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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 g_CRC(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 . However, this method limits the number of intra‐frequency eNodeBs that a UE can simultaneously use for positioning. The second solution is to provide a database of the network to the UE. In this method, the UE needs to search over all possible values of the cell IDs to acquire the right ones unless the UE knows its current position, which is not a practical assumption. The other solution, which is more reliable and overcomes the aforementioned problem, is to extract the neighboring cell IDs using the information provided in the SIB transmitted by the main eNodeB. Since other operators transmit on different carrier frequencies, the same approach can be exploited to extract the cell IDs of the neighboring eNodeBs from other operators. Knowing the eNodeBs’ cell IDs, the receiver only needs to know the position of the eNodeBs using a database or pre‐mapping approaches [37, 39].

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.

Figure 38.36 General structure of downlink physical channels (Shamaei et al. [64, 65]).

Source: Reproduced with permission of Institute of Navigation, IEEE.

The SIB contains information about (i) the eNodeB to which it is connected, (ii) inter‐ and intra‐frequency neighboring cells from the same operator, (iii) neighboring cells from other networks (UMTS, GSM, and cdma2000), and (iv) other information. The SIB has 17 different forms called SIB1 to SIB17, which are transmitted in different schedules. SIB1, which is transmitted in subframe 5 of every even frame, carries scheduling information of the other SIBs. This information can be used to extract the schedule of SIB4, which has the intra‐frequency neighboring cell IDs. To decode SIB1, the UE has to go through several steps. In each step, the UE needs to decode a physical channel to extract a parameter required to perform other steps.

In general, all the downlink physical channels are coded in a similar fashion before transmission, as shown in Figure 38.36. Although all the physical channels have the same general structure, each step in Figure 38.36 differs from one channel to another. Each step for the PBCH was discussed in the MIB decoding step. Further details are given in [61, 70]. In the following, the steps to retrieve information from SIB4 are summarized.

 PCFICH Decoding: The UE first obtains the control format information (CFI) from the physical control format indicator channel (PCFICH). The CFI specifies the number of REs dedicated to the downlink control channel and can take the values 1, 2, or 3. To decode the CFI, the UE first locates the 16 REs dedicated to the PCFICH. Then, it demodulates the obtained symbols by reversing the steps in Figure 38.36, which results in a sequence of 32 bits. Finally, this sequence, which can be only one of three possible sequences, is mapped onto a CFI value.

 PDCCH Decoding: The UE can identify the REs associated with the physical downlink control channel (PDCCH) and demodulate them by knowing the CFI. This results in a block of bits corresponding to the downlink control information (DCI) message. The DCI can be transmitted in several formats, which is not communicated with the UE. Therefore, the UE must perform a blind search over different formats to unpack the DCI. The right format is identified by a CRC.

 PDSCH Decoding: The parsed DCI provides the configuration of the corresponding physical downlink shared channel (PDSCH) REs. The PDSCH, which carries the SIB, is then decoded, resulting in the SIB bits. Subsequently, these bits are decoded using an Abstract Syntax Notation One (ASN.1) decoder, which extracts the system information sent on SIBs by the eNodeB.

 System Information Extraction and Neighboring Cells Identification: During signal acquisition, the frame timing and the eNodeB cell ID are determined. Then, the MIB is decoded, and the bandwidth of the system as well as the frame number are extracted. This will allow the UE to demodulate the OFDM signal across the entire bandwidth and locate the SIB1 REs. The UE moves on to decode the SIB1 message, from which the scheduling for SIB4 is deduced and is subsequently decoded. SIB4 contains the cell ID of intra‐frequency neighboring cells as well as other information pertaining to these cells. Decoding this information gives the UE the ability to simultaneously track signals from different eNodeBs and produce TOA estimates from each of these eNodeBs. Signal tracking and TOA estimation will be thoroughly discussed in the next two subsections. Figure 38.37 summarizes all the aforementioned system information extraction steps.