OFDM Transmission Technique Orthogonal Frequency Division Multiplexer

EE 578Assignment # 4 Mohammad Alkhodary ID: 200806080

Concept of parallel Transmission schemes

The maximum delayed time of the delayed waveform is greater than 2 symbols time, therefore booth the spectrum and waveforms are distorted.

Implementation Difficulties have been encountered in using adaptive equalizer at High rate.

Concept of parallel Transmission

Parallel transmission

Concept of parallel Transmission

the delay time of the delayed waves is suppressed to within 1 symbol time.

ISI and ICI Still problems

Concept of OFDM

The spectra of an OFDM signal are not strictly band-limited, the distortion, due to multipath fading, causes each subchannel to spread the power into the adjacent channels.

To reduce the distortion, a simple solution is to increase the symbol duration or the number of carriers. However, this method may be difficult to implement in terms of carrier stability against Doppler frequency and FFT size.parallel Transmission

ISI Solution

One way to eliminate lSI is to create a cyclically ex tended guard interval, where each OFDM symbol is preceded by a periodic extension of the signal itself.

When the guard interval is longer man the channel impulse response, or the multipath delay, the effect of lSI can be eliminated.

Usually

Concept of OFDM

Received signal and Channel Equalization

characteristic of delayed wave

OFDM Receiver

OFDM Transmission Scheme

The multipath fading environment in time domain is characterized by impulse responses with different delays an powers.

In frequency domain, it is characterized by attenuation or enhancement of different frequencies of the impulse responses

OFDM Transmission

The signal is given by:

The subcarrier is given by:

When Bs > Bc, distortion increases but the total received power variation is less.

When Bs < Bc, distortion is little but the total received power is attenuated.

We use parallel transmission to solve this problem.

The high rate is decreased as we multiplex the data while the symbol duration increases.

IFFT Circuit

Guard Bands

Used to eliminate ISI that appears in the received signal.

Ttotal= Tg + Ts

Tg < Ts/4

Results

BER & PER Performance

Thank You

Pilot Symbol Aided OFDM

Compensate for the phase and amplitude fluctuation.

Pilot symbols are inserted in at the transmitter at fixed time interval.

At the receiver, we estimate the channel behavior using the pilot symbols.

Fluctuation is independent of pilot symbols.

Frame Format Configuration

Sampling rate: 20 MHz .

Modulation scheme.

FEC: R= ½, K= 7

Frame format .

Radio channel model.

Frame Format Using Channel Estimation

BER & PER Performance Using Channel Estimation

OFEM BER

The BER depends on me level of the receiver's noise. In OFDM transmission, the orthogonality is preserved, and the BER performance depends on the modulation scheme in each subchannel.

OFDM Performance

Performance (BER)

Performance (PER)