Dear Rishabh,
Waiting for offer letter from Ericsson for almost 1 year is such a long period. There are many possibilities due to which the company is not releasing the offer letter. There is possibility that, the company may be thinking about importance of the position for which they are hiring or there may be certain delay in approval stage. Still these all are assumptions, better you should move ahead and look out for offer from any other organisation rather than relying only on Ericsson. This is just a suggestion which you may or may not use for your decision.