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Transmission Modes
WAVECOM Decoder W74PC, W-PCI/e, W-CODE, W-CLOUD Manual V9.1.0
Three different block lengths are used in SWED-ARQ. The short block containing three characters is essen-
tially the same as the SITOR-ARQ mode. If favorable propagation conditions prevail the system will switch
to medium block length (9 characters) or to long blocks (22 characters). In case of interference the block
length is automatically reduced. The block length is displayed on the status line as either short, middle or
long. Longer block lengths effectively increase the data transfer rate. The equivalent Baudot rates are 75
and 100 Bd for medium and long block lengths respectively.
The illustration depicts the protocol of data transmission for the SHORT, MEDIUM and LONG block types.
Given this timing scheme, the two stations can maintain the data link even after losing block length syn-
chronization by re-transmitting the change-over commands. These consist of the IDLE A, IDLE B and other
characters but are always three characters long.
In some RQ-cycles, SWED-ARQ makes use of bit center keying and maintains a request counter of cycles
deviating from the 3:4 mark-space ratio.
The SWED-ARQ mode may be started by selecting a baud rate. If the transmission channel is subject to
very strong interference, the block length change-over might be lost. By reselecting the baud rate, re-
synchronization may be reestablished.
When phasing the software automatically recognizes block length and polarity.
TETRA
TETRA (TErrestrial Trunked Radio) is a digital voice and data system for mobile services standardized by
ETSI and used for public safety and civilian PMR (Public Mobile Radio). It is closely related to GSM, but has
its own unique features, e.g., a direct mode allowing mobile stations to communicate with each other
without a base station and air interface encryption.
Parameter
Value
Frequency range
CEPT common EU allocation UHF (Public safety: 385 – 390/395 – 399.9
MHz, civil applications: 410 – 420/420 – 430 MHz)
Operation modes
Digital voice and data system
Symbol rate
18 kBd
Modulation
π/4-DQPSK
Receiver settings
BW = 25 kHz
Input format(s)
IF
Additional info
Data or voice
Operation modes
Trunked (TMO V+D (Voice + Data)), direct (DMO)
Access method
TDMA (Slotted Aloha)
Voice codec
ACELP
A Short Data Service similar to the SMS of GSM is available. SDS includes a number of protocols, and the
WAVECOM decoder supports Text Messaging. For this type of messaging the text itself and delivery status
reports are displayed and the messages saved to disk. The other data protocols are displayed as bits due
to the lack of detailed protocol descriptions.
TMO and DMO voice calls in all time slots are decoded and can be monitored in real-time as well as saved
to disk (in a .wav file) for later playback and analysis. The input bits to the vocoder are saved in Hex in a
