Current research on mobile wireless networks places substantial emphasis on scenarios in which primary (or licensed) and secondary (or unlicensed) terminals coexist in the same spectral resource, forming “cognitive radio” networks. Irrespective of the traffic class (multihop, uplink/ downlink, etc.), the main conventional approaches to enable such a coexistence are: (i) Underlay/ overlay/ interweave strategies, which are based on the paradigm of oblivious primary transmitters and enforce strict constraints on the secondary behavior in order to avoid interference to the primary; (ii) System-wide dynamic spectrum allocation/ sharing, which prescribes the optimization of a given system-wide utility function in order to allocate resources among distributed nodes, with possible priorities assigned to primary users. Both frameworks, while being promising in certain aspects, appear to have significant drawbacks for implementation of large-scale distributed cognitive radio networks, due to the technological and theoretical limits on the ability of secondary activity to perform effective spectrum sensing (for (i)) and on the stringent constraints on protocols and architectures (for (ii)).

Intellectual impact: To address the problems highlighted above, this project introduces the novel framework of distributed spectrum leasing via cross-layer cooperation (DiSC) as a basic mechanism to guide the design of Medium Access Control/ Data Link (MAC/DL) - physical (PHY) layer protocols in decentralized cognitive radio networks. According to this framework, dynamic “leasing” of a transmission opportunity (e.g., a time-slot) from a primary node to a secondary terminal is performed locally as driven by primary needs in terms of given Quality-of-Service (QoS) measures at the MAC/DLPHY layers (e.g., probability of outage or of MAC buffer overflow). Specifically, DiSC enables each primary terminal to “lease” a transmission opportunity to a local secondary terminal at MAC Protocol Data Units (MPDUs) granularity, in exchange for cooperation (relaying). The project aims at both a theoretical understanding of the potential of the approach and the design of MAC/DL-PHY protocols that effectively implements DiSC in a complex wireless environment. In the first thrust (core theory), the novel theoretical problems raised by the approach are investigated from the standpoints of network information theory and networking theory (queuing, resource allocation) by focusing on the local behavior of the system and considering increasingly more complete models that account for the PHY layer (e.g., fading, imperfect channel state information) and DL functionalities (e.g., queuing, ARQ). The fundamental problem is that of designing transmission/ reception strategies that optimize primary QoS requirements (rate, latency) under the spectrum leasing constraint that cooperating secondary nodes receive enough bandwidth to satisfy their respective QoS parameters of interest. Additional constraints include the possible competitive and malicious behavior of secondary users. In the second thrust, the design of the MAC/DL-PHY protocol stack and interplay with higher layers in complex multi-hop networks are investigated. Emphasis is put on the minimization of MAC signaling overhead by exploiting existing MAC messages (ACK, RTS/CTS, etc.) for the purpose of acquiring channel state information and piggybacking minimal additional information. Both a clean-slate design of the protocol stack for best performance and a suboptimal implementation that maximizes backcompatibility with current standards are studied.

Broader impact: The pervasiveness of wireless technology in the forthcoming “internet of things” is based on the envisioned open-spectrum paradigm for the utilization of radio resources. Spectrum leasing, and its decentralized and dynamic implementation proposed in this project, is a major candidate for the realization of this paradigm. It is both a transformative technology and one that provides a smooth transition from the current wireless standards. As such, the outcome of this research is expected to benefit the society as a whole towards the outlined evolutionary path. Moreover, the proposal enables the collaboration between the involved institutions and graduate students (via exchange and joint seminars), and makes provision for international cooperation with major international institutions.