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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 , representing the sector number of the eNodeB.

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 . There are 168 possible sequences for the SSS that are mapped to an integer number , called the cell group identifier. The FFT‐based correlation in Eq. (8.21) is also exploited to detect the SSS signal. Once the PSS and SSS are detected, the UE can estimate the frame start time, , and the eNodeB’s cell ID using [62]. The cell ID is used for data association purposes.

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 represents the CRS sequence; denotes the set of subcarriers containing the CRS, which is a function of the symbol number, port number, and the cell ID; and represents some other data signals.