Digital PMR Radio Processor
CMX7131/CMX7141
5.3.4 FEC and Coding
The CMX7131/CMX7141 implements all CRCs, convolutional codes, interleaving and scrambling required
by the NXDN standard for 4800bps Conventional Mode operation. CRC failures in control channel fields
and coded data blocks are indicated to the host by issuing an “Event” IRQ with a corresponding error code
in the Modem Status register, $C9. This relieves the host of a substantial processing load and has the
added advantage of reducing the complexity and timing constraints of interfacing between the host,
vocoder and CMX7131/CMX7141.
Table 8 Frame Types
Frame type
Un-coded data bits
Coded data bits
Notes:
(C-BUS interface)
(over-air)
LICH
7
16
SACCH
FACCH1
FACCH2
UDCH1
CAC
8 + 18
80
8 + 176
8 + 176
252
60
Radio Access Number in the SU field
144
348
348
252
72
Radio Access Number in the SU field
Radio Access Number in the SU field
No coding defined
TCHx
72
Coding applied by vocoder
Note: FACCH1, FACCH2, UDCH1 and TCHx are all regarded as payload data. If the “Softcoded” function
is enabled, in Rx mode this data will be reported as a 4-bit code per data bit.
5.3.5 Voice Coding
The CMX7131/CMX7141 supports an external vocoder by routing voice payload data over the main C-
BUS interface and through the host. In this mode, all vocoder control and data transfers must be managed
by the host.
Voice data transferred to the external vocoder in Rx mode may use soft decision (4-bit log-likelihood ratio)
format, although it increases the required data transfer rate over C-BUS by a factor of four.
5.3.6 Radio Performance Requirements
In LD mode, for optimum performance, the signal should not be significantly degraded by filters that are
excessively narrow and/or cause significant group delay distortion. Care should be taken when interfacing
the device to the radio circuits to maintain the frequency and phase response (both low and high end), in
order to achieve optimum performance. Test modes are provided to assist in both the initial design and
production set-up procedures.
In I/Q mode the recommended interfacing to the CMX994 should be used; see section 4.3. The
CMX7131/CMX7141 includes digital filters to provide adjacent channel rejection while compensating for
the in-band response of the CMX994 I/Q filters.
Further information and application notes can be found at http://www.cmlmicro.com.
2014 CML Microsystems Plc
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