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Technology Stocks : Qualcomm Incorporated (QCOM)
QCOM 163.32+2.3%Nov 21 9:30 AM EST
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To: Clarksterh who wrote (30114)5/16/1999 2:50:00 AM
From: engineer  Read Replies (1) of 152472
 
If the system has something like a viterbi decoder running in hardware to do the 9.6k bit stream, then if they wanted to change from rate 1/2 to say rate 7/8, it may not be possible, since the decoding memory would not be long enough to hold hte symbols, or if they found a new scheme to handle a better time search algorithm, they may not have the hardware to extract the data fast enough. (I know for a fact that the Viterbi decoder in the I* is in hardware...)

Since I* is digital at the system level and it is out of reach to the ground crew it would have to have anticipated these approaches and left extra componets to handle this.

With G* having the entire over the air system in analog and doing the digital at both the handset and groundstation, then they already have an infinite amount of bits out there and can upgrade the system level alot.

the isue of memory in space is a real tough one. Standard dynamic RAM cannot be used, as the charges in the bit cells is too small to survive space born radiation. SRAM must be used and in the case of something like Xilinix where the cells are reprogrammable RAM cells, the bit upset rate is way too high. I would be surprised if they had much in RAM based reprogrammability or had alot in computer systems.

the systems design is a real tradeoff problem, one which is hard to overcome. Put in too much programmable stuff and the computer becomes complex enought o suffer system failures in space (99.99% failure times the number of components), put in too much hardware and it is not upgradeable. If they put it all in DSP and tried to do baseband decoding (which they DO NOT do), then they would overrunt he pwoer budget for the LEO and would have to have giant solar panels to power it.

It is not an easy problem.
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