Land Surveyor Forum

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• Survey Legend

  ⚡Survenator⌁ January 25, 2016 at 3:09pm
• GEO Ambassador

  Jaybird January 24, 2016 at 8:38pm

  Here is some info on the Spread Spectrum Radios and ive attached info i had on UHF... hope this helps

  Spread-spectrum telecommunications

  This is a tech­nique in which a telecom­mu­ni­ca­tion sig­nal is trans­mit­ted on a band­width con­sid­er­ably larger than the fre­quency con­tent of the orig­i­nal in­for­ma­tion. Fre­quency hop­ping is a basic mod­u­la­tion tech­nique used in spread spec­trum sig­nal transmission.

  Spread-spec­trum telecom­mu­ni­ca­tions is a sig­nal struc­tur­ing tech­nique that em­ploys di­rect se­quence, fre­quency hop­ping, or a hy­brid of these, which can be used for mul­ti­ple ac­cess and/or mul­ti­ple func­tions. This tech­nique de­creases the po­ten­tial in­ter­fer­ence to other re­ceivers while achiev­ing pri­vacy. Spread spec­trum gen­er­ally makes use of a se­quen­tial noise-like sig­nal struc­ture to spread the nor­mally nar­row­band in­for­ma­tion sig­nal over a rel­a­tivelywide­band (radio) band of fre­quen­cies. The re­ceiver cor­re­lates the re­ceived sig­nals to re­trieve the orig­i­nal in­for­ma­tion sig­nal. Orig­i­nally there were two mo­ti­va­tions: ei­ther to re­sist enemy ef­forts to jam the com­mu­ni­ca­tions (anti-jam, or AJ), or to hide the fact that com­mu­ni­ca­tion was even tak­ing place, some­times called low prob­a­bil­ity of in­ter­cept (LPI).

  Fre­quency-hop­ping spread spec­trum (FHSS), di­rect-se­quence spread spec­trum (DSSS), time-hop­ping spread spec­trum (THSS), chirp spread spec­trum (CSS), and com­bi­na­tions of these tech­niques are forms of spread spec­trum. Each of these tech­niques em­ploys pseudo­ran­dom num­ber se­quences — cre­ated using pseudo­ran­dom num­ber gen­er­a­tors — to de­ter­mine andcon­trol the spread­ing pat­tern of the sig­nal across the al­lo­cated band­width. Ul­tra-wide­band(UWB) is an­other mod­u­la­tion tech­nique that ac­com­plishes the same pur­pose, based on trans­mit­ting short du­ra­tion pulses. Wire­less stan­dard IEEE 802.11 uses ei­ther FHSS or DSSS in its radio interface.

  • Techniques known since the 1940s and used in military communication systems since the 1950s "spread" a radio signal over a wide frequency range several magnitudes higher than minimum requirement. The core principle of spread spectrum is the use of noise-like carrier waves, and, as the name implies, bandwidths much wider than that required for simple point-to-point communication at the same data rate.
  • Resistance to jamming (interference). DS (direct sequence) is good at resisting continuous-time narrowband jamming, while FH (frequency hopping) is better at resisting pulse jamming. In DS systems, narrowband jamming affects detection performance about as much as if the amount of jamming power is spread over the whole signal bandwidth, when it will often not be much stronger than background noise. By contrast, in narrowband systems where the signal bandwidth is low, the received signal quality will be severely lowered if the jamming power happens to be concentrated on the signal bandwidth.
  • Resistance to eavesdropping. The spreading code (in DS systems) or the frequency-hopping pattern (in FH systems) is often unknown by anyone for whom the signal is unintended, in which case it obscures the signal and reduces the chance of an adversary's making sense of it. Moreover, for a given noise power spectral density(PSD), spread-spectrum systems require the same amount of energy per bit before spreading as narrowband systems and therefore the same amount of power if the bitrate before spreading is the same, but since the signal power is spread over a large bandwidth, the signal PSD is much lower — often significantly lower than the noise PSD — so that the adversary may be unable to determine whether the signal exists at all. However, for mission-critical applications, particularly those employing commercially available radios, spread-spectrum radios do not intrinsically provide adequate security; "...just using spread-spectrum radio itself is not sufficient for communications security".
  • Resistance to fading. The high bandwidth occupied by spread-spectrum signals offer some frequency diversity, i.e. it is unlikely that the signal will encounter severemultipath fading over its whole bandwidth, and in other cases the signal can be detected using e.g. a Rake receiver.
  • Multiple access capability, known as code-division multiple access (CDMA) or code-division multiplexing (CDM). Multiple users can transmit simultaneously in the same frequency band as long as they use different spreading codes.