RADIO PROPAGATION STUDIES IN A SMALL CITY FOR UNIVERSAL PORTABLE COMMUNICATIONS

Daniel M. J. Devasirvatham

Bell Communications Research Inc. Radio Systems Research Division

331 Newman Springs R o d . NVC 3X-343 Red B.nlr, NJ 07701-7020

ABSTRACT

Time delay spread and signal level measurements were made in the rtreets of a small city at 850 MHz. The maximum root mean square (rms) time delay rpread observed was 640 nanorcconds (na). However, this dropped to 330 nr for cell radii of 150 meters, a more likely rize for personal communications syrtems. Also. rms delay spread waa under 110 ns for cell radii under 75 meten or where there was a strong line-of-sight path. When both the transmitter and receiver were on the same road, received power levels followed a two-path model, together with about 0.02 dWm additional attenuation.

A weak but consistent relationrhip of a decade of increase in rms time delay rpreul for about 80 dB decrease in received power levels, was also found at thir site.

INTRODUCTION

The design of digital communications receivers for universal digital portable communications is a challenging task. The receivers must work in an environment which has multipath propagation, shadow fading, and moderate motion . These factors combine to give deep fades varying with time and physical motion. pulse spreading, and hence, intersymbol interference 01 .

The impulre response of the radio channel yieldr useful information about its capacity. The root mean square (rms) time delay spread of the impulse response can be related to intersymbol interference produced by the channel I3] I4] . Intersymbol interference limits the usable digital signPling rate in the medium for a given error rate. The first measurements of time delay spread at three office buildm s and two residences were reported earlier by Devasirvatham Is? 16] ('1 [*I f91 . In these studier, root mean square time delay spreads under 100 ns were consistently measured within the buildings when there was a direct linesf-sight path between the transmitter and receiver. Where there was no line of sight, rmr delay spreads of much as 250 ns were seen. On inside-to-outside paths in two reridencer and a medium-sized office building, rms delay spreads were again under 100 ns when there was a good direct path. Rmr delay spreads of up to 420 ns were meaaured where there was no direct line of sight.

This paper represents the continuation of these studies, and describes time delay spread and signal level measurements made in the rtreets of the central portion of a small city.

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TEEEXPERIMENT

The experiment to measure the impulse rerponse of the radio propagation channel has been reported in detail in In . It followr the princi les of an earlier measurement in the mobile radio environment . Briefly, 8SO MHz carrier, bi-phase modulated by a 40 Mbitls peudonoise code, is broadcast by a transmitter. The rignd ruffen time-smear in the propagation channel and is then correlated at the receiver with an identical preudonoire code, running 4 kbitls slower. Ai the codes sweep past each other, the receiver t races out the power-delay profile of the channel. In the ideal channel, the receiver OuQut would be a narrow triangle with a base width of two clock periods, i.e., 50 nr . In the prerence of multipath, it is spread out in time. The system, thus, acts like a bi-static radar, measuring the (un-normalized) impulse response of the medium. In this implementation of the experiment, only the envelope of the power-delay profile ir recorded.

The measurements reported here were made on the streets of Red Bank, NJ. Both the transmitter and the receiver were carried in vehicles. The receiver was in the Bellcore mobile radio research vehicle. The receiving antenna was a sleeve dipole. which was mounted vertically on a mast and raised to a height of 9.1 meten above ground level. This is approximately the height of a lamp port, which would be a possible location for a base station antenna in a portable communications system rerving users walking on sidewalks or within buildings.

The transmitter had an output power of 10 watts. It was housed in a mini-van and powered by an inverter. The tranrmitting antenna was also a sleeve dipole, fed by a 10 m length heliax cable. It was attached vertically to a horizontal 0.6 m motorized rotary arm mounted on a tripod. The tripod waa placed on the sidewalks of the streets of Red Bank so that the height of the antenna above ground level was about 1.8 m. This represented a typical height of a standing handset user.

At every measurement location, the transmitter antenna was rotated through a horizontal 1.2 m (4 foot) diameter circle, 01 rhown in Figure 1. A power-delay profile, as described above, was obtained at eight equally spaced positions on the circle. The receiver output waa sampled at an effective rate of 1 01 2 nanoseconds (ns) per point . Two thousand forty eight pointa, representing either 2048 or 4096 ma of the power-delay profile, depending on the time scale, were digitized and stored, after verifying that there was no signal at greater time delays.

Twelve receiver van sites were chosen around the city. Many transmitter locations were used for each of the twelve receiver sites. A total of 300 combinations of transmitter and receiver locationr were used. Figure 2 shows these locations. The rcceiver rites are shown by triangles and the transmitter locations are shown by circles in the figure. The measurement locationr were within an area of 1100 m x 750 m. Most

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FIgure 1. Measurement method to acquire eight power-delay profiles at a location.

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Figure 2. Map of area studied in Red Bank, showing receiver and transmitter locations.

buildings are three stories or lower, though there are a few buildings that have up to 10 stories in the northern and northwestern regions. The southern region has many houses, including some large houses which have been converted into offices. The southeastern side rises up to a small hill. Many of the streets have trees growing along the sidewalks.

The eight individual profiles were then power averaged at each time delay to calculate an averaged power-delay profile. This averaged profile characterizes the neighborhood of the circle for the particular transmitter-receiver location combination. An averaged power-delay profile is shown in Figure 3. Delays measured from the point at which the signal rises out of the noise floor will be referred to as " Excess Time Delay", or " Time Delay", for short.

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Figore 3. Averaged power-delay profile corresponding to the eight profiles measured at a location in Red Bank.

Subsequently, averaged data up to 1800 samples after the first arrival of the signal were used for analysis, again after verifying that there was no signal beyond this point. The rest of the 2048 samples were used to estimate the noise floor of the signal.

The "Root Mean Square Time Delay Spread", Td, is defined as the square root of the second central moment [61 17] of the averaged profile. The rms delay spread is the measure of channel dispersion that has the most significant effect on the digital error rate ['I 13] 141 .

It can also be shown ['I that the total power under the averaged power-delay profile is equal to the averaged power from a cw source that would be received by a cw receiver within the area. This was then normalized by the averaged power received at a distance of 0.3 m ( 1 foot ) from the transmitter, a convenient distance, to give a measure of the relative path loss.

RESULTS

Figure 4 shows the cumulative distributions of rms time delay spread measured in Red Bank. The solid curve shows the results for all locations measured. The rms delay spread exceeded 400 us for 5% of the locations. The maximum rms delay spread measured was 640 us. This corresponded to the power delay profile shown in Figure 3. There was no direct line of sight in this case. It is also clear from the geometry that the positions of the transmitter and receiver in this case would not be representative in a practical personal communications system in Red Bank.

The other five curves show the cumulative distribution of rms delay spread when the distance between the transmitter and receiver was limited to be less than 600, 450, 300, 150, and 75 meters, respectively. In the first three cases, the maximum rms delay spread still exceeded 500 us. At a 150 m cell radius, the maximum rms delay spread dropped to 330 us. Finally, when the cell radius was limited to 75 m the maximum rms delay spread measured was 108 us.

Figure 5 plots the same set of curves, but only for data where both the receiver and transmitter were on the same road. Ignoring any slight curves in the road, this may be defined as "line-of-sight" data. Now the rms delay spread dropped to 270 us even as far away as 600 meters. The maximum rms delay spread for distances up to 150 meters was only 62 ns.

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Figon 4. Cumulative distributions of rms time delay spread, by cell radius, for all the data.

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Pigore 5. Cumulative distributions of rms time delay spread, by cell radius, for same-road data.

Next we consider power levels. The average power which would be received by a cw receiver over the scanning area was calculated and normalized to the power at 0.3 m ( 1 foot ), a convenient distance. It might be expected that the propagation mechanisms would differ for the dose-in points, where there would be a strong line of sight. The larger distances would also include non-line-of-sight cases. Hence it would be reasonable to expect different propagation laws to dominate in this region. There was some evidence of th is behavior in the data. However, there was also a significant increase in the sprcad of the data at the larger distances, on the order of 40 dB, precluding such simple segmentation.

Hence, in Figure 6, only the subset of the data where both the transmitter and receiver were on the same road is considered. The received power level is plotted against distance, d. There data have much less spread, on the order of 10 dB at most. Further analysis showed that the slope changed at about 180 m range. Hence the data are plotted in two groups, with the crosses showing data at less than 180 m and the circles showing the data at greater ranges.

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M E IN MTERS Figore 6. Scatter plot of received relative power with distance

for same-road data, together with linear regression and two-path model fits.

Linear regression fits for the data in these two ranges gave

Pde = -15.7 -19.7 log d d < 180 m , (la)

PdB = 65.9 -56.0 log d d z 180m . (lb)

the following results.

These are shown as dashed curves in Figure 6.

The coefficient of determination was 0.96 in each case. While the short-range data suggest d-*, i.e., free space propagation, the d-5.6 law at greater distances is not explainable in simple physical terms. Propagation over a flat earth tends to a d-4 law at large distances.

Several approaches were tried to explain this. These included the curvature of the earth, as well as simple models of the rises and dips in the road. The best results were obtained by assuming a simpk two-path ( line-of-sight path plus ground reflection ) model for propagation, together with a uniform attenuation factor, a. This additional attenuation could be caused by the trees l i i g the streets, and other objects such as can and people on the road. This model was fitted to the data at all distances, while varying all parameters. The results of the regression were as follows:-

PdB = "O-Path mOdel(hT,hR,d) + a . d + Po . (2)

The best fit values were

a = 0.022dWm

Po = -17.8 dB . Here hT and hR are the transmitter and receiver heights, rapectively. PO is the constant of proportionality. It corresponds to the relative power received at unit distance. The b a t fit Values for hT and hR were 1.4 m and 9.7 m. While the bat-fi t transmitter antenna height is a little low, the receiver (base station) antenna height is very close. This result is shown as the solid curve in Figure 6. The coefficient of determination was 0.98 .

It should be further noted that a very deterministic model is being fitted to statistical data. As the superimposed Curve shows, the deep nulls formed by the perfect ground reflection assumed will reduce the accuracy of the fit. The slightest deviation from perfect reflection will wash out the nulls. Also, the m a l data is averaged over eight samples. Considering these limitations of the model, it is felt that the results indicate a reasonable approach to the 'same road" propagation case.

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Figure 7. Scatter plot of received relative power with distance for non-line-of-sight data, together with linear regression fit.

Figure 7 shows the rest of the power data, i.e., for the cases when the transmitter and receiver were not on the same road. Here, the propagation is by reflection and penetration through buildings. In some cases there could also be some leakage around corners and "guiding" by the buildings along the streets. Also, in some cases, though the transmitter and receiver were on different streets, one of them was, physically, at the corner of the intersection of the streets. These show up as data points where the received power is much higher than the rest of the data at that range. Clearly, these data are a more complex collection which is not amenable to simple modeling. Hence only a linear regression fit was attempted. The best fit was:-

( 3 4 PdB = 19.7 - 45.1 log d , where d is the distance in meters. The coefficient of determination was 0.82 .

Cox et a1 have made continuous wave attenuation measurements in and around eight houses, using similar antenna heights, at 900 MHz ['I1 [ I 1 . Their overall regression result, when converted to the same reference, is

(3b) PdB = 18.6 - 45 log d , where d is again in meters. These two results agree with each other within 1 dB over the measured distances. Considering the very different techniques used, the different measurement sites and the time difference, this closeness of the results is remarkable.

Next, attempts were made to relate rms delay spread to any of the observables. Considerable effort was made to relate delay spread to distance. Any results were cross checked for consistency with previous measurements made in houses and buildings. These efforts were unsuccessful. No consistent correlation was found between delay spread and distance. Indeed, the results were contrary to one another in different measurements. In some sites, there was no correlation whatsoever. In other places, the delay spread even decreased with distance. In Red Bank, there was a general increase with distance, but without any clear pattern. The wide variability in the measurement sites was reflected in the data.

Consequently, a fit between delay spread and received power was attempted next. This was also cross checked with the prrvious measurements from other sites. It was found that these results were consistent, although the relationship was still weak.

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Figure 8. Scatter plot of rms time delay spread with received relative power for all the data; logarithmic scale.

Figure 8 shows the plot of log( rms delay spread ) , log( Td ), with received power in dB, for all locations in Red Bank. The regression line for these data was

log( Td ) = 1.048 -1.298 lo-' X PdB . (4)

The coefficient of determination was 0.77 . The variance was approximately 0.2, corresponding to a fractional variation in Td of 2.8 . The inverse slope is equivalent to about 77 dB of power per decade of delay spread. When the data were partitioned into sub-groups such as the same-road data, the slope and the intercept varied by less than 5%. The results in other sites were within about 50% of tbese values. More importantly, the trend was consistent. Furthermore, the spread of the data about the regression line was also relatively constant.

DISCUSSION

The measurements in Red Bank must be viewed in the context of the ultimate goal of a radio system architecture. This is the provision of universal portable communications with low power transmitters and low antenna heights, using relatively small cells compared to today's mobile radio systems. In such a system, a more densely populated area such as a small city will have a larger number of smaller cells than a single-family- housing zoned suburb.

The results of the measurements in Red Bank reiterate those of measurements made earlier in very different environments. In general, measurements in all sites have shown the maximum rms delay spread in practical cell sizes to be less than 400 ns. In the case of Red Bank, this requires cells of under 150 m radius. Where there is line-of-sight, the allowable cell radius increases to 600 m; but this may not be usable from traffic density considerations. Where there was strong line-of- sight, as in cells of under 75 m radius in Red Bank, the maximum rms delay spread was under 110 ns.

The fit of a two-path model with attenuation to the same- road data is intuitively satisfying. The off-line-of-sight data fits well with previous residential data.

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The discovery of a consistent relationship between rms delay spread and received power is potentially useful. The lack of dependence on the environment may be attributed to the fact that both quantities are derived from the radio signal which had traversed the terrain. The terrain information is embedded in both. When either delay spread or power is related to distance alone, a variable which has no terrain information contained in it, one may naturally expect wider variations.

Time delay spread measurements are complicated, costly and slow. Power measurements, on the other hand, are simple and cheap. The ability to derive an order-of-magnitude estimate of delay spread from power measurements would be a very useful tool to design and field engineers.

Studies of the relationships between time delay spread and channel capacity show that radio links in these cnvironmenta could support a few hundred kbith in an unequaliied channel I3].

SUMMARY

Time delay spread and signal level measurement8 were made at 850 MHz in the streets of a small city over distances of up to 1100 m. The maximum rms time delay spread measured was 640 ns. However, this dropped to 329 ns for cell radii below 150 m. Considering the higher population density in the city, which would mandate a smaller cell, the worst-case rms time delay spread in such places is likely under 400 ns. The rms time delay spread was under 110 no for all cell sizes under 75 m, or in cases of strong line-of-sight. The latter is consistent with results from all previous measurements, whether inside office buildings or around houses, despite the wide differences in the locations themselves.

Received power levels in the city, when both the transmitter and receiver are in the same road, follow a two-path model, together with about 0.022 dB/m additional attenuation In off-lie-of-sight cases, the power-distance relationship fits a d-'.' power law, in good agreement with previous cw measurements in residential areas.

Rms time delay spread and distance are not well related. A weak but consistent relationship was found, however, between rms delay spread and relative received power. Generally, a higher received relative power is associated with a lower rms delay spread. The delay spread increases one order of magnitude for about 80 dB decrew in relative received power level. Most significantly, this type of relationship seems to be true in the city as well as in all previous measurements, whether inside office buildings or in and around houses.

These results continue to indicate that data rates of a few hundred kbith could be supported in these environments with an unequalized channel.

REFERENCES

1. Cox, D. C.: "Universal Digital Portable Radio Communications", Proc. IEEE, Vol. 75, pp 436-477, April 1987.

2. Bello, P. A. and Nelin, B. D.: "The Effect of Frequency Selective Fading on the Binary Error Probabilities of Incoherent and Differentially Coherent Matched Filter Receivers", IEEE Trans. Commun. Syst., Vol. CS-11, pp 170-186, June 1963.

3. Chuang, C-I.: "The Effects of Time Delay Spread on Portable Radio Communications Channels with Digital Modulation', IEEE JSAC, Special Issue on Portable and Mobile Communications, Vol. SAC-5, No. 5, pp 879-889. June 1987.

4. Jaker, W. C: editor, "Microwave Mobile Communications", John Wiley and Sons, 1974.

5. Devasirvatham, D. M. J.: "Time Delay Spread Measurements of Wideband Radio Signals Withii a Building", Electronics Letters, Vol. 20, No. 23, pp 950-951, 8th November 1984.

6. Devasirvatham, D. M. J.: "Time Delay Spread and Signal Level Measurements of 850 MHz Radio Waves in Building Environments", IEEE Trans. Ant. and Prop., Vol. AP-34, No. 11, pp 1300-1305, November 1986.

7. Devasirvatham, D. M. J.: "A Comparison of Time Delay Spread and Signal Level Measurements Within Two Dissimilar Office Buildings", IEEE Trans. Ant. and Prop.,

8. Devasirvatham, D. M. J.: "Multipath Time Delay Spread in the Digital Portable Radio Environment", IEEE Communications Magazine, Vol. 25, No. 6, pp 13-21, June 1987.

9. Devasirvatham, D. M. J.: "Propagation Time Delay Jitter Measured at 850 MHz in the Portable Radio Environment", IEEE JSAC Special Issue -on Portable and Mobile Communications, Vol. SAC-5, No. 5, pp 855-861, June 1987.

10. Cox, D. C.: "Delay Doppler Characteristics of Multipath Propagation at 910 MHz in a Suburban Mobile Radio Environment", IEEE Trans. AP-S, Vol. AP-20, No. 5, pp 625-635, September 1972.

11. Cox, D. C., Murray, R. R., and Norris, A. W.: "800 MHz Attenuations Measured in and around Suburban Houses", , AT&T Be11 Laboratories Technical Journal, Vol. 63, No. 6 ,

Vol. AP-35, NO. 3, pp 319-324, March 1987.

pp 921-954, July 1984.

ACKNOWLEDGEMENTS

Thanks are due to H. W. Arnold and D. C. Cor for their support, guidance and commentary on this work. Other members of the experimental study team were R. R. Murray, M. J. Krain, R. J. Dell, L. G. Schimpf, and C. Insuaste. This work would not have been possible without their dedication and cheerful effort. The assistance of the Red Bank police and city authorities is greatly appreciated.

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