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`US006868079B 1
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`(12)United States Patent
`
`
`Hunt
`
`(10)Patent No.:US 6,868,079 Bl
`
`Mar.15,2005
`(45)Date of Patent:
`
`(54)RADIO COMMUNICATION SYSTEM WITH
`
`
`REQUEST RE-TRANSMISSION UNTIL
`ACKNOWLEDGED
`
`
`
`(75)Inventor:
`
`Bernard Hunt, Redhill (GB)
`
`N.V.,(73)Assignee: Koninklijke Philips Electronic
`
`
`
`
`(NL)
`Eindhoven
`
`5,594,738 A * 1/1997 Crisler et al. ............... 370/347
`
`
`
`
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`5,790,535 A * 8/1998 Kou ........................... 370/337
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`
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`5,854,785 A * 12/1998 Willey ........................ 370/332
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`
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`5,926,469 A * 7/1999 Norstedt et al. ............ 370/329
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`
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`6,097,717 A * 8/2000 Turina et al. ............... 370/348
`
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`6,256,301 Bl * 7/2001 Tiedemann et al. ......... 370/342
`
`
`
`6,301,249 Bl * 10/2001 Mansfield et al. .......... 370/394
`
`
`6,320,869 Bl * 11/2001 Van Driel et al. .......... 370/443
`
`
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`6,463,298 Bl * 10/2002 Sorenson et al. ........ 455/552.1
`
`
`6,542,488 B2 * 4/2003 Walton et al. .............. 370/335
`
`OTHER PUBLICATIONS
`
`the term of this ( *) Notice: Subject to any disclaimer,
`
`
`
`
`
`
`
`patent is extended or adjusted under 35
`
`U.S.C. 154(b) by O days.
`
`(21)Appl. No.: 09/455,124
`
`(22)Filed:Dec. 6, 1999
`
`
`
`* cited by examiner
`
`Schwartz, Mischa; "Tellicommunication Networks", Nov.
`
`
`
`
`
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`1988, pp. 122-124.*
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`
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`
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`Primary Examiner-Nick Corsaro
`
`
`
`Assistant Examiner-Alan T. Gantt
`
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`Dec. 10, 1998 (GB) ............................................. 9827182
`
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`(74)Attorney, Agent, or Firm----Dicran Halajian
`
`
`
`(30) Foreign A pplication Priority Data
`
`
`
`(51)Int. Cl.7 ................................................... H04J 3/00
`
`ABSTRACT
`(57)
`370/328; 370/336; (52)U.S. Cl. ....................... 370/345;
`
`
`
`A method of operating a radio communication system in
`
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`370/343; 370/347; 455/434; 455/435.3;
`
`
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`which secondary stations use dedicated time slots to request
`455/451
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`
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`services from a primary station. A secondary station wishing
`(58)Field of Search
`
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`................................. 370/328, 329,
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`
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`to request a service sends a request in every time slot
`
`
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`370/336, 337, 345, 343, 230, 347, 346;
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`allocated to it until it receives an acknowledgement from the
`
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`455/434, 451, 435.3, 471, 435; 714/748
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`
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`primary station. The primary station can use combining
`
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`techniques on multiple time slots to identify the presence or
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`absence of a request from a secondary station with improved
`accuracy.
`
`(56)
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`4,414,661 A 11/1983 Karlstrom .................... 370/95
`
`
`
`
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`18 Claims, 3 Drawing Sheets
`
`
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`4,888,767 A * 12/1989 Furuya et al. .............. 370/243
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`302
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`START
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`REQUEST SERVICE
`304
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`y
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`STOP REQUESTS
`310
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`NEGOTIATE WITH B
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`312
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`SERVICES SET UP
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`314
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`REGUEST SERVICE
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`ACK
`RECEIVED
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`STOP REGUESTS
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`SERVICES SET UP
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`US 6,868,079 B1
`US 6,868,079 B1
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`MDR(%)
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`MDR(%)
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`SNR (dB)
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`1
`RADIO COMMUNICATION SYSTEM WITH
`REQUEST RE-TRANSMISSION UNTIL
`ACKNOWLEDGED
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`FIELD OF THE INVENTION
`The present invention relates to a method of operating a
`radio communication System, and further relates to Such a
`System and to primary and Secondary Stations for use in Such
`a System. While the present Specification describes a System
`with particular reference to the emerging Universal Mobile
`Telecommunication System (UMTS), it is to be understood
`that Such techniques are equally applicable to use in other
`mobile radio Systems.
`BACKGROUND OF THE INVENTION
`In a radio communication System it is generally required
`to be able to exchange Signalling messages between a
`Mobile Station (MS) and a Base Station (BS). Downlink
`signalling (from BS to MS) is usually realised by using a
`physical broadcast channel of the BS to address any MS in
`its coverage area. Since only one transmitter (the BS) uses
`this broadcast channel there is no acceSS problem.
`In contrast, uplink signalling (from MS to BS) requires
`more detailed considerations. If the MS already has an
`uplink channel assigned to it, for voice or data Services, this
`Signalling can be achieved by piggy-backing, in which the
`Signalling messages are attached to data packets being Sent
`from the MS to the BS. However, if there is no uplink
`channel assigned to the MS piggy-backing is not possible. In
`this case a fast uplink signalling mechanism should be
`available for the establishment, or re-establishment, of a new
`uplink channel.
`In conventional Systems, for example those operating to
`the Global System for Mobile communication (GSM)
`Standard, fast uplink Signalling is enabled by the provision
`of a random access channel using a Slotted ALOHA or
`Similar protocol. However, Such a Scheme works Satisfac
`torily only with a low traffic load, and is not believed to be
`capable of handling the requirements imposed by third
`generation telecommunications Standards Such as UMTS.
`To meet these requirements one UMTS embodiment
`includes a dedicated Signalling channel, which comprises
`frames including a time slot for each MS registered with the
`controlling BS. If a MS requires a service from the BS it
`transmits a request in its allocated slot then waits for an
`acknowledgement from the BS Setting up the required
`Service. Parameters which characterise the performance of
`the signalling channel include the false alarm rate (where the
`BS erroneously identifies a MS as requesting a Service), the
`missed detection rate (where the BS does not detect a request
`from a MS), and the delay between a request for a service
`by the MS and the provision of that service by the BS.
`SUMMARY OF THE INVENTION
`An object of the present invention is to improve the
`efficiency of the method by which a MS requests resources
`from a BS.
`According to a first aspect of the present invention there
`is provided a method of operating a radio communication
`System, comprising a Secondary Station transmitting a
`request for resources to a primary Station in a time slot
`allocated to the Secondary Station, characterised by the
`Secondary Station re-transmitting the request in at least a
`majority of its allocated time slots until an acknowledge
`ment is received from the primary Station.
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`This Scheme improves the typical time for a response by
`the primary Station to a request by a Secondary Station.
`Because there is no possibility of requests from different
`Secondary Stations colliding, a Secondary Station can retrans
`mit requests in each allocated time slot. In contrast, in prior
`art Systems a Secondary Station has to wait at least long
`enough for the primary Station to have received, processed
`and acknowledged a request before it is able to retransmit.
`Further, the primary Station can improve the accuracy
`with which it determines whether a request was sent by a
`particular Secondary Station if the received signal Strength is
`close to the detection threshold by examining the received
`Signals in multiple time slots allocated to the Secondary
`Station in question.
`According to a Second aspect of the present invention
`there is provided a radio communication System comprising
`a primary Station and a plurality of Secondary Stations, the
`primary Station having means for allocating a time slot for
`a Secondary Station to transmit a request for resources to the
`primary Station, characterised in that the Secondary Station
`has means for re-transmitting the request in at least a
`majority of its allocated time slots until it receives an
`acknowledgement from the primary Station.
`According to a third aspect of the present invention there
`is provided a primary Station for use in a radio communi
`cation System, the primary Station having means for allo
`cating time slots to Secondary Stations for requesting
`resources, characterised in that the primary Station has
`combining means for determining from a combination of
`received signals in a plurality of Successive time slots
`allocated to the Secondary Station whether the Secondary
`Station has transmitted a request for resources.
`According to a fourth aspect of the present invention there
`is provided a Secondary Station for use in a radio commu
`nication System including a primary Station having means
`for allocating a time slot for the Secondary Station to transmit
`a request for resources to the primary Station, characterised
`in that means are provided for re-transmitting the request in
`at least a majority of the allocated time slots until an
`acknowledgement is received from the primary Station.
`The present invention is based upon the recognition, not
`present in the prior art, that in a System having time slots
`allocated to a Secondary Station for requesting resources,
`improved performance can be obtained by the Secondary
`Station repeating the request until an acknowledgement is
`received.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`Embodiments of the present invention will now be
`described, by way of example, with reference to the accom
`panying drawings, wherein:
`FIG. 1 is a block Schematic diagram of a radio commu
`nication System;
`FIG. 2 illustrates a possible frame format for a dedicated
`uplink Signalling channel;
`FIG. 3 is a flow chart illustrating a method in accordance
`with the present invention of a mobile Station requesting a
`Service from a base Station;
`FIG. 4 is a complex phasor plot showing the output of a
`matched filter in a BS in the presence of noise;
`FIG. 5 is a graph of missed detection rate (MDR) in
`percent against Signal to noise ratio (SNR) in dB for a fixed
`Signal magnitude, the Solid line indicating results with no
`combining at the BS and the dashed line indicating results
`with combining at the BS; and
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`FIG. 6 is a graph of missed detection rate (MDR) in
`percent against Signal to noise ratio (SNR) in dB for a signal
`Subject to Rayleigh fading, the Solid line indicating results
`with no combining at the BS and the dashed line indicating
`results with combining at the BS.
`In the drawings the same reference numerals have been
`used to indicate corresponding features.
`DETAILED DESCRIPTION OF THE
`INVENTION
`Referring to FIG. 1, a radio communication System com
`prises a fill primary station (BS) 100 and a plurality of
`secondary stations (MS) 110. The BS 100 comprises a
`microcontroller (uC) 102, transceiver means 104 connected
`to radio transmission means 106, and connection means 108
`for connection to the PSTN or other Suitable network. Each
`MS 110 comprises a microcontroller (uC) 112, transceiver
`means 114 connected to radio transmission means 116, and
`power control means 118 for altering the transmitted power
`level. Communication from BS 100 to MS 110 takes place
`on a downlink channel 122, while communication from MS
`110 to BS 100 takes place on an uplink channel 124.
`The present invention is concerned with an uplink channel
`124 dedicated to the transmission of requests for Services by
`a MS 110 to a BS 100. One arrangement of such a channel
`for UMTS is illustrated in FIG. 2. The uplink channel 124
`is divided into a succession of frames 202, each of length 10
`ms, and each MS 110 registered with the BS 100 is allocated
`a time slot 204 in each frame in which it can transmit a
`request for service. Although only ten time slots 204 are
`shown in each frame 202, in practice there may be many
`more per frame.
`Although it is anticipated that a Single dedicated uplink
`channel 124 will provide Sufficient capacity in normal
`Situations, it is possible for there to be more mobile Stations
`110 registered with a BS 100 than there are available time
`slots in each frame. In Such circumstances the BS 100 can
`either make another uplink channel 124 available for fast
`Signalling purposes or increase the capacity of the existing
`channel by not allocating a time slot for every MS 110 in
`every frame.
`In traditional Signalling Schemes, for example that used
`for the random access channel in GSM, a MS 110 makes a
`request for service to a BS 100 and then waits for an
`acknowledgement from the BS 100. If no acknowledgement
`is received after a predetermined period of time, the MS 110
`assumes that the request was not correctly received and
`Schedules another request. This Scheme minimises traffic on
`the channel to minimise collisions between requests from
`different mobile stations 110 thereby avoiding loss of chan
`nel capacity.
`However, in a dedicated uplink channel 124 having time
`slots allocated to each MS 110 collisions will not normally
`occur. A more effective Signalling Scheme, in accordance
`with the present invention, is that shown in FIG. 3. The
`process starts at 302 when the MS 110 determines that it
`requires a service from the BS 100. The MS 110 then makes
`a request for service 304 in the next time slot 204 (FIG. 2)
`allocated to it. In a Code Division Multiple Access (CDMA)
`System, the request 304 is made by transmitting a predeter
`mined code Sequence. Requests 304 continue to be made in
`Successive allocated time slots 204 until a first test 306
`determines that no further requests should be made, for
`example by examining the value of a flag that was Set to true
`at the start 302 of the process.
`A second test 308 determines whether the MS 110 has
`received an acknowledgement from the BS 100. When the
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`second test 308 determines that an acknowledgement has
`been received from the BS 100, the MS 110 at 310 stops any
`further requests from being Sent, for example by Setting to
`false a flag that is checked by the first test 306. The MS 110
`then begins negotiations 312 with the BS 100 to define fully
`the required Services. Finally, at 314, the required Services
`are set up by the BS 100.
`This Scheme has the advantage that if a request is not
`received correctly by the BS 100 it can be repeated at the
`frame rate (100 Hz for the 10 ms frame defined in UMTS),
`or at least in every allocated time slot if the System is busy
`and a time slot is not allocated in every frame. In a
`traditional Scheme it is not guaranteed that a request could
`be received and processed by the BS 100 Sufficiently rapidly
`for an acknowledgement to be Scheduled for the immedi
`ately following frame, so the time that a MS 110 has to wait
`before re-transmitting the request is Substantially longer.
`A further advantage of the Signalling Scheme in accor
`dance with the present invention is that the quality of
`detection at the BS 100 can be improved by combining
`requests. Since the BS 100 knows that requests will be
`repeated in every frame, it can postpone making a decision
`about a possible received request when this request is near
`to the decision threshold and use information from the next
`(and Subsequent) frames to improve the confidence of the
`decision. This amounts to a form of time diversity, and will
`improve the robustness of the Signalling Scheme to the
`effects of fading, near-far effect and other interference.
`In one embodiment of UMTS, the dedicated uplink chan
`nel employs a CDMA technique. Using information trans
`mitted on a downlink broadcast channel 122 by the BS 100,
`each MS 110 is able to determine the uplink signalling
`Sequence it should use (thereby defining the dedicated
`uplink channel 124) and the time slot 204 it is allocated in
`a frame 202. The uplink Signalling Sequence is detected at
`the BS 100 by a matched filter, and the time at which a peak
`appears in the output of the matched filter indicates which
`MS 110 issued the request.
`Simulations have been performed to investigate the
`advantages of combining Successive transmitted requests at
`the BS 100. FIG. 4 is a complex phasor plot illustrating a
`model for the effect of noise on the output of the matched
`filter in the BS 100. Noise was modelled as complex
`Gaussian noise, with the result that the output 402 of the
`matched filter is the vector sum of a signal peak 404 and a
`complex Gaussian noise vector 406. In the absence of a
`Signal 402, the filter output is simply complex Gaussian
`OSC.
`In a first simulation the level of the signal 404 was held
`fixed and varying levels of noise 406 applied. In a scheme
`with no combining the magnitude of the filter output vector
`402 is compared to a threshold value, and if it exceeds this
`value a signal is assumed to be present. Two different error
`rates were measured: the False Alarm Rate (FAR), which is
`the probability of detecting a signal when only noise is
`present; and the Missed Detection Rate (MDR), which is the
`probability of failing to detect a signal which is present.
`A simple combining Scheme was also simulated, in which
`the magnitudes of two Successive matched filter outputs
`were added together and compared to a (different) threshold
`to determine whether a signal was present. For both Schemes
`the threshold was Set as a multiple of the rms noise magni
`tude Such that the FAR remained constant at 1%.
`The results are shown in FIG. 5, which is a plot of the
`MDR in percent against the ratio of the Signal magnitude to
`mean noise magnitude (SNR) in dB. The solid curve shows
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`the results for the Scheme without combining and the dashed
`curve the results with combining. The results of the com
`bining Scheme show a Significant improvement in MDR,
`equivalent to an improvement of typically 2 dB in SNR.
`In a second simulation the level of the signal 404 was
`Subjected to Rayleigh fading, to provide a more realistic
`mobile environment. The results are shown in FIG. 6, which
`is a plot of the MDR in percent against the SNR in dB. The
`Solid curve shows the results for the Scheme without com
`bining and the dashed curve the results with combining. In
`both cases the results show significantly higher MDR for a
`given Signal to noise ratio than the Simulation without
`fading, as might be expected. Again the results of the
`combining Scheme show a significant improvement in
`MDR, equivalent to an improvement of between 2 and 5 dB
`in SNR.
`Alternative combining Schemes could be used in a method
`in accordance with the present invention. The results dis
`cussed above demonstrate the improved accuracy resulting
`from combining two Successive filter outputs. Further
`improvements could be obtained by combining more
`outputs, although at the cost of increased delay in the BS 100
`responding to the requests from the MS 110.
`A multiple threshold scheme could offer further advan
`tages. Such a Scheme would operate by examining the
`magnitude of the filter output. If it is above an upper
`threshold then a request has been detected which the BS 100
`can acknowledge immediately, while if it is below a lower
`threshold no request has been detected. If the output lies
`between the two thresholds then Signal combining Schemes
`Such as those described above can be used to resolve the
`question of whether a request was sent.
`Some form of power control is also desirable. If a MS 110
`transmits a request at too high a power level it may Swamp
`other signals at the BS 100, while if it transmits at too low
`a power level the request will not be detected. Closed loop
`power control is not available until the requested Services
`are set up. Open loop power control is possible if the MS 110
`uses the characteristics of a broadcast channel from the BS
`100 to determine the initial power at which to transmit its
`requests. If no acknowledgement is received from the BS
`100 the power at which the requests are transmitted could be
`gradually increased, Subject to appropriate maximum power
`limits.
`In a Rayleigh fading environment there could also be
`advantages in having random or other variations in the
`transmitted power level.
`A further application of a method in accordance with the
`present invention is in the provision of paging or other
`services, where a BS 100 transmits messages on a downlink
`channel 122 to a MS 110, and continues retransmitting until
`an acknowledgement is received from the MS 110. In this
`application therefore the BS 100 functions as a secondary
`station and the MS 110 as a primary station.
`From reading the present disclosure, other modifications
`will be apparent to perSons Skilled in the art. Such modifi
`cations may involve other features which are already known
`in radio communication Systems and component parts
`thereof, and which may be used instead of or in addition to
`features already described herein.
`In the present Specification and claims the word “a” or
`“an' preceding an element does not exclude the presence of
`a plurality of Such elements. Further, the word “comprising
`does not exclude the presence of other elements or Steps than
`those listed.
`
`6
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`What is claimed is:
`1. A method of operating a radio communication System,
`comprising:
`allocating respective time slots in an uplink channel to a
`plurality of respective Secondary Stations, and
`transmitting a respective request for Services to establish
`required Services from at least one of the respective
`Secondary Stations to a primary Station in the respective
`time slots,
`wherein the at least one respective Secondary Station re,
`transmits the same respective request in consecutive
`allocated time slots without waiting for an acknowl
`edgement until Said acknowledgement is received from
`the primary Station,
`wherein the primary Station determines whether a request
`has been transmitted by the at least one respective
`Secondary Station from a combination of the received
`Signals in a plurality of Successive time slots allocated
`to the at least one respective Secondary Station.
`2. The method of claim 1, wherein the primary station
`determines whether said request has been transmitted only if
`the level of a received request is between lower and upper
`thresholds.
`3. The method of claim 1, wherein the at least one
`respective Secondary Station modifies the power of the
`re-transmitted requests in response to a lack of acknowledg
`ment from the primary Station.
`4. The method of claim 3, wherein the at least one
`respective Secondary Station increases the power of the
`re-transmitted requests in response to a lack of acknowledg
`ment from the primary station.
`5. The method of claim 1, wherein:
`the allocating of the respective time slots comprises
`allocating the respective time slots in frames in the
`uplink channel;
`each frame has a plurality of time slots, and
`the at least one respective Secondary Station re-transmits
`the respective request in the consecutive allocated time
`slots in a consecutive frames until the acknowledge
`ment is received from the primary Station.
`6. The method of claim 1, wherein:
`when the at least one respective Secondary Station has
`received the acknowledgement from the primary
`Station, the at least one respective Secondary Station
`Stops any further requests for Services from being
`transmitted, and begins negotiations with the primary
`Station to define fully the requested Services.
`7. The method of claim 1, wherein:
`the requests for Services comprise requests for establish
`ing a new uplink channel for voice or data Services.
`8. A radio communication System, comprising:
`a primary Station and a plurality of respective Secondary
`Stations,
`the primary Station having means for allocating respective
`time slots in an uplink channel to a plurality of respec
`tive Secondary Stations to transmit respective requests
`for Services to the primary Station to establish required
`Services,
`wherein the respective Secondary Stations have means for
`re-transmitting the same respective requests in con
`secutive allocated time slots without waiting for an
`acknowledgement until Said acknowledgement is
`received from the primary Station,
`wherein the primary Station determines whether a request
`has been transmitted by at least one of the plurality of
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`respective Secondary Stations from a combination of the
`received signals in a plurality of Successive time slots
`allocated to the at least one of the plurality of respective
`Secondary Stations.
`9. The radio communication system of claim 8, wherein:
`the means for allocating allocates the respective time slots
`in frames in the uplink channel;
`each frame has a plurality of time slots, and
`the means for re-transmitting re-transmit the respective
`requests in the consecutive allocated time slots in
`consecutive frames until the acknowledgement is
`received from the primary Station.
`10. The radio communication system of claim 8, wherein:
`when at least one of the respective Secondary Stations has
`received the acknowledgement from the primary
`Station, the at least one respective Secondary Station
`Stops any further requests for Services from being
`transmitted, and begins negotiations with the primary
`Station to define fully the requested Services.
`11. The radio communication system of claim 8, wherein:
`the requests for Services comprise requests for establish
`ing a new uplink channel for voice or data Services.
`12. A Secondary Station for use in a radio communication
`System, comprising:
`means for transmitting a request for Services to establish
`required Services to a primary Station in respective
`allocated time slots in an uplink channel;
`wherein the primary Station allocates respective time slots
`in the uplink channel to a plurality of respective Sec
`ondary Stations, and
`means for re-transmitting the same request for Services in
`consecutive allocated time slots without waiting for an
`acknowledgement until Said acknowledgment is
`received from the primary Station,
`wherein the primary Station determines whether a request
`has been transmitted by at least one of the plurality of
`respective Secondary Stations from a combination of the
`received signals in a plurality of Successive time slots
`allocated to the at least one of the plurality of respective
`Secondary Stations.
`13. The Secondary Station of claim 12, further comprising
`means for modifying the power of the re-transmitted
`requests in response to lack of acknowledgement from the
`primary Station.
`14. The secondary station of claim 12, wherein:
`the primary Station allocates the respective time slots in
`frames in the uplink channel;
`each frame has a plurality of time slots, and
`the means for re-transmitting re-transmit the request in the
`consecutive allocated time slots in consecutive frames
`until the acknowledgement is received from the pri
`mary Station.
`
`1O
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`15
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`25
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`35
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`40
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`45
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`50
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`US 6,868,079 B1
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`8
`15. The secondary station of claim 12, further comprising:
`means for Stopping any further requests for Services from
`being transmitted when the acknowledgement is
`received from the primary Station; and
`means for beginning negotiations with the primary Station
`to define fully the requested services when the
`acknowledgement is received from the primary Station.
`16. The secondary station of claim 12, wherein:
`the request for Services comprises a request for establish
`ing a new uplink channel for voice or data Services.
`17. A method of operating a radio communication System,
`comprising:
`allocating respective time slots in an uplink channel to a
`plurality of respective Secondary Stations, and
`transmitting a respective request for Services to establish
`required Services from at least one of the plurality of
`respective Secondary Stations to a primary Station in the
`respective time slots,
`wherein the at least one of the plurality of respective
`Secondary Stations re-transmits the Same respective
`request in consecutive allocated time slots without
`waiting for an acknowledgement until Said acknowl
`edgement is received from the primary Station,
`wherein the primary Station determines whether a request
`for services has been transmitted by the at least one of
`the plurality of respective Secondary Stations by deter
`mining whether a Signal Strength of the respective
`transmitted request of the at least one of the plurality of
`respective Secondary Stations exceeds a threshold
`value.
`18. A radio communication System, comprising:
`a primary Station and a plurality of respective Secondary
`Stations,
`the primary Station having means for allocating respective
`time slots in an uplink channel to the plurality of
`respective Secondary Stations to transmit respective
`requests for Services to the primary Station to establish
`required Services,
`wherein the respective Secondary Stations have means for
`re-transmitting the same respective requests in con
`secutive allocated time slots without waiting for an
`acknowledgement until Said acknowledgement is
`received from the primary Station,
`wherein Said primary Station determines whether a request
`for services has been transmitted by at least one of the
`respective is Secondary Stations by determining
`whether a signal Strength of the respective transmitted
`request of the at least one of the respective Secondary
`Stations exceeds a threshold value.
`
`IPR2020-00038
`MM EX1001, Page 8
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