This "tethering" idea is ludacris. A simplier solution would be one I saw in a Rutger Hauer movie. Attach explosive rings around the convicts neck. If they go outside the approved area, boom. Instant decapitation. Same thing happens if they tamper with the device.
I'm sure it would never occur to the criminals to form gangs and throw those they didn't like outside of the boundary, so they'd be instantly decapitated.