Digital PMR Radio Processor
CMX7131/CMX7141
180
160
140
120
100
80
60
40
20
0
-140
-120
-100
-80
-60
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0
20
Signal Strength (dBm)
Figure 8 RSSI in I/Q Mode
5.3 Introduction
This modem runs at 4800bps, occupying a 6.25kHz bandwidth RF channel. It has been designed such
that, when combined with suitable RF, host controller, vocoder and appropriate control software, it meets
the requirements of the NXDN 4-FSK standard.
The NXDN standard specifies the AMBE+2 Voice Coding algorithm which is supported by transferring all
payload data through the host using the main C-BUS interface.
The standard requires a 4-FSK modulation scheme with an over-air bit rate of 4800bps (2400 symbols per
second). FI-3.1.x supports 4800bps Conventional mode using RDCH channel formatting.
5.3.1 Modulation
The NXDN 4-FSK modulation scheme operates in a 6.25kHz channel bandwidth with an over-air bit rate of
4800bps (2400 symbols per second). RRC filters are implemented at both Tx and Rx with a filter “alpha” of
0.2. The maximum frequency error is +/- 625Hz and the CMX7131/CMX7141 can adapt to the maximum
time base clock drift of 2ppm over the duration of a 180-second burst. Figure 11 shows the basic
parameters of the 4FSK modulation, symbol mapping and filtering requirements.
Figure 9 and Figure 10 show a transmitted PRBS waveform, as recorded on a spectrum analyser in 36k
span and zero-span mode, having been 2-point modulated using a suitable RF transmitter.
2014 CML Microsystems Plc
Page 22
D/7141_FI-3.x/6
CMLMICRO [ CML MICROCIRCUITS ]
