## 3 Cognitive Mobile BTS  
Our private cognitive BTS consists of the three modules shown in Figure 2: (i) Private BTS, (ii) Spectrum Sensor Module (SSM) and (iii) Decision Module (DM).  
The private BTS module is the core element, comprising both the radio transceiver (_i.e._, the module implementing the GSM radio interface) and the switchboard functionalities allowing communications between registered Mobile Stations (MSs). As an additional functionality, it provides controlled channel change, that is, the ability of migrating current communications to a new frequency channel.  
The SSM also performs continuous spectrum sensing in order to detect used and empty spectrum channels. This information is passed to the DM, which is in charge of deciding when a channel change is necessary. Decision criteria must prevent communication degradation. One possible mechanism could be periodic change to an idle channel, but this would not ensure interference-free communications if the period was too long. Moreover, it would force the MSs to continuous channel changes if the period was too short, draining their batteries and reducing communications capacity during these periods. Therefore, the criterion is based on an estimation of the quality of communication in the current channel, similarly to what occurs with conventional GSM traffic channel changes. In other words, when high degradation is detected, the communication is transferred to a suitable new channel selected by the DM. The following procedures have been implemented:  
Figure 2: Cognitive BTS modules.  
* Downlink channel control: allows to detect interferences affecting each particular downlink connection, it is based on the measurement reports received from each MSs, like in conventional GSM networks.
* Uplink channel control: it is the counterpart of the previous procedure, allowing interference control for the uplink connections. It is performed by obtaining quality measurements of the channel, like conventional GSM networks.
* Idle mode control: permits detecting interferences while no active connection exists between the BTS and the MS. This control procedure is unavailable in conventional GSM networks, since in them MSs will tune to some neighbor cell of the same network to avoid the interference source. In our case, there is a single cell, and therefore the normal control procedure is not possible. We propose the following mechanism to minimize the impact of interferences in case of idle MSs: (i) force periodic communications from the MSs to the cognitive BTS and (ii) perform channel reselection if a given ratio of previously registered MSs do not transmit periodic messages.  
When the switchboard receives the change command from the DM, it notifies to all registered MSs of the newly selected channel. As we will explain later, channel changes are transparent to ongoing communications. This is a highly relevant feature for an SDR-based mobile BTS implementation in which just one cell is operative at any given time.  
The cognitive capability avoids possible interferences caused by other systems operating in the same channel, as it is illustrated in Figure 3. The private BTS deployed in a ship gives coverage to crew terminals. Once the system sensors detect another signal in the current operating channel, a new channel is activated and communications are redirected to it. A standard cargo ship travels at around 25 knots [29], _i.e._, 46.3 km\(\cdot\)h\({}^{-1}\). The GSM standard is designed to tolerate speeds of 250 km\(\cdot\)h\({}^{-1}\), so the Doppler effect is not relevant in our scenario, in which the users travel with the BTS inside the ship.  
An additional service, which is not available in commercial GSM networks, is the fast identification of the MSs that are reachable at a given moment by performing controlled channel changes: in them, active terminals will automatically respond, allowing the system to identify and potentially locate them quickly in case of emergencies.  
In-depth implementation details are provided in the next sections.