(12) United States Patent
`US 6,868,079 B1
`Hunt
`(45) Date of Patent:
`Mar. 15, 2005
`
`(10) Patent N0.:
`
`US006868079B1
`
`(54) RADIO COMMUNICATION SYSTEM WITH
`REQUEST RE-TRANSMISSION UNTIL
`ACKNOWLEDGED
`
`(75)
`
`Inventor: Bernard Hunt, Redhill (GB)
`
`(73) Assignee: Koninklijke Philips Electronic N.V.;
`Eindhoven (NL)
`
`( * ) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`(21) Appl. No.: 09/455,124
`
`(22)
`
`Filed:
`
`Dec. 6, 1999
`
`(30)
`
`Foreign Application Priority Data
`
`............... 370/347
`1/1997 Crisler et a1.
`5,594,738 A *
`8/1998 Kou ..............
`370/337
`5,790,535 A *
`5,854,785 A * 12/1998 Willey ...............
`370/332
`5,926,469 A *
`7/1999 Norstedt et a1.
`.. 370/329
`6,097,717 A *
`8/2000 Turina et a1.
`.........
`.. 370/348
`
`.. 370/342
`6,256,301 B1 *
`7/2001 Tiedemann et al.
`.......... 370/394
`6,301,249 B1 * 10/2001 Mansfield et al.
`6,320,869 B1 * 11/2001 Van Driel et al.
`.......... 370/443
`6,463,298 B1 * 10/2002 Sorenson et a1.
`........ 455/552.1
`6,542,488 B2 *
`4/2003 Walton et al.
`.............. 370/335
`
`
`
`OTHER PUBLICATIONS
`
`Schwartz; Mischa; “Tellicommunication Networks”; Nov.
`1988; pp. 122—124.*
`
`* cited by examiner
`
`Primary Examiner—Nick Corsaro
`Assistant Examiner—Alan T. Gantt
`
`Dec. 10, 1998
`
`(GB) ............................................. 9827182
`
`(74) Attorney; Agent; or Firm—Dicran Halajian
`
`Int. Cl.7 ................................................... H04J 3/00
`(51)
`(52) US. Cl.
`....................... 370/345; 370/328; 370/336;
`370/343; 370/347; 455/434; 455/435.3;
`455/451
`
`(58) Field of Search ................................. 370/328; 329;
`370/336; 337; 345; 343; 230; 347; 346;
`455/434; 451; 435.3; 471; 435; 714/748
`
`(56)
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`(57)
`
`ABSTRACT
`
`A method of operating a radio communication system in
`which secondary stations use dedicated time slots to request
`services from a primary station. Asecondary station Wishing
`to request a service sends a request
`in every time slot
`allocated to it until it receives an acknowledgement from the
`primary station. The primary station can use combining
`techniques on multiple time slots to identify the presence or
`absence of a request from a secondary station with improved
`accuracy.
`
`11/1983 Karlstrom .................... 370/95
`4,414,661 A
`4,888,767 A * 12/1989 Furuya et al.
`.............. 370/243
`
`18 Claims, 3 Drawing Sheets
`
`302
`
` 304
`
`REQUEST SERVICE
`
`ACK
`RECEIVED
`I7
`
`306
`
`STOP REQUESTS
`
`
`
`
`
`
`
`310
`
`
`
`314
`
`812
`
`
`
`SERVICES SET UP
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`ERICSSON v. UNILOC
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`Ex. 1001 /Page 1 of8
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`ERICSSON v. UNILOC
`Ex. 1001 / Page 1 of 8
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`

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`US. Patent
`
`Mar. 15, 2005
`
`Sheet 1 0f3
`
`US 6,868,079 B1
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`
`
`FIG. 1
`
`202
`
`202
`
`202
`
`202
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`*I
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`IIIIIMIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII /‘ 24
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`/
`204
`
`FIG. 2
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`ERICSSON V. UNILOC
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`US. Patent
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`Mar. 15, 2005
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`Sheet 2 0f3
`
`US 6,868,079 B1
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`302
`
`304
`
`REQUEST SERVICE
`
` ACK
`RECEIVED
`
`Y
`
`306
`
`STOP REQUESTS
`
`
`
`310 312
`
`
`
`SERVICES SET UP
`
`
` 314
`
`FIG. 3
`
`
`
`FIG. 4
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`ERICSSON V. UNILOC
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`US. Patent
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`Mar. 15, 2005
`
`Sheet 3 0f3
`
`US 6,868,079 B1
`
`MDR(%)
`
`MDR(%)
`
`ERICSSON V. UNILOC
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`Ex. 1001 /Page 4 of8
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`ERICSSON v. UNILOC
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`US 6,868,079 B1
`
`1
`RADIO COMMUNICATION SYSTEM WITH
`REQUEST RE-TRANSMISSION UNTIL
`ACKNOWLEDGED
`
`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
`
`invention is to improve the
`An object of the present
`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
`
`invention will now be
`Embodiments of the present
`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
`
`ERICSSON v. UNILOC
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`US 6,868,079 B1
`
`3
`
`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
`n01se.
`
`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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`US 6,868,079 B1
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`5
`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 setup. 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.
`a :a
`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.
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`What is claimed is:
`
`6
`
`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.
`least one
`3. The method of claim 1, wherein the at
`respective secondary station modifies the power of the
`re-transmitted requests in response to a lack of acknowledg-
`ment from the primary station.
`least one
`4. The method of claim 3, wherein the at
`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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`US 6,868,079 B1
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`7
`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.
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`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.
`
`ERICSSON v. UNILOC
`
`Ex. 1001 /Page 8 of8
`
`ERICSSON v. UNILOC
`Ex. 1001 / Page 8 of 8
`
`