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 Multicarrier 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 subcarriers 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