the symbol preceding the PSS, and on the same subcarriers as the PSS. The SSS is obtained by concatenating two maximal‐length sequences scrambled by a third orthogonal sequence generated based on CRS: The CRS is an orthogonal pseudorandom sequence, which is uniquely defined by the eNodeB’s cell ID. It is spread across the entire bandwidth (see Figure 38.29) and is transmitted mainly to estimate the channel frequency response. Due to the scattered nature of the CRS, it cannot be tracked with conventional DLLs [15, 63]. The CRS subcarrier allocation depends on the cell ID, and it is designed to keep the interference with CRSs from other eNodeBs to a minimum. Since the CRS is transmitted throughout the bandwidth, it can accept up to 20 MHz bandwidth.
The transmitted OFDM signal from the u‐th eNodeB at the k‐th subcarrier and on the i‐th symbol can be expressed as
(38.18)
where
38.6.1.3 Received Signal Model
Assuming that the transmitted signal propagated in an additive white Gaussian noise channel, the received signal in the i‐th symbol will be
where
38.6.2 LTE Receiver Architecture
A cellular LTE navigation receiver consists of four main stages: signal acquisition, system information extraction, tracking, and timing information extraction [64, 65]. This section discusses the various stages of the navigation LTE receiver depicted in Figure 38.30. Section 38.6.2.1 describes the acquisition of PSS and SSS. Section 38.6.2.2 discusses the extraction of relevant system information. Section 38.6.2.3 discusses the tracking stage. Section 38.6.2.4 describes the timing information extraction.
38.6.2.1 Acquisition
The first step in acquiring an LTE signal is to extract the transmitted frame timing and the eNodeB’s cell ID [66–68]. These two parameters are obtained by the PSS and SSS. To detect the PSS, the UE exploits the orthogonality of the Zadoff–Chu sequences and correlates the received signal with all the possible choices of the PSS according to
where
Figure 38.30 Block diagram of the LTE navigation receiver architecture (Shamaei et al. [65]).
Source: Reproduced with permission of IEEE.
where
After obtaining the frame timing, the UE estimates the frequency shift (Doppler frequency) using the CP in the received signal r(n). The apparent Doppler frequency, including the carrier frequency offset due to clock drift and the Doppler shift, can be estimated by the CP as
where NCP is the set of CP indices, and Ts is the sampling interval [69]. Upon estimating the Doppler frequency, the acquisition of the LTE signal is complete. Figure 38.31 summarizes the LTE signal acquisition process.
The normalized correlation of received LTE signals with locally generated PSS and SSS signals are presented in Figure 38.32. It can be seen that since the PSS is transmitted twice per frame, the correlation has two peaks in the duration of one frame, which is 10 ms. However, the SSS correlation has only one peak, since the SSS is transmitted only once per frame. The figure also shows that the highest PSS correlation peak was at
