The CDMA Principle The CDMA Principle 8.3 Pseudonoise (PN) Codes Let us take a closer look at the PN-code concept in CDMA. Each bit a is transmitted in n chips C1 through Cn. We derive each chip value by multiplying bit a by the PN code Pk so that the transmitted chip sequence is Ck = apk, for k = 1 n. The receiver adds up the n chips with the same PN code pk as multipliers in Eq. (8.2) to get the received bit arec (8.2) We can substitute Ck = apk into Eq. (8.2) to get Eq. (8.3): (8.3) Whether Pk is +1 or - 1, the value (pk)2 is always +1. In the absence of noise or inter- ference, this gives an exact answer arec = na, which will be positive when a is positive and negative when a is negative. At the same time a is being spread with Pk, the transmitter1 is processing b with qk so that the total chip value is Ck = apk + bqk. The receiver for a uses Eq. (8.2) to get the received bit arec in Eq. (8.4): (8.4) The receiver for b uses a similar calculation to get the received bit brec in Eq. (8.5): bree - Each (8.5) 1This could be either the same transmitter or another transmitter adjusted so that both signals reach the receiver at the same power level. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. The CDMA Principle Key Radio Concepts The secret of CDMA is that Pk and qk agree and disagree about the same amount, so the interfering cross term ( Pran) (8.6) is small compared to N. The PN code of CDMA has to send the signal message with minimal interference with other PN codes. This can be done by generating a sequence of digits that does not repeat itself for a long time or by using codes tailored not to interfere with each other. 8.3.1 Pseudorandom sequences Truly random sequences (without the pseudo- prefix) have three desirable statistical properties for CDMA over the long haul: 1. Half the bits are 0 and half are 1. 2. The numbers of same bits 