alt="Schematic illustration of the general structure of downlink physical channels."/>
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.
38.6.2.3 Tracking
After acquiring the LTE frame timing, a UE needs to keep tracking the frame timing for two reasons: (i) to produce a pseudorange measurement and (ii) to continuously reconstruct the frame. The PSS and SSS are two possible sequences that a UE can exploit to track the frame timing. The PSS has only three different sequences, making it less desirable to use in tracking the frame timing because (i) the interference from neighboring eNodeBs with the same sector IDs is high and (ii) the number of eNodeBs that the UE can simultaneously track is limited. The SSS is expressible in 168 different sequences; hence, it does not suffer from the same problems as the PSS. Therefore, the SSS will be used to track the frame timing. In this section, the components of the tracking loops are discussed, namely, an FLL‐assisted PLL and a carrier‐aided DLL.
FLL‐Assisted PLL: The frequency reuse factor in LTE systems is set to be 1, which results in high interference from neighboring cells. Under interference and dynamic stress, FLLs have better performance than PLLs. However, PLLs have significantly higher measurement accuracy compared to FLLs. An FLL‐assisted PLL has both the dynamic and interference robustness of FLLs and the high accuracy of PLLs [72]. The main components of an FLL‐assisted PLL are a phase discriminator, a phase loop filter, a frequency discriminator, a frequency loop filter, and an NCO. The SSS is not modulated with other data. Therefore, an atan2 discriminator, which remains linear over the full input error range of ±π, could be used without the risk of introducing phase ambiguities, given by
Figure 38.37 System information extraction block diagram (Shamaei et al. [64, 65]).
Source: Reproduced with permission of Institute of Navigation, IEEE.
where
where ωn, p is the undamped natural frequency of the phase loop, which can be related to the PLL noise‐equivalent bandwidth atan2 discriminator for the frequency as well. The frequency error at time step k is expressed as
where Tsub = 10 ms is the subaccumulation period, which is chosen to be one frame length. The transfer function of the frequency loop filter is given by
