About how USRP sets the sampling frequency below the minimum sampling frequency reached by the hardware

What communication is not good? We have to do underwater acoustic communication ... If you have less information, there are still a lot of problems ... have to , Get down to business ：

Catalog

1、 Sampling on the sending end

2、 Receiver downsampling

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Recently, I'm trying to make the underwater sound OFDM stay X310 Run in , The first problem we encounter is ： although X310 Support DC-6GHz The frequency of , but The lowest sampling frequency can only be 195.31 KHz, as follows ：

Hey, are you angry , If data is sent at such a minimum sampling frequency , Then my carrier frequency must be greater than  195.31 KHz Talent ！ This is true for underwater acoustic channels TM high ... See some use X310 The article about underwater acoustic communication doesn't explain how to use it 85KHz Data sent by carrier wave of level ... On the contrary , Use 85KHz The sampling rate of the data sent by the carrier of level must be less than 85kHz, Isn't that ridiculous ？ It's so unkind ！ Then I began to look for information , At last, I have something to look at , The link is still the one above , I'll cut a screen and come out as follows ：

It means you can use Resampling Method for data transmission . The following are personal opinions , The actual measurement is effective , If you have any questions, please contact us ~

1、 Sampling on the sending end

With OFDM System as an example . Send the generated baseband signal to USRP（UHD:USRP Sink） Before sampling and multiplying by carrier frequency transmission , Upsampling baseband signal , namely Interpolation operations （Interpolation）, The location of resampling operation is shown in the figure 1 Shown ：

chart 1 Interpolation resampling

The picture shows 5 Interpolation resampling , It is equivalent to repeating the original signal 5 Time . Baseband signal is entering USRP Before that, there were discrete data , There is no concept of symbol spacing , At a certain sampling rate , One baseband data is processed for each sampling , The sampling rate set here is 200khz, Meet the minimum limit of hardware . Therefore, the data interval is set to 1 / fs, namely 1/200k=5us . If you keep 200k The sampling rate of does not change , The original baseband signal is 5 Sample after times of interpolation , It can be considered that the duration of the original signal becomes the original 5 times , At this time, the data interval is  1 / (fs/5), namely 1/(200k/5)=25us . At this moment   Equivalent sampling rate   It becomes  200kHz/5=40kHz . This completes the purpose of continuing to lower the sampling rate of the hardware ！

2、 Receiver downsampling

By contraries , At the receiving end, the received data needs to be downsampled , namely Extraction operation （Decimation）. Pictured 2 Shown .

chart 2 Downsampling

Here we go 1:5 The extraction operation of is used to recover the original amount of data , in addition , The equivalent sampling rate after extraction also becomes 200khz/5=40khz , After that, the sampling rate is in accordance with 40kHz To set up . Be careful USRP The sampling rate at the time of reception still needs to be maintained 200khz.