原文链接：http://www.cnblogs.com/nickchan/archive/2011/10/02/3104476.html 2011-10-02     Introduce:  There are two RX chennels in USRP1. It can be implemented double receivers by us. Abstract: Yesterday, I rewrote usrp_rx_cfile.cc to implement double receivers. Now, I sum up how to write a multiple antenna receiver code. I use this code to receive GSM donwlink. We can use double RX to implement MIMO.   Environment : Hardware : PC T61, USRP1, DBXR1 * 2  Software : Ubuntu 11.04, GNURADIO 3.4.1, airprobe GSM-Receiver   When I found USRP1 having tow RXs, I always wanted to know how could they work together. Now, I use both RXs to receive the same GSM donwlink channel and save different files: received_chan1.dat and received_chan2.dat . Then, I use GSM-Receiver which is a open-source project in airprobe  to decode GSM signals and dump to wireshark. I contrast the different files result in wireshark and find they are the same. When you want to implement RX function, you should make sure something you have already done.

1. make  usrp_source_c_sptr  usrp 
2. set nchannels (how many antennas do you need)
3. select subdev (which daughterboard do you need)
4. set frequency (which IF do you need)
5. set mux (input mux configuration, more detail in GNURADIO API)
6. deinterleave function is called to separate RX1 data and RX2 data 

  1st step: usrp_source_c_sptr usrp;     // contains 4 Rx paths without halfbands and 0 tx paths.     std::string fpga_filename="std_4rx_0tx.rbf";       // use default values and add fpga_filename     usrp = usrp_make_source_c(d_which, d_decim,      1, -1, 0, 0, 0,      fpga_filename.c_str());     2nd step:  usrp->set_nchannels(2);   3rd step:   usrp_subdev_spec myspec1(0,0);   db_base_sptr mysubdev1 = usrp->selected_subdev(myspec1);   printf("\nSubdevice name is %s\n", mysubdev1->side_and_name().c_str());     usrp_subdev_spec myspec2(1,0);   db_base_sptr mysubdev2 = usrp->selected_subdev(myspec2);   printf("\nSubdevice name is %s\n", mysubdev2->side_and_name().c_str());   4th step:     usrp_tune_result r;   bool ok = usrp->tune(0, mysubdev1, freq, &r);//DDC0   if(!ok) {     throw std::runtime_error("Could not set frequency.");   }     ok = usrp->tune(1, mysubdev2, freq, &r);//DDC1   if(!ok) {     throw std::runtime_error("Could not set frequency.");   }   5th step:   unsigned int mymux = 0x00003210;   //usrp->set_mux(mux);   usrp->set_mux(mymux);   printf("mux: %#08x\n",  mymux);   6th step:  gr_deinterleave_sptr my_deinter = gr_make_deinterleave(sizeof(gr_complex));   Test: Start wireshark and listen to your network interface. Use the following capture filter:"port 4729"(Hint: Ctrl + i, then click Option on any and enter the capture filter)   Keep wireshark running and keep listening. To decode RX1 file and RX2 file and contrast both GSM decoders.  use GSM-Receiver and type : ./go.sh received_chan1.dat you will see

and then type : ./go.sh received_chan2.dat  you will  see antenna2 channel RX's GSM data. Conclusion : USRP1 has double RXs and they both can work well. We can also work in double TXs. However, if we use double antennas, USRP1 needs 4 RX channels and it can't work in TXs. In this experiment, we know how to write code of RX and learn how to use double antennas of USRP1 receive signal such as GSM downlink.  GNURADIO is interesting. Have fun!!                                                                                                      Reporter:Nick Chan                                                                                                            

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