1. OFDM Based WLAN System
Song Ziqi
Zhang Zhuo

2. Wireless Channel Wireless channel is always unpredictable
Multipath reception is the unique characteristic of wireless channels
impairments present at the channel

3. 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

4. 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!!

5. 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

6. 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

7. 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

8. 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

9. Transmitted in Parallel

10. 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

11. Cyclic Prefix

12. Disadvantages of OFDM Strict Synchronization Requirement
Peak-to-Average Power Ratio(PAPR)

13. 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

14. The OFDM Model
At the transmitter
At the receiver

15. Channel Model Assumed to have a finite-length impulse response
Including Additive White Gaussian Noise

16. 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

17. The system

18. More BPSK and QAM modulation Channel Estimation
Gain and phase compensation