A reader Naghshineh asks , ” I want to know concept information about Idle mode, cell selection and reselection in LTE first. I mean, in a network with several technologies such as 2G, 3G and 4G with the same frequency band (for example 900 band), when the UE is powered on, when UE scans the frequency band, how does a UE can recognize and select the technology? ”
Answer: Great question Nagshineh . I want to divide this question into three sections i.e., three posts.
1. What happens when you power on the UE and how it selects (recognizes) LTE even in presence of other technologies ?
3. Once PLMN is found. How an LTE cell is selected ? What is an Idle mode ? Specifically, what is cell selection and reselection in Idle Mode ?
In this post, I will discuss first section only i.e., What happens when you power on the UE and how it selects (recognizes) LTE even in presence of other technologies ?
Before we dive into it , let me ask you . If you are a coffee snob let’s say Starbucks is your favorite coffee, how do you find out a Starbucks location even if you in a town for the first time ? Even though there are plenty of other coffee shop around you , you will ultimatley find out which one is Starbucks just by identifying the dark green and white signature sign of Starbucks.
If you cannot find out, then you may settle for some random coffee shop.
What did I mean by that ? Just like we know what we want and if we don’t find it , then we may settle for something else.
In the same way an LTE UE knows, when powered on. It starts scanning the frequency band. It may not know, which one is an LTE network, but it does know how to identify and LTE network based on a pattern of Primary and Secondary Synchronization signals. Once it finds out the pattern , it realizes , yes it is looking for an LTE network and it’s about to camp on one.
The whole attach procedure can be quite lengthy.
However in order to go through this search to find LTE. When you power on the phone , UE will go through 4 basic steps to get onto LTE network,even if there are other technologies available in the neighbor , such as 2G, 3G etc.
Step 1:Synchronize in Time Domain
The first thing for UE to realize is, it needs to get synchronize with LTE radio frame (In LTE, a radio frame is allocated both in time and frequency domain, every 10 msec.) on LTE. Without synchronizing with an LTE radio frame , UE cannot even proceed further. It cannot read LTE signals until it is synchronized. UE will start scanning the frequency spectrum on LTE. It does not know how to synchronize until it starts seeing the Primary Synchronization Signal (P-SS) and Secondary Synchronization signal (S-SS) pattern repeatedly in every radio frame. P-SS helps in slot synchronization , while S-SS helps in radio frame synchronization. Once UE is synchronized with radio frame in the Downlink, it can read information like Reference signal measurements in the radio frame. This information helps to identify the coverage , if Reference Signal Received Power (RSRP) value is good or bad etc.
Let’s see what happens in second step.
Step 2: Find Cell Identity(Physical Cell Identity ): Extract the P-SS and S-SS information
PCI is the identity for a cell in LTE. Before a UE can extract additional information from the cell. It needs to have this PCI information. Once a UE gets connected to the cell. PCI scrambling is used to send information to the UE, scrambled with PCI. How PCI is computed is shown in the figure below. PCI is very important specifically in LTE, in fact , doing PCI planning is more important in LTE rather frequency planning. Two cells can have the same frequency next to each, with difference PCI values.
Physical Cell ID is the basis for:
Step 3: Find Bandwidth used
Once UE has computed the PCI value and get synchronized with the downlink radio frame. Then it starts reading Broadcast system information. Specifically, in this case it will look for Master Information Block (MIB). MIB is sent over the air interface using PBCH (Physical Broadcast Channel) . UE will find what is the bandwidth of the network by reading MIB. MIB contains significant information in it. The main information which arrives in MIB is:
Step 4: Read System Information:
Once UE has gone through Step 1,Step 2 and Step 3 above. Then in Step 4, it will look for other system information blocks (SIBs). There are a number of system information blocks in LTE. However, when it comes to getting connected to the eNodeB SIB 1 and SIB 2 are most important in that regard.
How SIB 1, is used in PLMN selection and ultimately LTE selection. I will post it the next follow up post.
Until then, if you have any questions / comments, put them here below.
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