OFDM Based WLAN System

Song Ziqi

Zhang Zhuo

Wireless Channel

• Wireless channel is always unpredictable

• Multipath reception is the unique characteristic of wireless channels

• impairments present at the channel

What is Multipath• More than one

transmission path between transmitter and receiver

• Received signal is the sum of many versions of the transmitted signal with varying delay and attenuation

Effect of Multipath on Received Part

• Delay Spread leads to ISI (Inter Symbol Interference)

-Received signal at any time depends on a number of transmitted bits-Needs equalizer to recover data-Data Rates increase: Even WORSE!!

Multi-Carrier System

• Single carrier system– signal representing each bit

uses all of the available spectrum

• Multicarrier system– available spectrum divided

into many narrow bands– data is divided into parallel

data streams each transmitted on a separate band

What is OFDM?

• OFDM is a multicarrier system– uses discrete Fourier– Transform/Fast Fourier– Transform (DFT/FFT)– sin(x)/x spectra for subcar

riers

• Available bandwidth is divided into very many narrow bands

• Data is transmitted in parallel on these bands

Why is OFDM?

• Most broadband systems are subject to multipath transmission

• Conventional solution to multipath is an equalizer in the receiver– high data rates - equalizers too complicated

• With OFDM there is a simple way of dealing with multipath– relatively simple DSP algorithms

How does OFDM solve Multipath Problem

• Data is transmitted in parallel– longer symbol period– e.g. for N parallel streams, symbol period is N

times as long

• Cyclic prefix– trick to avoid residual ISI

Transmitted in Parallel

Transmitted in Parallel Without Interference

• Each subcarrier has a different frequency

• Frequencies chosen so that an integral number of cycles in a symbol period

• Signals are mathematically orthogonal

Cyclic Prefix

Disadvantages of OFDM

• Strict Synchronization Requirement

• Peak-to-Average Power Ratio(PAPR)

What shall we achieve?

• Simple OFDM model including transmitter and receiver

• Input random data with several modulation schemes for testing

• Channel model

• Demodulate the output data then compare it with the input random data

The OFDM Model

• At the transmitter

• At the receiver

Channel Model

• Assumed to have a finite-length impulse response

• Including Additive White Gaussian Noise

OFDM Model Design Parameters

• Number of subcarriers: 64

• CP length:8

• Subcarrier frequency spacing:

sample rate/64

• Modulation type per subcarrier: QPSK

• Number of multipath: 5

• SNR of the channel: 30DB

The system

More

• BPSK and QAM modulation

• Channel Estimation

• Gain and phase compensation