A Little Is-136 Background Information - Wavetek IS-136 MS Operating Instructions Manual

A Little IS-136 Background Information

IS-136 MS
A Little IS-136 Background Information
All IS-136 mobiles are dual mode, meaning that a mobile can use the analog and
the digital network. To maintain compatibility with the analog (E)AMPS network,
channel spacing is 30 kHz. This results in the requirement to limit the bandwidth
of a carrier to ±15 kHz in the modulation spectrum.
A digital traffic channel (DTC) is divided into six time slots. In full-rate data
transmission these time slots are used as pairs (1st and 4th, 2nd and 5th, 3rd and
6th) with each time slot carrying half the speech data. Full rate means that three
users share one frequency (DTC).
With the introduction of half-rate data transmission it is possible to use each time
slot independently. That means a doubling of capacity in digital networks because
six users share one frequency. This technique is also refered to as time-division
multiple access (TDMA), because several mobiles/users share one frequency at
the same time.
The digital modulation used for IS-136 is
π/4 DQPSK (digital quadrature phase-
Magnitude
Error
shift keying). This modulation process Error
Vector
shifts phase in steps of ±π/4 depending
on bit-pattern changes. With each phase
step you can transmit two bits (special
Phase
IS-136 terms: 2 bits = 1 symbol; 1 bit = 1
Error
unit). In a vector diagram of the modula-
tion signal the ideal end positions of the
45˚
eight possible vectors describe a circle.
The error vector is the line drawn be-
Ideal
tween the ideal signal vector and the real
Vectors
signal vector. Modulation accuracy is
measured by determining the RMS dif-
ference (root mean square) between the real signal and the ideal signal. For this
the individual vector error on each symbol in a slot is measured and computed to
the sum-square vector error. Finally the RMS vector error is computed as the
square root of the sum-square vector error divided by the number of symbols in
the slot (156 for transmission from mobile to base station).
DQPSK modulation is error-tolerant. This means, for example, that if the vector 10
error of the base-station transmitter becomes too high, the mobile receiver will
still be able to reconstruct the correct bit stream with an intelligent bit-error-cor-
rection mechanism. But if this correction function is operating at its limits, speech
quality could suddenly turn from good to bad. Therefore, well adjusted modula-
tors are necessary nevertheless.
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