1. OFDM based for WIFI Ziqi SONG Zhuo ZHANG Supervisor: Prabin

2. Overview Introduction –Problem of Multipath transmission –OFDM solution Realization in Simulink –System overview –Channel Model Fixed FIR Rayleigh –Channel Estimation Block-Type Comb-Type Conlusion

3. Multipath Transmission Multipath is the unique character of wireless system 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 Signal ISI (Intersymbol Interference) –Received signal at any time depends on a number of transmitted bits

5. Effect of Multipath on Received Signal Frequency Selective Fading –Multipath fading causes some frequencies to be attenuated –Causes changes both in Amplitude and Phase –Approximately constant over narrow band

6. How OFDM Solves Problem Multi-Carrier Approach –Data transmitted in parallel –Cyclic prefix –Channel estimation and equalization

7. Cyclic Prefix If multipath delay is less than the cyclic prefix –no intersymbol or intercarrier interference –amplitude may increase or decrease

8. CP Interval vs. Multipath Delay If multipath delay is equal to or smaller than CP interval-perfect equalization

9. CP Interval vs. Multipath Delay If multipath delay is larger than CP interval, e.g. 1% larger –ISI emerging

10. OFDM System Overview

11. Channel Estimation Block-type (Training Symbol) Estimation symbols are transmitted periodically, all subcarriers are used as pilots Uses the estimated channel condition until next pilot symbol arrives Estimation can be based on LS, MMSE

12. Channel Estimation Comb-Type Pilots are uniformly inserted into subcarriers apart from each other Efficient interpolation technique is needed Based on linear, second-order, low- pass, spline cubic interpolation and so on.

13. Channel Model-FIR Ten taps and fixed complex coefficients

14. Without Equalizer BER=0.66 mainly caused by phase shift.

15. After Equalized Left :block-type training every 3 symbols Right: comb-type a pilot every 3 subcarriers

16. Channel Model-Rayleigh Impulse response

17. Channel Model-rayleigh Frequency response

18. Without Equalizer Rayleigh Channel plus additive Gaussian noise BER=0.795

19. After Equalized Using block-type estimation At the condition of 1)The pilot to data ratio is 1:2 2)The signal to noise is 20DB Get BER=0.005

20. After Equalized Using comb-type estimation At the condition of 1)The pilot to data ratio is 1:2 2)The signal to noise is 20DB Get BER=0.117

21. Block-Type Simulation Result

22. Comb-Type Simulation Result

23. Conclusion In our channel condition, block-type estimation is better The weightier pilots, the better estimation But trade-off between BER performance and efficiency