Linux/WL1835MOD: how to improve bluetooth communication distance

Tool/software: Linux

Hi, I am trying to work with BT of wl1835 on linux 3.12

With the help of TI Guide ,the basic BT function is OK,

I start SS1BTPM&  LinuxSPPM

two BT devices can find each other ,can connect  together and send/receive data successfully 

But  the BT communication distance is too short (about less than 40cm, RSSI about -60~-80dB).

when the distance of  two BT devices become more than 100cm , RSSI drop to -96~-100dB,and 

can not communicate successfully.

the wifi module works well , wifi and BT use the same antenna.

Below is my custom board info:

1. custom am437x_evm  board with  SDK AM437xSDK  07_01_00_00

2. TI Bluetooth Stack  --AM335xBluetopiaLinuxProduction-4.2.1.0.1.0

3. TI bt service package --wl18xx_bt_sp_v4.2

So, Is there any suggestion or solution to this problem ?

Thanks!