Perhaps you should try some spelling variations:
"modified channel estimation algorithms" » "modified channel estimation algorithm", "modified channel optimization algorithms", "modified channel optimisation algorithm", "modified channel excitation algorithms"
"modified parameter approximation algorithms" » "modified parameters approximation algorithms"

You may be able to get more results by adjusting your search query.

• If you are trying to use Boolean operators, they must be ALL CAPS: (("modified channel estimation algorithms") OR ("modified parameter approximation algorithms"))~.
• Removing quotes may allow a broader search: (( modified channel estimation algorithms ) or ( modified parameter approximation algorithms ))~.
• Adding a wildcard symbol may retrieve word variants: (("modified channel estimation algorithms") or ("modified parameter approximation algorithms"))*.