Communications, Radar & Electronic Warfare (201.. - Index of

30 Communications, Radar and Electronic Warfare• Assignments for particular up-links and down-links (where they both exist; forexample in broadcast there is no uplink). Assignments are normally determined bytechnical means, based on factors such as:- distance from other stations using the same frequency;- geographic limits of interfering signals from stations, determined usually bycomputer predictions;- time-based, when some networks do not need to operate continuously;- time slot de-confliction, for systems that use Time Division Duplexing (TDD);- use of spreading codes (also known as PN codes);- use of polarisation in some cases.It can be seen from this list that spectrum management is partly an administrativeprocess and partly a technical process. Systems that rely more on the administrativemethods tend to be conservative and less flexible, as well as being less optimal thanthose systems based on technical de-confliction. However, technical de-confliction ismore demanding to achieve. It requires advanced spectrum management tools, experts touse them and all of the logistics and planning required to achieve and retain the level ofknowledge required. However, given that spectrum management in the civil spherenormally changes very slowly and licences tend to be issued to last for a number of years.This is necessary for licence holders to be able to commit to the large investmentsrequired to implement large networks and for them to recover their investment. As wewill shortl y see, this is very different from spectrum management for military operations.2.3 Military Spectrum Management2.3.1 IntroductionCivil spectrum management can be characterised in the following way:• It occurs in a known geographical location.• The assignment process can be slow, but this is not a problem because the time toacquire a licence is normally known in advance.• Licences, once assigned, last a long time; usually years.• Plans can be made years in advance.• Information on spectrum users is easily obtainable and all important parameters areknown explicitly.• The national regulator has complete control over the spectrum; with the limitedexception of pirates, normally in radio broadcast, there will be no unauthorised orthird-party spectrum users.Military spectrum management includes assignments with the same characteristics;for example, air defence networks to protect a nation are unlikely to change rapidlyand the area remains the same. However, military operations have very differentrequirements: