Channel Estimation in OFDM Systems Zhibin Wu Yan

Channel Estimation in OFDM Systems Zhibin Wu Yan Liu Xiangpeng Jing

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

OFDM Overview p Divides high-speed serial information signal into multiple lower-speed sub-signals: n p p p p Transmits simultaneously at different frequencies in parallel. Modulation ( BPSK, QPSK, 16 QAM, …). 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 p Impulse response of the channel shorter than Cyclic Prefix p Slow fading effects so that the channel is time-invariant over the symbol interval p Rectangular Windowing of the transmitted pulses p Perfect Synchronization of transmitter and receiver p Additive, white, Gaussian channel noise p

System Architecture

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

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

Block-type Channel Estimation p LS: Least Square Estimation

Comb-type Estimation 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) LMS - + e(k)

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

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

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 p Symbol Error Rate p p Combating multipath rayleigh fading with RLS adaptive equalization A detail simulation with MATLAB 20 multipath, random phase, and weibull distribution of amplitutde

Comb-LS Estimation

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

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