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`
`US006356771Bl
`
`(12)United States Patent
`
`
`Dent
`
`(10)Patent No.:
`US 6,356,771 Bl
`
`Mar.12,2002
`(45)Date of Patent:
`
`(54)RADIO COMMUNICATIONS SYSTEM W ITH
`
`ADAPTIVE POLARIZATION
`
`5,838,670 A 11/1998 Billstrom
`
`
`
`
`
`
`
`6,222,503 Bl * 4/2001 Gietema et al. ............ 343/890
`
`
`
`
`
`(75)Inventor: Paul W. Dent, Pittsboro, NC (US)
`
`
`
`
`
`FOREIGN PATENT DOCUMENTS
`
`
`
`0210254 A2 11/1986
`EP
`
`Ericsson, Inc., Research Triangle Park,
`(73)Assignee:
`
`0847209 A2 6/1998
`EP
`NC (US)
`*cited by examiner
`
`( *) Notice: Subject to any disclaimer, the term of this
`
`
`
`Primary Examiner-Vivian Chang
`
`
`
`
`
`patent is extended or adjusted under 35
`Mehrp our
`
`Assistant Examiner----Naghmeh
`
`
`U.S.C. 154(b) by O days.
`
`
`
`(74) Attorney,Agent, or Firm-Wood, Phillips, Van Santen,
`Clark & Mortimer
`
`(21)Appl. No.: 09/113,316
`
`(57)
`
`ABSTRACT
`
`
`
`(22) Filed: Jul. 10, 1998
`
`
`
`
`
`A radio base station having a plurality of directional sector
`
`
`(51) Int. Cl.7 .................................................. H04B 1/38
`
`
`
`antennas for providing communications with outstations
`
`
`
`
`
`lying at different azimuth angles to the base station is
`
`
`
`
`(52) U.S. Cl. ..................................... 455/562; 455/277.1
`
`
`
`
`
`disclosed in which the antennas transmit signals using one or
`
`
`Field of Search .............................. 455/561, 277.1,
`(58)
`
`
`both of two orthogonal polarizations such as left hand or
`
`
`
`455/277.2, 278.1, 229.1, 273, 275, 450
`
`
`right hand circular antennas and polarization and which also
`
`
`
`measures interference levels on different frequency channels
`
`
`
`
`that are used for allocating an optimum channel and polar­
`
`
`ization for connecting a call. Each of the outstations com­
`
`
`
`prise at least one antenna of selectable polarization.
`
`
`
`3,956,699 A * 5/1976 Leahy ......................... 325/15
`
`5,491,837 A 2/1996 Haartsen
`
`5,724,666 A 3/1998 Dent
`
`(56)
`
`
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`
`
`2 Claims, 9 Drawing Sheets
`
`11
`
`
`
`Adaptive polarization selection
`
`13
`
`14
`
`RIIC J ___ _ --�---..,·
`
`Loop
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`
`Directive Patch
`
`Array 10
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`Telephone lnstrumen,
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`I
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`90 Degree
`Coupler
`- �
`Receiver
`----\
`Polarization
`selection
`switch
`12
`
`--,
`
`Control
`Transmittur
`
`Processor &
`Interfaces
`
`Ex.1016
`APPLE INC. / Page 1 of 16
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`U.S. Patent
`U.S. Patent
`
`Mar.12, 2002
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`Sheet 1 of 9
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`US 6,356,771 B1
`US 6,356,771 BI
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`U.S. Patent
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`Mar. 12, 2002
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`Sheet 2 of 9
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`US 6,356,771 B1
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`U.S. Patent
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`Mar. 12, 2002
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`Sheet 3 of 9
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`US 6,356,771 B1
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`U.S. Patent
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`Mar.12, 2002
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`Ex.1016
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`U.S. Patent
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`Mar. 12, 2002
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`Sheet 5 of 9
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`US 6,356,771 B1
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`U.S. Patent
`U.S. Patent
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`Mar.12, 2002
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`Sheet 6 of 9
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`US 6,356,771 B1
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`U.S. Patent
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`Mar. 12, 2002
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`US 6,356,771 B1
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`U.S. Patent
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`Mar. 12, 2002
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`US 6,356,771 B1
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`U.S. Patent
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`Mar. 12, 2002
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`Sheet 9 of 9
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`US 6,356,771 B1
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`US 6,356,771 B1
`
`1
`RADIO COMMUNICATIONS SYSTEM WITH
`ADAPTIVE POLARIZATION
`RELATED APPLICATION
`This application is related to U.S. Pat. No. 5,724,666 to
`Dent, the Subject matter of which is incorporated by refer
`CCC.
`
`2
`When different frequencies are used for transmission and
`reception, correlation between their interference environ
`ments cannot be assumed. The polarization to be used by
`outstations and base stations is not defined in the DECT
`Systems. Thus, a potential doubling of System capacity by
`polarization reuse is not available.
`U.S. Pat. No. 5,491,837 issued to Haartsen describes
`adaptive channel allocation methods for use in a two
`frequency duplex System. The outstations measure signals
`received on channels in a first frequency band from various
`base Stations and transmit the Signal measurements to the
`Serving base Station. The Serving base Station knows the
`power transmitted by all base Stations and can, therefore,
`determine the path loSS from every base Station to the
`outstation in the first frequency band. The Serving base
`Station also knows the Signals that are transmitted and the
`channels on which these Signals are transmitted by each of
`the base Stations and can, therefore, compute the interfer
`ence Scenario at the outstation on every channel, including
`channels not measured by the outstation. The base Station
`also measures interference levels on channels in a Second
`frequency band used for the communications from the
`outstation to the base Station. The base Station then deter
`mines a channel in the first frequency band for transmitting
`to the outstation and combines it with a channel in the
`Second frequency band for receiving from the outstation.
`This combination results in good signal quality in both
`directions. Haartsen, however, is not concerned with deter
`mining a best polarization to use for Serving a given out
`Station.
`U.S. Pat. No. 5,548,813 issued to Charas et al. and U.S.
`Pat. No. 5,619,503 issued to Dent also describe the use of
`multiple-beam antennas for cellular and Satellite communi
`cations Systems. The Subject matter of these documents is
`hereby incorporated by reference. These documents
`assumed that the communication path length was long So
`that various non-free-space propagation effects could arise
`to distort Signal polarization. Therefore, it was difficult to
`count on frequency re-use with different polarization to
`increase capacity for communicating with mobile terminals.
`Polarization reuse is, however, disclosed in the context of
`communicating between a Satellite and a fixed ground
`Station. This type of polarization reuse is non-adaptive and
`both polarizations are used in the same directional beam.
`The 666 patent also describes cellular base stations in
`which the polarization is alternated between adjacent Sectors
`of a multiple-sector directional antenna, the main purpose of
`which is to obtain uncorrelated fading of the same signals
`received at two adjacent antennas.
`
`15
`
`35
`
`45
`
`50
`
`25
`
`BACKGROUND
`The present invention relates to radiotelephone Systems
`Such as cellular phone Systems, indoor cordleSS Systems and
`wireless in the local loop (WLL) Systems, and more par
`ticularly to methods of allocating channel parameterS Such
`as frequency, timeslots, polarization and power level in
`order to optimize Signal quality.
`The Digital European Cordless Telephone system
`(DECT) is an existing example of a wireless telephone
`System employing adaptive allocation frequency channels
`and timeslots for connecting a call. In the DECT Systems,
`Single-frequency duplex operation is employed by alter
`nately transmitting a TDMA burst from a base station to an
`outstation and from the outstation to the base Station. This is
`known as Time Division Duplex (TDD). The use of TDD
`and a common frequency for both directions of communi
`cations means that both the base and Station and the outsta
`tion experience a common interference environment, par
`ticularly in indoor, wireless PABX applications for which
`DECT was designed. Thus, either the base station or the
`outstation can choose a frequency channel and a timeslot
`having momentarily minimum interference levels with near
`certainty that the chosen channel will be a good channel for
`communicating in both directions. In DECT, the outstation
`is allowed to choose the frequency and timeslot without a
`prior warning to the base Station. The base Station listens on
`all frequencies and timeslots in order to ensure that it always
`receives the Signal. Signal bursts from different outstations
`are identified by means of a short ID code so that the base
`Station can assemble bursts received from the Same outsta
`tion on different channels. The base station transmits to the
`outstation using a timeslot in the transmit half of its TDD
`40
`frame period corresponding to the timeslot in which the
`immediately previous data was received from the same
`outstation. The outstation listens for the base Station on the
`receive timeslot in the receive half of its TDD frame
`corresponding to the transmit timeslot it used immediately
`previously to transmit to the base Station. In this way, fast
`adaptation to changing interference ScenarioS is achieved in
`DECT.
`The DECT System also employs adaptive antenna Selec
`tion (space Selection diversity) in order to mitigate slow
`fading caused by a wireleSS telephone user moving inside a
`building at walking pace, for example.
`The base Station may transmit a timeslot using a first or a
`Second antenna, Spaced So that fading of the path from the
`first antenna to the outstation is uncorrelated with fading
`from the Second antenna to the outstation. The antenna used
`for transmitting a slot is indicated by a data bit contained in
`the slot. The outstation receives slots intended for it as well
`as slots intended for other outstations and determines
`whether it can receive a slot transmitted by one antenna
`better than a slot transmitted by another antenna. The
`outstation then Selects a channel frequency and timeslot
`containing the lowest measured signal level for use in
`transmitting to the base Station. This indicates that the
`channel is not in use nearby and transmits data to the base
`Station in that Slot including an indication of the base Station
`antenna it preferS for receiving a reply.
`
`SUMMARY
`It is an object of the present invention to overcome the
`deficiencies described above by providing a method for
`increasing System capacity. This is achieved by utilizing
`alternating polarization for short range communication, e.g.,
`on the order of one mile, in order to increase capacity of the
`System by allowing frequency reuse for different signals
`with different polarization in adjacent Sectors.
`A radio base Station comprises a number of directional
`Sector antennas for providing communications with outsta
`tions lying at different azimuth angles to the base Station.
`The directional antennas may transmit Signals using one or
`both of two orthogonal polarizations, Such as left or right
`hand circular polarization. A channel allocation unit allo
`cates Spectral resources, Sector antennas and polarization for
`communicating with each outstation in a manner designed to
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`US 6,356,771 B1
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`4
`maximum distance at which an outstation can lie from a
`given Site along the line between two Sectors being half the
`maximum distance at the center of a Sector. For Stations
`lying further away, an adjacent site (not shown) takes over
`communications. Given the normally assumed fourth power
`of distance propagation law, only one-sixteenth of the power
`(-12 dB) is required to communicate with Stations lying at
`half the distance along the line between two sectors. The
`patterns illustrated in FIG. 1 are thus tailored to provide
`roughly equal communications performance around the
`periphery of the area Served by the site.
`FIG. 2 illustrates alternating polarizations applied to a site
`of twelve Sectors according to an exemplary embodiment of
`the present invention. Six of the sectors employ Right Hand
`Circular polarization (RHC) while the other six in between
`employ Left Hand Circular polarization (LHC). Thus, each
`of the twelve Sectors employ polarization that is opposite
`each of its two adjacent Sectors in order to reduce the
`interference between Sectors.
`Table 1 below illustrates the signal to interference ratio
`(C/I) at an outstation lying at various angles from the base
`Station Site without alternating polarization and with alter
`nating polarization. The outstation is Served by the best of
`the Sectors, as indicated, to provide the desired signal. The
`outstation employs an antenna with polarization that
`matches that of the Serving Sector. The Signals reaching the
`outstation due to the non-Zero Sidelobes of other Sectors
`represent interfering Signals. The ratio of the desired signal
`power to the total interfering power in dB represents the
`Signal to interference ratio. Since there is a twelve-fold
`symmetry in the system, the variation of C/I only needs to
`be plotted over one repetition cycle of 30 degrees of azi
`muth.
`
`3
`minimize interference between different communications
`Signals. Allocating spectral resources can include allocating
`a channel frequency, a timeslot or a spread-spectrum acceSS
`code.
`Each outstation comprises at least one antenna to receive
`at least one polarization, but preferably an antenna of
`Selectable polarization. The Outstation antenna may be a
`directional antenna, in which case it is oriented to provide
`maximum directional gain towards a base Station that is
`Selected to provide radiocommunication Service. Each out
`Station compriseS receiver means for receiving control chan
`nel Signals from the Selected base Station indicative of
`incoming call alerts and transmitter means for transmitting
`responses to call alerts or call initiation requests. The
`outstation receiver is equipped to measure interference or
`Signal quality levels on different frequency channels or
`timeslots using Selected polarizations and the transmitter is
`equipped to transmit interference or Signal quality measure
`ments made by the receiver to the base Station.
`The base Station is also equipped to make measurements
`of interference levels on different frequency channels or
`timeslots using Selected polarizations, and to provide these
`measurements to the channel allocation unit along with
`measurements received from the outstation. These measure
`ments enable the channel allocation unit to allocate an
`optimum channel and polarization for connecting a call.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`These and other objects, features and advantages of the
`present invention will be readily apparent to one skilled in
`the art from the following written description, read in
`conjunction with the drawings, in which:
`FIG. 1 illustrates a conventional 3-sector cellular base
`Station;
`FIG. 2 illustrates a 12-sector Site having alternating polar
`ization;
`FIG. 3 illustrates a site having twelve sectors for each of
`two groups of channels with a half-sector offset between the
`tWO groups,
`FIG. 4 illustrates a site having twelve sectors for each of
`two groups of channels with a half-sector offset between the
`tWO groups,
`FIG. 5 illustrates E-plane and H-plane directivity patterns
`of an antenna with a circular aperture where directive gain
`is plotted as a function of angular offset from peak,
`FIG. 6 illustrates a wireless terminal with adaptive polar
`ization Selection;
`FIG. 7 illustrates a terminal with polarization diversity;
`FIG. 8 illustrates an exemplary WLL base station for
`determining antenna polarization; and
`FIG. 9 illustrates a phasing network for the arrays of FIG.
`8.
`
`DETAILED DESCRIPTION
`FIG. 1 illustrates the radiation patterns produced by a
`conventional three-sector cellular base station. The 360
`degrees of azimuth around the base Station antenna Site are
`divided into three, 120-degree Sectors. Each Sector has an
`asSociated directional antenna with the polar radiation pat
`tern shown, for transmitting to or receiving Signals from the
`outstations Such as mobile phones, for example. It is cus
`tomary in three-sector cellular Systems for radiation patterns
`of the three Sectors to croSS each other at around -10 to -12
`dB relative to peak antenna directivity. This results from the
`
`15
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`45
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`50
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`
`TABLE 1.
`
`40
`
`CII (dB)
`Outstation Position (without alternating
`polarization)
`(in Degrees)
`
`CII (dB)
`(with alternating
`polarization)
`
`O
`5
`1O
`15
`2O
`25
`3O
`
`23.6
`12.3
`5.5
`-O.4
`5.5
`12.3
`24.7
`
`26.O RHC
`22.3 RHC
`17.5 RHC
`12.8 RHC/LHC
`17.5 LHC
`22.3 LHC
`26.1 LHC
`
`RHC - Rigbt Hand Circular Polarization
`LHC - Lefi Hand Circular Polarization
`
`Serving
`Sector
`
`1.
`1.
`1.
`1 or 2
`2
`2
`2
`
`It can be seen that the C/I, while acceptable for stations at
`0 degrees (greater than 20 dB) and tolerable at 5 degrees
`(greater than 10 dB) from the center of their Serving Sector,
`is much lower (5.5 dB and -0.4 dB) at locations between
`two Sectors when adjacent Sectors use the same polarization.
`With adjacent Sectors using opposite polarization, however,
`the C/I is always greater than 12.8 dB.
`Another method for reducing interference is to utilize a
`Sector only at angles where it provides an acceptable C/I,
`Such as +/-7.5 degrees from center. The gaps in between are
`then filled by having a set of twelve sectors displaced by 15
`degrees and using a different non-interfering channel, e.g.
`different frequency or timeslot.
`Table 2 below illustrates the C/I improvement obtained by
`this method, which is disclosed in Applicant's 503 patent.
`
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`US 6,356,771 B1
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`S
`
`TABLE 2
`
`Outstation
`Position
`(Degrees)
`
`C/I (dB)
`without alternating
`channel group
`
`C/I (dB)
`with alternating
`channel group
`
`Serving Channel
`Sector
`Group
`
`5
`
`O
`5
`1O
`15
`2O
`25
`3O
`
`23.6
`12.3
`5.5
`-0.4
`5.5
`12.3
`24.7
`
`23.6
`12.3
`12.3
`23.6
`12.3
`12.3
`24.7
`
`1.
`1.
`15*
`1.5
`1.5
`2
`2
`
`1.
`1.
`2
`2
`2
`1.
`1.
`
`interstitial sector
`In table 2, the worst case angular position of the outsta
`tions of +/-7.5 degrees is not shown. The C/I at these
`positions would be somewhat lower than 12.3 dB.
`FIG. 3 illustrates the arrangement of twelve Sectors using
`channel group 1 plus twelve interstitial Sectors using channel
`group 2. Although this appears to be a twenty four Sector
`system, the beamwidth of the sectors is that of a twelve
`Sector System, So the antenna aperture is not increased. The
`incorporated references describe how the same number of
`antenna elements may be used for forming any number of
`Sets of Staggered beams with the aid of digital beam forming,
`for example. Therefore, the provision of interstitial Sectors
`can be accomplished by beam forming Software rather than
`additional antenna hardware.
`In order to increase the C/I further, the arrangement of
`interstitial sectors of FIG. 3 and the arrangement of alter
`nating polarizations of FIG. 2 may be combined to achieve
`the arrangement illustrated in FIG. 4. The result is a first
`group of Six SectorS Spaced 60 degrees apart using channel
`group 1 and RHC, a Second group of Six Sectors displaced
`15 degrees from the first using channel group 2 and RHC, a
`third group of Six Sectors displaced a further 15 degrees
`using channel group 1 and LHC, and a fourth group of Six
`Sectors using channel group 2 with LHC. Thus, adjacent
`ones of the twenty four Sectors use different channel groups
`with no interference between them. Sectors that are sepa
`rated from each other by two other Sectors use the same
`channel group but different polarizations which limits the
`interference between them in part by pattern isolation and in
`part by polarization isolation. Only Sectors that are separated
`by four other Sectors use the same channel and polarization
`but have by then Sufficient pattern isolation to provide a high
`C/I. The C/I resulting from using both interstitial beams and
`alternating polarization is illustrated in Table 3 below, where
`the notation 1.5 for a Serving Sector refers to an interstitial
`Sector between SectorS 1 and 2 and the notation 2.5 for a
`Serving Sector refers to an interstitial Sector between Sectors
`2 and 3 respectively.
`
`TABLE 3
`
`Outstation
`Position
`
`CII (dB)
`using both channel and Serving
`polarization isolation
`Sector
`
`Channel
`Polarization
`
`O
`5
`1O
`15
`2O
`25
`3O
`35
`40
`45
`50
`55
`
`26.0
`22.3
`22.3
`26.0
`22.3
`22.3
`26.1
`22.3
`22.3
`26.1
`22.3
`22.3
`
`1.
`1.
`1.5
`1.5
`1.5
`2
`2
`2
`2.5
`2.5
`2.5
`3
`
`1 RHC
`1 RHC
`2 RHC
`2 RHC
`2 RHC
`1 LHC
`1 LHC
`1 LHC
`2 LHC
`2 LHC
`2 LHC
`1 RHC
`
`FIG. 5 illustrates typical E- and H-plane directivity pat
`terns of an antenna with a circular aperture, Such as a patch
`
`15
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`35
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`40
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`45
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`50
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`6
`antenna or an array of Such patches. The beamwidth of the
`antenna is determined by the total aperture of the antenna.
`The aperture may be set, for example, by choosing the
`number of elements in the array. FIG. 5 illustrates the
`directivity patterns for vertical and horizontally polarized
`components of an RHC or LHC wave. The gain for the
`Vertically and horizontally polarized components is not quite
`the same in the Sidelobes, which lead to imperfect Suppres
`sion of the unwanted polarization in the sidelobes. This
`effect has been taken into account in computing the C/I
`values of Table 3 by using these patterns for both the base
`Station Sector antennas and for the outstation antennas,
`suitably scaled in beamwidth.
`In a wireless in the local loop (WLL) embodiment of the
`present invention, which uses a radio System to deliver
`telephone Service by wireleSS communications to residential
`areas, it is envisaged that fixed terminals installed at indi
`vidual homes will use directional antennas, akin to TV
`antennas. This results in the System being better able to
`discriminate Signals between those from a Serving base
`Station and those from interfering base Stations. Interference
`from other Sites was not taken into account in computing the
`C/I values of Table 3.
`The values for C/I in Table 4 below were computed taking
`into account interference from Six Surrounding sites. The C/I
`values of Table 4 account for interference arriving from
`different angles through use of exemplary directivity pat
`terns such as those illustrated in FIG. 5. The directivity
`pattern was scaled in beamwidth to be -4 dB at +/-15
`degrees from Sector center for base Station use, and Scaled to
`be -4 dB at +/-45 degrees for Outstation use. Interference
`arriving from Sites at different distances was accounted for
`by use of the free-space propagation law, i.e. a distance
`Squared law. Free Space propagation is a reasonable assump
`tion for short range WLL applications up to about one mile,
`and a free Space law is also a necessary assumption for
`obtaining polarization isolation. When a signal propagates
`by non-free Space laws. Such as diffraction or reflection,
`polarization changes can occur. For this reason, adaptive
`selection of polarization will be described later.
`TABLE 4
`
`Degrees
`
`Azimuth
`
`O
`5
`1O
`15
`2O
`25
`3O
`
`C/I (dB)
`
`C/IE (dB)
`
`C/I (dB) C/ID
`(dB)
`
`6.17
`4.99
`2O2
`-2.61
`1.90
`4.79
`5.87
`
`8.32
`7.91
`6.38
`3.60
`7.55
`9.22
`9.60
`
`6.1
`4.9
`4.9
`6.O
`4.9
`4.8
`5.9
`
`8.3
`7.9
`7.9
`8.8
`8.0
`9.2
`9.6
`
`C/I taking account of surrounding cell/site interference
`CIA = 12 sectors using same channel and polarization (all sites)
`C/IE = 12 sectors using same channel and alternating polarization
`C/Ic = 12 sectors using a first channel plus 12 interstitial sectors using a
`second channel
`C/I = 12 sectors using alternating polarization on a first channel plus
`twelve interstitial sectors using alternating polarization on a second chan
`nel
`According to the values in Table 4, the same channel may
`be used twelve times over at every site as long as the
`modulation and coding chosen for communications function
`satisfactorily at a C/I of around 7.9 dB. For example, the
`coherent TDMA modulation used in the European digital
`cellular System known as GSM can provide acceptable
`performance at this C/I when the fading environment is a
`Ricean fading environment of short-range WLL.
`Even with the use of a pattern of fixed polarization re-use,
`an outstation should still Select the polarization to be used
`
`Ex.1016
`APPLE INC. / Page 13 of 16
`
`

`

`7
`depending on the Outstation's location and the Selection of
`the Serving Sector. This can change for mobile Stations, but,
`even when constructing fixed installations, it is difficult to
`know in advance, absent a Survey, which Sector will be the
`best Serving Sector. Furthermore, Outstations should, under
`ideal conditions, be of identical design, be capable of
`operating with any polarization and on any channel fre
`quency with the optimum polarization being automatically
`adapted in operation after installation.
`In addition, it may only be possible to assign a fixed
`pattern of polarization and channel re-use to base Stations
`only in cases of regular spacing. Since a System can come
`into being by the addition of base Stations as the need for
`capacity increases, irregular networks can result over a
`period of time. Thus, it may also be desirable for a base
`Station to choose the polarization to be used in a given Sector
`and a given radio channel adaptively in order to fit in with
`ongoing communications with minimum interference.
`A method will now be described for enabling both out
`Stations or base Stations to dynamically choose the optimum
`polarization. When only the Outstation has to choose its
`polarization to match the fixed polarization of a Serving base
`Station, it may do So as follows:
`An outstation is installed with an antenna capable of being
`Selected to receive Signals of either polarization and to
`provide Signals of the Selected polarization to the receiver.
`The polarization is Selected by a microprocessor controller.
`Upon installation, the outstation receiver Searches for a
`Special control-channel Signal radiated permanently by base
`Stations and provides an indication of control channel Signal
`Strength found on various channel frequencies using both
`polarizations. The indication may, for example, be displayed
`on a special installation aid or instrument to enable the
`installing engineer to adjust the antenna orientation to
`receive the best control channel Signal Strength, Signal-to
`interference ratio or other quality measure, Such as lowest bit
`error rate. Once the installation is complete, the apparatus
`will listen to the control channel providing the best Signal
`using the optimum receiver polarization for that channel. If
`the apparatus transmits to the base Station, it may initially do
`so using a Random Access Channel (RACH) which can be
`located in relation to the selected “best control channel. For
`example, the RACH may be at a constant frequency offset
`from the control channel receive frequency, or a constant
`time offset from a control channel receive timeslot, or both.
`Upon detecting the RACH transmission from an outstation,
`a base Station transmits a traffic channel allocation to the
`outstation. This can include a frequency channel Selection, a
`timeslot Selection, a CDMA access code Selection and a
`polarization Selection. Although the Sector Serving a particu
`lar fixed outstation may remain constant along with the
`polarization of that Sector on a given channel frequency, the
`polarization can change between different channel frequen
`cies. One reason for this is to facilitate the multiplexing of
`different channels into the transmit Sector antenna. In U.S.
`Pat. No. 5,584,057, herein incorporated by reference, Appli
`cant describes the difficulties encountered in attempting to
`couple transmitterS operating at adjacent channel frequen
`cies into the Same antenna and Solves the problem by
`coupling even numbered channels to a first antenna and odd
`numbered channels to a Second antenna. According to an
`exemplary embodiment of the present invention, a base
`Station antenna is provided with a first transmit connection
`for Signals to be transmitted using RHC and a Second,
`isolated input for LHC Signals. Alternate frequency channels
`can be coupled to alternate inputs, thus facilitating the
`coupling as well as enhancing the adjacent channel rejection
`
`5
`
`15
`
`25
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`US 6,356,771 B1
`
`8
`by ensuring that adjacent channels in the same beam do not
`use the same polarization. This can be achieved by flipping
`the use of LHC and RHC between adjacent channels of the
`first channel group of FIG.3 or FIG. 4, and likewise between
`adjacent channels of the Second channel group. In this
`manner, the adjacent channels are used either with opposite
`polarization in the same beam or with the same polarization
`in different beams. The adjacent channel isolation require
`ments are relaxed while permitting a tighter channel Spac
`ing. Thus, depending on the assigned channel frequency, the
`outstation may also have to adapt its transmit and receive
`polarization.
`In an irregular network topology, for example, where it is
`not possible to assign fixed polarizations to fixed base
`Station Sectors, an adaptive polarization Selection mecha
`nism can be used. A base Station can choose a fixed
`polarization for the transmission of control channels and the
`reception of RACH signals, but Such transmissions can be
`provided with additional coding protection using error cor
`rection coding to allow operation at lower C/Is. When a
`traffic channel is to be assigned in response to a call
`initiation, however, the base Station comprises a channel
`allocation unit which determines the best Set of channel
`parameterS Such as, frequency, timeslot, polarization and
`power level, to assign for the call.
`A base Station can determine the unwanted Signal level in
`presently unassigned channels (i.e., frequency/timeslot
`combinations) in the sector in which the RACH message
`was deemed received from the outstation requesting or
`accepting Service, the unwanted Signals being received from
`other outstations that are active in different Sectors or in
`different cells, and the unwanted Signal level can be deter
`mined for both RHC and LHC polarization. The base station
`can then assign the channel and polarization that gives the
`lowest interference measurement for receiving data from the
`outstation. The channel to use for transmitting to the out
`Station may then be determined by a fixed frequency or time
`offset from the receiving channel. This method assigns a
`channel only with regard to uplink quality, and assumes that
`the downlink quality will be acceptable.
`FIG. 6 illustrates a radiotelephone terminal suitable for
`adaptive polarization selection operation. A patch array (10)
`is an exemplary method for constructing directional anten
`nas. A number of resonant conductive discS disposed over a
`ground plane can be fed at either of two alternative places on
`the disc to provide horizontal or vertically polarized radia
`tion or reception. An array that forms only a Single directive
`beam can have the discS cophased by a feed line connecting
`the horizontally polarized drive points and another feed line
`connecting the vertically polarized drive points. The com
`bined feed points are then connected to a 4-port, 90-degree
`coupler (11) to form LHC and RHC polarization drive
`points. A Polarization Selection Switch (12) is connected to
`select either the RHC or the LHC drive point to be connected
`to transmitter/receiver (13) that is controlled by the control
`processor (14). The control processor Selects the polariza
`tion assigned for receiving a call by the base Station Sending
`a channel assignment message to the terminal of FIG. 6.
`Control processor (14) may select the polarization for
`receive and the polarization for transmit Separately when the
`radio protocol is of a Time Division Duplex (TDD) type in
`which the transmitter transmits bursts of data between the
`bursts of data being received.
`If the terminal operates in a non-TDD mode in which
`transmissio