TAILIEUCHUNG - Adaptive WCDMA (P8)

CDMA network In this chapter, we initiate discussion on CDMA network capacity. The issue will be revisited again later in Chapter 13 to include additional parameters in a more comprehensive way. CDMA NETWORK CAPACITY For initial estimation of CDMA network capacity, we start with a simple example of single cell network with n users and signal parameters defined as in the list above. If αi is the power ratio of user i and the reference user with index 0, and Ni is the interference power density produced by user i defined as αi = Pi /P0 , i = 1, . | Adaptive WCDMA Theory And Practice. Savo G. Glisic Copyright 2003 John Wiley Sons Ltd. ISBN 0-470-84825-1 8 CDMA network In this chapter we initiate discussion on CDMA network capacity. The issue will be revisited again later in Chapter 13 to include additional parameters in a more comprehensive way. CDMA NETWORK CAPACITY For initial estimation of CDMA network capacity we start with a simple example of single cell network with n users and signal parameters defined as in the list above. If ai is the power ratio of user i and the reference user with index 0 and Ni is the interference power density produced by user i defined as ai Pi Po i 1 . n 1 Ni Pi Rc PiTc aiP0Tc then the energy per bit per noise density in the presence of n users is . Eb n 1 No Ni i 1 8-2 If Eb N0 R is the required single-user Eb N0 necessary to make the n-user signal-tonoise ratio SNR namely Eb N0 n equal to Eb N0 1 then we have t n Eb No R 1 G 1 Eb No R n J a n-1 Eb No R1 G 1 a i 1 -1 218 CDMA NETWORK where G Tb Tc Rc Rb is the so-called system processing gain. At the point where Eb N0 Eb No i equation gives Sr Eb No i 1 - G-1 Eb N0 i a j 8-4 and the degradation factor DF can be represented as 1 _ Eb N0 R DF ---------- Eb N0 1 1 - G-WN0 i g w 8-5 For n equal-power users and no coding we have ai 1 for all i and equation becomes DF 1 1 - n - 1 G-1 Eb N0 1 S n Eb No R 1 - n - 1 G-1 Eb No R For large values of Eb N0 R lim Eb N0 R N n 8-7 G -------- n 2 n - 1 - This is the largest value that the SNR Eb N0 n can attain. With this motivation we define the multiple-access capability factor MACF as G n 1 normalized by the SNR Eb No n. MACF G n 1 Nj n which can also be expressed as MACF G G n 1 Nj n n 1 NJr As long as the desired SNR namely Eb N0 n is such that the left-hand side is greater than or equal to one we can achieve that SNR by appropriately adjusting Eb N0 R in the right-hand side. If the left-hand side is less than one however no value of Eb N0 R will give the

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