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To: Dexter Lives On who wrote (123646)	8/30/2002 1:15:27 AM
From: Ruffian	Read Replies (1) \| Respond to of 152472

So................pump the OFDM thread..........

To: Dexter Lives On who wrote (123646)	8/30/2002 7:40:14 AM
From: qveauriche	Respond to of 152472

robv- i have submitted your post to the good folk at Gilder.I will post any response I get. Thanks for raising the issues.

To: Dexter Lives On who wrote (123646)	8/30/2002 12:46:29 PM
From: Clarksterh	Respond to of 152472

Multipath: CDMA can tolerate long delay spreads but it captures only a fraction of the energy of the multipath signal because of the limited number of rake fingers. In OFDM, as long as guard interval is long enough, all inter-symbol-interference is absorbed. Moreover, multipath self-interference affects CDMA but not OFDM. True - OFDM definitely has an advantage here. But I severely doubt it to be the factor of 3 that Flarion claims - even in the worst of CDMA circumstances I would guess it to be a factor of 2 or less. And when comparing to 1XDO the advantage disappears because MUD (multi-user detection) becomes much easier with just a few users at a time. But they have a corresponding disadvantage in that they are much more sensitive to frequency problems associated with motion. I would expect them to work substantially more poorly from a moving car or near a freeway. * Narrowband Interference: only a few tones are affected or lost in OFDM, whereas in CDMA the interference affects all symbols. Utter bunkum. Spread Spectrum (of which CDMA1x is one type) is noise tollerant - that is the whole point. In fact even Flarion recognizes this which is why they use frequency hopping although I'd be willing to bet that in order to keep the advantages of OFDM (e.g. simple rcvr) they don't hop all that often and thus are substantially more susceptible to noise spikes. * Impulse Noise: OFDM spreads the impulse noise over a burst reducing its effect, whereas in CDMA several symbols may be lost. Again utter bunkum. Timing Acquisition: CDMA is very sensitive to timing and requires fast acquisition. This results in complex algorithms and overhead unlike OFDM. Certainly CDMA acquisition is tough, but anyone who wants a frequency reuse of 1 is using a form of CDMA (e.g. Flarion's Freq Hopping) and is going to have most of the same problems. * Complexity: the CDMA rake receiver is more complex than OFDM digital front end (FFT). Implementation of equalization, interference cancellation, and adaptive antenna array algorithms is simpler in OFDM. Probably true that the OFDM front end is simpler than the CDMA rake receiver. But there are some interesting questions about how much of a 'rake' type receiver is needed for OFDM which does frequency hopping. It depends on how often it does the hopping. * Power Control: CDMA requires fast and precise power control as it is very sensitive to received power fluctuations which is not the case for OFDM. Anything with a frequency reuse of 1 is going to require fancy power control and BTW is thereby caught by Qualcomm's most important and most blocking patents. * Smart antennas: CDMA technology is less sensitive to capacity enhancement by using smart antenna techniques than OFDM technology because of CDMA intra-cell interference behaviour. Probably true to some degree since smart antenna only get rid of inter-cell interference, and a larger proportion of noise in CDMA2000x1 is intra-cell. But not true for 1xDO (see previous answers). Bottom line - As I said a long long time ago OFDM has some potential advantages. Perhaps as much as a doubling of capacity and perhaps some simplification of the rcvr. But this will be mitigated as they try to get a frequency reuse of 1. Are they sufficient to overthrow the backwards compatibility, easy upgrade costs, ... of CDMA? Probably not in the US or Asia, perhaps in Europe. JMO. Clark