## 4 Simulation Results and Discussion  
In this Section, some experimental results characterizing the effectiveness of the proposed method are presented. The dimensions of the video-sequences employed in our experimentations have been properly chosen in order to simulate a multimedia service in a UMTS scenario, _i.e._ typical 3G video-calls. Therefore, QCIF (144x176) video sequences, which well match the limited dimensions of a mobile terminal's display, have been employed and a frame rate of 15 fps has been chosen. The marked video is then transmitted over noisy channels, simulated by [PERSON]'s generators of random transmission errors. Specifically, wireless channels characterized by different levels of bit error rate (BER) have been designed, considering the customers near and/or far from the base station (BS), respectively. Specifically, we take into account the _near-far-effect_ typical of cellular systems: when the user is near the BS, the uplink communication has been considered worse than the downlink path, in terms of BER while, when the user is far from the BS, the uplink path has been considered better than the downlink (_i.e._ the mobile transmits with much more power to compensate the distance from the BS). For users closer to the BS, we have imposed an uplink BER of \(10^{-4}\) and a downlink BER of \(10^{-5}\), while for users far from the BS, the uplink BER has been considered equal to \(10^{-4}\) and the downlink BER to \(10^{-3}\). Our simulation trials are proposed to test the effectiveness of the double boomerang approach, which can be used for error control purposes in e.g. Hybrid ARQ techniques in UMTS  
Figure 2: Principle scheme of the double boomerang approach for QoS purposes. applications. Four situations are simulated: -two mobile agents far from the BS (_scenario 1_); -two mobile agents near to the BS (_scenario 2_); -the first mobile agent is near while the second is far from the BS (_scenario 3_), -the first mobile agent is far from the BS while the second is near (_scenario 4_). In all the following simulations, we have used the common test video-sequences such as _Carphone_, _Foreman_, _Miss America_ and _Suzie_, all MPEG-4 coded. In particular, we have supposed two scenarios: a video-call between the users [PERSON] and [PERSON] and another video-call between [PERSON] and [PERSON]. We have first verified that with this double boomerang approach is still possible to trace the alterations endured by the data in the communication link. Then, we have evaluated an empirical relationship (obtained with the videos under investigation) between the MSE of the watermark and of the multimedia content transmitted trough the network, as a proof of concept of the objective QoS assessment method proposed here. Hence, we have used four different video-sequences and, in particular, each MSE value of our simulation trials is obtained after \(10^{6}\) Montecarlo runs. Moreover, we have evaluated the confidence level of our simulations, verifying that the confidence interval varies from the \(\pm 3\%\) up to the \(\pm 10\%\) corresponding to the different operating scenario under investigation. In particular, we have verified that the confidence interval varies (as expected) with the BER of the channel and hence is strictly connected to the link quality of the considered scenario: in fact, the confidence level becomes relevant when considering the worst channel conditions (_i.e._ scenarios 3 and 4) and it is in such conditions that the minimum MSE of the watermark may not correspond to the minimum MSE of the video (as it will be shown in the following graphs). Figure 3(_a_) shows here the watermark MSE of the video-communication between MissAmerica-Suzie, while Figure 3(_b_) depicts the watermark MSE of the Carphone-Foreman video-communication. The MSE metric is evaluated using (2), comparing the received watermark (embedded using the double-boomerang approach explained in Section 3) and its original counterpart, _i.e._ the original frame opportunely adapted (_i.e._ cropped and binarized).