I am trying to build an ofdm flow graph with usrp, as shown below
Flowgraph
The flowgraph works fine without any errors. However, there is no received signal at the receiver end as shown in results(1) and results(2).
It seems that the USRP does not get data/signal from the OFDM Trasnmitter, and the signal shown at Rx spectrum is just noise.
Any recommendation to solve this problem?
your sampling rate is too low. The USRPs you're using probably aren't running at the rate - look for the warnings at the beginning of your flow graph execution about other sampling rates being used
The gain is too low, probably, for over-the-air transmission.
The analog bandwidth settings make no sense. Since this seems to be a network-connected USRP: only very few of these have adjustable frontend bandwidths, anyway, but using a frontend bandwidth much larger than your sampling rate makes no sense.
Most importantly: This is communications, with a random channel, noise, interferers and device imperfections. It's very normal for packets to get lost.
Related
I have two USRP B200 connected to two Raspberry Pi's that I want to communicate via AX.25. Here are the flowgraphs:
TX:
RX:
They work well and are able to communicate. However, if I change the samp_rate to 200k on both the TX and RX, the RX is not able to receive the messages sent. However, if I use Direwolf with RTL-SDR, I am able to receive the messages sent at 200k. Can anybody help me how to receive the data sent at 200k?
Thanks!
This is really an outdated version of GNU Radio. gr-ax25 is ported to GNU Radio 3.9 and 3.8; please use a modern GNU Radio (3.8 or 3.9), that solves a lot of problems you'd otherwise run into later on.
Also, as UHD will print to the console, 150 kS/s is lower than any sampling rate than any USRP actually supports, and hence is substituted for a different one – which works, because both ends of the communication do the same!
It's still not "proper". You need use a higher rate (recommendation: 1 MS/s) and use a resampler to go down to the very low AX.25 rates; the "rational resampler" block will do that!
Then, you misconfigured both your NBFM TX and RX – your quadrature rate is not the 48 kHz you configured, but your actual sampling rate / 4.
Of course, as long as both ends of the communication are "wrong in the same way", they have a chance of working together. But any actually correct receiver implementation won't be able to make out what you meant to send.
I'd recommend you look into the apps/ subofolder of dl1ksv's gr-ax25. That contains a properly set up APRS transceiver, for a sampling rate of 192 kS/s; I'd recommend you replace the fcdproplus blocks with your UHD USRP source/sink blocks and use a rational resampler, so that at the USRP Source you decimate by 16, and on the USRP Sink you interpolate by 16. Your sampling rate for both Sink and Source would then be 192000*16.
I am trying to set up 5 USRP1 and some daughterboards on 2.4 and 5 GHz.
Some of them are out of order and some work properly, but I don't know which is which. I am trying to send a symbol (QAM modulation) sequence then I am trying to pass it with a file source to both a USRP sink and an FFT sink.
I am trying to find articles and tutorials, how sample rates are connected and how to set up them but I can't understand what am I missing. Could somebody please help with the schemes?
128 MS/s is not a rate that is possible with the USRP1. The console will contain a UHD warning that a differen, possible rate was chosen (most likely, 8MS/s).
Now, you contradict that rate by having a "Throttle" block in your flow graph - that block's job is only (and nothing more) to slow down the average rate at which samples are being let through – and that is something your "USRP Sink" already does. In fact, modern versions of GRC will warn you that using a throttle block in the same flow graph as a hardware sink or source is a bad idea.
Now, you'll say "ok, if the USRP sink actually needs to consume but 8MS/s, and my interpolator makes 128 MS/s out of my nominally 1M/s flow (really, signals within GNU Radio don't have a sampling rate), then that's gotta be fast enough to satisfy the 8MS/s demand!".
But the fact is that a 128-interpolator is really a CPU-intense thing, and the resulting rate might not be that high, made even worse by the choppy nature of how Throttle works.
In fact, your interpolator is totally unnecessary. The USRP internally has proper interpolators for integer fractions of its master clock rate 64MS/s, which means that you can set the USRP Sink to a sampling rate of 1MS/s and just directly connect the file source to it.
I am doing a project on indoor localisation using fingerprinting. Is it possible to build a system in LabView which can scan the entire spectrum and provide me the RSSI measurements of different types of signals?(say FM, GSM, DVB-T and so on.) In case it has to be done separately, can someone please point me to some resources that would help me to find the RSSIs of say, FM signals? I am new to SDRs and would really appreciate some help. I have used this paper as a reference:
http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=7444902
There can be no general method. RSSI is inherently signal-specific, and hence, for each signal type, you will need a different estimator. An estimator that estimates the received signal strength of FM broadcasts will only see noise in a DVB-T signal at incredibly high noise power with no stable signal at all, whereas a DVB-T RSSI indicator would only see a slowly moving interfere in an FM signal.
I see you're using the indoor-positioning-system tag. That is a very bad thing: In an indoor scenario, fading is extremely important, and your received signal strength allow absolutely no conclusions on the distance to the transmitter. This is pretty much the definition of the indoor channel, where lots and lots of reflections overlap and interefere, and there's not always a direct line of sight between transmitter and receiver.
I'm afraid you still have quite some theory to read up on.
Im currently creating a satellite ground station which will be used to control our cubesat in coming months. The modulation scheme used is GFSK and the baud rate is 9600. I have tried to run some tests by using a USRP board before I could try to communicate with the satellite by directly connecting the tx and rx blocks in the flowgraph.I was able to send and receive a png file using this flowgraph.
However, when I connect the tx and rx output to my USRP B210 TX/RX(transmission sink) and RX2(reception source) as shown below, I receive no data even though the two the source and sinks have been connected to each other carefully by RF cables with attenuators.
Below are the assumptions I took into account when I was making the second flowgraph. Please tell me if im on the right path.
Transmitter side : The packet decoder and GFSK mod blocks use 20 samples per symbol. Baud rate is 9600 and sample rate is 20*baud rate = 192K. Since the expected symbol rate by the satellite is baud_rate = 9600, I included a rational resampler and set UHD symbol rate to baud_rate. Is my logic correct?
GFSK mod and demod : For both of these blocks, I calculated sensitivity as S = Pi * Modulation_index/Samples_Per_Symbol. The default BT value of 0.5 is used. Are my calculations sound? Is there a link for to find documentation for GFSK blocks? My derivations are based on the GFSK python source code which is a poor substitute for documentation.
Packet Encoder/Decoder : The output of packet decoder is null even though the GFSK demod block give some kind of output which is rather meaningless. Is this normal? What is the meaning of the threshold variable and why its value is -1?
I'm a newbie in GNU Radio as well as GFSK in general. So please drop me any further references.
Thanks in advance.
Moses.
I was finally able to solve the problem. All I did was re-implementing the GFSK demod in GRC. If you go into source of gfsk.py, you will find out that the blocks used are Quadrature Demod --> M&M Clock recovery --> Binary slicer which can easily be connected in GRC directly. As Marcus suggested in my other thread, GFSK demodulation with Xlating filter in GNU Radio , I replaced the M&M Clock recovery block with PFB block. My flowgraph is shown below.
Even if I can not answer all of your questions, I provide below some thoughts:
When using hardware devices the Throttle MUST be removed from the flow-graph. The hardware device is now responsible for the rate limiting. Mixing hardware device and the Throttle block may break the real-time boundary of your flow-graph required by the device. Underflows or Overflows messages should be produced by the UHD driver in such a case.
Are you sure that the USRP can support the requested sampling rate? You may need also to change the master_clock_rate of the device, if the requested sampling rate is not an integer decimation of the clock. If this is not possible consider some kind of re-sampling.
EDIT: The B200 can not provide 192e3 sampling rate with the default clock. You can set the master_clock_rate at 19.2e6. The hardware will apply then the proper decimation. The master_clock_rate can be changed either by the device specific arguments or the Clock Rate field of the UHD Sink/Source blocks that presents in the latests GNU Radio versions.
Hi Im using the following RF module
http://www.apogeekits.com/rf_receiver_module_rx433.htm
on an embedded board with the PIC16F628A. Sadly, I realized that the signal strength was in analog form and couldn't get any ideas to get the RSSI reading off the pin because well my PIC is digital DUH!.
My basic idea was
To get the RSSI value from my Receiver
Send it to the PIC
Link the PIC to a PC via RS232
Plot a graph of time vs RSSI of the receiver (so I can make out how close my TX is to my RX)
I thought it was bloody brilliant at first but ive hit a dead end here. Any ideas on getting the RSSI data to my PC from this receiver would be nice.
Thanks in Advance
You can get a PIC that has an integrated ADC for sampling the analog signal. Or, you can use an external ADC chip to do the conversion. You would connect that to your PIC using SPI or I2C.
The simplest thing to do is obviously to use a more appropriate microcontroller - one with an ADC! There are many (most), including PICs (though that wouldn't be my first choice).
Attaching an external SPI or I2C ADC might be a bit tedious since having no SPI or I2C on your part, you'd have to bit-bash it. If you do that, use an SPI part - its simpler. Your sample rate will suffer and may end-up being a bit jittery if you are not careful.
Another solution is to use a voltage controlled PWM, then use the timer input capture to time the pulse width. That will give you good regularity and potentially good resolution. You can get a chip (example) to do that, or grow your own. That last option requires a triangle wave input as well as the measured (control) voltage, but on the same site...
In a similar vein, you could use a low frequency VCO (example) and use the output to clock one of the timers, then using a second timer periodically sampling the first and reset it. The count will relate to the voltage, though not necessarily a linear relationship, linearisation could be none on the PIC or at the receiving PC - I'd go for the latter - your micro will suck at arithmetic (performance wise) - even integer arithmetic, especially if it involves division.