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The data transmitted on the forward link channel in cellular CDMA systems is modulated through quadrature phase shift keying (QPSK) and then spread using direct‐sequence CDMA (DS‐CDMA). However, for the channels of interest from which positioning and timing information is extracted, the in‐phase and quadrature components, I and Q, respectively, carry the same message m(t) as shown in Figure 38.4. The spreading sequences c_I and c_Q, called the short code, are maximal‐length PN sequences that are generated using 15 linear feedback shift registers (LFSRs). Hence, the length of c_I and c_Q is 2¹⁵ − 1 = 32, 767 chips at a chipping rate of 1.2288 Mcps [47]. The characteristic polynomials of the short code I and Q components, P_I(D) and P_Q(D), are given by

where D is the delay operator. It is worth noting that an extra zero is added after the occurrence of 14 consecutive zeros to make the length of the short code a power of two.

In order to distinguish the received data from different BTSs, each station uses a shifted version of the PN codes. This shift is an integer multiple of 64 chips, and this integer multiple, which is unique for each BTS, is known as the pilot offset. The cross‐correlation of the same PN sequence with different pilot offsets can be shown to be negligible [46]. Each individual logical channel is spread by a unique 64‐chip Walsh code [48]. Therefore, at most 64 logical channels can be multiplexed at each BTS. Spreading by the short code enables multiple access for BTSs over the same carrier frequency, while orthogonal spreading by the Walsh codes enables multiple access for users over the same BTS. The CDMA signal is then filtered using a digital pulse shaping filter that limits the bandwidth of the transmitted CDMA signal according to the cdma2000 standard. The signal is finally modulated by the carrier frequency ω_c to produce s(t).

Figure 38.4 Forward link modulator (Khalife et al. [18]).

Source: Reproduced with permission of IEEE.