Sync Problem with 256-QAM with USRP B2xx in GRC

Dear GNU Radio Community,

since a couple of months i try to get a 256-QAM Transmission over Air to work.

Now i reached a dead point, because i could not get the receiver locked on the phase of the QAM.

4-QAM is not a Problem, it works fine like it is described in the Tutorial.

I have tried Polyphase Clock Sync, Costas Loop ect. (what ever i found under Synchronizers) in quite various combination,

but this leads to nothing. 

My guess is that Costas Loop is not working for higher QAM anyway.

I did not find any description, how this could be done right, neither in the Internet (Google, Youtube) nor in the library of our

University.

Helpful would be:

- a generic description or example how this is done

- a reference one a book or paper which describe that (even if this book is expensive)  

- link to an online-resource 

Thanks in advance

Best regards

Leonhard