OFDM: Advantages and Disadvantages of Orthogonal Frequency Division Multiplexing

This page explores the benefits and drawbacks of OFDM (Orthogonal Frequency Division Multiplexing) as a data modulation technique.

What is OFDM?

Introduction: OFDM stands for Orthogonal Frequency Division Multiplexing. It’s a type of FDM (Frequency Division Multiplexing) where the subcarriers are arranged very close together, but in a way that makes efficient use of the available bandwidth.

OFDM achieves high data rates by transmitting data in parallel across multiple orthogonal subcarriers simultaneously. This method is also known as a digital multi-carrier modulation scheme.

OFDM transmitter with spectrum

An OFDM transmitter uses sophisticated modulation techniques to pack more data into each OFDM symbol. The amount of data carried by each subcarrier depends on the modulation level. For example, 16-QAM modulation maps 6 bits to a complex symbol, and this symbol is then transmitted using a single subcarrier within the OFDM symbol. In OFDM transmission, multiple such subcarriers are transmitted with each carrying different data symbols to increase the data rate and consecutively the throughput. This is shown in the figure-1 above.

OFDM transmitter and receiver chain

As shown in Figure 2, an IFFT (Inverse Fast Fourier Transform) is used at the OFDM transmitter, while an FFT (Fast Fourier Transform) is used at the OFDM receiver. A Digital-to-Analog Converter (DAC) is needed at the transmitter to convert the digital signal from the IFFT output into an analog signal. This analog signal is then sent to the RF front-end (which includes the RF transmitter, power amplifier (PA), and antenna) before being transmitted over the channel.

At the receiver, the OFDM signal passes through the RF front-end (which includes the Low Noise Amplifier (LNA) and RF receiver) before reaching the Analog-to-Digital Converter (ADC). The ADC converts the analog signal back into a digital form for baseband processing. Initial equalization algorithms are then applied to correct for time, frequency, and channel distortions.

There are several variants of OFDM, including f-OFDM (Flash OFDM), Hybrid-OFDM, OFDMA, COFDM, VOFDM, and WOFDM.

Benefits or Advantages of OFDM Data Modulation

Here’s a list of the advantages of using OFDM:

• Efficient Spectrum Usage: OFDM utilizes the frequency spectrum efficiently with its overlapping narrow subcarriers, making it better than traditional FDM methods.
• Resistant to Frequency Selective Fading: By dividing the broadband channel into smaller narrowband subchannels, OFDM is less susceptible to frequency-selective fading. Furthermore, channel encoder/decoder and interleaver/deinterlaver chain helps in recovering lost OFDM symbols due to fading.
• Robust Against Multipath Fading: OFDM uses a cyclic prefix to eliminate Inter-Symbol Interference (ISI) caused by multipath channels.
• Simplified Channel Equalization: Channel estimation and equalization are simplified by using known patterns (preambles) and embedded pilot carriers within the symbol. This is more efficient than channel equalization in Single Carrier (SC) systems.
• Easy Time Offset Estimation: Time offset estimation and correction algorithms are straightforward due to the correlation technique used.
• Scalable Bandwidth: Bandwidth can be allocated according to resource requirements, making OFDM bandwidth-scalable.
• Efficient Implementation with FFT: OFDM data modulation and demodulation can be implemented using computationally efficient FFT techniques.
• Less Sensitive to Sampling Time Offset: OFDM is less sensitive to sampling time offset impairments compared to SC systems.
• Robust Against Narrowband Interference: OFDM is resilient to narrowband co-channel interference.
• No Tuned Sub-Channel Filters Required: Unlike FDM, OFDM receivers don’t need tuned sub-channel filters.
• Facilitates SFNs: OFDM supports Single Frequency Networks (SFNs) for transmit macro diversity.

Drawbacks or Disadvantages of OFDM Data Modulation

Here are the disadvantages of OFDM:

• High Peak-to-Average Power Ratio (PAPR): OFDM signals have a high PAPR because of their noise-like amplitude with a large dynamic range. This requires the use of RF Power Amplifiers (PAs) with a higher PAPR in OFDM-based transmission systems.
• Sensitive to Carrier Frequency Offset (CFO): OFDM is more sensitive to CFO than SC systems due to different Local Oscillators (LOs) and DFT leakage. This necessitates complex frequency offset correction algorithms at the OFDM receiver.
• Prone to ISI and ICI: OFDM is susceptible to Inter-Symbol Interference (ISI) and Inter-Carrier Interference (ICI). This requires time and frequency offset correction algorithms.
• Guard Band Overhead: OFDM spectra travel through multiple paths, requiring a guard band to avoid ISI errors caused by timing offsets. The use of cyclic prefix leads to a loss of efficiency.
• Sensitive to Doppler Shift: OFDM performance can be affected by Doppler shift.
• Linear Transmitter Circuitry Required: OFDM requires linear transmitter circuitry, which can suffer from poor power efficiency.