In "LTE Cell Search, MIB and SIB1 Recovery with Two Antennas": Shouldn't we reacquire the signal, once we know the full bandwidth of the system?

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Laura F. 2018-4-27

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编辑： Graham Freeland 2018-5-1

I was going through the example "LTE Cell Search, MIB and SIB1 Recovery with Two Antennas" and changed it so that it acquires a signal from a known (commercial) base station with the USRP, without needing the transmission part of the example.

I noticed that in the example, it says at one point "Restarting reception now that bandwidth (NDLRB=%d) is known...", but there's no reacquisition of the signal at that point, just a resampling. In the given example it works only because they don't need to resample, but in a real-case scenario, shouldn't we acquire new frames with the full bandwidth?

For instance, let's say that after decoding the MIB I found out that the system's bandwidth is actually 10 MHz. Then resampling without reacquiring the signal shouldn't bring me any new information, because the SIB1 could be anywhere in the 10 MHz bandwidth, whereas the signal that I had at that point was only in a bandwidth of ~1.9 MHz and an upsampling will just re-space that signal.

Am I right or am I missing something? If I am right, then shouldn't the example be readjusted so that it works also in cases other than that where we don't need resampling?

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Answer 1

Graham Freeland 2018-5-1

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编辑：Graham Freeland 2018-5-1

The example is designed to work with a single input waveform, for example, an actual capture or a synthetic waveform. This allows the example to be used immediately by people without ongoing access to data acquition hardware. Often users will have sets of offline captures or test waveforms rather than online hardware.

In terms of content and functionality, the example was intended to show the initial stages of LTE system acquition (sync and cell ID, MIB and then on to SIB1 PDSCH) and some of the minimum bandwidth requirements for these steps and the reasons behind them.

You are correct that, to perform all the processing steps on a single capture, in the general case, the data in the file would have to have been acquired at the full carrier bandwidth. If only the sync and cell ID steps were required then a narrower capture bandwidth could be used. If you have access to measurement hardware then, given on the device capabilities, it could be possible to dynamically reacquire data at different rates, depending on the processing stage.