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Real-Time I/Q Baseband Generator User’s and Programming Guide
Overview of Modulation Formats
ESG Family Signal Generators
GSM
Options UN8 and 202
GSM
The Global System for Mobile Communication (GSM) digital communications standard
defines a voice and data over-air interface between a mobile radio and the system
infrastructure. This standard was designed as the basis for a private radio
communications system. A central control center is linked to multiple base station sites
which provide the required coverage.
The GSM digital communications standard employs a 8:1 Time Division Multiple Access
(TDMA) allowing eight channels to use one carrier frequency simultaneously. The
270.833 kbits/second raw bit rate is modulated using a system known as Gaussian
Minimum Shift Keying (GMSK). This standard defines the channel separation as 200 kHz
at carrier frequencies between 935 and 960 MHz (downlink transmissions) and 890 to
915 MHz (uplink transmissions) at a power level ranging between 3.7 mW and 20 W. The
standard includes multiple traffic channels, a control channel, and a cell broadcast
channel.
The GSM framing structure is based on a hierarchical system consisting of timeslots,
TDMA frames, multiframes, and hyperframes. One traffic channel consists of 156.25 (157)
modulated bits including tail, training sequence, encryption, and guard time, and timeslot
bits. Eight of these bursts make up one TDMA frame. Twenty-six TDMA frames make up
one multiframe. Frames 13 and 26 in the multiframe are dedicated to control channel
signaling.
Guard time appears in the visual representation of the timeslot as an 8.25-bit field. In the
actual implementation, the guard time field in timeslots 0 and 4 are 9 bits long and the
remaining timeslots contain 8 bit fields. (This implementation is documented in the GSM
standard “GSM REC. 05.10 Section 5.7” as follows: “Optionally, the BS may use a timeslot
length of 157 bit periods on timeslots with TN=0 and 4, and 156 bit periods on timeslots
with TN=1, 2, 3, 5, 6, 7, rather than 156.25 bit periods on all timeslots.”)
Summary of Contents for ESG series
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