The doctoral dissertations of the former Helsinki University of Technology (TKK) and Aalto University Schools of Technology (CHEM, ELEC, ENG, SCI) published in electronic format are available in the electronic publications archive of Aalto University - Aaltodoc.
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Channel and Delay Estimation Algorithms for Wireless Communication Systems

Marius Sirbu

Dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Department of Electrical and Communications Engineering for public examination and debate in Auditorium S4 at Helsinki University of Technology (Espoo, Finland) on the 5th of December, 2003, at 12 o'clock noon.

Overview in PDF format (ISBN 951-22-6838-8)   [833 KB]
Dissertation is also available in print (ISBN 951-22-6837-X)

Abstract

This thesis addresses the problem of channel and propagation delay estimation in wireless communication systems. Channel estimation and equalization compensate for channel distortions. Consequently, transmitted data may be reliably recovered. A feasible communication link, in both single user and multi-user communications, requires synchronization between the transmitter and the receiver. Traditional channel estimation and synchronization methods use training data, therefore decreasing the effective data rates. More efficient methods which use smaller amounts of training data are of great interest. In particular blind equalization methods, as well as receiver based synchronization methods enable higher effective data rates.

In Global System for Mobile Communications (GSM) more than 22 % of the transmitted signal is used for channel estimation and synchronization purposes. If blind equalization methods could be applied in GSM, this part of the signal could be used for transmitting information bits. Blind channel identifiability problems in GSM are investigated in this thesis. The performance of several blind equalization methods is also evaluated, for both the Gaussian Minimum Shift Keying (GMSK) modulation and for the 8-Phase Shift Keying (8-PSK) modulation proposed for the future evolution of the GSM, Enhance Data Rates for Global Evolution (EDGE). Blind equalization methods are feasible for GSM in low mobility scenarios.

The uplink (mobile to base station (BS) link) in direct-sequence code division multiple access (DS-CDMA) wireless networks is asynchronous. A DS-CDMA receiver has to simultaneously estimate channel impulse responses (CIR) and propagation delays for the active users. Commercial CDMA based systems use long spreading codes, with the period much longer than the symbol period. In this thesis, a novel uplink multi-user adaptive receiver is developed for long-code DS-CDMA. It is also capable of tracking time variations of the channels. Multiple antennas are considered at the receiver end, taking advantage of the signal to noise ratio (SNR) gain and the antenna diversity gain. A specific system model is developed, characterized by a channel matrix which also includes the effect of the propagation delays. Estimating the channel matrix leads to the implicit estimation of the propagation delays. Algorithms for the explicit estimation of the propagation delays are also derived. The proposed receiver structures are capable of estimating and tracking the impulse responses of the channels and synchronizing the active users by using low complexity adaptive techniques.

This thesis also addresses the problem of channel estimation and time synchronization in Orthogonal Frequency Division Multiplex (OFDM) systems. OFDM is robust with regard to frequency selective channels but is very sensitive to time and frequency synchronization errors. A novel low-complexity iterative method is developed for channel and time-offset estimation in OFDM by using a system model specific to fixed wireless links, e.g. wireless local area networks (WLAN) IEEE 802.11 standard.

This thesis consists of an overview and of the following 9 publications:

  1. Sirbu M., Mannerkoski J., Zhang Y. and Koivunen V., 2001. Feasibility of fractionally spaced blind equalization with GMSK modulated signals. In: Proceedings of the 15th European Conference on Circuit Theory and Design (ECCTD'01). Espoo, Finland, 28-31 August 2001, Vol. 1, pages 45-48.
  2. Sirbu M., Mannerkoski J., Zhang Y. and Koivunen V., 2001. On the feasibility of blind equalization for EDGE systems. In: Proceedings of the Third IEEE Workshop on Signal Processing Advances on Wireless Communications (SPAWC'01). Taoyuan, Taiwan, 20-23 March 2001, pages 90-93.
  3. Sirbu M. and Koivunen V., 2002. Channel estimation and tracking in asynchronous uplink DS/CDMA using multiple antennas. In: Proceedings of the 14th International Conference on Digital Signal Processing (DSP'02). Santorini, Greece, July 2002, pages 635-638.
  4. Sirbu M. and Koivunen V., 2002. Multichannel stochastic gradient method for adaptive equalization in uplink asynchronous DS/CDMA. In: Proceedings of IEEE Sensor Array and Multichannel Signal Processing Workshop (SAM'02). Rosslyn, VA, USA, August 2002, pages 303-307.
  5. Sirbu M. and Koivunen V., 2002. Propagation delay estimation in asynchronous DS/CDMA using multiple antennas. In: Proceedings of the 2002 European Signal Processing Conference (EUSIPCO'02). Toulouse, France, September 2002, pages 653-656.
  6. Sirbu M. and Koivunen V., 2003. Multichannel estimation and equalization algorithm for asynchronous uplink DS/CDMA. Wireless Personal Communications 26, No. 1, pages 33-52.
  7. Sirbu M. and Koivunen V., 2003. Low complexity synchronization method for uplink long-code DS/CDMA. IEEE International Symposium on Intelligent Signal Processing and Communication Systems (ISPACS'03). Awaji Island, Japan, December 2003, accepted for publication.
  8. Sirbu M. and Koivunen V., Delay estimation in long-code asynchronous DS/CDMA systems using multiple antennas. Helsinki University of Technology, Signal Processing Laboratory, Technical Report 42. Eurasip Journal on Wireless Communications and Networking, submitted for publication.
  9. Sirbu M. and Koivunen V., 2003. One-step refinement method for joint channel estimation and timing acquisition in OFDM transmission. In: Proceedings of the IEE & IEEE International Workshop on Signal Processing for Wireless Communications (SPWC'03). London, UK, May 2003, pages 261-264.

Keywords: DS-CDMA, GSM, OFDM, blind equalization, synchronization, multi-user system, statistical signal processing, adaptive filtering, channel estimation

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© 2003 Helsinki University of Technology

Last update 2011-05-26