OFDM Basics

OFDM stands for Orthogonal Frequency Division Multiplexing. OFDM is a technique that allows a base station to split a chunk of radio spectrum into sub-channels. The signal strength of the sub-channels and the number of channels assigned to different devices can be varied as needed. OFDM allows high data rates, even far from a base station, and it copes well with the type of radio interference that is common in urban areas, where signals reflect off walls to produce confusing echoes.

This is a method to transmit data using a large number of carriers that are separated on different frequencies to carry one data stream which has been broken up into many signals.

Speeds that are lower are easier to detect. It was discovered that by using multiple subcarriers the receiver could detect signals easier in environments with interference. Subcarriers transmit a lower speed signal which is converted by to its original high-speed signal at the other end. The subcarriers for OFDM are modulated by several methods including QAM and QPSK.

The basic principle of OFDM is to split a high-rate datastream into a number of lower rate streams that are transmitted simultaneously over a number of subcarriers. Because the symbol duration increases for lower rate parallel subcarriers, the relative amount of dispersion in time caused by multipath delay spread is decreased. Intersymbol interference is eliminated almost completely by introducing a guard time in every OFDM symbol. In the guard time, the symbol is cyclically extended to avoid intercarrier interference.

In OFDM design, a number of parameters are up for consideration, such as the number of subcarriers, guard time, symbol duration, subcarrier spacing, modulation type per subcarrier. The choice of parameters is influenced by system requirements such as available bandwidth, required bit rate, tolerable delay spread, and Doppler values. Some requirement are conflicting. For instance, to get a good delay spread tolerance, a large number of subcarriers with small subcarrier spacing is desirable, but the opposite is true for a good tolerance against Doppler spread and phase noise.

OFDM is present in

 – LTE

 – Mobile WiMax IEEE 802.16e

 – xDSL

 – Wireless LAN IEEE 802.11a,g,

What is OFDM?

• Orthogonal FDM – it’s multiplexing

• It’s more: – Multi Carrier – Digital modulation (PSK, QAM) – Digital processing

• Demultiplexing

In a conventional serial data system, the symbols are transmitted sequentially, with the frequency spectrum of each data symbol allowed to occupy the entire available bandwidth. A When the data rate is sufficient high, several adjacent symbols may be completely distorted over frequency selective fading or multipath delay spread channel.

In OFDM

       The spectrum of an individual data element normally occupies only a small part of available bandwidth. Because of dividing an entire channel bandwidth into many narrow subbands, the frequency response over each individual subchannel is relatively flat.

A parallel data transmission system offers possibilities for alleviating this problem encountered with serial systems.

    - Resistance to frequency selective fading.

The process of mapping the information bits onto the signal constellation plays a fundamental role in determining the properties of the modulation. An OFDM signal consists of a sum of sub-carriers, each of which contains M-ary phase shift keyed (PSK) or quadrature amplitude modulated (QAM) signals. 

Modulation types over OFDM systems

 - Phase shift keying (PSK)

 - Quadrature amplitude modulation (QAM)

OFDM communications systems are able to more effectively utilize the frequency spectrum through overlapping sub-carriers. These sub-carriers are able to partially overlap without interfering with adjacent sub-carriers because the maximum power of each sub-carrier corresponds directly with the minimum power of each adjacent channel.

As an example , figure below shows four subcarriers from one OFDM signal. In this example, all subcarriers have the phase and amplitude, but in practice the amplitudes and phases may be modulated differently for each subcarrier. Note that each subcarrier has exactly an integer number of cycles in the interval T , and the number of cycles between adjacent subcarries differs by exactly one. This properly accounts for the orthogonality between subcarriers.