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Technology Stocks : Qualcomm Incorporated (QCOM)

QCOM 173.62

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11:21 AM EST

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To: Maurice Winn who wrote (22952)	2/16/1999 4:21:00 AM
From: Clarksterh	Read Replies (3) of 152472

Maurice - Don't count CDMA out yet. Essentially OFDM is trying to overcome a weakness of CDMA - the reverse link (mobile to basestation) is not synched between all of the users and thus they interfere with one another. To get the complete benefits of CDMA ideally the basestation receiver would start getting a bit from all of the transmitters at the same time. However, because everyone is moving around this is very difficult to do on the reverse link (on the forward link everything is being centrally coordinated at the basestation so the mobile does indeed see all of the channels (only one of which they care about - their channel) starting at the same time.). When the bits are all received at the same time everyone's channel but the one you are interested in cancel out due to a mathematical trick (orthogonality) - otherwise they add to the noise. OFDM essentially tries to make the orthogonality something that occurs in the frequency domain instead of the time domain. The assumption here is that frequency is easier to control among many moving users. This may or may not be true - it is largely dependent on how fast things are moving in the cell and how you implement the OFDM (e.g. how many carriers). For instance a jet screaming by overhead could really screw up this assumption depending on what the parameters of the system are (and if the parameters are tight enough maybe even speeding down the expressway could screw it up.) I've tried to find papers looking at this issue, but so far haven't found any (although I haven't looked extensively yet.). So, what is poor CDMA to do? There is at least one thing that I think no currently deployed CDMA system does that could significantly increase its capacity on the reverse link. It could use MUD (Multi-User Detection - Great acronym eh?) which essentially says that the basestation would process the signals one at a time and then subtract them out of the remaining signal. This would then remove this 'noise' from the remaining signals. I'm not sure what the capacity of a system like this would be but it would probably be close to the forward link. Note that the reason that this has not been done, to my knowledge, is that it is extremely computationally complex. But by the time OFDM became practical, I'm sure that technology would have advanced enough to let CDMA match or surpass OFDM's performance in a multicell system (in a single cell system OFDM may always have an advantage(?), but like TDMA systems they cannot reuse frequencies in a neighboring cell.) In any case OFDM bears watching, and for that reason I have been keeping up to speed. Hope this helps. Clark PS As far a Qualcomm is concerned, the real problem with OFDM is that it probably offers performance comparable to CDMA in spectral efficiency, but using very different technology which would probably bypass most of Qualcomm's CDMA patents.

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