Increasing energy consumption is a result of the rapid growth in cellular communication technologies and a massive increase in the number of mobile terminals (MTs) and communication sites. In cellular communication networks, energy efficiency (EE) and spectral efficiency (SE) are two of the most important criteria employed to evaluate the performance of networks. A compromise between these two conflicting criteria is therefore required, in order to achieve the best cellular network performance. Fractional frequency reuse (FFR), classed as either strict FFR or soft frequency reuse (SFR), is an intercell interference coordination (ICIC) technique applied to manage interference when more spectrum is used, and to enhance the EE. A conventional cellular model's downlink is designed as a reference in the presence of inter-cell interference (ICI) and a general fading environment. Energy-efficient cellular models,such as cell zooming, cooperative BSs and relaying models are designed, analysed and compared with the reference model, in order to reduce network energy consumption without degrading the SE. New mathematical models are derived herein to design a distributed antenna system (DAS), in order to enhance the system's EE and SE. DAS is designed in the presence of ICI and composite fading and shadowing with FFR. A coordinate multi-point (CoMP) technique is applied, using maximum ratio transmission (MRT) to serve the mobile terminal (MT), with all distributed antenna elements (DAEs), transmit antenna selection (TAS) being applied to select the best DAE and general selection combining (GSC) being applied to select more than one DAE. Furthermore, a Cloud radio access network (C-RAN) is designed and analysed with two different schemes, using the high-power node (HPN) and a remote radio head (RRH), in order to improve the EE and SE of the system. Finally, a trade-off between the two conflicting criteria, EE and SE, is handled carefully in this thesis, in order to ensure a green cellular communication network.

Η χρήση διαφορισμού σε συστήματα κινητών επικοινωνιών είναι μία ευρέως χρησιμοποιούμενη τεχνική για την καταπολέμηση των διαλείψεων που παρατηρούνται στις ραδιοζεύξεις. Στην παρούσα διατριβή παρουσιάζονται διάφοροι τρόποι και τεχνικές για την μελέτη της απόδοσης συστημάτων που κάνουν χρήση διαφορισμού στον δέκτη. Οι τεχνικές διαφορισμού που μελετώνται είναι ο διαφορισμός ίσου κέρδους (equal-gain combining) και ο διαφορισμός επιλογής (selection combining). Επιπλέον, στα πλαίσια της διατριβής αυτής, υποθέτουμε ότι ο δέκτης λειτουργεί σε κανάλια μη-επιλεκτικών συχνοτήτων με βραδείες διαλείψεις (flat and slow fading channels), ενώ το εκπεμπόμενο σήμα υφίσταται διαλείψεις κατά Rayleigh, Nakagami-m, Rice ή Hoyt. Αποδεικνύεται ότι η απόδοση του συστήματος στο οποίο ο δέκτης κάνει χρήση διαφορισμού εξαρτάται σε μεγάλο βαθμό από: • τη συσχέτιση μεταξύ των καναλιών διαφορισμού και • τη διαφορά της ισχύος μεταξύ των καναλιών διαφορισμού. Για να μελετηθεί η απόδοση ενός ΔΕ που λειτουργεί σε συσχετισμένα Nakagami-m κανάλια παρουσιάζεται παρουσιάζονται για πρώτη φορά σε κλειστή μορφή η συνάρτηση πυκνότητας πιθανότητας (ΣΠΠ) Nakagami-m μεταβλητών με εκθετική συσχέτιση, ενώ η αντίστοιχη συνάρτηση αθροιστικής κατανομής (ΣΑΚ) που προκύπτει έχει τη μορφή πολλαπλών άπειρων αθροισμάτων. / Diversity reception has been successfully used in wireless communications systems to mitigate the negative effect of channel fading. This thesis deals with the performance analysis of wireless systems employing receiver diversity. The diversity schemes studied are selection combining (SC) and equal-gain combining (EGC). We assume that the receiver operates in flat and slow fading channels and that the signal undergoes Rayleigh, Nakagami-m, Rice or Hoyt fading. As it will be shown, the performance of the diversity system depends greatly on: • the correlation among the diversity branches, • the average power imbalance of the received signals In order to study the performance of a SC receiver operating in correlated Nakagami-m fading channels, we first present a statistical analysis of correlated Nakagami-m random variables (rvs). The probability density function (pdf) of exponentially correlated Nakagami-m rvs is presented in closed-form, while the corresponding cumulative distribution function (cdf) has the form of multiple converging infinite series. Furthermore, bounds for the truncation of the infinite series.

The technique of cooperative communications is finding its way in the next generations of many wireless communication applications. Due to the distributed nature of cooperative networks, acquiring fading channels information for coherent detection is more challenging than in the traditional point-to-point communications. To bypass the requirement of channel information, differential modulation together with non-coherent detection can be deployed. This thesis is concerned with various issues related to differential modulation and non-coherent detection in cooperative networks. Specifically, the thesis examines the behavior and robustness of non-coherent detection in mobile environments (i.e., time-varying channels). The amount of channel variation is related to the normalized Doppler shift which is a function of user’s mobility. The Doppler shift is used to distinguish between slow time-varying (slow-fading) and rapid time-varying (fast-fading) channels. The performance of several important relay topologies, including single-branch and multi-branch dual-hop relaying with/without a direct link that employ amplify-and-forward relaying and two-symbol non-coherent detection, is analyzed. For this purpose, a time-series model is developed for characterizing the time-varying nature of the cascaded channel encountered in amplify-and-forward relaying. Also, for single-branch and multi-branch dual-hop relaying without a direct link, multiple-symbol differential detection is developed. First, for a single-branch dual-hop relaying without a direct link, the performance of two-symbol differential detection in time-varying Rayleigh fading channels is evaluated. It is seen that the performance degrades in rapid time-varying channels. Then, a multiple-symbol differential detection is developed and analyzed to improve the system performance in fast-fading channels. Next, a multi-branch dual-hop relaying with a direct link is considered. The performance of this relay topology using a linear combining method and two-symbol differential detection is examined in time-varying Rayleigh fading channels. New combining weights are proposed and shown to improve the system performance in fast-fading channels. The performance of the simpler selection combining at the destination is also investigated in general time-varying channels. It is illustrated that the selection combining method performs very close to that of the linear combining method. Finally, differential distributed space-time coding is studied for a multi-branch dual-hop relaying network without a direct link. The performance of this network using two-symbol differential detection in terms of diversity over time-varying channels is evaluated. It is seen that the achieved diversity is severely affected by the channel variation. Moreover, a multiple-symbol differential detection is designed to improve the performance of the differential distributed space-time coding in fast-fading channels.

Single Carrier Frequency Division Multiple Access (SC-FDMA) is a multiple access transmission scheme that has been adopted in the 4th generation 3GPP Long Term Evolution (LTE) of cellular systems. In fact, its relatively low peak-to-average power ratio (PAPR) makes it ideal for the uplink transmission where the transmit power efficiency is of paramount importance. Multiple access among users is made possible by assigning different users to different sets of non-overlapping subcarriers. With the current LTE specifications, if an SC-FDMA system is operating at its full capacity and a new user requests channel access, the system redistributes the subcarriers in such a way that it can accommodate all of the users. Having less subcarriers for transmission, every user has to increase its modulation order (for example from QPSK to 16QAM) in order to keep the same transmission rate. However, increasing the modulation order is not always possible in practice and may introduce considerable complexity to the system. The technique presented in this thesis report describes a new way of adding more users to an SC-FDMA system by assigning the same sets of subcarriers to different users. The main advantage of this technique is that it allows the system to accommodate more users than conventional SC-FDMA and this corresponds to increasing the spectral efficiency without requiring a higher modulation order or using more bandwidth. During this work, special attentions wee paid to the cases where two and three source signals are being transmitted on the same set of subcarriers, which leads respectively to doubling and tripling the spectral efficiency. Simulation results show that by using the proposed technique, it is possible to add more users to any SC-FDMA system without increasing the bandwidth or the modulation order while keeping the same performance in terms of bit error rate (BER) as the conventional SC-FDMA. This is realized by slightly increasing the energy per bit to noise power spectral density ratio (Eb/N0) at the transmitters.

Single Carrier Frequency Division Multiple Access (SC-FDMA) is a multiple access transmission scheme that has been adopted in the 4th generation 3GPP Long Term Evolution (LTE) of cellular systems. In fact, its relatively low peak-to-average power ratio (PAPR) makes it ideal for the uplink transmission where the transmit power efficiency is of paramount importance. Multiple access among users is made possible by assigning different users to different sets of non-overlapping subcarriers. With the current LTE specifications, if an SC-FDMA system is operating at its full capacity and a new user requests channel access, the system redistributes the subcarriers in such a way that it can accommodate all of the users. Having less subcarriers for transmission, every user has to increase its modulation order (for example from QPSK to 16QAM) in order to keep the same transmission rate. However, increasing the modulation order is not always possible in practice and may introduce considerable complexity to the system. The technique presented in this thesis report describes a new way of adding more users to an SC-FDMA system by assigning the same sets of subcarriers to different users. The main advantage of this technique is that it allows the system to accommodate more users than conventional SC-FDMA and this corresponds to increasing the spectral efficiency without requiring a higher modulation order or using more bandwidth. During this work, special attentions wee paid to the cases where two and three source signals are being transmitted on the same set of subcarriers, which leads respectively to doubling and tripling the spectral efficiency. Simulation results show that by using the proposed technique, it is possible to add more users to any SC-FDMA system without increasing the bandwidth or the modulation order while keeping the same performance in terms of bit error rate (BER) as the conventional SC-FDMA. This is realized by slightly increasing the energy per bit to noise power spectral density ratio (Eb/N0) at the transmitters.