Hi Israel, can you please try to keep this discussion on the mailing list? It's just that I'm not the only one who can help you, and certainly not even the most competent :)
> 1- I have a very short distance between USRPs (approximately half > a meter), so I’m surprised that I have so many errors. Besides, in the > virtualization environment, it’s assumed that the channel model is > perfect (without channel noise, frequency offset, timing offset) and, > therefore, I shouldn’t haver errors...but I do. Well, no matter how short your transmission is, pushing something through quantization, digital-to-analog-conversion, through mixers, amplifiers (which might be compressing!), the RF channel, amplifiers, mixers, an ADC (which might be clipping) and hence, quantization always has a non-zero noise figure. I assume with "virtualization" you mean simulation, if I'm misunderstanding, please correct me. So, the GMSK demod is designed to give the best possible BER under certain assumptions. That might mean that under perfect conditions, it gives more than a 0 BER. (to give you a bit more detail: on the inside, it does timing recovery, which will solve the real world problem of unknown symbol timing in a real receiver, but will look like something's jittering in a noise- and delay-free simulation). You set your packet decoder to automatic thresholding, which is fine, but means that it might wrongfully assume a different threshold for different parts of your sample stream, based on the energy of the data you're transmitting (in a wider sense, that should be fine, because JPEG is Huffman-encoded internally, and I'd hence expect a nice power distribution). > 2- Assuming I always have errors and the packet decoder will drop > any packet it can't properly decode, I find it odd that the top of my > picture is always right. I mean, the errors should be random and in > all transmission tests I can receive the first piece of the picture > correctly (I attached a example of this) > On the contrary! Assume (that's not inherently the case) that errors would actually be random, and assume that continued packet detection errors would require a certain number of bits to be flipped -- that will very seldom happen right at the start of your observation, but its probability will increase with the number of transmitted bits; Bernoulli is nobody's friend. > 3- On the other hand, I have added a FLL Band-Edge block and a > Costas Loop block to my receiver flowgraph, but it hasn’t worked > either. Isn't there any exampleof transceiverthat uses GMSKmodulation > (or other, it doesn'tmatter)andimplementsblocks against allthese > errorsin order to I can study how to solve the problem? FLL Band-Edge and Costas Loops can be used with many constellations, but one has to be careful not to damage one's own signal when correcting shifts! I don't understand your question about there being an example of GMSK transceiving -- you're actively working with it! Another, more recent, approach to digital packet transceivers can be found in the gr-digital/examples folder as OFDM GRC flowgraphs. These pull a lot more tricks, more explicitly: * Schmidl & Cox estimators based frequency offset correction and timing detector, * channel estimation and separate header and payload equalization Best regards, Marcus On 13.11.2015 15:34, Israel . wrote: > > > > > > Hi Marcus, > > > > Thank you very much for your quick answer. I understand my transceiver > isn’t very robust, but: > > > > 1- I have a very short distance between USRPs (approximately half > a meter), so I’m surprised that I have so many errors. Besides, in the > virtualization environment, it’s assumed that the channel model is > perfect (without channel noise, frequency offset, timing offset) and, > therefore, I shouldn’t haver errors...but I do. > > > 3- On the other hand, I have added a FLL Band-Edge block and a > Costas Loop block to my receiver flowgraph, but it hasn’t worked > either. Isn't there any exampleof transceiverthat uses GMSKmodulation > (or other, it doesn'tmatter)andimplementsblocks against allthese > errorsin order to I can study how to solve the problem? > > > > Regards >
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