RADIO SIGNAL PROPAGATION MEASUREMENTS INSIDE BUILDINGS

M. Otmani*, M. Lecours** and A. Benkrid*

*Electronic Dept., College of Technology, P.O.Box 7650, Dammam, Saudi Arabia,(Tel.: 966 3 861 1910, Fax: 966 861 1979)

** Dept. of Elect. And Comp. Eng., University Laval, Quebec, Canada, G1X 7P4Tel: (418) 656-2984, Fax: (418) 656-3159, Email: mlecours@gel.ulaval.ca

Abstract: Indoor radio networks operating in the UHF and in the millimeter-wave frequency range mayoffer large information transport capacity and more sharply defined cell boundaries. The main features ofthe propagation in an indoor radio environment are multipath, due to reflection or diffraction from the wallsand surrounding objects, and shadowing of the direct or line-of-sight propagation path interveningobstacles. This paper reports the results of radio propagation measurement made within two location, bytransmitting a 900 MHz (cw) vertically polarized signal modulated by a train of 2 ns pulses. The receiver isequipped with a two branch diversity system (cross-polarized antenna), which makes possible theapplication of polarization diversity in order to reduce the deep fades that affect the signal envelope. It iswell known that diversity reception [1][2] is effective for reducing excessive deep fades that affect the signalenvelope.

1. Introduction

Next generation personal communicationsystems are designed for operation from 800 MHzto 2.0GHz frequency range. There is a growingdemand for such systems and this demand hasprompted research into propagation characteristicsof radio signals within buildings.

The propagation within buildings is verystrongly influenced by the local features i.e. thelayout of the particular building underconsideration and the building constructionmaterials used for the walls, floors and ceilings.The radio waves are attenuated as they passthrough walls and objects. There is multipathpropagation, since components of the signalmay arrive at the receiver at slightly differenttimes due to different delays after reflection anddiffraction from the surrounding. Thedestructive interference between the direct rayand the other multipath ray of the same order ofmagnitude can results in deep fades varyingwith time and physical motion. A useful method for studying themultipath radio channel to obtain moreinformation than with transmission in the CWmode is to record its impulse response. Thistime domain study is more tractable, bothexperimentally and analytically, than thefrequency response method. One approach is to

broadcast very short pulses, using a pulsedtransmitter.

The measurement system transmitter wasthus stationed in one location while thereceiver, synchronized to the transmitter, wasmoved to consecutive fixed positions, andrecorded the direct and reflected pulses ofenergy as a function of time.

Experiments were conducted using twodifferent transmit/receive configurations, amedium size room and a long corridor. All weremade in the second floor of the electricalengineering department of Laval University(Canada).

2. Measurement Apparatus

Radar like impulse response system (Fig.1)is used to perform the wide band impulseresponse measurements at frequency of 900MHz with a 7 dBm output power. The signal ismodulated by a train of 2 ns pulses with arepetition period longer than any delay observedin the test. The signal is radiated from avertically polarized antenna, which isomnidirectionnal in horizontal plane withheights of 1.5 m above the floor, and about 3 dBgain. The receiving antenna is a two branchdiversity reception cross-dipole (two crossedelements). The cross-antenna characteristic,verified in the anechoic room, gave a cross-polarization isolation of the order of 20 dB. The

received signal by each element of the cross-dipole antenna, once preamplified by a low noiseamplifiers (LANs) chain, is sent to the RF portof two mixers.

The bandwidth of each branch that is notlimited by filtering but solely by the componentscharacteristics is of the order of 500 MHz. Ouracquisition card can not directly treat suchsignals whose maximum tolerated samplingfrequency does not reach 300 kHz. To solve thisproblem, we have used the dual channel sampler(model 9605) from Lab Volt didactic radar [6].This sampler allows the pulsed signal receivedon each branch to be widened, and by means ofan adequate sub-sampling allows the originalsignal to be grossly restored, and henceeffectively reducing its bandwidth. In this way,the data acquisition can be made by means ofthe analogue to digital conversion card of thecomputer. Note that the receiver performs asimple demodulation and not a coherentdemodulation with signals Q and I.

The sampler is triggered by a clock signalthat is sent through a cable from the transmittersystem. Synchronization of both the transmitterand the receiver RF sources is achieved using theexternal reference inputs outputs. Synchronousoperation is made possible by running a 10 MHzreference signal from one synthesizer to the other.

Figure1: Measurement system

3. Description of sites

Experiments were conducted in a mediumroom and a long corridor in the second floor ofthe Electrical Engineering building at LavalUniversity, for different combinations oftransmitter-receiver locations. In the first case,the transmitter (Tx) and the receiver (Rx) werelocated in a corridor, in a line of sight (LOS)propagation path. The corridor is 2.3m wide, 4.5m high and 30 m long. The experimental siteshave the same characteristics: the floor is incement tile, the ceiling in metallic tile and thewalls in rugose plaster.

Both antennas have height of 1.5 m abovethe floor. The room plan under test is sketchedin Fig. 2. This room is filled with equipment; ithas also a Faraday cage of dimensions(2.5X3.1X2.5m) and an air conditioning system.

Figure 2: Measurement site

4- Experimental results

The first observation we can make from theindoor results measurement that at a shortdistance between the stations, the LOScomponent arrives not only first but has a clearpredominance over all other components,reflected or diffracted by the local environment.This level superiority diminishes with theincrease of distance between the stations. On thecontrary, in the case where signal is obstructedby the different obstacles of the measuring site(NLOS), the first intercepted impulse and thedifferent successive echoes, all come fromreflections or diffractions and have most of thetime relatively comparable levels.

The measurement were taken for distancesbetween transmitter and receiver varyingbetween 2 and 7 meters, except for the corridor

where the minimum distance was chosen to be 3m, in order to favor the LOS component over theother components. The limitation of the distanceto 7m is imposed by the measurement system.

At every measurement location, thetransmitter was thus stationed in one locationwhile the receiver was moved to consecutivefixed positions. A recording is made every 5 cm,which gives 6 recordings over a distance of 30cm. A mean value calculation is then performedover some individual consecutive signals inorder to evaluate the local mean of the signalsover each interval [3-5].

The different measurements are presentedin Figures 3 to 9. The horizontal scale of thesefigures corresponds to the number of samples ofthe data acquisition system. On the dual channelSampler, each 48 ns duration corresponds tonearly 175 samples. To show the repetition of theimpulses, we have taken for these figures some300 samples for the horizontal axis. We thereforesee 48 ns of a first pulsing signal followed by thestart of a second pulsing. The signal received overthe vertical channel allows the origin of time to befixed on these graphics.

Figure 3:The absolute value of four successivesignals received in LOS by the vertical element

Figure 4: The absolute value of foursuccessive

Signals received in LOS by the horizontalelement

Figures 3 and 4 show the absolute value of thesum of four successive signals received on thedirect LOS respectively in the room by the verticaland horizontal elements of the crossed dipoleantenna. For these figures, the recordings weremade over a distance of 20 cm, hence the series offour signals.

The same comment can be made concerningfigures 5 and 6, with the difference that in thesefigures the signal series is composed of fivesuccessive signals rather than four and that theywere obtained in the same room but without lineof sight (NLOS).

Figures 7 and 8 are composed of a series offive signals. These signals were obtained in thecorridor, in LOS condition.

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Figure 7: Signal received at the corridor(LOS) by the vertical element

Figure 5: Signals received at the room(NLOS) by the vertical element

Figure 8: Signals received at the corridor(LOS) by the horizontal element

In each case of Figure 9, (a) show the originalsignal and (b) its absolute value received in thecase of NLOS.

It can be seen from Figures 3 and 7 (b)that the direct path (first impulse) is predominantover all others rays. The second impulse of Figure3 which is first in Figure 4 is probably due to theFaradays cage which is the nearest obstacle to theemitting station. The other delayed paths arereceived via reflections or diffractions by the localenvironment. Both results show amplitudesdecaying exponentially from the direct path.

The multiple paths in Figures 5 and 6appear to be associated with a non-resolvablenumber of reflective and diffractive surfaces. Thiswidening of the impulses can be explained by asuperposition of retarded echoes. The last impulse

Figure 9: (1) Signal received by thevertical element at the corridor (NLOS)

(2) Signal received by the horizontalelement at the corridor (NLOS)

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in Figures 7 and 8 is probably due to the rayreflected from behind.

A visual comparison between thesefigures shows at first sight that the level of thedirect component of Figures 3 and 7 is nearlytwice as large as the highest of the reflected anddiffracted components. As a second remark, wecan generally say that the level of the componentsof signals received by the vertical element of thecrossed antenna is slightly higher than that of thecomponents of signals received by the horizontalelement.

5. Conclusion

Our measurement data reveals interesting andimportant multiray propagation for microcells at900 MHz. The propagation within buildings isvery strongly influenced by the local features, i.e.the layout of the particular building underconsideration and the building constructionmaterials used for walls, floors and ceilings. Thereare substantial variations in signal strength fromplace to place within a building. The signal can behighly attenuated after propagation a few metersthrough walls, ceilings and floors or my still bevery strong after propagation several meters alonga corridor.

The number of multipaths at the receiveris generally large, but the power content of eachpath would be different due to different physicallengths, caused by the propagation mechanism ofreflection and diffraction. The interference ofthese additional rays is a function of the relativeposition of the receiver and emitter with respect tothe room (corridor) walls. Both the transmitterand the receiver were stationary during theacquisition of each pulse response.

References

1. W. C. Y Lee, Y. S. Yeh, Polarizationdiversity system for mobile radio,IEEETrans. Comm., Vol. COM-20, no. 5 pp. 912-922, 1972.

2. W. C. Jakes, Microwave mobilecommunications, New York: Wiley, 1974.

3. D. C. Cox, Delay Doppler characteristics ofmultipath propagation at 910 MHz in asuburban mobile radio environment, IEEETrans. Ant. Prop., Vol. AP-20, pp. 625-635,Sept 1972.

4. Devasirvatham, D. M. J., Multipath timedelay spread in the digital portable radioenvironment, IEEE Commun. Mag. June1987, 25, pp. 13-2.

5. Devasirvatham, D. M. J., Time delay spreadand signal level measurements of 850 MHzradio waves in building environments, IEEETrans. Ant. Prop., Vol. AP-34, pp. 1300-1305, Nov. 1986.

6. Principes de fonctionnement des radars, vol.1, Lab-Volt, Quebec, Canada, 1991.

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