Midamble Aided OFDM Performance Analysis in High Mobility Vehicular Channel

1. Midamble Aided OFDM Performance Analysis in High Mobility Vehicular Channel Authors: Date: 2008-01-14 Hanbyeog Cho (ETRI)

2. Abstract • We present the Mid-amble based approach for the channel estimation and its performance analysis of OFDM signal in high mobility vehicular radio channel. WAVE system experiences outdoor time-variant and frequency- selective fading channel. It means that channel coefficients will change with time in the vehicular environment due to relative motions between vehicles. • The training symbol is assigned to a particular OFDM symbol, which is sent periodically in time domain. Mid-amble simply reuses the long training sequence of preamble. The number of data symbols between two Mid-ambles is varied. • The preamble is used for packet detection and channel estimation. But time-variant channel due to the user mobility may cause some errors in the progress of compensation. If tracking of the time-varying channels and updating of the channel estimation are not provided, the performance can be degraded dramatically. • Proposed scheme utilizes the preamble to the initial channel estimation and the Mid-amble to update the channel coefficients for the next symbol until a new Mid-amble is received. Hanbyeog Cho (ETRI)

3. Contents Objective and Approach Channel estimation Proposed Algorithm Simulation Results Conclusion Hanbyeog Cho (ETRI) Slide 3

4. Objectives • Time-variant character of channel in WAVE(IEEE 802.11p). • Channel coherence time ,which is the maximum packet length duration without being distorted during the transmission.* • In IEEE 802.11p, the maximum information data size transferred from MAC layer is about 4000 bytes. • Current training sequences, at beginning of each packet, are not sufficient to estimate the channel correctly for a long packet. **fd : Doppler spread ex) velocity v = 200km/h, carrier frequency fc = 5.9GHz : fd = 1kHz => Tc = 423μs * T.S.Rappaport, Wireless Communications: Principle and Practice, Chs.5, Upper Saddle River, NJ: Prentice-Hall, 1996. Hanbyeog Cho (ETRI) Slide 4

5. Approaches The time-varying channel creates the different channel condition between the preamble and the behind of the transmission packet. In the case of long-sized packet transmission, preamble based if channel estimation may occur critical errors. So, tracking the time-varying channel and updating the channel estimation are needed. We propose midamble aided transmission scheme for WAVE. Hanbyeog Cho (ETRI) Slide 5

6. Channel estimation Recovers data which is corrupted by channel. The channel frequency response (CFR) is calculated by using the known training sequences or pilot signals. yn= xn * hn + wn Y(k) = FFT(yn ) Y(k) = X(k)H(k) + W(k), 0≤ k ≤ N-1 yn : received signal xn: transmit signal hn: channel impulse response wn :AWGN N : FFT length : Channel frequency response Hanbyeog Cho (ETRI) Slide 6

7. Channel estimation • Conventional method(only preamble) • channel estimated by preamble at the beginning of the packet. • it compensates every data symbol in a packet. Preamble Data ··· Data ··· Data ··· Data Data Data Equalizer Channel estimation OFDM Training Structure (WAVE) Channel Estimation Method Hanbyeog Cho (ETRI)

8. Proposed Algorithm • OFDM Training Structure Midamble Midamble DATA Preamble(long training symbol) OFDM subcarrier(freq.) ∙∙∙ ∙∙∙ ∙∙∙ Data subcarrier OFDM symbol(time) Pilot subcarrier Hanbyeog Cho (ETRI) Slide 8

9. Proposed Algorithm • Proposed method (preamble + midamble) • Preamble :initial channel estimation • Midamble : channel estimation(tracking) & update the equalizer coefficients N symbol Preamble Data ··· Data Midamble Data ··· Data Midamble Data ··· Data Equalizer Equalizer Equalizer Channel estimation Channel tracking & Update Channel tracking & Update Channel Estimation Method Hanbyeog Cho (ETRI) Slide 9

10. Simulation Description Used MATLAB WAVE(802.11p) and 802.11 specs Differential modulation (QPSK, 16QAM) Simulated 1000-byte packet length Interval between two Midmables : 4(16QAM) and 10(QPSK) No. of Paths = 4 Velocity : 50, 100, 150 and 200km/h Result : BER vs. SNR Hanbyeog Cho (ETRI) Slide 10

11. Simulation Block Diagram In this simulation model, channel coding is not considered and perfect synchronization is assumed. Channel coding Modulation IFFT Add GI Input Data Channel BER calculation Synchronization De-modulation Equalizer FFT Remove GI Channel decoding BER + AWGN Channel Estimation Hanbyeog Cho (ETRI) Slide 11

12. Simulation Results • Performance analysis • varying the packet size • conventional method(only preamble) : packet length increasing  BER increasing Hanbyeog Cho (ETRI) Slide 12

13. Simulation Results • Performance analysis • varying the velocity • differential tracking method : Pilot vs. Midamble Hanbyeog Cho (ETRI) Slide 13

14. Conclusions • We proposed a new frame structure for long packet size transmission. • Conventional scheme is not sufficient to estimate the channel correctly for a long packet. • The proposed scheme improves the performance in comparison with conventional schemes under high mobility environment. • We concluded that the midamble aided transmission scheme suitable solution to support WAVE system. Hanbyeog Cho (ETRI) Slide 14