*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
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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.
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Mqurice
PS: I bet Ramsey thinks this is Wayyyyy too technical. |
