 (28.2) says that a system with users has - 1 interferers for each user: ar = ( o - 1)r + n (28.2) which we can rephrase in Eq. (28.3): mr = n (28.3) where m =a+1-o. We can solve for r in Eq. (28.4): (28.4) Naturally, this equation is meaningful only when the number of users is less than a + 1. As approaches this limit, the system increases every user's transmit power to ensure sufficient Eb/No. Looking at the graph in Fig. 28.1, we can make an important observation. Until gets very close to a + 1, the receiver power levels stay very low. These equations (28.1 through 28.4) tell most of the story for the reverse direction. Each user terminal has to transmit with enough power to achieve r received power at the antenna face. Thus, if Gub is the radio path gain from user U to antenna face b, then Number of Users Figure 28.1 CDMA reverse power requirement as a function of number of users. 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. CDMA Principles for Multicellular Systems CDMA Principles for Multicellular Systems the received power is the transmit power multiplied by the path gain. Equation (28.5) shows the relationship in absolute terms rather than decibels: r=tuGub (28.5) Thus we can express the required transmit power for each user U in Eq. (28.6): (28.6) The forward direction is a little more complicated. We start with Eq. (28.1) and use the radio path gain relationship ru = tuGub to solve for the base station transmit power tu for user U in Eqs. (28.7) and (28.8): (28.7) (28.8) Since our forward power limitation is the total transmitted power T for all the users served by the antenna face, we add up all the users' transmit power in Eq. (28.9) and re- alize that the double summation contains every user - 1 times to get Eq. (28.10): (28.9) (28.10) where N = Now The noise component N has a physical meaning: It is the total noise of all th