Real-time frequency management system for hf communication networks

REAL-TIME FREQUENCY MANAGEMENT SYSTEM FOR HF COMMUNICATION NETWORKS

Inventors: HAIM D. PAUL, Tel Aviv, Israel

Dr. JOSEPH M. PERL, Rishon־le-Zion, Israel

Assignee: TADIRAN ISRAEL ELECTRONICS INDUSTRIES LTD.

ABSTRACT

Disclosed is a frequency management system to enhance communication reliability over HF channels in the 2 to 30 MHz band. Real time channel monitori ng, evaluating, sounding and frequency allocating procedures are integrated into a single HF radio system using a single range of equipment. The best of available HF channels is automatically acquired by means of a single two-way exchange of unique sounding signals between members of an HF network.

THE PRIOR ART lonospherically 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 interferencefree, 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 readily 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 source 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 timesynchronized 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 overspecified.

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 addon 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. 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 'good¹ 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 using the same frequency hopping transmission.

Accordingly, the invention relates to a high-frequency (HF) frequencymanagement 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 a frequency-management processor means for:

-- continuously 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! for a 'quiet' channel or a binary 0 for a ^,noisy* channel (or vice versa), based on a predetermined set of criteria;

-- storing and updating the resulting binary word wherein each bit represents an 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 on each one of the scanned group of frequencies;

-- hard-labeling of each one of the said channels as either a bi nary1 for a ^,good' or 'acceptable¹, and a binary 0 .for a 'bad' or 'not-acceptable' communication quality (or vice versa), 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 channel s;

-- majority-detecting said redundant answer-back sounding message by the first, controlling station receiver processor;

-- performing link quality measurements by the controlling station receiverprocessor, on each one of the said scanned group of frequencies;

-- 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 relevant information 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;

The timing and control means comprise:

-- means for randomly selecting N channels from within a specified HF subband given its limits f_10w to f_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;

-- 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 AGO 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;

- ר 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 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;

-- means for quantizing the parameters: signal-to-noise-ratio and bit-errorrate, if desired 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 correspond!ng 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;

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.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete description of this invention may be had by reference to the accompanying drawings, illustrating a preferred embodiment of the invention to be described in detail, wherein:

Fig. 1 depicts three block diagrams illustrating three different configurations of HF communications systems in which the system of the present invention is integrated.

Fig. 2 outlines the format for sounding messages between net stations.

Fig. 3 outlines the format used in the network synchronization transmission cycle.

Fig. 4 is a simplified block diagram of the frequency management systern according to an embodiment of the present invention.

Fig. 5 is a functional block diagram of the system according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Before going into a detailed description of the figures a brief overview will be given describing the environment and general features of the systern.

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 ofoptimal 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 fj to f₂ of the HF spectrum, for any sized time period t^ to t₂ 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 nonlimitative manner.

This ensemble of 125 automatically pre-assigned frequencies constitutes the frequency management single, widewband , operati ng channel, kithin 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.

These 125 frequencies are continuously monitored by each of the net stations and the channel noise and interference is evaluated. The controllingstation initiates the sounding transmission. The sounding signal consists of local noise information analyzed at the controlling-station location. During the sounding cycle the control 11 ngstation 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 Fj to Fg then, for any subset of numbers Έ= >ημ...^mi25] randomly chosen from the set ׳R’ = 41,2..., where / 111a a 1 ^_ax — —Εζ._Ξ.Ε.Ι_ and £ being in MHz) .004

The corresponding frequency set is

The net controlled-station step synchronously with the controllingstation and performs transmission quality evaluations pertaining to each of the 125 channels. The controlled-station will sequentially respond by a repeat sounding cycle, scanning again all 125 frequencies. The sounding signal will now carry local reception quality information back to the controllingstation, again frequency-hopping over all 125 channels, in a random sequence. The controlling-station performs its own transmission 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 assign optimum operating HF frequencies to each communicating link.

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. A freqency management terminal would normally be closely integrated with the radio equipment.

In Fig. la 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 system is shown to comprise a remote control unit 22 and a processor 23 which is connected to another conventional but dedicated HF radio system. The two control units are interconnected by 25 to allow automatic frequency assignment and control. In this configuration the information channel 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 system channel which uses 125 frequencies operates simultaneously and continuously on a non-interference basis.

In Fig. lb the HF radio system 29 is shown to comprise separate HF transmitter system 30 and an HF receiver system 31. These may. be physically widely separated. The frequency management system, 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 system 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 receiver system 35 that serves both.

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 communication 3-kHz bandwidth. This monitoring process shall go on at all available times, by means of the HF system communication receiver. Each terminal 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 will be defined. Measurements will continuously indicate which of the eight thresholds has been crossed at each of the 125 3-kHz . channel s. 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 ineluding always a fixed, 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 interferencestate 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 measurewent pattern, is used as the primary sounding signal by the controlling terminal. A sounding transmission will comprise a single cycle of 125 pseudorandomly 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.

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 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 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.

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 1/2, or open when their BER = B < 1/2.

Under these simplifying assumptions if N = 2n+l 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:

<img file="IL67379A_D0001.tif" />

<img file="IL67379A_D0002.tif" />

N-H K

A further approximation takes into consideration two types of open channels: ^,Good', when the BER = B^ and 'Bad', when the BER = 10“^ = Bp

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| channels being 1^):

<img file="IL67379A_D0003.tif" />

K־FL+1-j-L

It should be emphasized that the above is derived under the assumption of uniformly distributed independent errors which is a fair assumption. Namely, the bits received on channel i are independent identically distributed (i.i.d) with respect to the same bits (in the message) received on channel j (j = i). This assumption would not be accurate under flat, very wideband fading conditions of extremely long duration (tens of seconds), but these conditions are rarely encountered.

One can evaluate the average bit error probability over a discrete

<td> distribution of channel</td><td> qualities</td><td> for the</td><td> ιι_Λopen</td><td> channels.</td><td> If a typical</td>

<td> channel distribution is as</td><td> follows:</td><td></td><td></td><td></td><td></td>

<td> BER: 1/4</td><td>¹־10</td><td>²־10</td><td>³־10</td><td>⁴־10</td><td>⁵־10</td>

<td> % of open channel s: 5</td><td> 10</td><td> 30</td><td> 40</td><td> 10</td><td> 5 .</td>

The average bit error probability after majority decoding for N=125

14־ and M=50 percent would be: 5x10 , 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+l channels and n-1 control bits),

PE = 1 - (1 - P^M)³ⁿ

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 nonlinear 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 poing 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 cond ר ti ons.

The synchronized pseudo-random sequence generators at all frequency management terminals 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 invention 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 systemchannel performance degradation in a digital communication system is the biterror-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:

log P_p = K + 10gP_E where P is the bit pseudo-error probability and P_r is the actual error r C.

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 Pp and P^.

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:

logPp = K + logPg- + where is a compensating factor whose value is derived from the burst error statistics of the HF channel.

When the sounding cycle has ended, the controlled-terminal receiver 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 .ζ 10³ or BER>׳

Following the Doppler correction and AGC settling, the receiver 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-terminal, C, initiate a sounding braodcast cycle. C uses as a sounding signal its most recent IMP. The control 1 ed-terminal, c, derives Cs IMP error-free, which provides it with the noise and interference levels measured at Cs 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:

<td> Channel</td><td> No. - - ------24</td><td> 25</td><td> 26</td><td> 27</td><td> 28</td><td> 29</td><td> 30</td><td> 31</td>

<td> C's IMP</td><td> (received) ---- 0</td><td> 0</td><td> 1</td><td> 0</td><td> 1</td><td> 1</td><td> 0</td><td> 1</td>

<td> c's IMP</td><td> (measured)---- 1</td><td> 0</td><td> 0</td><td> 0</td><td> 1</td><td> 0</td><td> 1</td><td> 1</td>

<td> c's LQP</td><td> Decisions ---- G</td><td> B</td><td> B</td><td> G</td><td> B</td><td> G</td><td> B</td><td> G</td>

<td> (G for </td><td>¹Good and B for Bad)</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td>

<td></td><td colspan="6"> From the above data c can immediately deduce</td><td colspan="2"> that:</td>

a.

b.

c.

d.

e.

f. 9.

h.

At the controlling terminal C, frequencies 24, 25, 27 and 30 are noisy. Frequencies 26, 28, 29 and 31 are quiet.

At the controlled terminal c, frequencies 25, 26, 27 and 29 are noisy, while frequencies 24, 28, 30 and 31 are quiet,

Reception quality was good at frequencies 24, 27, 29 and 31 and bad at frequencies 25, 26, 28 and 30.

At frequencies 27 and 29, although the channels were noisy at c's end, reception was good, probably because the signal overpowered the noise level.

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.

For transmission in the C-to-c direction, frequencies 24 and 31 may be a good choice.

For transmission in the c-to-C direction, frequencies 29 and 31 may be a good choice.

Depending on the nature and level of the noise at the receiver's end other frequencies may al so 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 allocatng function, over the communication link, may be assigned to the controlled terminal 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 terminal. This frequency allocation must be securely burst-transmitted to the controlling terminal to allow normal HF communications to proceed.

Following the reception of the sounding transmission cycle, the controlled terminal 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, the controlled terminal shall automatically construct a reporting message made up of just the one selected C-to-c communication frequency (till the next update).

When responding, only upon arrival at the time slot that coincides with the selected c-to-C frequency (in the FH sequence) will the controlled terminal 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. Controlling and controlled terminals have now automatically tuned their fixed-channel HF communication receivers and transmitters to the selected pair of operational frequencies.

Normally the system operation over an HF link will involve a two-way sounding process, with the controlling terminal assuming frequency assignment authority. Following the first C-to-c sounding cycle, the controlled terminal, having formed its Link Quality Pattern, 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. The controlling terminal 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 either a sounding broadcast bursttransmission or the current operating secure communications channel.

Before a terminal 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 and a certain agreedto cycle start-time is also set in. This is used with the actual time to determine automatically the elapsed time of the operation for frequencyhopping and key synchronization purposes. The actual time is acquired from a suitable reference external source having second-accuracy, such as coordinated universal time, 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 all set to within maximum +D (t+T) seconds of real time, where t is the system hop-time between frequencies and T is the system dwelltime 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 terminals automatically advance their set time-of-day by D time-slots in time. The terminals' 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 crisscross each other until acquisition is achieved and the search procedure ends.

The average waiting time (Tq) between meetings of the terminals during the search procedure is given by:

<img file="IL67379A_D0004.tif" />

4b + l

<img file="IL67379A_D0005.tif" />

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

Mq different frequencies, where:

<img file="IL67379A_D0006.tif" />

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+l)-(2D+2)-(2D+l)-...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-a.larm probabi 1 ities.

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:

<img file="IL67379A_D0007.tif" />

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^. When three suecessive 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.

Referring to Fig. 4 which is a block diagram of the frequency management 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 system's timing waveforms, and controls the sounding, and secure radio functions.

3. Computer Module 11, 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 interface controls and indicators.

From the.radio interface connector 15 the radio receiver AGC signal is fed through conductor 71 to the computer module 11. The received audio FSK signal is applied through conductor 73 to monitor filters 31 and R/T control deyice 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 initiates a sounding transmission, a SEND/REC-ON signal appears, through 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 dual bandpass filters 33 for signal shaping and improved isolation. Conductor 78 feeds the two FSK outputs to the radio modulator.

When the terminal reverts to the receiving mode, the 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 sensing the doppler frequency shift and applying an adaptive compensation 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, through conductor 85. The measured doppler shift as well as the processed corrections are transferred via conductor 82 to the computer module interface 22.

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 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.

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 keyvariables 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.

Front panel control module 13 provides a manually operated interface and comprises a time-of-day display 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-way transmission, etc.

A functional block diagram of the frequency management terminal is depicted in Fig. 5. It contains two functional groups: receiver group and transmitter group. Functional modules numbered 601 to 610 are part of the transmitter, while functional modules 611 and 625 (excluding 616 and 617) are part of the receiver. The timing of the terminal 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, the received audio and the AGO. The audio is the input to the receiver where the time recovery (611) and the detection (612) functions are being performed. Auxiliary functions like frequency shift corrections (613) and pseudo - BER measurement (614) are also part of the receiver. The audio and the AGO are being monitored (604) and the IMP or LQP is. generated (605). 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 (6 20) are performed in the receiver on the received data. The hop-sync of the receiver (621) is initialized during the acquisition 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 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, a self-test cycle can be initialized (628), the results of which are stored (for further statistics) and communicated to the operator as well.

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/or described, it is aparent 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.