Channel Estimation in OFDM Systems Zhibin Wu Yan Liu Xiangpeng Jing

OUTLINE  OFDM System Introduction  Channel Estimation Techniques  Performance Evaluation  Conclusion

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

Introduction to OFDM Systems

Time-Frequency View

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

System Architecture

System Architecture (cont’d) 1. Input to time domain 2. Guard Interval 3. Channel 4. Guard Removal 5. Output to frequency domain 6. Output 7. Channel Estimation ICIAWGNChannel Estimated Channel

Pilot for Channel Estimation Time Carriers Time Carriers  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

Block-type Channel Estimation  LS: Least Square Estimation

Comb-type Estimation N p pilot signals uniformly inserted in X(k) L=Number of Carriers/N p x p (m) is the m th 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 EstimateLMS Estimate Xp(k) LMS + e(k)- Yp(k)

Interpolation for Comb-type  Linear Interpolation  Second Order Interpolation

Simulation Parameters ParameterSpecifications FFT Size64 Number of Carriers64 Pilot Ratio1/16 Guard Length16 Guard TypeCyclic Extension data rate of OFDM signal1Mbps/sub-carrier Signal Constellation16QAM

System structure in MATLAB Simulation

OFDM Transmitter OFDM Receiver

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

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

Comb-LS Estimation

Filter length.vs. Sample Rate 1.Keep the ratio of F/S, increase S 2.Keep S, increase F. Observed Symbol error rate with F ( filter length ) and S ( samples per symbol)

Conclusion  OFDM System Introduction  Block 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