LTE Tutorials - 4 - Some more about OFDM

Some facts about OFDM

·         OFDM was invented more than 40 years ago.

·         OFDM has been adopted for several technologies:

o        Asymmetric Digital Subscriber Line (ADSL) services.

o        IEEE 802.11a/g, IEEE 802.16a.

o        Digital Audio Broadcast (DAB).

o        Digital Terrestrial Television Broadcast: DVD in Europe, ISDB in Japan

o        4G, IEEE 802.11n, IEEE 802.16, and IEEE 802.20.

Why OFDM

·         High bit rate needs are clumped by the nature of communication channels.

·         Multi-path Propagation effects forbid increasing of transmission rates.

Basics

·         OFDM = Orthogonal FDM

·         Carrier centres are put on orthogonal frequencies

·         ORTHOGONALITY - The peak of each signal coincides with trough of other signals

·         Subcarriers are spaced by 1/Ts

·         Channel bandwidth is divided into multiple sub channels to reduce ISI and frequency-selective fading.

·         Multicarrier transmission: Subcarriers are orthogonal each other in frequency domain.

·         Time-domain spreading:

o        Spreading is achieved in the time-domain by repeating the same information in an OFDM symbol on two different sub-bands => Frequency Diversity.

·         Frequency-domain spreading: 

o        Spreading is achieved by choosing conjugate symmetric inputs for the input to the IFFT (real output)

o        Exploits frequency diversity and helps reduce the transmitter complexity/power consumption

 

Modulation

 Frequency Division Multiplexing 

OFDM frequency division

Mathematical Representation

The baseband OFDM signals can be written as

Where

is the central frequency of the mth sub-channel and

is the corresponding transmitted symbol.

The signals

are orthogonal over [0, T ] as illustrated below: 

Generic OFDM Transmitter

Proprietary OFDM flavors