How to add Order by expression on Link Entity attribute in Query Expression?

Hi,

I need to add order by on link entity attribute in query expression. 

My query in fetch xml format is as below-

<fetch mapping='logical'>
<entity name='policymember'>
<all-attributes/>
<filter type='and'>
<condition attribute='memberid' operator='eq' value='8B7E5B4C-EE47-DE11-A167-0015177CD626'/>
</filter>
<link-entity name='policy' from='policyid' to='policynoid' alias='MemberPolicy' link-type='natural'>
<filter type='and'>
<condition attribute='enddate' operator='ge' value='09-18-2011'/>
</filter>
<order attribute='modifiedon' descending='true'/>
</link-entity>
</entity>
</fetch>

Thanks in Advance.