Ulllted States Patent [19]
`Smith et al.
`
`[54] METHOD AND APPARATUS FOR
`TRANSMITTING COMMUNICATION
`SIGNALS USING TRANSMISSION SPACE
`
`DIVERSITY AND FREQUENCY DIVERSITY
`
`[75] Inventors: David Anthony Smith, Guildford,
`United IQHgdOIIl; Hz‘ikan Gunnar
`Olofsson, Stockholm; Knut Magnus
`Almgren, Sollentuna, both of Sweden
`
`[73] Assignee: Telefonaktiebolaget LM Ericsson
`(pubn’ Stockholm’ Sweden
`
`US006006075A
`[11] Patent Number:
`[45] Date of Patent:
`
`6,006,075
`Dec. 21, 1999
`
`5,329,548
`5,430,713
`5,507,035
`5,542,107
`5,648,968
`
`7/1994 Borg ...................................... .. 375/202
`7/1995 Gupta et a1.
`370/50
`4/1996 BantZ et al.
`375/299
`7/1996 Kay ........... ..
`.. 455/101
`7/1997 Reudink ................................ .. 375/299
`
`7/1998 Suzuki .................................. .. 455/101
`5,787,122
`FOREIGN PATENT DOCUMENTS
`
`0 213 780 A2 3/1987 European Pat O?- -
`0 364 190 A2 4/1990 European Pat. Off. .
`‘218253255613?
`$155899“ Pat Off-
`WO 97/01044 12/1997 WIPO I
`
`[21] Appl' NO‘: 08/665’501
`[22] Filed:
`Jun_ 18’ 1996
`
`Primary Examiner—EdWard F. Urban
`Attorney, Agent, or Firm—Jenkens & Gilchrist, PC.
`
`[51] Int. Cl? ..................................................... .. H04B 7/06
`
`[57]
`
`ABSTRACT
`
`U-S. Cl. ........................ ..
`375/299
`_
`[58] Fleld of Search ................................... .. 455/101, 102,
`455/103> 560> 561> 562; 375/299
`_
`R f
`Ct d
`e erences l e
`Us PATENT DOCUMENTS
`
`56
`[
`]
`
`4,479,226 10/1984 Prabhu et a1. ........................ .. 375/321
`1;;
`IS\I£:’1SeSIIelnet";1'""
`375/232
`4/1993 Larsson
`455/101
`1/1994 Nakahi et a1.
`1/1994 Dent et a1. .............................. .. 380/23
`
`572047878
`5,280,631
`5,282,250
`
`A transmitter diversity assembly and an associated method
`for a radio transmitter. When coupled to a radio transmitter
`to form a portion thereof, Signal diversity is Created to
`overcome multi-path fading. A plurality of antennas are
`freely selectable to be coupled to receive bursts of a com
`munication signal, modulated to be transmitted on any of
`selected carriers. By altering the carriers upon Which the
`bursts of the communication signal are transmitted and the
`spatially_separated antennas used to transduce the bursts of
`the communication signal, both transmission space diversity
`and frequency diversity is Created‘
`
`17 Claims, 5 Drawing Sheets
`
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`Page 1 of 16
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`

`
`U.S. Patent
`
`Dec. 21, 1999
`
`Sheet 1 of5
`
`6,006,075
`
`TRNSMTTR
`
`TRNSMTTR
`
`16)
`
`Signal
`
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`
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`
`CONTROLLER
`(Determines
`coherence BW)
`
`RX
`CIRCUITRY
`
`FIG. 1
`
`Page 2 of 16
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`

`
`U.S. Patent
`
`Dec. 21, 1999
`
`Sheet 2 of5
`
`6,006,075
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`High coherence BW
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`Page 3 of 16
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`

`
`U.S. Patent
`
`Dec. 21, 1999
`
`Sheet 3 0f5
`
`6,006,075
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`Page 4 of 16
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`Page 4 of 16
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`

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`U.S. Patent
`
`Dec. 21, 1999
`
`Sheet 4 0f5
`
`6,006,075
`
`12
`
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`Page 5 of 16
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`Page 5 of 16
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`

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`U.S. Patent
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`Dec. 21, 1999
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`Sheet 5 of5
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`6,006,075
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`# OF ANTENNAS A = 5
`
`COHERENCE BW = 38
`
`# OF FREQUENCIES F = 4
`
`available
`frequencies
`
`E‘
`
`1
`102
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`
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`
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`Page 6 of 16
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`

`
`1
`METHOD AND APPARATUS FOR
`TRANSMITTING COMMUNICATION
`SIGNALS USING TRANSMISSION SPACE
`DIVERSITY AND FREQUENCY DIVERSITY
`
`The present invention relates generally to apparatus and
`methods for overcoming multi-path fading of communica
`tion signals transmitted upon a communication channel
`susceptible to multi-path fading. More particularly, the
`present invention relates to a transmitter diversity assembly,
`and an associated method for creating signal diversity, such
`as transmission space diversity or frequency diversity, for a
`transmitter Which transmits the communication signals upon
`the communication channel. The signal diversity created
`through operation of the assembly mitigates the effects of
`fading of the communication signals transmitted upon the
`communication channel.
`The transmitter diversity assembly, and its associated
`method of creating signal diversity, is advantageously
`utiliZed, for example, at a base station of a cellular commu
`nication system. The transmitter diversity assembly, and its
`associated method, is advantageously utiliZed to create sig
`nal diversity both in a conventional cellular communication
`system Which transmits analog communication signals and
`in a digital cellular communication system. When used at a
`base station of a digital cellular communication system, or
`at a transmitter of another digital communication system to
`create transmission space diversity, communication signal
`bursts are transmitted from different antennas of an antenna
`assembly, thereby to create the transmission space diversity.
`The transmission diversity assembly, and its associated
`method, is also advantageously utiliZed to create frequency
`diversity in conjunction With a communication system
`Which utiliZes a frequency hopping scheme. Use of the
`transmission diversity assembly to create both frequency
`and transmission space diversity further mitigates the effects
`of fading of the communication signals transmitted upon the
`multi-path channel.
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`BACKGROUND OF THE INVENTION
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`A communication system is formed, at a minimum, of a
`transmitter and a receiver interconnected by a communica
`tion channel. Communication signals generated by the trans
`mitter are transmitted upon the communication channel to be
`received by the receiver.
`A radio communication system is a type of communica
`tion system in Which the communication channel is formed
`of a radio frequency channel. Aradio communication system
`is advantageous for the reason that a ?xed, or hardWired,
`connection is not required to form the communication
`channel extending betWeen the transmitter and receiver.
`Communication can be effectuated betWeen remotely
`positioned transmitters and receivers Without the need to
`form the hardWired or other ?xed connections therebetWeen.
`A cellular communication system is a type of radio
`communication system. When the infrastructure, hereinafter
`referred to as the netWork, of the cellular communication
`system is installed in a geographical area, a subscriber to the
`cellular system is generally able to communicate telephoni
`cally in the system When positioned at any location Within
`the geographical area encompassed by the cellular netWork.
`Technological advancements and economies of scale have
`contributed to loWered costs of communicating pursuant to
`a cellular communication system. Concomitant With the
`decreased costs of communicating pursuant to a cellular
`communication system has been an increase in usage of such
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`systems. In some instances, conventional cellular commu
`nication systems have been operated at their maximum
`capacities. When operated at their maximum capacities,
`access is sometimes denied to additional users attempting to
`communicate pursuant to such systems. Ongoing calls are
`sometimes also adversely affected.
`To avoid capacity problems and to permit an increase in
`the number of users permitted to utiliZe a cellular commu
`nication system, attempts have been made to increase the
`communication capabilities of such systems. To increase
`communication capacities, some conventional, cellular com
`munication systems using conventional, analog technologies
`have been converted to digital, cellular communications
`systems Which utiliZe digital coding and modulation tech
`nologies. Other radiotelephonic, and other communication
`systems, have been similarly converted.
`Because digital communication systems generally utiliZe
`the radio frequency transmission channels upon Which the
`communication signals are transmitted more efficiently,
`increased numbers of communication signals can be trans
`mitted upon the radio frequency channels allocated for such
`communication systems.
`By digitiZing an information signal, Which is modulated
`to form a communication signal, signal redundancies can be
`removed out of the information signal Without affecting the
`amount of information transmitted in a communication
`signal formed therefrom. Also, once an information signal is
`digitiZed, a communication signal formed therefrom can be
`transmitted in discrete, discontinuous bursts. TWo or more
`communication signals can thereby be multiplexed together
`and transmitted sequentially upon a single frequency chan
`nel. A tWofold, or greater, increase in capacity is thereby
`possible When the communication system is converted into
`a digital communication system.
`The communication signal transmitted upon a radio fre
`quency channel is susceptible to scattering, diffraction,
`re?ection, and attenuation. Signal re?ection of the transmit
`ted signal causes the signal actually received by a receiver
`to be the summation of signal components transmitted by the
`transmitter by Way of, and some instances, many different
`paths, in addition to a direct, line-of-sight path.
`The communication channel is sometimes referred to as a
`“multi-path channel,” as the signal actually received by the
`receiver is the summation of a plurality of signal compo
`nents transmitted to the receiver on a plurality of different
`paths. Values of the signal components transmitted upon the
`multiple numbers of paths are dependent, in part, upon their
`relative phases. Therefore, the value of the summation of the
`plurality of signal components received by the receiver is
`dependent upon the position at Which the receiver is located
`When the signal is received. The receiver might be posi
`tioned such that signals transmitted on the plurality of signal
`paths add together destructively. Signal “fading” occurs
`When the signals add together destructively, and fading
`“dips” or “nulls” occur When the summation of the received
`signals results When such destructive addition makes
`dif?cult, or prevents, accurate determination of the informa
`tional content of the received signal.
`Because fading deleteriously effects the quality of
`communications, attempts are sometimes made to mitigate
`the effects of fading. The deleterious effects of fading are
`particularly problematical in a cellular communication sys
`tem When a subscriber unit operable to communicate therein
`is maintained in a stationary, or sloWly moving, position. If
`positioned at an area in Which a fading dip is signi?cant, a
`signi?cant amount of sequential information might be lost.
`
`Page 7 of 16
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`

`
`3
`More particularly, various kinds of diversity are created at
`selected locations of the digital communication system to
`mitigate the effects of the multi-path fading. Time diversity,
`frequency diversity, receiver space diversity, and transmis
`sion space diversity are all types of diversity Which can be
`created to mitigate the effects of multi-path fading.
`When time diversity is created, signal bits of an informa
`tional signal are spread out, or interleaved together With
`other signal bits, over time, thereby to spread the bits over
`a time period. When interleaved or otherWise spread out
`over time, the likelihood that all of the bits are received at
`a receiver at a fading null is reduced. To be effective, the bit
`spreading requires that the fading nulls of the multi-path
`channel not last for signi?cant time periods. If the nulls last
`for lengthy time periods, spreading out of the bits of the
`informational signal does not create diversity of levels
`effective to mitigate the effects of fading.
`Receiver space diversity is also sometimes created. To
`create receiver space diversity, tWo or more receiver anten
`nas are positioned at tWo or more spaced apart positions, or
`at tWo or more different angles. The creation of receiver
`space diversity requires at least minimum physical separa
`tion distances betWeen the receiver antennas. Such mini
`mum separation distances cannot be provided at a subscriber
`unit operable in a cellular communication system due to the
`small siZe of the subscriber unit. Also, the conventional need
`for redundant receiver circuitry portion for each of the tWo
`or more receiver antennas is siZe-prohibitive in a subscriber
`unit Which must be of minimal dimensions. Therefore,
`receiver space diversity sometimes cannot be utiliZed to
`create necessary levels of diversity to mitigate the effects of
`fading.
`Frequency diversity is also sometimes created to mini
`miZe the effects of multi-path fading. Frequency hopping,
`i.e., transmitting bursts of a communication signal on car
`riers of different frequencies, spreads the communication
`signal over various frequencies. In a digital cellular com
`munication system, such as the Group Special Mobile
`(GSM) communication system, an adequate level of fre
`quency diversity is sometimes unable to be created as
`sometimes only a limited number, as feW as, for instance,
`tWo, different carriers are available to transmit communica
`tion signals betWeen a radio base station and subscriber unit.
`As multi-path fading is generally frequency-selective, the
`transmission of bursts of the communication signal on a
`different frequency carriers provides a diversity effect.
`HoWever, the number of different frequency carriers upon
`Which the bursts of the communication signal can be trans
`mitted are sometimes limited. Such limitations limit the
`amount of frequency diversity Which can be created.
`Also, When successive carriers upon Which successive
`bursts of a communication signal are transmitted are of
`similar fading characteristics, little frequency diversity is
`created. The coherence bandWidth is a frequency range
`Which exhibits similar fading characteristics. When succes
`sive bursts of communication signals are transmitted upon
`carriers Which are Within the coherence bandWidth, little
`frequency diversity is created by such frequency hopping. If
`successive bursts of the communication signal are not trans
`mitted on carriers Within the same coherence bandWidth,
`communication quality degradation occurring as a result of
`multi-path fading is of less of a problem. Appropriate
`selection of the carriers upon Which to transmit successive
`bursts of the communication signal Would therefore advan
`tageously better overcome the deleterious effects of multi
`path fading.
`Transmission space diversity is sometimes also created.
`To create transmission space diversity, tWo or more trans
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`mitting antennas are positioned at spaced-apart positions. In
`one manner of creating transmission space diversity, the
`same information is transmitted by each of the tWo or more
`transmitting antennas, but the information is transmitted at
`offset times. In another manner by Which transmission space
`diversity is created, bursts of the communication signal are
`transmitted at only one antenna at a time, but shifting
`betWeen the antennas occurs so that the bursts are transmit
`ted sequentially to different ones of the antennas. Such a
`manner of creating the transmission space diversity is
`referred to as antenna hopping. Existing circuitry for creat
`ing transmission space diversity, hoWever, does not typically
`provide for complete freedom of selection of antennas,
`particularly in instances in Which statically-tuned transmitter
`elements are utiliZed to modulate the communication signal.
`While the creation of receiver space diversity is imprac
`tical in the mobile subscriber units of a cellular communi
`cation system and similarly in some other communication
`systems, and While the creation of time diversity is some
`times unable to overcome the effects of multi-path fading,
`frequency and transmission space diversity can be created at
`a base site of a cellular communication system to overcome
`the effects of multi-path fading. Other communication
`devices of other communication systems Which are suscep
`tible to multi-path fading similarly can create frequency and
`transmission space diversity to overcome the deleterious
`effects of fading. Circuitry and methodology for a commu
`nication device Which permits greater freedom in the cre
`ation of transmission space diversity and frequency diversity
`Would be advantageous.
`It is in light of this background information related to the
`creation of signal diversity to overcome the effects of
`multi-path fading that the signi?cant improvements of the
`present invention have evolved.
`
`SUMMARY OF THE INVENTION
`
`The present invention advantageously provides a trans
`mitter diversity assembly, and an associated method, Which,
`during operation to transmit a communication signal, creates
`signal diversity to mitigate the effects of transmission of the
`communication signal on a multi-path, fading channel. The
`transmitter diversity assembly is utiliZed in conjunction With
`any of many different types of transmitters including trans
`mitters operable to generate conventional, analog commu
`nication signals and those Which generate digitally-encoded
`communication signals.
`The transmitter diversity assembly and its associated
`method is advantageously utiliZed, for eXample, at a base
`station of a cellular communication system to create signal
`diversity in doWnlink signals transmitted by the base station
`to mobile subscriber units. Bursts of the communication
`signals generated by transmitter circuitry of the base station
`are transduced by alternate ones of a set of spatially
`separated antennas, thereby to create transmission space
`diversity.
`In one aspect of the present invention, the communication
`signals are transmitted upon successive carriers in Which
`selection of Which of the carriers upon Which to transmit
`successive portions of the communication signals is made
`responsive to transmission characteristics of the communi
`cation channels upon Which the communication signals can
`be transmitted.
`In another aspect of the present invention, the transmitter
`diversity assembly creates both frequency diversity and
`transmission space diversity. The communication signal
`generated at the base station is generated pursuant to a
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`Page 8 of 16
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`

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`6,006,075
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`5
`frequency hopping scheme to create diversity, and transmis
`sion space diversity is created through operation of a diver
`sity antenna assembly. The frequency diversity created by
`the frequency hopping of the communication signal together
`further mitigates the effects of fading of the communication
`signal transmitted upon a multi-path communication chan
`nel.
`In yet another aspect of the present invention, the trans
`mitter diversity assembly forms a diversity antenna assem
`bly Which creates transmission space diversity. The diversity
`antenna assembly of an embodiment of the present invention
`and its associated method for creating transmission space
`diversity are particularly advantageously utiliZed When other
`manners by Which to create diversity to mitigate the effects
`of multi-path fading are unavailable. For instance, in a
`cellular communication system, transmission space diver
`sity is sometimes the only type of diversity Which can be
`created to mitigate adequately the effects of multi-path
`fading. Receiver transmission diversity can sometimes not
`be adequately created at a subscriber unit due to the small
`dimensions of many subscriber units. Time diversity also
`sometimes can not be adequately created depending upon
`the fading characteristics of the multi-path channel upon
`Which the communication signal must be transmitted, and
`sometimes frequency diversity cannot be adequately created
`due to the limited number of different carriers Which might
`be available at a particular time.
`The diversity antenna assembly provides a multi-antenna
`assembly. Communication signals are transduced by
`selected ones of the antennas When connected to transmitter
`circuitry Which generates the communication signals.
`SWitching betWeen selected antennas of the antenna assem
`bly is effectuated quickly and, in one embodiment, switching
`occurs at baseband frequencies. When used in conjunction
`With a frequency hopping scheme, selection of the antennas
`at Which the communication signal is transduced is selected,
`in part, responsive to the frequency of the carrier upon Which
`the communication signal is to be transmitted. Thereby,
`synergistic bene?ts of both transmission space diversity and
`frequency diversity are provided.
`In these and other aspects, therefore, a transmitter diver
`sity assembly, and an associated method of creating trans
`mission space diversity, is provided for a communication
`station operable at least to transmit communication signals
`upon at least one communication channel. Antennas of a set
`of antennas are selectively coupled to the communication
`station to transduce the communication signals formed at the
`communication station. SWitching circuitry is actuatable
`into selected sWitch positions for selectively coupling
`selected antennas of the set of antennas to the communica
`tion station to transduce the communication signals formed
`at the communication station. A selector is coupled to the
`sWitching circuitry. The selector determines transmission
`characteristics of the at least one communication channel
`and actuates the sWitching circuitry into the selected sWitch
`positions responsive to the transmission characteristics
`determined thereat.
`A more complete appreciation of the present invention
`and the scope thereof can be obtained from the accompa
`nying draWings Which are brie?y summariZed beloW, the
`folloWing detailed description of the presently-preferred
`embodiments of the invention, and the appended claims.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`FIG. 1 illustrates a functional block diagram of a portion
`of a cellular communication system Which includes an
`embodiment of the present invention.
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`FIG. 2 illustrates a graphical representation Which plots
`the fading pattern of an exemplary communication system
`Which exhibits a high coherence bandWidth.
`FIG. 3 illustrates a graphical representation, similar to that
`shoWn in FIG. 2, but of an exemplary communication
`system Which exhibits a loW coherence bandWidth.
`FIG. 4 illustrates a functional block diagram of a com
`munication station Which includes a transmitter diversity
`assembly of an embodiment of the present invention.
`FIG. 5 illustrates a communication station Which embod
`ies a transmitter diversity assembly of another embodiment
`of the present invention.
`FIG. 6 illustrates a communication station Which embod
`ies yet another embodiment of the transmitter diversity
`assembly of the present invention.
`FIG. 7 illustrates an exemplary communication scheme
`utiliZing frequency hopping.
`FIG. 8 illustrates a table Which lists the antenna selections
`selected to transduce communication signals upon the fre
`quency channels of the communication scheme illustrated in
`FIG. 7.
`
`DETAILED DESCRIPTION
`
`Referring ?rst to FIG. 1, a communication station 10, here
`a radio transceiver, is operable to transceive communication
`signals With a remote communication device (not shoWn).
`The communication station 10 includes a transmitter diver
`sity assembly 12, formed of the elements positioned Within
`the block shoWn in dash, of an embodiment of the present
`invention. The transmitter diversity assembly 12 is operable
`to create signal diversity of communication signals, herein
`after sometimes referred to as “doWnlink” signals, to over
`come or otherWise mitigate the effects of multi-path fading
`of a communication signal during its transmission to the
`remote communication device.
`In the illustrated embodiment, the transmitter diversity
`assembly 12 is operable to create both frequency diversity
`and transmission space diversity. As mentioned previously,
`frequency diversity is created by transmitting the commu
`nication signals upon carriers of different frequencies at
`different times, i.e., frequency hopping the communication
`signal. Transmission space diversity is created by transmit
`ting the doWnlink signal, at different times, from different,
`spatially-separated antennas.
`The communication station 10 includes a signal source
`14, here shoWn in block form. In an embodiment in Which
`the communication station 10 forms a base station of a
`cellular, communication system, the signal source 14 is
`representative of communication signals transmitted to the
`base station by Way of, for example, a public service
`telephone netWork (PSTN). The signal source 14 forms the
`communication signal Which is applied by Way of a plurality
`of lines 16 to a plurality of single-frequency transmitter
`elements 18.
`The transmitter elements 18 each modulate the commu
`nication signal applied thereto. Each of the transmitter
`elements 18 is tuned to a different frequency, i.e., are
`statically tuned, such that modulated signals modulated by
`the various transmitter elements are modulated to be of
`different frequencies. The modulated signals generated by
`the transmitter elements are applied, by Way of lines 22, to
`?rst sides of a radio frequency (RF) sWitch 24. Second sides
`of the RF sWitch 24 are coupled to antennas 26. The antenna
`elements 26 are spatially-separated from one another. While
`three antenna elements 26 are illustrated in the ?gure, a set
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`Page 9 of 16
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`6,006,075
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`7
`of antenna elements 26 formed of another number of
`antenna elements can instead be utilized.
`The RF sWitch 24 permits connection of any of the lines
`22 With any of the antenna elements 26, thereby to permit the
`communication signal modulated to be of any of the fre
`quencies generated by any of the transmitter elements 18
`together With any of the antenna elements 26. Connection of
`any of the transmitter elements With any of the antennas is
`freely selectable, i.e., a free mapping is permitted. Selection
`of the sWitch position of the RF sWitch 24 is controlled by
`a controller 32 Which is connected to the RF sWitch 24 by
`Way of lines 34. In one embodiment of the present invention,
`the sWitch positions of the RF sWitch 24 are sWitched
`through a selected sequence to connect successive ones of
`the transmitter elements 18 With successive ones of the
`antenna elements 26. The RF sWitch 24 may, for example, be
`formed of a sWitch matrix.
`In the embodiment illustrated in the ?gure, the controller
`32 is further coupled to receiver circuitry 38 by Way of lines
`42. The receiver circuitry 38 receives “uplink” signals
`transmitted to the communication station 10 from the remote
`communication devices. In a cellular communication
`system, the remote communication devices are formed of
`mobile subscriber units. TWo receiver-diversity antennas 44
`are illustrated in the ?gure to detect the uplink signals
`transmitted to the communication station. Indications of an
`uplink signal detected by an antenna 44 are provided to the
`controller 32 by Way of the receiver circuitry 38 and the lines
`42.
`The controller 32, as shall be described in greater detail
`beloW, determines a coherence bandWidth of the communi
`cation channels formed betWeen the communication station
`10 and the remote communication devices. Responsive to
`the coherence bandWidth and previously-used carriers
`paired together With selected antenna elements, the control
`ler selects the sWitch position into Which the RF sWitch 24
`should be actuated. Indications of previously-used carriers
`are stored in a memory element 46, coupled to the controller
`32 by Way of lines 48. In one embodiment, the carriers are
`selected according to a selected frequency hopping scheme.
`The antenna elements from Which the communication signal
`is transmitted are selected for each carrier selected pursuant
`to the frequency hopping scheme. Selection of the antenna
`elements is made responsive to the coherence bandWidth and
`carrier and antenna element pairings made previously to
`transmit preceding portions of the communication signal.
`Because the RF sWitch 24 permits any of the transmitter
`elements 18 to be coupled to any of the spatially-separated
`antennas 26, connection of any of the transmitter elements
`18 With any of the antennas 26 is freely selectable. Bursts of
`the doWnlink communication signal modulated by any of the
`transmitter elements 18 can be transmitted from any of the
`antennas 26. The controller 32 is further used to select the
`carriers upon Which to transmit the bursts of doWnlink
`signals. Such selections are responsive to the transmission
`characteristics of the channels formed betWeen the commu
`nication station 10 and the remote communication devices,
`thereby to minimiZe the possibility that successive bursts of
`the doWnlink communication signal are transmitted upon
`carriers Which exhibit high levels of multi-path fading.
`In one embodiment, the RF sWitch 24 permits a plurality
`of different transmitter elements 18 concurrently to be
`coupled to a plurality of different antennas 26.
`FIG. 2 illustrates graphically the characteristics of a
`communication system in Which a high coherence band
`Width is exhibited and in Which a loW coherence space is
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`10
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`15
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`20
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`25
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`30
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`35
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`40
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`45
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`55
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`60
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`65
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`8
`exhibited. The abscissa axis 56 indicates positional
`locations, and the ordinate axis 58 indicates frequency.
`Elongated elliptical forms 60 are representative of areas
`Which exhibit high levels of multi-path fading. Point 62 is
`representative of the fading characteristic at a particular
`frequency and at a particular location. Point 62 is positioned
`Within an elliptical form 60. Therefore, communications
`betWeen the tWo communication stations at the frequency
`and location identi?ed by the point 62 exhibit signi?cant
`levels of multi-path fading.
`If the communication signals transmitted betWeen such
`communication stations are transmitted on a different carrier
`located loWer in frequency, indicated by the arroW 64,
`signi?cant multi-path fading Would still degrade the quality
`of the communication transmission. That is to say, by
`repositioning the point 62 in a direction indicated by the
`arroW 64, the communications Will still exhibit high levels
`of fading, if the point is repositioned Within the elliptical
`form. In the illustrated communication system Which exhib
`its a high coherence bandWidth, signi?cant levels of multi
`path fading are exhibited over a large range of frequencies,
`resulting in the elongated ellipsis 60. Altering the position,
`as indicated by the arroW 66, hoWever, removes the com
`munications out of the area of high multi-path fading. In the
`illustrated communication system Which exhibits a loW
`coherence space, by repositioning the point 62 in a direction
`indicated by the arroW 66, the communication is reposi
`tioned beyond the elliptical form.
`FIG. 3, conversely, graphically represents a communica
`tion system Which exhibits a loW coherence bandWidth and
`Which exhibits a coherence space similar to that shoWn in
`FIG. 2. The abscissa and ordinate axes 56 and 58 correspond
`to their correspondingly-numbered counterparts shoWn in
`FIG. 2. Here, areas of high levels of multi-path fading are
`represented by elliptical forms 70. In contrast to the elliptical
`forms 60 pictured in FIG. 2, the elliptical forms 70 are not
`elongated.
`Communications represented by point 62 are again trans
`mitted in an area Which exhibits high levels of multi-path
`fading. In contrast to the high coherence bandWidth repre
`sented in FIG. 2, here alteration of the frequency channel in
`the direction indicated by the arroW 74 removes the com
`munications out of the areas Which exhibit high multi-path
`fading With a lesser amount of frequency change than that
`required in the communication system represented in FIG. 2.
`Again, altering the position, as ind