OFDM (Orthogonal frequency division multiple access)

OFDM (Orthogonal Frequency Division Multiplexing) is a digital modulation technique widely used in modern communication systems. It enables efficient transmission of data over wireless and wired channels by dividing the available bandwidth into multiple orthogonal subcarriers. Each subcarrier carries a portion of the data, allowing for simultaneous transmission and reception of multiple data streams.

The fundamental idea behind OFDM is to combat the adverse effects of multipath propagation, which causes intersymbol interference (ISI) and reduces the overall spectral efficiency of the system. By using a large number of closely spaced subcarriers, each with a relatively low symbol rate, OFDM mitigates the effects of ISI.

In OFDM, the overall channel bandwidth is divided into multiple narrowband subcarriers, with each subcarrier having a specific frequency and carrying a data symbol. These subcarriers are orthogonal to each other, meaning they have no interference between them. Orthogonality is achieved by carefully selecting the carrier frequencies and spacing them in such a way that the subcarriers' sinusoidal waves have integer multiples of the fundamental frequency.

The transmission process in OFDM consists of three main stages: modulation, multiplexing, and transmission.

1. Modulation: Each subcarrier in OFDM employs a modulation scheme, typically quadrature amplitude modulation (QAM) or phase shift keying (PSK). These modulation schemes allow multiple bits to be encoded onto each subcarrier, enabling high data rates. The choice of modulation scheme depends on factors such as the desired data rate, signal-to-noise ratio (SNR), and the available channel conditions.
2. Multiplexing: In OFDM, the data stream is divided into parallel streams, with each stream modulating a separate subcarrier. The division of the data stream is achieved through a process called inverse fast Fourier transform (IFFT), which converts the data from the frequency domain to the time domain. IFFT essentially spreads the data symbols across time, converting them into a time-domain OFDM symbol.
3. Transmission: The time-domain OFDM symbols are then transmitted over the channel. The transmission process includes adding a cyclic prefix (CP) to each OFDM symbol. The CP is a copy of the last part of the OFDM symbol that is appended to the front, acting as a guard interval. It helps combat inter-symbol interference caused by multipath propagation. The CP ensures that even if there is some delay spread or delay spread variation in the channel, the orthogonality between the subcarriers is maintained, allowing for efficient demodulation at the receiver.

At the receiver, the received OFDM signal undergoes the reverse process of the transmitter. First, the CP is removed, and the time-domain signal is transformed into the frequency domain using a fast Fourier transform (FFT). The FFT extracts the individual subcarriers and their corresponding symbols. Demodulation is then performed on each subcarrier to recover the original data.

OFDM offers several advantages that make it a popular choice for modern communication systems:

1. Spectral Efficiency: By using closely spaced subcarriers, OFDM allows efficient utilization of the available spectrum, enabling high data rates.
2. Robustness to Interference: OFDM's orthogonality property ensures that subcarriers do not interfere with each other, making it resistant to frequency-selective fading and narrowband interference.
3. Mitigation of Multipath Effects: OFDM's ability to handle multipath propagation through the use of a cyclic prefix helps combat the adverse effects of ISI, improving the overall system performance.
4. Flexibility: OFDM can adapt to varying channel conditions by dynamically allocating subcarriers to different users or services. This flexibility enables efficient resource allocation and supports multiple access techniques like OFDMA (Orthogonal Frequency Division Multiple Access).
5. Compatibility: OFDM is compatible with various digital modulation schemes and can be easily integrated into existing communication standards.

Overall, OFDM is a versatile modulation technique that enables high-speed and reliable data transmission in various communication systems, including wireless LANs, digital television broadcasting, 4G and 5G cellular networks, and broadband internet access. Its ability to combat multipath propagation, achieve high spectral efficiency, and support multiple access techniques makes it a crucial technology in modern wireless communications.