Space-time block coding (STBC) has added a new dimension to broadband wireless communication systems. Applications such as wireless Internet access and multimedia require the transmission of high data rates over frequency selective fading channels. The reliability of the wireless communication system can be increased by using diversity techniques combined with an equalizer at the receiver to eliminate the inter-symbol interference caused by multipath propagation. Generalizing Alamouti's famous STBC method to frequency selective channels, Time Reversal-Space Time Block Coding (TR-STBC) was first introduced in [1] and has since been shown to be an effective transmit diversity scheme [2, 3, 4]. TR-STBC-based schemes are considered promising candidates for indoor transmission [5] as well as for the enhanced data rates of the global evolution (EDGE) system [2, 3]. The optimal equalizer for a TR-STBC-based transceiver is the Maximum Likelihood Sequence Estimator (MSLE), realized using the Viterbi algorithm. Unfortunately, a Viterbi equalizer is difficult to implement in real-time due its exponential increase in complexity with the number of antennas and the length of the channel impulse response. Thus, we consider an adaptive algorithm-based Decision Feedback Equalizer (DFE). Such a DFE requires only linear processing complexity while maintaining good performance. Theoretically, the two output streams of a 2 x 1 TR-STBC decoder are uncoupled in terms of the input signal statistics and uncorrelated in terms of the channel noise statistics. The standard approach to removing the inter-symbol interference from these streams is to use either two parallel independently-adapted Single-Input Single-Output (SISO) equalizers or to use a single Multiple-Input Multiple-Output (MIMO) equalizer. By exploiting the common second-order statistics of the two output streams, we proposea novel hybrid equalizer structure which shares the statistical information between two SISO equalizers while constraining them to have common tap weights. To accommodate various levels of performance versus computational complexity, we propose novel Least Mean Square (LMS), Normalized Least Mean Square (NLMS), and Recursive Least Squares (RLS)-based adaptive algorithms for this new equalizer architecture. We use both statistical analysis and Monte Carlo simulations to characterize the dynamic convergence of these algorithms and to compare our new structure with the conventional uncoupled SISO equalizers and fully-coupled MIMO equalizer. We show that our new equalizer outperforms the other two equalizers using a reduced computational complexity similar to the uncoupled SISO equalizers. As expected, with increasing complexity, we find that the novel RLS-based algorithms converge the fastest followed by the novel NLMS- and LMS-based algorithms. We also consider alternative packet structures and kick-start methods to increase the convergence speed and reliability of the equalizer at realistic complexity. Finally, adding multiple receiver antennas to our system, we extend our equalizer structures and algorithms to the 2 x NR case. Using analysis and simulations, we demonstrate that the added receiver diversity in this case yields even greater reliability.

Η χωροχρονική μπλοκ κωδικοποίηση (STBC) αποτελεί μία αποδοτική και ευρέως διαδεδομένη τεχνική διαφορετικότητας μετάδοσης για την αντιμετώπιση του φαινομένου της εξασθένησης στις ασύρματες επικοινωνίες. Χαρακτηριστικό παράδειγμα είναι ο ορθογώνιος κώδικας του σχήματος Alamouti, ο οποίος με δύο κεραίες μετάδοσης επιτυγχάνει τη μέγιστη χωρική διαφορετικότητα στο μέγιστο δυνατό ρυθμό μετάδοσης, για οποιονδήποτε (πραγματικό ή μιγαδικό) αστερισμό συμβόλων. Ωστόσο, το σχήμα Alamouti έχει σχεδιαστεί για συχνοτικά επίπεδα κανάλια. Στην παρούσα εργασία μελετούμε την εφαρμογή STBC σε κανάλια συχνοτικά επιλεκτικής εξασθένησης. Εστιάζουμε κυρίως στο συνδυασμό του σχήματος Alamouti με τεχνικές εξάλειψης της διασυμβολικής παρεμβολής, εξετάζοντας τα σχήματα OFDM-STBC, FDE-STBC και TR-STBC, που έχουν προταθεί στη βιβλιογραφία. Επιπρόσθετα των συμβατικών δεκτών, για τα δύο τελευταία σχήματα περιγράφουμε και προσαρμοστικούς δέκτες, οι οποίοι παρακολουθούν τις μεταβολές του καναλιού, χωρίς να απαιτούν την ακριβή εκτίμησή του. Η έρευνα πάνω στα προηγούμενα σχήματα οδήγησε σε ορισμένα αξιόλογα αποτελέσματα. Κατ’ αρχήν, αποδεικνύουμε ότι τα σχήματα FDE-STBC και TR-STBC είναι ισοδύναμα, μολονότι καθένα εφαρμόζει διαφορετική κωδικοποίηση στα μεταδιδόμενα δεδομένα. Επίσης, σχεδιάζουμε έναν νέο δέκτη για το σχήμα TR-STBC, τον οποίο αναπτύσσουμε και σε προσαρμοστική μορφή. Βασικό πλεονέκτημα του προτεινόμενου δέκτη είναι ότι εκμεταλλεύεται τους κυκλικούς πίνακες συνέλιξης για τη μείωση της πολυπλοκότητας αποκωδικοποίησης. Τέλος, η απόδοση κάθε σχήματος και δέκτη αξιολογείται σε διάφορες συνθήκες εξασθένησης μέσω προσομοιώσεων σε υπολογιστικό περιβάλλον. / Space-time block coding (STBC) is an effective and widely used transmit diversity technique to combat multipath fading in wireless communication systems. A prominent example of STBC is the orthogonal code of Alamouti scheme, which achieves full spatial diversity at full transmission rate for two transmit antennas and any (real or complex) signal constellation. However, Alamouti scheme has been designed only for frequency-flat channels. In this thesis we study the application of STBC in frequency-selective channels. We mainly focus on combining Alamouti scheme with techniques for mitigating intersymbol interference, by studying several schemes (OFDM-STBC, FDE-STBC and TR-STBC) that have been proposed in literature. In addition to the conventional receivers, for FDE-STBC and TR-STBC we describe adaptive receivers too, which have the ability of tracking channel variations, without requiring explicit channel estimation. Research made upon the above schemes has come to some remarkable results. First, we prove that TR-STBC and FDE-STBC are equivalent, although each one encodes differently the transmitted data. Then, we design a new receiver for TR-STBC, which exploits the circulant convolution matrices, in order to reduce decoding complexity and we, also, develop an adaptive structure for the proposed receiver. At last, we evaluate the performance of all the described schemes and receivers in different fading conditions, by using computer simulations.