I have Huawei E/173 usb modem connected with the mikrotik CCR.
SMS sending working fine. I am trying to running USSD code but its always giving error. I tried different channels as well but no use.
how can i solve it?

Hi you should use a GSM 7bit PDU converting.
Try here:
http://smstools3.kekekasvi.com/topic.php?id=288

On mikrotik device open terminal and run:

/system/serial-terminal port=usb2 channel=0

When you have this error "failure: port's channel is already in use", you must disable receiving SMS in /tool/sms/ section.

Then you can send a 7bit instructions using AT commands:

AT+CUSD=1,"*101#",15
it's the same as:
AT+CUSD=1,"AA182C3602",15

Regards

Ryz