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

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    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

    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).

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    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

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    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

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    1966 GOLDBERG:

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    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

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    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.

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    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.

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    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.

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    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-

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    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

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    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

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    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.

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    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.

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    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