What are Closed Subscriber Group Cells ?

If you live in a place like this

Dream location for Telecom operators to sell their plan and packages

Or  maybe here.

Wonder what will happen if all the residents want to make a call at the same time or watch live streaming on their smart phones simultaneously

Or if You don’t like any of the above places,  How about the one below.

Want to lose weight, live in one of these buildings and don’t use elevators. Caution, you need not to have a pre-existing heart conditions

That’s all very good

Except there is one problem

You may have to get familiar with lack of cell phone signal coverage. ( I am using it as an example, it doesn’t mean the above buildings have lack of cell phone coverage)

Imagine one day something like that happens …

Frustrated , you call your telecom operator and yell at them , ” Why don’t you erect some more towers at this location so that your subscribers in this community can  get better signal coverage ” .

Rather they answer your question in a subtle way and upsell you on $250+ small cell , which you can install in your home to overcome coverage hole.

Welcome to home eNodeB.

Small cells come in all forms, shapes and sizes

You come home very happy. Unpack the box, to install it.

” Oh no, I thought it was plug and play setup. It’s not “, you say

Frustrated you call a tech-savvy friend of yours to come over , configure and install it for you.

Your friend arrives and looks at the femtocell first and later at the price tag,

”  What in the world were you thinking to spend $250 on this item  ? You could have ended the current operator contract and purchased a new connection from another operator who has great coverage in this area , with no contract, unlimited text, unlimited talk and data. Still you would not spend up to $250 ”

We all have friends, who want to sell you on their money saving ideas. Rationally or Irrationally

You get defensive a little and later change the subject indicating you are interested in getting the Home eNode B setup. You don’t care about spending $250, end of story.

So finally, you got the home eNodeB setup, signal strength and coverage has improved greatly.

However, you just want the coverage to yourself and don’t want to share it with next door neighbors or any passing traffic 🙂

What to do Now ?? How do you do that ?

Closed subscriber group is the solution to your tragedy .. (Not tragedy , or whatever )

Closed subscriber group (CSG) concept applies to cells which only allow specific UEs to access the cell.

Dick does not win in this case

How only certain UEs are allowed to access a cell ?

USIM of your phone (UE)  can be programmed to include one or more allowed CSG identities , i.e., there can be a whitelist which defines CSG identities. So basically you will call your small cell operator and they will add your UE(s) to the CSG. Sweet !!!

How your UE will know (search) about CSG every time ?

SIB 1 broadcasts the CSG identity of a cell when a cell belongs to CSG. Your UE’s  USIM is programmed to include your own CSG. Therefore, you can access it. But your neighbors and friends cannot. Yay !!

SIB 1 broadcast will help your pre-programmed UE

UEs which have at least 1 allowed CSG identity within their USIM, use an autonomous search function to detect, at least previously visited CSG cells on other frequencies and /or Radio Access Technologies (RATs). This is how your UE will search it every day when you come back to your place.

Keep in mind this autonomous search function operates in parallel to normal cell reselection. The UE can also use an autonomous search function on serving frequency.

What about Mobility in CSG cells ?

If a UE detects a CSG cell on a different frequency or RAT, the UE completes cell reselection onto that CSG cell irrespective of the priority of the frequency as long as the CSG cell is the highest ranked cell on that frequency or RAT

UE uses the normal intra-frequency cell reselection rules to camp on intra-frequency CSG cells.

Once a UE is camped on a CSG cell, the normal cell reselection rules are applied.

Image credits : here and here

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