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