(12) United States Patent
`Young
`
`(10) Patent No.:
`(45) Date of Patent:
`
`US 6,643,522 B1
`Nov. 4, 2003
`
`USOO6643522B1
`
`(54) METHOD AND APPARATUS PROVIDING
`SIMULTANEOUS DUAL MODE
`OPERATIONS FOR RADIOS IN THE
`SHARED SPECTRUM
`
`(75) Inventor: Song-Lin Young, Vancouver, WA (US)
`(73) Assignee: Sharp Laboratories of America, Inc.,
`Camas, WA (US)
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`(*) Notice:
`
`(21) Appl. No.: 09/536,212
`(22) Filed:
`Mar 27, 2000
`(51) Int. Cl." ................................................. H04M 1100
`(52) U.S. Cl. ................ 455/552.1; 455/90.2; 455/422.1;
`455/412
`(58) Field of Search ................................. 455/552,553,
`455/575, 550, 73,422, 41, 78,552.1, 90.2,
`442.1, 41.2; 370/441, 442, 464, 335, 342,
`344, 296, 465; 375/133, 141
`
`(56)
`
`References Cited
`U.S. PATENT DOCUMENTS
`5,390,345 A 2/1995 Wada et al.
`5,392,300 A 2/1995 Borth et al.
`5,630.224. A
`5/1997 Swail
`5,710.986 A
`1/1998 Obayashi et al.
`5,765,113 A 6/1998 Russo et al.
`5,794,159 A * 8/1998 Portin ........................ 455/553
`
`5,884,189 A 3/1999 Yamazaki et al.
`6,058,137 A * 5/2000 Partyka ...................... 375/131
`6,108,313 A
`8/2000 Lee et al. ................... 370/294
`* cited by examiner
`Primary Examiner Edward F. Urban
`Assistant Examiner Sonny Trinh
`(74) Attorney, Agent, or Firm-Robert D. Varitz, P.C.
`(57)
`ABSTRACT
`A dual mode RF radio includes a first transceiver operating
`under a first transmitter/receiver protocol, and having a first
`receiver portion and a first transmitter portion therein; and a
`Second transceiver operating under a Second transmitter/
`receiver protocol, and having a Second receiver portion and
`a Second receiver portion therein; wherein both transmitter/
`receiver protocols are spread spectrum protocols, a shared
`antenna operatively connected to both Said first transceiver
`and Said Second transceiver; and an isolation mechanism for
`isolating Said first and Second receiver portions from a
`transmitter portion while Said transmitter portion is trans
`mitting. A method of Simultaneously operating a dual mode
`RF radio includes operating a first transceiver under a first
`transmitter/receiver protocol; operating a Second transceiver
`under a Second transmitter/receiver protocol, wherein both
`transmitter/receiver protocols are spread spectrum proto
`cols, sharing an antenna between the first transceiver and the
`Second transceiver; and isolating the receiver portions of the
`transceivers from the transmitter portion of the transceivers
`while a transmitter portion is transmitting, including pro
`Viding a circulator for connecting the receiver portions and
`the transmitter portions to the shared antenna.
`10 Claims, 6 Drawing Sheets
`
`50-
`
`
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`
`
`(a)
`POWER 60
`SPLTTER
`
`
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`BAND PASS
`FILTER
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`TX/RXSWITCH CONTROL. A
`
`84a
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`54
`/
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`RADIO ADOWN
`CONVERTER 8
`DEMODULATOR
`
`RADiO BDOWN
`CONVERTER&
`DEMODULATOR
`
`RADIO A
`MODULATOR &
`PREAMPFER
`
`RADIO A
`BASEBAND
`PROCESSOR
`
`RADIOB
`BASEBAND
`PROCESSOR
`
`MAN
`PROCESSOR
`
`
`
`MODULATOR &
`PREAMPFER 86
`
`
`
`
`
`TXIRXSWITCH CONTROLB
`
`86a
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`U.S. Patent
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`Nov. 4, 2003
`
`Sheet 1 of 6
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`US 6,643,522 B1
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`f
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`f
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`f
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`CHANNEL 1 CHANNEL2 CHANNEL3
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`Pout(dBm)
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`PSat
`
`Pin(dBm)
`Pth
`O
`1Hom-b-
`LINEAR OPERATION
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`s
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`Fig. 3
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`Ex. 1011 / Page 2 of 13
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`US 6,643,522 B1
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`U.S. Patent
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`Nov. 4, 2003
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`MOIWITACIOWEQ ?
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`Sheet 2 of 6
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`U.S. Patent
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`Nov.4, 2003
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`Sheet 3 of 6
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`US 6,643,522 B1
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`Ex. 1011 / Page 4 of 13
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`Ex. 1011 / Page 4 of 13
`ERICSSON v. UNILOC
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`U.S. Patent
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`Nov. 4, 2003
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`Sheet 4 of 6
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`US 6,643,522 B1
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`Ex. 1011 / Page 5 of 13
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`U.S. Patent
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`Nov. 4, 2003
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`Sheet 5 of 6
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`US 6,643,522 B1
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`HomeRF
`
`SUPERFRAME-20ms - T T
`
`-
`0.625ns
`BLUETOOTH
`TIME SLOT
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`Fig. 10
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`PROBABILITY
`NO. OF COLLISIONS
`
`
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`Ex. 1011 / Page 6 of 13
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`U.S. Patent
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`Nov. 4, 2003
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`Sheet 6 of 6
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`US 6,643,522 B1
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`Ex. 1011 / Page 7 of 13
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`US 6,643,522 B1
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`1
`METHOD AND APPARATUS PROVIDING
`SIMULTANEOUS DUAL MODE
`OPERATIONS FOR RADIOS IN THE
`SHARED SPECTRUM
`
`15
`
`2
`communication units which are capable of protecting data
`transmission with encoderS/decoders in either error detec
`tion or forward error correction modes. A mode Selector
`Switch is used for Selecting the desired mode of operation.
`U.S. Pat. No. 5,630,224 to Swail, for Method and appa
`ratus for avoiding desensitization of a radio frequency
`receiver, granted May 13, 1997, describes a techniques for
`avoiding receiver desensitization by changing the frequency
`being used by a Subscriber unit or delaying the data trans
`mission. The detection of interference is achieved by com
`paring measured BER (bit error rate) vs. expected BER, for
`RSSI (received signal strength indicator) of the same level.
`U.S. Pat. No. 5,710,986 to Obayashi et al., for Dual mode
`radio communication apparatus having function of Selec
`tively designating analog or digital mode, granted Jan. 20,
`1998, describes use of a mode designation switch selectively
`designating analog mode or digital mode.
`U.S. Pat. No. 5,765,113 to Russo et al., also entitled
`Method and apparatus for avoiding desensitization of a radio
`frequency receiver, granted Jun. 9, 1998, describes tech
`niques for avoiding receiver desensitization.
`U.S. Pat. No. 5,884,189 to Yamazaki et al. for Multiple
`modes adaptable radiotelephone, granted Mar. 16, 1999
`describes a radiotelephone which uses a Single wireleSS
`transceiver for modulating and demodulating Signals in the
`transmitting/receiving frequency bands for both a cellular
`telephone system (AMPS) and a cordless telephone system
`(ISM band). A control unit, which incorporates the software
`for TIA/EIA/IS-94, is provided to control the wireless
`transceiver.
`The specification of Bluetooth System, Version 1.0 B,
`December 1999, may be found at bluetooth.com. The speci
`fication for HomeRF Shared Wireless Access Protocol
`(SWAP-CA) Specification Revision 1.2, October. 1999, may
`be found at http://www.homerf.org.
`SUMMARY OF THE INVENTION
`A dual mode RF radio includes a first transceiver oper
`ating under a first transmitter/receiver protocol, and having
`a first receiver portion and a first transmitter portion therein;
`and a Second transceiver operating under a Second
`transmitter/receiver protocol, and having a Second receiver
`portion and a Second transmitter portion therein; wherein
`both transmitter/receiver protocols are spread spectrum pro
`tocols, a shared antenna operatively connected to both Said
`first transceiver and Said Second transceiver; and an isolation
`mechanism for isolating Said first and Second receiver por
`tions from a transmitter portion while Said transmitter por
`tion is transmitting, including a circulator for connecting
`Said receivers and Said transmitters to Said antenna, a power
`Splitter for Splitting a received signal between Said first
`transceiver and Said Second transceiver, and a power, com
`biner for combining a transmitted Signal from Said first
`transceiver and Said Second transceiver.
`A method of Simultaneously operating a dual mode RF
`radio includes operating a first transceiver under a first
`transmitter/receiver protocol; operating a Second transceiver
`under a Second transmitter/receiver protocol, wherein both
`transmitter/receiver protocols are spread spectrum proto
`cols, sharing an antenna between the first transceiver and the
`Second transceiver; and isolating the receiver portions of the
`transceivers from the transmitter portion of the transceivers
`while a transmitter portion is transmitting, including pro
`Viding a circulator for connecting the receiver portions and
`the transmitter portions to the shared antenna.
`An object of the invention is to provide a single device
`which is capable of Simultaneous dual mode operations in
`the same shared frequency spectrum.
`
`FIELD OF THE INVENTION
`This invention relates to short range wireleSS communi
`cations technologies, such as HomeRF, 802.11 FHSS, and
`Bluetooth TM, and specifically to a method and apparatus to
`provide a non-deSensitizing, non-interfering dual transceiver
`device for use in Such technologies.
`BACKGROUND OF THE INVENTION
`Many wireleSS communication Systems make use of
`Spread spectrum technologies and operate in the unlicensed
`industrial, Scientific and medical (ISM) band, as regulated
`by FCC part 15 rules. While all such systems are allowed to
`use the Same frequency band, each has its own air interface,
`communication protocols, and applications. In the 2.4 GHz
`ISM band, there are many existing and emerging Standards,
`e.g., 802.11 WLAN, BluetoothTM and HomeRF. Bluetooth TM
`is a trademark/service mark of Telefonaktiebolaget LM
`Ericsson, of Stockholm, Sweden for telecommunication
`equipment, computer. communication equipment, including
`25
`radio modems, and telecommunication and computer com
`munication Services.
`802.11 WLAN products have been around for some time
`and are mostly used in an office infrastructure. Bluetooth TM
`and HomeRF, on the other hand, address the needs of
`cordless cable replacement for portable devices and home
`networking, respectively. It is not difficult to envision two or
`more wireless Systems co-existing in an office, in a residen
`tial area, or even public places. Given the foregoing
`Scenarios, each product must operate in the presence of
`interference generated by other co-existing Systems, and Still
`perform properly. Moreover, it is also desirable from the
`user's point of view to have only one wireleSS device, which
`may be used in various environments, rather than having to
`carry multiple wireleSS devices, and to have to change
`hardware and Software configurations, depending on loca
`tion and the presence of other ISM band devices.
`DeSensitization is a phenomenon wherein a receiver's
`Sensitivity degrades due to excessive signal Strength, which
`may overload the front-end circuits of the radio. The
`receiver will be desensitized if there is a second radio
`operating in close proximity, particularly if the two radioS do
`not transmit in Synchronization with each other.
`Attempts to Solve the problem of dual mode operation in
`a single transceiver include (1) Switching between the two
`modes, (2) time multiplexing different protocols of fre
`quency bands, or (3) reserving a certain period of time in one
`mode to allow transmission in another mode. There is no
`known prior art which provides a Solution for Simultaneous
`dual mode operations in the shared Spectrum using different
`transceivers in a Single enclosure.
`U.S. Pat. No. 5,390,345 to Wada et al. for Method for
`preventing desensitization and radio interference of radio
`receivers, granted Feb. 14, 1995, describes a method for
`using the level of croSS modulation from the output of a high
`frequency amplifier, or mixer, as an indication of Severe
`interference, and which controls the gain of the amplifier, or
`the attenuation of a variable attenuator preceding the
`amplifier, to avoid receiver desensitization.
`U.S. Pat. No. 5,392,300 to Borth et al., for Dual mode
`radio communication unit, granted Feb. 21, 1995, describes
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`3
`Another object of the invention is to provide multiple
`Services for users of different Systems.
`A further object of the invention is to provide an apparatus
`and method wherein frequency Selection in one System may
`be used by the other System through a shared backend
`processor.
`This Summary and objectives of the invention are pro
`Vided to enable quick comrprehension of the nature of the
`invention. A more thorough understanding of the invention
`may be obtained by reference to the following detailed
`description of the preferred embodiment of the invention in
`connection with the drawings.
`BRIEF DESCRIPTION OF THE DRAWINGS
`FIG. 1 illustrates channel partitions in a RF spectrum.
`FIG. 2 illustrates interference among communication SyS
`temS.
`FIG. 3 depicts the power input/output of a receiver.
`FIG. 4 is a block diagram of a typical radio transceiver.
`FIG. 5 is a block diagram of a radio architecture of the
`invention.
`FIG. 6 depicts adjacent channel interference.
`FIG. 7 depicts the GFSK power spectrum.
`FIG. 8 depicts frame timing of BluetoothTM and HomeRF
`Systems.
`FIG. 9 depicts the probability of collision from HomeRF
`interference.
`FIG. 10 depicts the probability of collision from Blue
`tooth interference.
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`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`The Industrial, Scientific, and Medical (ISM) band has
`become increasingly crowded, as wireleSS communication
`devices proliferate due to unlicensed usage allowed by FCC
`part 15 regulations. Mandatory use of Spread Spectrum
`technology has made it possible for different Systems to
`co-exist and operate in close proximity by controlling the
`power and time that each System is allowed to occupy the
`Spectrum, and by keeping any interference to a level which
`is acceptable to all systems in the RF spectrum. Beyond the
`interference issue, diverse requirements for different mobile
`users also drive the need for application/Service to co-exist.
`This invention addresses the problem of simultaneous
`operations of two co-located radioS which use different
`transmission protocols in a shared RF spectrum. This inven
`tion allows a device to include two radio transceivers, each
`using a different transmission/reception protocol, wherein
`both transceiverS operate at the same time. Either transmitter
`may transmit any time without desensitizing the receiver of
`the other radio. A Single device is therefore capable of
`Simultaneous dual mode operations in the Same spectrum
`and of providing multiple Services for users of different
`Systems. For instance, a HomeRF residential gateway hav
`ing BluetoothTM capability may communicate with a Blue
`tooth TM enabled personal digital assistant (PDA), which may
`be carried to home from the work place, and which will still
`allow any proximal networked computers, peripherals, and
`cordless phones to operate without disruption.
`Another advantage for an apparatus implementing the
`method described in this invention is that the information
`regarding frequency Selection in one System may be used by
`the other System through a shared backend processor. If a
`System incorporates the invention described herein, the
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`4
`exact timing of any potential frequency collisions may be
`determined in advance. Collision avoidance algorithms,
`although not part of this invention, may be implemented in
`a more efficient way than would be possible without such
`knowledge.
`This invention provides a method for using two radioS in
`a wireleSS communication device, Such that the device may
`offer multiple Services without compromising the perfor
`mance of either System incorporated therein. There are two
`issueS which must be resolved: desensitization and collision.
`These issues need to be considered when two radioS co-exist
`in close proximity and share the same frequency band. AS
`will be explained later herein, desensitization is a System
`independent effect and must be resolved in order for a dual
`System device to operate properly. This invention Solves the
`deSensitization problem by combining the front-end circuits
`of the transceivers in a novel arrangement So that two radioS
`are able to transmit and/or receive their respective signals
`without coordinating timing for Systems of different proto
`cols. Otherwise, the radio will not function properly when
`the receiver is desensitized due to the co-located radio that
`is transmitting.
`The collision issue arises when there are frequency and
`time Overlaps among Systems. The probability of collision in
`turn is dependent on certain parameters of Systems involved.
`Analysis of collision probability with Specific example SyS
`tems will demonstrate the effectiveness of the method of the
`invention. Algorithms for completely avoiding interference
`due to collisions, however, are not within the Scope of this
`invention, and will be well known to those of ordinary skill
`in the art.
`Traditionally, an allocated frequency band is divided into
`channels of equal bandwidth (BW) to allow efficient use of
`the Spectrum, as illustrated generally at 10 in FIG.1. A signal
`may occupy only one channel with center frequency, f, at
`one time, as illustrated by f, f, f . . . f.
`, f, as assigned
`by the system controller. The choice of bandwidth for each
`channel is pertinent to the requirements of an individual
`System. When a radio is within the range of another radios,
`operating the same or a different System, interference occurs
`whenever the radio receives, in addition to desired signal,
`the Signals from the other radio, which may be transmitting
`at the same time on the Same, or on a partially overlapped,
`channel.
`Interference, as a result of Spectrum sharing, dictates the
`performance of many wireleSS communication Systems.
`However, radio spectrum is a limited resource and needs to
`be shared by as many users as possible. The impacts of
`interference on System performance is primarily related to
`the power of desired receiving Signal relative to that of the
`interference producing radio. For instance, in FIG. 2, radio
`A and radio B communicate with each other in their System,
`referred to herein as a first System, while radio C and radio
`D transmit within their System, referred to herein as a Second
`System. The receiving capability of A is determined by the
`ratio of Signal Strength received:
`
`CB
`c + ID
`
`Eq. 1
`
`Where C is the desired carrier power transmitted by B and
`I, I, indicate interfering power from C and D, respectively.
`C and D may have different bandwidth from that of A and
`B. It should also be noted that they may interfere with Aeven
`if they are using a carrier frequency proximal to that used by
`A, Vice using exactly the same carrier frequency. Depending
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`on the transmitting characteristics of the interferers, any
`unwanted energy that enters a receiver's pass band may
`contribute to interference. The issue of co-channel and
`adjacent-channel interference will be discussed in more
`details later herein.
`
`Receiver Desensitization
`According to Eq. 1, the ratio of carrier power to interfer
`ence power determines the ultimate performance of a
`receiver. Increasing the distance between a receiver and an
`undesired signal Source may mitigate the impacts of inter
`ference. On the other hand, interference becomes more
`Severe when the distance between the radioS is decreased.
`When one radio is placed in proximity of another, the
`receiver may be overloaded due to transmission of the other
`radio. This condition is known as desensitization, and occurs
`regardless of carrier frequency Spacing. DeSensitization
`results because the front-end circuits of receivers have
`limited power-handling capability. If the total power enter
`ing the front-end circuit exceeds a certain threshold, the
`receiver will not function properly.
`FIG. 3 illustrates the power input/output relation of a
`typical receiver. In a linear operation region, where P is
`less than P, each dB of P, increment results in one dB
`increment of P. However, when P, exceeds P, this
`relation no longer holds, and P eventually Saturates at the
`level P. In this situation, the receiver's performance will
`degrade. It should be noted that the total power input to the
`receiver determines the operation point of a receiver.
`Therefore, the wanted Signal as well as any other Signals in
`the pass band needs to be considered when calculating P.
`A generic transceiver front-end circuit of a time division
`duplex (TDD) radio is depicted generally at 12 in FIG. 4.
`Transceiver 12 includes an antenna 14 and a band pass filter
`16, and is either in a transmitting mode or a receiving mode,
`according to the Setting of an antenna Switch 18 and a
`synthesizer Switch 20, which are both controlled by a
`TX/RX control signal 22 from a baseband processor 24.
`During a receiving period, any Signals picked up by the
`antenna will be processed by a receiver chain 26, which
`includes band pass filter 16, a low noise amplifier 28, a
`mixer 30, an IF channel filter 32, and an IF amplifier/
`demodulator 34.
`While the selection of desired signal is dependent on IF
`channel filter 32 to block all unwanted output from the mixer
`circuit, all the blocks in receiver chain 26 preceding the
`channel filter respond. to all signals in the whole pass band.
`A transmitter chain 36 includes a modulator/preamplifier 38,
`a frequency-Synthesizer 40 and a power amplifier 42.
`If the aggregate Signal power from interference exceeds
`the maximum power level, P, in FIG. 3, at which the low
`noise amplifier or mixer are designed to operate, the receiver
`Sensitivity will be degraded even though the wanted Signal
`has a power level in the normal dynamic range. As a result,
`deSensitization will affect the coverage of a wireleSS System,
`or completely prevent the receiver from functioning prop
`erly if excessive interference persists for an extended period
`of time. Therefore, any Strong Signal in the pass band, which
`does not occupy the same channel, may be a threat to the
`receiver.
`
`Transceiver Front End Circuit for Co-located
`Radios
`In order to Solve the desensitization problem, a novel
`front-end circuit configuration is provided. FIG. 5 depicts
`the architecture of a wireless device, or transceiver, 50,
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`6
`which incorporates two radios A, B, of different air inter
`faces. Instead of using a Switch, as in transceiver 12 of FIG.
`4, a circulator 52 is provided. Other components of trans
`ceiver 50 include, in a receive chain 54, a low pass amplifier
`56, and a power splitter 58, which divides an incoming
`signal into portions for Radio A and Radio B. Following
`power Splitter 58, the Signal proceeds along a divided
`receive chain for each radio, initially passing through
`transmit/receiver (Tx/RX) switches 60, 62, and a down
`converter/demodulator 64, 66, for Radios A and B, respec
`tively. Baseband processors 68, 70 are provided for each
`radio. A main processor 72 connects the two radioS to the
`remainder of the transceiver functions.
`A transmit chain 73 includes, for Radios A and B,
`respectively, a modulator and preamplifier, 74, 76, TX/RX
`Switches 78,80, a power combiner 82 and a power amplifier
`83, which is connected to circulator 52.
`Circulator 52 connects power amplifier 83, low noise
`amplifier 56, antenna 14 and band pass filter 16, providing
`isolation between receive chain 54 and transmit chain 73. A
`circulator is a 3-port device formed by a symmetrical Y
`junction coupled to magnetically biased ferrite material. It
`should be noted that circulators are used in prior art cellular
`base Stations for cascading power amplifiers of different
`channels to share the antennas. The circulator, as used in this
`invention, permits flow of Signal power in one direction
`only, e.g. from port 1 to port 2, from port 2 to port 3, and
`from port 3 to port 1. Isolation is defined as the power loss
`when Signal travels in the reverse direction, e.g., from 1 to
`3.3 to 2, and 2 to 1.
`In transceiver 50, output power from the transmitter
`power amplifier is directed to the antenna. None, or very
`little power, may pass through the circulator and reach the
`port connected to the receiver low noise amplifier. However,
`the Signals received by the antenna will reach the receiver,
`port with virtually no loss.
`In transceiver 50, both radio A and radio B baseband
`processors 68, 70 generate modulator/preamplifier control
`signals 84, 86 for turning their respective modulators and
`demodulators on and off. The outputs of these modulators
`are combined by power combiner 82 before being fed to the
`final Stage power amplifier 83. Unused outputs, e.g., during
`receiving mode, are terminated by the corresponding
`dummy loads 88, 90 with the control signals changing the
`state of TX/RX Switches. Similarly, the same set of control
`signals, 84a, 86a, select whether or not the output of the
`power splitter is connected to the demodulator for further
`processing. Power splitter 58 divides the signal from low
`noise amplifier 56 into halves So that each System may
`receive the necessary signal. An isolation mechanism
`includes circulator 52, low noise amplifier 56 and power
`amplifier 83, power splitter 58 and power combiner 82, as
`well as the control signals from baseband processors 68, 70,
`and TX/RX Switches 60, 62, 78 and 80.
`The circuit described herein enables two transceivers to
`be co-located in the same enclosure. Instead of using one
`antenna for each radio, which may result in desensitization
`of the receiver, a circulator is used to allow sharing of one
`front-end by both radios. Furthermore, the circulator isolates
`the transmitter power, keeping the transmitter power from
`passing through either receiver circuit, which would likely
`cause the receiver circuit to function improperly, or which
`may even permanently damaging the receiver circuit.
`The effectiveness of this invention dependent on the
`isolation property of the circulator. However, infinite isola
`tion does not exist in reality. For example, a commercially
`
`Ex. 1011 / Page 10 of 13
`ERICSSON v. UNILOC
`
`

`

`US 6,643,522 B1
`
`7
`available, single-junction circulator may have 30 dB of
`isolation over the entire 2.4 GHZ ISM band. To achieve
`higher isolation, two Such components may be cascaded in
`one package to provide 50 dB of isolation. One such
`commercially available circulator is that produced by Nova
`Microwave, Part No. 0245CHS (novamicro.com).
`AS previously noted, there are two emerging industry
`specifications: BluetoothTM, and HomeRF, which are used in
`the description of the operation of this invention. Table 1
`provides the pertinent System parameters for these two
`Specifications:
`
`TABLE 1.
`
`Radio Characteristics of Bluetooth and HomeRF
`
`15
`
`Frequency range
`No. of channels
`Channel bandwidth
`Hopping rate
`Output power
`Max. useable
`input level
`Modulation
`Bit rate
`Sensitivity
`
`Bluetooth TM
`
`HomeRF
`
`2.4-2.483 GHz
`2.4-2.483 GHz
`75
`79
`1 MHz
`1 MHz
`50 hops/sec
`1600 hop/sec
`
`20 dBmf4 dBm/O dBm 20 dBm/O dBm
`>= -20 dBm
`>= -20 dBm
`
`GFSK (BT = 0.5)
`1 Mbps
`-70 dBm
`
`GFSK/4GFSK (BT = 5)
`800 kbps/1.6 Mbps
`-80 dBmf-70 dBm
`
`25
`
`8
`example, frequency shift keying (FSK) type demodulators
`used by Bluetooth TM and HomeRF require a C/N>11 dB for
`proper operation. In the following analysis, this criterion
`(C/N>11 dB) is used with the Gaussian FSK (GFSK)
`modulation Spectrum to determine the number of adjacent
`channels which will affect the receiver.
`Having avoided desensitization using the circuit of FIG.
`5, it is also necessary to ensure that the interference level
`stays below required threshold. A Series of transmitter power
`spectrum for GFSK modulation, which are used by both
`Bluetooth TM and HomeRF (Table 1), of different Blue
`toothTM products are shown in FIG.7; BT indicates normal
`ized 3 dB bandwidth of the Gaussian low pass filter for pulse
`shaping. BluetoothTM uses the BT=0.5 Gaussian filter. Hom
`eRF uses the BT=5 filter, which has little effect on pulse
`shaping. As may be seen in FIG. 7, while all the curves
`Spread beyond one channel bandwidth, when normalized
`frequency =1, and contribute to adjacent channel
`interference, the curve for Bluetooth"M spectrum(BT=0.5)
`rolls off much more rapidly than that of the curve for
`HomeRF (BT-5).
`With the data in Table 1, the C/I ratios of a HomeRF
`receiver may therefore be calculated VS. a high power
`Bluetooth" device transmitting on co-channel and adjacent
`channels. Table 2 lists the resulting C/I ratios up to the fifth
`adjacent channel. Power levels of the adjacent channel
`interference (ACI) are derived from relative values in FIG.
`7, assuming a 20 dBm Bluetooth TM transmitter. It may be
`seen that despite the 50 db isolation provided by the
`circulator, it is not enough isolation to bring down CCI and
`ACI-1 to a level where the receiver meets the required C/I
`ratio, >11 dB. However, HomeRF receivers will have C/I
`ratios greater than 20 dB and will be virtually free from
`interference if the Bluetooth TM transceiver is transmitting at
`frequencies Separated by at least two channels, i.e., 2nd
`adjacent channel and up. Therefore, among the 79 available
`channels for Bluetooth TM, there are three channels, the
`carrier and the two 1st adjacent frequencies, which will
`interfere with a HomeRF receiver if a system is imple
`mented with the radio architecture of this invention.
`
`TABLE 2
`
`Interference from a co-located Bluetooth radio
`
`Bluetooth
`inter-
`ference
`(dBm)
`2O
`-10
`
`HomeRF
`Inter-
`Receiver
`ference
`Isola-
`tion at receiver sensitivity
`(dB)
`(dBm)
`(dBm)
`50
`-30
`-80
`50
`-60
`-80
`
`-50
`
`&-70
`
`&-70
`
`&-70
`
`50
`
`50
`
`50
`
`50
`
`-100
`
`<-120
`
`<-120
`
`<-120
`
`-80
`
`-80
`
`-80
`
`-80
`
`C/I C/I >
`(dB) 11 dB
`-50 No
`-20 No
`
`20 Yes
`
`>40 Yes
`
`>40 Yes
`
`>40 Yes
`
`Co-channel
`1 adjacent
`channel
`2" adjacent
`channel
`3' adjacent
`channel
`4" adjacent
`channel
`5" adjacent
`channel
`
`From Table 1, it may be seen that both specifications
`require the maximum uSeable input level to be as low as -20
`dBm. To guarantee that a receiver is free from
`deSensitization, it is necessary to keep any unwanted in band
`Signals 10 dB below the maximum uSeable input level, i.e.,
`–30 dBm for both Bluetooth TM and HomeRF. It is apparent
`that 50 dB of isolation, as provided by the circulator of FIG.
`5, will Satisfy this requirement, as the maximum output
`power is 20 dBm, in the worst case Scenario.
`Interference
`In addition to desensitization, interference also affects
`performance of the Systems. AS discussed previously, inter
`ference occurs whenever there is signal Overlap in both time
`and Spectrum. In particular, the Spectrum of a transmitter is
`not always confined to one channel, as shown in FIG. 1.
`Pulse Shaping, modulation Schemes, and unperfected power
`amplification are among the most important factors which
`contribute to the spreading of the Spectrum outside of the
`assigned channel. The impacts of non-ideal spectrum
`Spreading may be assessed by the amount of adjacent
`channel interference (ACI), as illustrated in FIG. 6, where
`ACP-1 and ACP-2 designate signals that are separated by
`one and two channels away from the wanted Signal, respec
`tively. The hatched regions indicate power transmitted by
`Signals on adjacent channels, which have encroached into
`the channel of the wanted Signal.
`AS described earlier, a receiver front-end circuit may not
`distinguish between individual in-band Signals. Therefore, a
`portion of the power from the interfering Signals will pass
`through the IF channel filter and enter the demodulator
`Section, together with the wanted Signal. This will eventually
`degrade the Sensitivity of the receiver.
`Another Source of interference is the undesired signals
`that are transmitted on the same channel as that used by the
`wanted signal. Co-channel interference (CCI) refers to this
`type of interference. CCI will produce similar, or even
`worse, effects on the receiver as that of ACI.
`Power-ratio-of-carrier to all in