APRIL 2002, PARISIPCN02 M. Ergen A Survey on Channel Estimation Techniques Based on Pilot Arrangement in OFDM Systems by Mustafa Ergen Authors: Sinem Coleri, Mustafa Ergen Berkeley Web Over Wireless Group University of California Berkeley

APRIL 2002, PARISIPCN02 M. Ergen Outline Motivation for OFDM OFDM System Architecture Channel Estimation Techniques Performance Analysis Conclusion

APRIL 2002, PARISIPCN02 M. Ergen Motivation for OFDM Disadvantages of FDMA –Bad Spectrum Usage Disadvantages of TDMA –Multipath Delay spread problem

APRIL 2002, PARISIPCN02 M. Ergen OFDM: Use of Frequency Spectrum Efficient use of spectrum –Overlap in frequency spectrum of subcarriers Null point of all other subcarriers at the center frequency of any particular subcarrier Frequency spectrum of the subcarriers

APRIL 2002, PARISIPCN02 M. Ergen Multipath Delay Spread Multi-path delay spread definition –Time spread between the arrival of the first and last multipath signal, seen by the receiver. Received radio signal consisting of a direct signal, plus reflections from objects Multi-path delay spread effect –Inter-Symbol Interference (ISI) when the delayed multipath signal overlaps with the symbols following it

APRIL 2002, PARISIPCN02 M. Ergen OFDM: Eliminating ISI Cyclic Prefix –Prepend the last part of the signal to the beginning of the signal Duration of the CP larger than multipath delay spread Orthogonality of the carriers not affected.

APRIL 2002, PARISIPCN02 M. Ergen 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 subcarriers used to prevent frequency and phase shift errors.

APRIL 2002, PARISIPCN02 M. Ergen Benefits of OFDM Higher data rates –Overlap of subcarriers Lower bandwidth than spread spectrum. –High spectral efficiency Lower multi-path distortion –Usage of cyclic prefix

APRIL 2002, PARISIPCN02 M. Ergen Our OFDM System 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

APRIL 2002, PARISIPCN02 M. Ergen System Architecture-1

APRIL 2002, PARISIPCN02 M. Ergen System Architecture-2 Input to Time Domain Guard IntervalChannel Guard RemovalOutput to Frequency Domain OutputChannel EstimationICIAWGNChannelEstimated Channel

APRIL 2002, PARISIPCN02 M. Ergen Pilot Arrangement Block Type – All sub-carriers reserved for pilots wit a specific period Comb Type –Some sub-carriers are reserved for pilots for each symbol

APRIL 2002, PARISIPCN02 M. Ergen Channel LS estimateMMSE estimate

APRIL 2002, PARISIPCN02 M. Ergen Channel Block-Type Block TypeDecision Feedback Interpolation H e -k th sub-carrier Channel Response Estimated X e (k) -> signal demapper -> signal mapper -> X pure (k) Use same channel estimation for the whole symbol duration

APRIL 2002, PARISIPCN02 M. Ergen Channel Comb-Type Pilot N p pilot signals uniformly inserted in X(k) L=Number of Carriers/N p {H p (k) k=0,1,…,N p }, channel at pilot sub-carriers X p input at the k th pilot sub-carrier Y p output at the k th pilot sub-carrier LMS EstimateLS Estimate

APRIL 2002, PARISIPCN02 M. Ergen Comb-Type Linear Interpolation Second Order Interpolation Low pass Interpolation Spline Cubic Interpolation Time Domain Interpolation

APRIL 2002, PARISIPCN02 M. Ergen Linear InterpolationSecond Order Interpolation Low Pass Interpolation (interp in MATLAB) Comb-Type Time Domain Interpolation Spline Cubic Interpolation (spline in MATLAB) Insert zeros into the original sequence Low-pass filter while passing original data unchanged Interpolation such that mean-square error between ideal and interpolated values min.

APRIL 2002, PARISIPCN02 M. Ergen OFDM Setup ParameterSpecifications FFT Size1024 Number of Carriers128 Pilot Ratio1/8 Guard Length256 Guard TypeCyclic Extension Sample rate of OFDM signal 44.1kHz Bandwidth17.5kHz Signal ConstellationBPSK, QPSK, DQPSK, 16QAM

APRIL 2002, PARISIPCN02 M. Ergen Channels Delay (OFDM samples)GainPhase(rad) DelayAmplitude Channel 1 Channel 2 Time Varying Channel (AR Model) ATTC (Advanced Television Technology Center) and the Grande Alliance DTV laboratory`s ensemble E model Simplified version of (Digital Video Broadcasting) DVB-T channel model

APRIL 2002, PARISIPCN02 M. Ergen Simulation-1 ModulationBPSK ChannelRayleigh Fading H(n)Channel 1 Doppler Frequency 70Hz

APRIL 2002, PARISIPCN02 M. Ergen ModulationQPSK ChannelRayleigh Fading H(n)Channel 1 Doppler Frequency 70Hz Simulation-2

APRIL 2002, PARISIPCN02 M. Ergen Modulation16QAM ChannelRayleigh Fading H(n)Channel 1 Doppler Frequency 70Hz Simulation-3

APRIL 2002, PARISIPCN02 M. Ergen ModulationDQPSK ChannelRayleigh Fading H(n)Channel 1 Doppler Frequency 70Hz Simulation-4

APRIL 2002, PARISIPCN02 M. Ergen Simulation-5 Modulation16QAM ChannelAR Fading H(n)Channel 1 Doppler Frequency 70Hz

APRIL 2002, PARISIPCN02 M. Ergen Modulation16QAM ChannelRayleigh Fading H(n)Channel 2 Doppler Frequency 70Hz Simulation-6

APRIL 2002, PARISIPCN02 M. Ergen Modulation16QAM ChannelRayleigh Fading H(n)Channel 1 SNR40dB Simulation-7

APRIL 2002, PARISIPCN02 M. Ergen Conclusion OFDM System Block Type Direct or Decision Feedback Comb Type LS or LMS estimation at pilot frequencies Interpolation Techniques Linear Second Order Low Pass Spline Time Domain Modulation BPSK,QPSK,16QAM,DQPSK Results: Comb Type performs better since it tracks fast fading channels. Low-pass interpolation performs better since mean square error between the interpolated points and their ideal values is minimized.