6 ms·

My understanding is that limitation is mostly technical and managerial at this point, to help manage things in the early stages. I would too imagine this being

• walrus01 6y ago
  They intentionally need to populate a certain number of terminals into specific geographic areas to test, measure and refine how time-sharing contention and the equivalent of TDMA is being setup. Each terminal has a GPS receiver in it and reports home with its location.
  • stefan_ 6y ago
    That sounds like more conspiracy theories. For all we know it has a GPS receiver as a precise reference clock.
    • walrus01 6y ago
      no, quite literally, if you move a beta terminal from your designated service location, it won't work. I know at least 4 people who also have beta terminals who have tried it. It depends on the size of your 'cell' - some people have been able to go a few km away, some people report it doesn't work even a few hundred meters away from their house. When you sign up for the beta you have to specify your street address and also click on an aerial map where you intend to install it. And of course a GPS receiver is useful for timing.
      • space_fountain 6y ago
        I have heard speculation that the issue is actually satellite signal steering. They’re using phased array antennas on both sides so the satellites need to know where to be listening
        • walrus01 6y ago
          They haven't released any real info on it. But there's been teardown videos of the CPE showing the phased array design. And the patent for the terminals, which is public, shows a dual beamforming phased array. It clearly needs to be in order to maintain a seamless make-before-break with rising and descending satellites that are illuminating specific areas on the ground with spot beams.
        • 6nf 6y ago
          Taat makes sense, with a small number of users they are not likely to spend CPU time doing fast Fourier transforms and whatnot for areas with zero subscribers
          • shaklee3 6y ago
            They don't use cpus. They use fpgas, and the signal processing is done regardless of whether a user is there.
            • sgtnoodle 6y ago
              Are you sure about that? I see there's an article from a couple years ago talking about quad-core A53 CPUs with FPGAs. That's most likely used for the satellite avionics, rather than for the starlink radios. While it's possible the avionics and the radios are tightly coupled, I would be surprised if they were. They're probably mostly separate for fault tolerance reasons. I suspect FPGAs wouldn't be fast enough to do the RF magic. I figure they likely partnered with a company like Qualcomm to develop some custom silicon, or adapted some existing cell tower silicon, since those chipsets are largely "software defined" these days anyway.
              • rcxdude 6y ago
                FPGAs are extremely commonly used in the RF telecomm space. You don't implement a multiplier in the free-form FPGA fabric, instead such FPGAs have banks of 'DSP blocks' which essentially give you hard silicon ALUs which you can then hook up however you want. This gives you extremely flexible and potentially massive parallelism with very tight latency guarantees, and you don't lose a huge amount of performance because most of the heavy lifting is in hard silicon anyway. Also FPGAs excel at high bandwidth I/O, which is usually extremely important in these use-cases. A DSP or even a GPU might give you more FLOPs per W or $, but they'll usually struggle with latency. The quad-code A53 CPUs with FPGAs sounds like Xilinx's RFSoCs, which are explicitly designed to basically give you a custom high-performance base station on a chip (though they can't directly convert to the frequency range Starlink is using).
  • sgtnoodle 6y ago
    Do you think it's TDMA? I just assumed it would be whatever the latest CDMA technology is. The download probably doesn't really matter much one way or another since there's so much space between receivers that there's no practical chance of collisions. The satellite certainly must be able to receive multiple uplinks simultaneously, even in order to maintain a modest 15Mbps for multiple customers.
    • shaklee3 6y ago
      Satellite typically doesn't use cdma. See dvb-s
      • sgtnoodle 6y ago
        There's no reason to assume starlink works like a typical satellite, though. Digital video broadcast in particular is in one direction from geostationary orbit. Latency is on the order of several seconds due to all the forward error correction that's necessary to tolerate bit errors in reception. There's not really any need for a "multiple access" scheme because "channels" are statically allocated frequency bands. I would think the radio technology in starlink should more closely resemble modern LTE and 5G cell networks, whether you want to call that CDMA, OFDM, or whatever the specific techniques these days are called.
        • anticensor 6y ago
          > There's no reason to assume starlink works like a typical satellite, though. And it does not. Starlink satellites do active switching unlike BSS satellites which only transmodulate and redirect.
          • shaklee3 6y ago
            Not true. Most modern geo satellites for internet have switching capabilities.
        • shaklee3 6y ago
          Satellites are interference limited and power limited. Dbv is not just for broadcasting, despite its name. It's used for pretty much all non-proprietary satellite waveforms.