Repeater units are designed to receive signals from a donor site, amplify and rebroadcast the donor sites signals into poor coverage areas or to extend the coverage range of the donor site.

Repeater

Repeater :

  • These repeaters are bi-directional and do not translate frequency and subsequently are limited in output power and gain.
  • Repeaters provide between 50 to 80 dB of gain.
  • There are two types of repeater band selective and channel selective.
  • Band selective repeater amplifies a band of frequency. Hence it amplifies any frequency that falls within its band.
  • Channel selective repeater allows selection of a number of individual channels to amplify and rebroadcast.
  • Typically a channel selective repeater allows selection of 2 to 4 channels.
  • If the GSM900 or DCS1800 network incorporates frequency hopping, then only band selective repeaters should be used.
  • Since repeaters are non-frequency translating, the isolation between the donor-side and mobile-side antenna needs to be much greater than the gain of the repeater.
  • If antenna isolation is less than or equal to the repeater gain, the repeater will begin to amplify its own feedback and oscillate. This condition must be avoided.
  • The isolation between the antennas needs to be at least 10 dB greater than the gain of the repeater.
    • Iso = Grep + 10dB
  • Following equation can be used to calculate antenna isolation
    • Vertical separation Iso = 28 + 40log( d / l )
    • Horizontal separation Iso = 22 + 20log( d / l ) – (GA + GB)
    • where d = Separation distance(feet)
    • l = Wavelength
    • GA = Gain of donor-side antenna in direction of Mobile side antenna.
    • GB = Gain of mobile-side antenna in direction of donor- side antenna.
  • Separation type is based on site type. If the site is a roof, then horizontal separation should be used while if the site is on a tower vertical separation should be used.
  • If directional antennas are used then the required horizontal spacing is given by
    • Iso = 22 + 20log( d / l ) – (Gamax – Gaback) – (Gbmax – Gbback )
    •                 and
    • Iso = Grep + 10dB
    • If gain of the directional antenna is 17dBi and front-to-back >25dB and the repeater( gain of 80dB) is used on GSM900 then
    • 80 + 10 = 22 + 20log(d/1) – (17-25) – (17-25)
    • 90 = 22 +20log(d) + 8 + 8
    • 22+20log(d) = 74
    • 20log(d) = 52
    • log(d) = 2.6
    • d = 398 feet
  • Hence a horizontal separation of 398 feet is required to provide a isolation of 90dB between the donor-side and the mobile-side antenna which is quite unreasonable.
  • Large antenna separation produces large cable losses and reduces repeater output power.
  • Some obstruction should be in between both the antennas to reduce this separation.
  • If vertical spacing is given by
    • Iso = 28 + 40log( d / l )
    •                 and
    • Iso = Grep + 10dB
    • if the repeater is used on GSM900 then
    • 80 + 10 = 28 + 40log(d/1)
    • 90 = 28 + 40log(d)
    • 40log(d) = 62
    • d = 35.48 feet