As a user roams about {HEAD ~ his cell}, he will at times reach {TAIL ~ the boundary} of good coverage 
As a user roams about { HEAD ~ his cell } , he will at times reach { TAIL ~ the boundary } of good coverage
OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH