DFTs-OFDM vs OFDM: Why DFT-Spread OFDM Reduces PAPR Effectively (with MATLAB Code)

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.

DFT-spread OFDM (DFTs-OFDM) has lower Peak-to-Average Power Ratio (PAPR) because it "spreads" the data in the frequency domain before applying IFFT, making the time-domain signal behave more like a single-carrier signal rather than a multi-carrier one like OFDM.

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.

Further Reading 

1. DFT, FFT, and PSD
2. OFDM for 4G & 5G
3. OFDM Symbols and Subcarriers Explained
4. OFDM Transmission in Practical LTE and 5G Systems
5. OFDM Waveform with MATLAB Code   
6. SC-OFDM
7. GFDM

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.