4G technology LTE_OFDM

Todays data rates becomes higher as telecommunication technology is evolving.

Today in LTE network it can be more than 100Mbps.

Then what is the technology behind ??

It’s OFDM

OFDM involves FDM and multi carrier communication.

1.FDM divides the available bandwidth into many subcarriers. 2.It allows multiple users to access the system simultaneously. 3.Each users data transmits on different subcarriers.

Between subcarriers there is a guard band to avoid interference.

In multicarrier FDM the data of a single user can be split into multiple sub-streams and transmit them in parallel through various bands. Which makes the data rate higher.

Well in OFDM too ,data is transmitted in parallel. Each subcarrier is modulated with a conventional digital modulation scheme(QPSK,16QAM etc)

The subcarriers are designed to be orthogonal. This allows the subcarriers to overlap.

Thus bandwidth is saved and higher data rate is achieved.

So OFDM differ with FDM in following ways: Multiple carriers carry the information of each user. The sub carriers are orthogonal to each other. A guard interval is added to each symbol to minimize the channel

delay spread and inter symbol interference.

Understanding Orthogonality:

In the frequency domain each subcarrier results in a SINC function spectrum with side lobes producing overlapping.

So interference occurs except at orthogonally spaced frequency points.

At orthogonal frequency , individual peaks of subcarriers all line up with the nulls of the other subcarriers.

This overlap of spectral energy does not interfere. The receiver multiplies the incoming signal by the known set of

sinusoids to recover original bite.

Orthogonality:

Simple analogue OFDM system implementation

OFDM advantages

Immunity to selective fading. Resilience to interference Spectrum efficiency Resilient to ISI, delay spread Resilient to narrow band effects.

OFDM disadvantage:

High peak to average power ratio. Sensitive to carrier offset and drift.

OFDM variants:

Coded OFDM Flash OFDM OFDMA(multiple access) Vector OFDM Wideband OFDM

Thanks to you allFouad Hasan Meraj