226 IEEE TRANSACTIONS ON BROADCASTING, VOL. 44, NO. 2, JUNE 1998

cast Analysis and rediction in the HF Ban S. Gokhun Tanyer, Member IEEE, and Cemil B. Erol

A b s t r a c t 4 comparative study of the commonly used methods for the prediction of the coverage area due to transmitter operating at the HF band is presented. Those statistical model based tools are ITU’s REC 533, Ionospheric Communication Analysis and Prediction Program; IONCAP, Voice of America’s VOACAP and Ionospheric Communication Enhanced Profile Analysis and Circuit Prediction Program; ICEPAC. The received signal due to a typical transmitter located at Cricklade, England operating around 7.9 - 19.1 MHz (HF Band I) is computed as a function of frequency, time and the sunspot number. The ray tracing method calculates the virtual height. The calculated results are compared with the experimental data and the discrepancies are examined.

I. INTRODUCTION

The demand for new frequency channels in broadcasting and telecommunications are illcreasing exponentially by the upcome of new technologies and applications. The channel width requirements are also increasing in parallel to the increase in information to be transferred in spite of the developments in digital signal processing and the VLSI technology. Wider channels and carrier frequencies beyond the VHF and UHF frequency bands are required to follow the new developments in several applications including digital TV and digital radio broadcasting, advanced personal communications, interactive digital services, remote learning on satellite and terrestrial transmission media. The coverage area of a transmitter reduces considerably for higher frequencies, which is due to increased atmospheric attenuation and decreased diffraction. For the coverage area constraint and an immediate need for frequency reuse for the spectrum, utilization of the HF band for communications is becoming increasingly important. HF propagation, Publisher Item Identifier S 001 8-93 16(98)05595-4

especially for the terrestrial communication, is very dependent to the location of the transmitters and receivers, the time where communication takes place and the level of the time-variant noise (see Fig. 1). For a reliable communication or broadcasting in the HF band one needs improved tools for accurate prediction of possible interference between systems [l-31. The aid of computer databases and the digital terrain maps are also very useful. Furthermore, the current tools of prediction need to be compared and individual regions of accuracy and validity should be studied which summarizes the scope of this work. In ths paper, the three methods are compared on a typical example; coverage of an approximately 2500 km. apart service point; transmission from a typical transmitter located a t Cricklade-England operating at 7.9 - 19.1 MHz (HF Band I) and receiving at Kandilh- Istanbul, Turkey. The experimental data is measured at this service point in comparison with the calculated results. Later, to predict the virtual height of the reflection boundary and the critieal frequency the ray- tracing technique is used. Those parameters are optionally input to the prediction programs to observe more accuracy. The experimental and the theoretical results are discussed in Results and Conclusions.

11. ELECTROMAGNETIC PROPAGATION PREDICTION METHODS

In order to plan a broadcasting and/or point to point communication system, the attenuation, multi-path fading etc. observed during the propagation should be predicted. For that purpose, prediction programs are widely used which can be grouped as statistical, empirical add numerical. The most widely used statistical methods are implemented in ITUs REC 533, Ionospheric Communication Analysis and Prediction Program; IONCAP, Voice of America’s VOACAP and Ionospheric Communication Enhanced Profile Analysis and Circuit Prediction Program; ICEPAC. The alternatives are the numerical methods in which the electromagnetic field is calculated

0018-9316/98$10.00 0 1998 IEEE

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-1 1 0 1 0 10.1 70.2 10.3 10.4 10.5 10.6 10.7 10.8 10.9 1 1

Hour (UT) Fig.1. A typical signal intensity variation measured at Kandilli-Istanbul, Turkey at September 28, 1992 due to a typical

transmitter located at Cricklade, England operating around 13.399 MHz (HF Band I)

exactly using a n approximate model for the medium parameters.

A. ITU's REC 533 REC 533 stands for the coverage area prediction software developed by ITU (Recommendation No.533). It is widely used where quick and rough estimates are required. REC 533 still stands as a n alternative in prediction of HF propagation.

B. Voice of America Communication Analysis and Prediction Program; VOACAP VOACAP is the modified version of IONCAP developed a t the Institute for Telecommunication Sciences [IO]. VOACAP could simply be stated as the IONCAP working on the Windows environment. It is an internationally used computer-based program, which predicts the performance of HF propagation system by calculating HF propagation parameters a t any location on the earth. The maximum usable frequency 'MUF', the lowest usable frequency (LUF), field strength a t the receiving point and the mode reliability parameters can be obtained. This program implicitly represents the morphological behavior of the ionospheric structure and the atmospheric noise expected by a set of numerical coefficients. Some of the inputs are transmitter and receiver location, transmitter power, transmitting antenna parameters, the universal time, and month and sunspot number. The program uses the parabolic layer theory and a

database, which includes geographic and temporal variations of the ionosphere, atmospheric noise levels and some knowledge about ionospheric characteristics, the propagation path geometry and signal attenuation. The more useful techniques were collected into one program as ICEPAC and the latest coefficients for representation of the ionosphere was used.

C. Ionospheric Communication Enhanced Profile Analysis and Circuit Prediction Program; ICEPAC ICEPAC is created for piediction of EM propagation in the low, mid and high-latitude and polar regions. The models used in ICEPAC are basically similar to those in IONCAP, but some modifications have been made to include some empirical and measurement corrections. ICEPAC uses Chapman layers for the E, Fi and F2 layers instead of the parabolic layers and also new coefficients recently approved by URSI are implemented. The Ionospheric Conductivity and Electron Density profile model (ICED) used by ICEPAC, is a statistical model of the large scale features of the northern hemisphere, namely, the subauroral through, the equator-ward portion of the auroral zone, the pole-ward region of the auroral zone and the polar cap zone. The semi-empirical method to include the recent measurements, especially backscatter measurement, in the calculation of ionospheric losses has , been developed in this program. The method includes the deviative and

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Fig.2. The ray trajectories originating from Cricklade-England, operating f'requency 7.9- 19.1 MHz, method use the refracting index data measured in April 1992. Cricklade-Istanbul trajectory corres'ponds to the ray having a range 2500 km.

non-deviative effects as well as sporadic-E effects. The average effects of magnetic field and of polarization changes are included implicitly by modifying the constants in the model equations to match the measured results. More details can be found in [4-61.

D. The Ray Tracing Method Technique Several numerical methods are available and widely used in UHF propagation and scattering problems. Some of those are parabolic equation modeling, shooting and bouncing ray technique and ray tracing technique [7-81 (see Fig. 2). If the pertinent characteristics of the ionosphere are known, then one of the above numerical methods can be used for the prediction of the HF propagation. Those become helpful when accurate site dependent data (ionospheric height, terrain height etc.) are available. Due to its practicality, ray tracing technique is used to calculate the virtual height and the critical frequency values in this work.

For a medium where the refractive index is smoothly varying, the behavior of the electromagnetic fields can be described by the ray approach. The classical laws of geometric optics can determine the trajectories. Then, the ray path can be predicted by the eikonal equation i.e.

-{ d n ( r $} = V n ( r ) ds

where s is the distance along the ray path, r is the position vector of a point on the ray path and Vn(r) is the gradient of the refractivity index of the medium. The eikonal equation can be written as

n(r)= n(h,x) (2) where h and x are the height and the range parameter corresponding to the position vector r, respectively. Eq.(2) and (3) yield

(3) ds ah and also

' (4) ds

where the transversal derivatives can be assumed to be zero. Eqs.(3,4) can be calculated on the computer by approximating the derivatives by finite differences. The rays launching from the transmitter located at Cricklade, England are shown in Fig.2. Note that the ray with a range of 2500 km. arrives to Kandilli, Istanbul.

111. RESULTS AND CONCLUSIONS

The most widely used coverage area prediction (CAP) methods; REC 533, VOACAP (equivalent to IONCAP) and ICEPAC (improved IONCAP) are

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Fig.3. The received signal intensity hourly median vs. time. Receiver is located a t Kandilli- Istanbul, Turkey in April 1992, transmitter is located a t Cricklade, England operating in the 7.9-19.1 MHz (HF Band I) range.

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Fig.4. The received signal intensity hourly median vs. frequency. Receiver is located a t Kandilli- Istanbul, Turkey in April 1992, transmitter is located a t Cricklade, England. Multiple values on the same frequency value correspond to calculation and

measurement a t different time instances.

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compared on a typical example; broadcasting in HF centered a t Cricklade-England. The ERP values for a single test pomt obtained using different programs are calculated and compared. The hourly median of the signal intensity is plotted in Fig. 3 as a function of time. The receiver is located a t Kandilli- Istanbul, Turkey and the transmitter is located a t Cricklade, England operating in the 7.9-19.1 MHz (HF Band I) range. The calculated results are compared with the experimental data obtained in April 1992. Both of the calculated results show some correlation with the measured data between 1 and 15 hours in universal time (UT) and ICEPAC predicts more accurately in 24-25 hours UT. In Fig.4 the hourly median of the signal intensity is plotted as a function of frequency where time, the receiver and the transmitter locations are the same. Multiple values on the same frequency value correspond to calculations and measurements a t different time instances. It is observed that the error on signal intensity prediction can be frequency dependent. The ratio of the operating frequency and predicted MUF and the ratio of the operating frequency and the measured MUF as a function of universal time is given in Fig.5. I t is observed that the ratios are within safety margin of approximately 0.85 percent between 19:OO - 07:OO UT. In this period of the day, the sun is completely down and the ionization rate is almost constant. On the other hand, safety

margin goes down to approximately 0.50 percent between 07:OO - 19:OO UT. In this time of the day, the sun is up and the ionization rate is highly variant and F2 layer is formed. The calculated results are observed to be more optimistic during the daytime. The estimated results are calculated using VOACAP and ICEPAC. The virtual height that is used by those programs is calculated using the ray tracing technique summarized in section 1I.D (see Fig. 2). The relative refractive index that is required by the ray- tracing algorithm is calculated using the electron density profile for the given time and operating frequency. Later, for that given relative refractive index, all the paths connecting the transmitter and receiving point is found using the eikonal equation given in Eq. (1). Then, the field induced at the receiving point is calculated by the complex sum of the field due to each path: Note that, the skip distance is about 700 km. and multi-path is observed approximately between 700-800 km range. Also note that the ray trajectories spreads apart in the direction of propagation, as expected. Using ray tracing technique, the amplitude, time delays and arrival angles of the various multipath components arriving a t the receiving end can be estimated. The use of site dependent data (ionospheric height, terrain height etc.) supplies some superiority over the statistical methods. More accurate prediction of HF propagation

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Fig.6. The hourly prediction error percentage in received signal power relative to the measured data vs. time. Receiver is located at Kandilli- Istanbul, Turkey in April 1992, transmitter is located at Cricklade, England.

is possible with the additional parameters; transversal variation of ionospheric density variation as a function of height so that 3D ray tracing can be done [9].

In Fig.6, the hourly prediction error percentage in received signal power relative to the measured data vs. time is given. Receiver is located at Kandilli- Istanbul, Turkey and the transmitter is located at Cricklade, England. The error values for VOACAP predictions are found to be less than ICEPAC predictions even if ICEPAC utilizes new ionospheric coefficients and new measurements. It is observed that the two methods could fail to accurately predict the changes in the coverage area about at least 20% of the time. To obtain more accuratee results, one needs to have additional and accurate information about the ionosphere layer. This emphasizes the importance of further studies on the ionosphere.

REFERENCES

VHF/FM Planning Parameters and Methods, Tech.3236-E, European Broadcasting Union, Technical Center, Brussels, July 1982. VHF and UHF Propagation Curves For The Frequency Range From 30 to 1000 MHz - Broadcasting Services, CCIR Recommendation

370-5, Vol. 5, XVIth Plenary Assembly, Dubrovnik 1986. R. Grosskopf, 'Comparison of field-strength prediction methods in UHF range', EBU Review- Technical, No. 229, June 1988. E. V. Thrane, 'Study of measured and predicted reliability of the ionospheric HF communication channel a t high latitudes', Radio Science, Vol. 29,

A. D. Spaulding, F. G. Stewart, 'The ionospheric communication analysis and prediction program (IONCAP); Reliability and service probability calculations, Radio Science, Vo1.30, No.3, 1995,

P. M. Hall, L. W. Barclay, M. J. Hewitt, 'Propagation of radio waves', IEE, London, 1996. R. J. McArthur, K. H. Craig, 'Propagation in the urban environment using building cover databases', U R S I Comm. F. Open Symposium,

H. Anderson, 'A ray-tracing propagation model for digital broadcast system in urban areas', IEEE Trans. on Broadcasting, Vo1.39, No.3, September 1993. R. M. Jones, 'A three-dimensional ray-tracing computer program (Digest of ESSA Technical Report, ITSA No.17), Radio Science, vo1.3, No.1, January 1968, pp.93-94.

No.5, 1994, pp. 1293-1309.

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UK, 8.1.1-5, 1992.

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[lo] L. R. Teters, J. L. Lloyd, G. W. Haydon, and D. L. Lucas, 'Estimating the performance of telecommunication systems using the ionospheric transmission channel-ionosp heric

University all in Elt

S. Gokhun TANYER was born in Ankara, Turkey, on September 9, 1966. He received his B.Sc. from the Middle East Technical University, M.Sc. from Bilkent University, Ankara, Turkey, and Ph.D. from Washington State

xtrical Engineering in 1988, 1990 and 1994 respectively. From 1994 to 1995 he worked for the Frequency Planning Research Center in Bilkent University. He later worked as an Assistant Professor a t the Department of Electronics Engineering a t the Ankara University. Since March 1997, he is currently working as an Assistant Professor in Department Electrical and Electronics Engineering a t the Bagkent University. His current research interests include electromagnetic scattering and propagation and EM1 / EMC related signal processing. Dr. Tanyer received in March 1994 the best research project award in 'Sixth Annual Graduate and Professional Student Research Exposition; Washington State University'.

communications analysis and predictions program user's manual', NTIA Rep.83-127, NTIS Order PB84-111210, 1983.

Cemil B. Erol was born in Ankara, Turkey, on December 12, 1967. He received his BSc., M.Sc. and Ph.D. from the Middle East Technical University all in Physics 1989, 1991. From 1994 to 1996 he worked for

Abant Izzet Baysal University as a vice- chairman of the Department of Physics. Since August 1996, he is currently working as an instructor in Department Electrical and Electronics Engineering a t the Baskent University. His current research interests include radiowave propagation in the ionosphere and its applications in communication and RADAR technology and ionospheric physics.