Greg - Correlator vs matched filter.
Some of the 3g proposals use a different system protocols than CDMAOne (and by extension I assume CDMA-2000, although I haven't actually checked). In CDMAOne the transmitter and receiver lock up with each other using a correlator filter and then stay locked by continuously transmitting at least some signal even when no one is talking. This wastes some bandwidth, but not much and it avoids the problem of having to lock up again with each burst of talk.
In contrast, some of the other 3g proposals will resynch for every burst of data. This allows the transmitter to transmit nothing when it has nothing to transmit, but on the downside it has to precede each data bust with a header which the matched filter can 'see' and lock to. This wastes spectrum just as the minimum signal in CDMAOne wastes spectrum.
So, which is a better system?
For extremely bursty data a matched filter system is probably somewhat better, and of course initial lock up at the start of a call is faster for a matched filter.
For a more steady stream (like voice or video) an 'always synched' system is probably somewhat more spectrum efficient. But even in the worst case I would expect no more than a few percent difference (Since this keeps coming up, I should just look at the CDMA-2000 spec and determine the ratio of the minimum transmission rate vs the max - that is the maximum spectrum inefficiency). As for correlators not working above a certain rate, this is just plain incorrect.
FWIW.
Clark
PS Note that Golden Bridge likes to talk about their great matched filter design, but they never explain why it supposedly allows greater data rates, so to some degree the above is speculation, but it is reasonably well informed speculation. In any case there is nothing new about 'matched filters' per se.
PPS 'Synchronization' is a factor in many different parts of a cell system design, but it should be noted that most of the time synchronization of one part of the system implies nothing about synch in another part. For instance, basestations in CDMAOne are all synch'd via GPS, but this has very little to do with the type of synch I discussed above. |
