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Actualités de l'école

07/12/2019

Soutenance de doctorat de Wafa Khrouf



Soutenance de thèse de doctorat le  07/12/2019 à 10H00 à l'amphi Ibn Khaldoun de Sup'Com.

Intitulé : Optimization of FBMC/OQAM Modulation Schemes

 



Présentée par : Wafa Khrouf


JURY


Président

M. Ridha Bouallègue

Professeur, SUP’COM, Université de Carthage, Tunisie

 

 

 

Rapporteurs

Mme Noura Sellami

Professeur, ENIS, Tunisie

 

M. Mohamed Lassaad Ammari

Maître de Conférence, ENISo, Tunisie

 

Examinateur

Mme Leïla Najjar Atallah

Professeur, SUP’COM, Université de Carthage, Tunisie

 

 

 

Directeur de Thèse 

Co-directeur de thèse

M. Mohamed Siala

Mme Fatma Abdelkefi

Professeur, SUP'COM, Université de Carthage, Tunisie

Professeur, SUP'COM, Université de Carthage, Tunisie


Abstract:

Multi-Carrier Modulations (MCM) attract a lot of attention for high speed wireless transmissions because of their capability to cope with frequency selective fading channels, turning the wideband transmission link into several narrowband sub-channels whose equalization, in some situations, can be performed independently and in a simple manner. Nowadays, Orthogonal Frequency Division Multiplexing (OFDM) with the Cyclic Prefix (CP) insertion is the most widespread modulation among all MCM, thanks to its simplicity and its robustness against multipath fading. Systems or standards, such as ADSL or IEEE 802.11a, have already implemented the CP-OFDM modulation. Other standards like IEEE 802.11n combine CP-OFDM and Multiple-Input Multiple-Output (MIMO) in order to increase the bit rate and provide a better use of the channel spatial diversity. Nevertheless, the CP-OFDM technique causes a loss of spectral efficiency due to the CP, as it contains redundant information. Moreover, the rectangular prototype filter used in CP-OFDM has a poor frequency localization. This poor localization makes it difficult for CP-OFDM systems to obey stringent specifications of spectrum masks.

To overcome these drawbacks, Filter Bank Multi-Carrier (FBMC) was proposed as an alternative approach to CP-OFDM. Indeed, FBMC does not need any CP, and it furthermore offers the possibility to use different time-frequency well-localized prototype filters, which allow much better control of the Out-Of-Band (OOB) emission. In the literature, we find several FBMC systems based on different structures. In this thesis, we focus on the Saltzberg’s scheme called FBMC/OQAM (or OFDM/OQAM). The orthogonality constraint for FBMC/OQAM is relaxed being limited only to the real field, while for OFDM, also known as OFDM/QAM, it has to be satisfied in the complex field. One of the common characteristics between FBMC/OQAM and OFDM/QAM is that the demodulated transmitted symbols are accompanied by interference terms caused by the neighboring transmitted data in time-frequency domain.

The aim of this thesis is to study the performance of FBMC/OQAM with respect to FBMC/QAM. First, we present a general review on multicarrier systems. Then, we compare the two schemes in the continuous- and discrete-time contexts. We show, theoretically, the excellent performance of FBMC/OQAM systems with respect to FBMC/QAM systems for critical densities. We also note that a slight decrease in spectrum efficiency, allows FBMC/QAM systems to outperform FBMC/OQAM systems. Nevertheless, in terms of waveforms localization in time and frequency, FBMC/OQAM remains significantly better than FBMC/QAM. But, we believe that FBMC/QAM should be recommended in practice. Indeed, the implementation complexity of FBMC/OQAM increases excessively in MIMO systems, and does not justify the slight gain of around 5 % procured in spectral efficiency, compared to FBMC/QAM.

Keywords:  OFDM, FBMC, QAM, OQAM, Filter Bank, Multicarrier, Inter-Symbol Interference (ISI), Inter-Carrier Interference (ICI), SINR, Optimization, Waveform.