Sign in Subscribe

ofdm signal generation

Last updated on 

Orthogonal Frequency Division Multiplexing (OFDM) is a modulation scheme used in digital communication to transmit data over radio waves or optical fibers. It's widely used in modern communication systems like 4G LTE, Wi-Fi, and digital television (DVB, ATSC). Let's dive into the technical details of OFDM signal generation:

1. Basic Concept:

OFDM is based on the idea of dividing a high-rate data stream into multiple parallel low-rate streams, each transmitted over a different subcarrier (a sub-channel within the main channel). These subcarriers are orthogonal to each other, meaning they do not interfere with each other, allowing for simultaneous transmission.

2. Steps in OFDM Signal Generation:

a. Data Mapping:

The input data stream is divided into parallel streams. Typically, a block of data is taken, and this block is divided into symbols. These symbols are then mapped to subcarriers. The mapping can be done using various modulation schemes like Quadrature Amplitude Modulation (QAM).

b. IFFT (Inverse Fast Fourier Transform):

After data mapping, the symbols are mapped into the frequency domain using an IFFT operation. This converts the data from the frequency domain to the time domain. The IFFT operation creates a time-domain OFDM symbol by converting the modulated subcarriers into a time-domain signal.

c. Cyclic Prefix Addition:

One of the key challenges in OFDM is dealing with intersymbol interference due to multipath propagation. To mitigate this, a cyclic prefix (CP) is added to the beginning of each OFDM symbol. The CP is a copy of the last part of the OFDM symbol, ensuring that the symbol appears periodic in time. This helps in preserving the orthogonality between subcarriers, even in the presence of multipath distortion.

d. Serial-to-Parallel Conversion:

After the cyclic prefix is added, the OFDM symbol is converted back from serial format (a single stream) to parallel format, preparing it for transmission over multiple subcarriers.

e. Upconversion and Transmission:

The parallel OFDM symbols for each subcarrier are upconverted to the desired frequency band for transmission. This involves modulating each OFDM symbol onto its respective subcarrier frequency. Once upconverted, the OFDM symbols are transmitted over the channel.

3. Key Features:

  1. Orthogonality: Subcarriers are designed to be orthogonal to each other, ensuring that they don't interfere with each other.
  2. Spectral Efficiency: By using many closely spaced subcarriers, OFDM can achieve high spectral efficiency.
  3. Robustness to Multipath Fading: Due to the cyclic prefix, OFDM is resilient to multipath fading, where signals take multiple paths to reach the receiver.

4. Receiver Side:

At the receiver, the reverse process takes place:

  1. Downconversion: The received OFDM signal is downconverted to baseband or intermediate frequency.
  2. Cyclic Prefix Removal: The cyclic prefix is removed, helping to mitigate intersymbol interference.
  3. FFT (Fast Fourier Transform): An FFT operation is performed on the time-domain signal to convert it back into the frequency domain.
  4. Data Demapping: The subcarriers are demodulated, and the data symbols are extracted.
  5. Decoding: The extracted symbols are then decoded to recover the original data stream.

OFDM is a powerful modulation technique that divides a high-speed data stream into multiple low-speed streams, each transmitted over orthogonal subcarriers. This allows for efficient and robust communication over various channels, especially those subject to multipath fading and interference.