Very low bitrate and low power RF communication? - Page 1
Forum section: electronics / beginners

author: Chris C
author_rank: Frequent Contributor
timestamp: August 22, 2015, 02:27:29 pm
content: Years ago I remember reading about some method of using a very low bitrate on the TX side, and a DSP technique on the RX side, by which communications could occur at great distances and/or extremely low received power - well below the noise floor. I seem to recall someone managing to bounce a signal off the moon, using modest TX power, and a speed measured in seconds-per-bit rather than bits-per-second.I've been Googling for it, trying to find it again, and I just can't seem to hit on the right keywords to find anything relevant. Can anyone point me in the right direction?For the curious, I have a potential use for it more modest than moon bounces. I have a series of sensor and automation nodes in my house, connected via a RF mesh network. Should one of these crash, I would like to record error information prior to it automatically resetting. But the MCUs used have no non-volatile memory other than FLASH, which I'd prefer not to write, in case the crash is severe enough that it might perform an erroneous write. And the crash will likely render the stack for the normal RF transceiver unusable as well. I was thinking it would be novel if I could hook a small wire antenna to a MCU pin, using PWM or a timed loop to generate a weak RF signal, that contains node ID and a crash code. The PC that controls all the nodes would then be fitted with a receiver (probably an SDR), capable of picking this up.

author: unitedatoms
author_rank: Frequent Contributor
timestamp: August 22, 2015, 02:46:14 pm
content: Try keywords like M-Sequence, correlation, analog correlator, autocorrelation.I once made a program back in 1990s to calculate the 32-bit sequence with brute force (because I did not have wiki and had no knowledge of polynomial method of M-Sequence generation). In space of 2^32 values there is only 1 best sequence (+1 inverted, +1 reversed, +1 inverted reversed). To check if it works, add all logical results of "equal" the boolean function of every shifted position of sequence against itself. Build a chart of correlation sum value from "-32..0..+32" bit shifts. Then see if chart has only single max=32. The rest of peaks will be below 2. That is the perfect sequence.If you want to share single frequency bandwidth in the same timeframe and location with use of multiple sequences, then minima for correlation could be worsen to fit more sequences. Those are named "orthogonal codes"/"orthogonal sequences". That what Qualcom made commercialized in CDMA. Before commercialization it was mostly for NASA and military.

author: PA0PBZ
author_rank: Super Contributor
timestamp: August 22, 2015, 02:53:40 pm
content: I think you mean something like WSJT or JT65?http://www.eham.net/articles/25604

author: unitedatoms
author_rank: Frequent Contributor
timestamp: August 22, 2015, 02:59:44 pm
content: Quote from: PA0PBZ on August 22, 2015, 02:53:40 pmThat method looks like above the noise still. Or correct me if I am wrong. If human eyes used as a correlator to decide on received bit, then it may be classified as below noise.

author: Chris C
author_rank: Frequent Contributor
timestamp: August 22, 2015, 05:43:43 pm
content: I'm not sure either of these methods are the one I originally saw, which I think may have been simpler. However, this is certainly better than nothing, and that's what I had before.Looking up JT65, it appears to have a payload of 13 characters (or more with compressable sequences) from a small list (numbers, uppercase letters, spaces, and basic punctuation). It adds forward error correction, and transmits over a 48 second window as 65-tone MFSK. I found a quote saying, "It's mathematically provable that JT65A signals can be detected with 100% certainty even if the received signal is -22 dB under the noise floor." The RX and TX side must be time-synchronized, with preferably less than a second difference. This I can do, seems simple enough, and I could possibly increase detection even further if needed by lengthening the window. The only apparent downside is that the remote node, once crashed, would have to wait for the beginning of the next window before even starting transmission. Wasted time when I might be waiting on it to reboot and become usable again. Perhaps I could eliminate that, but I'm not sure. I did not see any mention of the exact method used to perform tone detection, and whether the time synchronization is required for that, or just to allow multiple HAMs to simultaneously TX on the same frequency.As for ' method, wow... that is just Greek to me!I get the surface impression that a complex sequence is transmitted, and that the RX side is able to detect the presence of the slightest hint of that sequence (even under the noise floor) through statistical means, without any prior synchronization of time, state, or position in the sequence. I'll have to set aside some quiet reading time later today, hit Google, and see if this is something I'm going to be able to wrap my head around.

author: unitedatoms
author_rank: Frequent Contributor
timestamp: August 22, 2015, 09:46:27 pm
content: I just looked at NASA's paper about search for M-Sequence (brute force method b.t.w.)http://ipnpr.jpl.nasa.gov/progress_report/42-82/82O.PDFon p.143 "..Optimal sequences are ideal for search of extraterrestrial intelligence.."