u.o.c. 621.391.812: 621.394.5
`Indexing Terms : Data transmission, Radio transmission, Fading
`
`High data· rate
`transmissions over
`h.f. links
`
`N. M. MASLIN, M.A., Ph.D., M.lnst.P .•
`C.Eng., M.I.E.R.E. *
`
`SUMMARY
`A review is presented of the problems inherent in
`transmitting data over h.f. links. The propagation medium
`imposes characteristics of time and frequency dispersion,
`fading and delay distortion upon the transmitted signal,
`particularly when wide bandwidths are used. The
`magnitude and variability of these features are quantified
`and a simplified expression for the received signal is
`derived. Techniques that have been used to transmit high
`data rates over h.f. links are summarized and their relative
`merits compared. It is concluded that the ionosphere
`continues to be a limiting factor in the design of an
`efficient modem, but that recent developments in
`microelectronics provide the potential to make a
`significant improvement in the performance of future
`communication systems.
`
`• Software Sciences Ltd .• Defence Division. 57- 61
`High Street, Frimley. Camberley, Surrey GU16 5HJ
`
`The Radio and Electronic Engineer. Vol. 52. No. 2. pp. 75-87. February 1982
`
`1 Introduction
`The high frequency (2- 30 MHz) portion of the spectrum
`has long been recognized as a useful and economic
`medium for achieving wide distribution of energy over
`large distances. Although satellite communication
`systems are becoming more widely available, h.f. will
`continue to be extensively used by many nations for
`point-to-point
`transfer of
`information
`and
`for
`commercial shipping and aircraft communications;
`military forces rely heavily upon h.f. for land , sea and air
`operations. The major constraint for a satellite link is to
`maintain an adequate signal-to-noise ratio for the high
`data rates that are required. In contrast, constraints on
`h.f. links centre around the dispersive characteristics of
`the transmission medium and in the high levels of
`interference that may be encountered.
`The design of h.f. systems depends upon accurate
`predictions and new technology to improve circuit
`reliability. System planners should know what frequency
`ranges must be covered, what transmitter powers are
`necessary to overcome the background noise at the
`receiver and what antenna configurations are most
`suited to the applications required.
`The evaluation of h.f. link reliability has been detailed
`in a previous paper ;1 an air-to-ground link was chosen
`as an example to illustrate the concepts involved and the
`problems with which
`the communications system
`designer must cope. The resultant effects caused by time
`and frequency dispersion were not specifically addressed,
`however, and the conclusions reached assumed that such
`effects are not major. The present paper considers these
`dispe~sion effects in detail, and analyses the limitations
`they impose upon high data rate transmissions of an h.f.
`signal.
`Section 2 summarizes the operational constraints of
`using h.f. data transmissions for point-to-point and
`mobile applications. The properties of the propagation
`medium are analysed
`in Section 3; each major
`characteristic is discussed in turn , its physical cause
`identified and the magnitude of its resultant effect
`quantified . Section 4 considers some techniques that
`have been used to transmit data at high rates over 3 kHz
`voice bandwidth channels and compares their relative
`merits. Sections 5 and 6 extend the study to consider the
`use of wider bandwidths, the problems that may arise
`and some techniques that might be employed. Finally, in
`Section 7, some future trends are briefly examined and
`their possible impact upon communications systems are
`considered.
`2 Operational Constraints
`2.1 Point-to-paint Links
`The h.f. spectrum is used extensively for long-range
`point-to-point communications and broadcasting ; the
`characteristics of such links have therefore been widely
`studied. Commercial services are available 2·3
`for
`predicting the optimum working frequencies and quality
`of communications at those frequencies. Most point-to(cid:173)
`point land fixed b.f. communication circui ts use high(cid:173)
`gain rhombic or log periodic antennas, whilst arrays of
`horizontal dipoles, also with significant directivity, are
`popular for broadcasting using the sky wave.
`0033-7722/ 82/ 020075 + 13 $1 .50/ 0
`© 1982 Institution of Electronic and Radio Engineers
`
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`

`

`N. M . M ASLIN
`
`Many circuits employing data transmissions have a
`large frequency complement. This allocation can be used
`to advantage
`to choose frequencies close
`to
`the
`maximum usable frequency (m.u.f.) and thus ensure that
`differential delays between propagation modes are small
`enough to provide frequency flat fading over a 3 kHz
`channel. Post-detection diversity combining can be
`employed to combat such fading by using spaced
`receiving
`antennas
`and multiple
`transmission
`frequencies. The digital errors that remain are then
`caused predominantly by either wideband impulsive
`noise or man-made interference;
`the time varying
`dispersive effects of the channel are of secondary
`importance. In principle, therefore, the performance of
`these point-to-point links may be optimized by good
`engineering design and practice in respect of the
`equipment and antenna systems, whilst high transmitter
`power is often available.
`
`2.2 Mobile Applications
`Much more difficult problems are presented by h.f.
`communication to mobiles. Communication is often
`required at ranges from a few kilometres to hundreds or
`even thousands of kilometres over a wide variety of
`terrain, and this implies different modes of propagation
`according to range. Physical constraints are placed upon
`antennas
`that are used
`for manpack or vehicle
`application so
`that efficiencies may be seriously
`degraded; radiation patterns are obtained that may not
`be suited to the propagation mode, whilst transmitter
`power is often severely limited.
`When transmitting data to and from these mobiles, it
`is neither easy nor always possible to use frequencies
`close to the m.u.f. as in the case of point-to-point links
`with large frequency complements; time-varying channel
`dispersive effects can
`then become of primary
`importance. At the frequencies available to the mobiles,
`the resulting differential delays between propagation
`modes may be sufficient
`to produce narrowband
`frequency selective fading within a 3 kHz channel. To
`achieve satisfactory results over an h.f. link of this kind ,
`careful consideration must be given to the h.f. channel
`characteristics, the terminal radio equipment (including
`modulation techniques and error coding), the planning
`of operational
`links and
`the managemen t of the
`frequencies to be used over those links.
`
`2.3 High Data Rate Requirements
`Further complications arise when high data rate
`transmissions are required. For example, digital voice
`requirements
`imply data
`rate
`transmissions of
`2·4 kbit s - •. Higher data rates would give better quality
`from the speech synthesis aspect, but channel bandwidth
`considerations show that approximately 2·4 kbit s- 1 is
`the highest rate that can be tolerated in a 3 kHz channel.
`Military radio links may need to incorporate a high
`degree of immunity to electronic counter measures ;
`complex modulation schemes
`involving
`frequency
`hopping and spread spectrum must
`therefore be
`adopted. This, in turn, necessitates a detailed study of the
`propagation medium to determine whether various
`forms of wide bandwidth modulation techniques can be
`
`76
`
`Table l
`General h.f. channel characteristics
`
`Propagation
`mechanism
`
`Channel
`characteristics
`
`Relevant
`parameters
`
`Ground wave
`
`attenuation
`
`delay
`
`{
`
`Single mode
`sky wave
`
`Multimode
`sk y wave
`
`~:;:~uation
`
`fading
`delay distortion
`Doppler shifl
`Doppler spread
`
`(
`
`f time dispersion
`l interference
`
`fading
`
`soil conduc1ivity
`terrain type
`range
`wave polarization
`wave frequency
`
`time of day
`sunspot activity
`season of year
`range
`wave polarization
`wave frequency
`
`different modes
`different hops
`high/low angle rays
`magnetoionic splitting
`relative allenuation
`relative delay
`
`transmitted with fidelity. The problems inherent in the
`design of modems to achieve satisfactory transmissions
`at 2·4 kbit s- 1 oveP h.f. channels have not yet been
`adequately solved; it has been the ionosphere which has
`proved to be a limiting factor in the design of an efficient
`modem.
`
`3 Characteristics of t he Pro pagat ion Medium
`Good network and frequency management are vital,
`particularly for the successful performance of a mobile
`radio system. The m.u.f. increases with range and, if a
`choice of receiving stations is available, it may be
`advantageous4 to work to the more remote station so
`that higher working frequencies can be used and thus
`better antenna efficiencies achieved. The requirement for
`good frequency management of h.f. links is implicit
`throughout this paper.
`Groundwave communication is more straightforward
`than skywave ; it can be assumed that the groundwave is
`merely an attenuated, delayed but otherwise undistorted
`version of the transmitted signal. Ionospheric skywave
`returns, however , in addition to experiencing a much
`greater variability of attenuation and delay, also suffer
`from fading, frequency or Doppler shifting and spreading,
`time dispersion and delay distortion. These features are
`summarized in Table I and will be discussed in detail.
`
`3.1 The Received Signal
`Consider a complex transmitted baseband signal, E(1 ),
`traversing a single propagation path
`through
`the
`ionosphere. Let it experience a delay r. The medium is
`dispersive and thus the signal is subject to delay
`distortion, caused by the fact that the delay is a function
`of frequency. This distorted waveform is denoted by E(1).
`In addition, the signal experiences attenuation and
`random fading. This can be represented by multiplying
`the delayed, distorted signal by a
`random gain
`G(A , v, a, t) where A characterizes
`the attenuation
`(0 ~ IAI ~ I) and v, a represent the fading in terms of a
`
`The Radio and Electronic Engineer, Vol. 52. No. 2
`
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`

`

`HIGH DATA RATE TRANSMISSIONS OVER H .F. LINKS
`
`( a) GROUNOWAVC/S KYWAV[
`
`( b ) UlfFERE HT l AYE kS
`
`(c) OIFF(RENI HOPS
`
`( d) HIGH/LOW ANGLE
`
`(e) ~AGNEIOIONIC SPLI IIING
`
`Fig. I. Causes of multipath propagation.
`
`frequency shift and spread respectively. The received
`signal ER(t) thus becomes
`ER(t) = G(A, v, a, t) £(1 - t).
`(1)
`Components of this signal may be returned from both
`the E region and F regions of the ionosphere (the latter
`may include both high and low angle ray paths). There
`are skywave returns for the ordinary and extraordinary
`magneto-ionic components and for multiple hop paths
`(see Fig. I). Although many propagation modes are
`possible, all but a few experience a large attenuation; the
`number of 'effective' modes is generally small.
`Each mode has a different value of the characteristics
`of equation (1 ). For the jth mode, the received signal is
`ERit) = GiAj, vj, aj, t) Ej(t-t)
`Consider now the groundwave. This can be assumed
`to experience a delay r8
`, and a non-random gain , but no
`) and G
`distortion. Thus, E(t - r) becomes E(t - r
`8
`becomes A 8 (0 ~ IA 81 ~ 1 ). The total received signal is
`then
`£R(t)=A 8£(t - t 8)+ L Gi(Aj, vj,<1j,t)((t-t)
`
`N
`
`j = I
`
`(2)
`
`(3)
`
`where N represents the number o( 'effective' skywave
`modes. Expressions for these terms are derived in the
`Appendix. It is not necessary, however, to delve into the
`detailed mathematics to consider the effects imposed
`upon the received signal by the characteristics of the
`propagation medium. The relevant phenomena are
`summarized in Table 2. Each is now discussed in more
`detail in terms of its cause, magnitude, variability and
`resultant effect.
`
`3.2 Multipath Propagation and Time Dispersion
`Multipath characteristics can be described by the
`dispersion produced in the unit impulse response of the
`medium. Time dispersion can result from one or more of
`the following (see Fig. 1):
`(a) Groundwave and skywave paths,
`(b) Skywave returns from different ionospheric layers,
`(c) Skywave returns involving different numbers of
`'hops',
`(d) High and low angle skywave paths,
`(e) Splitting of
`the magneto-ionic components,
`ordinary and extraordinary, resulting from the
`effects of the Earth's magnetic field.
`
`February T 982
`
`77
`
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`

`

`N. M. MASLIN
`
`Table 2
`Causes of distortion on an h.f. channel
`(Parameters referenced are those used in equation (3))
`
`Effect
`
`Cause
`Withfo a single mode
`Between different modes
`
`Time
`spread of t 1• due 10
`dispersion slightly different
`constituent raypaths
`
`t 1 different for each
`propagation mode
`
`Fading
`
`different time dependence
`G1 a function of time
`- movement in ionosphere of each G;
`-
`polarization variations
`-
`absorption changes
`
`Frequency v1• u1 are non-zero
`dispersion
`-
`phase path is time
`dependent
`
`Delay
`distortion
`
`t; a function of
`frequency and/or time
`
`relative phases between
`modes changes with time
`
`t 1 may have a different
`frequency /time dependence
`for each mode
`
`its own
`Each propagation path or mode has
`characteristic group delay -r1. The time dispersion of the
`medium is caused by the difference in group delays
`between
`the different modes ; it can give rise
`to
`intersymbol
`interference when
`the signalling
`rate
`becomes comparable with the relative multipath delays.
`The maximum serial data transmission rate is thus
`limited to the reciprocal of the range of multipath
`
`propagation times. This is itself a function of frequency,
`path length, geographical location , local time, season
`and sunspot activity. The data rate can be maximized5
`by working close to the m.u.f., although this is extremely
`difficult to achieve in practice for mobile applications. As
`the operating frequency is decreased from the m.u.f., a
`frequency is reached at which the spread is a maximum.
`For a 2500 km path, the maximum time dispersion has
`been shown5 to be about 3 ms; for 1000 km it increases
`to 5 ms and for 200 km it is about 8 ms.
`Under some conditions, the transmission rate could be
`increased by a factor of 100 over the normal values 5 by
`judicious choice of operating frequency. In practice,
`however, the upper limit is approximately 200 bit s- 1
`when conventional detection equipment is used. Even
`within a single mode of propagation, there remains an
`approximately 100 µs spread due to the slightly different
`constituent ray trajectories caused by roughness o f the
`ionospheric layers and non-zero antenna beamwidths.
`Under anomalous conditions, such as spread F, when
`the ionosphere contains many irregularities, the time
`dispersion can be much greater.
`There are several important effects which multipath
`imposes upon a given communications technique and its
`associated equipment when transmitting high speed
`digital h.f. data:
`(a) The equipment is more complex, with special
`modems, diversity combining etc. For example, in
`phase shift keyed (p.s.k.) systems, abrupt phase
`changes occur as successive modes reach the
`
`Table 3
`Summary of fading characteristics
`
`Type
`
`Cause
`
`Flutter
`
`small scale irregu(cid:173)
`larities in F region
`
`Fading
`period
`
`Correlation
`bandwidth
`
`10--100 ms
`
`I kHz
`
`Diffraction
`
`movement of irregu(cid:173)
`larities in ionosphere
`
`10- 20 s
`
`50 kHz
`
`Remarks
`
`associated with
`spread F
`
`follows a Rayleigh
`distribution
`
`Polarization
`
`rotation of axes of
`polarization ellipse
`
`10- 100 s
`
`Skip
`
`time variation of m.u.f.
`
`Focusing
`
`curvature of reflecting
`layer
`
`Absorption
`
`time variation of
`ionospheric absorption
`
`Groundwave
`-skywave
`
`comparable strengths
`Skywave
`of different
`modes
`High and low propagation
`angle rays
`modes
`Magnetoionic
`splitting
`
`gene rally
`non-periodic
`15- 30 mins
`
`I hour
`
`2- 10 s
`
`1- 5 s
`! 2 s
`
`10--40 s
`
`25 kHz (night) only effective when
`400 kHz (day) both magnetoionic
`components present
`in approx. equal
`proportions
`
`avoided by working
`well below m.u.f.
`
`greatest at sunset
`and sunrise
`
`generally more
`severe than fo r
`skywaves alone
`
`JOO Hz
`-3 kHz
`
`78
`
`The Radio and Electronic Engineer, Vol. 52. No. 2
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`

`

`HIGH DATA RATE TRANSMISSIONS OVER H.F. LINKS
`
`OAWH
`
`Kl ODAY
`
`100
`
`80
`
`60
`
`40
`
`20
`
`[V( Nl Nu
`
`100
`
`80
`
`60
`
`40
`
`20
`
`5
`
`10
`
`15
`
`20
`
`2~
`
`30
`
`35
`
`5
`
`10
`
`l ~
`
`20
`
`25
`
`30
`
`35
`
`5
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`OAW N
`
`fAO{ OEPIH I N d8
`
`Kl ODAY
`
`100
`
`80
`
`60
`
`40
`
`20
`
`EVE NING
`
`100
`
`80
`
`60
`
`40
`
`20
`
`100
`
`80
`
`60
`
`40
`
`20
`
`100
`
`80
`
`60
`
`40
`
`20
`
`~ "' ..
`
`w z ..
`
`0-5 5- 10 10- 50 50-100 >100
`
`0-5
`
`5-10 10- 50 50- 100 >100
`
`0-5
`
`5-10 10-50 50- 100 >100
`
`f .\O[S PER HJ NUIE
`
`Fig. 2. Typical fade rates and depths recei ved by a monopole.
`
`receiver, necessitating the provision of a guard
`interval at the end of each signalling period. In
`bandlimited systems using multitone signalling,
`this reduces the number of tones available, since
`greater frequency separation is required.
`(b) The error rate is degraded, as a result of
`intersymbol interference, and high error rates may
`occur even at high signal-to-noise ratios.
`(c) The choice of operating frequency is limited to a
`small frequency band below the m.u.f. Working at
`frequencies too far below the m.u.f. increases the
`likelihood of encountering
`larger multipath
`delays.
`
`3.3 Fading
`Skywave signals characteristically fluctuate in amplitude
`and phase. No matter how irregular the ionosphere, the
`amplitude of the signal at a fixed receiver would remain
`steady if it were a static medium . The width of the
`received power spectrum (i .e. the fading rate) is then
`related to changes in the ionosphere.
`There are a number of different kinds of fading,
`defined according to their origin ; the main causes are
`movements and changes of curvature of the ionospheric
`reflector, rotation of the axes of the received polarization
`ellipse, time variations of absorption and changes in
`electron density. In addition to these effects which may
`be produced
`independently for each mode, more
`significant fading may be caused by interference between
`two or more modes, particularly when they are roughly
`of equal amplitude. The different types of fading, with
`their typical fading rates are summarized in Table 3.
`Figure 2 presents some average fading rates for a
`typical h.f. channel at mid-latitudes.6 It is clear that,
`
`February 1982
`
`particularly for the dawn and evening periods, the 10- 50
`fades per minute grouping is by far the most common.
`This is caused by interference between different skywave
`modes. For midday, the results are spread rather more
`evenly from O to 50 fades per minute, but again higher
`rates of fading are infrequent. Also shown in Fig. 2 is
`fade depth ; fades of less
`than 10 dB occur most
`frequently.
`For a two-path channel with relative delay d seconds,
`troughs
`in
`the amplitude-frequency
`response are
`separated by 1/d Hz and give rise to frequency selective
`fading ; signals with bandwidths greater than 1/d Hz are
`thus required for in band frequency diversity. The 1/d Hz
`bandwidth is known as the correlation bandwidth and is
`given for different types of fading in Table 3. As the
`distance between
`two closely spaced
`receivers
`is
`increased , the correlation coefficient between
`their
`respective received signals decreases. The distance at
`which the coefficient drops to 1/e is called the correlation
`distance ; it is of the order of a few wavelengths for
`skywave reception (i.e. ;;:: 100 m at h.f.).
`
`3.4 Frequency Dispersion
`For any given single propagation path, a shift vi in
`frequency can be caused by time variation of
`(a) height of the reflecting layer
`(b) electron density (and hence refractive index) along
`the path.
`Thus, if VI is the phase angle of a ray path at time t , then
`f dVJ
`v.= - - -
`1
`C dt
`for a fixed transmitter and receiver.
`The frequency (or Doppler) shifts experienced at niglit
`
`(4)
`
`79
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`

`N. M . MASLIN
`
`are small compared to daytime effects,7 whilst relatively
`large positive values occur at sunrise and large negative
`values at sunset. On 'quiet' days, values range8 from
`0·01 - 1 Hz. Shifts tend to be considerably less for E
`modes than for F modes, and slightly less for oblique
`than vertical incidence.
`Evidence from over-the-horizon radar9 shows that
`'quiet' conditions usually prevail, since resolution down
`to O· I Hz is often possible. When the ionosphere is
`disturbed, however, such as occurs during conditions
`giving rise to spread F, there is typically a continuum of
`shifts of sometimes 5- 10 Hz. During strong solar flares,
`deviations ofup to 50 Hz have been measured 7 but these
`are only caused for a matter of minutes and are most
`unusual. Typical shifts caused by flares are 1- 2 Hz.
`Now since each mode of propagation is composed of a
`number of rays which traverse slightly different ray
`trajectories, each ray path has a slightly different
`frequency shift. This results in a spread of received
`(see Appendix).
`frequencies, characterized by 2ai
`Measurements10
`have
`not,
`unfortunately,
`been
`concerned with the shift and spread of individual modes ;
`they have recorded composite Doppler values involving
`many modes. It has been estimated 8 that, under quiet
`conditions, spreads of 0·02 Hz would be applicable to E
`modes and 0·15 Hz to F modes.
`Continuous D oppler spread spuriously modulates
`each transmitted pulse and therefore contributes to the
`fading of the received pulses. The fading period has been
`found, 1 1 however, to be much greater than typical pulse
`durations.
`3.5 Delay Distortion
`Delay distortion occurs because the group delay is a
`function of frequency and is consequently not constant
`across a signal bandwidth. For a given ionospheric path,
`the oblique
`ionogram gives
`the
`frequency-time
`dispersion characteristics. T wo examples are shown in
`F ig. 3. The dispersion caused by the E layer is very small;
`it has been estimated 12 that the rate of change of group
`delay with frequency is typically 5 x 10- 6 µs Hz- 1• The
`F layer, particularly near its m.u.f. can give much more
`rapid changes of group delay with frequency, as evident
`from Fig. 3.
`
`( . )
`
`: [
`
`~
`
`a
`
`~
`
`:s
`~
`
`430 ko p>th
`
`Jr __.,.,
`
`2F
`
`_ )
`
`5
`
`6
`
`5
`
`6
`rR(QUfNCY ( MH,)
`
`lf ":::'::>
`
`1 E
`' ' ' '
`1 8 9 10 11 12
`
`(b)
`1365 k• path
`Fig. 3. Ionograms showing dispersion characteristics.
`
`fREOUC NCY (MHz)
`
`80
`
`for data
`importance of delay distortion
`T he
`transmission is concerned with the rate of change of
`delay with frequency and time. Ionospheric channels are
`non-stationary in both frequency and time, but if
`consideration is restricted to band-limited channels
`(say IO kHz) and sufficiently short times (say JO minutes)
`most channels are nearly stationary and can be
`adequately represented by a stationary model. 8 This
`means that, since propagation is limited to a discrete
`number of modes, the channel can be modelled by a
`delay line with a discrete number of taps, each of which
`are modulated in phase and amplitude by a time varying
`quantity.
`Correlation measurements 13 over an h.f. link have
`shown that:
`(a) the probability distribution of in-phase and
`quadrature components of
`the
`transmitted
`envelope followed a Rayleigh distribution with
`good fit,
`(b) approximately l · I seconds is necessary for the
`time correlation coefficient to reach 0·5.
`
`4 Techniques for 3 kHz Bandwidth Applications
`4.1 Non-adaptive Systems
`T he signalling rate must be kept well below the
`reciprocal of the multipath delay to avoid the effects of
`intersymbol interference. A higher data rate can be
`effected by multiplexing the data stream with a number
`of different tone frequencies.
`interference upon a
`The effect of narrowband
`multi tone signal is to produce a high error rate primarily
`on tones adjacent to the interfering signal. Under
`multipath conditions, different modes may interfere with
`each other, reducing the overall signal power and
`disrupting
`phase
`relationships
`between
`tones.
`Differential detection is therefore preferable to absolute
`phase modulation; a more up-to-date comparison can be
`obtained if it is made between adjacent tones. This is the
`basis of non-adaptive d.p.s.k. systems.
`Kineplex 14 is a four-phase multi tone time d.p.s.k.
`system in which the information is contained in the
`sequential bits
`relative phase difference between
`transmitted on the same tone. T he phase of the received
`bit is stored, and compared with the phase of the next bit
`and so on. Variations in the channel response which are
`slow compared with the bit duration give similar
`distortion to bits adjacent in time, so that the relative
`phase difference
`is approximately unchanged. One
`K ineplex version uses 20 tones, with each tone composed
`of two independent sub-channels. T ransmission occurs
`with 75 baud per sub-channel, providing a capacity of
`3000 bits - •. A tone spacing of 100 Hz gives a bandwidth
`occupancy of 2 k Hz for the transmission system.
`In frequency d.p.s.k. systems, the information is coded
`as the phase difference between two tones transmitted
`simultaneously on different frequencies. Differential
`comparison
`is
`thus performed under multipath
`conditions which are closely correlated. A better
`performance might be expected than is achieved for time
`d.p.s.k. systems at the expense of additional equipment
`complexity.
`
`The Radio and Electtonic Engineer. Vol. 52. No. 2
`
`HTC Corp., HTC America, Inc. - Ex. 1019, Page 6
`IPR2018-01555 and IPR2018-01581 (HTC and Apple v. INVT SPE)
`
`

`

`HIGH DATA RATE T RANSMISSIONS OV ER H.F. LINKS
`
`Andeft 27 is a four-phase frequency d.p.s.k. system
`which uses 40 Hz tone separation, with 66
`tones
`transmitted simultaneously, two of them being used for
`synchronization and as starting reference frequencies.
`Each tone is compared with the adjacent tone to detect
`the
`relative phase. Whole
`frames are detected
`simultaneously using 64 sets of correlators, modulators
`and phase detectors.
`
`4.2 Adaptive Systems
`Adaptive systems involve cascading the channel with
`some form of correlator at the receiver in order to
`·equalize' effects produced by the channel. The transfer
`function can be described in terms of either:
`(a) an impulse response in the time domain, or
`(b) a frequency response in the frequency domain.
`There are two essentially different strategies for (a).
`One approach attempts
`to combine all multipath
`components, whilst the other cross-correlates a single
`mode in an effort to reject all others.
`The approach for (b) uses a filter which produces a flat
`frequency response and a linear phase response (i.e. a
`filter with a frequency transfer function which is the
`reciprocal complex conjugate of the channel).
`For either (a) or (b), the equalization networks
`gradually become aged because of the time-varying
`nature of the channel. Thus, any adaptive system
`operating over an h.f. channel must be able to:
`(i) measure the channel parameters (which may
`involve transmitting a probe signal known to the
`receiver),
`the receiver
`(ii) set matched filter elements at
`according to the measured parameters,
`(iii) repeat (i) and (ii) sufficiently often to follow
`variations of the channel.
`The Adapticom system 1 5 applies
`time domain
`adaptation to the reception of a 2660 baud serial
`transmission for which the bit duration (about 0-4 ms) is
`considerably less than the multipath time spread; the
`spectrum of each bit occupies
`the whole voice
`
`bandwidth. The matched filter operates on the baseband
`and its response is determined by tap amplifier gains.
`An isolated probe pulse transmitted is received as a
`sequence of multipath signals. Instantaneous values of the
`composite received signal are sampled at the delay line
`and stored as tap amplifier gains. The matched filter
`equalizes the delays, squares all amplitudes of multipath
`spectral components and adds them coherently, thus
`compressing each signal into a duration comparable to
`that of the transmitted signal. The matched filter
`characteristics
`in
`the
`receiver must be updated
`frequently to follow changes in the channel transfer
`function.
`The Kathryn system16 applies frequency domain
`adaptation to the reception of a p.s.k. multi tone signal.
`An overall data rate of 2550 bit s - 1 is achieved using 34
`orthogonal tones transmitted in 75 baud frames, each
`frame carrying information a nd probe signals in phase
`quadrature. A l ms guard time is used to attempt to
`maintain orthogonality between tones under multipath
`conditions. The receiving terminal uses two receivers for
`diversity operation with optimum combining. A pseudo(cid:173)
`that at
`the
`random probe sequence,
`identical
`to
`transmitter, is generated in synchronism at the receiver.
`The measurement section of the receiver examines each
`received tone and determines the amplitude and phase
`distortion of the probe component. The latter is then
`removed from each tone and the remaining information
`component corrected in phase and modified in amplitude
`by
`the weighting section. The measurement and
`information weighting sections of the receiver form the
`adaptive matched
`filter. Continuous updating
`is
`provided using the probe sequence. However, the system
`possesses some
`inertia so that
`the receiver phase
`reference can never precisely compensate for the current
`state of the channel.
`
`4.3 Comparison of Adaptive and Non-adaptive Systems
`A detailed comparison of adaptive and d.p.s.k. systems
`for high data rate transmissions was presented1 7 some
`
`Table 4
`Summary of system characteristics
`
`System
`Name
`
`Processing Data rate
`domain
`(bits/sec)
`
`Modulation
`
`Adaptive
`processing
`
`Guard
`time
`
`Learning
`time
`
`Adapticom
`
`time
`
`26(i()
`
`d.p.s.k.
`
`matched filter
`
`Kathryn
`
`frequency
`
`2550
`(75 baud
`frames)
`
`p.s.k.
`(34 orthogonal
`tones)
`
`local
`
`diversity-
`reference
`co rrelation
`
`1·14 ms
`
`20 ms
`
`26 ms
`54 ms
`
`Kineplex
`
`time
`
`3000
`
`Andeft
`
`frequency
`
`4800
`
`d.p.s.k.
`4 phase
`(20 tones)
`
`d.p.s.k.
`4 phase
`(66 tones)
`
`Codem
`
`time
`
`d.p.s.k.
`2400
`(3750 coded) 4 phase
`(25 tones)
`
`relative phase
`comparison on
`same tone
`
`relative phase
`between two
`frequencies
`
`coding for
`channel
`
`4·24 ms 20ms
`
`1·67 ms
`
`33 ms
`
`2·67 ms
`
`!Oms
`
`February 1982
`
`81
`
`HTC Corp., HTC America, Inc. - Ex. 1019, Page 7
`IPR2018-01555 and IPR2018-01581 (HTC and Apple v. INVT SPE)
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`

`

`N. M. MASLIN
`
`years ago. It has been shown
`that serial data
`transmission with pulses narrow enough to resolve
`multipath can achieve18, with optimum equalization, an
`irreducible bit error rate (b.e.r.) that is a number of
`orders of magnitude less than the irreducible b.e.r. of
`parallel data transmission using multitone systems. ·
`The
`applicability of adaptive matched
`filter
`techniques relies on the channel multipath impulse
`response remaining constant for significantly longer than
`the duration of the echo train from a single sample. Thus,
`the product X of the multipath spread and Doppler
`spread must be appreciably less than unity for adaptive
`systems to be useful. It has been shown 18 that the
`transmission limit of communication for parallel data
`modems occurs for X ~ 1/ 2000, whilst for serial data and
`adaptive equalization, it occurs for X ~ 1/200. In
`practice, however, the performance of an adaptive
`system is worse than the performance of non-adaptive
`multitone systems. The major problem is undoubtedly
`caused by the rapid variability of the channel, which
`makes the greatest demands on equalizer performance
`when the signal is at its poorest. Of attempts such as
`Kathryn, Andeft, Adapticom. and Kineplex, 15 only
`Kineplex has entered service on point-to-point circuits.
`The characteristics of these systems are summarized in
`Table 4.
`
`4.4 Error Detection and Correction Techniques
`Interference often occurs in bursts in the h.f. band and
`this can result in the loss of si