India's back-office outsourcing business will post a growth of 15-16% in the year to March 2011 on growing demand from overseas clients, the head of a leading industry group said on Wednesday.            The export-driven sector has not seen any adverse impact from the European debt crisis, Som Mittal, president of the National Association of Software and Service Companies (NASSCOM), told reporters.                                            India's back-office outsourcing sector is estimated to have grown 6% to $12.4 billion in the last fiscal year that ended in March.