## 2 Related Work  
Traditional radio systems rely on hardware elements (HDR, Hardware Defined Radio), which are constrained to the fixed characteristics of the modules performing the radio functions. In SDR systems however, certain functions are implemented as modifiable software [12], which confers flexibility, and thus important advantages:  
* New products reach the market sooner.
* Software reuse reduces development costs.
* New features can be easily added to existing infrastructures.
* Logistics are simplified thanks to the use of the same platform for different markets.  
Some works such as [13] have proposed HDR systems for private communications inside large vehicles. However, their complexity has prevented a real implementation in most cases.  
SDR, on the contrary, is progressively drawing interest as a viable option for real implementations of mobile devices. [PERSON] _et al._[14], for example, presented a GPS SDR sensor with adaptive beam-steering capability for anti-jam applications, and [PERSON] _et al._[15] showed that SDR can be applied in true commercial BTS subsystems in a study in which they addressed the design of the hardware and software architectures of a smart antenna base station (SABS) operating in a cellular network.  
From a more general perspective, [PERSON] and [PERSON] [16] proposed a reconfigurable mobile architecture and identified the technologies needed to support reconfigurability. The architecture has intelligence to evolve according to changes in the network. In our work, the cognitive BTS actually follows this philosophy, as part of an intelligent serving mobile network.  
[PERSON] _et al._ proposed VillageCell [17], a low-cost, SDR open-source solution to provide free local and long-distance communications to remote regions. VillageCell is based on a GSM cellular network in conjunction with a local rural-area network for VoIP services, which are implemented using OpenBTS and Asterisk. However, unlike us, the authors did not consider channel interference sensing or handover, two scenarios for which we provide solutions.  
Several GSM-based safety systems have been already proposed for terminal detection. For example, [PERSON] _et al._[18] introduced the Helios Platform, an unmanned aerial vehicle used as a mobile base station. It is a cost-effective solution for covering low-user-density, impervious, or offshore locations, which minimizes interferences with an adaptive beam-forming smart antenna. However, it has no provision for cognitive spectrum reuse. Besides, if a rapid response were necessary (e.g., if a crew member fell overboard) the deployment time of Helios would be prohibitive, unless a communication system such as the one we propose was available between the vessel and the life jacket. [PERSON] _et al._[19] described AirGSM, a similar system with the same limitations. [PERSON] _et al._[20], in turn, described a search-and-rescue system that combines a jammer with an SDR BTS unit to disable normal communications and force terminals to register themselves with the new BTS, a relatively old concept [21]. However, there is no provision for cognitive channel allocation.  
Finally, we must remark that part of our work relies on the OpenBTS project [22], in conjunction with the Universal Software Radio Peripheral (USRP) [23]. Although several research works have employed this configuration (e.g., [24, 25, 26, 27, 28]), as far as we know none of them have implemented automatic channel selection or seamless migration between old and new channels.