Frequency management system

A high frequency (HF) frequency-management system for automatically selecting optimum HF frequency. A frequency management means controls the operation of a regular HF radio communications transmitter and receiver at each station. The frequency-management means transmits sounding signals synchronously and repeatedly over a finite group of HF frequencies, from a first radio communications station to a second radio communications station. Link quality evaluation (LQE) is carried out at the second station. Sounding signals are transmitted synchronously and repeatedly back to the first station. Optimum HF frequencies are selected based on the sounding signals and LQE. An HF communications path is automatically established between stations. The system performs the scanning of the HF frequencies, the detection and measurement of signals, noise and interferences on each frequency and the timing synchronization as required for the frequency management operation.

THE PRIOR ART 
Ionospherically propagated radio signals are frequently subjected to severe 
levels of amplitude and phase distortions from fading, multipath and noise 
phenomena as well as man-made interference effects. 
In attempting to maximize the availability and reliability of 
communications through the HF medium, it is now well recognized that the 
following two factors are predominant: 
1. The determination of the optimum propagating frequency for any selected 
path and time, and 
2. The validation that this selected channel is also interference free, 
primarily at the receiver's end. 
The most accurate means for specifying propagation conditions over a given 
HF circuit is attained through real-time oblique path sounding. The 
incorporation of real-time propagation data with accurate 
spectrum-interference data at a receiver provides the basis for a 
practical frequency management system. To be fully effective, however, HF 
frequency management would also require some means to rapidly disseminate 
recommended frequencies or spectrum information to multiple HF users. 
Moreover, this distribution of frequency assignments should be readly 
available, secure and not subject to the HF outages it is designed to 
avoid. 
The bandwidth that will support skywave communication between any two 
points is normally much less than the 28 MHz-wide HF spectrum. The 
available bandwidth changes cyclically on daily, annual, and eleven-year 
cycles and may be disturbed by unpredictable short-term effects. Frequency 
assignments are commonly made using forecasts based on the statistical 
variations of propagation expectancy cycles, the path and the frequencies 
available to the assigning authority. 
Interference may have components due to external causes (galactic, 
atmospheric or receiver noise) but it is actually man-made noise and 
particularly the widespread interferences from distant HF stations, that 
accounts for the main sources of errors in HF data communications. The HF 
bandwidth is heavily overcrowded especially at night and many observations 
have revealed that outages due to interferences from other users may 
exceed those due to propagation by a factor of five. Knowledge of the 
level of interference present in a communications channel is essential for 
channel optimization, as communications will take place on a channel 
showing the greatest value of signal-to-interference ratio. 
The most advanced HF frequency management system, typically consists of 
various combinations of three dedicated equipment items. Two of these 
items, an oblique sounder transmitter and sounder receiver 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 MHz 
band during the past 5-30 minutes. In a typical chirpsounder system the 
sounder transmitter sends a linear FM/CW test signal (2 to 30 MHz chirp) 
and is tracked by a time synchronized chirpsounder receiver at the other 
end of the communications path. 
Spectral analysis of the difference frequency between the sounder receiver 
local oscillator and the incoming signal yields a 
time-delay-vs-radio-frequency display. 
Specific conclusions with respect to the operational utilization of such a 
system indicate that: 
1. A close operational control and coordination is required between 
multiple users of the system. Simultaneous soundings requires careful 
transmitter synchronization. 
2. When the pool of assigned frequencies is not very large, the use of this 
system may prove to be counter-productive or over-specified. 
3. In a military environment, sounder transmitters have a very large, 
identifiable signature and must, therefore, be placed some distance from 
communication centers to minimize the risk of direction-finding, jamming 
or physical destruction. 
4. The simultaneous radiation of multiple sounding transmitters 
continuously scanning the entire 2 to 30 MHz band pollutes the HF 
spectrum, raises the RF noise floor and consequently self-jams friendly HF 
communication receiving equipment. 
5. The HF propagation path is not reciprocal, particularly with respect to 
the extraordinary modes. Resorting to two-way sounding per link will 
render the communication network operationally intractable and 
economically intolerable in view of the magnitude and high cost of such a 
system. 
6. Human judgement and analysis cannot entirely be replaced. Intelligent 
and experienced assessment of the dynamic ionogram is of paramount 
importance. This system requires the continuous intervention of a skilled 
operator. 
A radically different concept is therefore needed where a single add-on 
terminal controls and uses any standard modern HF communications equipment 
to automatically probe a large number of frequency channels within an 
assigned HF sub-band. It shall perform sounding, link quality evaluations, 
select and securely disseminate the best operating frequencies, to achieve 
rapid and reliable link connectivity. 
BRIEF DESCRIPTION OF THE INVENTION 
It is the primary object of this invention to provide a new real-time 
frequency management system which will permit the automatic selection of 
optimum operational frequencies in HF communication transmitters and 
receivers. It will establish communication links without the intervention 
of skilled operators, eliminate the need to resort to propagation 
predictions, and thus enhance the usefulness and reliability of high 
frequency communications systems. 
In accordance with the foregoing objects the invention herein is directed 
to frequency programmable HF communication systems which employ 
transmitters and receivers capable, in response to control signals, of 
remote tuning and scanning a plurality of channels. A high frequency 
communication network has one controlling station and a plurality of 
controlled stations. Means are provided for all stations to continuously 
monitor a large group of randomly selected frequencies within a given 
band, measure and analyze their noise and interference characteristics and 
hard-label each channel as either `noisy` or `quiet`, based on a set of 
criteria. The resulting binary word is used by the controlling station, in 
a preselected format, as the sounding message. Additional means are 
provided for the controlling station to redundantly broadcast the same 
sounding message sequentially over each one of the channels. During the 
sounding and the frequency dissemination cycles, the radio transmitters 
and receivers are synchronously hopped according to a pseudo-randomly 
coded sequence. 
The controlled station has the means for majority decoding the highly 
redundant sounding message and further means for measuring the link 
quality of each channel on which that message was received. The link 
quality analysis includes means for measuring bit-error-rates (BER), 
multipath delays, fading rates, interference levels and distributions and 
signal-to-noise ratios. 
The controlled station consequently generates another binary word in which 
each bit represents a hard-decision, based on a set of transmission 
quality criteria, as to whether the corresponding, scanned channel is 
accepted as `good` or `bad` for communications. This link quality pattern 
is now used by the controlled station as its answer-back sounding message. 
The controlling station majority decodes the repetitive sounding broadcast 
made by the controlled station while it synchronously sequences the entire 
group of frequencies. It performs its own link quality analysis and 
compares the data processed at both ends of the link. The controlling 
station now derives the optimal operating frequencies. The selected 
frequencies are then automatically disseminated usng the same frequency 
hopping transmission. 
Accordingly, the invention relates to a high-frequency (HF) 
frequency-management system with at least two stations, a controlling 
station and one or more controlled stations, each including an HF radio 
transmitter, HF radio receiver, a control unit for controlling the 
operation of the transmitter and receiver and frequency-management 
processor means for: 
monitoring the interference and occupancy of a finite plurality of HF 
channels, each channel tuned to a different frequency; 
hard-labeling of each one of the said channels as either a binary "1" for a 
`quiet` channel or a binary "0" for a `noisy` channel (or vice versa), 
based on a predetermind set of criteria; 
storing and updating the resulting binary word wherein each bit represents 
a channel occupancy evaluation of one of the frequencies visited; 
using this binary word as a sounding signal and transmitting this signal 
repeatedly, once over each of the said finite group of frequencies by 
having the transmitter scan said channels; 
synchronizing the remote station receiver so that it is sequenced through 
same said group of channels at an equal rate, being at each one of the 
channels at the same period of time as the transmitter, to allow the 
sounding message to be received; 
majority-detecting said redundant sounding message by the remote receiver 
processor; 
performing link quality measurements by the remote receiver processor 
during the reception of the sounding message on each one of the scanned 
group of frequencies; 
hard-labeling of each one of the said channels as either a binary "1" for a 
`good` or `acceptable`, and a binary "0" for a `bad` or `not-acceptable` 
communication quality based on another set of criteria; 
storing the resulting binary word at the remote station receiver-processor, 
to be used by it in forming the answer-back sounding signal; 
transmitting the answer-back sounding message repeatedly, once over each of 
the said group of channels by having the remote station transmitter scan 
said channels; 
majority-detecting said redundant answer-back sounding message by the 
first, controlling station receiver processor; 
performing link quality measurements by the controlling station 
receiver-processor, on each one of the said scanned group of frequencies 
during the reception of the back sounding (or reporting) message. 
selecting optimal frequencies by the controlling station processor, for 
reliable communications in both directions, controlling-to-controlled and 
controlled-to-controlling stations, based on the analysis of the received 
and derived link quality patterns; 
utilizing the synchronous frequency-hopping mode which is maintained 
between the stations, to disseminate frequency information by 
transmitting, over the selected optimal frequencies, the selected 
communication frequencies for the remote station; 
automatically tuning the communications transmitters and receivers to the 
selected preferred frequency or frequencies, to establish a reliable 
communication path between the stations; 
perform the communication by automatically performing the connectivity 
achieving function between any two net members, as a result of which these 
two members can communicate with each other on the best momentorily 
available frequency. 
The timing and control means comprise: 
means for randomly selecting N channels from within a specified HF sub-band 
given its limits f.sub.low to f.sub.high ; 
means for storing said N channels as alternate communication channels with 
each channel having a predetermined frequency; 
receive/transmit means for placing the station in a transmit mode; 
means 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 cycle operation, sounding 
cycle operation and signal processing algorithms; 
means for transmitting the sounding messages using an in-channel diversity 
of two FSK modulators-demodulators (or a multitone DPSK 
modulator-demodulator); 
means for generating a predetermined sequence based on the input of a key 
variable and real time of day. 
The noise and interference measurement means comprise: 
means for measuring the radio receiver AGC level and radio receiver noise 
output and distribution; 
means for measuring in-channel interference characteristics; 
means for classifying noise and interference present on the communication 
channel into a predetermined number of categories, according to a 
predetermined set of criteria; 
means for generating a corresponding number of binary words, each N-bit 
long, one for each category, wherein each bit represents a hard-decision 
qualifying each one of the N communication channels monitored. 
The link quality analysis means comprise: 
means for detecting noise representative of the noise present within the 
communication channel band as well as within two separate FSK channels; 
data detectors for providing a signal representative of the data levels 
that are present on the communication channel that the receiver is tuned 
to; 
means to measure the signal-to-noise ratio, the rms fading rate, and the 
rms multipath delay spread; 
means to use the demodulated and majority-detected sounding message to 
arrive at the actual bit-error-rate or predicted voice quality; 
means for quantizing the parameters: signal-to-noise-ratio and 
bit-error-rate, in combination with one or more of the parameters: fading 
rate, delay spread, channel noise, data levels, measured on the 
communication channel to define the desired predetermined number of link 
quality categories according to a predetermined set of criteria; 
means for generating a corresponding number of 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 where 
"good" channels are represented with a "1" and "bad" channels with a "0"; 
The advantages and further objects of the invention, and the means by which 
they are achieved may be best appreciated by referring to the detailed 
description which follows.

DETAILED DESCRIPTION OF THE EMBODIMENT 
Before going into a detailed description of the figures a brief overview 
will be given describing the environment and general features of the 
system. 
The operational situation typically assumes a network of HF radio users 
generally structured using a net controlling station in association with a 
plurality of widely scattered controlled stations, including relay 
stations. Each controlled net station is expected to continuously monitor 
the net traffic and to respond if polled by the controlling net station. 
Only one station at a time would then be transmitting, with transmit 
discipline being maintained by the controlling station. 
Centralized frequency management and control, with full frequency 
assignment authority would normally be the responsibility of the 
controlling station, within the single-net or the multi-net configuration. 
Operational configurations, however, using one-way transmissions only, 
utilize the capability of the present invention to assign the selection of 
optimal HF frequencies to the controlled terminal. 
The system according to the invention, can be used for the real-time 
management of HF communication networks having one controlling station and 
one or more controlled (or remote) stations. The system is adapted to 
provide a frequency management capability for any predetermined number of 
frequencies. Practical considerations show that generally, a number of 
from about 50 to about 150 frequencies provides a suitable system, 
depending on the conditions of use and the required speed and reliability. 
The operating sub-bands are chosen to have an adequate width for 
accomodating the predetermined number of frequencies, with an adequate 
spacing between the frequencies used. In the following, the invention is 
illustrated in an entirely arbitrary manner with reference to a system of 
125 channels. It ought to be understood that this is by way of example 
only, and that any reasonable and practical number of channels can be 
managed by such systems. 
As stated above, the invention is illustrated with reference to the system 
which provides a total capability of frequency managing a group of 125 
frequencies, randomly distributed between any sized sub-band f.sub.1 to 
f.sub.2 of the HF spectrum, for any sized time period t.sub.1 to t.sub.2 
of the day and night. The operating sub-band must be at least 500 kHz wide 
to accommodate 125 channels at 4 kHz spacing. Thus, the system can be 
programmed to process, excluding used or forbidden frequencies, the entire 
HF band all of the time or any smaller propagation windows of usable 
frequencies grossly predicted to be effective at certain corresponding 
time periods. It is to be clearly understood that this example is 
illustrative and ought to be construed in a non-limitative manner. 
This ensemble of 125 automatically pre-assigned frequencies constitutes the 
frequency management single, widewband operating channel. Within this 
channel information is time and frequency multiplexed, redundantly 
utilizing the `good` and the `bad` available frequencies. 
The system can be programmed to operate in either one band or two separate 
bands. 
1. One Frequency Band: One pair of frequencies shall define the limits 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. 
2. Two Frequency Bands: Four frequencies shall define two separate `DAY` 
and `NIGHT` operational bands, which may have overlapping 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 frequencies. 
A non-repeating, key-controlled permutation of numbers 0 to 125, determines 
the actual frequency locations and their transmission sequence within the 
defined operational bands. 
With two bands the system is actually processing 250 HF frequencies, during 
the 24-hour period. Alternatively, the 125 (250) discrete frequencies can 
be loaded via the front panel or the remote control loader. 
These 125 frequencies are continuously monitored by each of the net 
stations and the channel noise and interference is evaluated. The 
controlling station initiates the sounding transmission. The sounding 
signal consists of local noise information analyzed at the controlling 
station location. During the sounding cycle the controlling station scans 
through all of the 125 frequencies in a random sequence. 
The spacing between frequencies shall be in multiples of 4 kHz. Given a 
band F.sub.1 to F.sub.2 then, for any subset of numbers N={n.sub.1, . . . 
, n.sub.125 } randomly chosen from the set N={1, 2, . . . N.sub.max }, 
where 
EQU N.sub.max =(F.sub.2-F.sub.1)/0.004 
(F.sub.1 and F.sub.2 being in MHz) 
The corresponding frequency set is 
EQU {F.sub.ni }nieN, F.sub.ni =F.sub.1 +(ni/250)[MHZ]. 
The receiving net controlled-station steps synchronously with the 
transmitting controlling-station and performs channel quality evaluations 
pertaining to each of the 125 channels. The controlled station will 
sequentially respond by a back sounding cycle, scanning again all 125 
frequencies. The sounding signal will now carry local reception quality 
information back to the controlling-station, again frequency-hopping over 
all 125 channels, in a random sequence. The controlling-station performs 
its own link quality analysis and compounds it with the information 
received and processed through the sounding signal from the controlled 
station. 
A single two-way exchange of real-time sounding transmissions thus enables 
the controlling station to derive and reliably select optimum operating HF 
frequencies to each communicating link. Following the selection, an 
allocation cycle is initiated by means of which the best frequencies are 
allocated to the net. 
It is a central feature of this invention that the frequency management 
process acts as an automatic HF link control to achieve an adaptive 
channel and enhance communication reliability. 
FIG. 1 illustrates in a block diagram form three of the many other possible 
configurations of HF radio communications systems, incorporating the 
preferred embodiment of the present invention. The frequency management 
means would normally be closely integrated with the radio equipment. 
In FIG. 1a a conventional HF radio communications system is shown that 
includes a radio remote control unit 21, the HF transmitter/receiver 24, 
and a matching unit 26 to couple a narrowband antenna 29. This matching 
unit need not be used when a broadband antenna 27 is available. The 
frequency management means is shown to comprise a remote control unit 22 
and a processor 23 which is connected to another conventional but 
dedicated HF transmitter/receiver 20. The two control units are 
interconnected by 25 to allow automatic frequency assignment and control. 
In this configuration the information channel 21,24 is entirely 
independent from the frequency management system channel 22,23,20. The 
information channel 21,24 which normally uses one frequency (half-duplex) 
or two frequencies (duplex) will not be interrupted by the frequency 
management operation. The frequency management channel, which uses 125 
frequencies operates simultaneously and continuously on a non-interference 
basis. 
In FIG. 1b the HF radio system 29 is shown to comprise a separate HF 
transmitter system 30 and an HF receiver system 31. These may be 
physically widely separated. The frequency management means, however, uses 
only a dedicated but conventional HF receiver 32. The two receivers 
connect to a single receive antenna 37 through an antenna multicoupler 34. 
In this configuration the system shares the communications transmitter 
only, which is therefore used both for information transfer as well as 
frequency management transmissions. The frequency management receiver can 
thus uninterruptedly monitor the 125 channels. 
In FIG. 1c a single remote control unit 33 combines the communication and 
frequency management operations, audio and control, through the system 
processor 36 which connects directly to the HF radio transmitter/receiver 
35 that serves both. Since this configuration requires the least external 
equipment, it is the one implemented at present. 
As previously stated, each frequency management means in the net shall 
maintain a continuous evaluation of all 125 channels by examining the 
prevailing interference in the normal communication 3-kHz bandwidth. This 
monitoring process shall go on at all available times, by means of the HF 
system communication receiver. Each frequency management means shall 
sequentially scan the programmed list of 125 frequencies, continuously 
compiling and updating channel occupancy statistics. 
The number of available operational channels will depend first of all on 
the likelihood of finding any quiet frequencies (from the pre-assigned 
group) during all hours of the day and night, while the rate at which the 
channel must be evaluated will depend on the likely variability of the 
noise spectrum and propagation conditions with time. 
The term `quiet channel` generally implies a channel whose noise and 
interference level, inherently a variable quantity, only slightly exceeds 
some measured noise floor averaged within a limited bandwidth, or a fixed 
noise level that corresponds to a low-level signal induced into the 
antenna, or the threshold of atmospheric noise. 
However, the characteristics of the interference in the channel, depending 
on the traffic and mode of operation, will determine whether the channel 
could be expected to support an acceptable intelligibility of voice or an 
acceptable bit-error-rate. The power spectral density of interference from 
other HF users may be significantly non-white within HF voice channels. 
Low frequency CW, Morse Code or narrowband FSK may characterize an HF 
channel as `noisy`, while it may still support intelligible voice. 
According to the invention, a predetermined number of quantum states is 
defined, respective to a predetermined number of parameters, the main ones 
being signal-to-noise ratio and bit-error-rate, the others being channel 
noise, data levels, fading rate, delay spread. Advantageously, the 
parameters measured comprise at least the two main ones. These can be 
measured with one or more of the other parameters. Any combination of one 
of the main parameters with two or more of the other parameters can also 
be used. 
As stated above, the invention is illustrated with reference to a system of 
125 channels, and it is further illustrated with reference to eight 
quantum states classifying noise and interference present on the 
communication channel. 
Eight quantum states of noise and interference power/frequency distribution 
are defined. Measurements will continuously indicate which of the eight 
thresholds has been crossed at each of the 125 3-kHz channels. For each of 
these eight states a panoramic pattern will rapidly be formed, qualifying 
as `quiet` or `noisy` each one of the 125 channels that the entire net is 
currently evaluating. These patterns will be continuously updated 
throughout the monitoring periods. The labeling of a channel as `quiet` or 
`noisy` will represent a hard-decision, producing the best available 
choice and including always a fixed, a minimum number of the `quietest` 
channels in each pattern. With net stations dispersed over a wide 
geographic expanse, different interference conditions will be experienced 
at different locations, which will most likely result in a very different 
Interference Measurement Pattern (IMP). 
This IMP will thus constitute a sequence of binary measurements, 125 bits 
long, where each "1" or "0" corresponds to a hard-decision 
interference-state measurement. Each one of the monitored channels is 
labeled Quiet ("1") or Noisy ("0") at the terminal's location, based on 
the continuous monitoring and updating of channel occupancy statistics, in 
5 or 30 minute time-segments. Each bit position will correspond to the 
exact channel position in an automatically produced coded table of 125 
frequencies. 
The process of real-time HF channel selection normally involves a single 
two-way transmission exchange between a controlling frequency-management 
terminal and a controlled frequency-management terminal. However, reliable 
channel assessment may also be produced through a one-way transmission 
process. 
The IMP, the continuously compiled and updated interference measurement 
pattern, is used as the primary sounding signal by the controlling 
terminal. A sounding transmission will comprise a single cycle of 125 
pseudo-randomly selected HF frequency hops, repeating the same message in 
a burst of audio data, once every hop. During each successive frame period 
the same frequencies are visited but according to a different, 
non-repetitive PN-coded permutation, controlled by a non-linear sequence 
generator (NLSG). 
The identical, redundant sounding message will be sent over each one of 
these HF frequencies by means of noncoherent FSK, using 2-nd order in-band 
diversity, at a rate of 224 bits per second. The use of dual channel FSK 
contributes also to an increased correlation between assessed channel 
quality and voice quality. Alternatively, a multitone DPSK channel 
performs the same functions. 
FIG. 2 is a timing diagram of the selected burst format of the sounding 
frame 311+312+313 which has a hopping rate of 1/T hops per second. Each 
frame starts with a frequency time guard period 311 which is long enough 
to allow frequency change time, antenna match time and receiver AGC 
settling time. During the next time period 312, the receiver doppler 
correction loop (in the AFC circuit 35 in FIG. 4) utilizes the dual-FSK 
tones and filters to compensate for frequency drifts. The following time 
period 313 is devoted to the data block which consists of a total of 210 
bits. The first segment 420 of 64 bits each are the synchronization unique 
words used to provide frame sync. The next segment 421 of 24 bits is used 
for IDs of sender and destination. The following segment 422 of 125 bits 
accommodates the sounding message. Segment 423 of 3 bits indicates 1 of 8 
quality states to which the current sounding pattern belongs. The last 
segment 424 of 8 bits is the only one that varies with each frame as it 
indicates the frame number, from 1 to 125. The sounding message is sent by 
means of dual-FSK (or by multitone DPSK) transmissions at 224 bps and then 
125 times by hopping over each of the 125 channels. 
The sounding station (controlling or controlled) transmits its message on 
each frequency in turn, and all the remote receiving net stations being 
synchronized to the sounding station, repeatedly receive the identical 
message at each frequency. A unique majority-logic decoding algorithm 
(across the sounded frequencies) insures a very high probability of 
receiving all messages error-free, under extremely varying communications 
conditions. This capability of secure message transfer by redundant 
transmissions is a unique characteristic of the frequency management 
system embodied in the present invention, and its strength is shown by the 
following analysis. 
The same sounding message of N bits (N=125) is being received over N 
channels, each channel with its own bit-error-rate (BER). One can make a 
first approximation and classify HF channels as "blocked" when their 
BER.gtoreq.1/2, or "open" when their BER=B&lt;1/2. 
Under these simplifying assumptions if N=2n+1 is the number of tested 
channels, M of which are blocked, the probability of error in any one bit 
under an N/2 majority decision rule is: 
##EQU1## 
A further approximation takes into consideration two types of "open" 
channels: `Good`, when the BER=B.sub.2 and `Bad`, when the BER=10.sup.-1 
=B.sub.1. 
In addition to the M blocked channels, the proportion of the `Good` and 
`Bad` channels is known to vary considerably between day and night. During 
the day some 20 to 30 percent of the N-M channels may be considered `Bad` 
while during the night 40 to 70 percent of them may turn out to be `Bad`, 
on the average. 
Under these assumptions the probability of error in any one bit, after 
majority decoding is (the number of B.sub.1 channels being I.sub.1): 
##EQU2## 
One can evaluate the average bit error probability over a discrete 
distribution of channel qualities for the "open" channels. If a typical 
channel distribution is as follows: 
______________________________________ 
BER 1/4 10.sup.-1 
10.sup.-2 
10.sup.-3 
10.sup.-4 
10.sup.-5 
% of "open" channels 
5 10 30 40 10 5 
______________________________________ 
The average bit error probability after majority decoding for N=125 and 
M=50 percent would be: 5.times.10.sup.-14, namely, even under very severe 
conditions, with enough tested channels the bit error rate of the 
reference sounding message is remarkably low. 
Once the error probability for any one bit in the majority-decoded sounding 
message has been evaluated, one can evaluate the probability of receiving 
an errored sounding message, PE, and then the probability of receiving an 
exact sounding message which is given by 1-PE. For a total 3n bits of 
message (2n+1 channels and n-1 control bits), 
EQU PE=1-(1-P.sup.M).sup.3n 
Hence, this capability of secure acquisition of the sounding message, 
through utilizing the highly redundant transmission scheme, is a unique 
and a central aspect of the present invention. 
To maintain system synchronization all terminals must step their non-linear 
sequence generator (NLSG) clocks with their phases directly related to the 
transmitting terminal clock which takes the lead. The NLSG has several 
special features, in addition to the basic functions. It provides 
synchronizing or resynchronizing capability, the NLSG can be returned to a 
known starting point and then stepped to a predetermined point in time, in 
the process of initialization. 
The PR bit stream is based on a key-variable contained within the NLSG. The 
NLSG is programmable with respect to the variable in the sense that the 
current variable can be replaced with a new one as required, by means of a 
special external loader. A zeroizing function is also provided, should it 
become necessary to clear all stored data in the NLSG, under emergency 
conditions. 
The synchronized pseudo-random sequence generators at all frequency 
management means determine the same new frequency for each successive 
frame. The frequencies are selected from the string of bits generated by 
the NLSG each time the frequency is to be changed. 
During reception of the repeated sounding message, the system maintains an 
elaborate Link Quality Analysis, performing simultaneous measurements of 
all the parameters considered essential to the monitoring of communication 
traffic. Link quality analysis or in-band channel evaluation is a key 
process in enhancing HF frequency selection and communication system 
performance estimation/projection. 
The frequency management means incorporates advanced signal processing 
algorithms that permit measurements of all essential parameters within the 
time constraints imposed by the time-varying HF channel. A fundamental 
measure of system channel performance degradation in a digital 
communication system is the bit-error-rate (BER). In the period of time 
that the HF channel transfer function may be approximted as 
quasi-stationary, it is commonly difficult to accumulate sufficient bit 
errors to characterize near-instantaneous data performance. The invention 
uses a modified approach of error rate extrapolation based upon 
pseudo-errors (PBER) to estimate the probability of error in a very short 
time. Pseudo errors may be generated by modifying the gain or phase 
threshold criterion in the error decisions process to obtain parameters 
which indicate apparently greater circuit degradation than really exists. 
The measurement period is shortened since the pseudo-error is designed to 
be larger than the corresponding actual error rate. The basic idea is that 
by narrowing the "good detection region" and widening the "error detection 
region" one measures a higher BER than the actual BER of the detector. As 
a result, high accuracy low BER values can be measured from (a) small data 
samples, and (b) without actually knowing the transmitted data. The PBER 
and the actual BER are related as follows: 
EQU log P.sub.P =K+log P.sub.E 
where P.sub.P is the bit pseudo-error probability and P.sub.E is the actual 
error probability. 
If the transmitted data is known, or derived from majority decoding of 
repetitive soundings, as is done in this invention, one can "scale the 
channel" by calculating K out of measured P.sub.P and P.sub.E. 
It should be emphasized that the above was developed mainly for linear, 
additive noise type fading channels. Since this is not always the case 
with the HF channel, a certain correction should be made which will 
account for this discrepancy. The channel BER model can be rewritten as: 
EQU log P.sub.P =K+log P.sub.E +K.sub.1 
where K.sub.1 is a compensating factor whose value is derived from the 
burst error statistics of the HF channel. 
When the sounding cycle has ended, the frequency management means has at 
its disposal a wide variety of information about each of the N sounded 
channels. The present invention takes advantage of its unique capability 
to fully recover the sounding message bit-sequence. This original message 
is used for error counting, PBER and BER determination. 
Measurements are performed also of the rms multipath delay spread, the 
fading rate, interference levels and distribution, and SNR. This data is 
processed and updated with every additional sounding transmission. A very 
reliable characterization of the HF communication channel results. 
Knowledge of the channel conditions and parameters enables the prediction 
of channel performance at high data rate transmissions based on low rate 
data transmissions. 
Tested channels are also ranked for various uses: voice, multi-tone DPSK 
modem, wideband FSK, narrowband FSK, etc. The intended operational use 
clearly affects the link quality determination since interferences have 
different effects in different applications. 
Based on the link quality analysis and operational mode, hard decisions are 
made by the receiving terminal, qualifying as "Good" or "Bad" each one of 
the 125 channels tested. A binary Link Quality Pattern (LQP) of 
transmission performance measurements is generated, where each one of the 
tested channels is labeled "1" (Good) or "0" (Bad). "1" to indicate an 
acceptable channel and "0" to indicate an unacceptable channel. 
Acceptability is determined based on eight quantum states of performance 
characterizing eight separate link quality patterns. These LQPs represent 
the best available choice and include always a fixed, minimum number of 
the `best` channels in each pattern. For example, in the limiting case, 
with all other measured parameters equal, "Good" or "Bad" may indicate, 
say, BER&lt;10.sup.-3 or BER&gt;10.sup.-3. 
Following the Doppler correction and AGC settling, the receiving frequency 
management means must perform the following functions: 
a. Recover clock timing for bit detection; 
b. Recognize the frame-sync unique synchronizing word to establish the 
basic frame timing reference and identify net number; 
c. Identify the ID patterns. These bits enable the receiver to verify the 
validity and legitimacy of the received burst; 
d. Accept the remaining portion of each message. Arrive at the correct IMP 
or LQP and process the necessary tests, evaluations and decisions; 
To illustrate the system's operation, let the controlling frequency 
management means, C, initiate a sounding broadcast cycle. C uses as a 
sounding signal its most recent IMP. The controlled frequency management 
means, c, derives C's IMP error-free, which provides it with the noise and 
interference levels measured at C's location, in each of the 125 channels 
monitored. In addition, c carries out 125 LQP tests, during the sounding 
cycle, to sort out the "Good" channels. The final results can be tabulated 
as in the following simplified example: 
______________________________________ 
Channel No. 24 25 26 27 28 29 30 31 
______________________________________ 
C's IMP (received) 
0 0 1 0 1 1 0 1 
c's IMP (measured) 
1 0 0 0 1 0 1 1 
c's LQP Decisions 
G B B G B G B G 
______________________________________ 
(G for "Good" and B for "Bad") 
From the above data c can immediately deduce that: 
a. At the controlling frequency management means C, frequencies 24, 25, 27 
and 30 are noisy. Frequencies 26, 28, 29 and 31 are quiet; 
b. At the controlled frequency management means c, frequencies 25, 26, 27 
and 29 are noisy, while frequencies, 24, 28, 30 and 31 are quiet; 
c. Reception quality was good at frequencies 24, 27, 29 and 31 and bad at 
frequencies 25, 26, 28 and 30; 
d. At frequencies 27 and 29, although the channels were noisy at c's end, 
reception was good, probably because the signal over-powered the noise 
level; 
e. At frequencies 28 and 30, although the channels were quiet at c's end, 
reception was bad, probably because of no propagation or a very low 
signal; 
f. For transmission in the C-to-c direction, frequencies 24 and 31 may be a 
good choice; 
g. For transmission in the c-to-C direction, frequencies 29 and 31 may be a 
good choice; 
h. Depending on the nature and level of the noise at the receiver's end 
other frequencies may also be considered when the expected received signal 
level can be estimated. 
In applications where, most of the time, only one-way transmissions are 
conducted (C-to-c), the frequency changing or allocating function, over 
the communication link, may be assigned to the controlled frequency 
management means c. Operation relies on just the one-way sounding 
broadcasts (C-to-c). A reliable and rapid decision will be made, 
determining the best pair of transmit/receive operational frequencies, for 
communication with the controlling frequency management means. This 
frequency allocation must be securely burst-transmitted to the controlling 
frequency management means to allow normal HF communications to proceed. 
Following the reception of the sounding transmission cycle, c will wait a 
fixed number of time slots before attempting to respond to allow its 
communication transmitter time to tune to the frequency chosen for the 
c-to-C transmission. In preparation to respond, c shall automatically 
construct a reporting message made up of the selected C-to-c communication 
frequencies (till the next update). When responding, only upon arrival at 
the time slots that coincides with the selected c-to-C frequencies (in the 
FH sequence) will c turn-on its transmitter RF power for a 
burst-transmission of this message. This will automatically reveal to C 
the selected c-to-C frequency. c and C have now automatically tuned their 
fixed-channel HF communication receivers and transmitters to a selected 
pair of operational frequencies. 
Normally the system operation over an HF link will involve a two-way 
sounding process, with the C assuming frequency assignment authority. 
Following the first C-to-c sounding cycle, having formed its Link Quality 
Pattern, c automatically responds with a c-to-C sounding broadcast. Again, 
within the single frame of 125 frequency hops, c's LQP sounding message 
will be repeated once every hop. This two-way sounding process will take 
less than 5 minutes. 
C will now be looking at two LQPs which provide simultaneously the measured 
communication performance at both ends of the link and, therefore, enables 
a straightforward selection of optimal operating frequencies. The 
dissemination of frequency information will be conducted using a sounding 
broadcast burst-transmission followed by an acknowledge cycle by the net 
members. 
Before the frequency management means can be used in an actual exchange of 
signals, some preparatory operation is required. Necessary data must be 
entered and stored: the frequency band or bands to be used, operational 
modes, IDs of net sender destination, key-variables, initial operating 
frequencies, longest frequency change time for the transceiver, and a 
certain agreed-to cycle start-time is also set in. This is used with the 
actual time to determine automatically the elapsed time of the operation 
for frequency hopping and key synchronization purposes. The actual time is 
acquired from a suitable reference external source having 
minutes-accuracy, such as an electronic watch, a count down over voice 
radio, etc. 
To ensure proper net initiation under seach mode conditions, when a new 
number joins the net or transmissions have not taken place for many hours, 
a special Synchronization Cycle is provided. During this cycle a unique 
sync message, broadcast by the controlling terminal, is repeated once over 
each of the 125 channels. 
FIG. 3 illustrates a simplified transmission timing diagram of the Sync 
Cycle. In a typical frame, the first and last 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 field the blocks 502, 503 and 504 of 96 bits comprise three 
eight-bit characters, alpha or numeric, devoted to the sender's 
ID+Destination, repeated three times. 
Following initialization, which includes loading the terminal's NLSG with 
the common key, net synchronization is rapidly achieved if time-of-day 
internal clocks are wall set to within maximum +D (t+T) seconds of real 
time, where t is the system hop-time between frequencies and T is the 
sysem dwell-time at each frequency. The order in which the system is 
sequenced through the group of channels, is controlled by the NLSG's 
output. 
Upon entering the search mode, the frequency management means automatically 
advance their set time-of-day by D time-slots in time. The NLSGs are 
therefore forced to be within (0, 2D) time-slots ahead of the real time of 
day. 
The search receiver will be taking unequal steps, jumping always ahead of 
the sounding transmitter, and waiting for the transmitter to arrive. The 
receiver waits 2D+1 time-slots on its present frequency, then jumps ahead 
2D frequencies, then waits again 2D+1 time-slots, then jumps ahead 2D+2 
frequencies, waits another 2D+1 time-slot, then jumps ahead again 2D 
frequencies, etc. 
Following an initial shift of +D time-slots, and assuming t=0 and T=1 
second, the optimal search procedure is: wait=2D+1 and search=at 2D, then 
at 2D+2. As a result of this search pattern, the controlling-terminal and 
the controlled-terminal meet on various frequencies, in other words they 
criss-cross each other until acquisition is achieved and the search 
procedure ends. 
The average waiting time (T.sub.D) between meetings of the terminals during 
the search procedure is given by: 
##EQU3## 
The maximum waiting time until the first meeting for a given D, is T 
max=2D. 
During the synchronization period, the terminals meet on an average of 
M.sub.D different frequencies, where: 
##EQU4## 
where N is the number of assigned frequencies, N=125. 
Frame synchronization exploits the systematic nature of the search 
detection process to realize a very reliable and rapid frame-sync 
recovery. 
A digital correlator will detect arriving frame sync sequences and full 
utilization will be made of the so-called window technique. This method 
takes advantage of the fact that the sync sequences are periodic and that 
legitimate correlator outputs will have to be spaced in time according to 
the (2D)-(2D+1)-(2D+2)-(2D+1)- . . . pattern. Acquisition will be declared 
after detection of 3 sync sequences. The detection thresholds will 
determine the average synchronization time, as well as the max. sync. time 
for specified miss/false-alarm probabilities. 
If the probability of detecting a sync sequence on a channel is Ps, the 
average probability of detecting 3 consecutive syncs at proper spacings 
is: 
EQU P.sub.3 =(M.sub.D -2)P.sub.s.sup.3 (1-P.sub.s).sup.M.sbsp.D.sup.-3 
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 channel 
BER. 
Hence, the average synchronization time is P.sub.3.sup.-1. When 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 receiver is in full synchronism with the 
transmitter and hops with it at the regular rate. The frame-sync detector 
maintains a continuous tracking process and monitors the end of the 
sounding transmission. 
This unique synchronization algorithm is another important aspect of this 
invention. 
INVENTION IMPLEMENTATION 
Referring to FIG. 4 which is a block diagram of the frequency management 
means, it is shown to comprise four major modules: 
1. Analog Module 12, which includes the terminal's data link and basic 
sensors used for signalling in calling and channel monitoring; 
2. Process Control Module 14, which generates the timing waveforms, and 
controls the sounding, and radio functions; 
3. Computer Module 11, which is responsible for the signal processing, 
channel quality analysis and overall system control; 
4. Front Panel Control Module 13, which includes all the operator's manual 
interface controls and indicators. 
When not communicating or performing active sounding or calling functions, 
the frequency management means perfoms channel monitoring and calls 
searching. From the radio interface connector 15 the radio receiver AGC 
signal (s-meter) is fed through conductor 71 to the computer module 11. 
The received audio is passed through conductor 73 to monitor filters 31 
and bi-directional analog gate or R/T control 32. The monitor filters 31 
are examining discrete segments in the 300-3300 audio band and signals 
present are delivered to the processor module 11 via conductor 72. These 
audio signals are A/D converted in the interface 22 and processed by the 
microprocessor 21. The microprocessor evaluates the spectra density, noise 
level and occupacy profile of each monitored frequency. These channel 
parameters are stored and updated in the memory 21. 
When the frequency management means initiates a sounding transmission, a 
signal appears, through conductor 74, at the input of device 32, which 
reverses the direction of the analog signal flow. Out of the computer 
module 11 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 dual bandpass 
filters 33 for signal shaping and improved isolation. Conductor 78 feeds 
the combined FSK outputs to the radio modulator via the radio interface 
15. During the sounding, the monitoring is stoped, device 32 blocking the 
reception path. 
When in receiving mode, device 32 is placed in receive mode by conductor 74 
and it routes the two FSK signals received from the radio demodulator via 
the radio interface 15, 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 sounding or calling digital message is 
fed through conductor 81 to the processor module 22. 
The automatic frequency control device 35 provides a means of sensing the 
doppler frequency shift and applying an adaptive compensation to improve 
the bit detection capability of the FSK demodulators. In fact, instead of 
FSK signalling multitone DPSK signalling is also available, for which 
frequency shift correction is necessary. To help synchronize the local 
clock to the incoming digital burst, the bit synchronizer device 36 
continuously interacts with the central timing source 41, through 
conductor 85. The measured doppler shift as well as the processed 
corrections are transferred via conductor 82 to the computer module 
interface 22 for link quality evaluation. 
Under program control a multiple of unique algorithms and functions are 
simultaneously being processed in the microcomputer module 21. These deal 
with the rapid signal measurements, evaluations, and frequency management 
decisions that must be accomplished in almost real time, while visiting 
each of the 125 frequencies. Testing of noise and interface 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-encryption/decryption, 
secure protocol, frequency assignments, etc., all these activities are 
computer controlled. Specifically, the sampled audio is used by the 
microcomputer to measure the signal to noise ratio (SNR) of the received 
sounding signal. This is done by cummulatively summing the signal power in 
various bands of the spectrum. The spectral monitoring is performed by an 
FFT routine performed by the microcomputer 21. The frequency slots that 
are known to carry signalling are summed up and compared to the remaining 
frequency slots. Thus, an estimate of the SNR for each sounded channel is 
obtained and updated with a "fading memory". Two major parameters of each 
channel that are also estimated by the microcomputer 21 are the rms fading 
rate and the rms multipath delay. Using the outputs of consecutive FFT 
outputs V.sub.q,m these two parameters are evaluated by a "time and 
frequency sampling" of the tested HF channel. The employed algorithms 
perform differentiation in time and frequency when evaluating the 
estimated fading rate D and multipath delay M: 
##EQU5## 
Timing-and-process-control device 41 distributes all timing waveforms, 
stores and controls all initialization data serially inputted, through 
conductor 94 and remote control interface 48. It receives the output of 
the non-linear sequence-generator device 44. By means of an external 
loader key variables are serially fed 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 SEND/REC control information to the radio system, which 
enables the radio hopping through the plurality of HF frequencies for 
monitoring and sounding/calling purposes. 
Front panel control module 13 provides a manually operated interface and 
comprises a time-of-day display frequency, address and data indicator 
device 51, a function switch 52 for testing, initialization, time setting, 
band selection, etc., and a mode switch 53 to select automatic/manual 
operation, one/two way sounding, etc. 
A functional block diagram of the frequency management system is depicted 
in FIG. 5. It contains two functional groups which are the receiver group 
and the transmitter group. Functional modules numbered 605 to 610 are part 
of the transmitter, while functional modules 601 to 604 and 611 to 625 
(excluding 616 and 617) are part of the receiver. The timing originates 
from 616 which provides the required clocks to thr various functions. Two 
inputs are provided by the external radio receiver 650, namely, the 
received audio and the AGC. The audio is inputed to the receiver group 
where the bit synchronization. (611) and the detection (612) functions are 
being performed. Auxillary functions like frequency shift corrections 
(613) and pseudo-BER measurement (614) are also part of the receiver 
group. The audio and the AGC are being sampled (603) and monitored (604) 
and the IMP or LQP is generated (605). In the transmitter group, following 
the sync pattern transmission (606) the IMP or LQP as a sounding message 
is being transmitted (607) through the modulator (608). The frequency 
hopping of the radio units (receiver and transmitter) is being controlled 
by the control function (617). The sync search (618), acquisition (619) 
and tracking (620) are performed in the receiver group on the received 
data. The hop-sync of the receiver (621) is initialized during the 
acquisition phase, while the crypto (secure) sync (622) is initiated from 
the timing unit (616), the key value and time being loaded externally 
(623). The sync tracking unit (620) tracks the frequency hopping pattern 
following acquisition. Once the sounding cycle has ended and the receiver 
has analysed (624) the sounding message, a decision concerning the best 
frequency subset is performed (625). This decision is communicated to the 
operator (human or automatic) via the remote control I/O (627) and is 
displayed on the display (626). Via the control panel (626) or remote 
control port (627), a self-test cycle can be initialized (628), the 
results of which are stored (for further statistics) and communicated to 
the operator as well. 
SUMMARY 
Based on the above descriptions the frequency management system can be 
summed up as follows: The system uses frequency management means at each 
net member connected to the local HF transceiver (FIG. 3c). At each site 
the radio frequency is controlled via the radio interface connector (FIG. 
4, unit 15) by the radio control unit (FIG. 4, unit 42). The radio audio 
I/O is connected to the analog module (FIG. 4, unit 12), the PTT to the 
radio being activated from the same unit when transmission is required. 
When not transmitting the radio is hopped on a predetermined set of 
frequencies monitoring the channel activity level and searching for 
incoming calls (FIG. 5, unit 611-616). The accumulated channel occupancy 
statistics (IMP) is used when sounding is performed for choosing the best 
subset of communication frequencies. As long as the current set of 
communication frequencies is of satisfactory quality, no sounding is 
required. When the communication quality deteriorates, the controlling 
station performs a sounding cycle. During this cycle, its IMP is 
transmitted as a sounding message (FIG. 5, units 605-610). This message, 
after synchronization, relates to the controlled station the channel 
occupancy statistics at the controller while the propagation qualities of 
each sounded frequency are measured by the controlled station. Thus, a 
Link Quality Pattern (LQP) is established at the controlled site (FIG. 5, 
unit 624). In the one-way sounding mode (set by the panel--FIG. 4, unit 
53), the controlled station chooses the best new set out of the tested 
frequencies (FIG. 5, unit 625) and allocates them to the net (including 
the controller). The signal processing and some of the decisions are 
performed by a microprocessor such as the TMS-32010 type micro-computer 
(FIG. 4, unit 21), while the actual real-time operation is controlled by 
the timing and control unit (FIG. 4, unit 41) which contains a number of 
standard programable counters and logic arrays. In order to protect the 
operation sequence against jamming, the hopping sequence as well as the 
transmitted data (FIG. 2) are advantageously encrypted by an NLSG sequence 
(FIG. 4, unit 44). In order to enable the connection to any HF radio, the 
"personality module" (FIG. 4, unit 42) contains an additional 
microcomputer as well (such as the Intel 8031 type). The remote control 
unit (FIG. 4, unit 48), which enables remote control operation of the 
frequency management means, shares some of the features of the 8031 which 
is located on the same module (FIG. 4, unit 14). In order to start the 
operation sequence, a key-loader or the front panel (FIG. 4, unit 52) is 
used (FIG. 5, unit 623) to load the initialization variables: time (TOD), 
crypto K, sounding range or sounding frequencies, communication 
frequencies, forbiden frequencies, addresses (net-members), function 
(controller, or controlled, or passive member) and frequency transition 
time of the slowest transmitter. Finally, the frequency management means 
contains a comprehensive set of on line and off-line self-tests (FIG. 5, 
unit 628), whose results are displayed on the front panel (FIG. 4, unit 51 
and FIG. 5, unit 626) and reported via remote control (FIG. 5, unit 627). 
Any of the functions described herein, given the 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 described, it is 
apparent that changes and modifications may be made thereon without 
departing from the invention in its broadest aspects. The foregoing 
Detailed Description is intended to be merely exemplary and not 
restrictive.