United States Patent [19]
`Wilkinson
`
`Patent Number:
`[11]
`[45] Date of Patent:
`
`4,532,636
`Jul. 30, 1985
`
`[73] Assignee:
`
`[54] RADIO COMMUNICATIONS RECEIVERS
`[75] Inventor: Dent P. Wilkinson, Hampshire,
`England
`The Marconi Company Limited,
`England
`[21] Appl. No.: 389,905
`[22] ‘ Filed:
`Jun. 18, 1982
`[30]
`7
`Foreign Application Priority Data
`
`Jun. 22, 1981 [GB] United Kingdom ............... .. 8119215
`
`[51] Int. CLJ ............................................. .. H04B 1/10
`[52] US. CL- ...................................... .. 375/1; 375/104;
`455/304; 455/312
`[58] Field of Search ................... .. 360/381, 41; 375/1,
`375/102, 104; 371/31, 65, 69; 455/303, 304,
`312; 358/314
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`2,996,576 8/1961 1 Dolby ...... ..
`
`i .................. .. 360/381
`
`3,347,984 10/1967 Holmberg
`3,688,039 8/1972 Ishiguro ..... ..
`..... .. 371/31
`455/312
`3,700,812 10/1972 Springett .
`360/38.1
`3,824,620 7/1974 Langer
`3,947,636 3/1976 Edgar ................................ .. 328/163
`
`360/381
`
`371/31
`4,291,405 9/1981 Jayant et a1.
`.1 360/381
`4,376,289 3/1983 Reitmeier et al.
`4,398,296 9/1983 Gott et a1. ............................. .. 375/1
`
`FOREIGN PATENT DOCUMENTS
`
`2916127 11/1979 Fed. Rep. of Germany .
`2297524 8/1976 France .
`
`Primary Examiner-—Benedict V. Safourek
`Attorney, Agent, or Firm—Kirschstein, Kirschstein,
`Ottinger & Israel
`
`ABSTRACT
`[57]
`In radio communications receivers for use on frequency
`hopping radio communications networks, the received
`signal in each hop period is analyzed and assigned a
`quality value. If more than one signal is present during
`the hop period it is determined that more than one net
`work has changed to the same frequency. Similarly if
`the received signal is out of synchronism it is deter
`mined that more than one network has changed to the
`same frequency. When more than one network has
`changed to the same frequency, the received signal is
`not output by the receiver but is replaced by an earlier
`(or later) received signal.
`
`8 Claims, 4 Drawing Figures
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`U.S. Patent Jul. 30, 1985
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`U.S. Patent Jul. 30,1985
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`Sheet 2 of 2
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`1
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`RADIO COMMUNICATIONS RECEIVERS
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`15
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`The present invention relates to radio communica
`tions receivers and in particular to such receivers for
`use in frequency hopping radio communications net
`works.
`One technique of overcoming deliberately intro
`duced radio interference signals (jamming) in a radio
`communications network is to change the frequency on
`which the transmitters and receivers operate at periodic
`intervals. When such frequency changes occur at peri
`odic intervals of the order of milliseconds with each
`frequency change occurring by the transmitter and
`receiver in synchronism with each other the technique
`provides some con?dentiality to the transmissions as
`well as some immunity to jamming. This method of
`operation is referred to herein as “frequency hopping”.
`When frequency hopping communications networks
`are in use with several groups of radio sets using a simi
`lar set of frequency channels interference between
`groups of sets hopping in pseudo random fashion may
`occur due to two of the groups hopping to the same
`channel simultaneously. Such frequency coincidences
`will usually generate an interference burst of one “hop”
`period. When the network is being used for voice com
`munication with a hop period of, say, a few milliseconds
`the interference bursts are characterised by a noise
`background of apparent clicks superimposed on a voice
`message. As the number of groups of radio sets using
`the frequency channels available increases the probabil
`ity of two or more groups selecting the same channel
`increases and consequently the intelligibility of voice
`communications in the networks decreases.
`In practical tests it has been determined that if 25% or
`greater obliteration of the voice signal waveform oc
`curs the voice communication is no longer intelligible.
`Without special processing of the voice signal the intel
`ligibility limit is reached when approximately N/ 3
`groups of radio sets are independantly hopping between
`the same N frequency channels.
`It is an object of the present invention to provide
`radio communications apparatus in which the tolerable
`level of signal obliteration is increased.
`According to the present invention a radio communi
`cation receiver for use in a frequency hopping radio
`communication network comprises store means to store
`a representation of a received signal for a period of T2;
`selection means which is responsive to a control signal
`selectively to output either a currently received signal,
`or the stored representation from said store means of a
`received signal received T2 earlier; and signal quality
`measurement means which determines from at least one
`characteristic of the received signal in each of a succes
`sion of said periods Tl within those periods, whether
`the received signal comprises more than one transmit
`ted signal and, if so, forwards said control signal to said
`selection means such that, for any of the periods T1 in
`which the received signal has a low signal quality value,
`an alternative signal is output by the receiver.
`Preferably the period of T2 is in the range of eight to
`ten milliseconds and the means to store is a delay line.
`The received signal may be delayed by a period of T1
`before being selected for output and for storage by the
`delay line.
`In a preferred embodiment the received signal is
`stored in a delay line having a plurality of tappings at
`intervals of T2 and the tapping having the highest of
`
`4,532,636
`2
`said signal quality values is selected as the output signal
`if the received signal quality value is less than a prede
`termined value.
`The received signal may be delayed by an additional
`period of multiples of T2 so that a later received signal
`may be selected as the output signal if the quality values
`so require.
`Radio communications apparatus in accordance with
`the invention will now be described with reference to
`the accompanying drawings of which:
`FIG. 1 is a block schematic diagram of a frequency
`hopping radio receiver;
`FIG. 2 is a block diagram of a signal processing cir
`cuit for use in the apparatus;
`FIG. 3 is a block diagram of an alternative form of
`signal processing circuit for use in the apparatus; and
`FIG. 4 is a block schematic diagram of a quality
`assessment circuit used in the processing circuits of
`FIGS. 2 and 3.
`It will be appreciated that speech waveforms com
`prise a basic frequency on which a number of harmonics
`are superimposed. The average pitch period of the basic
`frequency of an adult male speaker has been determined
`to be in the range from eight to ten milliseconds.
`It is convenient to note that frequency hopping radio
`communications apparatus may change frequency (hop)
`at intervals of less than the average pitch period previ
`ously referred to.
`For the avoidance of doubt it is noted that the period
`between each change of frequency is referred to herein
`after as a “hop period”.
`For the purpose of description the signal arriving at a
`radio receiver in a hop period in which two or more
`radio communications sets are transmitting on the same
`frequency is referred to hereinafter as a “corrupt hop”.
`Similarly the signal arriving at a radio receiver in a hop
`period in which only an associated group of radio com
`munications sets have switched to the channel is re
`ferred to as a “valid hop”.
`Referring to FIG. 1 radio signals received at an aerial
`20 of the typical frequency hopping receiver are ampli
`fled by a radio frequency ampli?er 21 and the selected
`radio signal is converted to an intermediate frequency
`by mixing in a converter/oscillator 22. The intermedi
`ate frequency signal is ampli?ed by an intermediate
`frequency ampli?er 23 before being applied to a dis
`criminator 24 and an audio ampli?er 25.
`As the frequency of the wanted signal from the aerial
`20 changes (hops) at periodic intervals a channel con
`trol circuit 26 causes the frequency of the oscillator 22
`to change in a pseudo-random manner in synchronism
`with channel changes of the transmitted signal. A chan
`nel control circuit suitable for use as the channel control
`circuit is described in co-pending patent applicaton Ser.
`No. 386,296 ?led June 8, 1982.
`Since in the present system the received signals repre
`sent digitally encoded speech signals a clock reconstitu
`tor circuit 27 is used to extract the digital data which is
`then stored in a signal processing circuit 28. Digital data
`selected by the circuit 28 as hereinafter described is
`converted to analogue form by a digital to analogue
`converter 29 before ampli?cation by the ampli?er 25.
`Referring now to FIG. 2 a digital signal received by
`the radio communications set is demodulated and fed to
`an input 1 of the signal processing circuit 28 (of FIG. 1).
`The signal is delayed in a delay line 2 for a period of T1
`which is one hop period. A quality measurement circuit
`3 determines from the strength and synchronisation of
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`the received signal on the lead 1 a value representing
`the average quality of the signal received in the immedi
`ately preceding period T1.
`The quality measurement value determined by the
`quality measurement circuit 3 is compared by a compar
`ator 4 with a threshold signal supplied on a connection
`5. If the quality value exceeds the threshold signal value
`the signal from the delay line 2 is switched through an
`electronic changeover circuit 6 to an output line 7
`which is connected to the digital to analogue converter
`29 (of FIG. 1).
`If the quality value does not exceed the threshold
`signal value the comparator 4 provides a signal on lead
`10 to cause the changeover circuit 6 to switch to pass a
`previously received signal to be fed from a lead 9 from
`a second delay line 16 to the output line 7 until the
`average quality of the signal determined by the quality
`measurement circuit 3 again exceeds the threshold.
`The delay line 16 is fed from the output line 7 by a
`line 8 and introduces a delay “T2” of from eight to ten
`milliseconds, the average male pitch period between its
`input on the lead 8 to its output on the lead 9.
`Thus the signal processing circuit of FIG. 1 is ar
`ranged to replace any corrupt hop, as detected by the
`quality measurement circuit 3, with a previously re
`ceived valid hop. The valid hop replacing a corrupt hop
`is one received a period of “T2” earlier, the delay being
`introduced by the delay line 16.
`It has been determined that if T2 is close to the “hop”
`period causing each corrupt hop to be replaced by an
`immediately preceding valid hop signal obliteration of
`up to 40% may be achieved before the received signal
`becomes unitelligible. However, if T2 is close to the hop
`period the voice output of the apparatus taken on an
`unnatural sounding monotonic pitch since the voice
`pitch tends to be replaced by the pitch of the hop fre»
`quency.
`By arranging T2 to be close to the natural pitch the
`correct fill-in waveform is provided approximately
`90% of the time due to the nature of speech waveforms.
`~.Although pitch tracking may improve the voice output
`it has been found that arranging “T2” to be of the order
`of eight to ten milliseconds provides a natural sounding
`output from the radio receiver.
`The signal processing circuit of FIG. 3 to which
`reference is now made is arranged to extend the tech~
`nique of replacing corrupt hops with a previously re
`ceived valid hop to either a previously received or later
`received valid hop.
`The signal from the delay line 2 is fed to a tapped
`delay line shown as a number of individual delays 11,
`11' to 11" and the accompanying quality value from the
`quality measurement circuit 3 is fed to a respective
`tapped delay line 12, 12’ to 12". Each delay period of
`the tapped delay lines 11, 11’ to 11" and 12, 12' to 12” is
`of length “T2” which is preferably eight to ten millisec
`onds.
`The quality value from each output of the delay line
`12, 12’ to 12" is fed to a quality comparison device 15
`which determines by “weighting” the quality value of
`60
`each of the previously received signal segments stored
`in the delay line 11, 11’ to 11" and each subsequently
`received signal segment against the value of the signal
`segment which is due to be output (the current signal
`segment). If the quality value of the current signal seg
`ment is acceptable the comparison device 15 causes a
`one out of n selector 18 to output the current signal
`segment to the line 7.
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`If the current signal segment is corrupt the compari~
`son device causes the signal segment previously re
`ceived or the signal segment next received to be output
`by the one-out-of-n selector 18 on the lead 7 in depen
`dence upon their respective quality values as deter
`mined by the quality measurement circuit 3. If both the
`previously received and next received signal segments
`are also corrupt, the next nearest signal segment is se
`lected for output if its quality is acceptable.
`Two further delay lines 16 and 17 are provided each
`having a delay of “T2”. The delay line 16 is arranged to
`store the signal segment last output to the lead 7 whilst
`the delay line 17 stores its respective quality value. Thus
`if none of the signal segments stored in the delay line 11,
`11’ to 11" has an acceptable quality value the signal _
`segment output to the line 7 a period of “T2” millisec
`onds previously is repeated.
`The value of the delay “T2” introduced by the delay
`lines 11, 11', to 11", 12, 12' to 12", 16 and 17 is prefera
`bly in the range of eight to ten milliseconds if the appa
`ratus is for use by male persons.
`The quality comparison device 15 may be made up of
`analogue or digital voltage comparators or may be im
`plemented by use of a micro-processor.
`In an analogue measuring technique used by the qual
`ity measurement circuit 15, since it is known that the
`variation in loss on the propagation path between differ
`ing frequencies remains within a predictcable range,
`say, plus or minus six decibels about an average, any
`increase of the received signal strength above six deci
`bels (dB) indicates that another unwanted signal is pres
`ent. Thus any hop in which such an increase occurs is
`determined as a corrupt hop and the output signal is
`derived from a valid hop as hereinbefore described.
`Another method of utilising signal strength to deter~
`mine whether the received hop is a corrupt hop or a
`valid hop is to utilize the signal strength distribution of
`a wanted signal. In-‘such a method the quality measure
`ment circuit 15 is arranged either to accummulate the
`signal strength distribution or to assume a signal
`strength distribution. By comparing the accumulated or
`assumed signal strength distribution for a hop with the
`average value of the signal strength of a received hop a
`probability value of the received hop may be deduced.
`The probability value of the received hop is then
`compared with a probability threshold and if the proba
`bility value does not exceed the threshold, the hop is
`determined to be a corrupt hop and is replaced by a
`valid hop as hereinbefore described.
`Whilst the above methods are adequate when two or
`more transmitters in different networks are transmitting
`at the same time it is possible that two or more networks
`may have changed to the same frequency together and
`at that time only one transmitter or one of the networks
`is in use.
`The network in which that transmitter is working is,
`of course, unaffected but the other networks may re
`ceive a perceivable click as the signal would be identi
`?ed as a valid hop.
`Thus the signal strength quality measurement is aug
`mented by a digital comparison.
`It will be appreciated that a digital transmission has a
`certain regularity in the transitions from one binary
`value to the other. Thus from a received transmission a
`clock reconstitutor may generate a clock signal which is
`in synchronism with the data transitions. This clock
`signal is more usually used to determine the optimum
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`If the quality value indicates a valid hop a signal is
`time at which to sample the incoming signal to deter
`mine the respective value of each data bit.
`provided at the output 39 of the processor 31 to cause
`the signal to be used at the appropriate time. However,
`The clock reconstitutor makes only small phase cor
`_if the quality value indicates a corrupt hop the output
`rections of the clock signal over a relatively long aver
`signal from the processor 31 will indicate which previ
`aging period so that it is immune to noise or interference
`ously or subsequently received signal is to be used in
`in the signal.
`lieu of the currently received signal.
`Using the clock signal it is possible to assign to each
`I claim:
`received bit quality designations of valid or corrupt and
`1. A frequency hopping radio communication re
`late or early. Data bits received in synchronism with the
`ceiver which has a hop period of Tl and comprising
`clock signal (which 'will have been synchronised to the
`(A) store means to store a representation of a received
`digital signal over a period of time) are valid. If the
`signal for a period of T2;
`period between pulses of the clock signal is designated
`(B) selection means which is responsive to a control
`as the bit period then data bits received within a quarter
`signal selectively to output either
`of the bit period either side of the clock signal are desig
`(i) a currently received signal or
`nated valid, early or valid, late. Similarly data bits re
`(ii) the stored representation from said store means
`ceived between a quarter of the bit period and one-half
`of a received signal received T2 earlier; and
`of the bit period either side of the clock signals may be
`(C) signal quality measurement means which deter
`determined as corrupt, early or corrupt, late.
`mines from at least one characteristic of the re
`The early and late decisions may be used to apply
`ceived signal in each of a succession of said periods
`small corrections to the phase of the clock signal to
`Tl within those periods, whether the received
`synchronise the clock reconstitutor over a period of
`signal comprises more than one transmitted signal
`time.
`and, if so, generates and forwards said control sig
`The valid and corrupt decisions are used to determine
`nal to said selection means such that, for any of the
`whether the hop is a valid hop of a corrupt hop. The
`periods T1 in which the received signal comprises
`“valid” decisions are used to cause a counter to count
`more than one transmitted signal, the stored repre
`up one each time a nominally valid data bit is received
`sentation from said store of an earlier received
`whilst the “corrupt” decisions are used to cause the
`signal is output by the receiver.
`counter to count down one each time a nominally cor
`2. A radio communication receiver as claimed in
`rupt data bit is received. The value of the count at the
`, claim 1 in which T2 is in the range of eight to ten milli
`end of a hop period provides a measure of the quality
`seconds.
`value of the hop.
`3. A radio communication receiver as claimed in
`It will be appreciated that when a wanted and an
`claim 1 in which the store means is a delay line.
`unwanted signal are present at similar signal strengths
`4. A radio communication receiver as claimed in
`’ corrupt hops are determined by the digital discrimina
`claim 1, in which means is also provided to delay a
`tion in the quality measurement circuit 15 whilst when
`received signal for a period of duration T1 to enable an
`more than one signal is present and affecting the signal
`average value of the quality of a received signal in that
`strength corrupt hops are distinguished by threshold
`period to be used to determine which representation of
`discrimination.
`the signal is output by the receiver.
`Referring to FIG. 4 the intermediate frequency am
`5. A radio communication receiver as claimed in
`pli?er is arranged to provide a digital signal on a num
`claim 1 in which a tapped delay line having a plurality
`ber of leads 30 to a decision processor (for example a
`of taps at intervals corresponding to T2 constitute said
`microprocessor) 31. In the clock reconstitutor 27 the
`store means and the selection means, in addition to
`late/early, and valid/corrupt decisions are made. Any
`selecting as aforesaid either the‘ currently received sig
`late or-early decision is fed back to the clock reconstitu
`nal or the stored representation of a received signal
`45
`tor 27 on a lead 32 to cause the adjustment of the clock
`received T2 earlier, alternatively selects from a tap on
`as previously described. The valid and corrupt deci
`said delay line the stored representation of a received
`signal received an integral multiple of T2 earlier.
`sions are passed to a counter 33 which counts up for a
`valid decision and down for a corrupt decision. If a
`6. A radio communication receiver as claimed in
`counter over?ow is reached it is fed back on a lead 34
`claim 1 in which the quality measurement means deter
`to cause the counter 33 to preset to its maximum value.
`mines the presence of more than one transmitted signal
`Similarly if a counter under?ow is reached the counter
`in the received signal from the average value of the
`signal strength in each of the intervals of duration T1.
`33 is preset to its minimum value over a lead 35.
`A down biassing count may be applied on a lead 36 to
`7. A radio communication receiver as claimed in
`cause the counter to count down to its minimum value
`claim 1 in which the quality measurement means deter
`in the presence of a pure noise signal.
`mines the presence of more than one transmitted signal
`The count reached by the counter 33 is passed to the
`in the received signal for each of the intervals of dura
`tion T1 in dependence on a comparison of the arrival
`processor 31 which also receives signals on a lead 38
`time of each data digit de?ned by the signal with a clock
`indicating each occurrence of a frequency change
`(hop)
`signal synchronized to the expected arrival time of each
`data digit in a wanted signal.
`The count reached on the current hop is compared
`with the count reached on the previous hop by the
`8. A radio communication receiver as claimed in
`processor 31 to determine the digital quality value of
`claim 7 in which the quality measurement means also
`the signal in the hop period. The processor 31 also
`determines the presence of more than one transmitted
`signal in the received signal in dependence on average
`determines an analogue quality value from the signal
`value of the signal strength in each of the intervals of
`strength indicated on the leads 30 and uses a combina
`duration Tl.
`tion of the two values to determine the quality value to
`be assigned to the hop.
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