Increasing Spatial Spectrum Utilization Through Opportunistic User-to-User Communications Dimitris Tsolkas • Nikos Passas • Lazaros Merakos Received: 13 March 2012 / Accepted: 9 June 2012 / Published online: 27 June 2012 Ó Springer Science+Business Media, LLC 2012 Abstract The requirements for ubiquitous and highly reliable wireless services, combined with the low utiliza- tion of licensed spectrum, call for flexible and efficient spectrum management schemes. To this end, a lot of attention is paid in the literature on allowing secondary- external users to opportunistically access the licensed spectrum. In parallel to these efforts, the question is whe- ther the licensed users could further improve their own spectrum utilization. In this paper, we focus on increasing the spatial spectrum utilization of an infrastructure-based wireless system by adding autonomous functionality to the primary (system) users. An opportunistic operation mode for the uplink (UL) period, totally transparent to the base station (BS) of the system, is introduced. Users operating in this mode identify spatial spectrum UL opportuni- ties by interpreting BS broadcast messages, and exploit these opportunities by establishing direct connections. It is shown that multiple direct connections can take place in parallel with a single standard UL transmission. Moreover, significant additional throughput is achieved, and in most of the cases, the energy consumption for the direct con- nections is lower than that of the conventional ones (using the standard mode). Keywords Spatial spectrum reuse  Uplink opportunities  User-to-user communications  Direct communications  Opportunistic spectrum access 1 Introduction Modern wireless communication systems, such as long term evolution (LTE), are facing many challenges in addressing the requirements for ubiquitous and highly reliable wireless services, increased data rates, better quality of service (QoS), and lower energy consumption. These requirements, combined with recent measurements and reports showing the underutilization of a large portion of the assigned spectrum [1], call for flexible and efficient spectrum management schemes. In the literature, several promising approaches that aim at increasing the licensed spectrum utilization have been proposed [2–15]. On the one hand, the coexistence of different systems in the same band is utilized for increasing spectrum utiliza- tion. The most popular approach here is the opportunistic spectrum access (OSA) which introduces a hierarchical coexistence between primary and secondary networks [2] under the principles of cognitive radio technology [3]. Based on this approach, [4] focuses on the downlink transmissions and provides some fundamental limits for opportunistic spectrum reuse in a frequency-planned envi- ronment, such as that of cellular networks. The transmis- sion-capacity trade-off for coexisting cellular and random access networks is studied in [5], leading to a linear rela- tion between the two targets. Also, the ability of spatially reusing the spectrum is studied in [6], where topology informed users transmit concurrently and in the same band with users of a carrier sense multiple access with collision avoidance (CSMA/CA) system. D. Tsolkas (&)  N. Passas  L. Merakos Department of Informatics & Telecommunications, University of Athens, 15701 Athens, Greece e-mail: dtsolkas@di.uoa.gr N. Passas e-mail: passas@di.uoa.gr L. Merakos e-mail: merakos@di.uoa.gr 123 Int J Wireless Inf Networks (2013) 20:1–12 DOI 10.1007/s10776-012-0181-5