# Orthogonal Frequency-Division Multiplexing (OFDM)

MATLAB video I watched to learn about OFDM.

- Article for follow up

OFDM is a digital transmission scheme that is commonly used in WiFi, LTE, and 5G.

https://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing

OFDM is a type of digital transmission and a method of encoding digital data on multiple carrier frequencies.

Time is organized into OFDM symbols, slots, subframes and frames.

### The Process

The OFDM transmission scheme can be broken down into several components.

- We first code and modulate the data (usually into Quadrature Amplitude Modulation symbols).
- These symbols are loaded into equally spaced frequency bins and
- Inverse Fast Fourier Transform (IFFT) is applied to transform the signal into orthogonal overlapping sinusoids in the time domain.
- $x(n)=N1āk=0āNā1āX(k)e_{iN2Ļxtā}$

**OFDM symbol duration (Ts)**
Ts = 1/SCS, For SCS = 15 KHz, Ts = 1/15x103, Ts = 66.7 Āµs and so on.

### OFDM vs. FDM

OFDM system provides higher data rate compare to FDM system in the same bandwidth usage.

The basics of 5Gās modulation, OFDM (eeworldonline.com)

### Cyclic Prefix

- Cyclic prefix refers to the prefixing of a symbol, with a repetition of the end. The receiver is typically configured to discard the cyclic prefix samples, but the cyclic prefix serves two purposes:

- It provides a guard interval to eliminate intersymbol interference from the previous symbol.
- It repeats the end of the symbol so the linear convolution of a frequency-selective multipath channel can be modeled as circular convolution, which in turn may transform to the frequency domain via a discrete Fourier transform. This approach accommodates simple frequency domain processing, such as channel estimation and equalization.