Adaptation and Optimization in Multi-Carrier Modulation Systems

In recent years, we have assisted to the dawn of many wireless and wireline communication technologies that have adopted multi-carrier modulation (MCM) at the physical layer. The basic idea ofMCMs is to transmit a high rate data stream by dividing it into low rate streams that are used to generate low rate signals each modulated at a given carrier frequency. The use ofMCMs allows for dividing the frequency selective channel into a set of narrow-band sub-channels. Consequently, the transmitted signal experiences, in each sub-channel, a quasi flat frequency response, so that, the equalization task simplifies to a sub-channel filtering. In addition to the simplification of the equalization task, there are several benefits deriving from the use of MCMs that, in general, depend upon the considered transmission medium. The most important ones are the low complexity digital implementation, and the possibility to use bit and power loading algorithms that allow for nearly achieving channel capacity or for minimizing the transmitted power. Several MCM schemes have been presented in the literature in the last fifty years. Some examples are: the orthogonal frequency division multiplexing (OFDM); the pulse shaped OFDM (PS-OFDM); the filtered multitone (FMT). These technologies have to be reliable, namely, they have to be able to work over all the possible conditions (channel realizations) that can occur over a given medium. Consequently, these are designed according to a ?worst? case criterion, i.e., they adopt a set of parameters that allow the system to achieve, also in bad conditions, acceptable performances. However, in some cases, this design criterion is not optimum. In fact, the practice shows that there exist scenarios, where the channel slowly changes, in which the performances can be significantly improved by letting the systems to adapt their parameters to the experienced conditions. The first topic that we consider in this thesis is the optimization of the parameters and the resource allocation in adaptive-MCM systems whose received signals are affected by interference caused by signaling over a dispersive channel. To be more precise, we propose parameter design criteria for adaptive OFDM, PS-OFDM, and FMT. For these schemes, we also propose resource allocation algorithms that target the achievable rate maximization or the transmitted power minimization. The extension of the parameters adaptation to the multi-user case is also considered. In particular, resource allocation algorithms for the downlink case of networks using frequency division multiple access (FDMA) and time division multiple access (TDMA) is discussed. We also consider the case where the modulation scheme is adapted to the channel condition. More precisely, we make use of a hybrid architecture that depending on the channel conditions wisely switches between adaptive-OFDM and FMT so that the transmitted power is minimized. Extensive numerical results are shown for typical wireless LAN and in-home broadband power line channels. Besides the use of adaptive MCM schemes, another interesting advanced communication technique that allows for improving the performances of a MCM system is the use of relaying techniques. This is the context where we find the second objective of the thesis, i.e., to propose optimal resource allocation algorithms to provide capacity improvements, power savings, and coverage extension in two hops relay networks where half duplex time division relay protocols are used. More precisely, we consider a network where the communication between the source and the destination node follows an opportunistic protocol, namely, the relay is used whenever it allows w.r.t. the direct transmission (DT): for capacity improvements; or for power saving. Opportunistic decode and forward (ODF and opportunistic amplify and forward (OAF) are considered. At the physical layer, we assume the use of MCM. Under these assumptions, we find the optimal resource allocation, namely, the optimal power and time slot allocation, between the source and the relay nodes that maximizes capacity, or minimizes the total transmitted power for both ODF and OAF. We apply the proposed algorithms to the in-home power line scenario. According to this scenario, the relay can only be placed in accessible points of the network, i.e., in the main panel, or in derivation boxes, or in outlets. For each opportunistic protocol, we also determine the best relay position that maximizes capacity or minimizes the transmitted power.