Wavecom W61PC V7.5.0 - Manual (Page 170)

160  Transmission Modes WAVECOM Decoder W61PC/LAN Manual V7.5
In addition to the decoded output, several files are generated in the WAVECOM-DATA-OUT directory. In
the subdirectory “GW-OFDM[SN]”, all decoded emails are stored, either as text file (OutFileTXT) or in a
folder as .SSP-file (OutDir). If stored as OutFileTXT.txt or as .SSP file in the OutDir depends on the
transmission variant. The .SSP file can be opened with every text editor. The subdirectory RAW_DATA is
used by the decoder for intermediate files. The data stored in the RAW-DATA directory can be used for
advanced examination of the transmission.
Due to the compression employed by GW-OFDM, decoded data is not available until the end of a transmis-
sion; the transmission will be decoded in its entirety after it has been received. As long as a transmission
is in progress, IDLE or DATA frames are indicated in the status bar. After recognizing an END or END ACK
frame, the decoder will handle the ARQ protocol and then decompress the data. If the transmission is in-
terrupted, the decoding session will time out; however, the decoder will still attempt to determine call
signs, type and transmission ID. As the GW-OFDM-signal contains no redundancy, sufficient reception
quality is important: every DATA packet must be received at least one time without corruption. In the
case of packet repetition, the decoder can handle multiple received packets. It should be noted that the
decompressor has no error correction - even one wrong bit can produce a completely false decoded out-
put.
As the decoder is highly automated, no user configuration is necessary. Nonetheless, to improve demodu-
lation results the passband filter should be set to the expected bandwidth of the signal (approximately
2200Hz).
GW-PSK
GW-PSK is used by Globe Wireless on the Maritime Data Network of the company. The network works with
several digital modes in an adaptive system. GW-FSK is always used for opening a connection. Depending
on propagation conditions, it may switch to GW-PSK or other digital modes.
Parameter Value
Frequency range HF
Operation modes Simplex ARQ
Modulation DQPSK, D8PSK
Symbol rate 200 Bd
Bandwidth 400 Hz
Receiver settings DATA, CW, LSB or USB
Input format(s) AF, IF
Additional Info ASCII
GW-PSK is DQPSK and D8PSK modulated with a bandwidth of 400 Hz. The bit rate is 400 bits/s or 600
bits/s with a fixed symbol rate of 200 Baud.
A GW-PSK frame consists of 288 or 432 bits. Each frame contains 25 characters or data bytes. For error
detection a 16 bit CRC-checksum is required. For smaller packets, a filler ‘U’ for traffic or ‘<’ for last pack-
ets is used.
A GW-PSK signal is tuned in the same way as a PACTOR-II signal. The precision of the center frequency is
very important and influences directly the performance of the decoding.
Most of the traffic is compressed. From time to time uncompressed GPS position data in NMEA format is
transmitted on ship frequencies. GW-PSK stations can be found on all maritime bands and also in the fixed
service bands. More Information is available on http://www.globewireless.com.
HC-ARQ
HC-ARQ is a simplex system operating at a symbol rate of 240 Bd on the radio link.
Parameter Value
Frequency range HF