Architectures and Protocols for Flexible Physical Layers in Wireless Networks

Emerging wireless technologies are characterized by an increasing level of flexibility and programmability, not only in terms of core network functionalities, with the consolidated paradigms of software-defined-networks and function virtualization, but also in terms of radio access functionalities. Although the concept of software-defined PHY and MAC protocols is not new, exploiting flexibility at the lower layers of the protocol stack is not an easy task, because of complexity and performance constraints. Indeed, dealing with software-defined implementations of the radio implies managing complex software routines, often tightly inter-dependent and difficult to reuse, and poses some performance limitations because software implementations are inevitably less efficient than hardware ones. In this thesis, we focus on the theme of PHY flexibility, by proposing innovative architectures of the radio, in which a limited set of parameters and functionalities is programmable, in order to achieve a trade-off between the complexity of the hardware and software architecture of the receiver, and the performance improvements that can be enabled in different network scenarios, characterized by specific topologies or interference conditions. More specifically, by considering that most modern communication systems are based on Orthogonal Frequency Division Multiplexing (OFDM), we decided to focus on the generalization of a typical OFDM transceiver, in which we introduced different levels of programmability: i) the possibility of dynamically adjusting the total bandwidth, for a given number of subcarriers, even on a per-packet basis, and without an explicit control channel between the transmitter and the receiver; ii) the possibility of mapping dynamically pilot and data symbols, with a symbol-level resolution, in order to increase robustness to interference and jammers; iii) the possibility of transmitting special tone signals, on desired sub-carriers, for coding simple control messages to be exploited for network-wide coordination. The first capability has been designed in order to add more granularity to PHY adaptations (usually limited to the tuning of the transmission power or per-carrier modulation format). The second capability has been conceived in order to improve physical layer robustness to intentional attacks. The are different jammer types, but some kind of their are more disruptive than others. Indeed, in an OFDM based communication, estimation and equalization of the channel’s frequency response is crucial for a correct decoding of the packet at the receiver side. Estimation and equalization are done by the insertion of equal power and equally spaced pilot tones in the signal. Some types of jammers attack pilot tones in order to destroy information used by the equalization algorithm. For this reason, we designed and implemented some mechanisms in order to mitigate as much as possible some of the jamming strategies that are very problematic in an OFDM-based communication. Finally, the last capability has been exploited for designing an efficient contention mechanism based on the concept of Repeated Contentions, called ReCo. We demonstrate that running multiple contention rounds in random access networks in the frequency domain improves the channel utilization efficiency. Ultimately, thanks to advantages of flexibility of the physical layer, we provide a robust medium access control (MAC) protocol, which is not depending on the number of the contending stations. All the proposed extensions to a reference OFDM transceiver have been validated with real implementations and experiments. For the prototyping activities, we worked on two different platforms: the well known WARP software-defined board and the USRP platform. Experiments have been planned and analyzed by focusing on reproducibility of the results and by providing comparisons with benchmark scenarios.