Satgen 367 Bandpass Bandplan Pt2 by GM4IHJ 6th April 96 As described in Satgen366, The digital transmission speed of current low earth orbit amsats Uo22 and Korea23 is restricted to 9600 bps because of the bandwidth limitations of present radio amateur transmitters and receivers. If Rx Tx bandwidths could be increased it might be possible to use higher digital keying speeds . But this is unlikely for two reasons. Reason one. Commercial manufacturers show no signs presently of providing extra bandwidth. Indeed they have not yet standardised their systems to a common design for 9600 bps operation. Reason Two. The low earth orbits presently used by amsats operating with 70 cms downlinks, mean that each satellite takes up a considerable slice of the available freqency band. Noting that we must add to the 15 Khz or so of signal bandwith the 18 Khz or so of doppler shift. All of which adds up to a 33 Khz of bandplan to ensure that satellite signals do not overlap. Any increase in bandwidth to accomodate higher data speeds will only increase this problem. All of which raises the question of how this situation might be overcome. We could go higher in uplink and downlink frequencies. The next higher possibility is 1260 up 2400 Mhz down . But this nearly 6 fold increase in link frequency entails a nearly 6 fold increase in doppler shift if we stay in low earth orbit. Eg a LEO microwave digisat would take up roughly 130 Khz minimum of the available band at 9600 bps. At 28,800 bps ( the common internet access speed) this would spread to more than 160 Khz of the band for each satellite. If however we are prepared to put the satellites into higher orbits , we will need a lot more transmitter uplink power and better satellite downlink antennas, but we will have the advantage of much lower doppler shifts. So perhaps that is the way to go. Either via the Phase3D Molniya orbit with its low doppler for the hours when the satellite is near apogee orbit high point. Or via a geosynchronous or geostationary satellite with almost no doppler shift ( albeit with the disadvantage of loss of worldwide coverage). Hopefully we will soon know. Phase 3D will have 9600 bps FSK for us to start out with, and the possibilities for ongoing developement offered by Phase3Ds Digital Signal Processing features should allows lots of experiment. In addition, there is some excitement in commercial circles concerning the viability of HDB3, a method of digital keying which uses half the sine wave. Whether this can allow doubling the data rate without doubling the bandwidth is presently a matter of some debate. If it does allow some saving of bandwidth, perhaps this is the way forward once we have conquered 9600 bps on Phase3D.