alt="images"/>, and the highest SSS correlation peak was at Figure 38.31 Signal acquisition block diagram (Shamaei et al. [64, 65]).
Source: Reproduced with permission of Institute of Navigation, IEEE.
Figure 38.32 PSS and SSS normalized correlation results with real LTE signals (Shamaei et al. [64, 65]).
Source: Reproduced with permission of Institute of Navigation, IEEE.
38.6.2.2 System Information Extraction
Parameters relevant for navigation purposes include the system bandwidth, number of transmitting antennas, and neighboring cell IDs. These parameters are provided to the UE in two blocks, namely, the master information block (MIB) and the system information block (SIB).
The UE starts acquiring with the lowest possible bandwidth of LTE, since it has no information about the actual transmission bandwidth. After acquisition, the signal is converted to the frame, and the bandwidth is obtained by decoding the MIB. Then, the UE can increase its sampling frequency to exploit the high bandwidth of the CRS. The UE can also utilize signals received from multiple eNodeB antennas to improve the TOA estimate.
Since the frequency reuse factor in LTE is 1, it may not be possible to acquire the received PSS and SSS signals from eNodeBs with low C/N0. This phenomenon is called the near‐far effect. In this case, one can use the neighboring cell IDs obtained by decoding the SIB to reconstruct the CRS sequence [65]. This section discusses the decoding of MIB and SIB.
MIB Decoding: In order to exploit the high‐bandwidth CRS signal, which improves the navigation performance in multipath environments and in the presence of interference, the UE must first reconstruct the LTE frame from the received signal. To do so, the actual transmission bandwidth and number of transmitting antennas, which are provided in the MIB, must be decoded. The MIB is transmitted on the physical broadcast channel (PBCH) and consists of 24 bits of data: 3 bits for downlink bandwidth, 3 bits for frame number, and 18 bits for other information and spare bits. The MIB is coded and transmitted on four consecutive symbols of a frame’s second slot. However, it is not transmitted in REs reserved for the reference signals. Figure 38.33 shows the steps the MIB message goes through before transmission [61, 70].
Figure 38.33 MIB coding process (Shamaei et al. [65]).
Source: Reproduced with permission of IEEE.
In the first step, a CRC of length L = 16 is obtained using the cyclic generator polynomial gCRC(D) = D16 + D12 + D5 + 1. The number of transmitting antennas is not transmitted in the 24‐bit MIB message. Instead, this information is provided in the CRC mask, which is a sequence used to scramble the CRC bits appended to the MIB. The CRC mask is either all zeros, all ones, or [0, 1, 0, ⋯, 0, 1] for 1, 2, or 4 transmitting antennas, respectively. In order to obtain the number of transmitting antennas from the received signal, the UE needs to perform a blind search over the number of all possible transmitting antennas. Then, by comparing the locally generated CRC scrambled by the CRC mask with the received CRC, the number of transmitting antennas is identified.
In the second step, channel coding is performed using a convolutional encoder with constraint length 7 and coding rate 1/3. The configuration of the encoder is shown in Figure 38.34. The initial value of the encoder is set to the value of the last six information bits in the input stream. The method illustrated in Figure 38.35 is used to decode the received signal [71]. In this method, the received signal is repeated once. Then, a Viterbi decoder is executed on the resulting sequence. Finally, the middle part of the sequence is selected and circularly shifted.
In the next step, the convolutional coded bits are rate‐matched. In the rate matching step, the obtained data from channel coding is first interleaved. Then, the outcomes of interleaving each stream are repeated to obtain a 1920‐bit‐long array [70]. Next, the output of the rate matching step is scrambled with a pseudorandom sequence, which is initialized with the cell ID, yielding unique signal detection for all eNodeBs. Subsequently, QPSK is performed on the obtained data, resulting in 960 symbols which are mapped onto different layers to provide transmission diversity. To overcome channel fading and thermal noise, space‐time coding is utilized. This process is performed in the precoding step. Finally, the resulting symbols are mapped onto the predetermined subcarriers for MIB transmission [70].
SIB Decoding: When a UE performs acquisition, it obtains the cell ID of the ambient eNodeB with the highest power, referred to as the main eNodeB. For navigation purposes, the UE needs access to multiple eNodeB signals to estimate its state. One solution is to perform the acquisition for all the possible values of
Figure 38.34 Tail biting convolutional encoder with constraint length 7 and coding rate 1/3 (Shamaei et al. [65]).
Source: Reproduced with permission of IEEE.
Figure 38.35 MIB channel decoding method (Shamaei et al. [65]).
Source: Reproduced with permission of IEEE.
