IEEE - Aerospace and Electronic Systems - September 2022 - 24

An Experimental Analysis of Cyclic and Reference Signals of 4G LTE for TOA Estimation and Positioning in Mobile...
Figure 4.
Example LTE signal waveforms and spectra. (a) Baseband LTE signal waveform. (b) Spectrum of signal at 789 MHz. (c) Spectrum of signal
at 2135.2 MHz. (d) Another baseband LTE signal waveform. (e) Empty slot with only 1st and 5th OFDM symbols. (f) CP for 1st OFDM
symbol.
resampled at 15.36 MHz, which has significant variations
in its amplitude (42 spikes over 21 slots). Figure 4(b)
shows the spectrum of a signal at 796 MHz with a 5 MHz
bandwidth and an adjacent signal clearly visible. Figure 4
(c) shows the spectrum of another signal at 2135.2 MHz,
whose bandwidth seems larger than 5 MHz but narrower
than 10 MHz.
Figure 4(d) shows another example of the real part of a
complex baseband waveform resampled at 1.92MHz where
periodic gaps are visible, which correspond to transitions
between subframes (two slots over 1 ms), the schedulable
units for continuous transmission. Note that the purple start
and yellow circle o mark the start ofPSS and SSS symbols,
respectively, while the red cross x indicates the start ofseven
OFDM symbols in a slot. The particular slot shown in
Figure 4(e) happens to be an empty slot where only the 1st
and 4thOFDMsymbols are populatedwith reference signals
while other five OFDM symbols are empty. Figure 4(f)
shows the detail ofan OFDM symbol where the CP of nine
samples is highlighted with a purple line segment as compared
to the original signal at the end of this OFDM symbol
highlighted with a green line segment. There are six clusters,
excluding the CP, visible in this waveform.
CP for OFDM Symbol Timing. The detection of
OFDM symbol boundary via CP correlation is shown in
Figure 5. Figure 5(a), (b), and (c) shows the detection of a
single CP peak in a clean environment, three CP peaks in
a clean environment, and multiple CP peaks in a cluttered
24
environment, respectively. These peaks may originate
from different cells or be caused by multipath. If the
OFDM symbol boundary can be detected from CP, it can
be used to reduce the computations otherwise required in
search for PSS/SSS/CRS. In a clean environment, it provides
a reasonably good TOA estimate. However, such a
blind search cannot reveal the origin ofany detectable signal
and thus cannot be used for TOA estimation per se.
Detection of PSS and SSS for OFDM Symbol and
Frame Timing. The ideal PSS for Nð2Þ
Id ¼ 0 is shown in
Figure 6. Figure 6(a) shows the ZC sequence in the frequency
domain where both the real and imaginary components
are symmetric about dc (at the index 64 in the plot).
It has constant unit amplitude, as shown in Figure 6(b)
with its autocorrelation in Figure 6(c). The time-domain
waveform of PSS is complex and also symmetric about
the sequence center, as shown in Figure 6(d) but the magnitude
is no longer constant in the time domain, as shown
in Figure 6(e). The time-domain autocorrelation is shown
in Figure 6(f).
Similarly, for the ideal SSS in subframe 0 for PCI ¼
346, Figure 7(a) shows the binary sequence in the frequency
domain (dc at the index 64). The time-domain
waveform of SSS is conjugated symmetric about the
sequence center, as shown in Figure 7(b). Note that the
magnitude is not constant, as shown in Figure 7(c).
To search for PSS, the incoming signal resampled at
1.92 MHz is correlated with three possible PSS sequences
IEEE A&E SYSTEMS MAGAZINE
SEPTEMBER 2022
IEEE - Aerospace and Electronic Systems - September 2022

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