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The received OFDM signals are arranged in multiple blocks, which are called frames. In an LTE system, the structure of the frame depends on the transmission type, which can be either frequency‐division duplexing (FDD) or time‐division duplexing (TDD). Due to the superior performance of FDD in terms of latency and transmission range, most network providers use FDD for LTE transmission. Hence, this section considers FDD for LTE transmission, and for simplicity, an FDD frame is simply called a frame.

A frame is a major component in LTE communication, which is a 2D grid representing time and frequency. A frame is composed of 10 ms data, which is divided into either 20 slots or 10 subframes with a duration of 0.5 ms or 1 ms, respectively. A slot can be decomposed into multiple resource grids (RGs), and each RG has numerous resource blocks (RBs). Then, an RB is broken down into the smallest elements of the frame, namely, resource elements (REs). The frequency and time indices of an RE are called subcarrier and symbol, respectively. The LTE frame structure is illustrated in Figure 38.28, and the composition of a single LTE frame with six RBs is depicted in Figure 38.29 [61].

The number of subcarriers in an LTE frame, N_c, and the number of used subcarriers, N_r, are assigned by the network provider and can only take the values that are shown in Table 38.4. The subcarrier spacing is typically Δf = 15 kHz. Hence, the occupied bandwidth W′ can be calculated using W ′ = N_r × Δf. To allow for a guard band, the allocated bandwidth W is chosen to be slightly higher than the W′ bandwidth (e.g. W = 1.4 MHz is chosen for a W ′ = 1.08 MHz). Note that N_c is chosen to be a power of two to exploit the computational efficiency of the FFT.

When a UE receives an LTE signal, it must reconstruct the LTE frame to be able to extract the information transmitted in the signal. This is achieved by first identifying the frame start time. Then, knowing the frame timing, the receiver can remove the CPs and take the FFT of each N_c symbol. The duration of the normal CP is 5.21 μs for the first symbol of each slot and 4.69 μs for the rest of the symbols [61]. To determine the frame timing, the PSS and SSS must be acquired, which will be discussed in the next section.

Figure 38.29 Composition of a single LTE frame. The slots represent time, while the RBs represent frequency (Shamaei and Kassas [15]).

Source: Reproduced with permission of Institute of Navigation.

Table 38.4 LTE system bandwidths and number of subcarriers

Allocated bandwidth W (MHz)	Total number of subcarriers, Nc	Number of subcarriers used, Nr
1.4	128	72
3	256	180
5	512	300
10	1024	600
15	1536	900
20	2048	1200