NTIA-REPORT-79-24

Investigation of Digital

Microwave Communications

in a Strong Meteorological

Ducting Environment

R. W. Hubbard

u.s. DEPARTMENT OF COMMERCE

Juanita M. Kreps, Secretary

Henry Geller, Assistant Secretary

for Communications and Information

August 1979
Preface

This study was conducted with the support

and cooperation of the Capabilities

Development Department, Pacific Missile

Test Center (PMTC) Point Mugu, California

93042 under MIPR No. N6312678MP31106.

ii
1.

2.

3.

4 •

5.

6 •

7.

TABLE OF CONTENTS

ABSTRACT

INTRODUCTION

BACKGROUND

EXPERIMENT DESIGN

3.1 Link Description

3.2 Impulse Response Measurements

3.3 Error Rate Measurement

3.4 Angle Diversity Measurements

SUMMARY OF RESULTS

4.1 Meteorological Conditions

4.2 Impulse Response Data

4.3 BER Performance vs. Channel Conditions

4.4 Angle Diversity Measurements

CONCLUSIONS AND RECOMMENDATIONS

ACKNOWLEDGEMENTS

REFERENCES

APPENDIX

iii

Page

1

1

2

4

4

6

9

10

11

11

12

30

52

56

61

62

63

LIST OF FIGURES

Page

5

7

14

Figure 1.

Figure 2.

Figure 3.

The Laguna Peak (LP) receiver terminal.

Simplified block diagram of the EN Channel

Probe. The receiver is dual channel; only

one channel is shown.

Examples of the power impulse response

measured over the SNI-LP link with a P~

clock rate of 15_0 MHz.

Figure 4. The theoretical frequency transfer function

for a channel with a multipath component

equal in magnitude to the direct component,

at the delay time indicated. 17

Figure 5(a). Refractive index profile for Pt. Mugu

at 1058 PST, 31 August 1978. 20

Figure 5(b). Refractive index profile for SNI at 0904

PST, 31 August 1978. 21

Figure 6( a) . Refractive index profile for Pt. Mugu

at 1549 PST, 31 August 1978.

Figure 6 (b) . Refractive index profile for SNI at

1458 PST, 31 August 1978.

22

23

Figure 7. An example of the dynamic magnitude change

between multipath components. 25

Figure 8. Examples of the power impulse response

where the ffiultipath component delay is

within the resolution of the probe. 27

Figure 9. Sample chart record. Channel nomenclature

shown in Figures 10 - 13. 32

Figure 10(a). The average power impulse response

measured between 1255 and 1300 PST,

31 August 1978, at a PN clock rate

of 50 MHz. 34

Figure 10(b). Strip chart record for 1255 - 1300 PST,

31 August 1978. 35

Figure 11(a). The average power impulse response

measured between 1300 and 1305 PST,

31 August 1978, at a PN clock rate

of 50 MHz.. 37

Figure ll(b). Strip chart record for 1300 13D5 PST,

31 August 1978. 38

v
Figure A-2.

Figure A-3.

Figure A-4.

Figure A-5.

Page

Figure 12(a). The average power impulse response

measured between 1305 and 1310 PST,

31 August 1978, at a PN clock rate

of 50 MHz. 41

Figure 12(b). Strip chart record for 1305 - 1310 PST,

31 August 1978. 42

Figure 13(a). The average power impulse response

measured between 1310 and 1315 PST,

31 August 1978, at a PN clock rate

of 50 MHz. 43

Figure 13(b). Strip chart record for 1310 - 1315 PST,

31 August 1978. 44

Figure 14. Time-lapse photographs of the power

impulse response measured during the

fading period in Ch. 1 beginning at

1308 PST, 31 August 1978. 47

Figure 15. Strip chart record; nomenclature

same as Figure 10 - 13. 48

Figure 16. Strip chart record; nomenclature

same as Figures 10 - 13. 49

Figure 17. Time-lapse photographs of the power

impulse response measured between

1317 and 1346 PST, 31 August 1978. 50

Figure 18. The probability density functions (pdf)

for the tilted beam experiment, 1520 to

1730 PST on 30 August 1978. 55

Figure 19. The cumulative probability functions for

the data of Figure 18. 55

Figure A-I. The probability density functions of

the power impulse width for the low

antenna on 31 August 1978. These data

correspond to Figures 10 through 13. 67

Power impulse width distributions for

22 Auqus t; 1978. 68

Power impulse width distribution functions. 69

Power impulse width distributions for

25 August 1978. 70

The power impulse width distribution

functions for the data of 31 August

1978. These data were measured between

1112 and 1200 PST at a PN clock rate of

150 MHz, just prior to the period

discussed in Section 4. 71

vi
Page

Figure A-6. Power impulse width distributions for

1 September 1978. 72

Figure A-7. Recordings of the PN probe RSL for the

multipath conditions depicted by the

distribution functions in Figure A-6.

Note that the phase interference fading

is worse for the afternoon data. This

fact is reflected in the higher

probability that the power impulse width

was less than standard, as shown by the

distribution functions of Figure A-6(b). 73

Figure A-8. Power impulse width distributions and

RSL record for a night run on 5 September 1978. Note that the distribution

function for the impulse data on the

low antenna is quite similar to that of

Figure A-6. The fading however is much

slower, indicating less dynamic change

in the multipath components. Two very

deep fades are seen around 1945 PST,

with a signal enhancement between them. 74

Figure A-9. Power impulse width distributions for

6 September 1978. 75

Figure A-IO~ The RSL records of the PN probe data for

the low antenna distributions of Figure

A-9. The delay spread of the multipath

is similar for each period; however, the

dynamics of the channel cause deeper

fading over the sunset period. 76

Figure A-II. Power impulse width distributions and

the RSL record for an all night run

on 7-8 September 1978. The multipath

delay was essentially the same for both

diversity channels; however, the fading

was significantly different. As seen

in (b), the low antenna RSL was very

stable. The fading in the high antenna

channel was severe, as noted from the

impulse width distribution in (a). 77

Figure A-12. Power impulse width distributions and the

RSL record for the early morning and

sunrise period on 8 September 1978.

Both channels began to fade during this

period, and the impulse response for

the low antenna indicated a three-path

structure as noted in the distribution

function of (a). 78

vii
Figure A-15.

Figure A-16.

Figure A-17.

Figure A-18.

J?age

Figure A-13. Power impulse width distribution functions for the data of 10 and 11 september 1978. Note that the distribution

functions for the low antenna data

indicate a near standard impulse width

with high probability. 79

Figure A-14. Power impulse width distributions for

the data of 13 and 14 September 1978.

Minor multipath and moderate fading

were characteristic for 13 Sept.

Little or no multipath was seen in the

final run on 14 Sept. 80

Angle diversity data, 0920 to 1100 PST,

31 August 1978. 81

Angle diversity data, 1155 to 1435 PST,

1 September 1978. 82

Angle diversity data, 1710 to 1910 PST,

5 September 1978. 83

Illustration of possible reflection type

multipath from a strong ducting layer

on 1 September 1978. 84

Figure A-19. Illustration of possible reflection type

multipath from a strong ducting layer on

1 September 1978. 85

viii
INVESTIGATION OF DIGITAL ~ICROWAVE COMMUNICATIONS

IN A STRONG METEOROLOGICAL DUCTING ENVIRONMENT

R. W. Hubbard*

~his report describes an experiment c9nducted over a LOS microw~~e link off the

coast of southern California. The objective

was to determine the effect of known anomalous

propagating conditions, caused by persistent

temperature inversions in the atmosphere,

on a wideband digital:communications system.

A channel probe was used to measure the

effective impulse response of the transmission channel, and bit-error-rate (BER)

measurements were made on the same probe test

signal. In addition, a dual receiver was

used to investigate the application of

angle diversity on the reception of signals

in this propagation environment.

The results of the experiment indicate that

the test link should, if properly configured,

support digital transmission at a mission rate

on the order of 50 Mb/s. Recommendations for

implementing a digital system for the specific

link are given in the conclusions.

Key words: Angle diversity; channel characterization; digital microwave communications; impulse response

1. INTRODUCTION

Wideband digital ~ommunication links in the microwave

frequency band are rapidly being implemented in both

commercial and military networks. However, in many areas

of the world with differing atmospheric co~ditions, the

performance of these systems has been found to be less than

ideal. The performance is degraded from the theoretical

primarily due to various forms of frequency selective fading,

or multipath. A number of examples have recently been cited

*The author is with the Institute for Telecommunication Sciences,

National Telecommunications and Information Administration,

U.S. Department of Commerce, Boulder, Colorado 80303.
in the literature by Barnett (1978), Anderson et ale (1978);

Dougherty & Hartman (1977) ,and Samson et ale (1976). Performance results similar to those above are discussed by Hubbard

(1972) in relation to the measured channel impulse response

function. The latter can be measured using a pseudo-random

noise (PN) probe that is described by Linfield et ale (1976).

Because of the performance anomalies that have been

observed, there is a new caution among potential users of

these digital communication systems, and manufacturers of

microwave digital radios are actively seeking adaptive

techniques to overcome the problems. In order to be successful, however, additional information is needed in order to

characterize the transmission channel, and a detailed assessment of the impact on various modulation techniques should be

undertaken. It is in this vein that the experiment described

in this report was conducted. It is a first step toward

assessing the propagation and transmission characteristics of

a par.ticular microwave link for wideband digital communications.

2. BACKGROUND

The Pacific Missile Test Center (PMTC) of the u.S. Navy

at Pt. Mugu, CA, operates a n~mber of microwave communication links in direct support of mission activities. These

links carry both data and voice information critical to

range test programs. The microwave links are part of a complex

communication network (PMTC, 1976) linking Pt. Mugu centers to

the Channel Islands and other rang~ facilities. Personnel at

PMTC who are responsible for both the planning and operational

aspects of the microwave links have been studying the alternatives for improving the performance ·and increasing the capacity

of the system. A likely alternative to the p~esent systems is

to shift eventually to an all-digital communications system,

which should improve tandem link performance and provide

greater cap~city and flexibility. This alternative presents a

significantptoblemor question to resolve because of anamolous

2
propagation conditions that develop along the California coast

at certain times of the year. The question is "What form of

digital transmission should be used, and at what rates?" This

question forms the bac~ground for the experiment discussed in

this report.

The PMTC personnel have had many years of experience with

their current microwave links that use analog systems, and they

are well aware of propagation factors that have degraded

these systems in the past. For example, strong temperature

inversions are known to build up consistently off and along

the coast line near Pt. Mugu in the months of August and

September. These inversions cause superrefractive layers,

either elevated or surface based, that lead to ducting and/or

trapping conditions for microwave signals. These conditions

can be overcome to a great extent through the use of both

frequency and space diversity systems. However, the resultant

fading observed in the propagated signals can be a result of

either power fading or multipath fading (or both) under these

particular atmospheric conditions. Multipath causes frequency

selective fading across the transmission bandwidth, and is

generally more detrimental to a digital signal than to its

analog counterpart. The problem is compounded by the fact that

a digital system degrades very rapidly beyond some threshold

level, whereas an analog system is characterized by a more

gradual or so-called "graceful" degradation curve.

It is therefore very important to understand and evaluate

the potential transmission degradation expected for a digital

communications link. The PMTC personnel recognized this fact

in their planning phases, and initiated the project discussed

in this report. A decision was made to measure the transmission

quality of one of the longer over-water links at PMTC during

the most dominant period of propagation anomalies. The measurements were conducted by the Institute for Telecommunication

Sciences (ITS), National Telecommunications and Information

3
Administration (NTIA), u.s. Department of Commerce, in Boulder,

CO. The objective of the project was to develop sufficient

information about the link to guide PMTC personnel in making

their decisions regarding digital transmission rates.

3. EXPERIMENT DESIGN

The experiment conducted by ITS was planned to provide

three pieces of information about the particular microwave

'link. These are:

1. Characterization based on the impulse response

of the transmission channel.*

2. Bit-error-rate (BER) performance under various channel

conditions.

3. Evaluate possible performance improvement using a form

of angle diversity at the receiving antennas.

3.1 Link Description

The test link for the experiment operates between Laguna

Peak (LP) near the PMTC to San Nicholas Island (SNI), approximately 65 miles (104.6 km) offshore. The existing microwave

system is implemented for transmission in one direction only

from SNI to LP, using lO-ft (3-m) parabolic antennas in all

locations. Frequency diversity is used at the transmitter on

7.17 and 7.47 GHz, each with a power of 5 watts. The receiver

terminal at LP is configured for vertical space diversity

reception, with the antennas spaced approximately 125 ft

(38 m) apart on the tower.

The elevation at SNI is 912 ft (278 m) '0 and at LP it is

1400 ft (426.7 m) above msl. The transmitting antenna at SNI is

70 ft (21.3 m) above the surface. The lower antenna at LP is

approximately 30 ft (9 m) above the surface. The lower antenna

is just above a sloping ridge in the immediate foreground, and

the terrain from the site slopes steeply to the coast line. Two

photographs of the LP site, taken from the beach at Pt. Mugu and

*Transmission channel is used here to connote the microwave

propagation path, including the terminal antennas.

4
a close-up below the tower, are shown in Figure 1, We have not

included a profile plot of the link, as it is entirely over water

beyond the LP terminal shown in Figure 1.

Figure 1. The Laguna Peak (LP) receiver terminal.

5
3.2 Impulse Response Measurements

The instrumentation used for the impulse response measurements was the ITS channel probe mentioned previously, implemented

for BPSK operation at three transmission rates of 10 Mb/s,

50 Mb/s and 150 Mb/s. The highest data rate is used to provide

a 6 ns resolution to the impulse response measurements. The

lower rates were included primarily for making bit-error-rate

(BER) measurements over the link, using the same PN code as used

for the impulse response measurements. The impulse resolutions

for these data rates are 100 ns and 20 ns respectively. However, as will be seen in the data section of this report, the

50 Mb/s transmission rate prov~ded very useful data, when the

impulse response and BER were measured simultaneously.

The channel probe was operated at 8.6 GHz, and was

duplexed over the existing microwave link with the PMTC microwave signals. The duplexers were installed in the antenna feed

lines at both terminals, using waveguide to coax adapt~rs and

short sections of low-loss cable. The channel 1 ports of the

duplexers are designed to pass 7.15 to 7.7 GHz signals, and the

channel 2 port is relatively narrowband for the 8.6 GHz probe

. test signal. At least 50 dB of isolation between the two. ports

is provided, and no interference to the PMTC operating frequencies was experienced.

A simplified block diagram of the channel probe is shown

in Figure 2. The PN code used 511 bits per sequence (2 9- 1 ) .

The final power amplifier stage of the transmitter is a TWT

amplifier capable of up to 20 watts output. However, the power

output for these exper~mehts was held to one watt (30 dBm).

The receiver for the' system is a dual channel instrument

(only one channel is shown in Figure 2}, so that the impulse

response could be simultaneously measured over both receiving

antennas. The maximum PN clock rate is 150 MHZ, providing

an impulse resolution of approximately 6.7 ns. However,

the operation of the system is such that the measurement

is performed in a millisecond time frame with an equivalent

6
SHIFT REGISTER

PN GENERATOR

PHASE

MODULATOR

R.F.

t Ar,1PLIFIER 8~6 GHz

XMTR.

ANT. 8.0 GHz

5 MHz

600 MHz

FREQUENCY

SYNTHESIZER

REFERENCE

OSCILLATOR

(0) TRANSMITTER

CLOCKING

PULSES

RCVR.

ANT.

R. F. AMPLIFIER

&FILTER

8.0 GHz

POWER

DIVIDER

h (T)

r

R ...---..-..-. ~..--. ENVELOPE

OUTPUT

h. (T) 1 .

REFERENCE

OSCILL.ATOR

FREQUENCY 'SLOW PN

SYNTHESIZER 1--------..... GENERATOR

5 MHz CLOCKING

PULSES

PN

GENERATOR

(b) RECEIVER'

Figure 2. Simplified block diagram of the PN Channel' Probe. The

receiver is dual channel i only one channel is shown. .

7
resolution in the nanosecond range. This process is described

in Linfield et ale (1976). The processing rate in the receiver

is variable; i.e., from 1 to 50 impulses per second can be

selected. This rate is chosen by the operator to be commensurate

with channel variability. For this experiment it was found that

2 responses per second were adequate to keep pace accurately

with changes in the channel, and to provide good resolution within the primary response. The window length in this case is

500 ms, corresponding to 3.4 ~sec (511 x 6.66 ns) in total delay

resolution. Two lower clock rates were also used in the experiment, primarily for the BER performance measure~ents described

below. At the lower rates of 10 and 50 MHz, resolution is lost

in the impulse function, but the transmission rate is more typical of the 4esired digital system at PMTC. The 50 MHz rate (as

seen later) did provide significant impulse data however. All

three rates provide impulse data that can be conveniently

recorded on analog magnetic tape for later pLaybaok vand analysis.

During the measurements, the impulse responses measured for both

transmission paths (space diversity) were continuously monitored

on a storage oscilloscope. A polaroid scope c amer a was also

used to record the responses periodically~

The receiver of the probe is designed to be self calibrating. A PN modulated signal identical to that of the transmitter

is generated in the rf portion of the receiver at a fixed and

known level. In the calibrate mode, this signal is fed to both

receiver channels through accurate step attenuators for a

calibration up to 70 dB in dynamic range. Both the received

signal level (RSL}, meas~red at the 600 MHz IF of the system,

and the magnitude of the impulse response are calibrated at the

same time with this signal.

The probe transmitter is equipped for remote control,

using a touch-tone signal system over a voice channel. In the

case of the PMTC, a circuit was provided through other radio

facilities from LP to SNI. Touch-tone codes are provided for

the following operations:

8
1. Transmitter (TWT) : ON-OFF

2. PN code or cw operation

3. Selection of PN clock rate (10, S·O or ISO MHz) .

Thus, once installed, the transmitter was left unattended by ITS

personnel, and controlled remotely from the LP receiver site.

3.3 Error Rate Measurements

Prior to the PMTC measurements, the probe receiver was

modified from its original configuration to include a digital

data detector and a phase-locked loop (PLL) on one receiver

channel. The PN code used in the ·test signal to derive the

power impulse function was detected and compared in a ·bit-errorrate system with the locally generated code. A commercial

instrument was used for these measurements which was capable of

clocking up to ISO MHz. The clock for the error detector was

derived from the S-MHz reference oscillator of the receiver

shown in Figure 2. In addition, the reference oscillator was

phase locked to the received signal, dividing down from the

600 MHz IF for the locking loop.

Since a bi-phase modalation is used in the probe transmitter for the PN code, the error performance is relative to a

bi-phase transmission mode. No signal conditioning was used in

the receiver detection scheme, and the signal-to-noiseratio

(SiN) was not ideal. As a result, the error rate was not as low

as desired for a clear channel (no multipath). However, since

relative performance of the digital transmission with respect to

the channel .response was the primary objective, the measurement

scheme proved adequate. The error performance achieved for

back-to-back operation is noted in the data section of this

report for reference purposes.

As noted previously; it· was anticipated that the measurements would be continued during December 1978. However, the

ducting conditions d~d not develop. during that month, and our

measurements were limited to those obtained during August and

September 1978. In the period between Septe~ber and December,

9
the receiver detector was improved so that the December BER

measurements would have been more direct rather than relative.

3.4 Angle Diversity Measurements

Following the approach used by Hartman and Smith (1975),

the experiment was configured to include a tilted-beam antenna

measurement to investigate the fading distribution with angle

diversity. As was shown in the cited report, under certain

multipath conditions the fading range on a microwave link can

be significantly reduced using this technique.

For this portion of the experiment, a second dual-channel

receiver was used at LP. The two receiver channels could be

tuned to either of the PMTC operating frequencies. Two parabolic

antennas, each 4 ft (1.2 m) in diameter were mounted side-byside, on the tower at LP at approximately the same height as the

lower antenna for the operating receivers. To establish a

baseline for the data on this system, both antennas were

originally aligned for maximum RSL and the signal was recorded

for some period of time. The two received signals were nearly

identical in this configuration, and the receiver gains were

adjusted to provide as nearly identical calibration ranges as

possible. A signal generator was used to perform the calibrations at the operating frequency. Low-loss foam-flex cable was

used between the receivers and the antennas.

After the baseline data had been obtained, one of the antennas was tilted upward to the point that a nominal 2 dB loss in

signal level was received on that particular channel. When a

multipath signal is received with sufficient difference in the

vertical angle of arrival with respect to the more direct path,

the tilted antenna pattern provides some magnitude discrimination

for a multipath signal arriving from below the more direct path

(say from a surface reflection). This was the case in the experiment reported by Hartman and Smith (1975). However, as will be

seen from the data section, the multipath observed over the SNILP link is not generally due to surface reflection. Thus the

angle diversity was not particularly advantageous in this case.

10
Both individual events and distribution functions of the data

are presented in Section 4 and in the Appendix. Plans had been

made (for the December 1978 measurements) to tip one antenna

downward to verify our results more conclusively. However, as

noted above, no measurements were performed in December.

4. SUMMARY OF RESULTS

The measurements began on 20 August 1978 after equipment

installation and check-out. Arrangements were made with PMTC

personnel to obtain radiosonde data taken daily at both Pt. Mugu

and at SNI. In addition, a forecast statement was issued by the

Navy each day which described the expected meteorological conditions. These data were used to schedule measurement periods.

The measurements continued through 18 September 1978. At that

time the marine layer, the dominant factor in the propagation

mechanism, began to subside. The experiment was stopped, with

expectation of continuing during December 1978 as noted

previously.

Specific examples of the data analyses for the experiment

are presented in this summary and in the Appendix to this report.

In this section we will present a summary of our results,

and a discussion of the data analyses that lead to the conclusions given in Section 5.

4.1 Meteorological Conditions

During the measurement period in August and the first few

days of September 1978, the meteorological conditions relative

to the microwave path consistently included eith~r an elevated

ducting layer, or ducting conditions that begin near the

surface. Typically, the refractive index profile from both

Pt. Mugu and SNI consisted of a subrefractive or near zero

lapse rate (N/kft) for the first few hundred feet above the

surface, and a superrefractive (or ducting) layer, extending to

heights near 4000 ft (1219 m). In many cases the ducting layer

appeared continuous. other profiles indicated that the layer

was broken into two or more sections. A brief summary of the

11
profiles most relevant to the radio data are given in Table 1.

It should be noted from this table that both terminals of the

microwave link were frequently within the noted duct.

4.2. Impulse Response Data

During periods when strong refractive layering orducting

was present over the path, the impulse response data indicated

strong multipath conditions. For example, Figure 3 shows two

examples of the measured power impulse response of the channel.

These responses were photographed directly from the storage

oscilloscope during the measuremertts. The top part of the

figure (a) illustrates a clear channel (no multipath) condition,

while part (b) shows a very strong multipath component ~ith

a delay time of nearly 10 ns. These responses were measured

with the highest resolution 'of the probe system, at a clock

rate of 150 MHz. Thus, the base width of the response for a

clear channel would be approximately 13. 3 ns as shown .i.n

Figure 3(a). In general, the delay time of a multipath component can be measured as the difference between the two (or

more) peaks in the response. This difference, however, can be

misleading for delays shorter than the resolution due to the

relative phase of the multipath and the more direct signal. In

these cases, the base width of the response will indicate the

delay difference when compared with the clear channel width.

There are two distinct ways in which the power impulse

response function can be viewed. First, the resultant pulse

can be viewed as the actual pulse that would be seen by a

receiver if a 6.67 ns impulse had been transmitted. In other

words, it displays the distribution' of the power in the received

pulse ~s a function of delay time. Thus we note for the

response in Figure 3(b) that the pulse has been spread (by the

multipath) so that it would extend into the adjac~nt bit-time

had the transmission rate for a digital system actualiy been

150 Mb/s (the rate for this particular response mee s ur-ement.) •

The second way of viewing the response is in the more c La s si.caL

sense that it is the equivalent of the impulse response of the

12
Table 1. Summary of Refractive Index Profile Data.

DATE STATION TIME REFRACTIVITY CONDITIONS:

(1978)

.. 8(21 PM 1633 Duct: 750' to 1400'

Duct: 150.0' to 2250'

SNI 1500 Subre,f: 0 to 1000'

Stand: 1000' to 1750'

Duct: 1750' to 2750'

'.

8/22 PM 1545 Stand: 0 to 1500'

Duct: 1500' to 3500'

SNI 1455 Subre,f: o to 800'

Duct: 800' to 1700'

8/23 PM 0500 Sub'ref: 0 to 400'

Duct: 400' to 1900'

Duct: 2600' to 3500'

PM 0915 Stand: o to 1400'

Duct: 1400' to 3400'

SNI 0858 Subref: 0 to,' 1000'

Duct: 1000' to 3600'

PM 1555 Subref: 0 to 400'

Duct: 400' to 650'

1800' to 2100'

3200' to 3900'

8/24 PM 0500 Subref: 0 to 300'

Duct: 300' to 2300'

2600' to 3700'

PM 1024 Stand: 0 to 1400'

Duct: 1400' to 1650'

2400' to 2800'

3250' to 3700'

SNI 0856 Subref: 0 to 800'

Duct: 8,00' to ,1500'

3600' to 3950'

PM 1553 Stand: o to 600'

Duct: 600' to 3700'

SNI 1455 Subref: o to 900'

Severe Duct: 900' to 1000'

8/25 PM 0504 Sllbref: 0 to 2,50'

Stand: 250' to 1050'

Duct: 1050' to 1700'

2200' to 2700'

3150' ~c· 3:,)00' :

PM 1139 Duct: 0 to 2400'

SNI 0900 Subref: o to 1200'

Duct: 1300' to 43 09'

PM 1605 Stand: o 'to 1200'

Duct: 1200' to 2000'

SNI 1411 Subref: 0 to 1000' ,

Duct: 1000' to 4000'

8/28 PM 0507 Subref: 0 to 250'

Duct: 250' to 3000'

PM 1040 Stand: 0 to 500'

Duct: 500' to 2500' ,

SNI 0854 Subref: o to 800'

Duct: 800' to 850'

PM 1657 Duct: 0 to 1900'

SNI 1453 Subref: o to 800'

Duct: 800' to 2200'

DATE STATIO~ TIME REFRACTIVITY CONDITIONS:

(1978)

8/29 PM 1557 Stand: 0 to 900'

Duct: 900' to 1350'

Severe Duct: 1800' to 1950'

SNI 1455 Subref: o to 1500'

Duct: 1500' to 5000'

8/30 PM 1556 Stand: 0 to 2000'

Severe Duct: 2200' to 2300'

Duct: 2450' to 2850'

SNI 1452 Stand: 0 to 1750'

Duct: 1750' to 3100'

8/31 PM 0509 ' Stand: o to 1250'

Duct: 1250' to 1900'

PM 1058 Stand: o to 1450'

Severe Duct: 1450' to 1650'

Duct: 1650' to 2950'

SNI 0904 Stand: o to 1200'

Duct: 1200' to 3700'

PM 1549 Stand: o to 1100'

Duct: 1250' to 2000'

2600' to 3200'

SNI 1458 Stand: o to 1100'

Duct: 1350' to 3700'

9/1 PM 0515 Stand: o to 1350'

Severe Duct: 1350' to 2150'

PM 0910 Stand: o to 1250'

Duct: 1250' to 1850'

SNI 0901 Duct: 550' to 1000'

PM 1607 Stand: 0 to 1200'

Severe Duct: 1200' to 2050'

SNI 1357 Subref: o to 1500'

Severe Duct: 1500' to 1700'

9/5 PM 0505 Subref: 0 to 300'

Duct: 300~ to 3150'

SNI 0846 Duct: 0 to 500'

Stand: 500' & above

J PM 1550 S t2.r:~~: ,~ ..... ~.

i i

SNI 1518 Stand: 0 to 7000'

9/6 PH 1004 Duct: 0 to 1000'

Stand: 1000' to 5000'

SNI 0851 Subref: 0 to 500'

Stand: 500' & above

PM 1546 Duct: 0 to 500'

Stand: 500' & above

SNI 1434 Subref: 0 to 500'

Stand: 500' & above

9/7 PM 0000 Super:' 0 to 1850'

Subref: 1850' to 2400'

Duct: 2400' to 2850'

SNI 0005 Super: 0 to 1450'

Stand: 1450' to 3400'

Duct: 3400' to 4000'

9/8 PM 0505 Duct: o to 2400'

9/12 - 18 All Minor ducting above 2400'

13
(a) Clear channel response; impulse width is 13.3 ns.

(b) Response with a multipath component delayed 10 ns.

Figure 3. Examples of the power impulse response measured over the

SNI-LP link with a PN clock rate of 150 MHz.

14
channel (filter). Each peak in the response represents signal

energy received over the link that has propagated over a

different path, and the delay time is directly equivalent to

the path length difference. For a time delay of 1 ns, the path

length difference is almost 1 ft (actually 0.984 ft/ns).

Thus, the measured delay can be used to define an ellipsoid on a

plot of the propagation path that will bound the region for

the multipath component. This concept is particularly useful if

the multipath is a result of a point reflection, but is of no

value in the case of refractive multipath. However it can be

used to investigate possible reflections from meteorological

boundaries. An example of this is presented in Appendix A.

As a simple example of the application of these data to

the question of digital transmission performance in a given

channel, we consider the response shown in Figure 3(b). We

noted above that if this channel were used to transmit digital

information at a rate of 150 Mb/s, the performance would be

degraded by the channel since one bit is spread to the adjacent

bit-time, which results in intersymbol interference. However,

if the transmission rate were on the order of 50 Mb/s (one

bit-time of 20 ns), the delay spread of 10 ns in the response

does not result in intersymbol interference. The received

pulse is merely distorted by the multipath, and in most

instances it would be interpreted correctly by the receiver.

This view of the multipath situation is over simplified, but

is a useful concept for initial evaluation of the channel.

Obviously, the prolonged performance must be based on a statistical summary of the response; how frequently does the multipath

exist and how does it vary with time?

The impusle response may also be used to develop a

frequency domain view of the transmission channel (RummIer,

1978). This concept is frequently convenient, as most communication engineers are more familiar with frequency functions than

their equivalent time domain functions. Also, the frequency

domain concept of the transmission channel is used in recent

15
literature concerning the, performance of digital communication

systems. Some of this literature is referenced in the Introduction. The frequency transfer function of a channel or device is

derived from the Fourier transform of its impulse response.

Other statistics of the channel characteristics may be derived

from various operations on the impulse response, as presented

in a paper by Gallager (1964). For example, the familiar

frequency correlation function is the Fourier transform of the

power impulse function measured by the channel probe. Examples

of this transform are included in the paper by Hubbard (1977),

where the results show the coherent bandwidth of the channel.

Transforming a delta-function impulse response on a theoretical

basis provides a very useful method of analyzing the channel

behavior, and determining some limiting performance values. For

example, if we use the response information of Figure 3(b), where

there is an init.LaL (direct) response and a multipath component

of nearly equal magnitude at a delay of 10 ns, we can construct

the corresponding frequency transfer function assuming the

impulse function to be composed of a delta function at a delay

T = 0 and another at T l o .' = 10 n.s. The transform is sketched in

Figure 4. The nulls in the frequency response function are

separated by

1 1 ~fn = T = 10 ns = 100 MHz,

and the frequency of the nulls are found from

(1)

n f n = (2)

where n is a series of either even or odd integers depending

on the relative phase of the multipath component. If the

multipath is derived from a reflection with an associated

phase reversal the series is even, and the first null of the

transfer function will occur at f = liT. If the multipath is

n

derived in the channel from refraction, or diffraction, then

the delayed signal does not undergo a phase reversal and the

series becomes odd. The first null of the transfer function

16
I--'

.......,]

ro

::J

8

H

t3

g;

H

8

F::t:

....:l

&1

7.1 7.2 7.3

FREQUENCY - GHz

7.4 7.5

Figure 4. The theoretical frequency transfer function for a channel with a multipath component equal in

magnitude to the direct component, at the delay times indicated.
in this case will fall at f = 1/2T. Both of these conditions

n

are depicted in the functions in Figure 4 for a delay of 10 ns.

The first case is shown as the solid-line function, and the

second with the short dashed curve. Phase relationships between

these extremes result in a shift of the nulls along the frequency

axis with a slight change in their separation. This concept is

important to our later conclusions for the SNI-LP path.

Another important concept to visualize is the effect of

changing time-delays in the multipath channel. For example, we

are assuming for the moment that the functions depicted in

Figure 4 are representative of a nominal (observed) delay of

10 ns. However, as seen from the data below, the relatively

stable delay over the link ranged in value from around 4 ns to

as long as 14 ns. The theoretical transfer functions for

delays of these values are also sketched in Figure 4. It is

difficult to convey the dynamics of the channel using these

static descriptions. However, using the on-site observations

and the analysis of the data summarized in the Appendix, we can

discuss system performance aspects and re~ch some conclusions.

The delay magnitudes of 4 and 14 ns are representative of the

"worst case" conditions observed. The first important consideration is to note that with a delay component of 14 ns, the

coherence bandwidth of the channel could become relatively

narrow compared to the required bandwidth for a 50 Mb/s (nominal)

mission bit stream; the theoretical transfer function seen in

Figure 4 has a null-to~null width ~f = 71.4 MHz for this case.

n

Obviously the mission bit-stream spectra would suffer some

in-band distortion at either of the two operating frequencies

(noted in the figure) for the PMTC system, even when the channel

conditions remain fairly static as shown. However, as discussed

later in this report, there are techniques under development for

digital microwave systems to combat adaptively these in-band

distortions. The most severe problem for system performance

would occur whenever a deep null of the channel transfer

function "moves" across the signal passband. Thus, we must,

18
consider the dynamics of the channel, and the following paragraphs summarize our analysis and r-e.suLts. Details of the

analysis are given in the Appendix.

The "worst case" conditions observed during the experiment

occurred on 31 August 1978 at a time when a fairly strong

ducting-type refractive layer existed between the SNI and LP

terminals. The radiosonde data indicated that the layer

extended over the entire length of the propagation path, with a

steeper, more defined trapping gradient near the LP terminal and

a broken gradient toward the SNI terminal. The refractivity

profiles for these conditions are shown in Figure 5, as

measured during the morning hours. Note that the layer thickness was about 200 ft (61 m) near the LP terminal, and that at

least for a time, both terminals were probably embedded in the

layer. The stability of the layer during the day can be

assessed from the profile plots shown in Figure 6, which were

obtained from the afternoon radiosonde launches. Note that

the very strong ducting gradient at LP during the morning

run has changed to some extent, and the profiles for the two

ends of the path are more nearly the same. In addition, the

morning and afternoon profiles from SNI are seen to be quite

similar, indicating a fairly stable condition during the day.

Before discussing the channel dynamics in any detail, we

wish to characterize the impulse response data for this particular date with the following statements:

1. The channel could be readily characterized with a twopath model most of the time.

2. Delays measured in the multipath structure remained

relatively fixed for periods of minutes and longer.

3. Delay components were rarely observed to change in the

space diversity channels at the same time.

4. The distribution of power in the impulse response

changes slowly over periods of several seconds.

19
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200 240 280 320 360

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80

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An example of the power distribution change is illustrated

with the photograph of the measured response shown in Figure 7.

This power impulse was measured at the 150 MHz clock rate,

yielding a response width of 13.3 ns. As seen from the figure,

two fairly equal signals were first observed with a relative

delay time on the order of 14 ns. The sequence of responses are

500 ms apart in observation time, progressing from top to bottom

of the figure. Note that the two components are initially of

nearly equal magnitude, but that the first response begins to

decay as the delayed response increases in magnitude. This

dynamic change results in possibly the worst case for system

performance, since the digital receiver would (perhaps) f~rst be

synchronized with .the first response signal. As this response

fades below threshold, the system would lose sync and be required

to re-sync on the delayed signal. This condition would result

in a period of high error during the response change and the reacquisition time required in the receiver. Error performance

measurements associated with this dynamic response will be

shown in Section 4.3.

In the time domain, the above channel condition illustrates

a sudden change in total path transmission time as the signal

energy shifts from the initial response to the delayed response

or vice versa. In the frequency domain it can be visualized as

a collapse of the frequency transfer function, followed by a

transient recovery ,of the function with a different total path

transmission time. After the digital system has been able to

re-sync to the dominant signal path, then only the relative path

delays are important.

Referring again to Figure 4, the theoretical transfer

function for the two-path model can be used to illustrate two

additional forms of performance degradation. The first to be

considered is the effect of a change in delay-time for the

multipath component relative to the more direct signal. We

note from the figure that as the delay-time changes, nulls in

the transfer function will appear to move through the passband

24
(a) Sequential r'e spons e s , 500 .ms apart from top to bottom.

(b) Time-lapse photograph of t~e sequence in (a) on an expanded scale.

Figure 7. An example of the dynamic magnitude' change between multipath

components.

25
of the transmitted signal. The extreme of this example is to

assume a change in delay from 14 ns to 4 ns in the figure. The

concept on the frequency axis can be visualized by considering

the transfer function baseline as a stretching rubber band with

the cycloid shape of the function retained as it is stretched.

The nulls become farther and farther apart, and a great many of

them will move through the signal passband as the function is

stretched. For example, we note from the figure that the first

null (for T = 14 ns) to move through a signal at 7.17 GHz is that

labeled for n = 200, and the last one would be (for T = 4 ns)

where n = 58. Thus, in this case we would visualize a total of

71 nulls moving through the signal passband over the time

required for the delay change from 14 to 4 ns. Similarly, 74

nulls would have been observed at the 7.47 GHz frequency for the

same delay-time change. However, as the delay time changes so

also does the relative phase of the two components, and thus the

effect of any given null in the dynamic sense would be difficult

to predict. It is clear that in-band distortions to the signal

will occur, and notches will cause frequency selective fading.

The second change we wish to consider is one brought about

by a relatively fixed delay time between the two components,

but the relative phase slowly changes. Before discussing this

situation, however, we should point out the manner in which

phase information can be observed from the power impulse response measurements. For example, in the impulse measurements

shown in Figure 7 we note that the delayed multipath is observed

as an almost distinct response; i.e., it is delayed more than the

resolution of the probe system. Thus, in this case the relative

phase of the two components cannot be directly seen from the

power impulse response; it can only be determined from the coand quadrature correlation functions measured in the probe

receiver. However, when the lower clock rates such as the

10 MHz and 50 MHz are used for the impulse measurements, the

resolution is less and the relatiye phase is registered in the

power impulse. For example, assume the delayed component with

26
a delay of 14 ns is observed in a measurement using the clock

rate of 50 MHz. The resolution of the system for a distinct

separate response for a multipath component is then 20 ns. A

component with less delay than this will produce an impulse

response that overlaps ~he more direct signal. · If the delayed

component is nearly in phase with the earlier one, the overall

response will be broadened and the delay component will

generally be seen as a distortion or with a distinct peak on

the trailing edge of the response. If the delayed component

approaches an out-of-phase condition, the power impulse will

begin to split into two distinct pulses; i.e., with a notch

developing within the resolution time. Examples of these two

types of response are shown in Figure 8. Because of this

characteristic the relative phase of the two components can be

judged from individual responses, and delayed components less

than the resolution time can usually be detected.

(a) Components near an inphase relationship.

(b) Components approaching an outof-phase relationship.

Figure 8. Examples of the power impulse response where the multipath component delay is within the resolution of the probe.

27
A change in relative phase between components can be

transformed to the frequency transfer function asa shift of

the function along the frequency axis. For example, in Figure

4 we have plotted the theoretical function for a delay of 10 ns

when the two components are considered to be both in and out

of phase with each other." Thus, a change in the relative phase

with time will be visualized by a gradual shift in the one

function toward the position of the other, and one null of the

transfer function will move through the signal passband. This

phenomenon seemed to be quite prevalent in the data for the

SNI-LP measurements, i.e., the delay times.were quite stable for

periods of minutes and longer, but the relative phase could

change many. times during the period. Fading of the received

signal level (RSL~ could invariably be correlated with these

phas~ changes, and the bit-error-rate (BER) performance measured

on the probe signal (see Section 4.3) could be traced usually to

an out-of-Iock condition as the null moved through the signal

passband. Examples of these data are presented below.

One other observation should be made before concluding this

discussion. We note again from Figure 4 that multipath with

shorter delays will produce fewer null frequencies in the

transfer function, and thus fewer of them will impede the signal

passband as the delay time changes by small degrees. However,

we also note that the in~band distortion produced by a null

resulting from a short delay time will be more severe; the

attenuation of the signal components at a given level spans a

broader frequency range. This characteristic might be summarized by observing that shorter path delays result in a lower

probability of in-band distortions to a digital system, but

that a null in the transfer function can produce significantly

higher distortions for longer periods of time. Examples of

this were observed ~any times in the SNI-LP link measurements,

made possible only by the fact that the experiment permitted

simultaneous bbservati6nofthe im~ulse response of the channel,

28
the corresponding RSL, and the BER measure of the test signal

bit stream.

It is almost impossible to convey to paper a complete

description of the dynamics of the" channel measurements, as it

'depends on a combination-of the three pieces of information

noted above. We have attempted in'this section to illustrate

the dynamic situation, based on the analysis and visual observations of the measurements. Examples to aid the reader have

been presented, and more appear in later sections and the

Appendix of this report~ We have tried to include a sufficient

sample and resume of each of these three elements combined to

demonstrate at least the results that have led to our conclusionsfor theSNI-LP link given in Section 5.

Before going to a discussion of the BER 'and RSL measurements, we wish to emphasize two important surnmaryobservations

stated earlier in this section. All of the discussion and

illustrations to this point have been relevant to a single

propagation path; i.e., for a non diversity implementation.

The SNI-LP path is, however, instrumented for both space and

frequency diversity. The impulse response measurements were

performed on both space diversity paths simultaneously. Results

of these simultaneous observations bore heavily on our final

conclusions. The measurements on each of 'the two path~ were

very similar. However, two distinctions could be made: first,

the ~rev~lence of multipath was higher for the lo~er receiving

antenna at LP, and secondly, multipath with the same rrelative.

phase was only occasionally seen 'in both paths at the same time,

and the dynamic changes wereuncorrelat~d., TheBER performance

measures were scanned critically during these occas~ons and

found' to be acceptable on the monitored,channel during the

stable condition. The lack of correlatio~ duririg~impulse'

response changes indicates that good diversit~ perfor~ance can

be" expected". This conclusion is discus.sed· in more detail in

29
Section 5. Statistical summaries of the impulse shape and

width distributions are presented ~n the Appendix.

4.3 BER Performance vs. Channel Conditions

The limitations of the BER·performance measurements for

this experiment were pointed out in Section 3. A modification

was made to the probe receiver to permit this measurement with

a minimum of components. Prior to this modification, the PN

code used for the impulse response test signal was not directly

detected in the receiver. The minimum requirements for the

BER feature consisted of providing a phase-lock loop on the

receiver oscillator, a balanced-mixer detector for the recovery

of the received PN code, and generation of a clock signal

related to the received code for use with the error detector.

The latter was a commercial instrument set so that the internal

PN code length and format matched that generated in the probe

system. The phase-lock loop (PLL) was closed between the

received 600 MHz IF and the 5 MHz reference oscillator, and a

data clock signal was generated from the PLL controlled oscillator. The PN code detection was made without benefit of any

signal conditioning or shaping. In addition, the receiver was

designed to restrict the signal bandwidth for only a PN clock

rate at 150 MHz. The bandwidth was not restricted when measurements were made at the lower clock rates. As a result, the

signal-to-noise ratios (E/N ) for any of the data rates were not

o

ideal. The back-to-back error performance at 150 Mb/s was

-6

on the order of 1 x 10 , and a decade better at the 50 Mb/s

and 10 Mb/s rates.

However, it was not intended to perform any detailed BER

measurements for statistical summaries in this experiment.

The BER measure was primarily intended for use as a reference

monitor of the channel performance in respect to the channel

impulse function. For this application, the BER data were

valuable on a relative basis, and for correlating various

channel conditions. The prime objective was to characterize

the transmission channel without regard to any particular

30
system or transmission mode. The BER measure is, however,

representative of BPSK transmission in a nondiversity channel,

since this is the modulation scheme used for the channel probe

system.

To illustrate the application of these data with respect

to the channel response functions and the RSL, we have chosen

a few typical examples to present in this section. The data

were taken from a period around 1300 in the afternoon of

31 August 1979, for which the refractive index profiles for the

path spanning this time were given in Figures 5 and 6. This

period was selected because it was representative of the "worst

case" conditions found during the experiment, and it also

embodied most of the channel conditions discussed in Section 4.2

with respect to the impulse response. Before presenting these

data however, we wish to describe the recording and analysis

features that are common to all of the figures presented.

The BER was recorded simultaneously with other data

signals on a multi-channel chart recorder. Three channels of

the recorder were used to register the BER count from the

commercial error detector. The first two channels registered

the first two significant figures (integer and d.ecimal) of

the BER, and the third channel registered the exponent value.

These values were recorded as voltage level changes over a

decade in each channel. The other data signals were the RSL

for each channel of the probe receiver, the RSL for each

channel of the tilted beam antenna experiment (discussed in

the following section), and a time signal. The latter was

derived from an IRIG Code B time-code generator which was used

for precise timing on the associated magnetic tape recordings.

The magnetic tape was used to record the impulse response

data, the tilted beam data, and one RSL record from the Probe

receiver. Thus, the timing signal and duplicate data provided

a ready means of correlating the tape and chart records. A

sample of the complete chart record is shown in Figure 9.

31
CHAN·~

NO.

1

2

3

4

5

6

7

8

Figure 9. Sample" chart r·ecord. Channel- nomenc.Latnrre shown in Figures 10 - 13 .

. 32
The data in the following figures are blocked in 5-minute

intervals. The impulse response for each probe receiver

channel was reproduced from the magnetic tape and averaged

over this interval in a digital time-series analyzer in the

ITS laboratory. The result was then plotted on an X-Y recorder.

At the same time, the impulse response during the interval

was monitored on a storage oscilloscope, and a photograph(s)

was made during the interval in order to depict the dynamic

situation. These data were then compared with the RSL record

and the BER data in each interval. The results are shown in

the composite Figures 10 through 13.

The first 5-minute data interval is taken from 1255 to

1300, and the average of the impulse response for each diversity

chan~el is shown in Figure 10(a). Both responses indicate a

fairly clear-channel condition. Evidence of small magnitude

multipath is seen only in the trailing edge of the two averages.

A time-lapse photograph made from a magnetic tape playback for

a 20 sinterval at 1256 is shown in the lower half of the

figure .. This photo as well as others to follow were made by

setting the scope sweep for an A + B display, and using the

sync signal derived from the magnetic data tape. In this

display, the time separation between the response from the

lower receive antenna (left-hand response in the photo) and

that from the upper antenna is due to the difference in the

transmission time over the disparate length of waveguide

between the antennas and the receiver.

The corresponding strip-chart record showing the RSL for

both channels, and the BER record is shown in Figure lO(b).

The RSL's during this period show moderate fading on the lower

antenna signal (Ch. 1), and a fairly steady signal for the upper

antenna (Ch. 2) with exception of the 8 to 10 dB fade just prior

to 1300. The BER data were measured on the Ch. 1 signal only,

so our comparisons between this performance monitor and the

channel conditions are always with respect to Ch. 1. The fading

in Ch. 1 is typical of slow power (or flat) fading, and the

33
High

Antenna

Low

Antenna

50 ns

(a) Five-minute time averages.

-.j 50 ns

(b) Time-lapse photograph of 20 s period at 1256 PST.

Figure 10(a). The average power impulse response measured between

1255 and 1300 PST, 31 August 1978, at a PN clock rate

of 50 MHz.

34
t f

-9

-6

-3

-0

-9

-6

-3

-0

-9

-6

-3

-0

Figure 10(b). Strip chart record for 1255 - 1300 PST, 31 August 1978.

35

f

E-i

S

o

o
impulse data verify only short-delay multipath of relatively

small magnitude. The channel conditions can be typified from

Figure 4 by considering the transfer function for a component of

short delay and slowly changing in either delay and/or relative

phase. The fading is caused by the skirt attenuation of the

transfer function that has a shallow slope over the signal pass

band. The BER* is also quite stable over the period, ranging

-4 -4 from a high of 8 x 10 to a low of 2 x 10 . As we will see

later in this discussion, the deeper fade in Ch. 2 prior to

1300 is typical of a relative shift in multipath where a null

of the transfer function impinges on the signal passband with

steeper slope. Note that as this condition appears in Ch. 2,

the RSL in CH. 1 becomes more steady. This is more evident in

the next data interval.

The data of Figure 11 are for the period 1300 to 1305.

In the averages of the impulse response we see that Ch. 1 now

displays a very strong and stable multipath component. The

delay in the average response is seen to be about 10 ns. The

high and stable RSL for the period also confirms the stability

of the multipath. Measured at the 150 Mb/s rate, this impulse

response would resemble that shown in Figure 7, where a more

definite separation of the two components results from the

higher resolution. Also the clear responses shown in Figure 10

would indicate the small scale multipath with the higher

resolution measurements. However, we noted previously that

the 50 Mb/s response was quite useful in our analyses, and

Figure 11 is an example. The relative phase between the

components is seen to be opposing as the cusp is developed

within the resolution time.

The response for Ch. 2 has not changed significantly from

the previous interval, but the RSL record indicates that the

short-delay components in that channel have become less stable.

*For these comparisons we will generally ignore the decimal

value, and read only the first significant figure and the

exponent value from the chart record. .

36
Low

Antenna

T 10 ns

-.j 50 ns

(~) Five-minute time averages.

--.f 50 ns

High

Antenna

(b) Time-lapse photograph of 20 s period at 1304 PST.

Figure ll(a). The average power impulse response measured between

1300. and 1305 PST, 31 August 1978, at a PN clock rate

of 50 :MHz.

37
E-i

S

o

o

Figure 11(b). Strip chart record for 1300 - 1305 PST, 31 August 1978.

38
The photograph at the bottom of Figure ll(a) is a 20 s time

lapse of the impulse data made at 1304. The dynamic change in

relative phase between components in Ch. 1 is quite apparent,

while only a slight distortion to the trailing edge of the

Ch. 2 response is seen in the photo. The BER during this

interval shown in Figure ll(b), is similar to the previous

record with two exceptions. First, note that there are fewer

intervals where the rate has dropped to the 10-3 level and

those intervals are more random in the first few minutes of

the block. Secondly, near 1303 we observe that the RSL in Ch. 1

has become very steady and the error rate has improved to values

-5 between 5 to 9 x 10 for short periods. The cause for the

first observation is not tractable in any sense, but one can

speculate on the probable cause. The impulse response between

the two intervals in Ch. 1 indicates that a multipath component

has gradually changed from a short delay to near 10 ns. As

discussed in Section 3, the dynamics in the transfer function

would cause a number of nulls to move across the signal passband. The resulting power notches in the received signal spectra will increase the error rate, and in-band distortions as the

null begins to impinge add additional errors (see Barnett, 1978).

We assume this change to be taking place during the time (1255 -

1303) when the RSL is'increasing and becoming more steady. At

around 1303 the delayed component is well established, and the

relative phase was seen to be very stable at the time the best

error performance was measured. This condition was directly

observable in the data, ,where the impulse response remained

stable and had the same general shape as the average shown in

Figure ll(a). The photograph in the lower half of the figure

was taken over a few seconds beginning at 1304. It indicates

that the phase relationship began to change again with time,

and the BER 'shown in Figure ll(b) returned to the pattern

seen earlier in the interval. A few measures in the 10-5

range were still seen around 1305. One should note from this

very short example the lack of correlation between RSL and

39
BER performance, a basic characteristic of multipath propagation. Also note that the multipath change in the two channels

is not well correlated in any of these data displays.

The interval from 1305 t.o 1310 is depicted in Figure 12.

The averages of the impulse functions are shown in the (a)

part of the figure. The multipath in Ch. 1 is again seen to

be quite stable, and the average delay-time has increased to

approximately 12.5 ns. The first few minutes of the RSL record

and the BER record in Figure 12(b) follow the characteristics

from the previous interval until the deep fade in Ch. 1 just

prior to 1308. At the null point of this RSL fade, the error

detector lost synchronization and did not recover for several

minutes (except momentarily around 1310). We discuss the

details of this event later in this section. From the photograph of the Ch. 1 response in the lower half of Figure 12(a),

we note that the relative phase is again changing somewhat more

rapidly with time. For example, within the time-lapse display,

we see ind.ividual functions typical of those for a near inphase condition (broad top to the pulse) and several approaching an out-of-phase state (null within the resolution). This

is again visualized in the frequency domain as a shift of the

transfer function null along the frequency axis, with the

associated in-band signal distortions. The BER is seen to step

again between exponent values of 10-3 to 10-5 prior to entering

the out-of-lock state caused by the variability of the multipath phase.

The data interval from 1310 to 1315 is shown in Figure· 13.

The average impulse response for Ch. 2 has not changed

essentially throughout the total sequence. The Ch. 1 response

average now indicates a multipath component with a stable

delay of abo~t 7.8 ns, and nearly in-phase with the earlier

response. The time lapse photo at the bottom of the figure

illustrates the development of this function, and also shows

the relative stability at 1311. The RSL at this time is also

stable, but the BER measure was lost becuase of the out-of40

. ~~--_.~~~-----------_._--._....__.~~. ~- -'--~-'-'--'--'~--"~-""---~'-~'-
T

50 ns

Low

Antenna

12.5 ns

-..j 50 ns

High

Antenna

\

-------

(a) Five-minute time averages.

(b) Time-lapse photograph of 20 s period at 1307 PST.

Figure 12(a). The average po~er impulse response measured between

1305 and 1310 PST, 31 August 1978, at a PN clock rate

of 50 MHz.

41
f

+ f

E-t

S

o

o

Z

L'J

H

U)

o

H

3

o

9

6

3

t

9

9

6

6

3

o

o

Figure 12(b). Strip chart record for 1305 - 1310 PST, 31 August 1978.

42
Low

Antenna

"[ 7.8 ns

50 ns

(a) Five-minute time averages.

50 ns

High

Antenna

(b) Time-lapse photograph of 20 s period at 1311 PST.

Figure l3(a). The average power impulse response measured between

1310 and 1315 PST, 31 August 1978, at a PN clock rate

of 50 MHz.

43
o

r-f

E-t

S

o

o

f

-9

-6

-3

-0

-9

-6

-3

-:"0

-9

-6

-3

-0

Figure 13(b). Strip chart record for 1310 - 1315 PST, 31 August' 1978.

44
lock condition reached earlier. A note on this particular

case is included in the discussion to follow.

We observe from Figure 13(b) that the RSL in Ch. 1 began

to fade rather severely after 1311, and the BER detector resynchronized on the data stream at 1314 during one of these

fades. Even though the depth of some fades were several dB

greater than the one observed at 1308 that caused the loss of

sync, the BER performance was not greatly degraded over that

during steady signal periods. Periodically, the BER fell to

a few bits in 10-2 at the depths of the fades, but returned to

10-4 levels without loss of sync. The difference between the

performance during these fades and others can be seen from the

channel response functions presented below. Before presenting

them however, we should note a characteristic of the bit error

system. The data clock was derived from the PLL (as stated

above) rather than from the received data. Thus when a change

in the reference phase takes place within the PLL, the phase

of the data could reverse creating a data (or Lnver t.ed ) bit

stream to the error detector. This is the condition that was

created by the first fade at 1308. A time-lapse photo of the

impulse response at this time is shown in Figure 14(a). The

dynamics are somewhat obscured in the pho~o, but referring to

Figure 12(a) we can see that prior to 1308 the dominant signal

path was the first response in the two-path function. At the

time of the deep fade, the signal power slowly faded in the

initial response and shifted to the delayed component, changing

the phase reference of the PLL to that of the more dominant

delayed signal. Thus the PLL tracked the change, and caused

the phase of the data signal to reverse. In normal operation

of the experiment, the operator of the probe manually changed

the reference data stream in the error detector to reestablish

the data sync. However, in this case the manual change was

not made in order to follow the channel performance more

directly.

45
The photograph shown in Figure 14(b) was made at the end

of the interval where the data resynchronized at about 1314.

At this time a reverse shift between the two components took

place, and the dominant signal energy returned to the earlier

component. In both (a) and (b) photographs there is also

evidence that the total path changed in propagation time (or

length) during the time-lapse. This can be seen in the shift

in the envelope of the responses. The shift is greater in (b),

and gives the impression that the delay between components in

Ch. 1 is greater than it actually was. The photograph in part

(c) of the figure was taken at 1310 where the error detector

resynchronized for a few seconds, and then again went out of

lock. The magnitude of the Ch. 1 response shows a power fade

effect as well as the shift in energy between the two signal

components.

The multipath fading continued with similar characteristics

for the remainder of the afternoon, increasing somewhat in

depth and rate in both channels. This can be seen in Figure 15

and 16 which are sections of the chart record following those

periods discussed above. We see that the BER performance

continues at about the same level, and with a pattern similar

to that above. The error detector was however manually resynchronized at the out-of-lock points seen in Figure 16. In

o rder to show more of the dynamic multipath effects, we present

additional time-lapse photographs of the impulse functions in

Figure 17. The first photo in (a) illustrates that the Ch. 2

characteristics are similar to those discussed for Ch. 1. The

data were recorded during a fade in Ch. 2 a few seconds past

1317 (see Figure 13b), and the response shows a shift in signal

energy from one signal component to the other. Figure 17(b)

shows that the fade in Ch. 1 at 1331 (Figure 15) was a power

fade across the passband, and disturbed the BER count for a

very short time at the bottom of the fade. The photo in (c)

of the figure again illustrates the phase shift of signal

energy in Ch. 1. At the same time, a minor change took

46
(a) Time-lapse response measured at 1308 PST, corresponding to the

fade in Ch. 1 and the loss of synchronization in the data stream

shown in Figure 12(b).

(b) Time-lapse response measured at 1314 PST, where the data stream

resynchronized.

(c) Time-lapse response measured at 1310 PST, where the data stream

resynchronized for a few seconds.

Figure 14. Time-lapse photographs of the power impulse response

measured during the fading period in Ch. 1 beginning

at 1308 PST, 31 August 1978.

47
Figure IS. Strip chart record; nomenclature same as Figures 10 - 13.

48
Figure 16. Strip chart record; nomenclature same as Figures 10 - 13.

49
(a) Dynamic mu1tipath measured in Ch. 2 (high antenna) at 1317 PST.

(b) A flat-fade response observed in Ch. 1 (low antenna) at 1331 PST.

(c) Another example showing a power shift between components in Ch. 1,

with only minor change in the Ch. 2 response at 1346 PST.

Figure 17. Time-lapse photographs of the power impulse response

measured between 1317 and 1346 PST, 31 August 1978.

so
place in Ch. 2. This photo is for the data a few seconds after

1346 during a very sharp fade in Ch. 1. The data illustrate an

important observation that was stated in Section 4.2; i.e.,

the multipath character of the diversity channels was similar,

but rarely correlated in time. This ~s the desirable situation

for effective space diversity and combined performance

improvement.

To compare in some sense the BER performance in Ch. 2

(upper diversity receive Channel) with that in Ch. 1, the

error detector system was switched to Ch. 2 around 1520 on

the afternoon of 31 August. This change was accomplished by

exchanging the IF signal cables from the rf head of the

receiver to the two IF channels. Thus, the PLL and error

detector followed the conditions from the upper antenna signal.

The fading became more rapid and severe as the afternoon passed,

but the BER performance remained essentially the same as that

described above for Ch. 1. It was also possible to verify

visually the same type of multipath effects. This switch in BER

measurements was repeated several times throughout the experiment with the same result.

Most of the data obtained on 31 August were measured at

the 50 Mb/s data rate in order to evaluate the BER performance

characteristics. The resolution of the impulse response data

is obviously not as high as the 150 Mb/s rate, but it can be

seen from the above discussion that enough detail was available

at the lower rate to permit some tractable comparisons. The

meteorological conditions on the following day (1 September

1978) were very similar, and some of the data were recorded

at the 150 Mb/s rate on that date. A discussion of these

results is included in the Appendix. The results confirm

the conclusions drawn on the basis of the example presented in

this section at the lower clock rate.

51
4.4 Angle Diversity Measurements

The angle diversity measurements were made on the higher

of the two operating .frequencies of the PMTC microwave system

(7.47 GHz) , with the measurement apparatus as described in

Section 3.4. The system was originally aligned for the lower

frequency (7.17 GHz) , but changed on 25 August 1978 to 7.47 GHz

because of problems experienced by the Navy with the lower

frequency transmitter.

The RSL levels for both receivers were initially recorded

on chart paper only, and monitored for an extended period so

that the receiver gains and calibrations on the two (aligned)

antennas could be matched. One of the two antennas was later

tilted for a nominal 2 dB loss in mean RSL. The data were then

recorded on the magnetic tape system (as well as the strip

chart recorder) beginning on 29 August 1978. Examples of the

strip chart records can be seen in Figures 10 through 13 and

Figures 15 and 16 in the previous section. They are the signal

traces in Ch. 3 and Ch. 4 of these figures. Although the

polarities of these signals are reversed from those for the

two probe receiver channels (top two traces), the signal

fading can be compared between the sets. with the exception of

some o~ the sharper, deep fades seen in the 7.47 GHz signals,

the fade range for the two frequencies is comparable. Since

the probe system was operated at 8.6 GHz, the frequency

separation is 1.13 GHz. This is almost 4 times the separation

used between the PMTC frequency diversity channels. However,

the separation between the two measured frequencies is not

considered to be so large that some meaningful performance

observations cannot be made. For example, just a scan of the

records shown in the figures noted above will convey that the

fading is not highly correLated in time between the two

frequencies. If we assume that the multipath observed by

the probe at 8.6 GHz is appropriate to a wide range of transmission frequencies, then we coul~ expect a degree of spacial

correlation for the two frequencies with some time lag between

52
the fading events. This cannot be verified easily however

from the data. One can peruse the records and find some fade

events that appear similar at the two frequencies, with time

separation qn the order of m~nutes, but the pattern is not

consistent. There are of course two reasons that one would

not expect this to prevail. First, the structure of the frequency transfer function changes with respect to a given

operating frequency and thus the dynamic effects should be

different. We rely on this fact for improvement in frequency

diversity systems. In addition, we must keep in mind that the

RSL for the probe signal is derived from a broadband transmission while that from the angle diversity measurement is from a

relatively narrow band signal. Notches in the transfer function

of the channel will produce fades of different character in the

two records, even if the same dynamic multipath change is

responsible for the fade. The important aspect in this comparison is the lack of correlation in time between the signals at

the two frequencies; again a desirable result. It is actually

a by-product of the experi.ment, but should not be overlooked.

Returning to the original objective for this part of the

experiment, we wish to summarize the angle diversity data.

Again looking back at the figures in the previous section, and

scanning the two angle-diversity signals in' the strip chart

records, we can ascertain certain fades that are of less depth

in the tilted-beam record (Ch. 4) than in the direct-beam

record (Ch. 3). However, the reverse is also true on many

occasions. Thus the final conclusion can only be drawn from a

statistical summary. In general we have found that there is no

statistical improvement. This is based on analyses of the

magnitude distributions of the two RSL data records. The signals were played back from the magnetic tape recordings into

the time-series analyzer mentioned previously. A probability

density function (pdf) and a cumulative distribution (cd)

function were developed in the process for both signal channels.

53
Examples of these results are shown in Figures 18 and 19. Figure 18 presents the pdf for each channel, which have been

aligned on a relative power scale to match their peak magnitudes

(the tilted antenna signal peak is in reality approximately 3 dB

lower). There is no significant difference between these plots.

The cumulative distributions are shown in Figure 19. They

indicate that the tilted beam result is actually poorer than

the direct antenna signal, as the probability of a lower RSL is

higher in the tilted beam function than in its counterpart.

If one matches the median signal levels for the distributions

shown in Figure 19, it can be seen that the tilted antenna

provides a very slight improvement. However, this example and

others shown in the Appendix are not conclusive enough to warrant angle diversity as a viable option in configuration. In

essence, the results suggest that the angle-of-arrival for

multipath components in a refractive medium is a random variable.

Thus, no fixed angular offset to the receiving antenna will

provide significant improvement in fade margins.

Before concluding this discussion, one additional comment

should be made regarding the angle diversity measurements.

Even though the statistical analyses show relatively no

improvement in the signal distributions, a distinct difference

in character of some signal fades over the angle diversity

paths was observed. Some of these may be seen in the RSL

records (Ch. 3 and Ch. 4) of the figures presented in Section

4.3. In particular, we refer to Figures lO(b), 12(b), 13(b),

and 16. In each of these figures, particular fades are seen

that display different characteristics, indicating that

either the multipath phase relationship or delay was not

precisely correlated in both channels. Thus, as illustrated in

Section 4.2, the effect on a digital bit stream could conceivably be different and uncorrelated. This conclusion cannot be

verified from the measurements, however. The possibility was

recognized during the data analysis, and plans were made to

perform impulse response measurements on the angle-diversity

54
Tilted

Antenna

-60 -70

RSL - dBm

-80

Figure 18. The probability density functions (pdf) for the tilted

beam experiment, 1520 to 1730 PST on 30 August 1978.

RSL - dBm

Figure 19. The cumulative probability functions for the data of

Figure 18.

55
channels during December 1978. As pointed out previously, this

portion of ,the experiment was not performed because the desired

meteorological conditions did not develop. We mention this as

a potential diversity technique only for completeness, and' for

future reference. If the technique has performance merit it

can be conceivably implemented at lower cost than standard

space diversity configurations.

5. CONCLUSIONS AND RECO~mENDATIONS

The results of the measurements summarized in this report

permit some general conclusions to be reached, and specific

recommendations to be made for a digital communication system at

the PMTC. The conclusions and recommendations must be made on

the assumption that the anomolous meteorological conditions

encountered during the measurements are typical or representative

of those normally found in the Pt. Mugu region. One reservation

in this regard should be stated, based on information furnished

by meteorologists at Pt. Mugu. It was pointed out to the ITS

personnel that the marine layer which gives rise to the refractive structure was somewhat higher than normal during our

experiment. For example, the layer height during August and

September 1978 generally extended above the elevation of LP,

which was considered to be unusual. However, a cursory

comparison of refractive index profiles measured during the

experiment with those measured in previous years (private

corrmunication) indicated that the refractive structure that

would effect the microwave systems was quite similar. Some of

the earlier data were obtained from airborne refractometer

measurements between Vandenburg AFB and Santa Cruz Island in

August of 1969. Therefore, we have assumed that the higher

than normal marine layer does not have any significant impact

on our results.

It is concluded that the SNI-LP link would not support a

digital communication system opera~ing with a mission bit stream

on the order of 50 Mb/s, if configured for a single nbndiversity

56
path. The multipath problem would be severe enough to cause

unacceptable performance during most of the anomalous (ducting)

season. The impulse response measurements show that the

coherence bandwidth would not be adequate in the channel, and

of course, the nondiversity power fading would be as severe as

that known to degrade the present analog signals. The in-band

distortion caused by the refractive multipath would frequently

cause the BER to exceed threshold limits as reported by

Barnett (1978). For example, Barnett's measurements have shown

that BER's in excess of 10-3were experienced over a nondiversity path when a power distortion (assumed linear) across

the signal pass band was as small as 0.2 dB/MHz. These results

were obtained on a 78 Mb/s, 8-PSK system, operating at 6 GHz

over a 26.4 mile (42.48 km) test link. Transmission path

distortions such as these have also been treated theoretically

by Emshwiller (1978), and a method for estimating the fractional

time of unacceptable performance in a LOS nondiversity link is

given by this author for specific systems. Another estimate of

nondiversity multipath outage time as a function of path length

has been made in a paper by Prabhu and Greenstein (1979).

Their results also illustrate the importance of the multipath

degradation.

Comparing the real-time observations of BER performance

and the multipath structure presented in Section 4, we

conclude that significant performance improvement can be

expected with the use of space diversity. Rarely in our

measurements did we find the dynamic change in the multipath

(that caused the worst case performance) occurring in the space

diversity channels at the same time. This is not to say that

the multipath did not exist in both channels at the same time;

only that the dynamic changes were not correlated. Based on

this observation, and on the improvement in performance

reported by Anderson, et ale (1978) for a space diversity

configuration, we conclude that space diversity will yield an

acceptable level of performance in the SNI-LP channel with

57
proper single-link design. It should be noted that the results

reported by Anderson, et ale were obtained using an adaptive

phase combining technique at IF. It was beyond the scope of

this experiment to consider the combining techniques for

diversity systems. We thus restrict our consideration to the

channel conditions, and conclude that any properly adjusted

combining technique should be effective since the basic requirements for good space-diversity reception were inherent in the

measurements. For example, if a switched type combiner is used,

it also should be effective as long as it has been designed and

adjusted for "hitless" operation with respect to the digital

transmission stream.

The most restrictive frequency character in terms of the

coherent bandwidth of the channel develops when the multipath

delay approaches the 14 ns value. This was not a dominant

condition however; delays of this magnitude were seen infrequently as shown in the delay distribution functions in the

Appendix. In addition, this delay magnitude was practically

never seen simultaneously in the space-diversity channels. It

is interesting to note also, that this value of delay is quite

close to the 13 ns value one would calculate for the maximum

delay based on the path geometry and the 3 dB power points for

the parabolic antenna patterns over this link.

Improvement of performance through frequency diversity

was also noted as a distinct possibility in the discussion

in Section 4. Although the experiment was not designed to

evaluate specifically this aspect, the observation from the

data is significant. Based on theoretical developments

(Dougherty, 1967), the present frequency separation used on the

SNI-LP link should provide a fade protection on the order of

24 dB. Within the frequency assignments used for other links

in the PMTC network, the potential exists for much better

diversity protection (greater frequency separations).

To summarize the above, it is concluded that the SNI-LP

link will support a digital communication system with a

58
transmission bit-rate on the order of 50 Mb/s, if the system is

configured using both the space and frequency diversity techniques presently used with proper combining. Space diversity

alone would perhaps be adequate, but the frequency diversity

should assure even better performance.

In addition to the diversity reception considerations, the

above conclusion is also predicated on the assumption that some

form of adaptive equalization (or compensation) should be an

inherent feature of the digital microwave radio. We cannot

evaluate this adaptive performance technique from our measurements, but we can consider it from the characterization of the

measured channel and improvement results reported by others.

It is readily seen from the channel response measurements that

even with a space-diversity switched-combining technique, the

in-band distortion to the 50 Mb/s transmission stream can become

severe. For example, the linear distortion factor of 0.2 dB/MHz

reported by Barnett (1978) can be several times higher in this

channel when the multipath delay approaches the measured

extreme of 14 ns. Thus, in order to protect the system under

these conditions, an adaptive equalizer should be considered.

The expected performance improvement can be estimated from

two recent experiments. The first of these has been reported

in the literature by Anderson, et ale (1978). These authors

report that a simple adaptive linear amplitude equalizer

provides an improvement factor of about 2 when applied to a

nondiversity channel. This factor is relative to the probability of a system outage in the "worst fading month", which

was determined to be approximately 1 x 10-3. This performance

value was based on measurements performed on an 8 GHz,

91.04 Mb/s QPRS system, operating over a 51 km path. The base

line for system outage was with reference to 'long-haul availabilityof 99.98%; requiring an outage probability per loop of

<1.43 x 10-6 in the "worst fading month". These authors point

out that the non-diversity link performance (probability of

outage) was 700 times worse than the long-haul availability

59
objective. With the use of space diversity (phase adaptive IF

combining) this factor was reduced to the order of 18. The

combination of adaptive equalization and space d i ve.r si.t.y provided a combined improvement factor on the order of 20; adequate

to reduce the probability of out.aq e to a value "compatible with

long-haul availability objectives".

The second reported improvement based on in-band adaptive

processing has been reported informally by Rockwell International (Marketing Bulletin, ~1W-63A, Sept. 6, 1978; private

communication). In this case, an adaptive equalizer has been

designed and laboratory tested in the IF amplifier modules of

digital receivers. The reported results include a correction

of up to 14 dB of linear tilt in the IF bandpass of 30 MHz

width; a slope of 0.47 dB/MHz. We note that this represents an

improvement factor on the order of 2.5 over the in-band slope

distortion reported to be critical in Barnett's (1978) measurements. In addition, the equalizer is reported to be very

effective against frequency selective fades at or near the

bandpass center frequency, and is capable of responding to fade

rates as high as 75 dB/s.

It is recommended that the digital SNI-LP link be first

engineered on the basis of a nondiversity system, and an estimate of the expected outage time be computed using the techniques

suggested by Emshwiller (1978) and/or Prabhu and Greenstein

(1978). Multipath values from this report may be used as

typical for the link during the worst of the anomalous propagation. The time expected for the latter should be estimated

from previous meteorological data and experience, and should

include the expected conditions for December as well as the

dominant months of August and September. The nondiveristy

performance (outage) estimate can then be used to calculate the

expected outage or availability time for the system as implemented with space and frequency diversity, and including the

improvement factor for the recommended adaptive in-band processing. These methods should provide a reasonable and fairly

60
reliable performance estimate for the implemented digital

system. In this process, it is implied that the nondiversity

performance calculation be based on known statistics of the

fading encountered over the link for the present analog system,

and using deviations from the theoretical performance curves

given in the literature for the modulation techniques used

(i.e., QPSK, 8-PSK, QPRS, etc.).

6. ACKNOWLEDGEMENTS

The author wishes to acknowledge the following individuals

for their noted contributions to this project:

Mr. Jim Weblemo and Mr. Jim Kaness of PMTC for their support

and interest in the project. Mr. Kaness served as project

monitor.

Mr. Robert Bowker of PMTC for his excellent assistance

in making the necessary logistic and technical support

arrangements.

Mr. Lauren Pratt and Mr. John Wood of ITS for their

efforts in both installing the equipment, and performing

the measurements at Pt. Mugu. Their assistance in the t

data analyses and summaries is also appreciated.

Mrs. Olive Fritz of ITS for preparation of the

manuscript.

61
7. REFERENCES

Anderson, C.W., S. Barber, and R. Patel (1978), The effect of

. selective fading on digital radio, International Communications Conference, Toronto, Ontario, Canada, June.

Barnett, W.I. (1978), Measured performance of a high capacity

6 GHz digital radio system, International Communications

Conference, Toronto, Ontario, Canada, June.

Dougherty, H.T. (1967), Microwave fading with airborne terminals,

ESSA Tech. Rept. IER 58-ITSA 55, October.

Dougherty, H.T., and W.J. Hartman (1977), Performance of a

400 Mbit/s system over a line of sight path, IEEE Trans.

on Comm., Vol. COM-25, No.4, April.

Ernshwiller, M. (1978), Characterization of the performance of

PSK digital radio transmission in the presence of multipath

fading, International Communications Conference, Toronto,

Ontario, Canada, June.

Gallager, R.G. (1964), Characterization and measurement of time

and frequency spread channels, MIT Lincoln Laboratory,

Lexington, MA, Report 352.

Hartman, W.J., and D. Smith (1975), Titlting antennas to reduce

line-of-sight microwave link fading, Federal Aviation

Administration, Report No. FAA-RD-75-38, February.

Hubbard, R.W. (1977), Measurements in microwave telecommunication systems using a pseudo-random noise (PN) probe, ORSI

Measurement in Telecommunications, Lannion, France, October.

Linfield, R.F., R.W. Hubbard, and L.E. Pratt (1976), Transmission channel characterization by impulse response

measurements, OT Report 76-96, August.

PMTC (1976), Communications Capability, Capabilities Development

Department, Pacific Missile Test Center, Pt. Mugu, CA.

Prabhu, V.K., and L.J. Greenstein (1978), Analysis of multipath

outage with applications to 90 Mb/s PSK systems at 6 and

11 GHz, IEEE Trans. on Comm., COM-27, No.1, Janaury.

RummIer, W.D. (1978), A multipath channel model for line-ofsight digital radio systems, International Communications

Conference, Toronto, Ontario, Canada, June.

Samson, C.A., A.P. Barsis, and D. Smith (1976), Performance

measurement over long line-of-sight microwave links in

Colorado, OT Report 76-92·, June.

62
APPENDIX

This appendix contains a selected summary of the data

analyses used to reach the conclusions presented in the main

body of this report. Section A is a presentation of the pulsewidth distribution analyses performed on the power impulse

functions, measured for both channels of the space diversity

receiver at Laguna Peak. Section B contains a few selected

analyses of the tilted-beam experimental data, showing the

probability distribution functions of the RSL for both tilted

and untilted receiving antennas. Section C presents some

additional analyses that are discussed in relation to each

figure, and includes an example of a ray-tracing that depicts

the possibility of reflective multipath within a strong duct on

the SNI-LP link.

A. Impulse-Width Distribution Analyses.

The range of multipath delays reported in Section 4 has

been determined from the probability functions presented below.

The width of the measured impulse responses was analyzed using

a time-histogram algorithm in a special digital analyzer.

To avoid errors in cases where the impulse response splits

due to phase opposition of short-delay components, a trigger

circuit was designed to assure full pulse-width distribution

analyses. The distributions thus display the probability

density (pdf) of the total delay spread over the indicated

analysis period.

These distribution functions actually convey two pieces of

information about the dynamics of the channel response (see for

example Figure A-2). First, the probability of the delay spread

is shown in the right half of the function, to the value of

the pulse width in excess of the "clear channel" width. The

latter is 13.3 ns for the 150 MHz clock rate, and 40 ns for the

50 MHz data. The portion of the function to the left of these

values register periods when the response was narrowed due to

phase interference between the multipath components or by power

fading. It will be noted in some of the functions that the

63
pulse width has a finite probability density to a pulse width

of zero. This characteristic is typical of fading in the

channel, and indicates those periods where the phase opposing

ffiultipath causes complete cancellation of the received impulse.

Figure A-I presents the distribution functions for the

S-minute data samples shown in Figures 10 through 13 of

Section 4. The maximum delay spread is seen to be on the order

of 10 ns, with a fairly high probability of phase interference

fading (pulse widths < 40 ns).

Figures A-2 through A-14 present the pdf's of the power

impulse width for several data runs during the period 22 August

1978, through 14 September 1978. Specific notes regarding the

distributions are included in each figure. A few of the

presentations include samples of the RSL records taken during

the period of the pdf's. From these, one can see the relation

between the impulse width distributions and the signal fading

characteristics.

B. Angle Diversity Analysis.

As noted in Section 4, the data indicate that angle diversity does not provide any fade margin improvement for refractive

multipath. Although individual fades may be found in all of

the data that were less severe on the tilted antenna channel,

the converse was also true. The probability density analyses

of the RSL have shown that the tilted antenna had a higher

probability of a deeper fade. Figures A-IS through A-17 are

representative examples of these data, and were chosen to

illustrate essentially three different fading characteristics

found in the data. Examples of the RSL recording are included

in the lower half of each figure.

c. Additional Analyses.

The origin of multipath components is difficult to define

for most microwave links, especially when they are derived in

an anomalous propagation condition. For example, multipath in

a strong ducting environment is no doubt due to refraction/

64
diffraction through the atmosphere rather than any form of

reflection. However, the refractive structure found during the

morning hours of 1 September 1978 was stable enough to permit

at least a cursory examination of the possibility for a reflection from the ducting layer. The gradient of the duct was

severe with a dN/dh of approximately -160N/kft (~525N/km), which

is seen to be much greater than a tr~pping gradient. This steep

gradient is equivalent to an effective earth radius factor

k = -0.43. The boundaries of the ducting layer were also well

defined from the refractivity data. The basic propagation

conditions are illustrated in Figure A-18, based on a flatearth model of the LP-SNI link. As seen- from Figure A-6(a),

the multipath on both diversity channels during the morning

hours was on the order of 12 ns maximum. Using a nominal delay

of 10 ns, we have plotted the locus (ellipsoid) for a reflected

component with this delay. The ellipsoid is distorted on the

flat earth profile, but the solid-line locus is the lower half

of the ellipse around the k = -0.43 ray path (with the duct) .

In this example there is a possibility that the multipath

could be a result of a reflection from the upper boundary

of the ducting layer. The evidence is the near tangential

relation between the boundary and the reflection locus.

The standard atmospheric condition (k = 4/3) is included in

the figure for reference purposes. An important observation

to make is that the multipath with these conditions could not

be due to any surface reflections.

Another example of possible reflection multipath from a

layer is illustrated in Figure A-19. We note from Table 1 that

during the afternoon hours of 1 September 1~78, the ducting

layer was higher than during the morning, with the lower bound

about 500 ft above SNI. The nominal refractive profile from

the surface to the layer was substandard with dN/dh O. This

corresponds to a k = lover most of the path below the duct.

Referring to the impulse width distributions for the afternoon

of this date shown in Figure A-6(b), we see that the multipath

65
delays were between 4 and 8 ns. Thus, using a nominal delay of

6 ns, we have plotted the locus for a reflection in Figure A-19.

It is apparent that a reflection from the surface would require

a specific tilt to a wave, and is thus less likely than a

reflection from the lower bound of the layer. The latter can be

seen from the tangential relation between the layer boundary

and the 6 ns locus curve near 50 miles (80 km) in Figure A-19.

Surface reflections are also of importance in analyzing

this link. Under standard atmospheric conditions, the most

likely location of surface reflections would be from a point

about 40 to 45 miles from the LP terminal. The nominal delay

for a ~urface reflected component would be on the order of

2 to 3 ns. The most significant evidence of the surface

component is perhaps shown in the impulse width distributions

of Figure A-14(a) for the afternoon of 13 September 1978. Both

diversity channels show a higher probability of the impulse

width near 15 ns, which would correspond to the surface reflection delay. It is obvious however, that longer delay times

were prevalent with the highest probability of delay being

approximately 5 ns. This delay was probably caused by a small

layer that existed near the LP terminal, but below the LP

height.

The best evidence of a surface reflection was observed for

the data of 8 September 1978 in the early morning hours before

sunrise (Figure A-II). The refractivity structure was super

refractive (approaching ducting) with a gradient of -30 N/kft

(-98.4 N/km) , yielding a k = 2.7. Under these conditions a

surface component could be observed from a point approximately

50 miles from LP with a delay on the order of 5 ns. The

impulse distributions of Figure A-II indicate a high probability

that this component was present.

66
PN Clock:

50 MHz

Time:

1250 - 1255

1255 - 1300

1300 - 1305

1305 - 1310 L _

1310 - 1315

I ,

o 20 40 60

IMPULSE WIDTH - ns

Figure A-I. The probability density functions of the power impulse width

for the low antenna on 31 August 1978. These data correspond

to Figures 10 through 13.

67
30 ns

.' I·

ns o 10 20

Low High

Antenna Antenna

(J)

r--4

II

l-'

t

0 10 20 30

(a) Time: 1550 - 1735 PST PN Clock Rate: 150 MHz

I

0 10 20 30 40 50 0 10 20 30 40 ns

ns ns

(b) Time: 1740 - 1750 PST PN Clock Rate: 50 MHz

Figure A-2. Power impulse width distributions for 22 August 1978.

68
40 60

ns

20

High

en Antenna

Ll')

.

N

r-I

II

·f

I.

40 60 0

ns

20

Low

Antenna

o

(a) Date & Time: 23 August 1978, 0907 - 0930 PST PN Clock Rate: 50 MHz

o 20 40 60 o 20 40 60

ns . ns

(b) Date & Time: 24 August 1978, 1515 - 1645 PST PN Clock Rate: 50 MHz

Figure A-3. Power impulse width distribution functions.

69
o

Low

Antenna

20

ns

40 60 o

High

Antenna

20

ns

40 60

High

Antenna

(a) Time: 0900 - 0950 PST PN Clock Rate: 50 MHz

o 100

ns

200 o 100

ns

200

(b) Time: 1530 - 1600 PST PN Clock Rate: 10 MHz

Figure A-4. Power impulse width distributions for 25 August 1978.

70
Low

Antenna

8.7 ns

o

o

5

j

5

10

10

ns

High

Antenna

ns

15

15

20

20

25

8.7 ns

25

Figure A-5. The power impulse width distribution functions for the data

of 31 August 1978. These data were measured between 1112 and

1200 PST at a PN clock rate of 150 MHz, just prior to the

period discussed in Section 4.

71
High U} 'Antenna U}

s::

Low N N

Antenna r-f r-f

II II

l-'

I I

I I I I

0 10 20 30 0 10 20 30

ns

ns

(a) Time: 1005 - 1055 PST PN Clock Rate: 150 MHz

High

Antenna

I

U} U} I Low s:: s:: U} U}

Antenna 00 i s:: s::

/ II II \ 00

l-' l-' t II

f

, l-'

J )

0 20 40 60 0 20 40 60

ns ns

(b) Time: 1155 - 1435 PST PN Clock Rate: 50 MHz

Figure A-6. Power impulse width distributions for 1 September 1978.

72
1010

o

r-I

1055

(a) RSL record for 1005 - 1055 PST, 1 September 1978) low antenna.

o

r-I'

(b) RSL record for 1200 - 1400 PST, 1 September 1978) low antenna.

Figure A-7. Recordings of the PN probe RSL for the mu~tipath conditions

depicted by the distribution functions in Figure A-6. Note

that the phase interference fading is worse for the afternoon

data. This fact is re'flected in the higher probability that

the power impulse width was less than standard, as shown by

the distribution functions of Figure A-6(b).

73
40 60

ns

20

High

Antenna

I,

40 60 o

ns

20

Low

Antenna

I:

1\

I

, ~---)

o

(a) Time: 1705 - 2300 PST PN Clock Rate: 50 MHz

1900 1930 2000

(b) RSL record of the PN signal on the low antenna.

Figure A-8. Power impulse width distributions and RSL record for a

night run on 5 September 1978. Note that the distribution

function for the impulse data on the low antenna is quite

similar to that of Figure A-6. The fading however is much

slower, indicating less dynamic .change in the multipath

components. Two very deep fades are seen around 1945 PST,

with a signal enhancement between them.

74
High

Antenna

Low

Antenna T 8 ns

T 8 ns t

I

0 10 20 30 0 10 20 30

ns ns

(a) Time: 1635 - 1910 PST PN Clock Rate: 150 MHz

I

o

Low

Antenna

I

10

ns

20

T

,

9 ns

30 0

High

Antenna

10.

ns

20

\

T = 10 ns

30.

(b) Time: 1957 - 2155 PST PN Clock Rate: 150 MHz

Figure A-9. Power impulse width distributions for 6 September 1978.

75
1730 1800 1830 1900

(a) ~L for the period 1730 - 1900 PST, 6 September 1978.

2100 2130

I I I I

i I I i

I I I

- I

+- .. -

ru

L()

,

1

I I I I I

I

'(b) RSL for the period 2100 - 2130 PST, 6 September 1978.

Figure A-I0. The RSL records of the PN probe data for the low antenna

distributions of Figure A-9. The delay spread of the

mu1tipath is similar for each period; however, the dynamics

of the channel cause deeper fading over the sunset period.

76
High

Antenna

I

o

Low

Antenna

10

ns

20 30 0 10

ns

r

20

I

30

0100

(a) Time: 0030 - 0207 PST PN Clock Rate: 150 MHz

0124

t :

~r

0

M

l I I I I I I I I I I I I I

I

(b) RSL record for the low antenna between 0100 and 0124 PST. The steady

signal was typical for the entire night until early morning.

Figure. A-II. Power impulse width distributions and the RSL record for an

all night run on 7-8 September 1978. The multipath delay

was essentially the same for both diversity channels;

however, the fading was significantly different. As seen in

(b), the low antenna RSL was very stable. The fading in the

high antenna channel was severe, as noted from the impulse

width distribution in (a).

77
+J

(])

c

Low 0

Antenna 0

o

en

s:: §J

U1 I"(j High en r-I s:: s::

II 0

o Antenna r-I

l-' (]) r-I U)

II

l-'

,

0545 0610

I I

1-+++-. t

roo

o.

r-I

f I I

(b) RSL record for the low antenna between 0545 and 0610 PST.

Figure A-12. Power impulse width distributions and the RSL record for the

early morning and sunrise period on 8 September 1978. Both

channels began to fade during this period, and the impulse

response for the low antenna indicated a three-path structure

as noted in the distribution function of (a).

78·
o

Low

Antenna

I

10

ns

I

20

I

o

High

Antenna

I

10

ns

20

(a) Date & Time: 10 September 19/8, 0550 - 0740 PST PN Clock Rate: 150 MHz

High

Antenna

Low

Antenna

20

PN Clock Rate: 150 MHz

ns

10 20 o 10

ns

(b) Date & Time: 11 September 1978, 0947 - 1100 PST

o

Figure A-13. Power impulse width distribution fuunctions for the data of

10 and 11 ·September 1978. Note that the distribution functions

for the low antenna data indicate a near-standard impulse

width with high probability.

79
I

o

Low

Antenna

High

Antenna

, I t I I I I

10 20 30 0 10 20 30

ns ns

(a) Date & Time: 13 September 1978, 1630 - 1654 PST PN Clock Rate: 150 MHz

Low

Antenna

High

Antenna

) l

i I

0 20 40 60 0 20 40 60

ns

ns

(b) Date & Time: 14 September 1978, 1100 - 1200 PST PN Clock Rate: 50 MHz

Figure A-14. Power impulse width distributions for the data of 13 and 14

September 1978. Minor mu1tipath and moderate fading were

characteristic for 13 Sept. Little or no mu1tipath was seen

in the final run on 14 Sept.

80
, I f I I

-50 -60 -70 -80 -90

RSL - dBm

(a) Probability density functions.

-

Tilted Antenna

• I I

•

1100 I , 1 I I , , I ,

untilted Antenna

•

(b) RSL recording.

4- TIME I

•

I

0920

I

Figure A-IS. Angle diversity data, 0920 to 1100 PST, 31 August 1978.

81
-50 -60 -70

RSL - dBm

-80 -90

(a) Probability density functions.

Tilted Antenna

1300

untilted Antenna -t

(b)RSL recording.

.- TIME 1200

FigureA-16. Angle diversity data, 1155 to 1435 PST, 1 September 1978.

82
-50 -60

I

-70

RSL - dBm

-80 -90

(a) Probability density functions.

['i1ted Antenna I

• I I I I I

.noo"HSnYB

).815 I I I I I

unti1ted Antenra

•

I

I

1800

I

I

(b) RSL recording.

Figure A-17. Angle diversity data, 1710 to 1910 PST, 5 September 1978.

83
20

16

12

N

o

r-i

E-i

ril

10

z

H

E-i

::r:

o

8

6

4

2

o

Figure A-18. Illustration _of possible reflection, type multipath from

a strong ducting layer on 1 September 1978.

84
70

- . . ..- •. j

30 40 50 60

DISTANCE - MILES

20

I ..- _- I

I .-- .Locus fur 6 ·ns ;--- . _. I

; '-~~~lectti.~~_ __ . .

o 10

6

2

o

8

18

16

12

14...r--.;:-:;:;::-----+-------+----~--+-~---.:::!liooc:----+----,::=-"-===----+-----'----+------~

N o

r-i

X

8

rz:l 10

z

H

Figure A-19. Illustration of possible reflection type multipath from

a strong ducting layer on 1 September 1978.

85

FORM OT·29 (3-73)

u.s, DEPARTMENT OF COMMERCE OFFICE OF TELECOMMUNICATIONS

BIBLIOGRAPHIC DATA SHEET

1. PUBLICATION OR REPORT NO 2. Gov 't Accession No.3. Re c ip ie nt ls Accession No.

NTIA Report 79-24

4. TITLE AND SUBTITLE

Investigation of digital microwave communications in atstrong meteorological ducting enVlronmen

5. Publication Date

August 1979

6. Performing Organization Code

NTIA/ITS

7. AUTHOR(S) R.W. Hubbard 9. Project/Task/Work Unit No.

8. PER.FORMING OJ{GANIZAT10N NAME AND.ADDR.ESS SC1"ences Instltute Lor Te~ecommunlcatlon

National Telecommunications and Information

Administration, u.s. Department of Commerce

Boulder, CO '80303

9103543

10. Contract/Grant No.

MIPR N6312678MP31106

11. Sponsoring Organization Name and Address 12. Type of Report and Period Covered

ICapabilities Development Department

Pacific Missile Test Center (PMTC)

Point MuGu, CA 93042

14. SUPPLEMENTARY NOTES

NTIA Report

13.

FY 79

15. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography oe literature survey, mention it here.) This report describes an experiment

conducted over a LOS microwave link off the coast of southern California. The objective was to determine the effect of known amomalous

propagating conditions, caused by persistent temperature inversions in

the atmosphere on a wideband digital communications system. A channel probe was used to measure the effective impulse response of the

transmission channel, and bit-error-rate (BER) measurements were made

on the same p~obe test signal. In addition, a dual receiver was used

to investigate the application of angle diversity on the reception of

signals in this propagation environment.

The results of the experiment indicate that the test link should,

if properly configured, support digital transmission at a mission

rate on the order of 50 Mb/s. Recommendations for implementing a

digital system for the specific link are given in the conclusions.

16. Key works (Alphabetical order, separated by semicolons)

Apgle diversity; channel characterization; digital microwave

communications; impulse response

17. AVAILABILITY STATEMENT

UNLIMITED.

o FOR OFFICIAL DISTRIBUTION.

.* u.s. GOVERNMENT PRINTING OFFICE.;'979-0-68'-284/69

18. Securi ty Class (This report)

unclassified

19. Security Class (This page)

Unclassified

20. Number of pages

95

21. Price:

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