Multiple Access
Example: The power of a signal x(t) equals M-1?x2(mD), where the summation is over M time samples of x(t) taken D seconds apart. By extension, the power two signals x(t) and y(t) have in common, i.e., their cross-power, is given by M-1?x(mD)y(mD), which is the average of the product of x(t)y(t). If this average equals zero the two signals are said to be orthogonal and neither signal produces any interference when the other signal is recovered using a matched filter or correlator. In IS-95, the forward link transmits synchronized Walsh words which produce no mutual interference in the absence of multipath. The reverse link transmits Walsh words at the rate of 4800 per second. The Walsh words from each mobile are spread by different segments of the Long PN Code with each segment containing 256 PN chips. Denoting one segment by x(t) and another by y(t), the average of the product x(t)y(t) is zero. Therefore the signals are orthogonal. However, with one sample per PN chip, the term (1/256)?x(mD)y(mD) is not always zero. The mean is zero, but the variance of this term is 1/256 or 1/(16)2 when the power in both x(t) and y(t) is one. Hence, the cross-product power is often less than about 3 times 1/16 and is typically much less than one. See Correlators and Matched Filters for more discussion.
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© 1999 DR. ERNEST SIMO - SPACE2000
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