OFDM Based WLAN System
description
Transcript of OFDM Based WLAN System
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