Currently, there are no known ITU or North American error performance standards that address outage probability on allpacket point-to-point microwave radios According to both the !igants and ITU-" and ITU-" #$ #$%& %& models, the probability of outage 'ie, (everely )rrored (everely )rrored (econd "atio* (econd "atio* is inversely proportional to fade margin
Truth or Myth: Higher Fade Margins Equal Better Performance? This brings us to consider the following myth+ o higher fade margins improve error performance )ven though it makes sense intuitively, the concept of improving performance with high fade margins is not applicable to critical links.long links in low-lying, flat and humid regions /or this reason, a cautionary note needs to be disseminated among the global "/ planning community Fade Margin and its Meaning in Point-to-Point Design Point-to-Point Design uring the days of analog radios, high fade margins were re0uired because noise was additive on a per hop basis, and any disturbance affected performance It is important to recogni1e that annual or monthly outage time, not path fade margin, is the error performance ob2ective for all-packet microwave radios An all-packet radio will perform essentially error-free 2ust a few d3s above threshold Truth 1: ritical !in" or "-Factors #educe Fade Margin$ %ncrease &utage Time /or long '4&km567$-miles5* and flat paths deployed in low elevations '7&&m68$8-feet and lower* and humid areas, the geoclimatic model will yield a high geo-climatic factor 'C or k-factor* that will reduce fade margin and conse0uently increase outage time from %&& sec6year '99999: availability* to perhaps ;<$&& sec6year '9999$7: availability* The logic is that to reduce the outage time, large '=%m69>-foot* antennas would be re0uired Truth ': !arge (ntennas Ha)e *arro+ Beam+idth$ Decou,le at *ight ?owever, large antennas have a narrow beamwidth that would render the path unusable due to antenna decoupling because of dramatic changes of the k-factor at night Truth : High &ut,ut Po+er Does *ot (ccommodate High *octurnal "-Factors @n the other hand, high output power would not accommodate very high nocturnal k-factor values and as a conse0uence a high fade margin would be useless.not t o mention epensive to implementB Four Princi,les of .ritical/ #egion Path Engineering uring our $4 years of eistence in (ilicon !alley, !alley, Aviat Networks has accumulated vast eperience in the understanding of microwave radio propagation and performance in divergent geo-climatic conditions around the globe Conse0uently, Aviat Networks recogni1es the need to observe four path engineering commandments when implementing links in critical 'ie, low elevation, high humidity, ducting* regions as opposed to 2ust concentrating on fade margin+ < Ade0uate path clearance above suspected atmospheric boundary layers 7 @ptimi1ed antenna spacing % #roper antenna si1es and eacting alignments 4 /ade margin In critical regions, wide radio channels 'ie, 7> ?1D $8 ?1* are dramatically affected by divergent tropospheric dielectric boundaries, which cannot be mitigated by high "/ power or very large antennas /or these designs, sound path engineering is crucial, not necessarily high fade margin
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