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To: Maurice Winn who wrote (22952)	2/16/1999 12:06:00 AM
From: Maurice Winn	Read Replies (1) \| Respond to of 152472

OMC-CDMA Which means Orthogonal Multi-Carrier Code Division Multiple Access which might supersede that old-fashioned OFDM stuff. According to this url anyway, the contents of which I've now committed to memory. Mike, it's a tsunami of algorithms...are Q! and WWeb in trouble? These Berkeley people seem to have gone Berserkely past OFDM straight to some other concatenation of convoluted Fourier transforms [which were NOT fun even way back in 1972 though there was a simple beauty to them like snowflakes viewable only by being naked at the South Pole - where even the possession of an Iridium phone doesn't make one wish to stay long]. Have we become as comfortable in cdmaOne/cdma2000 as hagfish in a whale carcass? Is Q! ahead of the curve on this one? From: diva.eecs.berkeley.edu ---------------------------------------------------------------- WHAT IS ORTHOGONAL MULTI-CARRIER CDMA? There are many equivalent ways to describe MC-CDMA: MC-CDMA is a form of CDMA or spread spectrum, but we apply the spreading in the frequency domain (rather than in the time domain as in Direct Sequence CDMA). MC-CDMA is a form of Direct Sequence CDMA, but after spreading, a Fourier Transform (FFT) is performed. MC-CDMA is a form of Orthogonal Frequency Division Multiplexing (OFDM), but we first apply an orthogonal matrix operation to the user bits. Therefor, MC-CDMA is sometimes also called "CDMA-OFDM". MC-CDMA is a form of Direct Sequence CDMA, but our code sequence is the Fourier Transform of a Walsh Hadamard sequence. MC-CDMA is a form of frequency diversity. Each bit is transmitted simultaneously (in parallel) on many different subcarriers. Each subcarrier has a (constant) phase offset. The set of frequency offsets form a code to distinguish different users. P.S. Our MC-CDMA is NOT the same as DS-CDMA using multiple carriers. WHAT ARE THE ADVANTAGES OF MC-CDMA? Compared to Direct Sequence (DS) CDMA. DS-CDMA is a method to share spectrum among multiple simultaneous users. Moreover, it can exploit frequency diversity, using RAKE receivers. However, in a dispersive multipath channel, DS-CDMA with a spread factor N can accommodate N simultaneous users only if highly complex interference cancellation techniques are used. In practice this is difficult to implement. MC-CDMA can handle N simultaneous users with good BER, using standard receiver techniques. Compared to OFDM. To avoid excessive bit errors on subcarriers that are in a deep fade, OFDM typically applies coding. Hence, the number of subcarriers needed is larger than the number of bits or symbols transmitted simultaneously. MC-CDMA replaces this encoder by an NxN matrix operation. Our initial results reveal an improved BER. ----------------------------------------------------------------- Mqurice PS: I bet Ramsey thinks this is Wayyyyy too technical.

To: Maurice Winn who wrote (22952)	2/16/1999 4:21:00 AM
From: Clarksterh	Read Replies (3) \| Respond to 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.