3 ms·
There are more than 2 roundtrips. Any MAC that has to perform time division multiplexing on a shared uplink has to poll all base stations over time to figure o
by bcrl 6y ago
There are more than 2 roundtrips. Any MAC that has to perform time division multiplexing on a shared uplink has to poll all base stations over time to figure out which ones have data to transmit, and how much data is queued. Once the satellite knows how much data the ground station has to transmit, it assigns sufficient timeslots, transmits the assignment and then waits for data to come back. This is very similar to PON networks where upstream is shared, but the difference is sub-1ms latency vs 4-5ms latency. Sadly, this does have unfortunate latency implications for how long web pages take to load and render. Streaming video should, however, work swimmingly.
- spookthesunset 6y agoIt’s interesting they’d do TDMA over some flavor of code division multiple access (CDMA). From what I read it has to do with the fact the antennas are high gain directional antennas and not omnidirectional ones like on cell phones. With cell phones you are kinda walking around in a soup of cell signals all sharing the same spectrum at once... you and hundreds of other people are broadcasting in the same frequencies at the same time and they all tell each other apart because they all use a different “language”; the Wikipedia CDMA article does an excellent job explaining this. I would think that as more satellites get launched they could use WCDMA and signal from your station could be seen by multiple satellites in orbit much like a cell phone can reach multiple towers. Writing it out... I bet TDMA is required because the FCC would never grant a block of spectrum where hundreds of thousands of ground stations were using low gain, somewhat omnidirectional antennas to reach a constellation of satellites in space....
- bcrl 6y agoIt's even more complicated than that. Thanks to MIMO antenna arrays, signals from multiple ground stations can be received and decoded at the same time (MU-MIMO). The advances in radio MACs over the past 20 years is seriously impressive compared to what was considered high tech in the 1990s, and it's all a result of Moore's law making it cheaper to do more math in the same size and power envelope as older semiconductors.
- spookthesunset 6y agoHad to look up MIMO. That is actually really cool. So it is doing some combination of time division and spacial division.