SEDSAT-2 Communications Design Notes 20080509
From SEDSWiki
Contents |
SEDSAT-2 Communications Meeting: Agenda
Progress so far
We have come up with most of the specifications for the system, and we understand the high level problem very well and have got good high level solutions to most of the problems. However, we (and some of the other teams) need some more accurate analysis results, which we are now able to complete due to our better understanding of the system (compared to when we started).
As we move into the Engineering/implementation phase of the project, we should start making use of the people we mentioned as advisers on our SEDSAT-2 application forms - they should be able to help, or at least should be able to put us in contact with someone who can help with how we go about designing or implementing the parts of our subsystem.
Implementation
Re-evaluate link budget
We've got a detailed link budget, but the Ground team have been a bit unsure about some of the numbers, for example the amount of antenna gain, and the noise temperature of the receiver. They think we may need a 5m parabolic dish! Now that we have decided what antenna/transceiver we will use on the satellite, and Ground also have a couple of transceivers in mind, we should find out the noise performance of both ends of the link, as if we have a lower noise temperature, we can use a small ground station antenna and make up the loss in antenna gain by using low-noise-amplifier stages.
Comms MCU
We need to get the digital control system working. This involves the I2C interface to CDH, and working out what control commands we will provide to CDH. Currently we have talked about a 8-bit address and 16-bit data, which allows up to 256 commands. We need to define this in more detail - what exactly are the commands? Example commands: Power up, power down, reset, extend antenna, increase transmit power, etc... We also need to work out how we will physically interface the MCU with the other components.
Multiplexing
We need a way of sending both uplink and downlink data over the same radio channel. This means we need a multiplexing scheme. Our research has found four schemes: FDMA, CDMA and TDMA. FDMA won't work because our transceiver doesn't support it. CDMA requires a wide bandwidth so isn't suitable for use in the amateur radio spectrum. TDMA works and is reasonably simple to implement. Stop-and-wait ARQ is a type of TDMA, and we could implement it as follows. The ground station tells the satellite to transmit some data, the satellite transmits the data and then stops and waits for confirmation from ground that the data has been received. We still need the SWP in the satellite to track data, although it is a bit simpler than previously as data will never be sent out of order.
Channel Modelling
We have a list of channel effects (dispersion, fading, doppler shift and others), we need to develop a baseband channel model to implement these effects so we can simulate our system and predict how it will perform. We have tried to contact the SEDSAT-1 team to see how they modelled their channel (and to see how their system in practice compared with their channel model) but have had no response from any of the e-mail addresses on their website.
