OFDM vs SC-OFDM

 

The main difference between OFDM and SC-OFDM is that SC-OFDM transmits the signal using a single carrier, while OFDM uses multiple subcarriers. However, in SC-OFDM, the signal is generated with different sub-bands, but it is transmitted through a single carrier (more technically, through a wideband carrier signal).

Block Diagram of OFDM:

Data → Modulation → Serial-to-Parallel → IFFT → Add CP → Transmit

Received Signal → Remove CP → FFT → Parallel-to-Serial → Demodulation → Data

Block Diagram of SC-OFDM:

Data → Modulation → DFT → IFFT → Add CP → Transmit

Received Signal → Remove CP → FFT → Demodulation → Data 

 

In the case of OFDM, the input modulated data is converted from a serial stream to parallel streams, and different subcarriers are assigned to each chunk. Then, IFFT is applied to these chunks, and a cyclic prefix is added to each one. Each chunk is technically referred to as an OFDM symbol.

Unlike OFDM, SC-OFDM does not perform serial-to-parallel conversion on the modulated input data. Instead, it directly applies DFT (Discrete Fourier Transform) to the data, followed by IFFT. Like OFDM, a cyclic prefix is added before transmission. SC-OFDM uses DFT and IFFT to simulate the multi-frequency behavior of OFDM, but instead of transmitting over multiple subcarriers, the data is transmitted using a single carrier. After IFFT, the data is converted back to the time domain, where it can be transmitted over the channel.

 

MATLAB Code for Comparison of OFDM vs SC-OFDM

% The code is developed by SalimWireless.com clc; clear; close all; % Parameters N = 64; % Number of subcarriers Fs = 1000; % Sampling frequency (Hz) Ncp = 16; % Length of Cyclic Prefix SNR = 20; % Signal to noise ratio nSymbols = 10; % Number of symbols % Generate random data and modulate using QPSK data = randi([0 1], N, nSymbols); % Random data for N subcarriers modData = pskmod(data, 4, pi/4); % QPSK modulation % --- OFDM Baseband Generation --- ofdmSignal = []; for sym = 1:nSymbols symbol = modData(:, sym); ofdmSignal = [ofdmSignal; ifft(symbol, N)]; % IFFT to get time-domain signal end % Add cyclic prefix to OFDM signal ofdmSignalWithCP = [ofdmSignal(end-Ncp+1:end, :); ofdmSignal]; % --- SC-OFDM Baseband Generation --- scOfdmSignal = []; for sym = 1:nSymbols symbol = modData(:, sym); dftSymbol = fft(symbol, N); % Apply DFT to simulate frequency components scOfdmSignal = [scOfdmSignal; ifft(dftSymbol, N)]; % IFFT to get time-domain signal end % Add cyclic prefix to SC-OFDM signal scOfdmSignalWithCP = [scOfdmSignal(end-Ncp+1:end, :); scOfdmSignal]; % Time axis for plotting t = (0:length(ofdmSignalWithCP)-1) / Fs; % --- Plotting OFDM vs SC-OFDM Baseband Signals --- figure; % Plot OFDM Baseband Signal subplot(2, 1, 1); plot(t, real(ofdmSignalWithCP(:)), 'b', 'LineWidth', 1.5); title('OFDM Baseband Signal (with Cyclic Prefix)'); xlabel('Time (s)'); ylabel('Amplitude'); legend('OFDM Baseband Signal'); grid on; % Plot SC-OFDM Baseband Signal subplot(2, 1, 2); plot(t, real(scOfdmSignalWithCP(:)), 'r', 'LineWidth', 1.5); title('SC-OFDM Baseband Signal (with Cyclic Prefix)'); xlabel('Time (s)'); ylabel('Amplitude'); legend('SC-OFDM Baseband Signal'); grid on; web('https://www.salimwireless.com/search?q=ofdm', '-browser');

 

Output

 

 

Comparison of Steps for OFDM vs SC-OFDM (Baseband)

Here’s a breakdown of the steps involved in both OFDM and SC-OFDM for the baseband signal processing.

1. Input Data (Symbols)

OFDM: The input data is divided into multiple subcarriers. This data is usually modulated using a scheme like QPSK or QAM (e.g., 64-QAM).

SC-OFDM: The input data is also modulated (usually QPSK or QAM), but it's processed in a way that will later simulate single carrier transmission.

2. Mapping to Frequency Domain

OFDM: The symbols are directly mapped to multiple subcarriers. The mapping is done by placing each modulated symbol onto a separate subcarrier, which is orthogonal to the others.

This is typically done using the IFFT (Inverse Fast Fourier Transform).

SC-OFDM: The symbols are processed through DFT (Discrete Fourier Transform) to divide the data into frequency components, but instead of using multiple subcarriers, all frequency components are combined into a single carrier.

3. IFFT (Inverse Fast Fourier Transform)

OFDM: After mapping the symbols to subcarriers, an IFFT is applied to convert the signal from the frequency domain to the time domain. Each subcarrier carries a portion of the data.

SC-OFDM: The DFT is used to convert the data into frequency components, which are then combined using IFFT. The main difference is that the entire signal behaves like a single carrier, even though it’s represented in the frequency domain initially.

4. Cyclic Prefix (CP) Addition

Both OFDM and SC-OFDM: A cyclic prefix (CP) is added to the signal to prevent Inter-Symbol Interference (ISI) caused by multipath fading. The cyclic prefix is a copy of the last few samples of the signal, repeated at the beginning.

5. Baseband Signal

OFDM: The resulting baseband signal after IFFT is spread across multiple subcarriers in the time domain. This allows for parallel data transmission, which is very efficient for high data rates.

SC-OFDM: The baseband signal after IFFT still consists of a single carrier, but it contains frequency components that simulate multiple subcarriers (via the DFT). The signal is similar to traditional single carrier signals but with data distributed over a frequency range.

6. Results (Baseband Signal):

OFDM Baseband: The baseband OFDM signal will show multiple peaks (each corresponding to a subcarrier), and it will look more complex in the time domain due to the presence of multiple subcarriers.

SC-OFDM Baseband: The baseband SC-OFDM signal will show a single peak, representing a single-carrier signal, but its structure is still influenced by multiple frequency components that are DFT-based.
 

Further Reading

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