Orthogonal Frequency Division Multiplexing (OFDM) is a digital modulation and multiplexing technique that divides a high-speed data stream into multiple lower-speed substreams, each transmitted on a different subcarrier frequency. OFDM is commonly used in modern communication systems, such as wireless communication, digital television, and broadband internet.
The key idea behind OFDM is to divide the available spectrum into multiple narrowband subcarriers, each with orthogonal frequencies. These subcarriers are closely spaced and overlap, but their frequency spectra are designed to be orthogonal to each other. This orthogonality allows efficient and simultaneous transmission of multiple substreams without interference.
The process of Orthogonal Frequency Division Multiplexing involves the following steps:
- Data Encoding: The digital data to be transmitted is encoded and divided into multiple parallel data streams, each representing a different subcarrier.
- Subcarrier Generation: The subcarriers are generated by modulating the data streams onto individual sinusoidal waveforms with different frequencies. The frequencies of the subcarriers are carefully chosen to be orthogonal to each other.
- IFFT (Inverse Fast Fourier Transform): The parallel data streams are transformed from the frequency domain to the time domain using an inverse fast Fourier transform (IFFT) operation. This converts the data from subcarrier frequencies to time-domain waveforms.
- Guard Interval Insertion: A guard interval is inserted between each OFDM symbol to mitigate the effects of multipath interference. The guard interval is a period of silence or redundant data that helps in recovering the transmitted data accurately.
- Serial-to-Parallel Conversion: The time-domain waveforms from the IFFT operation are converted from a serial stream to parallel streams, with each stream representing a subcarrier.
- Parallel-to-Serial Conversion: The parallel subcarrier streams are combined into a single serial stream for transmission.
- Modulation and Transmission: The serial stream is modulated onto a carrier signal and transmitted through the communication channel, such as wireless channels or wired media.
- Reception and Demodulation: At the receiving end, the received signal is demodulated to extract the serial stream. The serial stream is then parallelized into individual subcarrier streams.
- FFT (Fast Fourier Transform): Each subcarrier stream undergoes a fast Fourier transform (FFT) operation to convert the time-domain waveforms back to the frequency domain.
- Data Decoding: The demodulated subcarrier streams are decoded to recover the original digital data.
OFDM offers several advantages and features:
- High Spectral Efficiency: OFDM allows efficient utilization of the available spectrum by dividing it into multiple orthogonal subcarriers. This enables high data rates and increased capacity.
- Robustness to Multipath Fading: OFDM exhibits good resistance to multipath fading and narrowband interference due to the use of a guard interval. The guard interval helps mitigate inter-symbol interference caused by delay spread in the channel.
- Flexibility: OFDM can adapt to different channel conditions by adjusting the modulation scheme and power allocation across subcarriers. This allows for adaptive modulation and coding techniques to optimize performance.
- Compatibility with FFT: The use of FFT and IFFT operations in OFDM enables efficient implementation using fast algorithms, such as the Fast Fourier Transform, which makes real-time processing feasible.
Orthogonal Frequency Division Multiplexing has become a fundamental technique in various communication standards, including wireless LANs (Wi-Fi), digital audio and video broadcasting (DAB/DVB), 4G/5G cellular networks, and digital subscriber line (DSL) systems. Its ability to provide high data rates, robustness, and spectral efficiency has made it a widely adopted modulation and multiplexing scheme in modern communication systems.