intro to hf ch_mod

27. Characterization and Modeling of the HF Communications Channel

Paul S. Cannon1,2, Matthew J. Angling1, and Bengt Lundborg3

1Centre for Propagation and Atmospheric Research, QinetiQ, Malvern, WR14 3PS, UK

Tel: +44-(0)1684-896468; Fax: +44-(0)1684-895341; E-mail: [email protected]

2also at Dept of Electronic and Electrical Engineering, University of Bath, Bath, BA2 7AY, UK

3Swedish Defence Research Agency (FOI), PO Box 1165, SE-581 11 Linköping, Sweden 1. ABSTRACT High frequency (HF) skywave-communication systems may exhibit low signal-to-noise (SNR) ratios, and may be subject to slow fading at mid-latitudes and fast fading at high and equatorial latitudes. Furthermore, for modern systems, the radio channel is almost always frequency selective. Until 10 years ago, it was common for the HF user to expect data rates of only ~75 bit/s and low availabilities. However, with the advent of digital signal processing, data rates have increased significantly to 2400 bit/s, 4800 bit/s, and beyond, even in the standard 3 kHz channel allocation. Furthermore, such is the progress that commercial digital HF broadcasting is now planned to replace conventional analog transmissions. In order to support these initiatives, it has been necessary to model, measure, and characterize the HF communications channel to determine the details of its multipath and Doppler characteristics. This paper reviews recent contributions that have been made to this subject, and outlines the areas requiring future research. 2. INTRODUCTION For many years, analog amplitude-modulated (AM) low-frequency (LF), medium-frequency (MF), and high-frequency (HF) radio systems provided the backbone of national and international broadcasting. In more recent years, these services have become marginalized, first by more sophisticated analog services, such as frequency-modulated (FM) radio broadcasts and direct-broadcast satellite television, and more lately by digitally modulated transmissions. There is now a general user expectation for high-quality monophonic and stereophonic sound with high availability – a service which analog LF, MF, and HF services cannot deliver. As a consequence, the International Telecommunications Union (ITU) initiated, via ITU-R Question 217-1/10 and services requirements document ITU-R BS.1348, a process that will lead to a world-wide standard for digital broadcasting in these frequency bands. The standard (ITU-R BS.1514) aims to provide listeners with improved quality and consistency of service, whilst providing the option of transmitting additional services. Broadcasters and network operators foresee the introduction of a digital system that will allow the

597

598 Paul S. Cannon, Matthew J. Angling, and Bengt Lundborg

continued use of many of their existing transmitters and that will provide lower operating costs. The intention is to produce a system that will reliably support transmission of approximately 2 bits/Hz in a 10 kHz channel. In addition to this revolution in LF, MF, and HF broadcasting, the last few years have seen a similar change in the requirements for point-to-point communications at HF. Until 10 years ago, it was common for HF users to expect data rates of little more than 75 bit/s and low availabilities. However, with the advent of cheap digital signal processing in the last few years, data rates have increased significantly to 2.4 kbps, 4.8 kbps, and beyond in a 3 kHz channel. Data rates on benign skywave channels and on groundwave paths can now reach 64 kbps, albeit sometimes using wider channel bandwidths, and there is an increasing desire to reliably achieve 16 kbps. The performance of skywave HF communication and broadcast systems is dependent on how well the system design is able to compensate for the propagation channel. For example, an appropriate choice of antenna gain and transmitter power can compensate for free-space losses and for excess attenuation in the D region. Of equal importance, however, and especially pertinent to the design of digital systems, is an appropriate choice of the signaling waveform, since it can, to a greater or lesser extent, compensate for radio-channel distortion. (In this context, the term signaling waveform includes the modulation type, error-correction code, interleaving, and other related issues.) In essence, the waveform aims to compensate for multipath and Doppler effects that can compromise the signal integrity. Multipath propagation arises because replicas of the transmitted signal arrive at the receiver after reflection from more than one ionospheric layer, and/or after multiple reflections between the ionosphere and the ground. Each signal (or propagation mode) generally arrives with a different time delay, causing either constructive or destructive interference, which, when viewed in the frequency domain, dictates the coherency bandwidth of the channel [e.g. Proakis, 1989]. Multipath is, of course, a worldwide phenomenon. However, at high and equatorial latitudes, the situation is further complicated because each received “mode” may exhibit time spreading or smearing. Such effects are generally associated with spread F, seen on vertical-looking ionosondes [e.g. Wright et al., 1996], and largely result from scattering of the signal from a distributed volume. Signal dispersion is usually insignificant, unless the bistatic link is operating close to the junction-frequency Frequency (Doppler) shifts and frequency-spread distortion [e.g. Basler et al., 1988; Flaherty et al., 1996] can be imposed on the transmitted signal by the temporal variability of the ionosphere, and this defines the coherency time of the channel. At high and equatorial latitudes, Doppler shifts and spreads of many Hertz are very common, and these are also often associated with spread returns, due to ionospheric irregularities, on vertical ionograms [e.g. Röttger, 1976; Röttger, 1979]. The effects of random irregularities on HF propagation has been discussed by a number of authors [Booker et al., 1987; Fridman et al., 1995; Wagen and Yeh, 1989a; Wagen and Yeh, 1989b; Zemov et al., 1992]. At mid-latitudes, the ionospheric variations are slow and are generally related to normal diurnal processes, together with the effects of traveling ionospheric disturbances (TIDs). Consequently, Doppler shifts of much less than a Hertz are most common, but still challenging for the communications-system designer, since slow fading can result in long periods of low signal strength. The complex ionospheric radio-propagation channel is a considerable challenge for the designers of new digital ionospheric-radio systems, and their ultimate success depends critically on a good understanding of the radio-channel multipath and Doppler characteristics. This review

27. Characterization and Modeling of the HF Communications Channel 599

will address recent measurements and theoretical progress in this sphere in respect to communication systems. Recent research has focused on the HF channel and, consequently, our review will likewise address this frequency band; the many common issues with the LF and MF channels will, however, be apparent. 3. A SIMPLE HF CHANNEL MODEL: THE WATTERSON MODEL Models of the signal distortions caused by the ionosphere are important because they facilitate the design and testing of new modems and radio receivers via their incorporation in channel simulators. Given an input signal, , the output signal, , after passage through a radio channel with an impulse response that is described by , is often represented by

( )x t ( )y t( , )h t τ

, (1)

( ) ( ) ( ) ( )* ,y t x t h t n tτ= +

)b

where t is the time variable, is the delay variable, n t is a noise signal, and * indicates convolution. Therefore, if the time-varying channel impulse response is available (either measured or simulated), it may be convolved with the input signal to provide the channel-modulated output signal. This process may be conveniently implemented as a tapped delay line, with time-varying tap gain functions.

τ ( )

The Watterson model has been the standard representation of the HF channel used in simulators for many years [Watterson et al., 1970]. This model was originally validated with just 36 minutes of mid-latitude data, taken on a 1300 km path during quiet ionospheric conditions on a single day in November, 1967. The model assumes that the channel fading is described by a Rayleigh amplitude distribution, and that the Doppler spread on each propagation mode has a Gaussian power spectrum. The model does not define a shape for delay spread, and most practical implementations assume that each mode exhibits no delay spread. The Watterson model considers the channel as an ideal tapped delay line, where at each tap, the delayed signal is modulated by a tap gain function, . In general, each tap gain function is defined by

( )iG t

. (2) ( ) ( ) ( ) ( ) (exp 2 exp 2i ia ia ib iG t G t j t G t j tπν πν= + The subscripts a and b indicate the two possible magnetoionic components, the exponentials allow Doppler shifts to be added to the signal, and the tildes indicate that the G terms are sampled functions of two independent complex Gaussian ergodic random processes, each with zero mean values, and independent real and imaginary components with equal RMS values. Such complex number sequences exhibit Rayleigh fading. The tap gain functions are also filtered, to produce a Gaussian Doppler spread in the signal’s power spectrum. Generally, one tap is used for each propagation mode. In practice, only a single magnetoionic component is simulated, and a limited number of propagation modes (taps) are allowed (generally four or five). Furthermore, the facility to include a specular mode is usually provided, which can be used to simulate a line-of-sight or surface wave.


4. MORE-COMPLICATED CHANNEL MODELS 4.1 INTRODUCTION The Watterson model was long considered too simple for applications pertaining to the high and equatorial ionosphere: for example, it is well known that at high latitudes, the Doppler spectrum is often not Gaussian [e.g., Wagner and Goldstein, 1995]. To overcome these limitations, a more-complicated ionospheric channel model has been proposed by the Institute of Telecommunication Sciences (ITS) in Boulder, Colorado, USA [Mastrangelo et al., 1997]. This was proposed as a wideband model, but can be used as a narrowband model.

Deterministicphase function

Delaypowerprofile

Doppler spread Doppler shift Delay spread

Time series ofCompleximpulse

responsesfor single mode

Channel parameters(measured or modeled)

R

R

R

Rayleightime seriesgenerators

DopplerFilters

Stochasticmodulating

function ......

...

Del

ay, τ

Time, t

......

...

......

...

Figure 1. A block diagram of impulse-response reconstruction [Angling et al., 2002]. In the ITS model, the overall channel impulse response is defined as a sum of the impulse responses of each propagation mode, and is a function of time, t, and delay, (Equation 3). The ITS model represents the impulse response of each mode as the product of three terms: a stochastic modulating function,

τ

( , )n tψ τ , which is defined by the Doppler spread and the spectral shape; a deterministic phase function, , defined by the Doppler shift and the rate of change of the Doppler shift with respect to delay; and the square root of a delay power profile,

, defined by the propagation mode’s time of flight, delay spread, and maximum power (Equation 4):

( , )nD t τ

( , )nP t τ

, (3) ( ) (, n

nh t h tτ = ∑ ),τ

and


( ) ( ) ( ) (, ,n n nn

h t P D t tτ τ τ ψ=∑ ),τ , (4)

where the summation n is over the number of modes. Each of these terms is examined in the following sections by reference to the implementation by Angling et al. [2002], a block diagram of which is shown in Figure 1. 4.2 STOCHASTIC MODULATION FUNCTION In order to model the fading of the impulse responses, the stochastic modulating function (SMF), ( , )tψ τ , is constructed from an ensemble of time series of random complex numbers. At each delay offset, two independent random-number sequences are constructed, representing the real and imaginary parts of the complex time series. Each is independent, white, and exhibits a Gaussian amplitude distribution. The corresponding complex random-number sequences have an amplitude distribution that conforms to Rayleigh statistics. To constrain the width of the random-number power spectrum – to simulate the required Doppler spread – the random-number sequence is convolved with a filter (Gaussian or Lorentzian), with a power spectrum width corresponding to the Doppler spread (set by the user) of the mode. Independent sequences are produced at each delay offset, thus producing a set of sequences that are uncorrelated in delay but that share the same fading characteristics. 4.3 DETERMINISTIC PHASE FUNCTION The Doppler shift of the mode is implemented by multiplying the stochastic modulating function by a deterministic phase function (DPF): , (5) ( ) ( )[ tmfi csetD ττπτ −+= 2, ]

where t is the time variable, is the delay variable, is the delay of the mode’s peak, m is the rate of change of Doppler shift with respect to τ, and is the Doppler shift at . This function allows for the inclusion of slant modes, i.e., modes with a Doppler shift that varies with delay.

τ cτ

sf cτ τ=

4.4 DELAY POWER PROFILE The delay power profile (DPP) is determined by the shape of the mode in the delay dimension, and is defined by the position (in delay) of its peak, , and the positions of two points of intersection with a threshold, and (Figure 2). The profile is defined by

Cτ

Lτ Uτ

, (6) ( ) ( )ln 1z zP Ae

ατ

+ −= where


, (7) ( ) ( ) 0/ >−−= LCLz ττττ and A is the peak power of the mode. The parameters and control the width and symmetry of the profile. They depend upon the delay spread and the threshold level, and are calculated by an iterative method [Vogler and Hoffmeyer, 1993].

α Lτ

DELAY

POW

ER

A

Ath

τL τc τu

Figure 2. The delay power profile [after Vogler and Hoffmeyer, 1993]. 4.5 OTHER SIMULATOR MODELS The ITS model has been described in some detail because of its wide interest for both research and within the ITU. However, a number of other researchers have studied the design of HF simulators. Lacaze [1998] proposed a model based on the Watterson model, but with random delays modeled by a Gaussian distribution. Milsom [2000] has proposed a simulator architecture that incorporates a model of the group-time-delay slope that will introduce phase distortion across the operating band. The suggested approach uses a filter with a quadratic phase response and a flat amplitude response across the band of interest. This was an extension of an implementation by Van der Perre et al. [1997], who proposed to split the operating band into sub-bands, each with a different delay. LeRoux et al. [2000a] have considered the issue of simulating the HF transmission channel when there are multiple antennas at one or more terminals. Salous and Bertel [2000] have also considered the architecture of wideband models. 5. SETTING SIMULATOR CHARACTERISTICS A limitation of most ionospheric HF simulators, such as those described above, is the absence of an associated model for multipath and Doppler occurrence statistics. Worldwide characterization of multipath has long been carried out, and is embodied within HF-prediction models such as described in ITU-R P.533.6 [1999]. Even so, the detailed characterization of multipath spread and variability, which is required by the radio-waveform designer, was not


available until quite recently (see below), particularly at high and equatorial latitudes. Poorer still is the characterization of global Doppler effects. Simulator testing can be conducted in two ways. A limited number of simulator tests may be devised, which, it is hoped, cover the most probable multipath and Doppler conditions. Appropriate test conditions are given in ITU-R F.1487 [2000]. These are based on various experimental measurements and theoretical ray-tracing studies. Using this strategy, a number of fixed condition tests are carried out. Alternatively, the simulator may be dynamically controlled by measurements previously made on representative ionospheric paths. This is known as a replay simulator, and it can be used to simulate varying ionospheric conditions, such as those that occur over a complete twenty-four hour period. Both methods have their problems. A limited number of simulator tests may not cover all eventualities, and the weight to be given to each test result may not be known. Conversely, a replay simulator must have data sets that are representative of a sufficient number of different paths so that any deployment may be tested. 6. THEORETICAL MODELING OF THE HF CHANNEL 6.1 INTRODUCTION The ideal HF-channel simulator would probably be one that – starting from a sufficiently detailed description of the ionospheric state and its evolution – solves the full wave-propagation problem in the medium. Wave propagation through the ionosphere is generally dealt with in the frequency domain, and is governed by the wave equation: , (8) ( )2 2 2 , , , 0E k n t Eω ω∇ + =r k ω

)

where n is the complex refractive index of one of the magnetoionic modes. The refractive index is a function of field-point position, as well as of wave frequency and direction. When the ionosphere changes with time, n may also be slowly time dependent. Simulation work usually involves the channel impulse response; cf. Equation (1). The monochromatic field, , resulting from Equation (8) can be identified with the time-variant channel transfer function at the fixed reception point. Hence, the impulse response, , is the Fourier transform of , with t expressing the temporal change of the channel, and delay, , being the Fourier conjugate of frequency, [Bello, 1963].

ωE

ωE( ,h t τ

τω

Unfortunately, the wave Equation (8), in most practical cases, must be treated by approximate methods. Some recent developments in such methods are presented in the following sections. 6.2 RAY TRACING Ray tracing – or geometrical optics – has a very long record in radio-wave propagation. In this technique, the field is described by the (real) amplitude, A, and the phase, , according to ϕ


. (9) ( ) ( ) ( )expE A ikω ϕ= −r r r Separate approximate equations for phase and amplitude are obtained by inserting Equation (9) into Equation (8) and expanding the amplitude in an asymptotic series in k (as ), keeping only the first term. The rays arise as solutions to another set of new equations, the ray equations, as part of the process of solving the equation for (the eikonal equation). They are trajectories describing the direction of energy transport of the electromagnetic waves. In a further step, the field amplitude may be computed from the ray-tube divergence. Ray tracing is a shortwave technique, valid when the medium changes slowly over the wavelength scale as well as the Fresnel-zone scale.

∞→k

ϕ

One class of ray-tracing models deals with idealized profiles of ionospheric electron density that permit analytic solution of the ray and eikonal equations. Those models always simplify the refractive-index expression by omitting the geomagnetic field. In general, losses due to electron collisions are also neglected. One such two-dimensional model with high flexibility has been published under the name SMART [Norman and Cannon, 1997]. Very general ionospheric profiles can be modeled in SMART by building them as a sequence of quasi-parabolic segments. Also, horizontal gradients in the plane of propagation can be incorporated. The major advantage of the analytic models is their high numerical speed.

Figure 3. The fading of a wideband frequency-selective channel, modeled with the ray-tracing tool RaTS [Västberg and Lundborg, 1997]. The fading pattern is a result of interference between the ordinary and extraordinary modes when the ionosphere is under the influence of a TID. When three-dimensional ionospheric structure (like traveling ionospheric disturbances, “TIDs,” and troughs) or geomagnetic effects (like magnetoionic splitting and polarization) are important, one has to resort to fully numerical ray tracing. A complete set of ray equations in


spherical coordinates was derived by Haselgrove [1955] to deal with such general situations. A recent implementation of these equations is the so-called RaTS software [Västberg and Lundborg, 1996, 1997]. An illustration of the detailed wideband channel description possible with this kind of tool is presented in Figure 3. The picture shows the wideband fading at 4 MHz caused by interference between the ordinary and extraordinary rays when a TID is rolling through the ionosphere. To produce this result, RaTS used three-dimensional ray tracing with homing of the rays to the fixed receiver position. The phase paths, as well as the homing, were calculated over the path length of 500 km to within a fraction of a wavelength. The amplitudes of the component waves were calculated by solving a set of variational equations, derived from Haselgrove’s equations, to describe the ray-tube divergence. In addition, the absorption loss was obtained by integrating the imaginary part of the refractive index along the rays. The full polarization vectors of the ordinary and extraordinary modes were used in the superposition of the vector fields of the two modes. 6.3 FULL-WAVE METHODS The geometrical-optics approximation ceases to give a valid description of the wave field when the ionosphere contains significant density fluctuations on scales less than the Fresnel-zone size: typically, a few kilometers or less. Such fluctuations are always present in the ionosphere to some degree, and are prominent in the high-latitude as well as the equatorial ionosphere, as discussed later in this paper. Then, diffraction effects become important, and to handle these a full-wave method is called for. However, that method must, at the same time, be able to handle the regular propagation effects of dispersion and ray bending in the large-scale ionospheric background layers. One such method is a generalization of Rytov’s method, introduced by Zernov [1980]. This technique essentially starts out from the geometrical-optics solution, Equation (9), of Equation (8), extended by a new complex phase, ψ , according to . (10) ( ) ( ) ( ) ( )expE A ikω ϕ ψ= − +r r r r

ω

The field is now a solution to the following wave equation for the perturbed ionosphere: ωE , (11) ( ) ( )2 2 2 2, , , , , , 0E k n t n t Eω ω ω∇ + + = r k r k

where the refractive index fluctuations, , are used to describe the density perturbations. It is then possible to obtain a perturbation solution for

2nψ . Formally, the fluctuations must be small

for this perturbation expansion to be valid; the resulting signal-level fluctuations must be much less than unity. These criteria are not always fulfilled, especially not for disturbed conditions in the high-latitude ionosphere. A technique with a similar scope is presented in a recent paper by Kravtsov and Tinin [2000].

2~n

In Zernov et al. [1992], the wave fields were derived for a few particular deterministic

density perturbations. However, in most applications considered so far, the density fluctuations are described by the correlation function of the spatial spectrum of . In those applications, the 2~n


second-order moments of the first two terms, 1ψ and 2ψ , are used to derive various statistical characteristics of the field, such as coherence functions (two-frequency, two-position, and time) and, in particular, the scattering function: [Gherm et al., 1997a, b; Gherm and Zernov, 1998]. Recently, the technique has been extended to treat geomagnetic field-aligned irregularities [Gherm et al., 2001]. Examples of the wideband scattering function for this situation are shown in Figure 4. Another application considers range errors for GPS signals for the same kind of anisotropic-density-fluctuations model [Gherm et al., 2000].

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1-0.05

-0.04

-0.03

-0.02

-0.01

0

0.01

0.02

0.03

0.04

0.05

Doppler, Hz

Del

ay, m

s

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1

-0.05

-0.04

-0.03

-0.02

-0.01

0

0.01

0.02

0.03

0.04

0.05

Doppler, Hz

Del

ay, m

s

Figure 4. An example of a wideband (500 kHz) HF scattering function for propagation in the plane of the magnetic meridian (left), and perpendicular to the magnetic meridian (right). The density fluctuations are aligned along the geomagnetic field [Gherm et al., 2001]. The developments reviewed here have so far only dealt with an unmagnetized background ionosphere, i.e., the refractive-index function in Equation (11) is expressed according to the isotropic case. The irregularity spectrum used in [Gherm et al., 2000, 2001] is hence only anisotropic in its spatial structure, not in the refractive-index definition. The way to overcome these limitations would be to merge the theoretical framework developed by Zernov, Gherm, and colleagues with a ray-tracing tool like RaTS. This is possible, in principle, but would require some further theoretical efforts. The generalized Rytov method focuses on deriving statistical characteristics of the propagation for a stochastic model of the density fluctuations. A further significant step forward would be the ability to perform propagation calculations for specified realizations of the ionospheric fluctuations. This would, in essence, make possible the theory-driven propagation simulator. The simultaneous presence of several scales of ionospheric-density variations is very demanding when treating the wave-propagation problem. The generalized Rytov method described here handles this in a satisfactory way. It fails, however, with strong fluctuations, such as the disturbed high-latitude ionosphere. This is still an unsolved problem, but possible ways to deal with this were discussed in [Gherm and Zernov, 1998].


7. CHANNEL-MEASUREMENT TECHNIQUES 7.1 INTRODUCTION An alternative to theoretically modeling the ionospheric channel is the experimental approach. Oblique measurements have been carried out over many years to quantify the HF channel. These have generally used continuous-wave (CW) and low-data-rate frequency-shift-keyed (FSK) waveforms [Shepherd and Lomax, 1967; Warrington et al., 1994]. Most of these experiments were, however, somewhat inappropriate for supporting the development of modern sophisticated modems. In this context, we do not consider conventional bistatic ionosondes, since these do not in generally provide detailed channel characterizations. Basler et al. [1988] and Wagner et al. [1988] were the first to comprehensively try to characterize the oblique HF channel through the measurement of the channel-scattering function [Bello, 1963]. This function describes the way that the signal energy is redistributed, in both delay and frequency, as a result of the transmission channel. As such, it meets some of the requirements of the modem designer. The scattering function can be approximately determined by a suitable channel sounder that repeatedly, at an interval , measures the complex-valued impulse response as a function of the path delay, . The samples at each delay value are processed with discrete power-spectrum estimation techniques to derive the Doppler spectrum. Signal integration is implicit in the processing and, consequently, such measurements can be conducted at relatively low powers.

Itτ

7.2 DESCRIPTION OF THE DAMSON EXPERIMENT The experiments reported by Basler, Wagner, and co-workers collected data for offline analysis; however, Davies and Cannon [1993] described a system known as DAMSON (Doppler and Multipath Sounding Network), which can generate the scattering function in real time, and for which a sophisticated data-analysis suite has been written. DAMSON is a relatively low-power pulse-compression ionospheric sounder. It uses pulse-compression sequences, on pre-selected frequencies, between remote transmitting and receiving sites to provide real-time HF channel measurements. It provides measurements of absolute time of flight (typically, up to 40 ms), multipath (typically, up to 12.5 ms with 600 µs resolution in a 3 kHz bandwidth), Doppler shift and spread (typically, up to Hz with 0.6 Hz resolution), signal-to-noise ratio (SNR), and absolute signal strength.

40±

DAMSON is based on commercially available equipment (such as HF communication receivers and transmitters, personal computers, etc.), and makes extensive use of digital-signal- processing (DSP) techniques. The Global Positioning System (GPS) provides accurate timing. This ensures that transmitting and receiving stations are synchronized to better than 10 µs, thereby allowing absolute time-of-flight (TOF) measurements to be made. DAMSON characterizes the propagation path using a number of sounding waveforms that can be flexibly scheduled; of these, the delay-Doppler (DD) waveform has been specifically designed to measure the channel-scattering function. For the delay-Doppler measurements (e.g., the central panel of Figure 5), a Barker-13 pulse sequence is used, binary-phase-shift-key (BPSK) modulated at 2400 baud. This offers a


reasonable compromise between sequence-processing gain (22.3 dB), duty cycle, and Doppler tolerance ( Hz). Each sequence lasts 5.4 ms, and a pulse-repetition interval (PRI) of 12.5 ms has been used. This PRI results in a Doppler frequency range of Hz. The sequence is sent 128 times, giving an integration time of 1.6 s, and a frequency resolution of 0.625 Hz. The processing gain is 32 dB for signals transmitted over a non-fading channel. Generally, six delay-Doppler measurements are incoherently averaged to increase statistical confidence in the scattering function. This process, which assumes that the channel is stationary over the measurement period (36 s), results in better mode definition.

~ 50±40±

Figure 5. The Doppler and multipath characteristics for a high-latitude path from Harstad to Kiruna [Angling et al., 1998] (also see Plate 10). Other DAMSON measurement modes include a passive noise-measurement period lasting 2 s, and a CW transmission lasting 4.3 s (Figure 5, second-to-lowest panel). Due to its long integration time and small measurement bandwidth (400 Hz), the CW measurement is a very sensitive detector of ionospheric support: nearly 17 dB better than the delay-Doppler measurement. However, it is unable to distinguish unambiguously between different propagation modes in either the delay-time or frequency domain. A time-of-flight (TOF) mode also exists, which is similar to the delay-Doppler mode. This is able to unambiguously measure up to 40 ms of multipath in its usual configuration. Like the delay-Doppler mode, the time-of-flight mode uses pulse-compression waveforms. However, the increased PRI limits the Doppler range to Hz, so that this mode is unable to provide an accurate measure of the Doppler shift and spread during disturbed ionospheric conditions.

12.5±


7.3 DESCRIPTION OF OTHER EXPERIMENTS SCIPION [Le Roux et al., 2000b] is a versatile sounder, which can be used to make a variety of ionospheric measurements, including vertical and oblique. In the latter mode, it can be used to determine the channel-scattering function, to obtain the channel impulse response, and to examine spatial coherence. Up to sixteen multiplexed receiving antennas can be used, with a minimum dwell time on each antenna of 10 ms. Darnell et al. [2000] has also described a system for measuring the HF channel-impulse response. This system has operated under the auspices of the ITU, and can measure a number of parameters pertinent to the HF radio-communications channel by virtue of the transmitted signal format. This consists of FSK, Morse, CW, and a pair of 256 complimentary sequences clocked at 1200 bit/s, thereby generating a channel impulse response every 0.4 s. 8. HIGH-LATITUDE EXPERIMENTS AND RESULTING STUDIES Wagner et al. [1988] and Wagner and Goldstein [1995] gave comprehensive reports on measurements taken using a wideband sounder in high latitudes; however, the quantity of the analyzed data was limited.

Figure 6. A map showing the positions of the DAMSON sites, and the auroral oval for at 00 UT (01 LT) [Angling et al., 1998].

4Kp =

More recently, a large number of DAMSON measurements have been made on high-latitude paths between four sites in Scandinavia. Transmitters were located at Isfjord Radio Station on Svalbard (78.06° N, 13.63° E) and at Harstad, Norway (68.48° N, 16.30° E). The receivers were positioned at Tuentangen, near Oslo, Norway (59.94° N, 11.09° E) and Kiruna, Sweden (67.84° N, 20.40° E). Figure 5 shows typical data collected using this high-latitude experiment. The central panel shows three modes, each spread in time, and each with different Doppler-shift and spread characteristics. Figure 6 shows the four sites, and also gives the approximate position of the


auroral oval for high geomagnetic activity at 00 UT. The lengths of the paths ranged from 194 km to 2019 km. Normally, the system employed ten frequencies, ranging from 3 to 22 MHz, with a dwell time of one minute on each. The Svalbard transmitter radiated ~250 watts from a southerly pointing rhombic antenna. A similar power was used by the Harstad transmitter, but with a southerly pointing, horizontal wideband dipole antenna. At Tuentangen, an 80 m sloping Vee antenna, pointing due north, was used, whilst at Kiruna, the receiving antenna was a sloping dipole tuned to around 2.9 MHz and beaming north-south. The high-latitude DAMSON system was run from 1995 to December 1999, but there have been a number of equipment downtimes during this period. Furthermore, special experiments were run at various times that broke the data set. Not withstanding these breaks, DAMSON almost certainly provided the best and largest high-latitude HF channel characterization to date. 8.1 GROSS CHARACTERISTICS OF THE HIGH-LATITUDE PATH Table 1. Doppler (Hz) and SNR (dB) for multipath spreads between 0 and 5 ms. The values are appropriate to 95th percentiles (Angling et al. [1998]).

Time (UT)

Frequency (MHz)

S - T S - K H - T H - K

All freq -14 -17 -15 -16 2.8 - 4.7 -16 -18 -12 -10

6.8 - 11.2 -5 -8 -8 -15 00 - 24

14.4 - 21.9 -16 -20 -18 -22 All freq -11 -13 -13 -14 2.8 - 4.7 -9 -13 -5 -9

6.8 - 11.2 -7 -10 -10 -11

SNR (3 kHz)

(dB) 19 - 01

14.4 - 21.9 -15 -16 -16 -20 All freq 8.5 10.0 2.5 26.5 2.8 - 4.7 11.5 8.0 1.5 5.0

6.8 - 11.2 9.0 9.0 2.0 32.0 00 - 24

14.4 - 21.9 6.0 15.0 4.0 64.5 All freq 10.0 12.5 4.0 52.5 2.8 - 4.7 13.0 9.0 2.5 7.5

6.8 - 11.2 9.0 11.0 4.0 51.5

Doppler Spread

(Hz) 19 - 01

14.4 - 21.9 7.5 18.5 4.5 67.0

S - T = Svalbard – Tuentangen; S - K = Svalbard - Kiruna H - T = Harstad – Tuentangen; H - K = Harstad – Kiruna

Data from the high-latitude DAMSON network were analyzed by Willink [1997] to determine the diurnal distribution of Doppler spread, multipath spread, and SNR on each high-latitude path. The seasonal variations in the channel characteristics were also considered in an analysis by Angling et al. [1998]. For this analysis, data were broken down into seasons, two time periods, and four frequency bands. The times used were all-day (0000-2400 UT) and 1900-0100


UT; the latter period approximately corresponded to 2100-0300 CGMLT (corrected geomagnetic local time), when the auroral oval was expected to be in its southernmost position and, consequently, when the largest disturbances were expected. A summary of the 95th-percentile multipath, Doppler spreads, and SNRs were presented for each path. Doppler spreads ranged from 2 to 73 Hz, while multipath spreads ranged from 1 to 11 ms. Table 1 is a typical example of the results presented. An important result of these studies was the specification of the operating envelope for the low-data-rate NATO HF modem, STANAG 4415 [STANAG 4415, 1998]. Jacobsen et al. [2000] has attempted to analyze the DAMSON data from a geophysical perspective, by correlation with data from a magnetometer network in the same geographic region. Long-delay signals (several ms) were regularly observed near midday at frequencies well above the predicted MUF (maximum useable frequency), and could possibly be caused by ground scatter. Large Doppler spreads (tens of Hz) were observed during disturbed conditions (substorms), when the ionospheric reflection point was located within the auroral oval. Warrington et al. [2000] also used DAMSON 3 kHz transmissions, but with an independent direction-finding receiving system at Kiruna. Within one Doppler and delay-time spread propagation mode, it was common to see a variation in Doppler frequency with azimuth. These measurements were frequently consistent with irregularities drifting across the path. For periods often in excess of several hours, different propagation modes were received on widely spaced bearings; best estimates of the great-circle bearing was usually associated with the E mode of propagation. 8.2 HIGH-LATITUDE MODEM AVAILABILITIES Testing of modems on actual radio paths always presents problems of repeatability and cost, and, consequently, simulator testing in the laboratory, under specific channel conditions, is very popular. However, how often such specific conditions occur determines the overall modem availability (quantified as a fraction or percentage of time that the modem will function satisfactorily). In order to determine this, a long data set of on-air measurements is indispensable. Fortunately, the DAMSON data set can provide these data; moreover, the one data set can be applied to virtually any modem. In order to achieve this, it is first necessary to characterize the modem using a simulator. As an example, Figure 7 shows a Doppler, multipath, and SNR characterization of a MIL-STD-188-110A [MIL-STD-188-141A, 1991] 1200 bps modem, using a technique described by Arthur and Maundrell [1997]. The surface describes the SNR required to maintain a bit-error rate (BER) of

over a channel simulated by two equal-power modes, with identical Doppler characteristics and a specified multipath separation.

3~ 1 10−×

Composite (overall) Doppler and multipath values can be determined from DAMSON measurements. In particular, the composite multipath spread is calculated as the separation between the leading edge of the first mode’s 80th power percentile (i.e., the central region of the mode containing 80% of its total power) and the trailing edge of the 80th power percentile of the last mode. Subsequently, the availability of a modem can be estimated by placing each DAMSON composite-channel measurement on the modem characterization to determine the difference


between the measured SNR and the SNR of the surface (the SNR difference). A positive SNR difference indicates that the channel would support the specified BER. Cotterill and Arthur [1998] have shown that this technique is in good agreement with on-air modem tests (albeit during moderate propagation conditions). For these tests, DAMSON transmissions were interleaved with data transmissions from a number of modems.

0 2 4 6 8 10 1214 16

18 2024 28

32 36 40

0

1

2

3

4

5

6

7

89

10

-10-8-6-4-20246810121416182022242628303234363840

SNR(dB)

Doppler Spread (Hz)

Multipath (ms)

-10

0

10

20

30

40

Figure 7. A typical modem characterization for a MIL-STD-188-110A serial, short interleaver, operating at 1200 bps [Arthur and Maundrell, 1997] (also see Plate 10). Using this technique, Willink et al. [1999] analyzed DAMSON data from April to December, 1995 (near sunspot minimum), in order to estimate the need for a robust waveform on these paths. For frequencies below the predicted maximum-usable frequency (MUF), channel conditions meeting the criteria requiring a robust, low-data-rate waveform were found to occur 35% of the time. When propagation occurred above the predicted MUF, a robust waveform would be required 77% of the time. A comprehensive analysis of modem availabilities has recently been undertaken by Jodalen et al. [2001]. Example results for a summer period for two modems, STANAG 4285-2400bps [STANAG 4285, 1990] and STANAG 4415-75bps [STANAG 4415, 1998], are presented in Figure 8. Jodalen et al. [2001] report that the 75 bps modem provides 60-75% higher availability than the 2400 bps modem. Jodalen et al. [2000] also modified the nominal DAMSON path configuration, described above, to include a path from Harstad to Abisko (90 km), as well as the original Harstad to Kiruna (190 km) path. These paths share the same azimuths, and the objective of the experiment was to determine if modem availability could be improved using geographical diversity. For the data analyzed, no benefit was found.


Figure 8. Modem availability on propagating channels for the Isfjord-Tuentangen path (2019 km) [Jodalen et al., 2001]. 8.3 OPTIMAL FREQUENCY SET FOR USE IN HIGH-LATITUDE ADAPTIVE HF SYSTEMS The stability of the HF propagation channel was investigated by Willink and Landry [1997]. The results were used to assess the benefits of employing a frequency-agile multi-user HF network that is able to change frequencies in response to changes in ionospheric conditions. Such a system would have to incorporate a probe signal that sounds each frequency sufficiently often to characterize the radio paths. The time series of Doppler-spread and multipath characteristics at three frequencies were analyzed to determine the period of time over which a channel characterization could be considered valid. For the Doppler analysis, the data were considered both in terms of a daily averages and for the period 1900-0100 UT, the latter addressing the period about magnetic midnight. For the auroral, north-south, Isfjord to Kiruna link, the average time for the Doppler spread to change by 3 Hz was ~2 hours, averaged over the whole day, and only


~1 hour at night. Similar characterizations of the multipath and SNR conditions were reported, with the latter showing significant decorrelation within the 10-minute time resolution of the measurements. The shorter west-east path, from Harstad to Kiruna, was less stable. This work was used in the development of the NATO standard on Automatic Radio Control Systems, ARCS [STANAG 4538, 2000].

Figure 9. The overall availability of modems when the frequency set consisted of 1,2,...,10 frequencies for the Harstad Kiruna path [Jodalen et al., 2001]. Jodalen et al., [2001] also used DAMSON data and modem-performance characterizations, coupled with the ICEPAC HF prediction code [Stewart and Hand, 1994], to determine the optimal number of allocated frequencies required by an adaptive HF system. Optimal was considered to be the minimum number of frequencies to achieve maximum reliability. On a 2000 km path, it was shown that five to six frequencies were sufficient, whereas on a short 200 km path, three to four frequencies were sufficient. Figure 9 shows an example from this study. 8.4 AN ASSESSMENT OF THE NEED FOR MORE SOPHISTICATED HIGH-LATITUDE SIMULATIONS Most narrowband HF propagation simulators are based on the model by Watterson et al. [1970], and modem testing is generally based on the specification of two equal-power modes, with identical Doppler characteristics and a specified multipath separation. Willink et al. [1999] demonstrated that below the MUF, the mean percentage of the power contained in the strongest mode is, however, 60-75%. Consequently, such equal-power testing strategies should be treated with care. The Watterson model is restricted to simulating Rayleigh fading, Doppler spread described by a Gaussian power spectrum, and no delay spread within a mode. Whilst simulators based on


this model have proven extremely useful, they may not be satisfactory for testing equipment designed for use under difficult propagation conditions, such as those experienced at high and equatorial latitudes. SUEDE (Simulator Using Extended Dispersion Envelope) [Angling et al., 2002] is a replay simulator for 3 kHz channels. It uses a parameterized version of data collected by DAMSON to drive the ionospheric-channel model that has been proposed by the Institute of Telecommunication Sciences (ITS) [Mastrangelo et al., 1997] for wideband applications. The ITS model was, however, modified in one important respect. The function suggested by Vogler and Hoffmeyer [1993] only allows the Doppler power profile (DPP) to be skewed so as to provide a fast attack and a slow decay. However, measurements conducted with DAMSON frequently contained examples of modes with a slow attack and a fast decay. These situations were simulated in SUEDE by swapping the values of and , generating the DPP, and then rotating it about the delay of the profile peak. This allows the DPP to vary from symmetric to highly skewed in either direction.

Lτ Uτ

Angling et al. [2002] compared the testing of a narrowband modem using both SUEDE and the Watterson technique described in Section 3. Ideally, high-latitude DAMSON data would have been used to control both the Watterson and SUEDE models, and a comparison made. However, the former does not lend itself to varying the channel parameters. Consequently, although DAMSON was used to drive SUEDE by defining the SNR and the 3 dB Doppler and multipath spreads for each mode, the Watterson model testing was conducted using the previously described (Section 8.2) modem-performance surfaces. Angling et al. [2002] found that there were periods of poor agreement between simulations (the assumption being that SUEDE was a more realistic simulation), and that these were generally associated with periods of large Doppler spread. The disagreement could, however, be overcome by using a weighted multipath spread in the Watterson process, instead of the simple composite multipath spread. This new spread measure again used the 80th power percentiles, but also incorporated a weighting proportional to the power in each mode. Further work is required to clarify this situation, but it appears possible that a Watterson simulation can give sufficiently accurate results for some modem testing of high-latitude propagation paths if the weighted multipath spread is used. 9. THE MID-LATITUDE AND EQUATORIAL HF CHANNEL-SCATTERING FUNCTION Few mid-latitude measurements have been reported in recent years. Arikan and Erol [1998] used CW transmissions to make a statistical study on a path from the UK to Turkey. They found that the mid-latitude ionospheric channel was slowly time varying and locally stationary, within a window length of 22 s. Davies et al. [2001] have recently reported measurements on a short (~150 km) path at mid-latitudes, using a wideband sounder with a bandwidth of 80 kHz. No substantive results have so far been obtained. There are extensive observations of spread F, but there are only a limited number of bistatic HF-channel measurements in the equatorial region [Basler et al., 1998; Flaherty et al., 1996]. Fitzgerald et al. [1999] made equatorial-region bistatic measurements – using phase-locked CW transmissions on different frequencies and multiple receiving antennas – in the evening, when spread F was common. Their path was ~700 km long, and measurements were made over a two-week period for two hours every night. Figure 10 shows a comparison of the Doppler spectra at two times: before spread F onset (0735 UT), and after onset (0900 UT). The earlier spectrum


shows a strong one-hop peak at a Doppler shift of Hz, and the later spectrum shows three peaks above a pedestal of Doppler spread. Fifty percent of the evenings showed Doppler spreads of greater than 6 Hz at the –10 dB level. These authors also investigated spatial coherence distances. They found that 40% of the evenings showed coherence distances of < m and

m, respectively, in the directions normal to, and along, the bistatic path. Seventy-five percent of the evenings showed coherence bandwidths of less than 1.5 kHz.

3−

18075<

Figure 10. A comparison of the Doppler spectra for August 15 at two times: before spread-F onset (0735 UT), and after onset (0900 UT). The earlier spectrum shows a strong one-hop peak at a Doppler shift of –3 Hz; the later spectrum shows peaks above a pedestal of Doppler spread [Fitzgerald et al., 1999]. Between August 26 and September 22, 1997, DAMSON was also deployed in Thailand, where measurements were made of the equatorial HF scattering function [Cannon et al., 2000]. For these experiments, the transmitter was located at 7.2° N, 100.6° E, and the receiver was located at 12.7° N, 101.0° E, giving a predominantly north-south, 600 km path. Unlike high-latitude paths, the propagation features were repeatable from day to day (e.g., Figure 11), with predominately single-mode propagation during daylight, and multi-mode propagation during the night. The median Doppler spreads were usually below 2 Hz, but Doppler spreads exceeded 8 Hz during the day on the highest frequency used (10.051 MHz). Such measurements could have implications for new wideband, digital HF broadcast modems, which may be required to operate with high availability on fixed frequencies. Le Roux et al. [1998] described the deployment of the SCIPION sounder in Senegal. These measurements used a long integration time of 11 s, resulting in a fine frequency resolution of 0.09 Hz. Measurements near sunrise showed Doppler shifts and spreads up to 0.5 Hz.


Figure 11. The Doppler spread (containing 80% of power) as a function of the time of day for the whole data set [Cannon et al., 2000]. 10. SUMMARY AND THE FUTURE The aspirations of HF radio modem designers increase year-on-year, and hand-in-hand with this goes a requirement for improved channel measurements and models. More detailed narrowband models are required, together with models appropriate to bandwidths higher than the standard 3 and 6 kHz allocations. There have been very few mid-latitude 3-6 kHz measurements in recent years, and this probably reflects the fact that the models are now adequate for most simulator designs. It is also likely that at mid-latitudes, this knowledge can be extrapolated to 10 kHz channels with reasonable confidence. There is undoubtedly a need, however, for mid-latitude models of the very wideband channel that can be applied to new systems, especially those employing frequency hopping. In the high-latitude region, the DAMSON experiment has provided much information relating to the 3 kHz channel, but these data have only addressed the channel characteristics described by the channel-scattering function. Given the complicated propagation paths and the temporal variability in this region, more information is needed on the detailed phase


characteristics. This would undoubtedly improve the design of HF simulators, and might also improve the design of HF modems. Similar considerations, requiring a detailed understanding of the propagation channel, apply to the equatorial region, where the physics of spread F has been studied for many years. In contrast, however, and somewhat surprisingly, there is also a general need for scattering-function measurements, such as those already obtained at high latitudes. It goes without saying that such measurements are required using bandwidths appropriate to digital HF broadcast initiatives and frequency-hopped systems. Ray tracing is today a very general and flexible tool, but it is restricted to modeling large-scale variations of the propagation medium. The merging of ray tracing with a full-wave method, capable of handling propagation through an ionosphere that is inhomogeneous on several scales – as is the situation in reality – is an interesting prospect for the future. Another interesting prospect is the theory-driven propagation simulator. This would utilize full-wave solutions of the generalized Rytov type for time series appropriate to specific realizations of the ionospheric fluctuations. The best contemporary ionospheric HF propagation full-wave methods are confined to weak density fluctuations. To deal with strong fluctuations, common in the disturbed high-latitude ionosphere, a new approach would be needed. 11. ACKNOWLEDGEMENTS We would like to take this opportunity to thank our colleagues for the information and help that they have provided to aid the production of this review. We are especially thankful to the international DAMSON team from Canada, Norway, Sweden, and the UK for their assistance. 12. REFERENCES M. J. Angling, P. S. Cannon, P. C. Arthur, and P. L. Cotterill and N. C. Davies [2002], “New Approaches to Time Varying Narrow Band HF Simulations,” submitted to IEEE Transactions on Communications. M. J. Angling, P. S. Cannon, N. C. Davies, T. J. Willink, V. Jodalen, and B. Lundborg [1998], “Measurements of Doppler and Multipath Spread on Oblique High-Latitude HF Paths and Their Use in Characterising Data Modem Performance,” Radio Science, 33, 1, pp. 97-107. F. Arikan and C. B. Erol [1998], “Statistical Characterisation of Time Variability in Midlatitude Single Tone HF Channel Response,” Radio Science, 33, 5, pp. 1429-1443. P. C. Arthur and M. J. Maundrell [1997], “Multi-Dimensional HF Modem Performance Characterisation,” in 7th International Conference on HF Radio Systems and Techniques, Nottingham, UK, CP441 (available from IEE, London, UK). R. P. Basler, P. B. Bentley, R. T. Price, R. T. Tsunoda, and T. L. Wong [1988], “Ionospheric Distortion of HF Signals,” Radio Science, 23, 4, pp. 569-579. P. A. Bello [1963], “Characterization of Randomly Time Variant Linear Channels,” IEEE Transactions on Communications Systems, CS-11, 4, pp. 360-393.


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intro to hf ch_mod