Source: Reproduced with permission of Institute of Navigation, IEEE.
38.6.1.1 Frame Structure
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, Nc, and the number of used subcarriers, Nr, 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 ′ = Nr × 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 Nc 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 Nc 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 |
38.6.1.2 Timing Signals
There are three reference signals in LTE systems: PSS, SSS, and CRS, which can be exploited for positioning purposes by acquiring and tracking their subcarriers. These signals are discussed next.
PSS: To provide the symbol timing, the PSS is transmitted on the last symbol of slot 0 and repeated on slot 10. The PSS is a length‐62 Zadoff–Chu sequence which is located in 62 middle subcarriers of the bandwidth excluding the DC subcarrier. The PSS can be one of only three possible sequences, each of which maps to an integer value
SSS: The SSS is an orthogonal length‐62 sequence which is transmitted in either slot 0 or 10, in the symbol preceding the PSS, and on the same subcarriers as the PSS. The SSS is obtained by concatenating two maximal‐length sequences scrambled by a third orthogonal sequence generated based on
CRS: The CRS is an orthogonal pseudorandom sequence, which is uniquely defined by the eNodeB’s cell ID. It is spread across the entire bandwidth (see Figure 38.29) and is transmitted mainly to estimate the channel frequency response. Due to the scattered nature of the CRS, it cannot be tracked with conventional DLLs [15, 63]. The CRS subcarrier allocation depends on the cell ID, and it is designed to keep the interference with CRSs from other eNodeBs to a minimum. Since the CRS is transmitted throughout the bandwidth, it can accept up to 20 MHz bandwidth.
The transmitted OFDM signal from the u‐th eNodeB at the k‐th subcarrier and on the i‐th symbol can be expressed as
(38.18)
where
38.6.1.3 Received Signal Model
Assuming that the transmitted signal propagated in an additive white Gaussian noise channel, the received signal in the i‐th symbol will be