OFDM vs. f-OFDM: Key Differences Explained

This article compares OFDM (Orthogonal Frequency Division Multiplexing) with f-OFDM (Filtered OFDM), highlighting the key differences between these two modulation techniques.

What is OFDM?

OFDM stands for Orthogonal Frequency Division Multiplexing. It’s a multicarrier modulation scheme where data is transmitted over multiple closely spaced subcarriers.

In OFDM, these subcarriers are densely packed and modulated with complex data symbols before being upconverted to the radio frequency (RF). Each subcarrier essentially carries a portion of the total data. The magic of OFDM lies in its ability to demodulate each subcarrier successfully in the absence of Inter-Symbol Interference (ISI) and Inter-Carrier Interference (ICI).

This is achieved using a Cyclic Prefix (CP). The CP is a copy of the end portion of the OFDM symbol, added to the beginning. The duration of the CP is chosen to be greater than the expected delay spread of the communication channel. While the CP eliminates ISI and ICI, it does reduce spectral efficiency, making it less ideal for multicarrier uplink scenarios and cognitive radio systems.

Introducing f-OFDM (Filtered OFDM)

f-OFDM, or Filtered OFDM, is an enhanced version of OFDM that incorporates filtering to improve its spectral characteristics.

As illustrated above, the f-OFDM transmitter includes a filtering module after the IFFT (Inverse Fast Fourier Transform) and CP insertion stages. This filter is applied to the time-domain OFDM symbol to reduce out-of-band emissions while preserving the orthogonality of the OFDM symbols.

Common filter types used in f-OFDM include:

• Soft-Truncated Sinc Filters (using either Hann window or RRC window)
• Equi-ripple filter (using Remez exchange algorithm)

Key Differences: OFDM vs. f-OFDM

The following sections and table detail the key distinctions between OFDM and f-OFDM.

• Bandwidth Occupancy: In LTE systems, the maximum bandwidth occupancy is typically limited to 90% of the allocated bandwidth. However, in 5G, f-OFDM is employed, removing this 90% restriction. 5G utilizes filtering to meet Out-Of-Band Emission (OOBE) requirements, leveraging what is known as Filtered OFDM or f-OFDM. The figure above depicts the Power Spectral Density (PSD) of both OFDM and f-OFDM.

• Benefits: f-OFDM retains all the advantages of standard OFDM. Additionally, it facilitates efficient spectrum utilization by effectively meeting OOBE requirements.

• Sub-band Division: In f-OFDM, the entire allocated bandwidth is divided into multiple sub-bands. These sub-bands can then accommodate different types of services, each utilizing the most suitable waveform/numerology. This enhances overall spectrum utilization.

• Throughput: f-OFDM offers a throughput gain compared to traditional OFDM.

• Guard Bands: f-OFDM significantly reduces the need for large guard bands, leading to more efficient use of the available spectrum.

Comparison Table: OFDM vs. f-OFDM