Post navigation in same ancestor category

I am in a situation where I need the post navigation to work in same ancestor category NOT ONLY IN SAME CATEGORY.
for example:

• CATEGORY-A

• subcat1
• subca1-I
• subcat1-II

• subcat2
• subcat2-I
• subcat2-II

In the above category structure I want the post navigation to work on all the posts in CATEGORY-A not just subcat1 or subca1-I…

According to the Codex
<?php previous_post_link($format, $link, $in_same_cat = true, $excluded_categories = ''); ?>
provides the navigation “previous post” in same category but how can i get the navigation in same ancestor category?
Please help