Re: ARLA/CLUSTER: DARC avisa que o novo modo FHSS-CDMA tem um elevado risco de interferencia em outras estações (tradução em espanhol)

[Sergio Matias]

Segue artigo original do Tom DF5JL, em inglês:


A new digimode jumps within an amateur band in the choice of QRGs back
and forth, with a high risk of interference to other stations. The day
before yesterday José Ros (EA5HVK) has unveiled its new mode called
FHSS-CDMA
(http://rosmodem.wordpress.com/2014/05/16/new-digital-mode-fhss-mode/) -
as an implementation within the new version of his digital mode software
ROS. Now a video published yesterday by Andy, K3UK, which shows clearly
the immense levels of possible interference by FHSS.

In Andy's video (http://www.youtube.com/embed/s0HVLxIQ2oE?vq=hd1080)
FHSS starts its procedure at 14115.0kHz. But within about only a minute
the following frequencies (kHz) are used by the hopping procedure rapidly:

14115.0
14063.0
14156.5
14266.5
14344.5
14055.0
14160.0
14300.5
14324.0
14065.0

A channnel detection ("busy"?) is not visible, i.e. FHSS obviously
doesn't evaluate whether the frequencies are in use by other stations /
modes or not. It turns out once again that as shown with ROS the
developer José Ros still do not seem to care about band plans and best
practice on ham's bands.

In the U.S. the use of spread spectrum by amateur radio below 222 MHz is
not permitted (see http://www.tapr.org/ss_fcc.html). And we may ask if
spread spectrum generally is really a digimode for amateur radio. The
FHSS-CDMA method has not been specified in public by José Alberto Nieto
Ros (EA5HVK). But even when publishing it seems not to be what we can
call "an open mode". Because the pseudo-random sequence of FHSS-CDMA is
not known (José Ros: "Every transmission uses a different CDMA code
randomly chosen") this appears as pseudo-randomly and has a function of
a cryptographic key.

Frequency-hopping spread spectrum (FHSS) is a method of transmitting
radio signals by rapidly switching a carrier among many frequency
channels, using a pseudorandom sequence known to both transmitter and
receiver. It is used as a multiple access method in the
frequency-hopping code division multiple access (FH-CDMA) scheme.
Frequency spreading is generally used in military communications
(MILCOM) as in the civilian sector. The second generation (GSM) and the
third generation of mobile telephony (UMTS ) also uses the frequency
spread contained in CDMA. The reasons lie in the advantages of the
spread spectrum method: Due to the spread obtained to a greater
robustness against narrow-band interference; on the other hand it gives
a certain protection against eavesdropping. A spread-spectrum signal may
simply appear as an increase in the background noise to a narrowband
receiver. An eavesdropper may have difficulty intercepting a
transmission in real time if the pseudorandom sequence is not known. It
can also be difficult to recognize that any transmission takes place
(important for MILCOM, counterproductive for ham radio use).


Best 73, Tom DF5JL
HF Dept. DARC (GER)
Section "Digimodes"


73,
--
Sérgio Matias, ct1hmn
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