The wireless market of the future will demand inexpensive hardware, expandability, interoperability, and the implementation of advanced signal processing functions--i.e. a software radio. Configurable computing machines are often ideal software radio platforms. In particular, the Stallion reconfigurable processor's efficient hardware reuse and scalability fulfill these radios' demands. The advantages of Stallion-based design inspired an FPGA-based software radio - the proto-Stallion receiver. This thesis introduces the proto-Stallion architecture and details its implementation on the SLAAC-1V FPGA platform. Although this thesis presents a specific radio implementation, this architecture is flexible; it can support a variety of applications within its fixed framework. This implemented single-user DS-CDMA receiver utilizes an LMS adaptive filter that can combat MAI and constructively combine multipath; most notably, this receiver employs an adaptive tracking algorithm that harnesses the LMS algorithm to maintain symbol synchronization. The proto-Stallion receiver demonstrates the dependence of adaptive tracking on channel noise; the algorithm requires significant noise levels to maintain synchronization. / Master of Science

The system capacity of the direct-sequence code division multiple access (DS-CDMA) system is limited mainly due to the multiple access interference (MAI), this is basically due to the incomplete orthogonality of spreading codes between different users. In wireless communication environments, the use of DS-CDMA system over multipath channels will introduce the effect of inter-symbol interference (ISI), thus the system performance might degrade, dramatically. To circumvent the above-mentioned problems many adaptive multiuser detectors are proposed, such as the minimum mean square error (MMSE) criteria subject to certain constraints. Unfortunately, with the MMSE receiver it requires an extra training sequence, which decreases the spectral efficiency. To increase the spectral efficiency, the blind adaptive receivers are adopted. In the conventional approach the blind adaptive receiver is developed based on the linear constrained minimum variance (LCMV) criteria, which can be viewed as the constrained version of the minimum output energy (MOE) criteria. Other alternative of designing the blind adaptive receiver is to use the linear constrained constant modulus (LCCM) criteria. In general, the LCCM receiver could achieve better robustness due to the changing environment of channel. With the above-mentioned adaptive linearly constrained multi-user receivers, we are able to reduce the effects of ISI and MAI and achieve desired system performance. However, for worse communication link, the conventional adaptive multi-user detector might not achieve desired performance and suppress interference effectively. In this thesis, we consider a new approach, in which the pre-coder similar to the Orthogonal Frequency Division Multiplexing (OFDM) systems is introduced in the transmitter of the DS-CDMA system. In the receiver, by using the characteristics of pre-coder we could remove the effect of ISI, effectively, and follows by the adaptive multi-user detector to suppress the MAI. Two most common use pre-coders of the OFDM systems are the Cyclic Prefix (CP) or Zero Padding (ZP). Thus the pre-coded DS-CDMA systems associated with the adaptive blind linearly constrained receiver could be employed to further improve the system performance with the cost of decreasing the spectral efficiency.

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.