inclusion criteria :
* COPD : FEV1/FVC \< 70%, no response to bronchodilator (FEV1 change \<12% and \<200ml)
* OSA : AHI \> 15/h, obstructive type predominant
exclusion criteria :
* any malignancy
* pregnancy
* claustrophobia
* age\<18
primary outcome : exercise capacity (walking distance (meter), measured by incremental shuttle walking test)
Method : For those fit in criteria. CPAP titration will be performed to determine the optimal pressure Baseline measurement : incremental shuttle walking test, urine catecholamine, heart rate variability, muscle power and other baseline characteristics Randomized those patients to group A and group B group A : CPAP treatment with optimal pressure for 1 month, repeat measurement change CPAP treatment to sham pressure 5cm H2O for 1 month, repeat measurement group B : CPAP treatment with sham pressure for 1 month, repeat measurement change CPAP treatment with optimal pressure for 1 month, repeat measurement
