A SOFTWARE -DEFINED RADIO IONOSPHERIC CHIRPSOUNDER FOR HF [608003]

A SOFTWARE -DEFINED RADIO IONOSPHERIC CHIRPSOUNDER FOR HF
PROPAGATION ANALYSIS
Pradeep B. Nagaraju*, Eric Koski¥, Tommaso Melodia*
*Department of Electrical Engineering, State University of New York at Buffalo, NY, USA
¥Harris Corporation, RF Communications Division, Rochester, NY, USA
SUMMARY
Advanced users of high frequency (HF) radio communications have long used systems known
as chirpsounders1 to obtain real -time information about the ionospheric propagation
conditions of different communication channels between a transmitter and a receiver.
Traditional chirpsounders are expensive to realize and lack the flexibility of state -of-the art
digital radio platforms. However, the advent of Software Defined Radio (SDR) technology
has created opportunities to prov ide a chirpsounder capability in a flexible, low -cost form. In
this paper, we report on our development of a prototype Software Defined Radio (SDR)
chirpsounder system based on a commercially -available SDR platform. The accurate real –
time picture of ionosp heric propagation provided at low cost by such a chirpsounder can be
instrumental in providing HF communications users with self -configuring link optimization
capabilities that dynamically select the best channels to be used in an HF link, to maximize
comm unications capacity and reliability.
1 INTRODUCTION
Services used by military, marine , aviation, and amateur radio users rely on HF signals that
propagate through the ionosphere over long distances. The ionospheric layers are subject to
variations caused b y factors like absence or presence of sunlight, seasons, sunspot cycle, solar
activity, polar aurora , among others . All these factors make ionospheric propagation
stochastic in nature and variable over relatively short time scales. For this reason, ionosph eric
chirpsounders have been traditionally used to provide an accurate estimate of the ionospheric
propagation characteristics. Chirpsounders are linearly varying frequency (LVF) modulators
that vary between 2.0 MHz to 30.0 MHz, designed to estimate the ca pacity and reliability of
high frequency (HF) links over different channels. A chirpsounder is one kind of ionosonde ,
a device for measuring the characteristics of the ionosphere by transmitting a sounding signal
and analyzing the signal returned to earth as a result of ionospheric refraction.
This paper describe s our experience in developing what is, to the best of our knowledge, the
first software -defined HF chirpsounder developed on a currently -off-the-shelf radio platform.
The chirpsounder is realized using the Flex -radio 5000A [1] (F5k) software defined radio [2]
(SDR) along with the powerSDR open source software platform. Compared to traditional
chirpsounders, the proposed SDR -based chirpsounder provides an inexpensive and
reconfigurable solution for automatic frequency management. The chirpsounder system
consists of a transmitter and a receiver. The transmitter transmits a 100W swept CW tone [11]
across much o f the entire HF spectrum, traversing the spectrum upward in frequency at 100
kHz per second, so that a complete sweep of the 2.0 MHz to 30.0 MHz HF spectrum takes
280 seconds to complete.

1 This work was supported by Har ris Corporation.

The receiver is time -synchronized to the transmitted chirp and tracks and analyzes it so as to
produce an ionogram, i.e., a chart of the received signal streng th as a function of frequency
and latency. By inspecting the ionogram, experienced users can quickly identify the
propagating layers of the ionosphere and their critical frequencies, i.e., the maximum
frequencies above which transmitted signals are not ref racted back to the earth. The
chirpsounder can be coupled with specialized communication software to generate an
automated frequency management system that adaptively controls the operation of the HF
communication link so as to dynamically allocate reliabl e channels for HF signals to
propagate. The proposed software defined chirpsounder can flexibly vary the sweep rate,
receive different chirp sounder formats used in some of the traditional chirpsounder systems,
enable the user to choose different chirp sou nding formats for transmission, manage the
forbidden frequencies and power as per the FCC standards and provide portability on
different platforms. To test the chirp sounders for various transmission formats in the indoor
milieu, we incorporate Wattersons [3] ionospheric channel modeling at the transmitter with
transmit power used less than 500 mW. The user can also incorporate and use alternative
channel models. All these functionalities are selectable at run time at user discretion. Finally,
we discuss te chniques to implement chirpsounders on software defined radio both for indoor
testing and field deployment. Extensive experimental performance evaluation results will be
reported to assess the performance of the software -defined chirpsounder.
The rest of t his paper is structured as follows. In Section 2, we review related work. In
Section 3, we discuss the software and hardware architecture of the proposed chirpsounder. In
Section 4, we discuss details of the chirpsounder transmitter and receiver .
2 RELATE D WORK
Barry Research (BR), now TCI, was th e first organization to develop commercially -available
chirpsounder s. TCI/BR chirpsounders use traditional analog techniques to realize a
chirpsounder. For this reason, the TCI/BR -designed chirpsounders cannot be modularized and
extended , and cannot be incorporated in to automated HF communication systems where
frequency management is the key for achieving high quality link s over the ionosphere.
Moreover , these chirpsounders are very expensive and no longer commerc ially available .
The Radio Oblique Sounding Equipment [4] (ROSE) 100/200/300 is another series of
commercial ionosonde . They use modern digital signal processing techniques to realize
ionosondes with less hardware dependency. ROSE ionosondes produce ionogr ams with
higher frequency resolution than those of TCI/BR. However, the hardware is specifically
designed for implementing the ionosonde . This makes ROSE to be unsuitable for extending
into automated HF communication systems.
Digisondes [5], developed at t he U Mass Lowell Centre for Atmospheric Research, are mai nly
used as vertical sounders. Vertical sounders do not propagate along the ionosphere . They are
used to determine the characteristics of the ionosphere at a specific geographic location rather
than a long a signal transmission path of interest. Hence, it is difficult to predict the
ionospheric propagation characteristics. Digisondes are characterized by high power
consumption since they use pulsed sounding techniques.
To provide a low-power and inexpe nsive solution, a software -defined -radio -based
chirpsounder is proposed in this paper. Due to rapid prototyping and real -time adaptation

capabil ities, the F5k based chirpsounder can be easily extended to other high frequency
ionospheric commun ication appli cations.
3 SOFTWARE DEFINED CHI RPSOUNDER
A software -defined radio is used to realize the chirpsounder. A generic SDR platform such a s
USRP2 (universal software radio peripheral 2) by Ettus Research [6] coupled with the GNU
Radio signal processing tool is a flexible platform to implement wireless communication
systems in software over a wide range of frequencies. However, chirpsounders are used to
analyse ionospheric communication channels in the frequency range between 2.0 MHz to 30.0
Mhz. There are frequen cy bands within 2.0 MHz and 30.0 MHz that are used for
broadcasting, emergency, amateur communications, and defence communications. According
to the tactical performance requirements of MIL -STD -188-141B [15], the harmonic
suppression should be -40dBc or be tter. To use generic SDRs, a power amplifier with filters
must be used to achieve the required harmonic suppression. At the receiver, an adequate
dynamic range for selectivity is required. This ensures that the receiver does not lose
sensitivity in the pre sence of strong out -of-band signals. Tactical desensitization should be
90dB according to the specifications of MIL -STD -188-141B. This requires an automatic gain
control (AGC) (with a good trade -off between speed and performance) to get the RF signals
to the A/D (analog -to-digital) converter within the dynamic range limits of the receiver. To
achieve these performance levels with a general -purpose system made up of off -the-shelf
components would be complex and expensive. The F5k is designed to provide a co st-
effective RF ‘front -end’ for an HF radio system meeting stringent RF performance
requirements such as those prescribed by the MIL -STD -188-141B standards. Thereby, we
have chosen to use F5k as the software defined radio platform for realizing the chirpso under.

Figure 1. Chirpsounder Architecture.
3.1 SOFTWARE DEFINED CHI RPSOUNDER ARCHITECTU RE
The overall architecture of the software -defined chirpsounder is shown in Fig.1. There are
two main components in the entire architecture, the host and the F5k.

3.1.1 Host
In the current chirpsounder implementation, the host is a 3 GHz Intel core2 -duo processor
with 6MB L2 cache and 4GB RAM. The host is responsible for all signal processing
activities, controlling the F5k and transceiving th e In-phase (I) and Quadrature (Q) samples to
and from F5k.
Figure 1 shows the software functionalities performed on the host. The controller is the
manager of the chirpsounder application on the host. It manages the PowerSDR [1] firmware ,
which is des igned to access the F5k for controlling it and exchanging the I /Q samples with it.
The DSP logic core comprises the signal processing logic of the chirpsounder transmitter an d
receiver . The samples that are exchanged with the F5k are processed within the DSP lo gic
core, as explained in detail in Section 4.

Figure 2. Host Architecture .
A graphical user interface (GUI) is managed by the controller to provide users with access to
the DSP logic core and the F5k. The chirpsounder transmitt er and receiver may be deployed
across the globe. Global positioning system (GPS) is use d by the chirpsounder appli cation to
synchronize both tran smission and reception of the chirpsounder. Both 1 pps and the National
Marine Electronics Association (NMEA) messages of the GPS are used to synchronize the
sweep time of both transmitter and the receiver. GPS time synchronization with the
chirpsounder application is also managed by the controller.
A modular approach , as shown in Fig . 2, is very important to test , debug and add new
functionalities to the chirpsounder without having to change the whole structure of the
application.

3.1.2 The F5k Radio
As shown in Fig. 1, the F5k consists of a transmitter block and a receiver block with full
duplex capabilities. Th ere are two Direct Digital Synthesizers (DDS) to tune the operating
frequency of the F5k. Each DDS has a resolution of 1 Hz with a total tunable bandwidth of
500 MHz. The F5k includes an antenna tunable unit (ATU) for coupling the antenna without
loss for varying frequencies. The chirpsounder bypasses this ATU, since tuning the ATU
might take significant time and the chirpsounder may not be able to sweep at the desired rate.
However, a broadband antenna is used to couple with the F5k to minimize the losses due to
antenna load mismatch and thereby achieve the desired sweep rates. The front end filters of
the F5k are designed to suppress the harmonics to match the tactical performance
requirements of MIL -STD -188-141B.
The F5k uses a firewire interface to both control the hardware and exchange I /Q samples
between the host and itself. However , the techniques adopted to address these two
function alitie s are different as explained in Section 3.2 . The F5k uses the Digital Interface
Communication Engine ( DICE ) contro ller chip from TC technologies [16] to provide firewire
interface to the host . DICE firewire chip communicates to the host through a physical
abstraction layer (PAL) firmware as explained in Section 3.2.1 . However, Flex -Radio has
wrapped the PAL firmware with F5k specific functionalities . Hence, the F5k hardware
functions are controlled through this Flex-Radio -DICE -PAL firmware .

Figure 3. Firmware Diagram.
3.2 F5K FIRMWARE
The powerSDR library provided by Flex Radio includes hardw are control functions like
adjusting the DDS for frequency, enabling the QSE/QSD [7] (Quadrature sampling
exciter/detector), enabling the DAC/ADC [8], and exchang ing data samples between the host

and the F5k. As a result of our exploration of F5k and power SDR, we have been able to
reconstruct a block diagram of the existing modular hierarch ies in the F5K firmware.
The block diagram shown in Fig. 3 describes the relationship between the Chirpsounder and
the hardware, and the different software modules that m akes it possible to access the radio
hardware. As discussed before, t he F5k interfaces to the host through a firewire connection.
The Flex-Radio driver is an interface between the computer and the hardware through
firewire. The application that needs acces s to the F5k interacts with the F lex-Radio driver in
two different ways : control the hardware functions through the F5k physical layer abstraction
(PAL) firmware and exchange the I/Q samples through the audio driver s on the host .
3.2.1 Hardware control
The basic hardware controls such as setting up the transceiver, hardware filters, frequency
tuning, enabling ADC/DAC, full duplexing, and accessing EEPROM are achieved through
the platform abstraction layer (PAL). The F5k also uses a musical instrument digital interface
(MIDI) interface for hardware control, but only for a limited set of functions such as manual
tuning.
3.2.2 Transceiving the digital samples
The IQ [9] samples are exchanged between the computer and the F5k through three different
interfaces. T he Chirpsounder interacts with the port audio for sending/receiving the samples ,
since port audio manages samples between various audio drivers within the computer. Unlike
the windows audio driver , the a udio streaming input output (ASIO) bypasses many laye rs of
the windows operating system to access the F5k. This helps in achieving high data rates
between the host computer and the F5k. Hence, the ASIO driver provided by Flex Radio is
configured to access F5k as its hardware device. ASIO interacts with the F lex Radio driver to
access the samples to/from the F5k. Thus, the Chirpsounder interacts with the ASIO to
send/receive from the F5k through port audio.
4 CHIRPSOUNDER DESIGN
This section represents the DSP core logic of the chirpsounder application on the host, and
illustrates t he design of the transmitter and receiver.
4.1 TRANSMITTER
The chirp signal is a LVF signal as shown in Fig. 4. There are at least two possible ways to
achieve the chirp signal at the transmitter and the receiver. The F5k radio uses a DDS to tune
the frequency of the carrier wave. The DDS can be tuned in steps of 1 Hz up to 500 Mhz with
a speed of 250 MHz/s. The DDS tuning is phase continuous , and hence it can be used in high –
speed varying -frequency applications like the chirpsounder. However, our experimental
evaluation showed that, with the current F5K firmware, the DDS can be retuned at most 55
times per second. This is attributed to the fact that Flex -Radio wrapped PAL code has a
overhead in accessing the DDS as fast as 250MHz/s. H ence, only retuning the DDS is not
enough to realize a chirpsounder sweeping at a rate of 100 kHz/s.

Figure 4. Chirp Signal .
The second alternative is to generate the LVF samples on the host and then send these across
the firewi re interface to the F5k. However, the front -end DAC of the F5k can process only
192000 samples/s [1]. This limits the bandwidth of the F5k to a maximum of 96 kHz. Since
the DDS can be retuned 55 times every second, in our design it is retuned in steps of 9 6 kHz
every 0.960 seconds to achieve a 100 kHz/s sweep rate. Figure 5 shows the spectrum that is
plotted as frequency vs time. Chirp samples are obtained for sweep rates of 100 kHz/s with a
sampling rate of 192000 samples/s. It can be noticed that the chir p signal runs out of samples
beyond 96 kHz. Hence, we need to tune the DDS every 0.960s in steps of 96 kHz. For
sweeping at higher rates than 100 kHz/s, the DDS must be re -tuned in less than 0.960s. The

Figure 5. Sweep Spectrum .
upper limit of DDS retune rate depends on the computing platform. To determine the upper
limit of the DDS retuning performance, a platform performance analysis routine is
implemented at the start -up of the chirpsounder program. This includes tuning the ha rdware
1000 times and calculating the time taken for each retune.

4.2 RECEIVER
Figure 6 shows the receiver operations. The samples from the F5k are received by the host via
firewire using the fi rmware, as explained in Section 3. The received digital sample s are time
stamped before they ar e processed. Since the constant -rate sweep tone is known at the
receiver, a matched filter technique is implemented to detect the received swept tone. The
matched filter produces peaks at locations where the correlation bet ween the received
sampled and the swept frequency sample at the receiver is maximized. By performing a
maximum likelihood ratio, the samples whose peaks are at places with maximum correlation
are extracted. An FFT is performed on these samples to determine the frequency of the
received swept samples. FFTW (FFT wisdom), an optimized C/C++ library, is used to
perform the FFT. For each of those frequencies received, we calculate its signal strength and
time lag. The time lag is a measure of the time elapsed wh en the receiver has swept that
frequency and the same frequency component is received by the receiver. Using this
information, a plot of the signal strength as a function of propagation delay and frequency ,
known as an ionogram , is plotted. This ionogram p rovides accurate information on the effects
of the ionosphere on various frequencies that are swept between 2.0MHz and 30.0MHz.

Figure 6. Receiver Operation .

4.3 TESTING THE CHIRPSOU NDER
The chirpsounder design is made testable by including the ionospheric channel models. To
test the chirpsounder under indoor conditions, the baseband samples at the transmitter are
applied to the Watterson ionospheric channel model [3] [14] . The Watterson model represents
the simulated Doppler and multipath spreads. The main assumption in the Watterson model is

that the HF channel is non -stationary in both frequency and time for bandwidths less than 10
kHz and for time less than 10 minutes. The output power of the transmitter is kept less than
500m W. The final version of this paper will include plots evaluating the performance of the
overall chirpsounder design .
5 CONCLUSION S AND FUTURE WORK
In this paper, the design of an ionospheric chirpsounder based on a software defined radio
was discussed an d illustrated. The F5k radio is used to implement the chirpsounder because of
its very rigid front end which strictly adheres to the regulations in using the frequency
channels between 2.0MHz to 30.0MHz. A modular architecture is illustrated to implement t he
chirpsounder with GPS interface to synchronize the transmitter and the receiver. The design
of the chirpsounder transmitter and receiver architecture was also briefly discussed.

Figure 7. Interface Diagram .
In the future , we plan to expose the chirpsounder functions using the Common Object
Request Broker Architecture [10] (CORBA) interface as shown in Fig . 7. This adds to the
capabilities of the chirpsounder to be extended into a n automatic link establishment (ALE)
[12], autom ated HF communication , cognitive networking [13] in high frequency
communication.
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