Single-Carrier Frequency Division Multiplexing

Description

Single-Carrier Frequency Division Multiplexing (SC-FDM) is a digital modulation scheme that forms the basis for the uplink multiple access technique in LTE (where it is called DFT-spread OFDM or DFT-s-OFDM) and 5G NR. While it shares the orthogonal subcarrier structure with Orthogonal Frequency Division Multiplexing (OFDM), its key differentiator is a Discrete Fourier Transform (DFT) precoding stage applied before the conventional OFDM modulation. This process transforms the multi-carrier signal into a single-carrier-like waveform in the time domain, which possesses favorable peak-to-average power ratio (PAPR) characteristics.

The architecture of an SC-FDM transmitter involves a specific signal processing chain. First, a stream of modulation symbols (e.g., QPSK, 16QAM) is grouped into blocks. Each block undergoes a DFT operation. This DFT-spread step distributes the energy of each original symbol across all the subcarriers in the frequency domain. The output of the DFT is then mapped onto a subset of the available orthogonal subcarriers within the system bandwidth. Following this subcarrier mapping, the signal is processed by an Inverse Fast Fourier Transform (IFFT) to convert it back to the time domain, just as in standard OFDM. Finally, a cyclic prefix (CP) is added to combat inter-symbol interference caused by multipath propagation. The receiver performs the inverse operations: CP removal, FFT, subcarrier demapping, and then an Inverse DFT (IDFT) to recover the original modulation symbols.

How SC-FDM works fundamentally relies on the properties of the DFT precoding. In pure OFDM, each subcarrier is independently modulated, leading to a time-domain signal that is the sum of many sinusoids. This can result in high peak power when they constructively interfere, leading to a high PAPR. High PAPR forces power amplifiers (PAs) to operate with a large back-off from their saturation point to avoid distortion, which is highly inefficient. SC-FDM's DFT precoding creates correlation between the subcarriers. This correlation shapes the time-domain output to resemble a single-carrier signal, which has a more constant envelope and thus a lower PAPR. A lower PAPR allows the UE's PA to operate closer to saturation with higher efficiency, translating to lower power consumption and/or higher transmit power for the same battery drain.

The role of SC-FDM in the network is critical for uplink performance, especially from a cost and coverage perspective. User Equipment (UE) like smartphones are power-constrained devices with less sophisticated, cost-effective power amplifiers. SC-FDM's low PAPR is a key enabler for practical uplink design in broadband cellular systems. It allows UEs to transmit at higher average power without exceeding linearity limits, thereby extending uplink coverage range and cell edge data rates. In 5G NR, SC-FDM (referred to as CP-OFDM with DFT-s-OFDM precoding) is supported as an optional uplink waveform, providing flexibility. For high-frequency bands or for coverage-limited scenarios, its power efficiency advantages make it the preferred choice, ensuring reliable uplink connectivity which is essential for symmetric services and the Internet of Things (IoT).

Purpose & Motivation

SC-FDM was developed to solve a critical problem in the design of the LTE uplink: the high Peak-to-Average Power Ratio (PAPR) inherent to standard OFDM modulation. High PAPR is detrimental for battery-powered user devices. It forces the device's power amplifier (PA) to operate inefficiently in its linear region to avoid signal distortion, wasting battery power and reducing the effective transmit power. This limits uplink coverage, data rates, and device battery life. The purpose of SC-FDM is to provide a multiple access scheme that retains the multipath resistance and spectral efficiency benefits of OFDM while dramatically reducing the PAPR for the uplink transmitter.

The historical context and motivation are directly tied to the choice of OFDM for the LTE downlink. OFDM was selected for its excellent performance in frequency-selective fading channels and its simplicity in equalization. However, using the same waveform for the uplink was recognized as impractical due to the UE power amplifier constraints described above. Previous systems like UMTS used Wideband CDMA (WCDMA), which has a continuous phase and good PAPR but different limitations in spectral efficiency and equalization complexity for wide bandwidths. SC-FDM was created as an innovative compromise, merging single-carrier advantages with the flexible resource allocation of OFDMA.

SC-FDM specifically addresses the limitations of both pure single-carrier systems and pure OFDM. It overcomes the high equalization complexity of wideband single-carrier systems in severe multipath by using the frequency-domain equalization simplicity of the OFDM structure after the FFT at the receiver. Simultaneously, it overcomes OFDM's high PAPR through DFT precoding. This engineering solution was motivated by the need to achieve high uplink data rates and capacity in LTE without making UE design prohibitively expensive or power-hungry. Its creation was essential for making high-performance, consumer-friendly 4G devices viable and continues to serve the same purpose in 5G NR, especially for coverage-critical and power-constrained use cases.

Key Features

• DFT precoding creates a single-carrier-like time-domain signal
• Significantly lower Peak-to-Average Power Ratio (PAPR) than OFDM
• Enables efficient power amplifier operation in User Equipment (UE)
• Retains orthogonal subcarrier structure and flexible resource allocation (DFT-s-OFDMA)
• Uses frequency-domain equalization at the receiver for simplicity
• Supported as the primary/uplink waveform in LTE and an optional waveform in 5G NR

Evolution Across Releases

Defining Specifications