1. Channel Estimation in OFDM Systems
Zhibin Wu Yan Liu Xiangpeng Jing

2. OUTLINE OFDM System Introduction Channel Estimation Techniques
Performance Evaluation
Conclusion

3. OFDM Overview
Divides high-speed serial information signal into multiple lower-speed sub-signals:
Transmits simultaneously at different frequencies in parallel.
Modulation ( BPSK, PSK,QPSK,16QAM, …).
Pilot sub-carriers used to prevent frequency and phase shift errors.
Usage of cyclic prefix for lower multi-path distortion
Controlled overlapping of bands in one channel
Max spectral efficiency (Nyquist rate)
Easy implementation using inverse FFTs
Easy time-freq. Synchronization
Modulate by switching between time and frequency domain

4. Introduction to OFDM Systems

5. Time-Frequency View

6. Some Assumptions Usage of cyclic Prefix
Impulse response of the channel shorter than Cyclic Prefix
Slow fading effects so that the channel is time-invariant over the symbol interval
Rectangular Windowing of the transmitted pulses
Perfect Synchronization of transmitter and receiver
Additive, white, Gaussian channel noise

7. System Architecture

8. System Architecture (cont’d)
Input to time domain
Guard Interval
Channel
Guard Removal
Output to frequency domain
Output
Channel Estimation
Channel
ICI
AWGN
Estimated Channel

9. Pilot for Channel Estimation
Comb Type:
Part of the sub-carriers are always reserved as pilot for each symbol
Block Type:
All sub-carriers is used as pilot in a specific period
Time
Carriers
Time
Carriers

10. Block-type Channel Estimation
LS: Least Square Estimation

11. Comb-type Estimation LS Estimate LMS Estimate
Np pilot signals uniformly inserted in X(k)
L=Number of Carriers/Np
xp(m) is the mth pilot carrier value
{Hp(k) k=0,1,…,Np} , channel at pilot sub-carriers
Xp input at the kth pilot sub-carrier
Yp output at the kth pilot sub-carrier
LS Estimate
LMS Estimate
Yp(k)
Xp(k) -
e(k)
LMS +

12. Interpolation for Comb-type
Linear Interpolation
Second Order Interpolation

13. Simulation Parameters
Specifications
FFT Size 64
Number of Carriers
Pilot Ratio
1/16
Guard Length 16
Guard Type
Cyclic Extension
data rate of OFDM signal
1Mbps/sub-carrier
Signal Constellation
16QAM

14. System structure in MATLAB Simulation

15. OFDM Transmitter
OFDM Receiver

16. Received and Recovered Signals
Received signal phases are distorted by multi-path fading

17. Comb-LS Estimation
Combating multipath rayleigh fading with RLS adaptive equalization
A detail simulation with MATLAB
20 multipath, random phase, and weibull distribution of amplitutde
Symbol Error Rate

18. Comb-LS Estimation

19. Filter length .vs. Sample Rate
Observed Symbol error rate with F ( filter length ) and
S ( samples per symbol)
Keep the ratio of F/S, increase S
Keep S, increase F.

20. Conclusion OFDM System Introduction Block Type Comb Type
Direct or Decision Feedback
Comb Type
LS or LMS estimation at pilot frequencies
Interpolation Techniques
Linear
Second Order
Time Domain
Modulation
BPSK,QPSK,16QAM,DQPSK
Some Results:
Comb Type performs better since it tracks fast fading channels.
RLS algorithm vs. LMS algorithm