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In LTE downlink transmission, data is encoded using orthogonal frequency division multiplexing (OFDM). OFDM is a transmission method in which the symbols are mapped onto multiple carrier frequencies called subcarriers. The serial data symbols are first parallelized in groups of length N_r, where N_r represents the number of subcarriers that carry data. Then, each group is zero‐padded to length N_c, which is the total number of subcarriers, and an inverse FFT (IFFT) is taken. The value of N_c is set to be greater than N_r to provide a guard band in the frequency domain. Finally, to protect the data from multipath effects, the last L_CP elements of the obtained symbols are repeated at the beginning of the data, called the cyclic prefix (CP). The transmitted symbols can be obtained at the receiver by executing these steps in reverse order. Since the frequency reuse factor in LTE systems is 1, all the eNodeBs of the same operator use the same frequency band. To reduce the interference caused by sharing the same frequency band, each signal is coded to be orthogonal to the transmitted signals from other eNodeBs. Using different frequency bands makes it possible to allocate the same cell IDs to the eNodeBs from different operators. Figure 38.27 represents the block diagram of the OFDM encoding scheme for digital transmission. The following subsections discuss the LTE frame structure and reference signals, which will be exploited for navigation.

Figure 38.27 OFDM transmission block diagram (Kassas et al. [6]).

Source: Reproduced with permission of IEEE.

Figure 38.28 LTE frame structure (Shamaei et al. [64, 65]).

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