300 KHz-30 MHz MF_HF(Goldberg1966)

8/17/2019 300 KHz-30 MHz MF_HF(Goldberg1966)

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IEEE TRANSACTIONS

O N

COMMUNICATION TECHNOLOGY VOL. COM-14,

N O .

6 DECEMBER 1966

In addition to the device reliability considerations, an

environmental study was made to verify that the system

could withstand shipping, storage, andoperating condi-

tions on the customer's premises. Laboratory tests which

included temperature shock, high relative humidity, and

vibrationwereused tostimulate he expectedenviron-

mental extremes to which the equipment would be sub-

jected in actual use. As a result of these tests minor design

modifications were incorporatedprior to initial production.

The majorityof equipment malfunctions a,re expected to

be repairable by merelynterchangingprintedwiring

boards. Trouble shooting

of

common circuitry consists of

the analysis of symptoms to narrow the troubledown to a

small number of possible circuit packs, and then the re-

placement of theseone at a imeuntil he defective

board is located. Troubles on traffic circuits, such as reg-

isters and centraloffice trunks, can be isolated by a feature

which enables a repairman to route test calls to specific

circuits. To facilitate testing, t,he built-in test equipment,

fuses, and alarms are located at the front of the cabinet ,

at eye level.

CONCLUSION

The 800A

PBX

was introduced into commercial service

in August 1966. Operational experience with he system

has been very good.

300

kHz-30

MHz MF/HF

Abstract-A

tutorial presentation is made in broad and general

terms regarding the properties of the

MF

and HF portions of the

radio spectrum as they pertain to and affect communication systems.

The fine grain behavior in terms of amplitude and phase variations

are presented in conjunction with the effects of fading periods, t ime

and frequency spread, and atmospheric noise.

A

discussion of both

theoretical and experimental bounds in error rate levels of digital

systemsas a function

of

the basic attributes of the ionospheric

channel is undertaken in connection with the adaptive approach to

communication system design. Two adaptive systems are described

briefly in terms of their ability to cope with the time variant dis-

persive ionospheric channel.

T

E PURPOSE of this paper will be to serve as

a

broad tutorial coverage of the elements and factors

employed for characterizinghe arious channels, as

segmented assignments, in he frequency spectrum extend-

ing from 300 kHz to 30 MHz. In th is connection, some of

the propert ies f significance such as the temporal behavior

in termsof signal levels and noise, channel transfer proper-

ties, interference, fine grain behavior, and system perform-

ance as exemplified by both theory and experimental data,

will be covered.

Liberal usewill be made of materia l already in the open

literature, material available to USAEL through their var-

ious contracts with industry andniversities, and data and

informationgeneratedasa result of USAEL'sown in-

houseprograms.

Although a good portion of the mater ial to be covered

will not be new to workers in this field, it is considered

Paper 19CP65-482 presented

at

the 1965

IEEE

Communications

Manuscript received March 28, 1966; revised August

I,

1966.

Convention, Boulder, Colo.

Thenthor is withhe Communications/ADP Laboratory,

U.S.

Army Electronics Command, Fort

R'lonmouth, N. J.

appropriate that it stillbe presented for the edification of

those people desiring to understand this area of activity

and for the sake of completeness.

In order to do justice to the broad spectrumovered by

this paper, it will be necessary to break apart the 0.3-30

nilHz frequency slot into four categories and then discuss

three of these categories (the medium frequencies) in

a

limited way while reserving the bulk of the discussion to

the 4th category 3.0-30 MH z (the high frequencies).

The so-called medium requency (MF) spectrum ex-

tending from 0.3 to 3

Hz

for the purpose of this presenta-

tion,as uststated, will bedivided into hreedistinct

regions approximated by region A00-550, kHz, region

B,

550-1650 kHz and region C, 1650 to 3000 kHz. Region A,

employing

CW

transmission almost exclusively, generally

is utilized for navigational purposes, for mobile, aero-

nautical and ship communications, for emergency survival

communications, and for time and frequency synchroniza-

tion. Region

B

is employed for standard broadcast service

and region C

is

and may be utilized for fixed and mobile,

land, maritime and aeronautical navigation, and commu-

nication purposes.

In

these three regions most of the useful distant field

energy is propagatedby heground

or

surfacewave.

The sky wavegenerally presents a sourceof trouble, how-

ever, it is occasionally utilized as the primarymode, espe-

cially for region C.

As far as the surface wave support is concerned, which

can be viewed as due toan earth-atmosphere wave guide,

the signal strength is reasonably well behaved. Generally t

follows an inverse distance law with the value of signal

strength being

a

function of the polarization, operating

frequency,and hegroundconductivityand dielectric

767


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765 I E E E TRANSACTIONS O N C O M M U N I C A I lO NE C H N O L O G YE C E M B E R

constant along the pathof propagation. In this regard, sea

water (conductivity4 X lo- EMU, ielectric constant80

ESU , rovides the pathwith the least attenuation. Aoor

earth , hat is, earthwith low conductivi.ty

EMU

and low dielectric constant

3-5 ESU

ields a path with

relativelyhigh ttenuation.There is littlediurnal or

annual variation in the ground wave characteristic.

In region A, under good conditions, ground wave propa-

gation can reach 000miles with only40 dB more loss than

that due to the inverse distance loss. Theoretical work of

significance in this area has been performed by Sommerfeld,

Morton,vanderPolandBrenner,Watson,andWait.

Sky wave propagation for this groupf frequencies exhibits

properties which are dependent upon he stateof the iono-

sphere with signals experiencing, change in level by a fac-

to r of from 2 to 5 as a function of sun spot activity. The

existence of the sky wave gives rise to fading and inter-

ference effects t locations where110th the groundwave and

sky wave are received. This interference effect tends to

tak e place with maximum severity at distances of a few

hundred miles from the transmitterwhere both theground

wave and the skywave are of equal strength.

I n general, sky wave signals experience diurnaland sea-

sonal variations superimposed upon the variations due to

the sun spot cycle. Fortunately, during the daylight hours

there is high absorption in the :D region, hence, the sky

wave tends to be aroblem only during nighttime whenhe

D layer disappears. The impact

o:i

ionospheric propagation

on medium frequencies and high frequencies (HF) will be

covered in the detailed discussion of the frequency region

from 3-30 MHz.

Some pertinent properties of sky wave transmission at

MF, however, are best cited at this time. The envelope of

the received signal n the majority.f cases tends to ollow

a

Rician distribution which could be viewed as the combina-

tion of a Rayleigh distribution and a specular component.

The fade rate is roughly 0.01 per second mplying ong

fades. Equally rough estimates of the correlation distance

forspaced antennas ndicates thatabout 20 km is re-

quired for decorrelation t o a value 1 / ~ .

As already tated for the lvIF region, theground

waves enerally the mostlnportant primarily be-

cause the energy is reasonably constant (nonfading) and

appears compacted as a specular ray. It is interesting to

note thatbecause of this specular nonfading characteristic,

diversity reception would not enhance system reliability

unless it could opera te on theresence of uncorrelated noise

or interfence. In the ower frequelncy portions of this spec-

trum limitationsdevelop n erms of antenna efficiency

with values of

10

percent being considered good and with

a communication bandwidth capa,bility n the ange

100

to

500 cycles being typical. Unfortunately, for this portion f

the radio spectrum (region A) atmospheric noise is quite

high being roughly about two ord ers of magnitude greater

tha n he level n the high requency 3-30 MHzband.

I n general, communications reliability in this region tends

to

be

limited by this noise factor which is further aggra-

vated by the ever present and generally increasing man-

made noise background rather than variationsn the signal

support mechanism.

Designs of communication systems in this region of the

spectrum are basically easily established in terms of rea-

sonably well behaved and understood factors.The additive

disturbances, characteristicsof applicable antenna systems,

transmission path loss, etc. are comfortably taken nto

account.

In general, RegionA tends to be elatively free from he

effects of sudden onosphericdisturbances which would

affect the signal support mechanism and is considered a

relatively reliable portion of the radio spectrum n his

regard. Both hephaseandamplitude of groundwave

signals tend to be of high stability. In th epresence of sky

wave there s a phase lagf 1-2 Hz representing he dirunal

variations of ionosphere layer heights.

Region B, the broadcast band, is known to all of us in

terms of the local range of coverage of the various broad-

cast stations. For the enter tainmentpurpose it is intended

to satisfy, there is relatively little one can complain about

(excluding program material) except during thunderstorm

activity or nighttime. This portionof the spectrum can be

considered as well disciplined and stat ic with its se deter-

mined by very rigid control. In this portion of the band,

exceptor nomalouspropagationbehavior and local

lightning activity, reception conditions are quite adequate.

The mode of modulation universally employed, using voice

or

music signals, is double sideband amplitude modulation.

Some activi ty is underway

t o

try to employ compatible

single sideband or this service. Generally, noisy signalse-

come a problem only near the service range fringes where

although intelligibility may still be igh, esthetic apprecia-

tion factors are quiteow. This port ion of the band has its

problems at nighttime when he high absorption properties

of the D layer are no longer available to reduce the un-

desired sky wave suppor t. It has been common experience,

especially while travelingn an utomobile, to hear stat ions

from distant points,

1000

miles or more, with clarity and

strength that at timesxceed the local station one hadeen

tuned to.

I n this portion of th e spectrum, information bandwidths

of

5

kHz are generally employed with some stations re-

ceiving authority for

10

kHz nformationbandwidths.

Most assignments in this frequency band are on aegional

basis with joint sharing f frequency coupled with reliance

on geographical separation for noninterference. t is neces-

sary a t nighttime

for

some stations to educe their radiated

power or even go off the air in order t o minimize the possi-

bility of theircreating nterference to a distant station

when undesired sky wave support would be prevalent.

The last region in the medium frequency range, region

C, basically employs groundor surface wave for itsropa-

gation support but s more seriously troubled by the pres-

ence, in most instances,of undesired sky wave propagation.

This portion of the spectrum is highly crowded as is the

entire range 0.3-30 MHz. Although t he atmospheric noise

level is lower in region C than at the region A portion of

medium requency ange, the groundwavedistance at


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1966 G O L D B E R G : MF/HF CO M M UNICATIO N SYSTEMS

acceptable a ttenuation levels is much shorter.

Generally, this region is used forcommunication dis-

tances of up to

100

to

175

miles. This frequency region

however is subjected to the effects of ionospheric disturb-

ances. Basically, thisportion of the spectrum is quite

stable n the daytime when the

D

layer s available to

attenuate thehigh angle radiation. At nighttime, however,

the level of interference due to sky wave support from

distant stations makes the situation in this portionof the

band something less than desirable. Largedirective an-

tenna arrays are mployed by marine operators inrder to

enhance reliability for communication. A large percentage

of the traffic carried in this region is amplitude modulated

3

kHz voice signals with possibly the inherent redundancy

in unprocessed speech making this segment of the spec-

trum useful a t night. This easonably stable segmentof the

M F

band is also employed for Loran purposes; however,

diurnalvariations nphaseandamplitude of received

signals are evident.

IONOSPHERICEFLECTION

Throughout thispreceding material i t has been indicated

that skywave upport epresents an undesirable phe-

nomenon. In th ematerial to ollow, which in essence repre-

sents the bulk of this paper , the mechanism of sky wave

support, which

is

essential for communication in the H F

range, will be explained and, more importantly, he impact

of the resulting effects on communication systems from

such support will be covered in detail.

The ionospheric mechanism provides forward upport in

the range

3-30

MHz bymeans of specular reflection, refrac-

tion, or byscatterwith in he ionized medium.About

60 years ago, the idea of an ionized layeraboveand

concentric wi th the Earth as conceived of independently

by Iiennelly and Heaviside as a means of explaining the

phenomenon of long-distancecommunicationorders of

magnitude beyond line of sight distances.

Th e upper regions of the Earth’s atmosphereecome less

dense as one proceeds away from the ea rt h. In the egion

from approximately 50 km to 450 km, one can find mole-

cules of oxygen, nitrogen, nitric oxide, and rarer gasses in

dispersion.

It

is generally believed t ha t ultraviolet radia-

tions and corpuscular bombardment from the sun are the

main agents in causing the gasses to ionize in the upper

atmosphere. Th e level of this ionization s not uniform

throughout the region from 50 to 450 km, in fact, the ion-

ization is distributed in layers having peak intensities a t

particular heights.

The ability of the ionosphere to provide propagation

support is related simplyo the ondition that itsefractive

index at radio frequencies is different €rom th at at free

space. A wave incident to the onospheric layer a t angle

4

will be bent toward the horizontal and thenack to Earth

with a rate that is dependent upon the electron density

and the ngle of incidence. This phenomenon canbe related

to the refractive index as follows:

769

(1)

where

= refractive index of ionospheric medium

N = electron density n electrons per cc

e , v

=

charge and mass of electron

Eo = permit tiv ity of freespace

w = radianrequency.

The electromagnetic wave will reach a maximum height

prior

to

returning to Earth at the point where

N

is large

enough to reduce t he value of o that

u

= sin 4 (2)

where 4, as defined previously, is the angle of incidence of

the electromagnetic wave with he ionized layer.

An important application of the above relations is in

their use in obtaining what is known as the critical fre-

quency for the case where the electromagneticwave s

vertically ncident,hence

4 =

0, sin

4

= 0, and

=O

Such a wavewill reach a height determinedby N and then

be returned to Earth .

Th e relationship is as follows:

when the proper constants are substituted. n this case f is

the frequency of the wave in MHz.

This critical frequencyfo obtained y rearranging

(3)

N

o

= 4 1 . 2 4

x

104

=

9

x

1 0 - 3 d R

is the highest frequency which can be reflected as a result

of vertical incidence. It is obviously only dependent upon

N , the electron density.

Soundings by pulse transmission probing with vertical

incidence provides a means for determining this critical

frequency. I t s use s undamental n engineering com-

munications circuits and estimating proper operating fre-

quencies by means of the relationship

f (MUF) =

fo sec

4

5 )

where f(MUF) is designated as the maximum useable fre-

quency foroblique transmission,

fo

is the critical frequency

from vertical sounding, and 4 is the oblique path angle of

incidence. This relationship is based upon ray theory and

neglects the Ear th’s magnetic field. It s use for prediction

purposes is quite adequate, in view of other assumptions

made in theprediction process.

Figure 1 depicts the phenomenon of refraction resulting

from the effect on the velocity of wave front propagation

in a mediumof changing refractive index.

The ionospheremedium has classically beendivided

into a number of regions. That portion below 90 km is

known

as

the

D

region; it s existence is predominantly a

daytime phenomenon. The level of ionization is approxi-


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I E E ET R A N SA C T I ON SO N C OM M U N I C A T lON T E C HN OL OGY

D E C E M B E R

/

Fig.

1.

Refraction

of

wave.

\

mately

lo2

electrons/cc at 70 k:m,

lo3

electrons/cc at

80

km, and l o4 electrons/cc at

90

lcm.

Of

necessity, electro-

magnetic wavesused for long distance and F layer propa-

gation pass through this region twice. Th e D region, be-

cause of i ts relatively higher concentration of neutral

particles and heavy ions, extracts energy from a passing

wave as a result of collisions with electrons excited by the

wave. As far asH F propagation js concerned, this region is

viewed asanattenuationband. However, atnighttime

when this attenuation is not present,we find phenomenal

propagation support for distant transmitters which gen-

erally cause a large increase n b,zckground interference.

The

E

region is considered as existing from

90

km to

approximately 160 km with a maximum region ionization

at about 110 km. Th e electron density at thi s height s

in the order of lo4 to lo5 electrons per cc during daylight

hours. At nighttime there is stillsome ionization,

but it

is

much weaker. The critical frequtmcy drops about an order

of magnitude from its dayt ime value. In addition to the

normal

E

layeronization, there ppears occasionally

patches of denser ionization a t

E:

layer heights that seem

to ravelas ionization clouds. Thisunpredictable phe-

nomenon s called Sporadic E and has been responsible

for creating nterference, because of it s superior support

for radiated energy, into areas not normally engineered to

allow for these signals. The E region is useful for propaga-

tion suppor t or distances up to2000 km, using frequencies

as high as

20

MHz.

The region above 160 km is hewn as theF region. This

region classically has been dividlxl into 2 layers known as

the

F1

and

Fz

layers. The

F1

layer generally exists during

daytime at about a height of 200 km, while the Fz layer

exists in the egion

250

to 4.50 knl.The F1 layer is not en-

erally considered as providing th'e basis for ell-engineered

long-distance communication. Th e F1 layer merges with

the

F2

layer a t nighttime to a height of about

300

km.

Th e electron density n this egion is in he order of lo6elec-

trons per cc. Th e 300-km layer height, called the F layer, is

usually considered as th e basis for circuit engineering. The

use of single hop transmission, because f the much greater

height of the F layer, can provide support to a distance of

4000

km or more. Frequencies as highs

50

MHz (when the

Fig.

2.

Electron density vs. height.

ionization level is high) can e utilized for his mode.

Figure 2 shows a profile of the threeregions and depicts

the electron density.

For various reasons, including it s high absorption and

low electron density, the

D

region has not been fully ex-

amined because of inst rumentation difficulties. However,

it is known that the D layer electron density varies with

the 11-year solar sun spot cycle and with the sun's zenith

distance. The electron density in this egion is a maximum

at noon and during theummer.

The

E

layer s generally well behaved except for the

unpredictableappearance of Sporadic E. The electron

density just before dawn rises from a low value a t night to

a maximumat noon then begins to fall again o a low value

after sunset. The E layer ionization does not change much

as a function

of

sun spot activity, nor does it vary much

with changes in season. The critical frequency of the E

layer hasempirically been determined o be

F , = 0.9 [(lSO

1.44

R

COS

y]lI4 (6)

where R is the sun spot number and

y

is the solar zenith

angle.

A

plot of the E layer critical frequency as a function

of the solar angleis shown in Fig.

3 .

The spread due to seasonal changes and time of day is

seen to be small. Generally, a lower critical frequency pre-

vails during the winter and summer. The variation, as a

function of solar activity, is depicted n Fig. 4. The change

is

even less pronounced.

When the short term behavior of t he E region is exam-

ined, there appearchanges in the rder of 10 percent in the

critical frequency which can be correlated with variation

in solar output. Magnetic storms do not materially affect

the

E

region. Short-haul circuit requirements for daytime

operation, based upon

E

layer reflection, can consequently

be easily satisfied.

The F1 layer, as shown in Fig.

2,

is not always sharply

defined.

Its

existence is most prominent during those times

when the critical F2 frequency is ow, as for example, during

the minimum of the sun spotycle.

It

is evident during he

summerand also during onosphericstorms. Thereare

small

changes in the

F1

critical frequency as a function

of

day to dayhanges in solar activity. Th e magnitude of the


5/18

1966 GOLDBERG:

MF/HF COMMUNICATION

SYSTEMS

771

.-

I

L

5

I

0.

0 1

.O.3 o . OS

o i

c.7 a a o

COS

SOLAR ZENIT# NGLE

Fig.

3.

Critical

E

layer, frequency

vs.

solar angle.

I L I

O 5 0

loo

IS0 t r 250

~McWTUIZD

&lNSPOr

NUMBER

Fig.

4.

Critical

E

layer, frequency

vs.

sunspot number.

change is approximately the same as th at experienced in

the

E

layer. This layer, as a efined enti ty, exists only in the

daytime.

The FZayer is he layer with theighest ionization evel

and plays a dominant role in long-distance communica-

tion. This layer is quite complex in its behavior. The Fz

layer critical frequency is not directly related to the solar

zenith angle. This layer exhibits what is known as anoma-

lous behavior. Th at is, it ac ts at times contrary to those

theories useful in explaining D,

E,

and F, layer behavior.

This

erratic

behavior occurs during the daytime n he

winter and has been labeled the

winter anomaly .

Figure 5 depicts

idealized

typical

Fz

layer critical fre-

quency behavior as a function of time of day andyear.

Figure

6

shows the dependence of the

Fz

layer critical

frequency on the sun spot number. The Fz layer shows a

direct dependence on he level of solar activity. During sun

spot maximum, the seasonal differences are enhanced. The

idealized MUF for various distances using Fz layer single

hop propagation as a function of time of day, season, and

sun spotmaximum and minimum are shown in Figs. 7 and

8.

The winter anomaly is quite evident.

The effects of changes in latitude and longitude on the

determination

of

critical frequencies relate essentially

to

the change in solar zenith angle at the geographical point

under consideration.

It

is

fairlyevident that one imitation o the useful

transfer of information from one point to another can be

expressed in terms of a signal-to-noise ratio. It is for this

Fig. 5 .

F

layer critical frequency vs. time of day.

Fig. 6.

F

layer critical frequency vs. sunspot number.

0 I

o

C

NODW

I 8

Lt

L O C L T IME Ar PATH CENTER

Fig.

7 . MUF

vs. time of day for winter.

NO *

18

LOCAL i M E

A T PAT H

c€NT€R

Fig.

8.

MU F vs. time of day for summer


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772

IEEE T R A N S h C l ’ I O N S O N C O M M U N I C A T I O N T E C H N O L O G Y D E C E M B E R

Fig.

10.

Signal propagation

by

E

and

F

layer sllpport.

0.

/ O

M c s

oo

Fig. 9. Noise level

as

a function

of frequency.

reason tha t noise, as a primary factor in communication

link design, is impor tant in itswn right. Th e following dis-

cussion will be limited to wha t is known as external radio

noise and specifically, relates to atmospheric,xtra-

terres trial and man-made noise. Figure 9 shows the rela-

tive levels of these three noises.

Atmospheric noise generally consists of short pulses of

high amplitude with random occurrence superimposed on

a lower level of random noise. The average value over a

period of a few minutes is used to develop an averagefor a

given hour. These values are generally constant for the

hour except during local thunders torm activity

or

iono-

sphericsunrise

or

sunset. The diurnalvariation n the

hourly median is related to thechanging propagation con-

ditions and the thunders torm activity. Generally, akmos-

pheric noise is greatest a t low frequencies, becoming rela-

tively unimportant above 30 R’II3z.

The extra-terrestrial noise may come from the sun, stars,

and interstellar space. Solar flares, when they occur, can

cause considerable ncreases in the oise level. This galactic

noise becomes greater th an the atmospheric noise in the

frequency region above 10 MHz.

In theHI? band, man-made noise can be a most signifi-

cant factor in the total noise contribution. This fact pin-

points th e need for proper siting when set ting up a re-

ceiving location. This noise is generated by any and all

electrical equipment. Generally, man-made noise is propa-

gated by power lines and byground wave, consequently, t

is notaffected by ionospheric conditions. The level of man-

made noise is highly correlated wi,thhe population density

of t he surrounding area. Man-nmde noise may be random,

periodic,

or

a combination of both, depending on the noise

sources.

It

is interesting to observehat for

HF

radio com-

munication, the front endreceiver noise (internal noise) is

not the limiting factor in erformance. The simple process

of connecting an antenna to HF’ radio receiver introduces

noise at a level considerably higher than that developed

by the eceiver.

Based upon ray tracing concepts, it is possible to define

the mechanism of electromagnetil: energy transfer between

the tra nsmitt er and receiver by simply extending direc-

tional lines to the eflecting ionospheric layer with an ngle

off the horizon equal to the propagat ion take off angle.

Figure

10

shows this technique

for

a number of different

take off angles. The particular layer involved in propaga-

tion support is utilized in defining the mode of propaga-

tion. For example, a single reflection from the F layer would

be known as the 1

F

mode, a double reflection from the E

layer would be known as 2

E

mode.

The transmission distance limit for single hop reflection

using ionospheric layers, based upon geometricalonsidera-

tions, is dependent upon the height of t he particular layer

being employed.

For

E layer propagation, this limit is in

the order of 2000 km; for

F

layer transmission, the distance

limit for one hop support is about 4000 km.

It is possible, and in practice happens oftennough, that

more than one path is available for the propagation sup-

port of the tr ansmitted signal. It is obvious that the time

taken by each path

is

different, hence the signals arriving

at the receiver a t a particular instant will represent differ-

ent instantaneous transmission epochs. This phenomenon

is known as nlultipath propagation andgives rise to one of

the major sources of trouble in long-distance communica-

tionby high requency adio. Th e technique of trans-

mission circuit design, based upon the concepts of maxi-

mum useable frequency and frequency of optimum traffic

is predicted uponminimizing the m ultipath suppor t and

layer absorption. The spread in arrival time of the trans-

mitted signal for circuits of 3000 to 5000 miles could be in

the order of 3 to 5 ms. Theseverlappingignals

generate destructive interference to the composite signal

applied to the receiver. This , of course, will decrease the

intelligibility of voice transmission and will create errors

in digital transmission. Both theoretical and experimental

work has shown that multipath is a maximum at trans-

mission path distances of about

2000

km.

Since the dlfferential path delay that could be tolerated

is dependent upon the natureof the communication signal

and the rate of it s transmission, i t is important that op-

erating frequencies be chosen in order not to exceed the

acceptable delay. Figure

11

shows the mul tipath eduction

factor as a function of path distance with the time delay

as a parameter. This factor is to be applied to the maxi-

mum useable requencydetermined or the path under

consideration.

When considering those factors affecting transmission

reliability, in a sense, the phenomenon of mu ltipath propa-

gation could in itself be viewed as a factor in creating tur-

bulence in the ransmission channel. However, he broader

meaning of turbulence is related to solar flares, magnetic

storms, and sudden onospheric disturbances (SID).


7/18

1966

GOLDBERG: M F / H FO M M U N I C A T I O NY S T E M S

773

10'

O IO IO

D I S T A N C E W

Fig. 11. H F multipath reductionfactors.

The solar flare usually lasts less than one hour andgen-

erally occurs most frequently during sun spot maximum.

During the flare, large amounts of ultraviolet and X-rays

are emitted which, in turn, cause large increases in the

D

layer electron density. This has the ffect of increasing the

absorption of electromagnetic energy passing through the

region and of significantly decreasing signal strength. The

immediate effect on theE and F layers appears to bemall.

However, since the energy for earth-bound stations must

pass through he

D

region at least twice, the effect on com-

munication is already felt. Th e SI D

or

short wave fade-

outs (SWF) last a relatively short time from minutes to

hours and arexperienced in the sunlitegions of the Earth.

The magnetic disturbances are usually experienced about

20 to 40 hours after thenset of a flare. This is attr ibutable

to the lower energy corpuscular radiation from the flare.

Th e magnetic effects last from two to five days.

It

is this

delayed effect th at is the most troublesome. The magnetic

and ionospheric storms re a worldwide phenomenon

which, in severe cases, affect practically all high-frequency

transmission employing the ionosphere. During he sun

spot maximum he turbulence is moreevere, but of shorter

duration (two days), while during sun spot minimum, al-

though the flare occurrence is rarer, its effects last for a

longer time-five days.

Th e most significant effect of an ionospheric storm is the

reduction of the Fz layer critical frequency. n addition, the

F

layer acts more like a diffuse-scattering surface rather

than a surface hat provides reasonably specular eflection.

The effects of

F

layer electron density reduction re greater

at

higher geomagnetic latitudes.

It

ishas been noted

that during sun spot minimum there appears to be a

27-

day cycle to he ionospheric disturbance.This period

corresponds to the solar period of rotation.

I n addition to he disturbancescorrelatedwith solar

flares, a t least three other phenomena are common causes

of transmission turbulence. The first of these is known as

Sporadic

E.

These ionization clouds located in the

E

layer

region support the propagation of electromagnetic energy

at frequencies considerablyabove the normal

E

layer

RIIUF.

The ionization clouds travel, and in numerous in-

stances have provided

E

layer height propagation support

a t night, when the normal

E

layer is absent.

A second disturbance known as Spread F is manifested

as a continuum of

F

layer height in that rather than having

a single, normal, sharply defined vertically sounded return

frequency vs. layerheight,manyheightsappear,hence

there is multiple support for each frequency. he effect on

signals propagated via Spread

F

is generally to introduce

upon the signal rapid fluctuations characteristic of scatter

communications.

For

the third disturbance within the eneral framework

of turbulence, it is appropriate to discuss fading. There are

various types of amplitude ading.Theseare generally

related o he period betweenminimums. The shortest

interval generally relates to polarization fading between

the ordinary and extraordinary waves and s known as in-

terference fading. Periods ranging from about 0.1 seconds

to a few minutes are responsible for both selective and fla

fading. Selective fades relate to specific frequencies within

a transmissionband fading out hile flat fades relateo th

entire band fading out. When the fadeeriods are approxi-

mately five minutes

or

more, the fades are generally

attributable to bsorption changes in the

D

layer.

Th e distribution of the amplitude of the signal envelope

for the common type of fading (both selective and flat)

which is generally due to multimode support of propaga-

tion, is best escribed by the resultant f a composite wave

made up of

a

Rayleigh distributed amplitude and a stead

component. This type of distribution is known as

a

Rice

distribution. This distribution has the attribute that hen

the specular component

is

small, the distribution is essen-

tially a Rayleigh type andhen the specular component is

large, the distribution is essentially Gaussian.

It is interesting to note tha t most of the information

relating to the critical frequencies for each layer, the level

of

electron density, the existence of Sporadic

E

and Spread

F,

and the general stat e of the ionosphere is obtained by

means of electromagnetic probing using a device known as

an ionospheric sounder. In theast few years, however, this

has been supplemented by rocket and satellite sounding

from both sides of the ionosphere.

Up to the ost recent time, the technique was to launch

a vertically directed wave (pulse) and monitor its return

on an oscilloscope. By measuring the time delay for he re-

turn over a band of frequencies, it is possible to develop an

electron densityprofile using the mathematical relationship

between the criticalfrequency and he electrondensity

noted in

(4).

More importantly, for communication, it

is

possible to observe the critical frequency for each ayer

directly. Considerable skill is required n order to inter-

pre t the results and much manual processing is needed.

Figure 12 shows an idealized return from a vertical iono-

spheric sounder. An ionogram such as this is quite rare,

most of the t ime the re isonsiderable interference present

and each return is made upf two lines due to the ffect of

the Earth's magnetic field splitting the electromagnetic

wave into twodifferently polarized waves, because each is

reflected by a different electron density.

These

two waves

are known as he ordinary and extraordinary rays.

Ionospheric sounders are located in field sites all around

the world. The job of these stations is o collect da ta using

15-minute intervals regarding the critical frequencies for


8/18

774

I E E ER A N SA C T I ON SNOM M U N I C A T I ONE C H N O L O G YE C E M B E R

1000

900

aoo

-

700

600

500

400-

300

200

100

-

0

-

Y

E LAVER

2 3 4 5 6

1 8 s

FREPUENCY mc/s

Fig.

12.

Vertical sounder ionogram.

the various ionospheric layers. The information from the

various sta tions are collated at the Bureau of Standards

and from th is data, world charts are developed showing

the critical frequency as a function of time of day and

geographical ocations. This information s then used to

form predictions about the R4UF' and FOT that hould be

used for particular communication paths.

A recent development in the ionospheric sounding ar t

is the oblique sounder.This device permits ionograms o be

madeusing the actual communication pa th th at would

normally be employed for traffic. The technique is to use a

stepped frequency transmitter for sending he probing sig-

nal, while at the dis tan teceiving site the receiving system

is ocked instepwith he ransmitter.This echnique-

actually permits the measurement of communication sup-

port to be determined at will. &lore importantly, it frees

the communicator from reliance on predictions which are

not always reliable. The potential for optimized ommuni-

cations frequency determination on a real-time basis is at

hand through the use of oblique sounders. This technique

is just beginning to develop.

Utilization of most of t he preceding information n

terms of establishing the interplay between the various

factors considered for engineering long-haul ionospheric

transmission networks can best be established by employ-

ing prediction charts issued by the Bureau of S tandards.

Although the procedure is well laid out , the esult achieved

makes the employment of this approach an art rather than

a science.

The usualprocedure s to employ prediction charts

issued by theBureau of Standard.s everymonth which pre-

dicts critical frequencies three months in advance for all

parts of the world. These predictions are updated by means

of a monthly, weekly, daily, and evenhourlyadvisory

issued by

NBS.

An impor tant factor used in developing

such charts is the solar activity index. An incorrect esti-

mate n his actor would came troublesomeerrors in

estimating MUFs for particular circuits.

The technique nabbreviated orm is essentially as

follows:

1) Estimate hegreat circle distancebetween rans-

mitter and receiver site and locate its midpoint, in terms

of it s geographic coordinates.

2) Determine midpoint local time.

3) Determine MUF from predictioncharts for particular

zone of interest as a function of t ime of day and midpoint

geographical location.

4) Plot these points or a full24 hours.

5)

The optimum working frequency is then taken as

85

percent of these values.

Another curve must be developed in order to define a

lower limit to the choice of frequencies available a t a par-

ticular time. This isnown as he LUF

r

lowest useful fre-

quency. This imiting frequency s determined by theignal

strength required at th e eceiving location. This, in turn, s

related to the local noise level which sets the threshold

against which the desired signal-to-noise ratio is established

for the required performance criterion. Th e received signal

strength is,

of

course, related to he ransmit ter power

available, antenna ystems involved, the transmission

distance involved, and the absorptionosses experienced by

the electromagnetic wave. The signal strength determined

in this manner ishighly dependent upon the frequency at

which it has been calculated. It is necessary to determine

the lowest frequency a t which the required signal strength

will be achieved. A plot of these values fordiff erent times

of day will then be the locus of the lower limit of useful fre-

quencies for the pat h under consideration in terms of t he

type of service required. It isnoted tha t he signal

strength required s significantly related o he ype of

modulation employed and the reliability required.

Figure 13 shows the result of a determination of the

FOT and LUF

for

different length circuits. Th e idea for

circuit operation is to choose operating frequencies which

fall within the bounds of FOT and LUF. I n general, the

complexities of propagation coupled with the eed fo r rely-

ing on predictions that are the esult of many approxima-

tions makes the choice of an optimum operatingrequency

dif€icult. However, some broad guidelines are possible. A

useful and obvious criteria is to adjust things so that an

adequate signal-to-noise ratio is achieved. The step t o be

taken in thisdirection is to use the highest frequency tha t

will propagate to the distantreceiver. This pays off, since

radio noise decreases as the frequency is raised while

absorption s likewise decreased. Th e use of frequencies

near the MUF, n addition,results in less likelihood of

multipathprogagation.

Th e ar t of engineering high-frequency communication

circuits is well documented by myriad publications fr om

the National Bureau f Standards and the Radioropag*

tion Agency of the U. S.Army.

We now reach the point where we can discuss what we

have learned about theHF ionospheric mode of communi-

cation. Equipments utilizing this mode mploy voice,

music, TTY, facsimile, data , and ven noise as modulation

sources. With proper conditions, these equipments can be


9/18

1966 GOLDBERG:

MF/HF

COMMUNICATION SYSTEMS 775

Fig. 13.

FOT-LUF

prediction.

used for essentially around the world communications with

distances being established by choice of operating fre-

quency, radiated power, and antenna ake-off angle.

Ionospheric propagation, as has been indicated, is gen-

erallycharacterizedbymultiplehop ionospheric layer-

ground reflection withboth specularand andom com-

ponents of energy arriving at he receiving antenna.

This energy, because of t he time variant dispersive proper-

ties of th e ionospheric medium, occupies a fading band-

widthfrom 0.05 to 15 Hz dependingupon th e level of

turbulence. Nonauroral path propagation generally has an

upper limit of about

2

Hz. The envelope of the composite

received signal exhibits Rician statistics with he Rayleigh

statis tic subset predominating as indicated earlier. Limited

da ta relating to measurement of th e correlation bandwidth

indicate that i t varies from about 100 to

3000

cycles de-

pending upon the channel turbulence. The time spread of

arriving energy varies fromess than 100ms to about4 ms.

With good pathsand properoperating frequencies the

multipath spreads are ess than 1ms. In this connection, it

is noted that ionospheric propagation via an auroral path

is generally much more turbulent than nonauroral trans-

mission. It is quite possible tha t t he fade rate may be as

high as

25

Hz while the correlation bandwidth may be

as narrow as

0

Hz or less over an auroral path. In general,

the ionospheric channel is limited in performance by both

additive disturbances such as atomspheric noise, friendly

interference, and basic propagation loss factorsandby

multiplicative effects such as fadeate and the Doppler and

time spread of the received energy.

By applying effective techniques uch as space, fre-

quency, or time diversity reception and the proper choice

of operating frequency, it is possible to have better thana

90 percent reliability factor forhis typeof channel.

Although R F bandwidths of up to

20

kHz (under good

conditions) can be adequately supported by this medium,

it isnoted tha t because of the hiah demand forassignment,

space and the already dense packing of users in this por-

tion of t he radio frequency spectrum, assignments are not

made that broad.

In general,

12

kHz of

RF

spectrum space s about as large

a slice tha t can be ssigned. In th emilitary this isenerally

utilized to carry four

3

kHz channels of information con-

sisting of either voice,

TTY,

facsimile, or data.

The most critical and fundamen tal ignal function th at

could be used to characterize the various forms of modula-

tion is considered to be a digital signal. Ultimately, it is

expected th at all information will be handled on a digital

basis. With this thought in mind, the

USAEL

undertook

a program to measure the properties of the ionospheric

channel in terms of it s fine grain behavior in both phase

and amplitude and n erms of the actual transmission

of digital signals. For the latter partf the work, both FSIi

and PSI


10/18

p I C 4

Fig. 14. Phasetability vs. weraging time,igh SNR.. Fig. 16. Phasetability vs.veragingime, low

SNR.

Fig.

15.

Phase stability vs. averagingime,edium

SNR.

Fig. 17. Phasetability, low fadingate.


11/18

0 20 LO 60 80

loo

120

a

Fig. 18. Phase stability, medium fading rate. Fig.

20.

Theoreticalhase stability for Rayleigh fading.

1 R 2

FB2

= ---

27r2

R2

where R = envelope of signal.

Using the theoretical curves in Fig. 0, it has een possi-

ble to extrac t f rom measured phase curves such as shown

inFigs. 14-19, estimates of th e fadingbandwidth. The

distributions for two classes of runs are shown in Fig. 21 .

It shouldbenoted tha t he ading bandwidth is not

the bandwidth of the power densityspectrum.For in-

stance, with a rectangularly shaped power density spec-

trum of width B, sayone cycle, the fadingbandwidth

would only be:

B

F B

=

~ FS 0.288.

2 6

(8)

The short-termamplitudecharacteristicsare also

of

significance. Figures 22 and 23 show th e distributions for

fine grainsignalamplitudemeasurements or

mild

and

sever e conditions with superimposed Rayleigh theoretical

curves. From a relatively large collection of da ta such as

this and the phase data cited earlier, it seems justified to

employ the statistics

of

narrow-band Gaussian noise as the

model of the time var ian t dispersive effect on ionospheric

transmission of signals.

The distribution of fading periods for m i l d and severe

conditions can be observed in Figs. 24 and

2.5.

It is appar-

ent that as the level of turbulence is increased, the fading

Fig. 19. Phase stxbility, high fading rate. period is decreased.


12/18

778 IEEE TRANSACTIONS ONOMMUNICATIONECHNOLOGY DECEMBER

5 -

4

3 -

1

1 -

o

I1

0

4 5

I

*I5

LI .

B

,i .LC . 3

35

2

(b)

Fig. 21. Dis tributions of fading brmdwidths.

(a)

Low-frequency

group

8

to 11 MHz, total number of 22 min runs: 25. (b) High-

frequency group

17

to 21 MHz, tot al number of 22 min runs:

33.

u l cR o vo tTs

INPUT sII;nAt

Fig. 22. Distr ibution

of

average input signal amplitude (mild con-

ditions).

MICROVOLTS INPUlIGNAL

Fig. 23. Distr ibution of average input signal ampli tude (severe

conditions).

J

FADING PERIOD SECONDS

2 0 30

4.0

5.0

FADINGERIOD SECONDS

Fig. 24. Distr ibution of t he signal fading period (mild condit ions)

iAOlNC

PERIOD

SECONOS

Fig. 25. Distr ibution of the signal fading period (severe conditions).

Figure 26 showsa distribution of fadedurations or

various hreshold crossings below a6-second verage.

We can see, for example, that if we were concerned about

fades of 20 dB below the 6-second average (which could

have somewherebetween

35

and

50

dB signal-to-noise

ratio), there will be a probabilityof approximately 10per-

cent that the fade ill last at least 300 ps It would appear

that about 22 bits at

a

75 bits per second signaling r ate

would be clobbered during this t ime. However, it mus t be

noted that thi s mould not be the case, since, during this

fade interval, in general, only the centr al bitswould have

been exposed to nstantaneous signal-to-noise ratios low

enough to cause the bit error rate to reach0.5. The other

bits in the intervalould have probabilit ies f error related

to heir nstantaneousbit signal-to-noise ratio.

As

an

estimate

of

what would have happened during this inter-

val, it is judged tha t itmould be unlikely for more than

3

bits out of th e 22 to be in error.

Thesemeasurementsweremadeover the Hawaii to

Deal,

N.

J., path, which isbasic,ally one

of

our better paths.


13/18

1966

GOLDBERG:

MF/HF CO M M U N ICA TIO N

SYSTEMS

779

Fig. 26. Distributions

of

the time duration

of

a fade.

Carrier frequency 20425 kHz

Tape 68 Average SNR 49 dB

Track

3

Averageignal0

p

Recorded February

Average fading rate

Time 1600 EST

23, 1963 0.126 Hz

Information such as this

is

basic to the ult imate design

of effective coding for H F ionospheric channel.

Over the years, it has been convenient for USAEL to

categorize the state of ionospheric turbulence dur ing tests

of communication equipment. Figure 27 shows this classi-

fication. Generally, conditionson a circuitare such that the

principal diagonal (left top to right bottom) receive th e

greatest number of data samples. As a qual itative classi-

fication the left column can beconsidered as representative

of mild, the middle column medium, and the rightcolumn

severe propagation conditions.

Figure 28 shows examplesof performance of an FSK

ys-

tem and a DPSK system under mild and severe propaga-

tion conditions. These tests were conducted using space

diversity reception with the FSK (AN/FGC-29) system

operating a t

1200

b/s and the

PSK

system (AN/FGC-54)

operating at

3000

b/s. Bothsystems were operated a t

equal power per system each using approximately a 3

kHz

portion of the

RF

spectrum.

10 aeo

Fade thresholds-

3

dB

0

l O d B

6 dB 20 d B - 0

Threshold lncreasmg

right to left

1-2

0-2

2

1-3

B-3

0 3

Fig.

27.

Propagationcategories.

The significant points to be made are that we see the

existence of bracketing regions of performance. Th e exist-

ence of asymptotes to performance of FSK and

PSI< sys-

temshas beenmeasured for some time now with the

implication th at finite ncreases in power would be

in-


14/18

780

IEEE TRA NSA CTIO NS ON COMMUNICATION TECHNOLOGY

DECEM BER

10

lo

10-

s

rn

n

lo-

10

\ , \

Fig. 28.

Comparison

ofFSK and PSK system performance.

effective in overcoming the loss in digital data reliability

due o he imevar ian t dispersive roperties of the

medium. I n this connection, Voelcker, i\/lasonson, and Bello

havemade significant contr ibut ions o he heory sup-

porting these observations.

During the lastew years th e underlying analytic mecha-

nism capable of accounting for th e measured performance

of FSK and PSI< systems havevolved with the esult tha t

useful and reliable predictions can now be made with re-

gard to system performance under dispersive channel con-

ditions.

Examples of such theoretical results for dispersive media

are seen in Figs. 29 and 30 where the bottoming effect is

quite evident.

It

should be noted that theseresults are for

nondiversity operation. An appropriatehift in scale would

be required t o utilize these curves for diversity reception;

the shape

of

the curves would not change.

It

must be

pointed out that theseesults are based upon consideration

of turbulence n th e channel hrough the fadingband-

width factor without considering the impact of multipath

propagation and its additional large contribution to the

irreducibleerror rate resulting rom th e generation

of

interchannel crosstalk and

oss

of signal set orthogonality.

I n Figs. 31 and

32 ,

we see examples of a lower bound of

performance due principally to atmospheric noise. Here we

see excellent agreementbetween the measured esults,

under mild and reasonably nonperturbed conditions where

atmospheric noise would be expected to limitperformance,

and the theoretical predictions for an

FSK

and a DPSK

digital da ta system under atmospheric oise conditions.

We now feel that withourpresentunderstanding of

ionospheric transmission tha t system performance can be

predicted quite closely once certain basic information

relating t o the turbulence of the ionospheric channel s

known.

It

appears that digital errors under high average SNR

are bounded at their lower error rate bound bytmospheric

(non-Gaussian) noise under nonperturbed conditions and


15/18

1966 GOLDBERG:

H F / M FO M M U N I C A T I O NY S T E M S

781

I

/\\

D P S K error-rate =

-

1 1

2 1 + M + m

2r2/FB2/BR2

in slow fading : - ~

2 1 + M '

F B = O '

at cross-over: ~

1

M

M

=

(BR2/20FB2)

in fading limited region: 10 FB2/BR 2,M =

FB

=

fading bandwidth,

RR =

bit rate

=

1/T

I =

mea n signal power-to-noise power.

Fig. 29. Theoretical PSK bit error rate vs. SNR and fading

band-width.

FSE; error-rate

= - -

-

1 1

2 2

1 1

1

in slow fading: { ( l + ) ( l + ~ ) , F B = 0

at their higher error rate bound, by an irreducible

error

rate dependent upon the time and Dopplerspread (fading

bandwidth).

In order to obtain a dat a base relating to ionospheric

channels tha t mould permit more precise estimates to be

made of system performance, USAEL has undertaken a

field test program with contractual help to measure and

then generate the following information in terms of the

diurnal variations on the measured values and the choice

of operating requency and ionospheric support mode:

1)

autocorrelation of phase angle

2) cross correlation between frequency spaced received

3 probability density of received signal

4) autocorrelation of received signal

5 ) probability density of signal envelope

6) autocorrelation of signal envelope

7) probability density of phase angle between different

8)

cross correlation between envelopes of diff erent tones

9 cross correlation between phases of different tones

10) cross correlationbetween envelope and phase of

11)

fading bandwidth

signals

tones

same tone

i n fading imited region: (FB/D)2, pprox ( M

=

m

Fig. 30. Theoretical

FSK

bit error rate vs. SNR and fading

band-width.

12)

coherent factor

13)

time spread

14) frequencyspread

15)

bit error rate.

We expect that his information willgo a long way

toward removing the need for speculation about the basic

behavior of the medium and permit substitution for this

speculation the quanti tative values obtained from meas-

urement.

The fundamentalpurpose of all this effort a nd hat

described earlier is related to specific needs of the military.

One use would be to permit the measurement of certain

critical parameters in real time so that predictions of sys-

tem performance in real time can e made without theeed

to actually examine the received digital data.

A second and more significant use would be to employ

the results to guide the development of optimum da ta


16/18

782

I E E ERANS ACTIO N S O N CO M M UNICATION TECH NO LO G YECEMBER

S Y S T E M

S N RG E N E R A L Y N A M I C S

T E S T

1-

Fig. 31. Comparison

of

theoretical an d measured FSK performance

in atmospheric noise.

terminals for the dispersive ionospheric medium. I n this

connection, the

USAEL

has alreadydeveloped, through i ts

supportingcontractors, wos,ystems designed tomatch

the changing data rate support

f

the perturbed ionospheric

channel.

These systems fall nto thecategory of what we call self-

automated adaptive comnmnication terminals responsive

to the data rate supportf the :medium.

One system already field tested, is known as the AN/

GSC-10. It

employs RAKE principles and reference track-

ing in addition to sophisticated processing known

t o

be

effective when designed specifically for time var iant dis-

persive ransmissionchannels. A much simplified block

diagram of the system is shownn Fig.

33.

Th e second approach to adaptive communication now

being fabricated, called ADAPTICOM, employs the means

for measuring the trans fer f uncti on of the pertu rbed me-

dium. This nformation is utilized at th e eceiving terminal

to create a matched filter to he medium and then operate

on the outputof the matched filter to reduce the side lobe

response of its essentially sinx/z output. This system in

simplified form is shown in Fig. 4.

Basically, the communication concept is o nterlace

probe signals with the data to be sent. The probe signal

sets up he receiving networlts

so

as

to

make the total

transfer function fromhe transmitter antenna through the

receiver terminal appear identical to that

of

a lossy linear

phase, constant time delay, nondispersive network. Once

the receiving networks are set data is transmitted serially

SYSTEM NR IGENERAL DYNAMICS TEST1 -

Fig.

32.

Comparison

of

theoretical and measured PSK performance

in atmospheric noise.

using shortbauds.Theentireoperation s completely

automaticwith channelprobe signals occurringoften

enough to follow the time var ian t ehavior of the channel.

The adaptive approach opens up a new concept in

HF

communications in that data under the proper conditions

may possibly be sent over a 3 kHz channel at the rateof

4800 to 9600 b/s whereas before, serious problems de-

veloped when we a.ttempted to end 2400 b/ s at an ccept-

able error rate level.

The use of shor t bauds in ransmission over a dispersive

HF ionospheric medium represents a major epartu re from

the heretofore accepted practice. In fa ct , ull exploitation

of th is concept requires basic da ta about the transmission

medium in terms of shor t baud transmission which is, at

this time, very scarce. We expect t o be adding to the data

base in this area lso in the near future.

The full ramfications of the adaptive approach to om-

munications have many useful ide effects.For example, in

theADAPTICOMapproach he existence of multiple

paths of propagation s actually employed as sources of

diversity input .which are processed so as to provide co-

herent gain in the equipment. I n this way, it appears that

a more optimum approach to a choice of operating fre-

quency

is

away from the RIUF toward the ordinarily un-

desired, henceunused byother communicators, par t of

the spectrum. Two advantagescould accrue from his fact,

one is th at th e available spectrum for communications is

broadened and, two, there would be less mutual nter-

ference.


17/18

1966

GOLDBERG: M F / H F C O M M U N I C A T I O N

SYSTEMS

V

783

(b

~ Fig. 33. Simplified block diagram AN/GSC-10 system. (a) Transmit terminal. (b) Receive terminal.

r - - - - - -

t

- - ,

Fig.

34.

Simplified block diagram AN FY C- 5 system. (a) ADAPTICOM ransmit terminal. (b) ADAPTICOM

receive terminal.


18/18

IEEE

T I t A N S h C ‘ l T O N SO N C O M M U N I C P . T I O N T E C H N O L O G Y VOL.

C O M - ~ ~ ,O . 6 DECEMBER 1966

CONCLUSION

At the present t ime we ha1.e a good handle on the con-

trol of the dispersive properties of the H F medium. Th e

possibility exists th at he use of adaptivesystems will

permiterror ates obe esponsive toand mprovable

upon by increases in received signal-to-noise ratio without

having

to

cope with

a

high ir.reducible error rate.

It

is ex-

pected thak th e remaining PI-oblem of atmospheric noise

will be ov’ercome by means of effective coding. We feel

that in the near future

HI?

ionospheric ransmission of

digital da ta will atta in a level

of

reliability a few orders of

magnitude beyond present capabilities reaching

a

state of

performance thought impossible just a few years ago.

ACKNOWLEDGMENT

The author would like to a,cknowledge he fine support

given to the

USAEL

general program in HF communica-

tion research by GeneralDynamics, nc. Stromberg-

Carlson Division), RCA Inc.,Defense Electronic Products,

Adcom Inc., and Communicstions Systems Inc. He also

acknowledges thecontributionsand assistance of

L. B.

Shucavage and J. Korte, both of USAEL, in the conduct

of the many programs thatave rise to the da ta resented.

REFER.ENCES

[ I ] “Radio propagation,” Department of the Army, Rept. TRI 11-

[2] F.

E.

Termon,

Radio ngineeringHandbook.

New York:

499, 1950.

McGraw-Hill, 1943.

[3] “Ionospheric radio propagation,” Nat’ l Bur . Std. Circular 462,

[4] K. G. Budden,

R a d i oWaves n theZonosphere.

Cambridge,

[5] S. K. Mitra,

TheUpperAtmosphere .

Calcutta,India: Asiatic

[6] ‘:Basic radio propagation predictions,” Nat’l

B u r .

Sld. CRPL

[7] “Reference da ta for radio engineers,”

ITT,

1956.

[8] Nat’l Bur. Std. RadioPropagation Course Notes, 1961-1962.

[9] B. Goldberg, “HF radio data transmission,” I R E T ra ns .

on

Communications Systems,

vol. CS-9, pp. 21-28, March 1961.

[lo] “Evaluation of a new high frequency adiocommunication

equipment,” General Dynamics Corp., Fina l ltep t.2 for task

1

Rept. AS 272 565, 1961.

[11] ‘Wtudy of fine grain fading and phase stabil ity of multiple CW

signals,” General Dynamics Corp., Rept.

3

for task 3, Rept.

AD 406 213, 1962,

[12] “Study of fine gram fading and phase stabili ty of multiple CW

signals,” General Dynamics Corp., Rept. 4 for task

3,

Contract

[13] J. Kort e and C. Jackson, “Evaluation of high frequency com-

municationsequipment using frequency stabilized receiver,”

UASELRDL, Test Rept. 1544 June 1963.

[14] J. F. Korte and J.

S

Koch, “Measurement of the phase per-

turbations of a CW slgnal over

a

long haul

H.

F. circuit and its

signal,” (Addendum to 1111 and 1121) USAEL Rept., April

1

comparison with nalyt ical resultsor

a

Rayleighading

1964.

[15] “Ionospheric transmission models, task 5 correlation between

transmission parameters of dispersive circuits and ystem

performance forpplication todaptive communications

systems,” RCA Defense Electronic Products,ContractDA

[IS] B. Goldberg, L. B. Shucavage, and

J.

Korte, “Fine grain iono-

spheric behavior,”

Globecom VZ Sympos ium Diges t ,

Philadel-

[17] ‘Character izat ion of radio channels,” Adcom Inc., Inte rim

phia, Pa., Ju ne 2-4, 1964.

Rept. Contract AD 28-043 AMC-00038 (E), 1964..

[18]

"Analytical

and experimental study of correlatlon function

over HF circuits,”Communicationsystems Inc.,inal

Rept. , Contract DA 2S-043-AMC-O0145(E), 1965.

1943.

England: Cambridge University Press, 1941.

Society.

Series D.

DA 36-039 SC-88943, 1963.

36-039 SC-87240.

Optimum 13inary

FSK

for Transmitted Reference

Systems Over Rayleigh Fading Channels

Absfracf-It is well known tha t in communicating over randomly

time-varying channels, a receiverwhich performs a channelmeasure-

men t can make a better decision than one that does not. Furthe r-

more, if th e channel characteristics vary relatively slowly in com-

parison to a arge number of adjacent message intervals,

a

small

portion of the transmittt er energy can be devoted to channel meas-

urement, and, in spiteof the loss of energy in the informat ion bear-

ing portion of the s ignal, the resulting sys tem performs bette r than

one with no measurement. This p,aper shows that improved system

performance from a channel measuring system occurs, even when

the channel characteristics are fixed only during the presentmessage

interval.

Th e randomly time-varying cha.nne1 stu died is that of a Rayleigh

fading medium with independently fading mark and space channels

whose fading i s fixed over one haud interval but is indepen dent

Manuscript received June 4,19fi5.

was formerly with the Air Force Cambridge Research Laboratories,

The author is with the University of California, Im ine, ‘Calif. He

Bedford, Mass.

from baud o baud. The transmission system s a modified fre-

quency shift keying (FSK) ystem such tha t during a portion of a

baud nterval, the m a r k and space frequencies are always trans-

mitted

so

as to a ct as reference signals. For this system, the follow-

ing has been established:

1

optimum receiver configuration

2 ) optimum ratio 01 of information energy to total signal energy

3 asymptotic optimum 01 for an M-diversity channel

4 ) error probabilities for item 2 and asymptotic error probabilities

for item 3 for oopt s a function of total SNR. The asymptotic results

show tha t by using reference echniques the order of diversity is

effectively doubled.

as

a

function of total available

SNR

for a single fading channel.

I

INTRODUCTION

NFORMATIONransmission verandondyime-

varying channels has been studied by many authors

[1]-[3].

Kailath has studied the Gaussian, randomlyime-

784

Documents

300 KHz-30 MHz MF_HF(Goldberg1966)