James Zeidler, PI Jittra Jootar and Haichang Sui, GSRs

1. James Zeidler, PI Jittra Jootar and Haichang Sui, GSRs Quantifying Performance Improvements Due to Spatial-Temporal Diversity in MIMO Spread-Spectrum Tactical Mobile Ad-hoc Networks

2. System of interest • DS or FH CDMA system with MIMO. • Coded system: block code or convolutional code with finite interleaver depth. • Time-varying frequency-selective channel at moderate to high fading speed. • Coherent reception • Correlation between closely spaced antennas • Realistic channel estimator • Channel estimation error • Imperfect feedback signal • Noncoherent reception • Differential unitary space-time modulation • Multiple-access interference (MAI) and partial-band interference (PBI)

3. Summary of the Main Results • MIMO DS-CDMA with coherent reception • We studied the effect of Doppler and spatial diversity in DS-CDMA systems with time-varying channels and noisy CSI • Trade-off among various system parameters are analyzed • MIMO FH-CDMA with noncoherent reception • We proposed a coded MIMO FH-CDMA PHY that is robust to unreliable CSI and unknown interference such as MAI and PBI (jamming). • Spatial and frequency diversity are achieved by space-time modulation and coding/interleaving, respectively. • A joint estimation-demodulation-decoding scheme was developed to estimate statistics of the channel/interference, combat fast fading, and suppress unknown interference. • The decoding error probability of the proposed scheme is analyzed.

4. Publications • J. Jootar, J. R. Zeidler, and J. G. Proakis, "Performance of Convolutional Codes with Finite-Depth Interleaving and Noisy Channel Estimates," IEEE Transactions on Communications, October 2006 • J. Jootar, J. R. Zeidler and J. G. Proakis, “On the Performance of Concatenated Convolutional Code and Alamouti Space-Time Code with Noisy Channel Estimates and Finite-Depth Interleaving”, accepted for publication in IEEE Transaction on Communications, in press. • J. Jootar, J. R. Zeidler, and J. G. Proakis, ``On the Performance of Closed-Loop Transmit Diversity with Noisy Channel Estimates,'' submitted to IEEE Transactions on Communications. • H. Sui and J. R. Zeidler, “Erasure Insertion for Coded MIMO Slow Frequency-Hopping Multiple-Access Networks”, Submitted to IEEE JSAC Special Issue on Optimization of MIMO Transceivers for Realistic Communications Networks • M. Zorzi, J. Zeidler, A. Anderson, B. Rao, and J. Proakis, A. L. Swindlehurst, M. Jensen, S. Krishnamurthy, “Cross-layer issues in MAC protocol design for MIMO ad hoc networks,” IEEE Wireless Communications Magazine, Vol 13, pp. 62-76, Aug. 2006.

5. Publications • H. Sui and J. R. Zeidler, ``Demodulation and Performance Analysis of Differential Unitary Space-Time Modulation in Time-Varying Rician Channels,'' IEEE Vehicular Technology Conference, (Montreal, Canada), September 2006. • H. Sui and J. R. Zeidler, ``Erasure Insertion for Coded DUSTM-FHSS Systems without A Priori Knowledge,’’ IEEE International Communications Conference, June 2006. • J. Jootar, J. R. Zeidler and J. G. Proakis, “On the Performance of Closed-Loop Transmit Diversity with Noisy Channel Estimates”, IEEE International Conference on Communications June 2006. • H. Sui and J. R. Zeidler, ``Erasure Insertion for Coded MIMO Slow Frequency-Hopping Systems in the Presence of Partial Band Interference,'' IEEE Global Communications Conference, pp. 3082-3086, Nov. 2005. • H. Sui and J. R. Zeidler, ``An explicit and Unified Error Probability Analysis of Two Detection Schemes for Differential Unitary Space-Time Modulation,'' IEEE Asilomar Conference on Circuits, Systems and Computers, pp. 1579-1583, Nov. 2005. • J. Jootar, J. R. Zeidler and J. G. Proakis, “On the Performance of Finite-Depth Interleaved Convolutional Codes in Time-Varying Rayleigh Fading Channels with Noisy Channel Estimates”, IEEE Vehicular Technology Conference (VTC) September 2005, vol. 1, pp. 600-605. • A. Anderson, J. Zeidler and M. Jensen, Performance of Transmit Precoding in Time –Varying Point-to-Point and Multi-User MIMO Channels, IEEE Asilomar Conference on Circuits, Systems and Computers, Pacific Grove, CA. November 2006

6. MIMO DS-CDMA: Coherent Systems • Objective • Two methods used to improve system performance for mobile applications are analyzed: • Finite-depth interleaved convolutional codes • Maximum time diversity for memoryless channels • Interleaver used to decrease channel memory • Smaller interleaver depth requires larger Doppler spread to achieve memoryless channel • CSI errors increase as Doppler spread increases • Transmit diversity • Alamouti space-time code (STC) • Closed-loop transmit diversity (CLTD) (Beamforming) • Performance of finite-depth convolutionally coded systems with and without transmit diversity are analyzed as a function of interleaving depth, Doppler spread and channel estimation accuracy.

7. Research Background (coherent systems) • Training requirements for MIMO Systems have been defined. ( Ref. Sung et. al. “Training for MIMO Communications”,in Space-Time Wireless Systems: From Array Processing to MIMO Communications, Cambridge Univ Press, 2006) • Estimation-Diversity trade-off in block-fading channel is studied by Stark et al from an information-theoretic viewpoint and by Milstein, et. al. from a communication systems viewpoint. • We study this trade-off under the following assumptions: • The CSI is estimated from pilots (cf. J. K. Caver et al) • Continuously time-varying channel instead of block fading channel • Convolutional codes with finite interleaving depth are accounted. Previous work assumes either perfect interleaving or perfect CSI. • Three transmit diversity algorithms are analyzed and compared: no transmit diversity (SISO), Alamouti STCs, and closed loop TD

8. MIMO DS-CDMA: Coded Alamouti pilot spreading 1 • System Model spreading 0 Alamouti Conv Enc Finite Depth Interleaver BPSK Mod spreading 0 spreading 2 pilot multipath channel Linear combining for Alamouti De- interleaver Conv Dec (ML) or Joint ML Alamouti and Convolutional decoder + deinterleaver Ch. Est. (FIR)

9. MIMO DS-CDMA: Alamouti without convolutional code • Comparison between no transmit diversity, and Alamouti STTD when CSI is noisy and channels are time-varying

10. MIMO DS-CDMA: Coded Alamouti • ρ denotes the fading correlation between the two transmit antennas. • The analysis has shown that the performance of LC-ML is comparable to JML in a non-idealistic environment (Doppler spread, channel estimation error, finite interleaving depth).

11. MIMO DS-CDMA: Coded Alamouti • Conclusions • The analysis has shown that the performance of the linear-combining decoder is comparable to ML decoder in a non-idealistic environment (Doppler spread, channel estimation error, finite interleaving depth). • As a function of the Doppler spread, interleaver depth and pilot SNR, there is a point at which the Alamouti scheme is outperformed by the system without transmit diversity.

12. MIMO DS-CDMA: CLTD • System Model pilot spreading 1 Tx BF spreading 0 Conv Enc Finite Depth Interleaver BPSK Mod spreading 0 PO or PA feedback spreading 2 pilot multipath channel Rx BF De- interleaver Conv Dec (ML) Ch. Est. (FIR)

13. MIMO DS-CDMA: CLTD • Different systems perform well under different circumstances. • PO-CLTD performs slightly worse than PA-CLTD but the former is significantly less complex and requires less feedback information • PA/PO-CLTD may be outperfomed by SISO in certain scenarios.

14. MIMO DS-CDMA: CLTD • Conclusion • The CLTD system is shown to provide significant improvements over Alamouti and SISO systems when the Doppler spread is small and the pilot SNR is large. • The advantage of CLTD is lost at high Doppler spreads and/or low pilot SNRs. • Performance of each system degrades differently as a function of the Doppler spread and pilot SNR. • There exists a point at which the degradations associated with CSI estimation errors result in no gain over the SISO system. For CLTD systems this occurs within the regions of practical interest.

15. MIMO FH-CDMA • Motivation • The study on MIMO DS-CDMA with coherent receiver suggests that, when CSIR is inaccurate, the gain from open/close-loop transmit diversity diminishes and the resulting MIMO system is outperformed by SISO. • For tactical MANETs, robustness in unpredictable, hostile environments is important. • Objective • To study the applications of MIMO for distributed MANETs that are robust in the view of unreliable CSIR and unknown interference. • Background • Previous studies on FH systems mainly focus on SISO/SIMO and FSK modulation.

16. MIMO FH-CDMA: PHY Features • We proposed a PHY for MANETs that combines FH-CDMA and noncoherent space-time modulation. • Features of the proposed MIMO FH-CDMA PHY • Anti-jamming, LPD/LPI, and operability in non-continuous spectrum from FH-CDMA • FH-CDMA is relatively more robust to the near-far problem than DS-CDMA. • Multiple-access from CDMA reduces overhead in MAC and also alleviates unfairness and throughput degradation in CSMA/CA protocols. • Diversity is achieved from both space and frequency without CSIR. • Scalable in bandwidth. • Distributed implementation.

17. MIMO FH-CDMA: Challenges & Approach • Challenges in decoding the received packet: • Fast time-varying channel with unknown time-correlation • Unknown SINR that are different for different dwells • Possible nonstationarity of fading/interference process in a dwell • The proposed receiver performs joint estimation-demodulation-decoding • Decision-directed adaptive estimation • Decision feedback demodulation • Erasure insertion decoding (for Reed-Solomon codes)

18. MIMO FH-CDMA: Transceiver Model Transmitter Model DUSTM Receiver Model

19. MIMO FH-CDMA: Summary of Results • The performance of DF demodulation in arbitrary time-varying Rayleigh and Rician channel was analyzed. A compact expression of the effective SINR was formulated. • The decision-directed adaptive estimation scheme was developed based on RLS to estimate unknown parameters for DF demodulation and erasure insertion (EI). • Bayesian EI rules were investigated to suppress interference. • A new EI rule (ESTT EI) was developed. The decoding error probability was obtained analytically. The analytical results also predict the performance of Bayesian EI closely.

20. MIMO FH-CDMA: Erasure Insertion (EI) • The decoding error probability benefits from proper EI since more erasures than errors can be corrected. • Bayesian EI • A ratio of symbol likelihood functions is used as a reliability measure. The symbol likelihood functions are derived for DF demodulation of noncoherent space-time modulation. • Effective SINR Threshold Test (ESTT) EI • The effective SINR captures the time-variation of the channel, the DF depth P, and the received SINR and is shown as • In either EI, the demodulated symbol is erased if its associated reliability measure falls below a predetermined threshold. • Decoding error probability is obtained for ESTT EI analytically.

21. MIMO FH-CDMA: Results • Both EI schemes effectively suppress MAI. • The performance is limited more by the user density than by the MAI power. • Good near-far resistance is observed. • Analysis of ESTT is accurate. • Similar results are obtained for PBI (jamming)

22. MIMO FH-CDMA: Results • The decoding error probability highly depends on the distribution/model of the interference, even the average signal to interference ratio is constant. EI decoding error probability under PBI and MAI with a constant signal to interference ratio

23. MIMO FH-CDMA:Further Discussion • The decoding error probability highly depends on the distribution of the interference power, even if the average SINR is constant. • For Bayesian EI, a fixed threshold can be found to be approximately optimal for interference power distributions (i.e., approximately optimal “on average”). • For ESTT EI, the threshold can be optimized easily, based on the analytical results for its decoding error probability. • The analytical decoding error probability of ESTT EI with optimized threshold predicts that of the Bayesian EI accurately.

24. Conclusion • MIMO DS-CDMA with coherent detection • Effects of noisy CSI estimation on the performance of Alamouti STTD and CLTD are studied for different receiver combining and feedback schemes. • The tradeoffs between the CSIR accuracy and the diversity gains are quantified as a function of the channel code, the interleaver depth, the pilot SNR, and the Doppler spread. • The point at which the degradations due to inaccurate CSI result in no gain over the SISO system is found to be in ranges of practical interest for CLTD. • MIMO FH-CDMA with noncoherent signaling • A robust coded MIMO FH-CDMA transceiver is developed for tactical MANET applications with high mobility. • Interference from jamming or multiple-access is suppressed by joint estimation-demodulation-decoding, without a priori knowledge of the fading or interference. • The proposed transceiver is robust to unknown CSI and effectively suppress MAI/PBI. • Analytical results are obtained for ESTT EI and predict of the Bayesian EI accurately when the interference is strong.

25. Future Work-Coherent Systems • Channel Estimation • Improved CSI Estimation Algorithms for Time Varying Channels • To be discussed by Professors Swindlehurst, Haykin, and Jafarkhani • Channel Distribution Information • Discussed previously by Professor Jensen • Waveform Selection for Multi-Access Channels • CSI estimation is waveform dependent • Professor Milstein will discuss MC-CDMA, OFDM, and FH-CDMA • Transmit Diversity • CLTD systems with quantization of the weights and feedback delay. • STC vs Beamforming Optimization • Both Issues will be discussed in Detail by Professors Rao and Jafarkhani

26. Future Work- FH SS Systems • Excellent near-far resistance is observed for both EI schemes. • Impact on the network will be studied. E.g. Long-hop may be considered in routing to reduce delay with the proposed PHY. What is the optimal transmission range? • When symbols in a packet are received at significantly different SINR as in FH-CDMA, the performance depends more on the distribution rather than the mean of the SINR. • Fundamentally different interference management schemes will be sought because conventional cross-layer design based on the average SINR in a packet could be misleading. • How to obtainknowledge about such distribution will be studied. • How to useknowledge about such distribution will be studied. • Network-level study with the proposed MIMO FH-CDMA PHY transceiver structure. • Information Efficiency of Ad Hoc Networks with FH-MIMO Transceivers • See poster by Kostas Stamatiou