## 1 Introduction  
Nowadays different cellular systems support worldwide wireless communications. The fusion of these systems with Internet provides flexible data services, enabling a new wave of mobile applications beyond traditional voice/message services. This great extent of flexibility at the upper communication layers contrast with the static and fixed core physical layers. Although in most cases this model is enough to satisfy users' needs, the use of flexible radios may offer new options and applications, thanks to real-time reconfiguration of radio resources. In this paper we introduce a new concept for communications inside vehicles based on software defined radio: a vehicular cell that is able to auto-reconfigure its operation channel to avoid interferences to/from neighbor cells. This cell provides both traditional services (voice/messages) and innovative ones such as user real-time detection. This mobile cell can be used in any large vehicle whose internal communications could be provided by means of a cell. There are no additional requirements to its application domain. Some illustrative examples are ships or trains. Vehicular communications involve data exchange within and between vehicles and between vehicles and infrastructure elements. Internal communications are especially useful in large vehicles, such as ships, where they may help to enforce safety. In contrast to cellular systems, VHF walkie-talkies indoor coverage is low. Although these systems operate at a lower frequency than second or third generation cellular systems, the higher sensitivity and transmission power of base station equipments, as well as the shorter and better communication paths, allow for better coverage in cellular systems. In [1], the authors conclude that broadband VHF signals inside vessels may not be feasible due to the rich multipath environment created by metallic structures, whereas in [2] the authors show that communication through ship compartments, despite these same effects, is possible in the frequency range of 0.8-2.5 GHz, where the cellular systems operate. In addition, VHF devices, mandatory in most vessels, are not allowed for personal communications. The main reasons are that VHF radiotelephones could interfere with safety-related VHF systems and that VHF frequencies are not suitable for communications in indoor locations. Therefore, a new technology for internal communications would enjoy high demand but, for outperforming current solutions, it should combine the following features:  
* **Wireless technology** (allowing ubiquitous communications within the vessel). * **Deployment simplicity** and **compatibility with inexpensive commercial terminals**. * **Low cost**, resulting from using off-the-shelf commercial components. * **Interference-free communication**, in the sense that it should not interfere with nearby in-shore communication systems or those of other vessels. The Global System Mobile (GSM) [3] is a well-known 2G technology which satisfies the first two requirements, and Software-Defined Radio (SDR), which allows a given physical system to take different operational roles along the time by reusing hardware, can meet the other two. First, SDR allows the implementation of low-cost mobile private Base Transceiver Stations (BTSs) as well as the switching functionalities required to enable communications in every ship. Although maritime wireless communications have motivated intense research [4, 5, 6, 7, 8], they focus on ship-to-ship and ship-to-shore communications. A low-cost GSM solution based on SDR has not been proposed for internal ship communications so far. Second, SDR flexibility allows the implementation of a Cognitive Radio (CR) module with spectrum sensing capabilities to avoid busy frequencies as the ship approaches the coast or other vessels, as we propose in this paper. Cognitive Radio (CR) [9, 10] relies on sensors to detect activity in the bands of operation. One of its main benefits is spectrum sharing.