Digital PMR Radio Processor
CMX7131/CMX7141
settings. In LD mode the signal is filtered using a Root-Raised Cosine filter and Inverse Rx Sinc filter
matching the filters applied in the transmitter, then passed to the AFSD (Automated Frame Sync Detector)
block which extracts symbol and frame synchronisation. During this process the 4FSK demodulator and
the data-processing sections that follow are dormant to minimise power consumption. When frame
synchronisation has been achieved the AFSD section is powered down, and timing and symbol-level
information is passed to the 4FSK demodulator which starts decoding the subsequent data bits. The
CMX7131/CMX7141 can detect the end of a burst by scanning the received control channel fields and will
automatically disable the demodulator and restart frame sync search when required without host
intervention.
In I/Q mode filtering is applied to the input signals and dc offsets are removed before an inverse tangent
function performs the FM demodulation function. The output of this stage has an offset depending on the
frequency error of the received signal compared to the nominal frequency of the receiver. This offset is
removed before RRC filtering, after which the signal chain is then the same as the LD case. In I/Q mode
the CMX7131/CMX7141 provides measurements of frequency error and RSSI (which are not available in
LD mode).
An NXDN™ call consists of a series of 80ms frames, each starting with a 20-bit Synchronisation Word
(SW). The first frame is preceded by a Preamble sequence, and the CMX7131/CMX7141 uses the last 18
bits of the Preamble together with the first Synchronisation Word to detect the start of a transmission. This
is reported to the host by setting the FS1 Detect bit in the Status register.
Last 9 Symbols of Preamble: xx11 0111 0101 1111 1101
Synchronisation Word: 1100 1101 1111 0101 1001
($375FD)
($CDF59)
The CMX7131/CMX7141 can optionally also detect the Synchronisation Word sequence in isolation to
perform “late entry” into an existing call. This is reported to the host by setting the FS2 Detect bit in the
Status register. The short length of the Synchronisation Word gives a high probability of false detections,
so by default the CMX7131/CMX7141 will only generate an initial FS2 Detect if two successive
Synchronisation Words are detected at the correct frame spacing in the received signal.
Additionally, once initial synchronisation has been achieved with an FS1 or FS2 Detect, it can later be re-
established from a single Synchronisation Word received within ±2 symbols of the original frame timing.
This mode slightly increases power consumption in order to maintain the symbol timer through Rx-Tx-Rx
or Rx-IDLE-Rx transitions and hence is selectable using b13 of the modem configuration register, $C7.
Note that this also requires b8 of $C7 to be set in order to enable FS2 detects.
When frame synchronisation has been achieved and the 4FSK demodulator has been enabled, Frame
Sync detection is switched off and any subsequent Preamble or Synchronisation Word sequences in the
received data are not reported to the host.
Table 5 NXDN Frame Format – Audio Communication
144
144
bits: >24 20 16 60
SW LI SACCH
FACCH1
TCH1
FACCH1
TCH3
P
TCH2
TCH2
TCH2
TCH2
TCH4
TCH4
TCH4
TCH4
SW LI SACCH
SW LI SACCH
SW LI SACCH
SW LI SACCH
TCH1
TCH3
TCH1
TCH3
TCH1
TCH3
Repeat until PTT released….
FACCH1
FACCH1
SW LI SACCH
2014 CML Microsystems Plc
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