Hello Mohammad Hasan:
I think you already posted this to the usrp-users list. I replied to you there, and I'll repeat it here.
The answer depends on what you want to do in your research project. There are many things to consider, such as the type of interface to the host computer you need/want, how much bandwidth you need to capture, whether you want to modify the FPGA, what kind of radio performance you need, what bit depth you need in your sample data, etc.
An LTE channel is at most 20 MHz, and with carrier aggregation, I think it may be up to 100 MHz, so you might need a lot of capture bandwidth, and that results in high data rates to the host.
Here's an example system configuration that would probably accommodate most of things that you would want to do:
* USRP X300
* SBX-120 or CBX-120 daughterboard
* 10 Gigabit Ethernet NIC
Note that you would need a powerful system to accommodate the high 10 GbE sample data rates. You would probably want an Intel Core i7 or Xeon CPU with a clock rate greater than 3 GHz and SATA-III SSD disks.
If you're just working with a single 1.4/3/5/10/15/20 MHz channel, then you could probably use a USRP N200/N210 with a SBX-40 or CBX-40 daughterboard. Also, a USRP B200/B210 might also work. Without knowing more details of what you want to do, it is hard to suggest a specific system configuration.
You will probably also want to use an external clock source, or a good 10 MHz and 1 PPS reference.
You can see all the USRP devices and daughterboards at:
https://www.ettus.com/product