Trying to clam the plot with Mother deed and survey plan

I'm the absolute owner of a plot measuring 15 ft * 30 ft since 15 years. I have the EC from 16 years and for the 15 years and till date the EC is in my name. I have the Khata certificate however I do not have a Khata extract. I do not have the Mother deed. I do not have the Survey plan or the revenue plan or the tenency court order to sell the land which is divided into same plots. I'm paying the property tax regularly to the municipality office. I have acquired this plot by Sale Deed registered in the Sub Registrar office in Bangalore, karnataka. Few days back some people came to me stating that the plot belongs to their late father and the signature and the thumb impression in my Sale deed is not that of his father. I have actually bought the plot though his father via registered Sale deed. Now, they want to grab this plot and have informed me that they will file a fraud case against me. What should I do now? What are the legal types of documents should I obtain? For where to obtain and apply for legal title deeds? What is the time limitation as per law for any one to claim the property?