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The pilot signal, which is purely the PN sequence, is used to acquire and track a cellular CDMA signal. The acquisition and tracking will be discussed in Section 38.5.2. Demodulating the other channels becomes an open‐loop problem, since no feedback is taken from the sync, paging, or any of the other channels for tracking. Since all the other channels are synchronized to the pilot, only the pilot needs to be tracked. In fact, it is required by the cdma2000 specification that all the coded channels be synchronized with the pilot to within ±50 ns [50]. Although signals from multiple BTSs could be received simultaneously, a UE can associate each individual signal with the corresponding BTS, since the offsets between the transmitted PN sequences are much larger than one chip. This is because the autocorrelation function has negligible values for delays greater than one chip. Therefore, the PN offsets, which are much larger than one chip delay, guarantee that no significant interference is introduced (the autocorrelation function is discussed in Section 38.5.2.3 and is shown in Figure 38.13).

The normalized transmitted pilot signal s(t) by a particular BTS can be expressed as

where ℜ{·} denotes the real part; C is the total power of the transmitted signal; c′_I(t) = c_I(t) * h(t) and c′_Q(t) = c_Q(t) * h(t); h is the continuous‐time impulse response of the pulse shaping filter; c_I and c_Q are the in‐phase and quadrature PN sequences, respectively; ω_c = 2πf_c, where f_c is the carrier frequency; and Δ is the absolute clock bias of the BTS from GPS time. The total clock bias Δ is defined as

Figure 38.8 Paging channel message structure (Khalife et al. [18]; 3GPP2 [50]).

Source: Reproduced with permission of IEEE.

where PN_offset is the PN offset of the BTS, s is the chip interval, and δt_s is the BTS clock bias. Since the chip interval is known and the PN offset can be decoded by the receiver, only δt_s needs to be estimated.

It is worth noting that the cdma2000 standard requires the BTS’s clock to be synchronized with GPS to within 10 μs, which translates to a range of approximately 3 km (the average cell size) [51]. Note that a PN offset of 1 (i.e. 64 chips) is enough to prevent significant interference from different BTSs. This translates to more than 15 km between BTSs. Subtracting 6 km from this value due to worst‐case synchronization with GPS (i.e. 3 km for each BTS), BTSs at 9 km or more from the serving BTS could cause interference (assuming all BTSs suffer from the worst‐case synchronizations). But 9 km is larger than the maximum distance for receiving cellular CDMA signals for ground receivers. Therefore, this synchronization requirement is enough to prevent severe interference between the short codes transmitted from different BTSs and maintains the CDMA system’s capability to perform soft hand‐offs [47]. The clock bias of the BTS can therefore be neglected for communication purposes. However, ignoring δt_s in navigation applications can be disastrous, and it is therefore crucial for the receiver to know the BTS’s clock bias. The estimation of δt_s can be accomplished via the frameworks discussed in Section 38.4.