonal to each other, and
3. A guard interval is added to each symbol to minimize the channel delay spread and intersymbol interference.

Image source: https://helpfiles.keysight.com/csg/89600B/Webhelp/Subsystems/wlan-ofdm/content/ofdm_basicprinciplesoverview.htm
In an OFDM (Orthogonal Frequency Division Multiplexing) system, the process starts with your data – a bunch of bits. These bits are grouped and converted into symbols, which are basically numbers that carry both amplitude and phase. These symbols represent points on a modulation scheme like BPSK or QAM.
These data sent on a different frequency channel (called a subcarrier). All these data go into the IFFT (Inverse Fast Fourier Transform).
The IFFT takes total number of subcarriers(N), and it converts them into a time-domain signal – something that can be actually sent over the air. You can think of the IFFT as creating N sine waves at different frequencies, each carrying one of your symbols, and then adding them all together to make a single complex signal.
Each of those sine waves (subcarriers) is spaced so that it doesn't interfere with the others – they are orthogonal. The result is a block of N samples called an OFDM symbol.
Once that's done, a few more steps happen (like adding a guard interval), and the signal is transmitted over the radio channel.
On the receiving side, the reverse happens. A device uses the FFT (Fast Fourier Transform) to separate the signal back into the individual frequency components, so it can recover the original data bits you sent.

Image source: https://helpfiles.keysight.com/csg/89600B/Webhelp/Subsystems/wlan-ofdm/content/ofdm_basicprinciplesoverview.htm
Benefits of OFDM
Robustness to multipath fading: Each subcarrier experiences flat fading, which simplifies equalization.
Spectral efficiency: Subcarriers are orthogonal, allowing overlapping spectra without inter-carrier interference (ICI).
Flexibility: Easily scalable to different bandwidths and data rates.
OFDM has become the modulation scheme of choice in numerous standards including 4G LTE, 5G NR, Wi-Fi, and DVB, due to its ability to handle frequency-selective fading and its support for high data rates. However, its multi-carrier nature causes high PAPR, posing challenges in the design of power amplifiers (PAs) and digital-to-analog converters (DACs).
How OFDM Results in High PAPR
What is PAPR?
For an OFDM signal x(t), the PAPR is defined as:

This measures how "peaky" the signal is—i.e., how much higher the peaks are compared to the average signal power.
Due to the superposition of multiple independently modulated subcarriers, the time-domain OFDM signal can have large peaks when the subcarriers constructively interfere. For example, in an 8-subcarrier OFDM system, if all carriers are in-phase:

Average power =A^2, so:

This shows even a small OFDM system can have significant PAPR.
PAPR Characteristics in OFDM
OFDM signals are composed of the sum of multiple subcarriers:

When all subcarriers align constructively (in phase), the peak power can be up to N times the average, making PAPR as high as:
PAPRmax=10 log10 N
This probability is low but non-negligible, especially in large-scale systems like 5G massive MIMO.
2. Why PAPR is a Problem
Power Amplifier Efficiency
PAs in wireless transmitters need to operate linearly to avoid signal distortion. A high PAPR forces the PA to operate in a back-off mode, reducing its efficiency and increasing power consumption.
Signal Clipping and Distortion
High PAPR can push the signal into the non-linear region of the PA, leading to:
In-band distortion: degrades BER performance.
Out-of-band radiation: causes adjacent channel interference.
Dynamic Range Requirements
High PAPR increases demands on DAC and ADC resolution and linearity, making hardware design more complex and costly.
Hardware Complexity
High PAPR increases the required dynamic range of ADCs and DACs, raising cost and design complexity.
3. PAPR Mitigation Techniques
Signal Distortion Techniques
Clipping and Filtering
Concept: Limit signal amplitude to a threshold and apply filtering to remove out-of-band components.
Example: Clipping level set to 3 dB above average power. Filtering reduces adjacent channel interference.
Pros: Simple to implement.
Cons: Causes in-band distortion and increases BER.
Peak Windowing
Concept: Apply window functions (e.g., Kaiser, Hamming) to smooth large peaks.
Example: Gaussian window applied to samples exceeding a threshold.
Pros: Less distortion than clipping.
Cons: Reduced spectral efficiency.
Probabilistic Techniques
Selective Mapping (SLM)
Concept: Generate multiple phase-rotated OFDM signals and select the one with the lowest PAPR.
Example: Four versions of OFDM signal generated with different phase vectors; one with lowest PAPR chosen.
Pros: No distortion.
Cons: Requires side information transmission.
Partial Transmit Sequence (PTS)
Concept: Divide input symbols into sub-blocks and apply independent phase shifts.
Example: Input divided into 4 sub-blocks; use exhaustive search over 4-phase values to minimize PAPR.
Pros: Good performance.
Cons: High complexity, side information needed.
Coding Techniques
Block Coding
Concept: Encode input data using special codes that avoid high PAPR patterns.
Example: Use Golay complementary sequences known for low PAPR.
Pros: Adds error correction capability.
Cons: Reduces data rate.
Tone Reservation
Concept: Reserve a few subcarriers to generate a signal that cancels peaks.
Example: 5% of subcarriers used to generate anti-peaks.
Pros: No BER degradation.
Cons: Slight loss in spectral efficiency.
Transform Techniques
Discrete Cosine Transform (DCT)
Concept: Apply DCT before IFFT to reduce time-domain correlation.
Example: Apply Type-II DCT to modulated data vector.
Pros: PAPR reduction up to 2–4 dB.
Cons: Additional complexity.
Companding
Concept: Use non-linear compression (e.g., µ-law) on the signal amplitude.
Example: Compress large amplitudes, expand small ones before IFFT; reverse at receiver.
Pros: Significant PAPR reduction.
Cons: Non-linear distortion.
4. Advanced and Hybrid Techniques
Machine Learning-Based Approaches
Concept: Train models (e.g., neural networks) to predict and minimize PAPR. A CNN learns mapping from input symbols to low-PAPR OFDM frames.
Pros: Adaptive and data-driven.
Cons: High training complexity.
Intelligent Reconfigurable Coding
Concept: Dynamically choose between coding strategies based on real-time metrics. Use SLM in good SNR, switch to companding in low SNR.
Pros: Environment-aware.
Cons: Requires channel estimation.
Deep Reinforcement Learning (DRL)
Concept: Model PAPR control as an MDP and use DRL to learn reduction policy. An agent learns to select phase vectors in PTS scheme.
Pros: Learns optimal actions.
Cons: Requires training time.
5. Practical Considerations
Standard Compliance: Techniques like SLM or PTS must preserve orthogonality and avoid excessive side information.
Implementation Complexity: Trade-offs between hardware feasibility and PAPR reduction.
Energy Efficiency: PAPR control becomes critical for battery-operated IoT and mobile devices.
6. Conclusion
While OFDM brings many advantages, high PAPR remains a central challenge, especially in next-generation systems. A range of mitigation techniques—ranging from classical methods like clipping to advanced AI-driven approaches—can be tailored for specific system constraints. The future lies in adaptive, hybrid, and context-aware methods that intelligently balance trade-offs in real time.
7 References
3GPP TS 36.211, Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation.
The Problem of Peak-to-Average Power Ratio in OFDM Systems, Martha C. Paredes #1 and M. Julia Fenandez-Getino Garcıa#2 https://arxiv.org/pdf/1503.08271
View of Improved PAPR Reduction Technique in 5G Systems: Addressing Limitations and Challenges with Computational Techniques, https://www.propulsiontechjournal.com/index.php/journal/article/view/1601/1116
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