A reduced complexity Ungerboeck type receiver for multi-code signaling in dispersive channels

Thesis Type: Doctorate

Institution Of The Thesis: Orta Doğu Teknik Üniversitesi, Faculty of Engineering, Department of Electrical and Electronics Engineering, Turkey

Approval Date: 2014




The main aim in this thesis is to propose multiple signaling waveforms (multi-code) based yet spectrally efficient modulation schemes and competent receiver architectures realizing soft-input-soft-output (SISO) detection. We search for generic suboptimal receiver architectures for Multi-Code Signaling (MCS), which can be represented as selection of one out of M waveforms per signaling interval. The proposed receiver architectures exhibit almost optimal performance at significantly reduced complexity in highly dispersive channels. First, an efficient reduced complexity implementation of the Ungerboeck type Maximum a Posteriori (MAP) receiver, directly operating on unwhitened channel matched filter and code matched filter outputs, is proposed for MCS by forming the factor graph (FG) and sum-product algorithm (SPA) framework. The proposed MAP receiver, generating the a posteriori probabilities by bidirectional reduced state sequence estimation (RSSE) recursions, is substantiated with symbol rate bidirectional decision feedback based on surviving paths in order to eliminate the post- and pre-cursor inter-symbol interference (ISI) as well as multi-code interference due to the non-ideal properties of the signaling waveforms and multipath channel. Second, we extend the proposed Ungerboeck receiver to be exploited in multiple access channel by unifying the bidirectional RSSE applied to each user and the mitigation of multi-user interference fulfilled by the SPA based on the obtained Ungerboeck type FG, resulting in linear complexity in the number of interfering users. Finally, error probability analysis, which provides significant insight on the success of the proposed reduced state Ungerboeck receivers in case of uncoded MCS transmission, and the packet error rate analysis based on the random coding approach that determines the cutoff rate for coded transmission are provided. These analyses help the designer determine system parameters and open up new possibilities for a performance enhancement of reduced complexity Ungerboeck receivers via a proper selection of a modulation scheme for the general class of MCS especially in long ISI channels. To sum up, the proposed receiver architectures here confirm, compare many previous works, and complement reduced complexity Ungerboeck structure by changing several system parameters generalized to MCS format with the help of the developed analytical tools.