NTIA Report 98-351 

Indoor Direction Diversity at 5.8 GHz 

Robert J. Achatz 

Yeh Lo 

Elizabeth E. Pol 

U.S. DEPARTMENT OF COMMERCE 

William M. Daley, Secretary 

Larry Irving, Assistant Secretary 

for Communications and Information 

July 1998 

iii 

CONTENTS 

Page 

ABSTRACT ........................................................................................................................................... 1 

1. INTRODUCTION............................................................................................................................ 1 

2. MEASUREMENT DESCRIPTION................................................................................................. 2 

 2.1 Equipment.......................................................................................................................... 2 

 2.2 Location............................................................................................................................. 5 

 2.3 Procedure........................................................................................................................... 5 

3. ANALYSIS DESCRIPTION ........................................................................................................... 8 

 3.1 Antenna Diversity Technique............................................................................................ 8 

 3.2 Example........................................................................................................................... 10 

4. RESULTS....................................................................................................................................... 11 

 4.1 Mobile Station Antenna Delay Spreads .......................................................................... 11 

 4.2 Diversity Algorithm Delay Spreads ................................................................................ 16 

5. CONCLUSION .............................................................................................................................. 17 

6. REFERENCES............................................................................................................................... 18 

APPENDIX: MEASURED IMPULSE RESPONSES......................................................................... 19 

INDOOR DIRECTION DIVERSITY AT 5.8 GHz 

Robert J. Achatz1

, Yeh Lo1, Elizabeth E. Pol2

Complex impulse response measurements of a warehouse building were made at 

5.8 GHz with a sliding correlator channel probe. These measurements were made 

with vertically and horizontally polarized directional antennas so performance 

improvement due to polarization- and direction-diversity could be evaluated. 

Improvement in performance was determined by changes in the radio channel’s delay 

spread. Performance improved with direction diversity; however, adding polarization 

diversity yielded little improvement over direction diversity alone. 

Key words: complex impulse response measurements; direction diversity; 

polarization diversity; indoor propagation; delay spread 

1. INTRODUCTION 

Signals transmitted indoors are reflected, diffracted, and scattered by a building’s walls, floor, 

ceiling, and furnishings. These phenomena cause the transmitted signal to arrive at the receiver via 

two or more paths with unique delays, direction-of-arrivals, and polarizations. The delayed signals 

degrade receiver performance by causing intersymbol interference (ISI). Intersymbol interference can 

be reduced with direction diversity by passing signals arriving from a desired direction and 

attenuating signals arriving from all other directions [1]. Direction/polarization diversity, which adds 

polarization discrimination to direction diversity, may reduce ISI further [2,3]. 

These diversity measures are important for two reasons. First, decreasing ISI at the antenna 

minimizes the need for traditional multipath mitigation techniques such as equalization, which may 

require high-speed signal processing, and spread spectrum echo cancellation, which increases radio 

spectrum requirements. Second, the directional antennas used with these diversity measures become 

smaller as frequency increases. This helps to alleviate spectral congestion at lower frequencies by 

making higher frequencies more attractive for applications where compactness is desirable. 

The Institute for Telecommunication Sciences (ITS) performed complex impulse response 

measurements at 5.8 GHz in a warehouse building with vertically and horizontally polarized, 

directional antennas in order to determine the effectiveness of direction- and direction/polarizationdiversity. Delay spread, derived from the complex impulse response measurements, was used to 

quantify the improvement in performance due to these antenna diversity techniques. 

 1

 The authors are with the Institute for Telecommunication Sciences, National 

Telecommunications and Information Administration, U.S. Department of Commerce, Boulder, 

CO 80303. 

2

 The author was with the Institute for Telecommunication Sciences when this work was 

done. 
2

2. MEASUREMENT DESCRIPTION 

In this section the equipment used to measure the complex impulse response is described, the 

warehouse building where the measurements were conducted is depicted, and the measurement 

procedure is explained. 

2.1 Equipment 

The ITS sliding correlator channel probe [4] was used to measure the complex impulse responses. 

Figures 1 and 2 describe the channel probe’s transmitter and receiver, respectively. Pseudorandom 

noise (PN) sequences, 127 “chips” in length, are generated at a rate of 100 million chips/s in the 

transmitter modulator. The chips bi-phase modulate a 1.5-GHz carrier to produce a signal that is upconverted to the 5.8-GHz carrier frequency. The transmitter’s final amplifier delivers 23 dBm of 

signal power to the antenna. 

The receiver downconverts the signal to 1.5 GHz, quadrature demodulates it into baseband inphase 

and quadrature channels, and correlates a copy of the transmitted PN sequence with each channel. 

The variable attenuator before the receiver’s low-noise amplifier is adjusted to avoid amplifier 

saturation. The recording level of the digital audio tape recorder is also adjusted to maximize 

recording dynamic range. 

The receiver has a 9-dB noise figure, a dynamic range of approximately 35 dB, a time delay 

resolution of 10 ns, and an unambiguous time delay range of 1.27 μs. The 5-MHz oscillators in the 

transmitter and receiver are phase locked to each other by a signal transmitted over a coaxial cable. 

The receiver’s chip rate is slightly offset from the transmitter’s chip rate so that all transmission time 

delays up to the unambiguous time delay range are periodically measured. 

The receiver was mounted in a fixed rack while the transmitter was mounted on a cart with wheels; 

thus, in this report, the receiver is called the fixed station (FS) and the transmitter the mobile station 

(MS). The antenna height at the FS and MS was 2.1 m and 1.2 m, respectively. The antennas used 

by the FS and MS are summarized in Table 1 and 2. Mobile station horn A and B antennas are 

identical; however, horn B was rotated 90° so that it would be horizontally polarized. 
3
4
5

Table 1. Fixed Station Antennas 

 Antenna Type Biconical Log Periodic 

 Gain (dBi) 3 8 

 Polarization vertical vertical 

 Vertical 3-dB Beamwidth 60° 60° 

 Horizontal 3-dB Beamwidth 360° 60° 

Table 2. Mobile Station Antennas 

 Antenna Type Biconical Horn A Horn B 

 Gain (dBi) 3 15 15 

 Polarization vertical vertical horizontal 

 Vertical3-dB Beamwidth 60° 29° 22° 

 Horizontal3-dB Beamwidth 360° 22° 29° 

2.2 Location 

A warehouse shelving area, depicted in Figure 3, was measured. The shelving area is located at the 

end of a large building on the Department of Commerce campus in Boulder, Colorado. Three of the 

shelving area’s sides are outside walls while the fourth is open to the rest of the building. The walls 

and floor are made of concrete and the roof is made of galvanized steel panels. The walls are 

approximately 22 m long and the roof is roughly 6 m high. 

The shelving area contains 8 large metal shelves, 3 m high and 2 m wide, that hold boxes of office 

supplies and other miscellaneous goods. Two large shelves extend along the back and side wall. 

The remaining 6 large shelves are located adjacent to paths 1 through 6. The shelf next to path 6, 

which separates the shelving area from the rest of the building, holds a large quantity of metal stock 

(e.g., brass rods) used to manufacture specialized scientific apparatus. 

2.3 Procedure 

Four measurement configurations, shown in Figure 4, were used in the experiment. Each 

measurement configuration has a unique FS antenna type, FS azimuth, and FS location as 

summarized in Table 3. Although the transmitter was moved from one MS location to another, the 

measurements at each MS location were collected while the transmitter was stationary. Thirteen
6
7

unique MS measurements identified by MS antenna type and MS antenna azimuth were collected 

at each MS location. The first MS measurement used the biconical antenna while the last twelve 

varied horn azimuth and polarization as summarized in Table 4. Approximately 100 complex 

impulse responses were collected for each MS measurement. 

Table 3. Measurement Configurations 

. Configuration FS Antenna FS Antenna FS Antenna 

Type Azimuth Location 

1 Biconical Omnidirectional Comer 

2 Log Periodic 300° Comer 

3 Biconical Omnidirectional Center 

4 Log Periodic 180° Center 

Table 4. Mobile Station Measurements 

MS MS Antenna MS Antenna 

Measurement Type Azimuth 

1 Biconical Omnidirectional 

2 Horn A 0° 

3 Horn B 0° 

4 Horn A 60° 

5 Horn B 60° 

6 Horn A 120° 

7 Horn B 120° 

8 Horn A 180° 

9 Horn B 180° 

10 Horn A 240° 

11 Horn B 240° 

12 Horn A 300° 

13 Horn B 300° 
8

3. ANALYSIS DESCRIPTION 

In this section, our antenna diversity techniques are described and an example MS location is chosen 

to illustrate how the diversity algorithms were implemented. 

3.1 Antenna Diversity Technique 

The multipath channel’s effect on the transmitted signal is dependent upon the transmitted signal’s 

bandwidth. The multipath channel is “frequency flat” if its transfer function’s amplitude and group 

delay are constant over frequencies in the signal’s bandwidth. Conversely, the multipath channel 

is “frequency selective” if its transfer function’s amplitude or group delay changes over the same 

frequencies. The frequency-flat channel transfer function degrades reception by attenuating the 

signal whereas the frequency-selective channel degrades reception primarily by introducing ISI. In 

this study we were concerned only with the frequency-selective channel effects; we therefore 

assumed that signal attenuation was not limiting the radio link’s performance and used the antenna 

diversity algorithm to select the antenna whose direction or direction/polarization minimized the ISI. 

Our first task was to identify an algorithm for quantifying ISI using the measured impulse responses. 

We considered two algorithms. The first algorithm quantified ISI with the minimum mean-squarederror (MSE) between the signal distorted by the frequency-selective channel and the same signal that 

had been attenuated, phase shifted, and time delayed by some optimum A, θ, and τ respectively: 

( ( ) ( ) ( )) 2 2 ξ =< ∗ − −τ > φ s t h t Ae s t j (1)

where s(t) is the complex baseband representation of the signal and h(t) is the measured complex 

impulse response [5]. The second algorithm computed the “delay spread” of h(t), a measure that 

corresponds to signal energy dispersion in time. Since ISI is defined as the amount of extraneous 

energy from one symbol’s time interval that interferes with the energy of a symbol in another time 

interval, we assumed that ISI is correlated to delay spread [6]. We chose the delay spread calculation 

for this study because it does not require computation of an optimum A, θ, and τ. 

There are several measures of impulse response delay spread [7,8]. In this study we used the area 

moment of inertia of |h(t)|

2

. Analytically, we let 

2

p(t) = h(t) , (2)

normalized p(t) to P(t) with unit area 

∫

=

= max

0

( )

( ) ( ) τ

t

p t dt

p t P t , (3)
9

determined the mean delay m of P(t) 

∫

=

=

max

0

( )

τ

t

m tP t dt , (4)

and calculated the square-root area moment of inertia σ of P(t) 

( ) ∫

=

= −

max

0

2 ( )

τ

σ

t

t m P t dt . (5)

Throughout these calculations the value of P(t) was set to zero when it was less than 20 dB below the 

maximum P(t) amplitude to minimize the effects of noise. The function p(t) was averaged over time 

to remove additive noise. However, since the transmitter and receiver were stationary during each 

measurement, the data does not represent a “spatially averaged power delay profile” that 

characterizes the MS location as in [9] and therefore was not used to predict the bit error rate of 

radios operating in Gaussian, wide-sense stationary, uncorrelated scattering, channels as described in 

[10]. 

Our second task was to define the direction- and direction/polarization-diversity algorithms. The 

direction diversity algorithm assumes the radio link has a vertically polarized, biconical or log 

periodic antenna at one end of the radio link and an array of vertically polarized, equally spaced in 

azimuth, horn antennas at the other. The direction diversity algorithm selects the horn antenna 

azimuth with the smallest delay spread. In our study the direction diversity algorithm selects the 

smallest horn A antenna delay spread from MS measurements 2, 4, 6, 8, 10, or 12. 

The direction/polarization diversity algorithm assumes the radio link has a vertically polarized, 

biconical or log periodic antenna at one end of the radio link and an array of vertically and 

horizontally polarized, equally spaced in azimuth, horn antennas at the other. The 

direction/polarization diversity algorithm selects the horn antenna azimuth and polarization with the 

smallest delay spread. In our study the direction/polarization diversity algorithm selects the smallest 

horn A or horn B antenna delay spread from MS measurements 2 through 13. 

The no-diversity case assumes the radio link has a vertically polarized, biconical or log periodic 

antenna at one end of the radio link and a vertically polarized, biconical antenna at the other. In our 

study the no-diversity case always uses the delay spread in MS measurement 1. 
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3.2 Example 

Table 5 shows the delay spreads for configuration 3, MS location 9, for each of the MS 

measurements. The measured p(t) functions used to compute the delay spreads are shown in 

Appendix A. The MS antenna delay spreads used in the antenna diversity algorithm are listed in 

Table 6. The horn A and horn B antenna delay spreads reported in Table 6 refer to the horn’s 

smallest delay spread over all measured azimuths. For this example, both horn antennas have lower 

delay spreads than the biconical antenna; however, the vertically polarized horn A antenna has a 

lower delay spread than the horizontally polarized horn B antenna. 

The diversity algorithm delay spreads for this example are summarized in Table 7. The no-diversity 

case corresponds to the biconical MS antenna delay spread, the direction diversity case corresponds 

to the minimum horn A MS antenna delay spread, and the direction/polarization diversity case 

corresponds to the minimum horn A or horn B MS antenna delay spread. 

Table 5. Delay Spreads for Configuration 3, MS Location 9 

MS MS Antenna MS Antenna Delay Spread 

Measurement Type Azimuth (ns) 

I Biconical Omnidirectional 25.9 

2 Horn A 0° 23.7 

3 Horn B 0° 22.9 

4 Horn A 60° 08.0 

5 Horn B 60° 22.1 

6 Horn A 120° 24.1 

7 Horn B 120° 18.1 

8 Horn A 180° 25.5 

9 Horn B 180° 22.7 

10 Horn A 240° 19.9 

11 Horn B 240° 32.0 

12 Horn A 300° 27.8 

13 Horn B 300° 30.3 
11

Table 6. MS Antenna Delay Spreads for Configuration 3, MS Location 9 

 MS Antenna Type Biconical Horn A Horn B 

 MS Measurement 1 4 7 

 Delay Spread (ns) 25.9 8.0 18.1 

Table 7. Diversity Algorithm Delay Spreads for Configuration 3, MS Location 9 

 Diversity No-diversity Direction Direction/ 

 polarization 

 MS Measurement 1 4 4 

 Delay Spread (ns) 25.9 8.0 8.0 

4. RESULTS 

The MS antenna delay spreads for all configurations and MS locations are presented. Cumulative 

distributions of diversity algorithm delay spreads are shown for all configurations to statistically 

quantify the effectiveness of the diversity measures. 

4.1 Mobile Station Antenna Delay Spreads 

Using the method described above, MS antenna delay spreads were computed for all configurations 

and MS locations. The results, which correspond to the example results in Table 6, are shown in 

Figures 5 through 8. The figures show that the vertically polarized horn A antenna had a lower delay 

spread than the biconical antenna at most MS locations. Exceptions to this include configuration 1, 

MS location 8; and configuration 2, MS location 23. Vertically polarized horn A antenna delay 

spreads were also lower than horizontally polarized horn B antenna delay spreads at most MS 

locations. Exceptions to this include configuration 1, MS locations 11, 14, and 15; and configuration 

2, MS locations 10, 11, 14, and 17. The biconical antenna delay spread of configuration 1, MS 

location 10 was approximately 70 ns, which exceeds the graph’s vertical scale. 

Delay spreads for MS locations 13-24 are generally higher than delay spreads for MS locations 1-12. 

This trend corresponds to observations [4] that delay spread typically increases with path distance. 
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15
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Table 8. Maximum Delay Spreads for 50% of Mobile Station Locations 

 Diversity Delay Spreads (ns) 

Configuration No-diversity Direction Direction/ 

 Polarization 

1 30 17 16 

2 23 15 15 

3 22 13 13 

4 15 9 9 

Table 9. Maximum Delay Spreads for 90% of Mobile Station Locations 

 Diversity Delay Spreads (ns) 

Configuration No-diversity Direction Direction/ 

 Polarization 

1 42 22 22 

2 42 25 23 

3 27 19 17 

4 18 17 17 

4.2 Diversity Algorithm Delay Spreads 

Diversity algorithm delay spreads, which correspond to the example results in Table 7, for each 

configuration and MS location can be obtained from Figures 5 through 8. The no-diversity delay 

spread is the same as the biconical MS antenna delay spread, direction diversity delay spread is the 

smallest horn A antenna delay spread, and direction/polarization diversity delay spread is the 

smallest of the horn A and horn B antenna delay spreads. Figures 9 through 12 show the cumulative 

distribution functions (CDF’s) of the diversity algorithm delay spreads. The CDF’s clearly show that 

direction- and direction/polarization-diversity lower the delay spread from no-diversity values. The 

CDF’s also show that adding polarization diversity to direction diversity does not improve 

performance significantly. Tables 8 and 9 summarize the maximum delay spreads for 50% and 90% 

of MS locations. 
17

5. CONCLUSION 

The results of this study indicate that direction diversity, implemented at one end of a 5.8-GHz 

indoor radio link, decreases the delay spread relative to the delay spread measured without diversity. 

These results were true for all four combinations of fixed station antenna types, azimuths, and 

locations depicted in Table 3. Figures 5 through 8 show that the horizontally and vertically polarized 

horn antennas have lower mobile station antenna delay spreads than the biconical antenna. The 

statistics of diversity algorithm delay spread, independent of mobile station location, are depicted in 

Figures 9 through 12. These statistics show that reductions in median direction diversity delay spread 

ranged from approximately 35% to 45% depending on the configuration. Adding polarization 

diversity to direction diversity decreased the delay spread in a small number of mobile station 

locations; however, these locations had little impact on the 50% and 90% quantiles of delay spread 

statistics summarized in Tables 8 and 9. 

The decrease in delay spread due to direction and direction/polarization diversity shows that arrays of 

directional antennas can be used to reduce intersymbol interference in a frequency-selective 

multipath channel. Radio link engineers know that propagation losses, which increase with 

frequency, can be compensated for by using higher gain, directional antennas. They also know that 

directional antenna size decreases as frequency increases if gain is held constant. These attributes of 

directional antennas combined with the results of this study indicate that wideband, indoor radio links 

using directional diversity may be well-suited to higher microwave frequencies bands. 
18

6. REFERENCES 

[1] J.E. Mitzlaff, “Radio propagation and anti-multipath techniques in the WIN environment,” IEEE 

Network Magazine, pp. 21-26, Nov. 1991. 

[2] T.S. Rappaport and D.A. Hawbaker, “Wide-band microwave propagation parameters using 

circular and linear polarized antennas for indoor wireless channel,” IEEE Trans. Commun., pp. 240-

245, Feb. 1992. 

[3] D.C. Cox, “Cross-polarization coupling measured for 800 MHz radio transmission in and around 

houses and large buildings,” IEEE Trans. Ant. Prop., pp. 83-87, Jan. 1986. 

[4] P.B. Papazian, Y. Lo, E.E. Pol, M.P. Roadifer, T.G. Hoople, and R.J. Achatz, “Wideband 

propagation measurements for wireless indoor communication,” NTIA Report 93-292, Jan. 1993. 

[5] M.C. Jeruchim, P. Balaban, and K.S. Shanmugan, Simulation of Communication Systems, New 

York: Plenum Press, 1992, pp. 479-486. 

[6] G.A. Hufford, “A characterization of the multipath in the HDTV channel,” IEEE Trans. 

Broadcasting, pp. 252-255, Dec. 1992. 

[7] G.A. Hufford, R.W. Hubbard, L.E. Pratt, J.E. Adams, and SJ. Paulson, “Wide band propagation 

measurements in the presence of forests,” U.S. Army Communications-Electronics Command Report 

CECOM 82-CS029-F, Jan. 1982. 

[8] J.D. Parsons, The Mobile Radio Propagation Channel, London: Pentec Press, 1992, pp. 162-188. 

[9] D.M. Devasirvatham, “Multipath time delay spread in the digital portable radio environment” 

IEEE Commun. Magazine, pp. 13-21, Jun. 1987. 

[10] J.C-I Chuang, “The effects of time delay spread on portable radio communications channels 

with digital modulation,” IEEE J. Selected Areas Commun., pp. 879-889, Jun. 1987. 
19

APPENDIX: MEASURED IMPULSE RESPONSES 

Figures A-1 through A-7 show the magnitude of the measured impulse response for configuration 3, 

mobile station location 9 for each of the 13 mobile station measurements listed in Table 4 of this 

report. 

Figure A-1. Magnitude of measured impulse response for MS measurement 1. 
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