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         Digital Modes for Ionospheric Radio Network
                                                      Ludwig López
                                Inter American University of Puerto Rico – Bayamón Campus

                                                                      In this paper, the HF band and channel characteristics will
   Abstract—This paper presents a recommendation of a modern         be discussed first. Then, the commonly used Watterson model
digital mode to transmit textual information over the HF band.       will be summarized as along with the CCIR 520-1 and ITUR
This mode will be used in an interstation communication system       F.1487 recommendations used to simulate the HF channel.
for a network of geophysical measurement stations in the             The methodology used and setup of the test will also be
Antarctic. An HF channel simulator was used to develop effective     explained in detail. Finally, the simulation results will be
comparative tests for all selected digital modes. The results from
                                                                     shown in conjunction with the conclusions drawn from them.
the channel simulator helped in choosing the correct digital mode
according to the most reliable text transmission.

 Index Terms—HF Channel, Watterson Model, Digital Radio                                    II. HF CHANNEL
Modes, HF Propagation                                                   The high frequency (HF) band ranges from 3 to 30 MHz
                                                                     and is used primarily for maritime, aeronautical, and military
                                                                     systems as well as for long-distance AM broadcasting and
                       I. INTRODUCTION                               amateur radio operations. The nominal bandwidth allocation is

A     S a propagation channel, the ionosphere has proven very
      useful for transmitting over long distances using the High
Frequency (HF) radio band. The ionosphere, however, is
                                                                     3 kHz. HF radio has been proven to be an inexpensive, low-
                                                                     power, yet reliable, means of long-distance communication
                                                                     without need for additional relay stations such as satellite or
typically characterized by interrelated temporal effects on          telecommunications line [4]. It is excellent for isolated and
communication, making it a highly variable medium over a             remote areas where other communication infrastructure is
wide range of time scales. These effects include, but are not        nonexistent.
limited to, common white noise, Doppler spread, Doppler                 The HF band, as well as other bands, possesses direct and
shift, and time delay [1]. These factors cause distortion in both    ground wave propagation, but with the addition of sky wave
                                                                     propagation, which is not present on other frequency bands
signal amplitude and phase, causing loss of synchronization
                                                                     higher than HF. This propagation mechanism is responsible
and data misinterpretation in a communication system. Digital
                                                                     for long-distance communication because the radio signals
mode selection is important as it affects error correction as
                                                                     that enter the ionosphere are refracted back toward the earth
well as character synchronization.                                   by the E and F layers of the ionosphere, depending on
   This paper formulates a recommendation for the use of a           ionospheric conditions, which can change with location, time,
modern digital mode to transmit textual information over the         and space weather conditions.
HF band. The recommended mode will be used in an                        The ionosphere as a channel of transmission is generally
interstation communication system for a network of                   characterized as a multi-path time-varying fading channel that
autonomous        and dynamically adaptive,           low-power      contains time delay, Doppler shift, noise, and interference,
geophysical measurement stations in the Antarctic [2].               resulting in a degradation of transmission quality.
   Even though on-air tests can be performed, ionospheric               The channel exhibits fluctuating signal-to-noise ratio (SNR)
conditions are impossible to control and do not yield                caused by several sources. Radio noise can arise from natural
satisfactory results for comparative tests between modes.            forms such as atmospheric noise, caused mainly by cloud-to-
What is needed is a way of simulating an ionospheric test            ground flashes, as well as cosmic noise that mostly affects the
medium where parameters can be controlled. A model that              higher portion of the HF band. In addition, manmade noise is
simulates an HF channel is the key to developing an effective        present and can range from electric power sources and electric
comparative test of all digital modes [3]. A computer                cables to computer clocks. Additionally, crosstalk caused by
simulation is the best way to obtain significant results.            signals in the same frequency at various power levels as well
   An HF channel simulator is used to choose the correct             as adjacent channels close in frequency with significant power
digital mode according to the most reliable text transmission.       can cause de-sensing in the receiver.
A station will send a predetermined message to all other               Ionospheric turbulence causes Doppler spread in the form of
stations on the chain when needed. A nearly error-free               fading of the signal, originating from multiple reflections
transmission is needed to ensure the message gets through in a       (multi-paths) of radio signals between the earth and different
timely manner.                                                       layers of the ionosphere. Furthermore, since the heights of the
                                                                     layers of the ionosphere change during the day, different
                                                                     frequency shifts on each of the multi-path components will
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                  III. HF CHANNEL MODEL
   The commonly used Watterson model forms the basis for
most modern HF channel simulations. This model proposes a
narrow-band HF channel that assumes it is both stationary and                                        TABLE I
                                                                                                 TESTED MODES
stable [5]. Although HF channels are nonstationary in both
                                                                        Modes Tested           Mode Production               Modulation
frequency and time, this model was validated with on-air                                      Capability of Modem
measurements and found to be valid for only short periods of        AMTOR FEC                AMTOR FEC & ARQ                     FSK
time (about 10 min) and small bandwidths (<12 kHz). This            ASCII 110                ASCII                               FSK
model helps to develop a controlled and repeatable
                                                                    CW 20                    CW                                 OOK
environment for performing tests.
   Based on the Watterson model, several channel conditions         PACTOR FEC               GTOR FEC & ARQ                      FSK
can be used for an HF ionospheric channel simulator. The            PSK31                    PACKET                              FSK
International Radio Consultative Committee (CCIR), now              RTTY 45                  PACTOR FEC & ARQ                    FSK
called the International Telecommunication Union (ITU), has
                                                                                             PSK31                              BPSK
listed various channel conditions for the HF band. This
document, CCIR Recommendations 520-1, defines values for                                     RTTY                                FSK
frequency spread and delay times [6]. These recommendations
include the commonly used CCIR Poor, Moderate, and Good
channel conditions. The ITU-R F.1487 [7] adds some
additional models based on latitude. It has conditions for low,
mid, and high latitudes, including three sub-conditions for
each location.

                     IV. METHODOLOGY
   The modes used for the tests were chosen according to the
modes the selected HF digital modem was capable of
transmitting and receiving. The tested modes are shown in
Table I. Note that the only modes considered are of the
broadcast type; in other words, those not requiring
acknowledgement from the receiving end since this cannot be
simulated. The goal of the tests was to determine suitability of
the modes to be used in an interstation communication system
in the Antarctic. Relative comparisons were made so as to          Fig. 1 Block diagram of components used to create one of three possible
make a recommendation, and compare their performance               simulation paths.
under certain conditions.
                                                                                                 TABLE II
   A message containing 330 characters, including spaces, was                      SELECTED CHANNEL CONDITION PARAMETERS
used for transmission. It contained a pangram repeated six
                                                                     Condition                          Differential       Doppler Spread
times. This message was generated using software called
                                                                                                        Delay (ms)             (Hz)
MultiPSK [8], which is used for digital mode operation and
                                                                     CCIR Good                              0.5                 0.1
creates corresponding audio tones for each mode. Each
message was then recorded in a wave audio file format. This          CCIR Moderate                           1.0                   0.5
audio file was then imported on the path simulator software
                                                                     CCIR Poor                               2.0                   0.5
[9] that did the work of simulating the ionosphere using the
Watterson model. Fig. 1 [10] shows the overall simulator data        High-Latitude Moderate                   3                    10
path. The parameters of the simulator were adjusted according
                                                                     High-Latitude Disturbed                  7                    30
to CCIR 520-1 and ITU-R F.1487 channel conditions. Table II
shows the selected conditions’ parameters. The end result of
the path simulator was played back onto another computer                               V. SIMULATION AND RESULTS
where MultiPSK decoded the message. The message received              Fig. 2 illustrates how the modes performed according to
was compared with the original, and the character error rate       different SNR conditions. All SNR values refer to a 3 kHz
(CER) calculated.                                                  BW. The graph depicts the modes’ behavior as they go from 0
   In some modes it was possible to change the values of the       to a 100 percent of CER as the SNR was decreased. PACTOR
baud rate and the frequency shift between tones. In these          FEC and ASCII 110 were found to be the most sensitive
cases, baud rates were adjusted according to the standard          modes. Meanwhile, PSK31 outperformed all the other modes,
conventions used for amateur radio operation; a 170-Hz shift       being able to get a 1.69% of CER at a SNR of -12dB. This is
was used.
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mainly because narrower bandwidth modes tend to be the best
performers at low SNRs.
   For CCIR Good through Poor conditions, the condition
name of the channel can create confusion because it was found
that, as the conditions decayed, some modes performed better
(Fig. 3 to 5). PSK31 didn’t seem to be particularly affected by
the different channel conditions as it had similar CER among
tests. RTTY 45 was observed to perform almost similarly,
with low CER, as the SNR conditions got better, being the
only two modes that consistently got good results with
approximately two percent of CER on moderate SNR
   When high latitude channel, quiet, and moderate conditions
were tested only PACTOR FEC and RTTY 45 had a CER                             Fig. 4 CCIR good conditions on character error rate (CER).
below four percent. Figs. 6 and 7 exhibit how the other modes
performed poorly even with a high SNR value. In disturbed
conditions only RTTY 45 demonstrated moderate
performance if the SNR was high; the remaining modes had a
100 % of CER.
   PSK31 performed very well under noisy channel
conditions. On the other hand, it started showing complete loss
of signal when Doppler spread was increased, which is due to
the nature of its modulation scheme. RTTY 45 is the other
protocol that showed overall good results, but it was found to
be sensitive to the noise present in the channel.
   Redundancy proved to be crucial when transmitting a
message. This ensures the message arrival at least most of the
time, especially when synchronization is lost for a brief                     Fig. 5 CCIR poor conditions on character error rate (CER).

Fig. 2 Character error rate (CER) of various digital modes as a function of
signal-to-noise ratio in AWGN conditions                                      Fig. 6 High-latitude moderate conditions on character error rate (CER).

Fig. 3 CCIR good conditions on character error rate (CER).
                                                                              Fig. 7 High-latitude disturbed conditions on character error rate (CER).

                          VI. CONCLUSION
   A recommendation for the use of a modern digital mode to
transmit textual information over the HF band was done. Tests
performed on an HF channel simulator were used to choose
the correct digital mode according to that which provided the
most reliable text transmission under different channel
   Based on the simulation results and analyses, the RTTY 45
digital mode was selected as the recommendation for the
interstation communications system for a network of
geophysical measurement stations in the Antarctic. This
digital mode was chosen because of its good performance on
all tests, provided that there is a strong enough signal.
   This paper was based on using MultiPSK for generating and
receiving the messages. Other tests could be done using actual
modems to compare the result with the simulations. Also,
different baud rates should be considered and tested to prove
the assumption that a slower baud rate will increase its

  The author thanks R. Nealy, Dr. R. Clauer, Dr. J. Baker, and
Dr. H. Vo. The author also extends his gratitude to Dr. T.
Bose, director of the Wireless@VT REU program.

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