  This paper presents directional and omnidirectional RMS delay spread
statistics obtained from 28 GHz and 73 GHz ultrawideband propagation
measurements carried out in New York City using a 400 Megachips per second
broadband sliding correlator channel sounder and highly directional steerable
horn antennas. The 28 GHz measurements did not systematically seek the optimum
antenna pointing angles and resulted in 33% outage for 39 T-R separation
distances within 200 m. The 73 GHz measurements systematically found the best
antenna pointing angles and resulted in 14.3% outage for 35 T-R separation
distances within 200 m, all for mobile height receivers. Pointing the antennas
to yield the strongest received power is shown to significantly reduce RMS
delay spreads in line-of-sight (LOS) environments. A new term, distance
extension exponent (DEE) is defined, and used to mathematically describe the
increase in coverage distance that results by combining beams from angles with
the strongest received power at a given location. These results suggest that
employing directionality in millimeter-wave communications systems will reduce
inter-symbol interference, improve link margin at cell edges, and enhance
overall system performance.
