Note: Descriptions are shown in the official language in which they were submitted.
~3(~Z~3
T11~ PRIOR ART
lonospherically propagated radio signals are frequen~.ly
subjected to soveL-e levels of amplitude and phase distortions
from fadine, multipath and noise phenomena as well as man--r11ade
inLc L' feronce ef~ct.s.
ln attempting to maximize the availability and relia-
bility o~ communications through the HE' medium, it is TIUW
1.0 woll recognizod that the following two factors are predominant:
l. Ti1e determination of the optimum propagating frequency
~OL any selected path and time, and
2. rho validaLion that this sclocLed channol is also
intorforcnco free, primarily at the recoiver's end.
'I'hQ most accu~aL~ mcans ~or sp~?cifying propagation
conc1itions ovor a give11 1iF circuit is attained through r~?al-
timc oblique path sounding. The incorporation of real time
propagation data wiLh accurate spectrum-interference data at a
receivcr provides the basis for a practical frequency manage-
ment system. To be fully effective, however, HF frequency
managemer1t would also require some means to rapidly disseminate
reco~unendcd freql1cncios or spectrum information to multiple 11F
~S users. Moreovor, this distribution of frequency assignments
should be roadily availablo, secure and not subject to the HF
oul.ages it is designed to avoid.
1ho bandwidth that will support skywave communication
botwc~:!(1 any two points is normally much less than the 28
MHz-wide 11~ spectrum. The available bandwidth cha11ges
cyclically Otl daily, annual, and eleven-year cycles and may be
dislurbod by unpredictable short-term effects. Frequency
assignn1ents aro commo11ly made using forecasts based on t.he
slal.i;l.ical variations of propagation expectancy cycles, t.he
path and the L`requencies available to the assi~ning authority.
"~
3~
-- 3 -
lntcr~erence may have cornponents due to external causes
(Kalactic, atmospheric or receiver noise) but it is actually
man-made noise and particularly the widespread interferences
from distQnL HL~ stations, that accounts for the main source of
errors in HL~' data con~,unications. The HE bandwidth is heavily
ovorcrowded especially at night and msny observations have
revealed that outages due to interferences from other users
muy exceed t;hose due to propagation by a factor of five.
Knowledge of the level of interference present in a communi-
cation~ channel is essential for channel optimization, ascommunications will take place on a channel showin~ the
groatc!st value of signal-to--interference ratio.
The most advanced H~' frequency management system,
typically consists of various combinations of three dedicated
eqllipment items. Two of these items, an oblique sounder
transmitter and sounder rece;ver provide an ionospheric "test
set" measuring the propagation of an HF signal vs frequency
over the communication path. The. third item, a spectrum
monitor, provides the extent of interference measured across
the entire 2 -- 30 Mllz bflnd during the past 5 - 30 minutes. Tn
a typical chirpsounder system the sounder transmitter sends a
linear lM/CW test signal (2 to 30 MHz chirp) and is track~d by
fl time synchronized chirpsounder receiver at tho other end of
the con~ nications paih.
SpocLrfll unalysis of the difference frequency between
the soundoL receiver local oscillator and the incoming signQl
yields a tirrle delay-vs-radio-frequency di~lay.
Specific conclusions with respect to tile operational
utilization of such fl system indicate thflt:
L. A close oporationfll control and coordination is required
I-ctween Tnultiple users of the system. Simultaneous
soullding~ requires careful transmitter synchronization.
~23~
- 4 --
2 When the pool of assigned frequencies is not very 1arge,
tl-"~ use Or t.his system may prove to be counter-productive
or over ~pcc;ficd.
3. Ln a mi].itary environmont, sounder transmitters havr-! a
S VeLy 1.arr,o, identifiable si~nature and must, therefore,
bo placcd some distance from communication conters to
minimize tho risk of direction-finding, jamming or
phys;cal destruit;on.
'1. rhll simultaneous radiation o~ multiple sounding trans
1.0 milters continuously scanning the entire 2 to 30 MHz
bnnd pollutos tho HF spectrum, raises the R~ noise floor
and consoquonlly se]r-jams friendly HF con~unicz~tion
roceiving equipment.
S. Tho HF propagaLion path is not reciprocal, particularly
LS wil.. h rospect to the extraordinary modes. Resorting to
two way snunding per link will render the communication
nelwl:,rk operat.iona].ly inlractable and oconomically
inl-ler.lhle in view o~ tho magnitude and hi8h cost of
such a systerll.0 6. Hulllall judeement and analysis cannot entir~.31y be
r~!E~laced. Intel].igent and exporienced asscssment o~ the
dynamic iono~ram is Or paramount importance. This
sy~l.t?m roquiros the continuous intervention of a sk;lled
~ o r .
S
A radica]ly di~erent concept is therefore needcd ~ tO
a singlo add-on torrn;nal contro1s and uses any standard rllodern
H~ communicaLions oquipment to automatically probe a laTt~e
number "r ~requoncy channels within an assigned HF sub-band.
:~0 lt sha].l perform sounding, 1.ink quality evaluations, select
nnd socu(oLy disseminate thc besl; operating frequencies, to
achiove rapid and re;iablc link connectivity.
~3~
Sl)MMAR~ 0~ Tli~ INVENTlON
Il is the priMary object of this invention to provide a
new real time ~requency management system which will p~:rmit
the automatic solection of optimum operational frequencies in
H~ conunullication transmitters and receivers.
It will estab]ish communication links without the intervcTIt.ion
of ski]led operators, eliminate the need to resort to
propagation predictions~ and thus enhance the usefulness and
reliability of high frequency communications systems.
ln accordancc with the fore&oing objects the invention
hcrcill is directed to frequency programmable HF conm~unication
systeins which employ transmitters and receivers capable, ;n
response to control si&nals, of remote tuning and scanning a
plura].ity Or channels. A high frequency communication network
has onc controlling station and a plurality of controlled
stulion.q. Moans are provided for all stations to continuously
motliLor a larga group of randomly selected frequencies wit:nin
a given band, measure and analy~e their noise and interf~reT?ce
characterisTics and hard-label each channel as either 'noisy'
or 'quiet', based on a set of criteria. The resulting b;TIarY
word is used by thc controlling station, in a prese]etted
formaT., as the sounding message. Additional means are provided
~5 fcr lhe conl.rolling station to redundantly broadcast the same
sounding mcqsage sequenTially over each one of the channels.
The radio trnnsmitters and receivers are synchronously hopped
~ccording to a pseudo-randomly coded sequ~rlce.
'I'he controL]cd station has the means for majority
decoding Lhe highly redundant soundin& message and furl:her
means for rncasuring the link quality of each channel on which
that mc~ssagc was received. The link quality analysis includes
mcalls ~or measuriny, bit-orror-rates (BER), multipath delays,
fading rate~, inter~ercnce lcvels and distributions and signal-
to noise ratios.
~f23C~
- 6
'IH~C! controlled station consequently generates another
binary word in which each bit represents a hard-decision,
bascd ~n a ~et of transmission quality criteria, as to whether
thc corre~ponding, scanned channel is accepted as 'good' or
S 'bad' ~or con~lunicat.iorls~
This link quality pattern is now used by the controlled
stution as its answer--back soundin~ messages-
rhe contro].l;ng station majority decodes the repetit.ive
sounding broadcast made by the control.led station while iSsynchronously sequences tho ent.ire group of frequencies. It
pcr~orrlls its own link quality analysis and compares the data
processed at both ends of the link. The contro]ling station
now derivcs tha optimal operating frequellcies. The selected
1.5 frequencies arc then ~utomatica].ly disseminated using the same
fcequency hopping l.ransmission.
Accordinely, tho invention rclates to a high-frequency
(H~) frequcncy-manngcment system wiLh ~t least two stations, a
control.].ing stalion and one or more contro]led stations, each
including an 11~ radio transmitter, HF radio receiver, a
conlrol unit t`or controlling the operation of the transmitter
and receivel and a frequency-management processor means for:
- continuously monitoring the interference and occupancy of
a ~inito plurality o~ H~ channels, each channel tuned t.o a
difrerent frequency;
-- hnrd-labeling of each one of the said channels as either a
binaly"L" for a 'quiet' channel or a binary 1l0ll for a 'noisy'
chanllel ~or vice versa?, based on a predetermined set of
criteria;
-- storing and updating the resultin~ binary word wherein
cucll bit represents an eva].uation of one of the freque7)c~ies
visited:
-- using this binary word as a sounding sign;ll and
tri~llsmitl..ing this signal repent.edly, once over each of the
said finite ~roup of trequencies by having the transm;t.ter
scun said ch~JIlrl~13;
~ ~3(~9;~3
-- synchronizing the remot0 station receiver so that it is
scquenced through same said group of channels at an equal
rate, bcing at each onc of the channels at the same period of
timc as the transmitter, to allow the sounding message to be
rcceived;
- majority-detecting said redundant sounding message by the
remotc recciver proce~or;
- performing link quality measurements on each one of the
scanned group of frequencies;
- hard-labeling Or each one of the said channels as either a
binary"l" for a 'good' or 'acceptable', and a binary "O" for a
'ba~' or 'not-acceptable' con~unication quality ~or vice
versa), based on another sct of crite~ia;
~S - storing the resulting binary word at the remote station
rcceivor-processor, to be used by it in formi-l~ the
answcr--back sounding signal;
- translllitting the answer-back sounding messa~e repeatedly,
once ovcr each of the said group of channels by having the
~0 rcmolc sLation transmitter scan said channels;
- rnajority-detecting said redundant answer-back sounding
mcssage by the first, controlling station receiver processor;
-- performing link quality measurements by the controlling
station receiver-processor, on each one of the said scanned
~S group of frequencies;
-- selecting optimal frequencies by the controlling station
processol, for reliable con~unications in both direct;ons,
contrnlling-to controlled and controlled--to-controllin~
stations, based on the analysis of the received and der;ved
link quality patt~rrls;
-- utilizing the synchronous frequency--hopping mode ~hich is
maintaincd between the stations, to disseminate frequency
informaticn by t~ansmitting, over the selected optimal
frcqucncies, the relevant information for the remote station;
- automatica]ly tuning the communications transmitters and
rcceivers to the selected preferred frequency or frequencies,
to establish a reliable con~unication path between the
stal;olls;
~;~3~
- 8 -
Thc timing and control means comprise:
--- mcans for randomly selecting N channels from within a
specified IIF sub-band given its limits fl to fhi h;
S ---- mealls for storing said N channels as alternate colllTnuni-
cal.ion channels with each channel having a predetern1;ned
frcquency;
-- reccive/transmit means for placing the station in a
transmiL mode;
1.0 --- mcans for sequencing and tuning the HF receiver and
transmitter through the group of N channels;
--- means for providing timing for the overall system
operation, bit synchronization, frame sync acquisition, sync
cycl.e operation, sounding cycle operation and signal pro-
cessin~ algorit.hllls;
- means for transmitting the sounding messages using an
in cllannel diversity of two FSK modulators-demodulators;
- means ~or ~cneraLing a predetcrmined sequence based on the
inpllt of a key variable and real time of day.
lhc noisc and interference measurement means comprise:
- mcans for measuring the radio receiver AGC level and radio
receiver noisc output and distribllt.ion;
~5 --- means for measuring in-channel interferencc
chatacteristics;
--- mcans for classifying noise and interference present on
the COnmllJniCatiOn channel into a predetermined number oE
catc~ories, according to a predetermined set of criteria;
~- means for generating a corresponding number of binary
words, each N-bit lon~, one for each category, wherein each
bit represents a hard-decision qualifying each one of the N
communication channels monitored;
~,3~
_ 9 _
The link quality analysis means comprise:
--- means for de'ectin" noj~e representative of the noisr3
prcsent within the communication channel band as well as
within two soparate ESK channels;
-- data detectors for prcviding a signal representative of
thc data levels that are present on the con~lunication channel
th~ the r~ceiver is tuned to;
-- means to measure the signal~to-noise r~-tio, the fading
rato, and the rms multipath delay spread;
-- mean~ to UCf3 the demodulated rnd màjority-dete(ted
sounding message to arrive at the actual bit-error-^rate;
-- mcans for quantizing tbe parameters: signal-to--noise-ratio
and bit-error-rate, iC desired in combination witn one or more
15 of ~I1C! parameters: fading rate, delay spread, channel noise,
datfl levels; measured on the communication channel to define
tho dosirod predotermined number of link quality categories
arcording to a predetermined set o~ criteria;
-- means for gcnerating a corresponding number o~ binary
words, each N-bit long, one for each category, wherein each
bit represents a hard-decision qualifying respectively one of
the N communication channels sounded;
Jhe advanta~es and further objects of the invention, and
71 thc mc;lns by which they are achiev3d may be best appreciatr~d by
referrin~ to ~hr3 detailod descripti~n which follows~
BRlEI` DUSCRIP'rION 0l~ Tll~ DRAWI~S
,~
~5 A more complete description of this invention may be had
- ~ by re~erence to the ~ccompanying draw-ngs, iL.~rr~ing
piererrr3d emhodilllf3rlt of the invention ts be drscribed in
~otail, wherein
~ Fig~ 1 depicts three ;J OCk diagralns ill~strating three
r , ./ ~i frSJrell~ configurQtior~ of IJF e~ 3unications systems in wbich
~i? s~fS~ o~ ~he present invrl~iorl i integra~ed~
... . . .
,. . ,. ~,~
2~
- 10 -
.;,~. 2 outlines the format for soundi.ng messages between
-not 5 tations.
~ ig. 3 outlines the format used in the network synchron-
i~ation transmission cycle.
Fig. 4 ;s a simplified block diagram of the frequencymanagement system according to an embodiment of the pr~st~nt
inve~ ion.
Fig. 5 is a functional block diagram of the system
accnrdirl~ to an embodiment of the present invention.
I:)k'TIll l.r~:D VESCRIPl~ION 0~' TIIE PREF'ERRP:D EM80DIMENT
15`
~ orore going into a detailed description of the figures
a brie~ overvicw will be given describing the environment and
gt)neral ~eatures of the syxt.~m.
~rhc oporational situation typically assumes a network of
HF radio USeL'S generally structured using a net controlling
station in association with a plurality of widely scat.t.~red
control.led stations, including relay stations.
~'ach controlled net station is expected to continuously
25 monitor the net traffic and to respond if polled by t.he
contro].lin~ net station. Only one station at a time wnuld
thon he transmitting, with transmit discipline being
maintained by the controlling stat.inn.
Centralized fret~uency management and control, with full
froquency assignment authority would normally be the rcspt~nsi-
bility of the controlling station, within the single-net or
thc multi--net configuration. Operational configurations,
howcver, using one-way transmissions only, utili7e the capa-
:~S bilil..y of the prescnt invention to assign the selection nf
optimal IIF frcquencies to the controlled terminal.
1~3~ 2~3
~ '~lC system according to the invention, can be used for
the real-time management of HF communication networks having
onc controlling station and one or more controlled (or rerllote)
stations. The system is adapted to provide a frequerlcy
S manaKometlt capability for any predetermined number of
frcquencies. Practical considerations show that generally, a
number of from about 50 to about 150 frequencies provides a
suitable sysLern, depending on the conditions of use and the
rcquirod speed and reliability. The operating sub-bands are
1.0 chosen to have an adequate width for accoEnodating the
prodctermined numbor of frequencies, with an adequate spac;ng
beLweon the frequencies used. In the eollowing, the invention
is il.l.ustrated in an entirely arbitrary manner with reference
to a system of 125 channels. It ought to be understood that
5 th i 5 i U by way of exaMple only, and that any reasonable and
practical numbcr of channels can be mana~ed by such systems.
As stated above, the invention is illustrated with
rofcrence to the system which provides a total capability of
~requcncy rnanaging a group of l25 frequencies, randomly
distributed betwQen any sized sub--band fl to f2 f the H~
spectrum, for any sized time period tl to t2 of the day
and night. The opcrating sub-band must be at least 500 kHz
widc to accon~odate 125 channels at 4 kHz spacing. Thus, the
system can bc pro~rammcd to process, excludinE, used or
fo~bidden Croquencies, the entire HP band all of the time or
any sma].l^r propagation windows of usable frequencies grossly
prodicted to bc effective at certain correspondin~ t.ime
pcriods. lt is to be clearly understood that this example is
illustLativo and ought to be construed in a non-limitative
malln"r. \,
r
This enscmble of 125 automatical.lv pre-assigned
frequencles_constitutes_the frequency mana~ement sing].e,
widcwband opeLatlnE cha_nel._ W_thin this channel information
is timo a_d_ Lcqu_ncv m_lti~lexed, redundantly utilizinlg__he
- '~ood' and tho 'bad'_available fre_uencies.
- 12 -
Thc system can be progran~led to operate in either one
band or two separate ~ nds.
1. Onc Frequency Band: One pair of frequencies shall
dc~inc the l.in)its of the expected operational band.
Within this band, 125 frequencies will always be
available for evaluation, regardless of the size of the
band, with 4--kHz minimum spacing.0 2. Two Frequency Bands: Four frequencies shall define two
scL~lrate 'VAY' and 'NIGHT' operational bands, which may
havo ovorlapping regions. A single transition hour will
be chosen for the transfer from the DAY band to the
NIGHT band, or from 125 DAY frequencies to 125 NIGHT
f L`Cqll~Jlll' i l'S .
~ non-repeating, key-contro].led permutation of numbs~rs O
to l25, ds~Le~mines the actual frequency locations and their
tcansmission scquence within the defined operational bands.
~0 WiLIl Lwo bands the system is actually processing 2.~0 I~F
t`rs~quencies, during the 24-hour period.
Thess~ 125 frequencies are continuously monitored by each
of the net stations and the channel noise and intereerence is
~S eva].uated. The cs~ntro].ling-station initiates the sour)d;ng
transmission. The sounding signal consists of local ns~;se
information analyzed at the contro].ling-station location.
Vuring ~hc sounding cycle the controllingstation scans through
a]l ot` the ].25 ~requencies in a random sequence.
lZ3~P92~
- 13 -
The spacing between frequencies shall be in multiples of
4 kH%. Given a band F1 to F2 then, for any subset o~
numl:)ers N= ~n~ 5~ randomly chosen from the set N-
1,2,..... N l , where
S max~
N = F~ - F (Fl and F2 being in MH~)
Tho corresponding frequency set is ~ F~
lo F~ 2~;0 ~ ~2J
The net controlled-station step synchronously with the
controlling--station and performs transmission quality evalu-
a~ios~s pertaining to each of the 125 channels. The controlled-
s~al.ioll wil]. sequentially respond by a repeat sounding cycle,scanning aeain al.l ].25 frequencies. The sounding signal will
now carry ].ocal reception quality information back to ~he
contLollin~-station, again frequency-hopping over all 12S
channels, in a random sequence. The controlling-station
~0 porforms its own transmission quality analysis and compounds
it with the information received and processed through t.he
sounding signal from the controlled station.
A single two-way exchange of real-time sounding trans--
~S rnissiolls thus ~nables the controlling station to derive andreliably assign optimum operating HF frequoncies to each
con~llunicating link.
lt is a central feature oE this invention that the fre-
quency managelllent process acts as an automatic HF link controlto achieve an adaptive cl~annel and enhance communication
reliabillty.
~ig. l illustrates in a block diagram form three of the
many other possible configurations of HF radio communications
sys~ms, incorporating the preferred embodiment of the p7esent
invention. ~ freqency management terminal would normally be
closely inte~ratoLI with the radio equipment..
-~ ~3~
- 14 -
In Fig. la a conventional HE' radio communications system
i5 ShOWII that includes a radio remote control unit 21, the HF
Lranslllitter/receiver 24, and a matchin~ unit 26 to cou~le a
narrowballd antenna 29. This matching unit need not be used
S whon a broadband antenna 27 is available. The frequency
management syst-:m is shown to comprise a remote control unit
22 and a processor 23 which is connected to another
convenLional but dedicated HF radio system. The two control
uniLs are interconnected by 25 to allow automatic freguency
assignment and control. In this configuration the information
channel is entirely independent from the frequency management
sysLem channcl (22-23-20). The information channel ~21-24)
which norrllally uses one frequency (half-duplex) or two
froquencies (duplex) wi].l not be interrupted by the ~requ~ncy
lS managelllent operation. The system channel which u~es l~S
frequencies operates simultaneously and continuously on a
non-interference basis.
In Fig. lb the HF radio system 29 is shown to comprise
separaLe IIF' transmitter system 30 and an Ht' receiver system
31. These may be physically widely separated. The frequency
mallagemenL system, however, uses only a dedicated but
convonLional ll~' receiver 32. The two receivers connect to a
single receive antenna 37 through an antenna multicoupler 34.
In t,his configurat.ion thc system shares the COnQlUniCationS
transmitter only, which is therefore used both for information
transfer as well as frequency management transmissions. The
sysLem receiver can thus uninterruptedly monitor the l25
c~ n~ s.
ln Fig. lc a single remote control unit 33 combines t.he
ConmlUniCation and frequency manaeement operations, audio and
colll,rol, through the system processor 36 which coTIn~ts
directly Lo the HF' radio receiver system 35 that serves both.
:3S
~ ~3~3~-
As previously stated, each terminAl in the net shall
maintain a continuous evaluation of all 125 channels by
examining the prevailing interference in the normal comr~ ni-
cation 3-kHz bandwidth. This monitoring process shall go on
S at all availablo times, by means of the HF system communication
eceivor. ~'ach tetDIinal shall sequentially scan the progra-nmed
list of L25 freguL?ncies, continuously compiling and updating
challn~l occupancy sTatistics.
The number of available operational channels will depend
fiLst oL' all on the likelihood of finding any quiet ~requeneies
(fLom tho pre-assigned group) during all hours of the day and
night, while the rate at which the channel must be evaluated
will dep-3nd on the likely variability of the noise spectrum
lS allcl pcl:lpaGation conditions with time.
'l`hc torm 'qu;et channel' generally implies a c'nannel
whose noise and interference level, inherently a variable
quan~ity, only slightly exceeds some measured noise fl(~or
avoL-~g(:~l within a liMited bandwidth, or a fixed noise level
that corresponds to a low-level signal induc~d into the
antorllla, OL~ the threshold of atmospheric noise.
Ilowever, the characteristics of the interference in the
~5 chanllcl, depcnding on the traf~ic and mode of operation, will
determitle wllether the channel could be expected to support an
accoptabl.e intelligibility of voice or an acceptable
bit--erLor-rate. The power spectral density of interfererlce
frotll oLheL IIF users may be significantly non-white within HF
voicc channels. Low frequency CW, ~orse Code or narrowband
FSK may characterizL? an HF channel as 'noisy', while it may
sl.ill support intelligible voice.
According to the invention, a predetermined number of
:15 quAIltum !.tates is de~ined, respective to a predeterll-ined
number of parameters, the main ones being signal--to-noise
~Z3~9~
- 1~
ratio and bit error-rate, the others being channel noise, data
levcl~, ~ading rate, dclay ~pread. Advantageously, the para-
mcLeLs mcasuted comprise at least the two main ones. These
can be measured with one or more of the other parameters. Any
combinution of one of the main parameters with two or more of
the othor parameters can also be used.
As stated above, the invention is illustrated with
rcrcrenco to a systom of 125 channels, and it i5 further
illustrated with reference to eight guantum sl;ates classifying
noise and interference present on thc communication channel.
~ l quanLum states of noise and interference power/
freqlJcncy distribution will be defined. Measurements will
lS conlinuously indicate which of the eight thresholds has been
crossed at oach of the 125 3-kHz channels. For each of these
eighL s~ates a panoramic pattern will rapidly be formed, qual-
irying a~ 'quiet' or 'noisy' each one of the 125 channels that
. thc ontire net is currently evaluating. These patterns will
bc cc~nlinuously updaLed throughout the monitoring periods.
Thc labcling of a channel as 'quiet' or 'noisy' will represent
fl hard-decisio_, producing the bcst available choice and in-
cluding a~wnys a fixed, minimum number of the 'quietest'
ch~ nQ~s in cach pattcrn. With net stations dispecsed over a
; ~S widc gcographic expanse, di~ferent interference conditions
will be cxperionced at different locations, which will most
likely result in a very diffcrent Interference Measurement
Pattc-.n (IMP).
Thi~: lMl' will thus constitute a sequence of binary
mcasurcmcnts, ]25 bits long, whcre each "1" or "0" corresponds
to a hard docision interference-state measurement. Each one of
tho Illonitored channels is labeled Quiet ~"1") or Noisy ~"0")
at lhe teTminal's location, based on the continuous monitl~rillg
and updating of channel occupancy statistics, in 5 or 30 m;nute
timo--scp~men~`s. Each bit position will correspond to the exact
channcl position in an automatically produced coded table of
l~S rroqu(~ s
~3L~32~
- ~7 -
Tho process of real time HF channel s~l~ction normally
involvos a single two-way transmission exchange between a
conlLo]ling frcquency-management terminal and a controlled
froquency-mana~f3ment torMinal. Howover, reliable channel
assessmont may also be produced through a one-way transmiss;on
~J ~ 5 .
Thc IMl~, the continuously compiled and updatad inter
forenco measuremcnt pattern, is used as the primary sounding
signal by the controlling terminal. A soundin~ transmission
will comprise a single cycle of 125 pseudo-randomly selected
11~ fcequency hops, repeating the same message in a buT-~t. of
auliio data, once every hop. During each successive fr~me
poriod tho samo frequencies are visited but according to a
dil:Ceront, non ropetitive PN-coded permutation, con~rolled by
a non-linoar sequence generator (NLSG).
Thc identical, redundant sounding message will be ~ent
over each o~o Or these HE' frequencies by means of noncoherent
LSK, using 2-nd order in-band diversity, at a rate of 224 ~its
peL' second. The use of du l channel FSK contributes also to
an incroased correlation between assessed channel quality and
voico quality.
~IG. 2 is a timing diagram of the selected burst format
of tho sounding framo 311~312~313 which has a hopping rate of
l/r hops per second. Each frame starts with a frequency time
guard pcriod 3]1 which is long enough to allow frequency ~ han~e
ti~ , antenna match time and receiver AGC sottling time.
During thc ncxt timc period 312, the receiver doppler correc--
tion loop (in the AFC circuit 35 in E'I~. 4) utilizes t;he
dual-ESK tonos and filters to compensate for frequency dr;fts.
Tho ~ol]owing time poriod 313 is devoted to tho data block
which CO!lSiStS oL a total of 210 bits. The first segmont 420
:~5 of 6~l bits each are tho synchroni~ation unique words used to
provi(:le frallle sync. Tho next soemont 421 of 24 bits is used
for I~s of sondor and destination. The foliowing segment 422
o~ 125 bi~s accon~lodates th~ sounding messa~.
J~Z3(~
- l8 -
Sc~nlent /l23 of 3 bits indicates 1 of 8 quality states to which
Ihc currollt soundin~ pattern belongs. The last segrnent 424 of
8 bits is the ollly one that varies with each frame as it
indicates the frame numberl from 1 to lZ5. The sounding
mc!ssage is sent by means of dual-FSK transmissions at 224 bps
and tbc-n 125 times by hopping over eaeh of the 125 channels~
.
Thc soundin~ station (controlling or controlled~ trans-
mits its mcssage on eaeh frequency in turn, and aIl the remote
receivin~ net stations being synchroni~ed to the sounding
staLiorl, repeatodly Leceive the identical message at each
froquency. ~ unique Majority-logic decoclin~ al~orithm insllres
a VCLy high probability o~ raceiving a].l messages error-free,
undcr extremcly varying con~unications conditions. This ~apa-
bil.iLy of socuro message transfer by redundant transmissions
is a unique chacacteristic of the frequency management system
ombodi~d in Ihe present inv~ntion.
The san~e sounding message of N bits (N = 125) is being
rcccivcd over N channels, each channel with its own bit-error-
ra~e (8~R). One can make a first approximation and classify
8~ challnels as "blocked" when their 8ER ~ 1/2, or "open" when
~, ~l C i L' U ~ 2.
llllcl(:!L thcse simp].ifying assumptions if N = 2n+1 is the
number o~ tcsted channels, N of which are blocked, the
probability of error in any one bit under an N/2 majority
dccision rule is:
.
p (B) ~ (2) ~ (~-e) ~
e-~ e
~ further approximation takes into eonsideration two
typos o~ "open" channels: 'Good', when the BER -~ n2 ~nd
'Ufld', whell the BI~K ~ lO = Bl.
~.~3~9;~8
- 19 -
In additioll to the M blocked channels, the proportion of
thc 'Good' ~nd 'l~ad' ch~nnels is known ~o vary considerably
bcl.wcoll day and night. ~uring the day SOMO 20 to 30 percent
Or l.hc N M cilanncls may be considered '8ad' while during the
nighL 40 to 70 porcell~ of them may turn out to be 'Bad', on
tlle avorage~
der these assumptions th& probability of error in any
one hit, aL'tor majority decc)ding is (the nun~ber of B
~0 channol 5 boin~
p~ -M5(/~ J~L~M
B K (~_ g ~ -I / lc~
I.t sllould be omphasi~ed Ihat the above is derivod under
thc assumptioll of uniformly distributed independent errors
Wlli(`ll i9 a fair assumption. Namely, the bits received on
cllal-lne]. i are indopcndcnt idontically distributed ti~i.d) with
~0 rc~:pect to the same bits (in the message) received on channel
j (j - i)~ 'I`his assumpt;on would not be accurate under flat,
vory widoband fading conditions of extremely lon& duration
tLells of soconds), but these conditions are rarely encountered~
One can ovaluate the average bit error probability ou~r
a discroto distribution of channel qualities for tho "open"
chanllc].s. ~.f a typical channel distribution is as follows:
Br':L~ ! 10 10- 2 ] 0 3 10--4 -5
~ c,~
channols: S lO ' 30 40 lO 5
~ rhe slverage bit crror probability after majority decoding
for N-l.~S and M-SO percent would be: 5xlO , namely, even
undc; vcry sovere conditions, with enough tested channels tho
bil. crror rato o' the ro'erence sounding message is remark.a~ly
low~
~ ~3~9~3
- 20 -
On~e ~he error probability for any one bit in the
majori~y - decoded sounding message has been evaluated, one
can ovaluate the probability of receiving an errored sounding
mcssap,e, Pl, and then the probability of receiving an exact
sounding mossa~c which is given by l-PE. For a totsl 3n bits
of messap,e (2n~1 channels and n-l control bits),
P~ ( 1 pM~ 3n
This capability of secure acquisition o~ the sounding
mcssage, through utilizing the highly redundant transmission
sl~home, is a unique and a central aspect of the present
i llV~-UIt~; on.
ro m~in~ain system synchronization all terminals must
stop ~heir non--linear sequcnce generator (NLSG) clocks with
t1lcir phasos directLy related to thc transmitting terminal
c]ock which takes the lead. l`he NLSG has several special
fcalures, in addition to the basic functions. It provides
~0 synchronizin~ or resynchronizing capability, the NLSG can be
ro~llrned to a known starting poing and then stepped to a
prclJet~:lmined point in tim~, in the process of initialization.
The YK bit st:ream is based on 8 key-variable contained
~5 witl~ ttlo NLSG. The NLSG is programmable with respect to t.he
valiahle in Ihe sense that the current variable can be replaced
with a new one as required, by means of a special ext.~rnal
loader. ~ zeroizing function is also provided, should it
bocomc necessary to clear all stored data in the NLSG, under
cmcr&~:l"::~y condi~ions.
Th~ synchronized pseudo-random sequence generators at
all froqu~ncy mana~oment torminals determine the same new
frcqucncy for each successive frame. The freguencies are
se~ected from the string of bits generated by the NLS~ each
timc t.he frequoncy is to be changed.
L)uring rec~ption o~ the repeated sounding message, the
sysl.cm maintains an elaborate Link Quality Analys;s,
peLLorming sin~u]taneous measurements of a]l the paralI~eters
co(Isiderod essenLial to the monitoring of con~unication
S tLaf~ic. Link qual.ity analysis or in-band channel evaluation
is a key process in enhancing HE` frequency selection and
conununication sysLem performance estimation~projection.
Tj1C invention incorporates advanced signal processing
a1goritl~ s that pcrmit measurcments of all essential paranIet.~rs
within tIlc time constraints imposed by the time-varyin~ IJF
ch~;Irlel~ ~ fundamental n~easure of system channel performance
dcL,rad~tioIl in a digital comn~unication system is the bit-~rror-
rate ~L3IK). In the period of timc that the ~iF channel transfer
ruIlction msly hc approxirnted as quasi-stationary, it is commonly
di~:ricul~ to accumu1.a~ su~icient bit errors to characteri~ie
noar-inslantaneous data performance. The invention us~s a
modifiod approach of error rate extrapolation based upon
pscIldo-errors (E~L3~R) to estimate the probability of error in 2
vf!Ly shorl. tirno. Pseudo errors may be generated by modifying
thc eain or phase threshold criterion in the error decisions
pL'OCe5S to obtain parameters which indicate apparently greater
circuit degradation than raally exists. The measurement period
is :hoLtened since the pseudo--error is designed to be larger
~S than the corLespoIlding actual error rate. The basic idea is
that by narrowing the "good detection region" and wi-dening the
"error detection region" one measures a highor BER than t.he
acLual 13Ii.R of the detec'i.or. As a result, high accuracy low
BIit vfllues can he measured from a) small data samples, and b)
wilholII. actua]ly knowing the transmitted data.
.
Thc l'L~I.R and Iho actual BER are related as ~o1io~s:
log P - K I loL,~
:~5 wheLc I' ;s tho bit pscudo-error probability arId PE i~ I.he
actua1 ~:rrI7r probabi1ity.
%~3 ~
22 -
Ir Iho transmitted data is known, or derived from
maJority decodin~ of repetitive soundings, as is donc in this
illVelltiOIl, ono can "scale the channel" by calculatin~ K out of
moasllrcd ~ and P~.
lt should be emphasized that the above was developed
ma;nly for linoar, additive noise type fading channels. Since
l.llis is not always the case with the HF channel, a cert.Rin
cortoction should be made which will account for this discre-
pancy. The channel BER model can he rewritteri as:
logP - K ~ logl'~ + KL
whore KL is a compensating factor whose value is derived
ftolll l.ho burst orror statistics of the HF chal)nrJI.
Whrn the sounding cycle has ended, the contro3.1ed-
Lcrminal recoivor has at its disposal a wide variety of
information abou~ each of tho N sounded channels. The present
invelltion takos advantage of its uniquo capability to fully
rc~covor the soundin~ mossage bit-sequence. This original
me SAI,O is usod ~or error counting, PBRR and 8ER dotermin~tion.
Moasurements are performed also of the rms multipath
dolay spread, the fading rate, interference levels and distri-
bution, and SN~. This data is processed and updated with every
additional sounding transmission. A ve~y reliable characteriz-
aliori of tho l~ conununication channel results. Knowledge of
tllo channel conditions and parameters enables the prediction of
channel perforn1ance aL high data rate transmissions based on
lo~ rato d~ita transmissions.
Tested channels are also ranked for various uses: voice,
rnulti-torlc D~'SK modem, wideband FSK, narrowband FSK, etc. The
inL(3rldod oporational use clearly af~ects the link quality
dClL'rnlinatiOIl Si.llCe interferences have different effect8 in
di rr~ "~ applicat.io"s.
- ~3 -
1~ascd on the link quality analysis and operational n1ode~
hnsd doci_ iOn5 are made by the receiving terminal, qualifying
as ~Good" or "Uad" each one of the 125 channels tested. A
bin~1ry L;nk Quality Pattern (rJQp) of transmission performance
S mC.l`SUrelllellt5 i5 gcnerated~ where each one of the tested chan-
ncls is labeled "l" (Good) or "0" ~Bad). "l" to indicat.e an
accoptable channel and "0" to indicate an unacceptable channel.
Accoptabi]ity is determined based on eight quantum states of
pcrrormance characterizing eight separate link quality
paller1ls~ These LQt's represent the best available choice and
includc always a fixed, minimum number of the 'best' channels
in f-~ach paLLern. ~`or example, in the ].imiting case, with all
oLhcr. measured parameters equal, "Good" or "Bad" may indicRte,
say, ~L1~ < 10 or BER ~ 10
following thc Doppler correction and AGC settling, the
rcceiver mu5f porform the following functions:
a. ~ccover clock timing for bit detection.
20 b~ Kccognizc thc frame-sync unique synchroni~ing word to
cslabli3h the basic frame timing reference and identify
ncl. nllmbcr.
c. lde1lliry the I~ patterns. These bits enable the re(:f?iver
to vcrify Lhe validity and legitimacy of the received
~5 bursL~
d~ ~ccopL 1he remaining portion of each message. Arrive at
Lhc~ corrccf~ IME' or LQP and process the nec0ssary tests,
evaluation3 and decisions.
To illu3trate the system's operation, let the controlling
Lor1n;nal, C, init:iate a sounding braodcast cycle C uses ax a
sol11(di11g signal its most recent IMP. The controlled-terminal,
c, derivfs C~s ;'MP error-free, which provides it with the noise
and inLer~crence levels measured at C's location, in each o~
j :~S Lhc 1~5 channcls n~onitored. In addition, c cafries out l~.~ r.QP
-~~ tcsts, during the sounding cycle, to sort out the "Good"
chnnnc]s. lhe final results can be tabulated as in Lhe
fo]lowi1lr, siMplified exan~ple
- 24 -
Chal~nal No. ~ 24 25 26 27 28 29 30 31 ~ - - -
C's MI~ (Lclc~ived) - 0 0 1 0 1 1 0 1 ~
c's TMI' (maasured) - l 0 0 0 1 0 1 1 - - - -
c's l.~P Decisions - G B B G U G 8 G - - - -
~ 5 (G for "Good" and B for "Rad")
:
~ ~rom the above data c can i~nediately deduce that:
.,
a. At th~ controlling terminal C, frequencies 24, 25, 27 and
30 are noisy. ~requencies 26, 28, 29 and 31 are quiet.
b. At the conLrolled terminal c, frequencies 25, 26, ?7 ar)d
29 are noisy, while fraquencies 24, 28, 30 and ~1 are
c. Ltocep~ion quality was good at frequencies 24, 27, 29 and
l.5 3l anl.l bad a~ erequencies 25, 26, 28 and 30.
d. At rreqllancies 27 and 29, although the channels were
noisy a~ c's end, reception was good, probably because
the signal over-powered the noise ]evel.
e. Al rreyuancies 28 and 30, although the channels were
yuiet at c's end, reception was bad, probably because of
no E~ropagation or a very low si~nal.
f. ~ur ~ransmission in the C--to-c direction, frequencies 24
and 31 may ba a good choice.
g. Ior ~ransmission in the c~to--C direction, ~requencies 29
and 3l ~nay be a good choice.
h. I.)oE)onl-lin~, on the nature and level of the noise at the
roceiver's end other frequencies may also be considered
whell the expccted received si~nal ].evel can be estimated.
.
~ Z3~D9;~3
- 2s -
In ~pplications where, most of the time, only one-way
transmissions are conducted (C-to-c), the frequency changing
or al~ocatn~ function, over the communication link, may be
asai~ncd to the controlled torm;nal c. Operation reli~ nn
5 just tho one-way sounding broadcasts (C-to-c). A reliable and
rapid docision will be made, determinin,~ the best pair of
trallSmit/locfaive operational frequencies, for cornmunication
witll thf~ controlling terminal. This frequency allocation must
bo sc~curely buLst-transmittf3d to the controlling terminal to
10 nl low norlllal Hf'~' con~lunications to prl)cf~d~
L~ollowin~ tho reception of the sounding transmission
cyc]o, tile controlled terminal will wait a fixed number of
tiUIlC! s].ots bo~ore attempting to respond to allow its communi-
.5 catioo Lransmittf2r time to tune to the f~equency chosen fortho c-to-C transMisslon. Tn prepar~tion to respond, tbe
conlLo:lled toralinal shall automatically construct a reporting
m,3s-:age mado up of just tho one selected C-to-c communication
freqllency (till the next update). Whan responding, only upon
arrival at the time slot that coincides with the selected
c-to-C froqu,3ncy (in the FH sequence) will the controlled
~rlll;nal turn--on its transmitter RF power for a bllrst-
tranamission of this mossage. This will automatically reveal
to C thf~ selected c~to-C irequency. Controlling and controlled
tolrninals have now automatically tuned their fixed-channel HP
COnlmUniCatiOn roceivers and transmitters to tho selected pair
! of` oporal.ional frequenci es .
NoLmally the aystem operation over an HF link will
:30 involvf3 a two--way sounding process, with the controlling
torminal assuming frequency assignment authority~ Following
tho fiL~t C-to-c soundine cycle, the controlled terminal,
havine formod its I,ink Quality Pattern, automatically responds
Wit~l a c to-C sounding broadcast. ~gain, within the single
frallle Or 125 ~reguency hops, c's LQP sounding message will be
repeat~cf onco ovory hop~ This two--way sounding process will
take lf39S I:han S minutos.
:~3~ZI~ `
- 26 -
rho colltrolling terminal will now be lookin~ at two T.QPs
which pLovide simultanQously the measured con~unie;lt.ion
porrotmance at both ends oE the link and, therefore, enables a
str~ight~orwald solection of optimal operating frequencies.
The dissemination of frequency information will be conducted
using either a soundin~ broadcast burst-transmission or the
current operating secure communications channel.
~efore a terminal can be used in an actual exchange of
si~n~lls, somo preparatory operation is required. Necessary
data n-ust be entered and stored: the frequency band or bands
to be ~sod, operational modes, IDs of net sender destination,
key variables, initial operating frequencies and a certain
agreed-Lo cycle start time is also set in. This is used with
the actual time to determine automatically the elapsed t;me of
the op()~ation for frequencyhoppin~ and key synchroni~ation
purE~oses. The actual time is acquired from a suit.ahle
refeLence external source having second-accuracy, such as
coordinated universal time, an electronic watch, a count down
OVflt voice radio, etc.
To onsure proper net initiation under seach mode
condiliolls, w11en a new number joins the net or transmissions
havo not taken place for many hours, a special Synchronization
Cycle is provided. During this cycle a unique sync message,
bLoadcast by the controll.in~ terminal, is rspeated once over
eacll o~ the 125 channels.
Ii'ig. 3 i]lustrates a simplified transmission timing
di~l~ran~ of the Sync Cycle. In a typical frame, the first and
lasL 64~bit data blocks are the two complementary unique words
501 and 505, designed to be detected as a doublet of a positive
folLowed by a negative correlation peak. In the central data
fiold the blocks 502, 503 and 504 of 96 bits comprise three
ei~,ht--bil. characters, ~lpha or numeric, devoted to the sender's
lUIL)esLillation, repeated three times.
1;~3~
- 27 -
I~ollowing initlalization, which includes loading the
terminal's NI~SG with the common key, net synchronization is
raEI;dly achicved i~ time-of-day internal clocks are all set to
within maximum ~D (t~T) seconds of real time, where t is the
syst~m hop-time between freguencies and T is the system dwell-
timc at each frequency. The order in which the system is
so(luenced through the gcoup of channels, is controlled by the
NLSG's output.
upon entoring the search mode, the frequency management
~orlninals au~on~atically advance their set time-of-day by D
tin~o-sloLs in time. The terminals' NLSGs are therefore forced
to ~o within (0, 2D) time-slots ahead of the real time of day.
1.5 ~hc sc!urch receiver will be taking unequal steps, jumping
fllw~ys ahead of the sounding transmitter, and waiting for the
tr.nnsmiLter to arrive. l'he receiver waits 2D+l time-slots on
its present frequency, then jumps ahead 2D frequencies, then
waits again 2D-Il time-slots, then jumps ahead 2D+2 frequencies,
waits another 2D-~1 time-slot, then jumps ahead again 2D
f e oqucnc ios, etc .
Following an initial shift of ~D time-slots, and
assusnill~ t 0 and T=l second, the optimal search procedure is:
wait . 2DIl alld S~arch = at 2D, then at 2Dt2. As a result of
~hi`: soarch pa~tern, the controlling-terminal and the
conlrollod--terminal meet on various frequencies, in other
words thoy criss-cross each other until acquisition is
achi~ved and the search procedure ~nds.
~ ho averago waiting time (T~) between meetings of the
tolminals during the search procedure is given by:
~.~3~9~28-
- 28 -
~2)(2D~ = 2 P
The maximum waiting time until the first meeting for a
giverl D, is l max = 2D.
DuriDg the synchroni~ation period, the terminals meet on
1 0 al- ~IVCL'-I~,C of
.
MU different frequencies, where:
D ~) where N i5 the number of
i 9 ~ (zp~/J2 ) assigned frequenr.ies,
1.5 N = 125.
Erame synchroni~ation exploits the systematic nature of
the soarch detection process to realize a very reliable and
raL~id frame--sync recovery.
~ digital correlator will detect arriving frame sync
sequences and full utilization will be made of the so-called
window tcchnique. This method takes advantage of the fact
tha~ the Syllc sequences are periodic ~nd that legitimate
co~relator outputs will have to be spaced in time according to
thc (2V)-(~D-~l)-(ZD-~2)-~D+l)-... pattern. Acquisition will
be dec]ared after detection of 3 sync sequences. The det~ct.ion
thLesholds will determine the average synchronization time, as
well as ~he n~ax. sync. time for specified miss/false-alarm
probabilities. t
I~ the probability of detecting a sync sequence on 8
channel is Ps, the averàge probability of detecting 3
consocutive syncs at proper spacings is~
' '` `
:~3~
- 29 -
P~ P5J
S Where Ps is given by the probability of detecting "over
the threshold" number of correct bits in a PN sequence; it is
a function of the channe] ~I~B.
Hence, the average s~nchroni2ation tiMe is p31
Whcn three successive hits are found, from among the channels
crossed (before one scanning cycle is complete) the operation
proceeds to the steady-state mode. In this mode the receiv~r
is in full synchronism with the transmitter and hops witb it
at the regular rate. The frame-sync detector maintains Q '
continuous trackin~ process and n~onitors the end of the
sounding transmission.
- This unique synchronization algorithm is another impor-
tant aspect of this inventiotl.
Re~erring to Fig. 4 which is a block diagram of the
frequency mana~ement terminal, the system is shown to comprise
four major modules:
1. Analog Module 12, which includes the terminal's data
link and basic sensors.
2. Process Control Module 14, which generates the syst~lm's
timing waveforms, and controls the sounding, and s~cllre
radio functions.
3. Computer Module ll, which is responsible for the system
signal processing, analysis and overall system control.
4. Front Panel Control Module 13, which includes all the
operator's manual inter~ace controls and indicators.
~z~
-- 30 --
Erom the radio interface connector 15 the radio receiver
AGC signal is fed through conductor 71 to the computer modulQ
11. The received audio FSK signal is applied through conductor
73 to monitor filters 3l and R~T control device 32. The
monitor filters are examining discrete segments in the
200-3200 audio band and signals present are delivered to the
processor module 11 via conductor 72. When the terminal
initiat0s a soundin~ transmission, a SEND/REC-ON ~ignal
appears; thrqugh conductor 74, at the input of device 32. Out
of the computer module the digital sounding message is applied,
through conductor 75 to the dual FSK modulator 37. This device
includes two widely spaced (in frequency) FSK modulators to
which the same message is fed simultaneously. Two FSK output
signals are then passed, via conductor 76, to the bi-
directional analog gate 32 which applies them to the dualbandpass filters 33 for signal shaping and improved isolation.
Conductor 78 feeds the two FSK outputs to the radio modu]ator.
When the terminal reverts to the receiving mode, th~ R/T
control device 32 routes the two FSK signals received from the
radio demodulator, through the dual bandpass filters 33,
gating their output via conductor 79 to the dual FSK
demodulator 34. The output of this device which is now the
restored digital message is fed through conductor 81 to the
processor module.
The automatic frequency control device 35 provides a
means of sensinK the doppler ~requency shift and applying an
adaptive compellsation to improve the bit detection capability
of the FSK demodulators. To help synchronize the local clock
to the incoming digital burst, the bit synchronizer device 36
continuously interacts with the central timing source 41,
throu~h conductor 85. The measured doppler shi~t QS well as
the processed corrections are transferred via conductor 82
to the computer module interface 22.
I
z~
- 31 -
Under program control a multiple of unique algorithms
and functions are simultaneously being processed in the micro-
computer module 21. These deal with the rapid signal measure-
ments, evaluations and frequency management decisions that
must be accomplished in almost real time, while visiting 0ach
of the lZ5 frequencies. Testing of noise and interf~ce
characteristic parameters as well as actual communication
quality parameters, the generation and grading of IMP and LQP
sounding signals, processing the synchronization acquisition
scheme, message block-encryptioD/decryption, secure protocol,
frequency assignments, etc., all these activities are computer
controlled.
..
Timing-and-process-control device 41 disLributes all
timing waveforms, stores and controls all initiali~ation data
serially inputted, through conductor 94 and remote corltrol
in~erface 48. .It receives the output of the non-linear-
- sequence-generator device 44. By means of an external loader
key variables are serially féd to the NLSG for the generation
of a random sequence which is used for digital encryption,
frequency translation and secure operation. The radio control
device 42 receives control data from the timing device 41 via
conductor 87, and couples frequency and SRND/REC control
information to the radio system.
~5
~ 'ront panel control module 13 provides a manually
operated interface and comprises a time~of-day display and
data indicator device 51, fl function switch 52 for testing,
initialization, time settinK, b~nd selection, etc., and a mode
switch 53 to select automatic/manual operation, one-way
transmission, etc.
A functional block diaKram of the frequency management
terminal is depicted in Fig. 5. It contains two functional
~5 groups: receiver group and transmitter group.
3.~3~
- 32 -
Functional modules numbered 601 to 610 are part of the
transmitter, while functional modules 6ll and 625 ~excluding
616 and 617) are part of the receiver. The ti~ning of the
teLminal originates from 616 which provides the required
clocks to control box (617), to processing ~615) and to the
modulator (608) and the demodulator (612) functions. Two
inputs are provided by the external radio receiver, namely,
tho received audio and the AGC. The audio is the iDpUt to ~.he
receiver where the time recovery (611) and the detection (6l2
functions are being performed. ~uxillary functions like ~re-
quency shi.ft corrections ~613) and pseudo -- BER measuremenL
(61~l) are also part of the receiver. The audio and the AGC
arc being monitored (604) and the lME' or LQP is generated
(605). Pollowing the sync pattern transmission (606) the TMP
L5 or LQP as a sounding message is being transmitted (607) through
the modulator (608). The frequency hopping of the radio un;ts
(receiver and transmitter) is boing controlled by the control
function (617). The sync search (618), acquisition (619) and
tracking (620) are perÇormcd in the receiver on the received
~0 data. The hop-sync of the receiver (621) is initialized
during the acguisition phase, while the crypto sync (622) is
initiated from the control and timing units, the key value and
time being loaded externally (623). The sync tracking unit
: (620) tracks the frequency keys following acquisition. Once
the sounding cycle has ended and the terminal receiver has
anulysed (62~) the sounding message, a decision concerning t.he
best frequcncy subset is pcrformed (625). This decision is
conununicated to the operator (human or automatic) via t.he
remole control I/O (627) and is displayed on the display
(626). Via the control panel (626) or remote control port, a
sel.f-test cycle can be initiali~ed (628), the results of which
are stored ~for further statistics) and communicated t.o t.he
operator as wel.l.
~LZ-3~928`
.
Any oE the functions descrihed herein, given t.he
teaching of the invention may be implemented by those skilled
in the art. Thus while a particular embodiment of the present
invention has been shown and/or describecl, it is aparent t.hat
changes and modifications may be made thereon without depar-
ting from the invention in its broadest aspects. The
foregoing Detailed Description is intended to be merely
exemplary and not restrictive.
