Is interference (intentional or otherwise) by other users a problem ATM? I haven't heard anyone mention in to date. I have seen examples (some on my blog posts) of good robustness to some overlap by SSB signals.
Those pitch/voicing/energy are protected by FEC, the parity bits for the FEC are at the opposite end of the frame to provide some protection from frequency selective fading. I've looked into interleavers but the nature of the HF multipath channel means the gains are small or zero. - David On Sun, 2014-03-02 at 14:48 -0600, Ham Radio Java wrote: > I thought I would put down on paper my thoughts about the current FDM > implementation. No performance charts or anything like that, just a > binary bit placement. > > > The QPSK is of course fed 2-bits, which are used to create one of four > phases to one of the sixteen subbands (carriers if you like). You > might ask yourself, are all subbands equal, and the answer from an > interference perspective is no. We've all had people creep-up to our > signals and get too close, and the edges of our signal then get > jammed. > > > In jamming you often analyze a protocol to find it's weakest point, > and then direct the jamming power towards that part. You might think > the BPSK pilot would be a good choice, but it has the most power. A > good jammer uses the least amount of power, so that it can hide it's > activity and not become a smoking hole in retaliation. > > > FDMDV is currently a very easy signal to jam. The voicing bits are > very important, as without them you would have to guess if each 10ms > frame is voiced or unvoiced. I suppose there is a mathematical > dice-rolling algorithm that can do this, but my hunch says to make it > more robust to keep Bubba and Jim-Bob at bay. > > > Here's the 1300 bps signal from the first of two frames: > > > Each subband is spaced 75 Hertz, or 125 Hz (wide): > > > V = Voicing (4-bits) > P = Pitch (7-bits) > E = Energy (5-bits) > F = FEC (12-bits) > L = LSP (36-bits) > > > Subband 0 =========================================== Bubba > Subband 1 ================== V1-V2 > Subband 2 ================== V3-V4 > Subband 3 ================== P1-P2 > Subband 4 ================== P3-P4 > Subband 5 ================== P5-P6 > Subband 6 ================== P7-E1 > Subband 7 ================== E2-E3 > Subband 8 ================== E4-E5 > Subband 9 -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Pilot > Subband 10 ================== F1-F2 > Subband 11 ================== F3-F4 > Subband 12 ================== F5-F6 > Subband 13 ================== F7-F8 > Subband 14 ================== F9-F10 > Subband 15 ================== F11-F12 > Subband 16 ================== L1-L2 > Subband 17 ================== L3-L4 > Subband 18 =========================================== Jim-Bob > > > The second half frame is just L5-L36 bits. > > > You can see I might have stopped after the first two subbands, because > with just a 150 Hz low-power jammer I can destroy your communications. > Nature and the ionosphere also make great jammers, so forget about > just having to worry about Bubba and Jim-Bob sliding up to your signal > with their 2 kW AM signals. > > > There are probably a dozen ways to scramble the bits, but I would > suggest first moving the voicing bits closer to the center. I'm > suggesting maybe Subband 5 and 7, and Subband 11 and 13. Then to not > put all your money on one horse, we could pair the bits with FEC bits. > So maybe V1-F1, F2-V2, V3-F3, F4-V4. Move all those FEC bits in > Subbands 10-15 around. Put some in the I-Channel and some in the > Q-Channel of the PSK signal. > > > Then maybe to mix it up a bit, but some of the LSP bits from Frame 2 > into Frame 1 and move some of the FEC, Pitch, and Energy bits from > Frame 1 to Frame 2. > > > 73, Steve > > > ------------------------------------------------------------------------------ > Flow-based real-time traffic analytics software. Cisco certified tool. > Monitor traffic, SLAs, QoS, Medianet, WAAS etc. with NetFlow Analyzer > Customize your own dashboards, set traffic alerts and generate reports. > Network behavioral analysis & security monitoring. All-in-one tool. > http://pubads.g.doubleclick.net/gampad/clk?id=126839071&iu=/4140/ostg.clktrk > _______________________________________________ Freetel-codec2 mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/freetel-codec2 ------------------------------------------------------------------------------ Flow-based real-time traffic analytics software. Cisco certified tool. Monitor traffic, SLAs, QoS, Medianet, WAAS etc. with NetFlow Analyzer Customize your own dashboards, set traffic alerts and generate reports. Network behavioral analysis & security monitoring. All-in-one tool. http://pubads.g.doubleclick.net/gampad/clk?id=126839071&iu=/4140/ostg.clktrk _______________________________________________ Freetel-codec2 mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/freetel-codec2
