`Mabey et a1.
`
`[54] POWER ECONOMISING IN MULTIPLE
`USER RADIO SYSTEMS
`
`[75] Inventors: Peter J. Mabey, Comberton; David J.
`Harrison, Cambridge; Diana M. Ball,
`Comberton, all of England
`[73] Assignee: U.S. Philips Corporation, New York,
`NY.
`[21] Appl. No.: 445,078
`[22] Filed:
`Nov. 30, 1989
`[30]
`Foreign Application Priority Data
`Dec. 23. 1988 [GB] United Kingdom ............... .. 8830200
`
`[51] Int. Cl.5 ............. ............................ .. H04B 17/00
`[52] US. Cl. ................................. .. 455/67.1; 455/231;
`455/343; 370/953
`[58] Field of Search ..................... .. 455/67, 68. 70. 89,
`455/231, 343. 53, 54; 370/951, 95.3, 104.1;
`340/525.44
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`4.577.315 3/1986 Otsuka .............................. .. 370/953
`4.612.637 9/1980 Davis et a1
`..... .. 370/953
`
`4.742.352 5/1988 lshii . . . . . . . . . . . . . .
`
`. . . . .. 455/89
`
`4.955.038 9/1990 Lee et a1, .......................... .. 455/343
`
`lllllllllllllllllllllllllllllllllllllllllllllllIIIIHIIIIlllllllllllllllll
`5,175,870
`Dec. 29, 1992
`
`US00517587OA
`[11] Patent Number:
`[45] Date of Patent:
`
`FOREIGN PATENT DOCUMENTS
`2063011 5/1981 United Kingdom.
`2069799 8/1981 United Kingdom.
`
`OTHER PUBLICATIONS
`"A Signalling Standard for Trunked Private Land Mo
`bile Radio Systems”, UK. Dept. Enterprise, Radio
`Communications Div., MPT 1327, Jan. 1988, pp. 9—l—
`9-3.
`Primary Examine'r—Curtis Kuntz
`Assistant Examiner-Edward Urban
`Attorney, Agent, or Firm-—Anne E. Barschall
`[57]
`ABSTRACT
`A multiple user radio system comprising at least one
`primary station and a plurality of secondary stations by
`way of a control channel. The primary station in use
`sets a value, in a transmitted message from which a
`secondary station wanting to economise on power can
`derive a period during which it can switch-off at least its
`radio section. The radio system may comprise a radi
`opaging system or a radio trunking system using an
`Aloha protocol. In the case of a radio trunking system
`using an Aloha protocol. the value comprises the Aloha
`numberv
`
`34 Claims, 1 Drawing Sheet
`
`“l/p
`SYSTEM
`CONTROLLER
`
`rnunmc
`
`PS
`
`I
`] _/
`1
`'1
`21
`J’
`16]22
`I ~/
`23
`
`MOBILES
`
`25
`PORTABLES
`
`2 2k
`
`26
`
`Z7
`
`+
`SECONDARY
`STATIONS
`
`.
`
`I
`|
`l
`l
`
`..____-l
`
`KllUPLEX
`RADIO
`RECEIVERS
`
`ARRIS GROUP, INC.
`IPR2015-00635 , p. 1 of 7
`
`
`
`US. Patent
`
`Dec. 29, 1992
`
`5,175,870
`
`FIXED POSITION \
`
`TRUNKING
`SYSTEM
`CONTROLLER
`
`MOBILES
`
`2S
`PORTABLES
`
`PS ALH (‘J
`SS
`
`AHY ALH(n)| mc
`ACK
`
`Fig. 3
`BSJ
`
`l
`801(9)
`
`P4512
`
`321
`
`502(6)
`
`SOHn)
`
`ARRIS GROUP, INC.
`IPR2015-00635 , p. 2 of 7
`
`
`
`1
`
`5,175,870
`
`POWER ECONOMISING IN MULTIPLE USER
`RADIO SYSTEMS
`
`2
`the primary station is arranged to address the transmit
`ted message containing the said indication to at least one
`of the subsets.
`Alternatively the population of secondary stations
`may be divided into subsets with each subset having its
`own primary station, the primary stations sharing a
`control channel such that only one primary station is
`transmitting at any one time. With such an arrangement
`each primary station prior to terminating its transmis
`sion may signal to its sub-set the anticipated time period
`when it will not be transmitting and the secondary sta
`tions can de-energise their radio sections in response to
`receipt of the terminating signal. The primary station
`prior to terminating its transmission may also send sepa
`rately or as part of the terminating signal an invitation
`to the secondary stations in its subset inviting them to
`transmit messages which can be responded to after the
`period of time when the at least one primary station is
`non-transmitting. Preferably the secondary stations of
`each sub-set re-energise their radio sections prior to the
`expiry of the anticipated time period in order that they
`will be able to re-synchronise themselves promptly
`when their primary station is operating.
`In the event of a secondary station having a task
`requiring it to for example initiate, continue or complete
`a signalling transaction with a primary station, it can
`refrain from switching-off or can re-energise its radio
`section.
`'
`According to another aspect of the present invention
`there is provided a multiple user radio system compris
`ing a primary station and a plurality of secondary sta'
`tions, each secondary station comprising radio and con
`trol sections, wherein the primary station comprises
`means for transmitting an indication of the number of
`time slots which it is making available for secondary
`stations wishing to transmit to make random access and
`wherein at least one of the secondary stations comprises
`means for receiving said indication in the message to be
`transmitted and for deriving'a time period from said
`indication and means for switching-off its radio section
`during a substantial proportion of said time period as a
`power economy measure.
`According to a further aspect of the present invention
`there is provided a multiple user radio system compris
`ing two or more primary stations sharing‘ a control
`channel between different subsets of secondary stations
`so that only one of the primary stations is transmitting at
`any one time, each of the secondary stations having a
`radio section and a control section, wherein each pri
`mary station has means for signalling to its subset of
`secondary stations the period of time it will be non
`transmitting, at least one of the secondary stations in the
`said subset has means responsive to said signal for de
`energising at least the radio section of the at least one of
`the secondary stations for at least a substantial propor
`tion of the period of time, each primary station com
`prises means for transmitting an indication of the num
`ber of time slots which it is making available for second
`ary stations wishing to transmit to make random access
`and wherein at least one of the secondary stations in the
`subset associated with the respective primary station
`comprises means for receiving said indication and for
`deriving a time period from said indication and means
`for switching-off at least the radio section of the at least
`one secondary station during a substantial portion of
`said time period as a power economy measure.
`
`DESCRIPTION
`The present invention relates to power economising
`in multiple user radio systems. particularly but not ex
`clusively, in trunking systems using an Aloha protocol
`and radiopaging systems.
`One such trunking system is the dynamic framelength
`slotted Aloha trunking system described and claimed in
`British Patent Speci?cation 20697998. A feature of this
`known system is that a trunking system controller form
`ing part of a primary station determines on the basis of
`the recent history of the operation of the system the
`number of successive time slots it will make available on
`a control channel in which a secondary station wanting
`to make a call can make a request. During busy periods
`the number of time slots is relatively large whereas in
`slack periods the number is small, frequently unity.
`Generally a secondary station is kept energised so that it
`is able to remain fully synchronised with the primary
`station and able to react promptly when the user wants
`to make a call. In the case of secondary stations pow
`ered from relatively high capacity power sources, the
`continuous energisation of the secondary station causes
`an insigni?cant power loss over the typical period of
`use. However the situation is different with secondary
`stations powered from a relatively low capacity power
`source having only a short life before the power source
`has to be recharged or replaced. Frequent recharging or
`replacement of the power source may be inconvenient
`to the user, particularly if required while the user needs
`service.
`An object ofthe present invention is to economise on
`the power consumed by the secondary station.
`According to one aspect of the present invention
`there is provided a method of operating a multiple user
`radio system comprising a primary station and a plural
`ity of secondary stations, the primary station transmit
`ting an indication of the number of time slots which it is
`making available for secondary stations wishing to
`transmit to make random access, wherein a secondary
`station wanting to economise on power utilises the indi
`cation to derive a time period during which it can
`switch-off at least its radio section.
`An embodiment of the multiple user radio system is a
`trunking system using an Aloha protocol. A bene?t of
`using the indication in the message transmitted by the
`primary station is that in the case of a trunking system
`using, for example, a slotted Aloha protocol, the indica
`tion can serve a dual function, that is, to indicate the
`number of time slots in an Aloha frame as well as inform
`a secondary station wishing to operate a power econo
`mising mode of the time period during which it can
`switch-off.
`The transmitted message may include a ?eld contain
`ing a signal indicative of whether or not a secondary
`station is permitted to operate in a power economising
`mode. This enables the primary station to control the
`service, which may be degraded by secondary stations
`being temporarily unavailable, and enables a primary
`station to commence operation without power econ
`omy control and subsequently be upgraded to include
`power economy control.
`If desired the population of the secondary stations
`having the capability of operating in a power economis
`ing mode may be divided into at least two subsets and
`
`O
`
`15
`
`45
`
`55
`
`65
`
`ARRIS GROUP, INC.
`IPR2015-00635 , p. 3 of 7
`
`
`
`3
`According to yet another aspect of the present inven
`tion there is provided a secondary station for use with
`the method in accordance with the present invention,
`comprising a radio section for providing communica
`tion with at least a primary station, and a control section
`for controlling the operation of the secondary station,
`the control section comprising means for deriving from
`said indication contained in the transmitted message a
`time period during which at least its radio section can be
`de-energised as a power economising measure.
`The present invention will now be described, by way
`of example, with reference to the accompanying draw
`ings, wherein:
`FIG. 1 is a diagram of multiple access trunking radio
`system,
`FIG. 2 illustrates the signalling between the primary
`station PS and a secondary station SS, and
`FIG. 3 illustrates the Aloha signalling when there are
`two primary stations serving respective populations (or
`subsets) of secondary stations, the levels of vtraf?c in
`each population being different.
`Referring to FIG. 1, the trunking system comprises a
`primary station PS including a trunking system control
`ler (TSC) 10 which is coupled by duplex or half-duplex
`links to a ?xed position 12 and to three duplex radio
`transceivers 14. 16, 18. In the drawings four secondary
`stations constituted by two mobiles 20, 22 and two por
`tables 24. 26 are shown. Each secondary station is
`equipped with a radio transceiver 21, 23, 25 and 27.
`capable of maintaining a half~duplex link with any one
`of the transceivers 14. 16, 18 of the primary station PS
`and a control section. It is pointed out that there will be
`many more secondary stations than the number of trans
`ceivers 14. 16, 18. Communication between secondary
`stations is via the transceivers 14, 16, 18 controlled by
`the TSC 10 which comprises a computer, modulators
`and demodulators.
`In operation most signalling between the primary
`station PS and the secondary stations 20, 22, 24 and 26
`is done on a signalling (or control) channel, which may
`or may not be a dedicated channel. Each secondary
`station has its own address or identity which is stored in
`its control section. Once a call has been set up by the
`computer in the TSC 10 of the primary station PS then
`a speech channel is allocated to the mobiles/portables
`involved in the call. Calls involving secondary stations
`can be arranged on a group basis or on a one-to-one
`basis. Some signalling, for example that necessary to
`terminate a conversation, takes place on speech chan
`nels.
`For wide area coverage, multiple ?xed sites can be
`used. At these sites the signalling channel may be oper
`ated for example quasi-synchronously or sequentially,
`or a separate signalling channel may be used at each site.
`However for the sake of simplicity of description only a
`single site example will be described.
`Suitable multiple access protocols which may be used
`in trunking systems operating in accordance with the
`method of the present invention are slotted Aloha pro
`tocols disclosed in for example British Patent Speci?ca
`tions 20630ll (?xed framelength) and 2069799B (dy
`namic framelength). When implementing this latter
`protocol the primary station monitors the number of
`garbled, successful and empty slots in each frame and
`65
`the call arrival rate so that the TSC 10 can determine,
`by means of feedback, the length of the next Aloha
`frame.
`
`35
`
`45
`
`60
`
`5,175,870
`
`10
`
`4
`The primary station from time to time broadcasts an
`Aloha message ALI-1(n) giving the number (n) of time
`slots available in which secondary stations can make
`requests for service. on a control channel. As mentioned
`above the TSC 10 determines the length of the next
`Aloha frame, that is, the number of time slots to be
`made available, based on the system performance. In
`the event of the radio traffic being light the Aloha num
`ber, n, is typically unity. This means that a secondary
`station on receiving this Aloha number can make its
`request (RQS) to the primary station in the next follow
`ing time slot. If the call is to another secondary station
`then the primary station sends an ahoy (AHY) message
`to the addressed secondary station which in the next
`time slot transmits an acknowledgement message ACK.
`Thereafter the primary station allocates a speech chan
`nel by sending a go-to-channel (GTC) message to both
`secondary stations. In the event of the radio traffic
`being heavy then the primary station may make longer
`frames available to reduce congestion (or call collision)
`or sub-divide the population of secondary stations.
`FIG. 2 illustrates by way of example the signalling
`between the primary station PS and a secondary station
`SS. In this example, the primary station PS transmits
`ALI-1(4), and a secondary station SS transmits a request
`RQS in the third of the four slots. Then the primary
`station transmits an ahoy (AHY) to an addressed sec
`ondary station in the next following slot. The primary
`station PS then transmits another Aloha message whilst
`in the same slot the addressed secondary station trans
`mits an acknowledgement ACK. In response to the
`ACK, the primary station uses the next slot to transmit
`a go-to-channel (GTC) message. Thereafter the two
`secondary stations switch from the control channel to a
`speech channel.
`Fuller details of suitable message structures are dis
`closed for example in MPT 1327 A Signalling Standard
`for Trunked Private Land Mobile Radio Systems pub
`lished by the Radiocommunications Division of the
`Department of Trade and Industry London 1988. Ac
`cordingly in the interests of brevity the basic signalling
`will not be described further.
`In order for the system to be able to minimise call
`set-up times it is preferred that the secondary stations be
`continuously energised to make requests to, or receive
`AHY messages from, the primary station.'Assuming
`that the power source for the transceivers in a second
`ary station has a sufficiently large capacity then there
`are no problems in keeping the transceivers fully energ
`ised. However with a portable secondary station, it is
`generally the case that the capacity of the internal
`power source, for example batteries, is marginal and
`accordingly it is desired that the trunking system be able
`to implement‘battery economising techniques.
`In accordance with the present invention, the pri
`mary station transmits an indication of the number of
`time slots which it is making available for secondary
`stations wishing to transmit to make random access, and
`a secondary station wishing to economise on power can
`derive from said indication a period during which it can
`switch-off at least its radio section but leave its control
`section energised. The said indication may for conve
`nience be the Aloha number (n). The transmitted mes
`sage may include an indication of as to whether a sec
`ondary station is permitted to operate in a power econo
`mising mode. The ability of the primary station to be
`able to send such a message enables the primary station
`to control the call set-up times, and also provides a
`
`ARRIS GROUP, INC.
`IPR2015-00635 , p. 4 of 7
`
`
`
`s
`
`15
`
`20
`
`25
`
`5,l75,870
`5
`6
`ready upgrade path between primary station without
`radiopaging systems. in which ?exible options for con
`trolling battery economy would be useful.
`and with power economy control.
`We claim:
`The control section of a secondary station which
`1. A method of economizing power in a multiple user
`wishes to initiate a call or continue a call can energise or
`maintain energised its radio section rather than adopt a
`radio system which includes a primary station and a
`plurality of secondary stations, the primary and second
`power economising mode. Once a call has been com
`ary stations each including respective transceiving
`pleted then the secondary station can revert to the
`power economising mode.
`means for communicating between the primary and
`secondary stations, the method comprising the steps of:
`When the radio tral'?c is light and the Aloha number
`a) in a primary station, making available a number, n,
`' is low, typically unity, then in order to avoid the sec
`ondary stations being fully energised substantially con
`of time slots in a framing protocol;
`b) transmitting by the primary station, following expi
`tinuously because the Aloha framelength corresponds
`ration of a last time slot of a preceding frame, an
`to one or two slots. the TSC 10 may transmit a signal
`which the secondary station may use to derive a time
`indication of a number, n, of time slots available to
`period during which it can practice power economising
`secondary stations in a following frame of time
`by switching-off its radio section.
`slots;
`During the period when a secondary station has
`c) choosing, in any secondary station, one of said it
`switched-off its radio section to economise on power,
`time slots at random;
`d) transmitting by such secondary station a request
`any requests for the economising secondary station
`either have to be stored and the AI-IY message transmit
`for access to the system in the one time slot;
`e) in one secondary station, deriving a time period
`ted after the secondary station has been re-energised or
`the calling station is requested to try again after the
`from said indication; and
`“off” time period has elapsed. When re-energising the
`i) in the one secondary station, switching off at least
`the transceiving means during said time period, to
`secondary stations, a time allowance has to be made for
`them to re-synchronise themselves. Generally the larger
`economize power when the one secondary station
`the number of timeslots which have been signalled by
`does not need access to the system.
`the primary station, the longer the time allowance
`2. A method as claimed in claim 1, characterised in
`that the radio system is a trunking system using an
`which has to be allocated for re-synchronisation.
`Aloha protocol.
`In the event of the trunking system having a single
`primary station and a large number of secondary sta
`3. A method as claimed in claim 1, further comprising
`the step of from the primary station, transmitting an
`tions. then their population can be divided into sub-sets.
`indication as to whether a secondary station is permit
`In operation only one sub-set at a time -is switched-off
`by a suitable signal produced by the primary station.
`ted to operated in a power economising mode.
`Division of the population can be done by giving each
`4. A method as claimed in claim 1, wherein those
`secondary stations having the capability of operating in
`sub-set a respective group address. By switching-off one
`power economising mode are divided into at least two
`sub-set at a time, the operation of the system is not
`subsets, and further comprising the step of, in the pri
`degraded too seriously. Secondary stations in the subset
`mary station, addressing a message containing the indi
`which is not switched-off can still practice power
`economising individually.
`cation of the number of available time slots to at least
`one of the subsets.
`In an alternative arrangement to the one just de
`5.4A method as claimed in claim 1. further compris
`scribed, each subset has its own primary station, only
`ing, in a secondary station which has a task requiring it
`one of which is energised at any one time. However the
`to be operating continuously,
`actual period during which each primary station is ac
`i) refraining from switching-off; or
`tive may be ?xed or may be variable and depend on the
`ii) if the respective transceiving means has been de
`level of radio traffic in the respective subsets. FIG. 3
`energised, re-energising the respective transceiving
`illustrates a number of time slots and how they are
`shared between two primary stations PS1 and PS2. In
`means.
`6. A method as claimed in claim 1, further comprising
`the last slot of the number of slots made available to
`each primary station, the respective primary station
`in a secondary station needing to initiate, continue or
`complete a signalling transaction with the primary sta
`transmits a value which constitutes a transmitter switch
`off signal, that is 501 and S02 for PS1 and PS2, respec
`tion
`'
`tively, indicating the number of slots, shown in paren
`thesis in FIG. 3, during which the radio sections of the
`secondary stations in its subset can be switched-off.
`Additionally power economising can be practised
`amongst the secondary stations forming the energised
`subset, for example by dividing each subset into at least
`two portions, only one portion being energised at any
`one time during the overall period allocated to the par
`ticular subset and/or by permitting the energised sec
`ondary stations to practice power economising on an
`individual basis. The value transmitted in the last slot
`also constitutes an invitation message for secondary
`stations to access the primary station which will re
`spond when it switches-on again.
`Although the present invention has been particularly
`described with reference to a radio trunking system,
`there are other multiple user radio systems, such as
`
`i) refraining from switching-off; or
`ii) if the respective transceiving means has been de
`energised, re-energising the respective transceiving
`means.
`7. A method as claimed in claim 1, wherein the sec
`ondary stations are divided into respective subsets with
`each respective subset having a respective primary sta
`tion, further comprising the steps of
`sharing a control channel among the primary stations
`such that only one primary station is transmitting at
`any one time, and
`in each respective primary station, prior to terminat
`ing transmission, signalling to the respective subset
`an anticipated time period during which the re
`spective primary station will not be transmitting.
`8. A method as claimed in claim 7, further compris
`ing, in at least one secondary station, re-energising the
`
`45
`
`50
`
`65
`
`ARRIS GROUP, INC.
`IPR2015-00635 , p. 5 of 7
`
`
`
`10
`
`25
`
`35
`
`5,175,870
`7
`respective transceiving means prior to expiration of the
`anticipated time period.
`9. A method as claimed in claim 7, further comprising
`the step of, from at least one of the primary stations,
`transmitting an invitation to the secondary stations in
`the respective subset inviting them to transmit messages
`which, if the messages are to be responded to, are to be
`responded to after the anticipated time period when the
`at least one primary station is not transmitting.
`10. A method as claimed in claim 9, characterised in
`that the signal indicative of the anticipated time period
`when the primary station will be switched-off and the
`invitation are one and the same message.
`11. A method as claimed in claim 1, further compris
`ing the step of storing messages, which are intended for
`the one secondary station when the one secondary sta
`tion is practising power economising, in the primary
`station.
`12. A method as claimed in claim 1 further compris~
`ing accessing the system by the secondary stations when
`the primary station is switched-off.
`13. A secondary station for use in the method as
`claimed in claim 1, comprising a radio section for pro
`viding communication with at least a primary station,
`and a control section for controlling the operation of
`the secondary station, the control section comprising
`means for deriving from said indication contained in the
`transmitted message a time period for switching off at
`least its radio section as a power economising measure.
`14. A secondary station as claimed in claim 13. com
`prising means responsive to the receipt of said indica
`tion for randomly selecting one of said time slots in
`which to transmit a request for access to the system for
`transmission by the secondary station.
`15. A secondary station as claimed in claim 13, cha
`racterised in that the control section further comprises
`means for deriving from a ?eld contained in a transmit
`ted message a signal indicating whether or not the sec
`ondary station is permitted to operate in a power econo
`mising mode.
`'
`16. A secondary station as claimed in claim 13, fur
`ther comprising means responsive to its control section
`requiring the secondary station to be operating continu
`ously for refraining from switching off or for re-energis
`ing its radio section.
`17. A secondary station as claimed in claim 13, fur
`ther comprising means responsive to the control section
`requiring the secondary station to initiate. continue or
`complete a signalling transaction with the primary sta
`tion for refraining from-switching-off or re-energising
`its radio section.
`18. A method as claimed in claim 1 wherein the num
`ber it varies from frame to frame.
`19. A method as claimed in claim 1 wherein the num
`ber n is not determined dynamically from frame to
`frame.
`20. A multiple user radio system comprising
`a) a plurality of secondary stations, each including
`i) a respective radio section; and
`ii) a respective control section;
`b) a primary station including transmitting means for
`transmitting an indication of the length of a frame
`including a number, n, of time slots which the pri
`mary station is making available for secondary
`stations wanting to transmit to make random ac
`cess; said transmitting means transmitting the indi
`cation after expiration of a last time slot of a pre
`ceding frame;
`
`8
`c) at least one of the secondary stations including
`i) means for receiving said indication:
`ii) means for deriving a time period from said indi
`cation; and
`iii) means for switching off the respective radio
`section during a substantial portion of said time
`period as a power economy measure.
`21. A system as claimed in claim 20, wherein the
`system is a trunking system using an Aloha protocol and
`in that the at least one of the secondary stations com
`prises means responsive to the indication in the trans
`mitted signal for deriving the number of timeslots in an
`Aloha frame.
`22. A system as claimed in claim 20, wherein
`the primary station has means for enabling the sec
`ondary stations to operate in a power economising
`mode and for signalling the enabling in a ?eld of a
`message to be transmitted; and
`the control section of at least one of the secondary
`stations includes means for detecting said field in a
`transmitted message and means responsive to the
`signal in said ?eld for permitting the secondary
`station to operate in a power economising mode.
`23. A system as claimed in claim 20, wherein
`those secondary stations having the means for switch
`ing off are divided into at least two subsets and
`the primary stations has means for addressing 9 trans
`mitted message containing said indication to at
`least one of the subsets.
`24. A system as claimed in claim 20, wherein a sec
`ondary station comprises means, responsive to the re
`spective control section requiring the secondary station
`to be operated continuously. for refraining from switch
`ing-off or for re-energising the respective radio section.
`25. A system as claimed in claim 20 wherein a second
`ary station comprises means, responsive to the respec
`tive control section requiring the secondary station to
`initiate, continue or complete a signalling transaction
`with the primary station, for refraining from switching
`off or for re-energising the respective radio section of
`the secondary station.
`26. A system as claimed in claim 20 wherein the num
`ber n varies from frame to frame.
`27. A system as claimed in claim 20 wherein the num
`ber n is not determined dynamically from frame to
`frame.
`’
`28. A multiple user radio system comprising
`a plurality of secondary stations, divided into at least
`two subsets, each secondary station having a re
`spective radio section and a respective control
`section;
`a control channel between the secondary stations;
`at least two respective primary stations, a respective
`one for each of the subsets, sharing the control
`channel between the subsets so that only one of the
`primary stations is transmitting at any one time,
`wherein
`each primary station has respective means for signal
`ling to its subset a period of time that primary sta
`tion will be non-transmitting,
`at least one of the secondary stations in each subset
`has means responsive to said signalling from the
`respective primary station for switching off at least
`the radio section of the at least one of the second
`ary stations for at least a substantial portion of the
`period of time,
`each primary station has respective means for trans
`mitting an indication of a number of time slots
`
`50
`
`55
`
`65
`
`ARRIS GROUP, INC.
`IPR2015-00635 , p. 6 of 7
`
`
`
`5,175,870
`
`10
`at least one of the subsets includes at least two of the
`secondary stations having the means for switching
`off and that at least one of the subsets is divided
`into at least two further subsets and
`the respective primary station of that at least one of
`the subsets has means for addressing a transmitted
`message containing said indication to at least one of
`the further subsets.
`32. A system as claimed in claim 28, wherein at least
`one of the secondary stations comprises means, respon
`sive to the respective control section requiring the at
`least one of the secondary stations to be operating con
`tinuously, for refraining from switching-off or for re
`energising the respective radio section.
`33. A system as claimed in claim 28, wherein at least
`one of the secondary stations comprises means-respon
`sive to the respective control section requiring the at
`least one of the secondary stations to initiate, continue
`or complete a signalling transaction with its respective
`primary station-for refraining from switching-off or
`for re-energising the respective radio section.
`34. A system as claimed in claim 28, wherein at least
`one of the primary stations includes means for transmit
`ting an invitation message to secondary stations inviting
`them to transmit messages which, if such messages are
`to be responded to, are to be responded to after said
`period of time during which the primary station is non
`transmitting.
`
`*
`
`1K
`
`*
`
`1‘
`
`*
`
`5
`
`15
`
`which that primary station is making available for
`secondary stations wishing to transmit to make
`random acces. and
`at least one of the secondary stations in each subset
`has
`means for receiving said indication and for deriving a
`second time period from said indication, and
`means for switching-off at least the respective radio
`section of the at least one secondary station during
`a substantial portion of said second time period as a
`power economy measure.
`29. A system as claimed in claim 28, characterised in
`that the system is a trunking system using an Aloha
`protocol and in that the at least one of the secondary
`stations comprises means responsive to the indication
`for deriving the number oi'time slots in an Aloha frame.
`30. A