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GSM Repeater what and How to Use 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 :
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. o Iso = Grep + 10dB Following equation can be used to calculate antenna isolation o Vertical separation Iso = 28 + 40log( d / l ) o Horizontal separation Iso = 22 + 20log( d / l ) – (GA + GB) o where d = Separation distance(feet) o l = Wavelength o GA = Gain of donor-side antenna in direction of Mobile side antenna. o 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 o Iso = 22 + 20log( d / l ) – (Gamax – Gaback) – (Gbmax – Gbback ) o and
o o
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 o 80 + 10 = 22 + 20log(d/1) – (17-25) – (17-25) o 90 = 22 +20log(d) + 8 + 8 o 22+20log(d) = 74 o 20log(d) = 52 o log(d) = 2.6 o 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 o Iso = 28 + 40log( d / l ) o and o Iso = Grep + 10dB o if the repeater is used on GSM900 then o 80 + 10 = 28 + 40log(d/1) o 90 = 28 + 40log(d) o 40log(d) = 62 o d = 35.48 feet
