BER remains constant while Eb/No increasing

Hello,

First sorry if my english is not proper as it should be. Well, I have an issue in Simulink. I'm trying to simulate a CPM Modulation to trace BER vs Eb/No using BERTOOL. When I am increasing the Eb/No (0 -> 12 dB), BER remains constant (I can see it in the display block). The system is the following one : - Random Integer Generator - Integer to Bit Converter (Tx) - CPM Modulator - AGWN Channel - CPM Demodulator (Rx) - Error Rate Calculation (between Tx and Rx) - To Workspace (an array in 3-D to save BER variable to use in BERTOOL) - Display to see result from the Error Rate Calculation (BER, Wrong bit, MAX Bits).

BER should decrease while Eb/no is increasing. Any ideas ?

Thank you in advance.