1. Synchronization issues for digital measurements¶
Usually, the USRP does not abide the “IQ sampling rate” that has been entered as an input parameter, and forces a value that is close to it. This “actual IQ sampling rate” is displayed on the user interface. As a result, there is a continuous drift of the sampling time instant regarding the optimal sampling time instant: at a given time, the optimal sampling time instant is reached and the constellation appears, later, the sampling time instant falls between two adjacent symbols and the constellation vanished. This phenomenon can be witnessed on the following video (to be downloaded). This is a hardware issue that we cannot fix. The error measurements derived from the constellation are only relevant when the constellation is well displayed.
On the other hand, assuming the actual IQ sampling rate is equal to the desired sampling rate, a manual control allowing synchronizing the initial sampling time instant with the optimal sampling time instant (see Figure 2) has not been implemented yet. Therefore, the initial sampling time instant is defined by the instant at which the program starts to run. In case the constellation does not appear as expected, one has to restart the program until getting the expected results. This is not user-friendly and the synchronization of the initial sampling time instant could be done manually by controlling a command that would shift the sampling time instant from 0 to Ts while the program is running. By controlling this shift, the aim would be to minimize the inter-symbol interference and the EVM while the constellation is being displayed.