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