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Doctorate thesis defense of Naoufel Debbabi

Doctorate thesis defense on December 01st 2016 at 10H00 AM ,in Amphi I, Sup’Com.


Presented by : Naoufel Debbabi 



Ridha Bouallegue

Professor at Sup'Com, Tunisia





Mohamed Slim Alouini

Professor at KAUST, Saudi Arabia


Mohamed Lassaad AMMARI

Associate Professor, ENISO, Tunisia



Leila Najjar Atallah

Associate Professor at Sup'Com, Tunisia



Mohamed Siala

Professor at Sup'Com, Tunisia



Ines Kammoun Jemal

Associate Professor at ENIS, Tunisia



Medium- and small-scale propagation fading in radio-mobile cellular environments is a serious impediment that can highly degrade the performance of a transmission scheme, mainly for users located at the cell borders. Cooperative relaying schemes, forming virtual antennas arrays, present promising techniques that can overcome this weakness, by reducing the fading effects through a combination of multiple replicas of the transmitted signal. However, the use of relays in a transmission scheme generates an additional Multiple Access Interference (MAI), which translates into a decrease in cellular system capacity, especially in case of Code Division Multiple Access (CDMA) communications, known to be both intra-cell and inter-cell interference limited.

In this thesis, we optimize cooperative relaying schemes based on multiple criteria. Firstly, we are interested on uplink CDMA communications. We start by defining a global MAI cost that reliably quantifies the additional intra-cell and inter-cell interference generated by a relayed communication to the whole cellular system. For uplink CDMA communications, this MAI cost hides an implicit and smart power control that takes into account not only the reception quality required at the serving base station but also the interference generated to other base stations of neighbor cells. Then, we propose a novel approach that optimizes the system performance of an Amplify-and-Forward (AF) relaying scheme while keeping the global MAI cost constant. Next, we extend the optimization to Decode-and-Forward (DF) relaying schemes and we compare the performance of both optimized AF and DF schemes for different scenarii.

Afterwards, we propose a new adaptive soft Decode-Remodulate-and-Forward (DRF) relaying scheme in order to maximize the system throughput for uplink CDMA communications. The proposed adaptive DRF relaying scheme is based on adaptive modulation schemes at the source and relay nodes. The modulations at the source and the relay can be different, which makes the combination of the received signals at the destination more complex. As a result, the destination combines th e signals received from the source and the relay using a soft combining approach instead of a classical and straightforward Maximum Ratio Combining (MRC). The proposed relaying scheme is based on a soft demodulation at both the relay and the destination nodes. The output of a soft demodulator is based on the a posteriori Log-Likehood Ratio (LLR). Moreover, we derive closed form expressions of the resulting a posteriori LLRs at the destination in case of DRF cooperative relaying with different modulation schemes at the source and relay nodes. The optimization is carried out for different combinations of the source and relay modulation schemes. Our optimized adaptive DRF cooperative scheme selects the appropriate modulation scheme that provides the maximal throughput at the destination after a soft combination of the signals received from the source and relay nodes.

All previous works are dedicated to improve the system performance for CDMA communications. To complete these works, we also consider the optimization process for uplink Orthogonal Frequency Division Multiple Access (OFDMA) communications, carried on a packet-per-packet basis. Then, the optimization is performed for AF and Incremental AF (IAF) relaying schemes. For OFDMA cellular systems, the MAI cost highly depends on the used frequency reuse pattern. The optimization, in terms of PER, is carried out while keeping the MAI cost constant, for both regular and Fractional Frequency Reuse (FFR) patterns. For FFR, different frequency reuse factors are applied to cell center and cell edge areas.

We finally tackle the optimization of the amplification function of AF relaying scheme. The optimization consists in the determination of the required optimal amplification function to ensure the best achievable performance at the destination. Our idea is that the relay adapts its amplification function based on the received signal from the source in order to minimize the Bit-Error-Rate (BER), resulting at the destination. We propose a new version of AF relaying, named Soft-Information-Amplify-and-Forward (SIAF) relaying, where the relay amplifies a soft decoding information on the detected symbol before forwarding it to the destination.


CDMA, OFDMA, Relaying, Optimization, Multiple Access Interference Cost, Incremental Relaying, Selective Relaying, Decode-Remodulate-and-Forward, Adaptive Modulation, Log Likehood Ratio, Amplification Function