## 4 Validation Experiments
### Cognitive BTS Testbed  
A full operative testbed with the main procedures in our solution was implemented. Its results are discussed next. Figure 9 shows the main elements in our setup. The following experiments were performed:  
Figure 8: Jammer building blocks. 1. **Connected mode--Normal cell reselection**. This experiment tested normal cell reselection with an ongoing call. A trace of the signaling involved has already been presented in Figure 7. Several tests were performed, with seamless changes of the cell channel in less than 570 ms in all cases. Speech communications continued fluently during the changes. Other tests with two simultaneous voice calls yielded the same results. 2. **Idle mode--Normal cell reselection**. This experiment tested cell reselection while the MSs were in idle mode. In this case, when the MS is aware of the channel change, it directly tunes to the new frequency without signaling exchange with the BTS. The maximum time measured in the experiments was 13.1 s. As an example of this procedure, Figure 10 shows how an MS first camps completely on a cell, which later changes its working channel. Once this occurs, the MS reselects that channel and continues to monitor broadcast and paging channels. 3. **Uplink loss and cell reselection**. This experiment tested the ability of the cognitive BTS to detect interference in the communication uplink. Obviously, this can only be detected when there are ongoing communications (otherwise, the MSs do not use the uplink). We emulated the effect of an interfering BTS nearby by activating our jammer in the corresponding part of the uplink spectrum. The jammer disabled any possible transmission from the MSs. Figure 8 shows the jammer tuned to 890.2 MHz, which is the uplink frequency of the channel with ARFCN #1. Figure 9: Experimental cognitive BTS setup. Figure 11 shows behavior of the MS. The upper part shows the received power in the downlink and the middle part shows the MS transmission power. In our experiment the jammer was activated twice at the instants shown in the figure. In both cases, the MSs stopped working, since they could not receive confirmation of their messages (both communication gaps can be clearly seen in the figure). Moreover, in each case, the cognitive BTS detected that call quality dropped below the critical level and issued a channel change. Note that channel frequencies are shown as different color lines (yellow, green and blue). Figure 10: Idle mode—Normal cell reselection, Location Update procedure samples. 4. **Downlink loss and cell reselection**. Similarly to the previous case, this experiment checked the detection of communication problems in the downlink channel. Figure 12 shows the corresponding traces. The jammer was activated several times to force the cognitive BTS to perform a channel reselection. Each activation is indicated in the figure. It can be clearly seen how there was a sharp decline in the quality of the downlink channel, and how the channel changed each time. Figure 11: Uplink loss and cell reselection. Figure 12: Downlink loss and cell reselection. 5. **Downlink loss in idle mode**. This last experiment tested the recovery from a failure in the downlink channel when there were not ongoing communications. After the MSs camped on our cell, the jammer was activated to block the corresponding downlink carrier. After waiting for 6 minutes at most, we checked that the cognitive BTS noticed the absence of connection attempts from the MSs trying to initiate a \"Location Update\" procedure. Then, the cognitive BTS changed to a new channel, which the MSs eventually found and they camped again. In this case, the time from when the channel change took place until the MSs camped on the new cell was approximately one minute. Note that this is longer than in the normal cell reselection procedure in idle mode. This is as expected, since, when unable to decode the BCCH, the MSs initiate a full cell search procedure, which takes longer that the cell reselection procedure.