Low cardinality in OLTP with high concurrency

I am struct with a problem of slow performance of a query. The problem is because of low cardinality. A table contains about 1 million rows which increases daily. there is a column which contains 16 distinct values say AOFA, AOFB,SIGN etc etc.

When i run a query with following clause

Where UPPER(Field) LIKE 'AOF%'

It takes to much time. I checked in the table in 1 million rows, about .5 million rows match this criteria. I have created a functional index on the table with Upper function. This improved the performance from 2 mins to 40 sec bt 40 sec is again not acceptable.

So i though of bitmap index. But since it is OLTP where inserts/ modifications run at night by utilities and selects is done by users in Day time, i have to drop this option to avoid any deadlock sort of things.

Any suggestion on this.

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Vipin