SC-FDMA vs. OFDM: A Detailed Comparison of Modulation Techniques

This article explores the differences between Single Carrier Frequency Division Multiple Access (SC-FDMA) and Orthogonal Frequency Division Multiplexing (OFDM), two popular modulation techniques used in wireless communication systems.

As illustrated in the figure above, the primary distinction lies in the inclusion of Discrete Fourier Transform (DFT) and Inverse DFT (IDFT) modules in the SC-FDMA transceiver chain. These modules are absent in a standard OFDM system. Essentially, adding DFT before the Inverse Fast Fourier Transform (IFFT) in the transmitter and IDFT after the Fast Fourier Transform (FFT) in the receiver transforms an OFDM system into an SC-FDMA system.

Key Differences Between SC-FDMA and OFDM

Here’s a summary of the key differences:

• PAPR (Peak-to-Average Power Ratio): SC-FDMA generally exhibits a lower PAPR compared to OFDM. This is a significant advantage in mobile devices where power efficiency is crucial.
• Frequency Offset Sensitivity: SC-FDMA is less sensitive to frequency offsets than OFDM, contributing to more robust performance in environments with Doppler shifts.
• Application: SC-FDMA is widely adopted in LTE subscriber terminals for the uplink (transmit) path. A variant, OFDMA, is employed in the eNodeB downlink (or the receive path of LTE subscribers). OFDM, on the other hand, is used in many broadband technologies such as WiMAX (802.16d/16e) and WLAN (802.11a/11n/11ac).

OFDM: A Multicarrier Modulation Technique

OFDM is often referred to as a multicarrier modulation technique. It divides the available bandwidth into multiple orthogonal subcarriers, each transmitting a portion of the data. This approach distributes the data stream across multiple parallel channels.

While it might resemble Frequency Division Multiplexing (FDM), OFDM differs significantly. In OFDM, the total subcarriers are divided into subchannels, and these subchannels are mapped to a single data/traffic source.

Merits of OFDM and SC-FDMA

Both OFDM and SC-FDMA offer distinct advantages:

SC-FDMA Merits: As mentioned earlier, SC-FDMA excels in low PAPR and reduced sensitivity to frequency offsets.

OFDM Merits:

• Frequency Selective Fading Resilience: Frequency selective fading primarily affects a subset of subchannels/subcarriers, leaving the rest of the band largely unaffected.
• Inter-Symbol Interference (ISI) Mitigation: OFDM effectively combats the impact of ISI, a common issue in multipath channel environments.