CNR combined 不等於相加平均

假設有四個Upstream 訊號要combined 進入CMTS
Upstream output from fiber receiver A: 35 dB
Upstream output from fiber receiver B: 32 dB
Upstream output from fiber receiver C: 26 dB
Upstream output from fiber receiver D: 29 dB

那combined 後，CNR 是多少? 是 (35 + 32 + 26 +29)/4 = ? 嗎????
錯~~~

答案就在圖片中~
資料來源: Cisco 這裡