Page 268 - From GMS to LTE

254 From GSM to LTE-Advanced Pro and 5G

4.6.1 Cell Search
When a mobile device is powered on, its first task from a radio point of view is to search
for a suitable network and then attempt to register. To speed up the task, it is guided by
information on the SIM card stored in the home network with access technology field
(EF‐HPLMNwAcT). With this field the network operator can instruct the mobile device
which radio access technology (GSM, UMTS, LTE) to search for first and use for regis-
tration. Older SIM cards that have not been updated still contain UMTS in this field,
which means that the mobile device will first search for and use the UMTS network
even if an LTE network is available and only switch to LTE once registration has been
performed. Newer SIM cards or cards that have been updated over the air instruct the
mobile device to first search for an LTE network of the network operator and use this
radio access technology for registration.
To shorten the search process, the mobile device stores the parameters of the last
cell it used before it was switched off. After the device is powered on, it can go
straight to the last known band and use the last known cell parameters to see if the
cell can still be found. This significantly speeds up the cell search procedure if
the device has not been carried to another place while it was switched off and the
last used radio access technology is the same as the network operator preference
stored on the SIM card.
In case the previous cell was not found with the stored information, it performs a full
search. UMTS and GSM cell search has been described in the previous chapters, so this
section focuses only on the LTE cell search mechanism.
For the first step, the mobile device searches on all channels in all supported fre-
quency bands for an initial signal and tries to pick up a primary synchronization signal
(PSS) that is broadcast every 5 milliseconds, that is, twice per air interface frame. Once
found, the device remains on the channel and locates the SSS, which is also broadcast
once every 5 milliseconds. While the content of the PSS is always the same, the content
of the SSS is alternated in every frame so that the mobile device can detect from the
pattern where to find the beginning of the frame. Figure 4.17 shows where the synchro-
nization signals can be found in a frame on the time axis.
To make cell detection easier, the PSS and SSS are broadcast only on the inner 1.25
MHz of the channel, irrespective of the total channel bandwidth. This way, a simpler
FFT analysis can be performed to detect the signals. Also, the initial cell search is not
dependent on the channel bandwidth. Hence, this speeds up the cell search process.
The PSSs and SSSs implicitly contain the PCI. The PCI is not equal to the cell‐ID as
previously introduced in GSM and UMTS but is simply a lower‐layer physical identity
of the cell. It can thus be best compared to the Primary Scrambling Code (PSC) in
UMTS. Like GSM and UMTS, LTE also knows a cell identity on higher layers, which is
discussed later on. The PCI is important to distinguish neighboring cells transmitting
on the same frequency. In practice, mobile devices, especially in cell‐edge scenarios,
receive several PSS and SSS and hence detect several PCIs on the same frequency.
After detection of the PSS and SSS, the mobile device is also aware if the cell uses a
normal or an extended cyclic prefix. As shown in Figure 4.17, the two signals have
different timing depending on the length of the prefix as only six symbols form a
slot when the extended cyclic prefix is used compared to seven symbols with a normal
cyclic prefix.