Baseband Processor For Mixed Mode Land Mobile Radio
CMX880
Receiving C4FM Signals
The CMX880 C4FM signalling scheme is intended to comply with the requirements of APCO 25 C4FM signalling,
in accordance with:
·
·
TSB102.BAAA, APCO-25 Recommended Common Air Interface;
TSB102.CAAB, Digital C4FM/CQPSK Transceiver Performance Recommendations.
APCO 25 messages, whether data or digitised speech, are formed into packets comprising frame sync (FS),
network ID (NID), header information and further coded data fields. The format of the data is dependent on the
type of message, as identified by the header.
The over-air signal is transmitted as C4FM (‘Phase 1’ implementation, 12.5kHz channel spacing) or CQPSK
(‘Phase 2’, 6.25kHz channel spacing). In either case the output of a receiving device’s FM discriminator is a 4-
level 4800 symbol/s signal. The 4-level signal is Raised Cosine (RC) and shape filtered before transmission, as
shown in Figure 8.
The CMX880 device extracts the symbols and clock and detects the 24-symbol FS pattern from the
demodulated C4FM/CQPSK signal. It does not perform error correction or extraction of the remaining data
packet fields, etc. These tasks must be done by an external DSP.
Symbol data is transferred over the Fast Serial Bus to the external DSP. The DSP must be capable of
processing the symbol data at the incoming symbol rate of 4800 symbols/s.
Filtered
baseband
signal
4-level
symbols
Figure 8 The 4-Level Symbols, Unfiltered and RC Filtered
The demodulated signal (without noise) at the receiver corresponds to that shown as the filtered waveform of
Figure 8. The signal is ‘integrate and dump’ filtered to reject HF noise and to equalise the signal to a form
suitable for extracting the 4-level symbols. The filter has a linear phase, sinc(p.f/4800) response up to 2880Hz.
The response is shown in Figure 9. The filter is followed by a stochastic gradient clock recovery device, which is
capable of extracting the symbol clock from a received signal that has up to ±40ppm deviation from the local
symbol rate clock. This allows for ±10ppm symbol rate accuracy of the transmitter and ±10ppm accuracy of the
local clock, plus a margin of ±20ppm for radio channel effects.
0
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-10
0
500
1000
1500
2000
2500
3000
Frequency (Hz)
Figure 9 Receiver Filter Frequency Response
ã 2001 Consumer Microcircuits Limited
21
D/880/1
CMLMICRO [ CML MICROCIRCUITS ]
