There are plenty of posts - and a bunch of good tips & helps - just do a search on "range" or "RF Range", or "CM15A range".
The CM15A does have very little range - and there are at least 2 things that can help.
1. Move the CM15A as far away from your PC as possible and as high as possible. I use a 16' USB extension and have it in a closet about 10' off the floor - plugged in or an outlet via a simple extension cord.
2. Cut an 18" piece of coat hanger (doesn't matter if it's painted or bare), and tape or wire-tie to the existing CM15A antenna - and have it parallel to the floor.
With that setup, I'm getting about 50' out to my garage door sensor through 2 outside walls, and about 35' to my front door sensor through interior walls.
Just for info, I added an EagleEye just outside that front door and it wouldn't get its signal through the metal door and back to the CM15A - so I added a TM715A (set to the Eye's HouseCode) - and plugged it in upstairs - that fixed the range for that HouseCode. - BUT - I am getting collisions - I've read the RR501 is much more polite. However to increase my overall range for all devices, I ordered a SR731 Smart RF Repeater ($25 on ebay from Digitronex) which is supposed to repeat all 256 codes AND avoid collisions. I should get it within the next few days.
Hope that helps....