Before we start with PSS and SSS, let us understand the Initial Access Procedure in LTE.
So when a UE is switched on it will go through Initial Access Procedure.
Initial Access Procedure = Cell Search + Decode System Information + Random Access Procedure.
For Cell Search and to Decode System Information it will have to perform:
1. Slot and Frame Synchronization
2. Read PBCH (MIB)
3. PLMN Search
4. Read PDSCH (SIB1, SIB2)
Once it gets all the information, it will do “s-criterion” calculation, to check if the particular cell suitable to camp on.
In this chapter we shall look at first 2 points. In the upcoming chapters we shall see how to do a PLMN search and camp on to a cell.
So to perform Slot and Frame Synchronization, it needs SSS and PSS.
In LTE downlink, you have 2 different types:
1. Physical Signals: PSS, SSS and DRS
2. Physical Channels: PDSCH (SIB1), PBCH (MIB)
So when the UE is switched on, it will scan all the bands from different telecoms vendors and calculate “s-criterion”. Then it will select the strongest among the signals and then it will read SSS and then PSS.
So SSS and PSS will help us to know the PCI (Physical Cell Identity).
Few points on PCI:
A network can have a total of 504 PCI’s. Within a network PCI values can be reused.
These 504 PCI’s can be derived using below formula from PSS and SSs:
PCI = 3 * SSS + PSS
PCI = 3 * Cell Group number + Cell Number.
where PSS = 0, 1, 2
SSS = 0, 1, 2, … 167
So PCI = 0, 1, 2, … 503
PSS and SSS
PSS: Primary Synchronisation Signal
SSS: Secondary Synchronisation Signal
UE will use these signals to:
1. Get the PCI
2. Get the radio frame, sub frame, slot and symbol synchronization in time domain.
3. Get the center of the channel bandwidth in the frequency domain.
4. By determining the position of PSS and SSS UE will know which modulation technique is used. If it a TDD or FDD.
5. SSS has 168 kinds of Synchronisation Sequences each referring to one unique group number.
6. PSS has 3 kinds of Synchronisation sequences, each referring to one unique cell number.
7. With the help of both PSS and SSS we can get PCI.
8. By decoding the Synchronisation signals, is a prerequisite to measure cell specific reference signals and decoding MIB on the PBSCH.
* PSS is broadcast twice during every radio frame and both transmissions are identical
* In Case of FDD, PSS is broadcast using central 62 sub carriers belonging ro the last symbol of time slot 0 and 10.
* In case of TDD, PSS is broadcast using the central 62 subcarriers belonging to the 3rd symbol of the time slot 2(subframe 1) and the third symbol of time slot 12 (subframe 5).
* PSS is used to achieve subframe, slot and symbol synchronisation in time domain.
* PSS cannot be used to achieve radio frame synchronisation
* PSS is used for slot synchronisation
* PSS is decoded on center frequency, it needs minimum 6RB. i.e 62 subcarrier.
* SSS is broadcast twice within every radio frame.
* The two transmission of SSS are different so that UE can detect which is the first and which is the second.
* In case of FDD, SSS is broadcast using the central 62 sub carriers belonging to the second to last symbol of time slot 0 and 10.
* In case of TDD, SSS is broadcast using the central 62 sub-carriers belonging to the last symbol of time slot 1(subframe 0) and the last symbol of the time slot 11 (subframe 5) both time slots 1 and 11 are always within downlink subframes.
* SSS can be used to achieve radio frame synchronisation
THe 5 Resource elements above and below the Synchronisation Signals are not used for transmission. They represent periods of DTX Discontinuous Transmission. They are present in both FDD and TDD.
By determining the position of PSS and SSS UE will know which modulation technique is used. If it a TDD r or FDD.
3GPP References: TS 36.211
The below topic gives an overview of UE idle mode. This topic is very small for a separate post. Hence I have included it here.
When a UE is switched on or if its not camped to a cell, UE will be in Idle mode.
UE Idle Mode Behavior:
* It increases the access success rate
* Improves QoS
* Load balancing
* Reduces Battery Consumption.
UE Idle Mode Procedures:
* PLMN Selection
* Cell search (PSS and SSS) & Cell Selection(Power On)
* Cell re-selection(UE moving)
* TA Update
The above topic will make sense once you complete Attach Procedure chapter.