Understanding PAPR in DFT-spread OFDM vs. Standard OFDM
In modern wireless communications like 4G LTE and 5G NR, managing the Peak-to-Average Power Ratio (PAPR) is critical for hardware efficiency. While OFDM is the gold standard for high-speed data, its high PAPR poses significant challenges for mobile devices. This is where DFTs-OFDM (also known as SC-FDMA) comes in.
Deeper Explanation:
| Aspect | OFDM | DFTs-OFDM |
|---|---|---|
| Signal Type | Multi-carrier | Single-carrier-like |
| Process | IFFT of QAM directly | QAM → DFT → IFFT |
| PAPR Level | High (due to many carriers adding up constructively) | Low (less fluctuation in amplitude) |
| Why PAPR is High | Subcarriers can add in phase, causing spikes | DFT "pre-spreads" data, smoothing it |
| Used in | Wi-Fi, LTE downlink | LTE uplink (as SC-FDMA) |
In OFDM, all subcarriers can align constructively → huge peaks → high PAPR.
In DFTs-OFDM, the DFT step spreads energy across subcarriers → smoother waveform.
MATLAB Code
clc; clear; close all;
% Parameters
N = 64; % Number of subcarriers
M = 16; % 16-QAM
numSymbols = 100; % Number of OFDM symbols
% Generate random data and modulate using QAM
data = randi([0 M-1], N*numSymbols, 1);
qamSymbols = qammod(data, M, 'UnitAveragePower', true);
qamSymbols = reshape(qamSymbols, N, numSymbols);
% --- OFDM ---
ofdmSignal = [];
for i = 1:numSymbols
x = qamSymbols(:, i);
ifftOut = ifft(x);
ofdmSignal = [ofdmSignal; ifftOut];
end
% Compute PAPR
paprOFDM = abs(ofdmSignal).^2;
PAPR_OFDM = max(paprOFDM) / mean(paprOFDM);
% --- DFTs-OFDM ---
dftsSignal = [];
for i = 1:numSymbols
x = qamSymbols(:, i);
dftOut = fft(x); % Spread data in frequency
ifftOut = ifft(dftOut); % OFDM modulation
dftsSignal = [dftsSignal; ifftOut];
end
paprDFTs = abs(dftsSignal).^2;
PAPR_DFTs = max(paprDFTs) / mean(paprDFTs);
% --- Display Results ---
fprintf('PAPR (OFDM): %.2f dB\n', 10*log10(PAPR_OFDM));
fprintf('PAPR (DFTs-OFDM): %.2f dB\n', 10*log10(PAPR_DFTs));
% --- Plot Time-Domain Envelopes ---
figure;
subplot(2,1,1);
plot(abs(ofdmSignal));
title('OFDM Time-Domain Signal');
ylabel('|x(t)|'); grid on;
subplot(2,1,2);
plot(abs(dftsSignal));
title('DFTs-OFDM Time-Domain Signal');
xlabel('Sample Index'); ylabel('|x(t)|'); grid on;
web('https://www.google.com/search?q=salimwireless.com+dft%20ofdm', '-browser');
Output
PAPR (OFDM): 10.35 dB
PAPR (DFTs-OFDM): 2.55 dB
Why Low PAPR is Critical for 5G and LTE Uplink
The primary reason DFTs-OFDM is used in the uplink (from your phone to the base station) is to preserve battery life. High PAPR requires the mobile phone's Power Amplifier (PA) to have a large "back-off" to avoid signal clipping and distortion. This makes the PA very inefficient, draining the battery rapidly. By using DFTs-OFDM, we reduce the PAPR, allowing the PA to operate closer to its saturation point, thereby increasing efficiency and coverage range. Read more about 5G Communication Technology (click here)
Comparison: OFDM vs. DFT-s-OFDM
- Primary Advantage: DFT-s-OFDM (SC-FDMA) offers a significantly lower Peak-to-Average Power Ratio (PAPR).
- Hardware Impact: Low PAPR allows the Power Amplifier (PA) to operate more efficiently, extending battery life.
- 5G Usage: 5G NR uses CP-OFDM for high-speed downlink and DFT-s-OFDM for power-efficient uplink in coverage-limited scenarios.
- Architecture: DFT-s-OFDM adds a "spreading" DFT step before the IFFT modulation.
Impact on Power Amplifier (PA) Efficiency
The main reason engineers obsess over PAPR is the Power Amplifier Back-off (PBO). In a standard OFDM signal, the high peaks force the PA to operate in its linear region, far below its maximum power capability.
Requires large "back-off" to avoid signal clipping. Result: Poor battery efficiency and heat generation.
Allows the PA to operate near Saturation. Result: Maximum coverage and longer smartphone battery life.
Common PAPR Reduction Techniques
While DFT-spreading is a structural fix, other Digital Signal Processing (DSP) methods are used in 5G and Wi-Fi 7 to manage power peaks:
- Clipping and Filtering: The simplest method, which manually cuts peaks above a threshold but increases Error Vector Magnitude (EVM).
- Selective Mapping (SLM): Creating multiple versions of the same data and transmitting the one with the lowest PAPR.
- Partial Transmit Sequence (PTS): Dividing the subcarriers into sub-blocks and phase-shifting them to minimize constructive interference.
- Tone Reservation (TR): Using specific "dummy" subcarriers to cancel out power peaks in the time domain.
| Metric | Standard OFDM | DFT-spread OFDM (SC-FDMA) |
|---|---|---|
| Waveform Type | Multi-carrier (Parallel) | Single-carrier-like (Serial) |
| MIMO Support | Very High / Simplified | Complex (Frequency Domain Equalization) |
| Spectral Efficiency | Maximum | Slightly lower (due to DFT overhead) |
| Typical Use Case | 5G Downlink, Wi-Fi 6/7 | 5G Uplink (Power Save Mode) |
| Sensitivity to CFO | High | Moderate |
Conclusion
Understanding the trade-off between spectral efficiency and power efficiency is key to 5G network optimization. While OFDM remains the king of high-speed data, DFT-s-OFDM is the unsung hero that keeps our mobile devices connected without draining the battery in minutes.
Want to know more about Power Spectral Density (PSD)? Click here.
Frequently Asked Questions
Q1: Is DFT-s-OFDM better than OFDM?
A: It is not correct to say one is universally better than the other because both serve different purposes in modern wireless communication systems like LTE and 5G. OFDM (Orthogonal Frequency Division Multiplexing) is generally preferred for the downlink (base station to device) because it is highly efficient in handling frequency-selective fading and supports high data rates with strong robustness in complex channel conditions.
On the other hand, DFT-s-OFDM (Discrete Fourier Transform-spread OFDM) is mainly used in uplink communication (device to base station) because it has a lower Peak-to-Average Power Ratio (PAPR). This makes the signal more power-efficient, which is important for battery-powered devices like smartphones. Lower PAPR reduces power amplifier stress, improving battery life and transmission efficiency.
In summary, OFDM is optimized for performance and capacity (downlink), while DFT-s-OFDM is optimized for power efficiency (uplink). The “better” choice depends on the use case and direction of communication.
Q2: Does 5G use DFT-s-OFDM?
A: Yes, 5G New Radio (5G NR) supports both CP-OFDM (Cyclic Prefix OFDM) and DFT-s-OFDM, depending on the scenario.
CP-OFDM is mainly used for both downlink and uplink because it supports high flexibility, advanced MIMO configurations, and high data rate services such as enhanced mobile broadband (eMBB).
However, DFT-s-OFDM is also supported in the uplink to improve power efficiency for user devices. This is especially useful for battery-constrained devices where reducing energy consumption is important.
In 4G LTE, SC-FDMA (based on DFT-s-OFDM principles) was strictly used for uplink. In 5G, both waveform options exist, allowing the network to adapt based on performance needs and device power constraints.