MULTIPATH PROPAGATION MEASUREMENTS IN RADIO LINKS AT 40 GHZ

Manuel García Sánchez (1), Iñigo Cuiñas (2), Ana Vázquez Alejos(3)

(1) Universidade de Vigo Dept. Teoría do Sinal e Comunicacións

Campus Universitario, s/n E36200 Vigo (SPAIN)

e-mail: manuel@com.uvigo.es

(2) As (1) above, but e-mail: inhigo@com.uvigo.es

(3) As (1) above, but e-mail: analejos@com.uvigo.es

ABSTRACT

The results of a measurement campaign conducted to determine the wideband behaviour of the radio channel assigned to Multimedia Wireless Systems are described in this paper. It has been found that small multipath components are present in the impulsive response of radio channels operating at 40 GHz with 80 MHz bandwidths. Multipath and delay spread are limited by the spatial filtering of directive antennas used in MWS and the low reflection coefficient of building materials at these frequencies.

INTRODUCTION

Multimedia Wireless Systems (MWS) are defined in [1] as terrestrial multipoint systems which have their origin in telecommunication and/or broadcasting, and which provide fixed wireless direct access to the end user for multimedia services. These MWS systems may offer different degrees of interactivity. The term “Multimedia Wireless Systems (MWS)” has been introduced to cater for the phenomena of convergence among terrestrial applications, whereby broadcasters are wishing to provide interactive services and telecommunications operators are wishing to supply broader band two-way services to wider markets. Therefore, MWS are wireless systems, which support information exchange of more than one type, such as text, graphics, voice, sound, image, data and video.

In 2001, the Electronic Communications Committee (ECC) of the Conference Européenne des Administration des Postes et des Télécommunications (CEPT) defined guidelines for the accommodation and assignment of Multimedia Wireless Systems (MWS) in the frequency band 40.5 - 43.5 GHz [2].

As in any other radio communication system, before MWS are designed, the radio propagation channel has to be measured. The attenuation, depolarisation, multipath and other propagation effects that the radio signal can suffer on its way from the transmitter to the receiver have to be know to be able to properly design the system.

In this paper the design, building and operation of a wideband radio channel sounder operating at 40 GHz are described. Wideband measurement results are completed with specific measurements of the scattering of electromagnetic waves by building materials

MEASUREMENT SYSTEM

The wideband channel sounder is based on the sliding correlation method [3]. A block diagram of the measurement system is shown in Fig.1. At the transmitter, a 40 GHz carrier is BPSK modulated by a pseudorandom binary sequence and then transmitted by means of a 20-dBi-gain sectorial horn antenna. For these measurements the pseudorandom sequence has been made 214 bits long, and its bit (chip) interval was fixed at 24 ns. The sequence occupies a 41.6 MHz base band bandwidth.

Another 20-dBi-gain standard horn antenna is used at the receiver, where the signal, after being amplified and filtered, is down converted to an intermediate frequency of 2.45 GHz. Then, it is demodulated to get the in-phase and quadrature components, which are sampled by an A/D card at a sampling rate of 625 Msamples/s. This is equivalent to 15 samples per chip. These data are stored in a computer for the subsequent off–line sliding correlation processing [4]. All signals are phase locked to rubidium clock references, both at the transmitter and the receiver, including the sampling signal of the A/D card.

Fig.1. Wideband sounder block diagram.

To get a reference sequence for the off-line correlation, transmitter and receiver were placed side-by-side and interconnected, and the sequence generated by the transmitter was recorded at the receiver. Then, transmitter and receiver were separated to measure the radio channel impulse response.

Several test were performed before the outdoor measurements were carried out. In one of these experiments, both transmitter and receiver were placed at one end of a laboratory, separated 1 meter, pointing towards a metallic plane reflector placed 8.5 meters away, as can be seen in Fig.2. Several echoes can be identified in the impulse response obtained from this experiment, that is presented in Fig.3. There is a component with short delay due to the direct coupling between the transmitter and receiver antennas. There is a component with longer delay is due to the metallic plane reflector. Different objects in the laboratory, as benches, computers and electronic equipment, generate the components with intermediate delays.

Fig.3. Wideband sounder test environment.

OUTDOOR RESULTS

Outdoor measurements were carried out with the transmitter and receiver at several university building roofs and terraces, since this is the kind of environment were fixed multipoint terrestrial MWS receivers are expected to operate. Transmitter and receiver directive antennas were pointed one towards the other, as will be usually done under actual operation conditions of MWS. Very small multipath components were found in this set of measurements. Apart from the direct component, just two other contributions with small excess delay and low power are present. These components are due to propagation paths generated by reflections on the building walls and/or roofs near the receiver location. The corresponding rms delay spread is 14 ns in this case, small compared with a 24 ns symbol interval, but not small enough to consider the system free of inter symbol interference. The delay spread can be made insignificant by increasing the symbol interval, that is, by reducing the channel bandwidth. For example, if the symbol interval is

increased to 50 ns (the signal bandwidth reduced to 40 MHz) the rms delay spread will be much smaller than the symbol interval and the inter symbol interference will be negligible.

Fig.4. Impulse response at the test environment.

MATERIAL CHARACTERISATION

In order to explain the small multipath components in the outdoor wideband measurements, additional experiments were carried out to analyse the scattering of electromagnetic waves at 40 GHz by building materials.

A channel sounder based on the swept frequency technique is the main element of the scattering measuring system [5]. This sounder is built around a vector network analyser (VNA) HP-8510-C, which is capable of measuring the S-parameters of a quadripole connected between its two ports. Two antennas are connected to the ports of the VNA test set: the transmitting antenna to the first port, and the receiving to the second one. Measuring the S21(f) parameter, the complex frequency response of the radio channel is obtained. If the channel is linear, the impulse response is computed from the complex frequency response by an inverse Fourier transform.

The sounder is used in co-ordination with an automatic positioning system. The receiving antenna was held on a motored tower, as can be seen in Fig.4, the movement of which is controlled by an indexer. This antenna was accurately moved along a semicircle around the circumference of a circle, which had the material at its centre. The transmitting antenna was located at several fixed positions. Both directive antennas were pointed to the centre of the material and the complex frequency response was recorded at each location. This information was then processed to calculate the impulse response and to identify the scattered component. The reflection coefficients were computed by comparing the reflected component due to the material under test with the one due to a perfect conductor plane placed at the same position. Custom software co-ordinates the movement of the tower, which is performed by an indexer-controlled step-by-step motor, and the operation of the electromagnetic measurement equipment.

The material under test for this experiment was a plane brick wall with no irregularities on it. From these scattering functions the reflection coefficient was calculated using to reflection models: Fresnel reflection model and the internal multireflection model. This later model takes into account the width of the obstacle [6-8]. It has been found that for incident angles smaller than 60º the reflection coefficient is below 0.5 [9]

CONCLUSIONS

A wideband measurement campaign has been conducted in order to determine the behaviour of the MWS radio channel. It has been found that small multipath components are present in an 80 MHz bandwidth radio channel. The corresponding rms delay spread is 14 ns. This is still high compared to a 24 ns symbol interval, but it is smaller compared to a 50 ns symbol interval. As a consequence, very little, or even zero, delay spread and inter-symbol interference can be expected in MWS if the channel bandwidth is below 40 MHz. These results are in agreement with those described in [10], where no multipath was found in absence of rain.

It has been found that the reason for the small multipath phenomenon is the combination of two effects: the spatial filtering of the directive antennas used for these links and the low reflection coefficient of building materials at MWS frequencies.

Fig.4. Scattering measurement system.

ACKNOWLEDGEMENTS

This work is outcome of project 1FD97-0960-C05-02, subsidized by the Spanish Ministry of Science and Technology.

REFERENCES

[1] European Radiocommunication Comitee (ERC), “ERC Decision of 1 June 1999 on the designation of the harmonised frequency band 40.5 to 43.5 GHz for the introduction of Multimedia Wireless Systems (MWS) including Multipoint Video Distribution Systems (MVDS)”, ERC/DEC(99)15, Conference Européenne des Administrations des Postes et des Télécommunications (CEPT).

[2] Electronic Communications Committee (ECC), “Recommended guidelines for the accommodation and assignment of Multimedia Wireless Systems (MWS) in the frequency band 40.5-43.5 GHz” ECC/REC/(01)04, Conference Européenne des Administrations des Postes et des Télécommunications (CEPT).

[3] J.D. Parsons, D.A. Demery, A.M.D. Turkmani, “Sounding techniques for wideband mobile radio channels: a review”, IEE Proceedings-I, vol.138, no.5, pp.437-446, october 1991.

[4] A.M. Street, A.P. Jenkins and D.J. Edwards, “High resolution time delay spread measurements on indoor channels using a spread-spectrum technique with off-line correlation”, IEE Colloquium on Microcellular Propagation Modelling, pp. 10/1-10/6. November 1992.

[5] I. Cuiñas, M.G. Sánchez, “Measuring, modeling and characterizing of indoor radio channels at 5.8 GHz”, IEEE Transactions on Vehicular Technology, vol.50, no.2, pp.526-535, march 2001.

[6] W.D. Burnside, K.W. Burgener, "High Frequency Scattering by a Thin Lossless Dielectric Slab", IEEE Transactions on Antennas and Propagation, vol. AP-31, no. 1, pp. 104-110, January 1983.

[7] L.M. Correia, P.O. Françês, "Estimation of materials characteristics from power measurements at 60 GHz", International Symposium on Personal, Indoor and Mobile Radio Communications, Den Haag (Netherlands), pp. 510-513, September 1994.

[8] J. Lähteenmäki, T. Karttavi, "Measurements of Dielectric Parameters of Wall Materials at 60 GHz Band", Electronics Letters, vol. 32, no. 16, pp. 1442-1444, August 1996.

[9] I. Cuiñas, M.G. Sánchez, “Permittivity and conductivity measurements of building materials at 5.8 GHz and 41.5 GHz”, Wireless Personal Communications, vol.20, no.1, January 2002.

[10] H. Xu, T.S. Rappaport, V. Kushya and H. Izadpanah, “Multipath measurements ans modelling for fixed broadband point-to-multipoint radio wave propagation links under different weather conditions”, IEEE 802.16.1pc-00/12r1, February 2000.