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Overview
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Frequency bands
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GSM 900
channels 0 – 124, 955 – 1023
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876.2 to 914.8 MHz (uplink),
921.2 to 959.8 MHz (downlink)
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GSM 1800
channels 512 – 885
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1710.2 to 1784.8 MHz (uplink),
1805.2 to 1879.8 MHz (downlink)
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GSM 1900 (Americas)
channels 512 – 810
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1850.2 to 1909.8 MHz (uplink),
1930.2 to 1989.8 MHz (downlink)
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GSM 850 (Americas)
channels 128 – 251
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824.2 to 848.8 MHz (uplink),
869.2 to 893.8 MHz (downlink)
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Channel spacing
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200 kHz
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Modulation
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GMSK (Gaussian Minimum Shift Keying), phase shifts of π/2 (90°) on symbol transitions
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Symbol rate
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270.833 symbols/s
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Bits per symbol
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1
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Access format
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TDMA (time-division multiple access)
TDD/FDD (time and frequency division duplex)
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Number of channels per carrier
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8 time slots per TDMA frame (4.615 ms)
148 bits per time slot, equivalent to 577 µs
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Speech
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Digital transmission with one out of several possible vocoders:
Full rate speech (13 kbit/s)
RPE-LTP (regular pulse excited code with long term prediction)
Enhanced full rate speech (12.2 kbit/s)
ACELP (algebraic code excited linear prediction)
Half rate speech (5.6 kbit/s)
VSELP (vector sum excited linear predictor)
Adaptive multirate (AMR), dynamic operation on full and half rate channel depending on capacity and reception quality
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Power control
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Yes, in 2-dB steps over a range of 30 dB
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Applications
GSM is the leading cellular phone standard in the world, with networks in most countries. GSM supports data connections through circuit switched data (up to 14.4 kbit/s), high-speed circuit switched data (HSCSD, up to 43.2 kbit/s by using more than one time slot) and general packet radio service (GPRS, with packet data channels on one or several time slots, up to 171.2 kbit/s). A 3G evolution path for higher data rates with GPRS is EGPRS, based on EDGE (enhanced data for the global evolution).
In addition, the short message service (SMS) is very popular for short offline communications – just like e-mail – and is used in a number of business applications such as flight check-in while approaching the airport via rapid transit.
A special version of GSM has been designed for the railways: GSM-R with broadcasts, multicast, semi-duplex calls, prioritisation.
Technology in brief
GSM is a TDMA system with eight time slots per carrier. A normal speech call uses one time slot; audio is typically transmitted on one time slot per frame. Each base station provides a base channel with basic information about the network and the base station in the first time slot of a carrier.
To accommodate frequency regulation in different regions, the original GSM bands at 900 and 1800 MHz were complemented in the USA by a 1900 MHz band and the 850 MHz that was originally reserved for analogue AMPS.
Frequency planning on the network side is critical to avoid interference and dropped calls. Interference with neighbouring cells can be minimized through power control in both mobile phone and base station.
First networks began commercial operation in 1992; expected lifetime at least until 2013. GSM is considered a 2G technology and is currently being complemented (rather than replaced) by 3G networks based on WCDMA/UMTS technology.
Typical measurements
Numerical measurements:
Peak power, rms and peak phase error, frequency error
Graphical representations:
power vs. time, phase error vs. time, modulation spectrum
Willtek Test Instruments
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Air Interface Test Products |
