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OFDM based for WIFI
Ziqi SONG
Zhuo ZHANG
Supervisor: Prabin
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
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
Effect of Multipath on Received Signal
• ISI (Intersymbol Interference)– Received signal at any time depends on a number of transmitted bits
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
How OFDM Solves Problem
• Multi-Carrier Approach– Data transmitted in parallel– Cyclic prefix– Channel estimation and equalization
Cyclic Prefix
• If multipath delay is less than the cyclic prefix– no intersymbol or intercarrier interference– amplitude may increase or decrease
CP Interval vs. Multipath Delay
• If multipath delay is equal to or smaller than CP interval-perfect equalization
CP Interval vs. Multipath Delay
• If multipath delay is larger than CP interval, e.g. 1% larger –ISI emerging
OFDM System Overview
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
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.
Channel Model-FIR
• Ten taps and fixed complex coefficients
Without Equalizer
• BER=0.66 mainly caused by phase shift.
After Equalized
• Left :block-type training every 3 symbols• Right: comb-type a pilot every 3 subcarriers
Channel Model-Rayleigh
• Impulse response
Channel Model-rayleigh
• Frequency response
Without Equalizer
• Rayleigh Channel plus additive Gaussian noise BER=0.795
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
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
Block-Type Simulation Result
Comb-Type Simulation Result
Conclusion
• In our channel condition, block-type estimation is better
• The weightier pilots, the better estimation
• But trade-off between BER performance and efficiency