What are the important things I need to know of USIM ?

In the lteenb documentation, it mentions about a test USIM card. Can I use a different SIM ?

UE and network need to authenticate each other based on a shared secret key (K).
On network side, the secret key, along with other USIM security parameters (authentication algorithm, milenage or tuak parameters etc ..) must be defined in MME data base config file, by default /root/mme/config/ue_db-ims.cfg
On UE side, the security parameters are stored in the USIM itself.

Unless you are an operator you probably do not get to know these USIM security parameters.
We provide Programmable Test Sim cards delivered with their security parameters.
You can refer to this link Amarisoft test sim card

How much would this SIM card cost ?

Please contact sales@amarisoft.com

I have several test cards but they all have the same IMSI and K values

> Can I use simultaneously several UE with thoses SIM ?

Yes, our MME has an extension that allows several simultaneous connection with same IMSI.
You only need one entry in ue_db MME config file for that IMSI.
This UE must also have multi_sim parameter set to true.
There's only one constraint : your Ue must have different IMEI.
 

My SIM card is having SQN 00000000000 but when I modified ltemme config SQN value to 000000000020, my lte router still able to attach successfully. I believe we should expecting to see Nas auth error with EMM cause #21. Please clarify.

The behavior is correct.With a test SIM card, the authentication algorithm is XOR and the SQN is ignore.With a "normal" SIM, the algo will be milenage and the #21 error code will be generated. Note that in this case MME will sync SQN to authenticate again and this should work.