USOO8442473B1
`
`(12) Unlted States Patent
`(10) Patent No.:
`US 8,442,473 B1
`
`Kaukovuori et a1.
`(45) Date of Patent:
`May 14, 2013
`
`(54) METHODS OF RECEIVINGAND RECEIVERS
`
`2010/0118923 A1
`2010/0210272 A1
`
`5/2010 P31
`8/2010 Sundstrom et a1.
`
`(75)
`
`Inventors: Jouni Kristian Kaukovuori, Vantaa
`(FI); AamoTapio Parssinen, Espoo (FI);
`Antti Oskari Immonen, Helsinki (Fl)
`
`(73) Assignee: Renesas Mobile Corporation, Tokyo
`(JP)
`
`“/2011 Park et al' """"""""""" 375/344
`2011/0268232 A1
`FOREIGN PATENT DOCUMENTS
`2 141 818 A1
`1/2010
`2 378 670 A2
`10/2011
`W0 00/ 11794 A1
`3/2000
`WO 2010/092167
`8/2010
`WO 2010/129584 A1
`11/2010
`
`EP
`EP
`W0
`W0
`W0
`
`( * ) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U’S’C’ 154(1)) by 35 days’
`(21) Appl No . 13/300 004
`.
`N
`’
`
`W0
`
`2/2013
`WO 2013/024450 A1
`OTHER PUBLICATIONS
`R4-113595, 3GPP TSG RAN WG4 Meeting #59AH, Bucharest,
`Romania, Jun. 27-Ju1. 1, 2011, ST—Ericsson/Ericsson, “Non-Con-
`tiguous Carrier Aggregation Configurations”, (5 pages).
`
`(22)
`
`Filed:
`
`NOV. 18, 2011
`
`(Continued)
`
`(30)
`
`Foreign Application Priority Data
`
`Nov. 17, 2011
`
`(GB) ................................... 11198884
`
`Primary Examiner i Nhan Le
`(74) Attorney, Agent, or Firm 7 Lucas & Mercanti LLP;
`Robert P. Michal
`
`(51)
`
`(2006.01)
`
`Int. Cl.
`H04B 1/26
`(52) US. Cl.
`USPC ......... 455/323; 455/334; 455/5501; 375/324
`(58) Field of Classification Search .................. 455/313,
`455/323, 324, 334, 339*341, 5501; 375/324
`See application file for complete search history.
`
`(56)
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`6 356 746 B1 *
`3/2002 Katayama ..................... 455/324
`6,785,529 132 *
`8/2004 Ciccarelli et 31, ,,,,,,,,,, 455/324
`7,526,052 B2 *
`4/2009 Davidoff et a1.
`........... 375/350
`7/5783375 B2 *
`8/2010 “fkayéima et 31~ ~~~~~~~~~~~ 455/333
`38843183323 :1
`13388131 31006:: a1
`'
`2007/1121 1837 A1
`9/2007 Zipper
`2008/0112470 A1
`5/2008 Cleveland et a1.
`2010/0104001 A1
`4/2010 Lee et al.
`
`100
`g
`
`ABSTRACT
`(57)
`Data transmitted Via a combination of radio frequency RF
`signals using carrier aggregation CA is received, each RF
`signal OCGUPying a respectiVe RF bands the bands being
`arranged in two groups separated 1n frequency by a. first
`frequency region, the first of the two groups occupying a
`wider frequency region than the second group. Inphase and
`quadrature components of the RF signals are filtered using a
`first bandpass filter bandwidth to give first bandpass filtered
`components and filtered using a second bandpass filter band-
`width, different from the first bandpass filter bandwidth, to
`give second bandpass filtered components. A reconfigurable
`receiver is configurable to a first mode to receive the combi-
`nation of RF signals, and is also configurable to at least a
`second mode. A filter is configured, in different modes, to use
`a first or a lowpass bandpass filter bandwidth.
`
`20 Claims, 18 Drawing Sheets
`
`L 102
`
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`

`

`US 8,442,473 B1
`
`Page 2
`
`OTHER PUBLICATIONS
`
`R4-113494, TSG-RAN Working Group 4 (Radio) Meeting #59AH,
`Bucharest, Romania, Jun. 27-Jul. 1, 2011, ZTE, “Handling the Inter-
`ferences in the NCiGAP in NC74CHSDP ”, (3 pages).
`3GPP TS 25 .101 (see Table 7.2C), User Equipment (UE) radio trans-
`mission and reception (FDD) Specification Detail (4 pages). Avail-
`able on-line: http://WWW.3gpp.org/ftp/specs/html-info/25101.htm,
`Feb. 21, 2012.
`Ericsson, 3 GPP TSG-RAN Meeting #51, RP-110416, Kansas City,
`Missouri, USA Mar. 15-18, 2011, “Non-contiguous 4C-HSDPA
`Operation, Core-Part, 4C Performance Part” (13 pages).
`Nokia Corporarion, 3GPP TSG-RAN Meeting #52, RP-110732,
`Bratislava, Slovakia, May 31, Jun. 3, 2011, “LTE Carrier Aggregation
`Enhancements, Core-Part, Performance” (17 pages).
`Ericsson, 3GPP TSG-RAN Meeting #60, Tdoc R4-114401, Athens,
`Greece, Aug. 22, 26, 2011, “Feedback on non contiguous carrier
`aggregation feasibility,” (2 pages).
`Renesas Electronics Europe, 3GPP TSG-RAN WG4 Meeting #58,
`RF-111965, Shanghai, China, Apr. 10-15, 2011, “Considerations on
`RSTD measures and carrier aggregation.” (9 pages).
`Ericsson, 3GPP TSG-RAN WG4 Meeting #61, R4-115583, San
`Francisco, California, Nov. 14-18, 201 1, “Scenarios for non-continu-
`ous intra-band CA,” (7 pages).
`Non-Final Office Action dated Feb. 28, 2013 issued in a related U.S.
`Appl. No. 13/677,776 (12 pages).
`UKIPO Combined Search and Examination Report under Sections
`17 and 18(3) dated Mar. 15, 2012 issued in a related British Appli-
`cation No. GB1119887.6 (5 pages).
`
`UKIPO Combined Search and Examination Report under Sections
`17 and 18(3) dated Mar. 19, 2012 issued in a related British Appli-
`cation No. GB1119888.4 (5 pages).
`UKIPO Combined Search and Examination Report under Section 17
`and 18(3) dated Dec. 7, 2012 issued in a related British Application
`No. GB1219626.7 (6 pages).
`PCT Notification of Transmittal of the International Search Report
`and the Written Opinion and PCT International Search Report, PCT
`Written Opinion mailed Feb. 25, 2013 issued in a related PCT Appli-
`cation No. PCT/IB2012/056441 (15 pages).
`Pietro Andreani, et al., “A CMOS gm 'C Polyphase Filter with High
`Image Band Rejection,” Solid-State Circuits Conference, 2000, Pro-
`ceedings of
`the 26 Round European,
`IEEE, pp. 272-275,
`XP031952366 (4 pages).
`Nokia et
`a1., 3GPP TSG-RAN WG4 Meeting 2010 AH#4,
`R4-103677, Xi’an, China, Oct. 11-15, 2010, “Image Rejection in
`intraband carrier aggregation” (8 pages).
`Toru Kitayabu., “Concurrent Dual-Band Receiver for Spectrum
`Aggregation System,” Radio and Wireless Symposium, 2009, RWS
`2009, IEEE, Piscataway, NJ, USA, pp. 634-637 XP031457487 (4
`pages).
`PCT Notification of Transmittal of the International Search Report
`and the Written Opinion and PCT International Search Report, PCT
`Written Opinion mailed Mar. 13, 2013, all of which was issued in a
`related PCT Application No. PCT/IB2012/056440 (15 pages).
`
`* cited by examiner
`
`

`

`US. Patent
`
`May 14, 2013
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`Sheet 1 0118
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`US 8,442,473 B1
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`US. Patent
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`May 14, 2013
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`Sheet 2 of 18
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`US 8,442,473 B1
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`US. Patent
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`May 14, 2013
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`Sheet 3 of 18
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`US 8,442,473 B1
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`US. Patent
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`May 14, 2013
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`Sheet 4 of 18
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`US 8,442,473 B1
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`May 14, 2013
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`Sheet 5 of 18
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`US 8,442,473 B1
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`May 14, 2013
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`U.S. Patent
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`May 14, 2013
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`US. Patent
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`May 14, 2013
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`US 8,442,473 B1
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`US. Patent
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`May 14, 2013
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`Sheet 9 of 18
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`US 8,442,473 B1
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`US. Patent
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`May 14, 2013
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`Sheet 10 of 18
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`US 8,442,473 B1
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`US. Patent
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`May 14, 2013
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`US. Patent
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`May 14, 2013
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`US. Patent
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`US 8,442,473 B1
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`US. Patent
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`May 14, 2013
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`Sheet 18 of 18
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`US 8,442,473 B1
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`US 8,442,473 B1
`
`1
`METHODS OF RECEIVING AND RECEIVERS
`
`CROSS REFERENCE TO RELATED
`APPLICATION
`
`This application claims benefit under 35 U.S.C. §119(a)
`and 37 CFR 1.55 to UK Patent Application 11198884, filed
`on NOV. 17, 2011.
`
`TECHNICAL FIELD
`
`The present invention relates to methods of receiving and
`receivers for radio communication systems, and in particular,
`but not exclusively, to non-contiguous carrier aggregation
`schemes.
`
`BACKGROUND
`
`Long Term Evolution (LTE) Advanced is a mobile tele-
`communication standard proposed by the 3rd Generation
`Partnership Project (3GPP) and first standardised in 3GPP
`Release 10. In order to provide the peak bandwidth require-
`ments of a 4th Generation system as defined by the Intema-
`tional Telecommunication Union Radiocommunication
`
`(ITU-R) Sector, while maintaining compatibility with legacy
`mobile communication equipment, LTE Advanced proposes
`the aggregation of multiple carrier signals in order to provide
`a higher aggregate bandwidth than would be available if
`transmitting via a single carrier signal. This technique of
`CarrierAggregation (CA) requires each utilised carrier signal
`to be demodulated at the receiver, whereafter the message
`data from each of the signals can be combined in order to
`reconstruct the original data. CarrierAggregation can be used
`also in other radio communication protocols such as High
`Speed Packet Access (HSPA).
`Carrier signals are typically composed of a carrier fre-
`quency that is modulated to occupy a respective radio fre-
`quency carrier signal band. Contiguous Carrier Aggregation
`involves aggregation of carrier signals that occupy contigu-
`ous radio frequency carrier signal bands. Contiguous radio
`frequency carrier signal bands may be separated by guard
`bands, which are small unused sections of the frequency
`spectrum designed to improve the ease with which individual
`signals can be selected by filters at the receiver by reducing
`the likelihood of interference between signals transmitted in
`adjacent bands. Non-contiguous Carrier Aggregation com-
`prises aggregation of carrier signals that occupy non-contigu-
`ous radio frequency carrier signal bands, and may comprise
`aggregation of clusters of one or more contiguous carrier
`signals. The non-contiguous radio frequency carrier signal
`bands are typically separated by a frequency region which is
`not available to the operator of the network comprising the
`carrier signals, and may be allocated to another operator. This
`situation is potentially problematic for the reception of the
`carrier signals, since there may be signals in the frequency
`region that separates the non-contiguous carriers which are at
`a higher power level than the wanted carrier signals.
`A Direct Conversion Receiver
`(DCR)
`is
`typically
`employed to receive cellular radio signals, and typically pro-
`vides an economical and power efficient implementation of a
`receiver. A DCR uses a local oscillator placed within the radio
`frequency bandwidth occupied by the signals to be received
`to directly concert the signals to baseband. Signals on the high
`side of the local oscillator are mixed to the same baseband
`
`frequency band as signals on the low side of the local oscil-
`lator, and in order to separate out the high and low side
`signals, it is necessary to mix the signal with two components
`
`10
`
`15
`
`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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`2
`
`of the local oscillator in quadrature (i.e. 90 degrees out of
`phase with one another) to produce inphase (I) and quadrature
`(Q) signal components at baseband. The I and Q components
`are digitised separately, and may be processed digitally to
`reconstruct the separate high side and low side signals. The
`reconstructed high and low side signals may be filtered in the
`digital domain to separate carrier signals received within the
`receiver bandwidth of the DCR.
`
`The presence of a higher power signal in the region sepa-
`rating non-contiguous carrier clusters poses particular prob-
`lems if a DCR is to be used to receive a band of frequencies
`comprising non-contiguous Carrier Aggregation signals. In
`particular, since the higher power signal is within the receiver
`bandwidth, the dynamic range ofthe receiver need to encom-
`pass the powers of the wanted carrier signals, which are
`typically received at a similar power to each other, and the
`higher power signal. This may place severe demands on the
`dynamic range of the analogue to digital converter (A/D) in
`particular. Furthermore, due to inevitable imbalances
`between the amplitudes and phases of the I and Q channels,
`the process of reconstructing the separate high side and low
`side signals suffers from a limited degree of cancellation of
`the image component; that is to say, some of the high side
`signals break through onto the reconstructed low side signals,
`and vice versa. The degree of rejection of the image signal
`may be termed the Image Reject Ratio (IRR). If the higher
`power signal is a high side signal, it may cause interference to
`received low side signals due to the finite IIR, and similarly if
`the higher power signal is a low side signal, it may cause
`interference to received high side signals.
`One conventional method of receiving Non-contiguous
`Carrier Aggregation signals is to provide two DCR receiver
`stages, each having a local oscillator tuned to receive a cluster
`of contiguous carriers, and so rejecting signals in the fre-
`quency region between the clusters before digitisation. How-
`ever, this approach is potentially expensive and power con-
`suming, and may suffer from interference between the closely
`spaced local oscillators.
`It is an object ofthe invention to address at least some ofthe
`limitations of the prior art systems.
`
`SUMMARY
`
`In accordance with a first exemplary embodiment of the
`present invention, there is provided a method of receiving
`data transmitted via a combination of at least a plurality of
`radio frequency signals using carrier aggregation, each radio
`frequency signal occupying a respective band of a plurality of
`radio frequency bands, the plurality of radio frequency bands
`being arranged in two groups separated in frequency by a first
`frequency region, the first of the two groups occupying a
`wider frequency region than the second group, the method
`comprising:
`downconverting said plurality of radio frequency signals
`using quadrature mixing to give inphase and quadrature com-
`ponents;
`filtering said inphase and quadrature components using a
`first bandpass filter bandwidth to give first bandpass filtered
`inphase and quadrature components; and
`filtering said inphase and quadrature components using a
`second bandpass filter bandwidth, different from the first
`bandpass filter bandwidth, to give second bandpass filtered
`inphase and quadrature components.
`In accordance with a second exemplary embodiment ofthe
`present invention, there is provided a receiver for receiving
`data transmitted via a combination of at least a plurality of
`radio frequency signals, each radio frequency signal occupy-
`
`

`

`US 8,442,473 B1
`
`3
`ing a respective band of a plurality of radio frequency bands,
`the plurality of radio frequency bands being arranged in two
`groups separated in frequency by a first frequency region, the
`first of the two groups occupying a wider frequency region
`than the second group, the receiver comprising:
`at least one downconverter configured to downconvert said
`plurality of radio frequency signals using quadrature mixing
`to give inphase and quadrature components;
`at least one first bandpass filter configured to filter said
`inphase and quadrature components using a first bandpass
`filter bandwidth to give first bandpass filtered inphase and
`quadrature components; and
`at least one second bandpass filter configured to filter said
`inphase and quadrature components using a second bandpass
`filter bandwidth, different from the first bandpass filter band-
`width, to give second bandpass filtered inphase and quadra-
`ture components.
`In accordance with a third exemplary embodiment of the
`present invention, there is provided a reconfigurable receiver
`capable of receiving data transmitted via a combination of at
`least a plurality ofradio frequency signals using carrier aggre-
`gation, each radio frequency signal occupying a respective
`band of a plurality of radio frequency bands,
`the receiver being configurable to a first mode to receive
`radio signals in which the plurality of radio frequency bands
`are arranged in two groups separated in frequency by a first
`frequency region, the first of the two groups occupying a
`wider frequency region than the second group, and to at least
`a second mode,
`receiver comprising:
`at least one downconverter configured to downconvert said
`plurality of radio frequency signals using quadrature mixing
`to give inphase and quadrature components;
`at least one first filter arranged to be configured, in the first
`mode, to filter said inphase and quadrature components using
`a first bandpass filter bandwidth to give first bandpass filtered
`inphase and quadrature components and, in the second mode
`to filter said inphase and quadrature components using a first
`lowpass filter bandwidth to give first lowpass filtered inphase
`and quadrature components;
`at least one second filter arranged to be configured, in the
`first mode, to filter said inphase and quadrature components
`using a second bandpass filter bandwidth, different from the
`first bandpass filter bandwidth, to give second bandpass fil-
`tered inphase and quadrature components.
`Further features and advantages of the invention will be
`apparent from the following description ofpreferred embodi-
`ments of the invention, which are given by way of example
`only.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a schematic diagram showing the transmission of
`carrier aggregation signals by the radio access network of a
`first operator and transmission of a signal from another a radio
`access network;
`FIG. 2 is amplitude-frequency diagram showing carriers in
`a non-contiguous carrier aggregation method and a carrier
`from another operator received at a higher level;
`FIG. 3 is a schematic diagram showing a conventional
`direct conversion receiver;
`FIG. 4 is a diagram illustrating an effect of a finite image
`rejection ratio in a direct conversion receiver;
`FIG. 5 is a diagram illustrating reception of non-contigu-
`ous aggregated carriers in a low IF receiver.
`FIG. 6 is a schematic diagram showing a conventional low
`IF receiver;
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`4
`
`FIG. 7 is a diagram illustrating problems with reception of
`non-contiguous aggregated carriers in a direct conversion
`receiver;
`FIG. 8 is an amplitude-frequency diagram illustrating
`reception of non-contiguous aggregated carriers in a direct
`conversion receiver in an embodiment of the invention;
`FIG. 9 is an amplitude-frequency diagram illustrating
`reception of non-contiguous aggregated carriers in an
`receiver having different passband filters for the high side and
`low side signals in an embodiment of the invention;
`FIG. 10 is a schematic diagram showing a receiver having
`two zero IF branches each having different bandpass filters in
`an embodiment of the invention;
`FIG. 11 is a schematic diagram showing an alternative
`receiver having two zero IF branches each having different
`bandpass filters in an embodiment of the invention;
`FIG. 12 is a diagram illustrating a conventional direct
`conversion receiver as implemented in an RFIC;
`FIG. 13 is a frequency-amplitude diagram illustrating
`problems with reception of non-contiguous aggregated carri-
`ers in a direct conversion receiver, showing image frequencies
`at the equivalent position in RF frequency;
`FIG. 14 is a frequency-amplitude diagram illustrating a
`conventional solution for the reception of non-contiguous
`aggregated carriers, by the use of two receivers, each having
`a different local oscillator frequency;
`FIG. 15 is a schematic diagram illustrating an RF IC imple-
`mentation for the reception of non-contiguous aggregated
`carriers, by the use of two receivers, each having a separate
`RFIC and a different local oscillator frequency;
`FIG. 16 is a schematic diagram illustrating an RF IC imple-
`mentation for the reception of non-contiguous aggregated
`carriers, by the use of two receivers, each having a different
`local oscillator frequency on a single RFIC;
`FIG. 17 is an amplitude-frequency diagram illustrating
`reception of non-contiguous aggregated carriers, with a
`single signal from another operator between the wanted car-
`rier signals in an embodiment of the invention;
`FIG. 18 is an amplitude-frequency diagram illustrating the
`reception of non-contiguous aggregated carriers, showing a
`single signal from another operator between carrier aggrega-
`tion clusters and the effect ofimage frequencies in an embodi-
`ment of the invention;
`FIG. 19 is an amplitude-frequency diagram illustrating the
`reception of non-contiguous aggregated carriers, showing
`two signals from another operator between carrier aggrega-
`tion clusters and the effect ofimage frequencies in an embodi-
`ment of the invention;
`FIG. 20 is an amplitude-frequency diagram illustrating the
`reception of non-contiguous aggregated carriers, showing
`three signals from another operator between carrier aggrega-
`tion clusters and the effect ofimage frequencies in an embodi-
`ment of the invention;
`FIG. 21 is an amplitude-frequency diagram illustrating the
`reception of non-contiguous aggregated carriers, showing a
`different filter bandwidth used for the reception of high side
`and low side signals in an embodiment of the invention;
`FIG. 22 is an amplitude-frequency diagram illustrating the
`reception of non-contiguous aggregated carriers, showing a)
`the use of a complex filter characteristic b) the effect of the
`complex filter characteristic shown with a digital filter char-
`acteristic superimposed and c) the combined effect of the
`complex and digital filters;
`FIG. 23 (upper part) is schematic diagram showing a
`receiver architecture having complex filters and a digital data
`path;
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`US 8,442,473 B1
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`5
`FIG. 23 (lower part) is schematic diagram showing a
`receiver architecture having real filters and a digital data path
`having image reject mixing;
`FIG. 24 is a schematic diagram showing a reconfigurable
`receiver having two branches, a first branch having a filter
`with a selectable low pass or band pass characteristic and a
`second branch having a band pass characteristic; and
`FIG. 25 is a schematic diagram showing an alternative
`reconfigurable receiver having two branches, a first branch
`having a filter with a selectable low pass or band pass char-
`acteristic and a second branch having a band pass character-
`istic, the two branches sharing the same I and Q mixers.
`
`DETAILED DESCRIPTION
`
`By way of example an embodiment of the invention will
`now be described in the context of a wireless communications
`
`system supporting communication using E-UTRA radio
`access technology, as associated with E-UTRAN radio access
`networks in LTE systems. However, it will be understood that
`this is by way of example only and that other embodiments
`may involve wireless networks using other radio access tech-
`nologies, such as UTRAN, GERAN or IEEE802.l6 WiMax
`systems.
`FIG. 1 shows the transmission of radio frequency signal
`signals 10a, 10b and 100 by the radio access network to a
`receiver 8. The radio frequency signals each occupy a respec-
`tive carrier signal band, as shown in the amplitude-frequency
`diagram ofFIG. 2.A carrier signal band is the part ofthe radio
`frequency spectrum occupied by a modulated radio fre-
`quency carrier comprising the radio frequency signal. Radio
`frequency signals 10a, 10b, and 100 occupy radio frequency
`bands 14a, 14b and 140 as shown in FIG. 2. Data is received
`using the combination ofthe radio frequency signals 10a, 10b
`and 100, and the bands 14a, 14b and 140 shown in FIG. 2
`represent a set ofradio frequency signals, that may be referred
`to as component carriers, transmitted using Carrier Aggrega-
`tion. It can be seen from FIG. 2 that non-contiguous Carrier
`Aggregation is used, since a radio frequency signal from
`another operator, other than the operator sending the data, is
`present in a frequency region separating bands 14b and 140.
`In FIG. 1, the radio frequency signals are sent from a first base
`station 4, operated by Operator A. A second base station 6,
`operated by a different operator, Operator B, is situated within
`the area of coverage 2 of the first base station 4, and transmits
`a radio frequency signal 12 that is received by the user equip-
`ment 8. It can be seen that the second base station is closer to
`the user equipment 8 than is the first base station. As a result,
`it can be seen from FIG. 2 that the radio frequency signal is
`received at the user equipment 8 at a significantly higher
`power level, as shown by the amplitude of the band 16 trans-
`mitted by operator B.
`FIG. 3 is a schematic diagram showing a conventional
`direct conversion receiver. A signal is received by an antenna
`100, and filtered by a front end filter 102, which removes out
`of band signals, protecting the Low Noise Amplifier (LNA)
`104 from saturation by strong out of band signals. A local
`oscillator 106 is typically set to a frequency in the centre of a
`desired radio frequency (RF) band. RF signals that are both
`higher than (high side) and lower than (low side) the local
`oscillator frequency are mixed with the local oscillator to
`downconvert the RF signals to baseband frequencies, which
`are the difference between the RF and local oscillator fre-
`quencies. These difference frequencies, for signals within an
`intended receive band, are arranged to fall within the pass-
`band of the low pass filters 114, 116 of the direct conversion
`receiver. In order to distinguish between RF signals that origi-
`nated on the high side of the local oscillator and RF signals
`that originated on the high side of the local oscillator, it is
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`necessary to mix the RF signal with two components of the
`local oscillator which are in quadrature (i.e. 90 degrees out of
`phase with one another) to produce inphase (I) and quadrature
`(Q) signal components at baseband. As shown in FIG. 3, the
`local oscillator is split into 0 and —90 degree components in a
`splitter 108 and each component is mixed with the incoming
`RF signal in a respective mixer 110, 112. The I and Q com-
`ponents are separately filtered low pass filtered, and each
`filtered signal is converted to the digital domain in an Ana-
`logue to digital converter (A/D) 118, 120, to produce a data
`stream with I and Q components 122, 124. The I and Q
`components may be processed digitally to reconstruct the
`separate high side and low side signals. The reconstructed
`high and low side signals may be filtered in the digital domain
`to separate carrier signals received within the receiver band-
`width of the DCR. However, as already mentioned, due to
`imbalances between the amplitudes and phases of the I and Q
`channels, the process of reconstructing the separate high side
`and low side signals suffers from a limited degree of cancel-
`lation of the image component, so that some of the high side
`signals break through onto the reconstructed low side signals,
`and vice versa. The degree of rejection of the image signal
`may be termed the Image Reject Ratio (IRR).
`FIG. 4 shows the effect of a finite image rejection ratio in a
`direct conversion receiver, in the case where two bands 202,
`204 are received at approximately the same power level at
`radio frequency. As can be seen, the two bands are mixed with
`a local oscillator 206 and downconverted to a band encom-
`passing Zero frequency, which may be referred to as DC
`(Direct Current). In FIG. 4, the high side signal 204 is shown
`as being downconverted to positive frequency 210, and the
`low side signal 202 is shown as being downconverted to a
`negative frequency 208. This is a matter of convention, and
`the designation of positive and negative frequencies may be
`transposed. The concept of positive and negative frequencies
`has meaning only within the complex signal domain, in which
`signals are represented by I and Q components. A negative
`frequency has a phasor defined by its I and Q components that
`rotates in the opposite direction to the phasor of a positive
`frequency. By distinguishing between positive and negative
`frequencies by signal processing, for example using a Fast
`Fourier Transform (FFT) or a complex digital mixer, signals
`originating as high side RF signals may be separately
`received from signals originating as low side RF signals. So,
`as shown in FIG. 4, data may be extracted from two received
`carrier signal bands, provided that the signal to noise ratio
`(SNR) is not degraded unacceptably by the image component
`214 of the high side signal 204 that is in the same band 208 as
`the downconverted low side signal 202, and the image com-
`ponent 212 ofthe low side signal 202 that is in the same band
`210 as the downconverted high side signal 204. For signals
`received at approximately the same power level, SNR is not
`usually degraded unacceptably by the finite image reject
`ratio.
`FIG. 5 is a diagram illustrating reception of non-contigu-
`ous aggregated carriers. In this example, wanted component
`signal bands 302 and 302 are separated by a higher power
`radio frequency signal 318, which may originate from
`another operator. As can be seen from FIG. 5, a local oscillator
`306 may be placed in the middle of a receive band defined by
`the three component signal bands 302, 304, 318. As can be
`seen from FIG. 5, images ofthe higher power radio frequency
`signal resulting from the finite image reject ratio do not fall on
`top of the downconverted weaker signals in this case, but fall
`within the downconverted components 320 of the higher
`power radio frequency signal.
`FIG. 6 is a schematic diagram showing a conventional low
`IF receiver that may be used to receive the signals illustrated
`in FIG. 5. It can be seen that the low IF receiver differs from
`a conventional DCR receiver in that the low pass filters of a
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`US 8,442,473 B1
`
`8
`quency on the low frequency side, it may be determined that
`the local oscillator offset should be set at a position that
`causes the least total interference with the wanted signals.
`This may be determined on the basis of signal to noise plus
`interference ratio measurements for each of the wanted sig-
`nals.
`
`FIG. 9 shows that that setting of the local oscillator may be
`used in conjunction with a receiver having two bandpass filter
`characteristics 540, 538 one of which 538 is wider than the
`other 540. The bandpass characteristics may be set to be
`appropriate to receive the component signal bands in the
`respective groups of signals.
`FIG. 10 is a schematic diagram showing a receiver having
`two bandpass filter characteristics as illustrated in FIG. 9 in an
`embodiment of the invention. The receiver has two branches.
`
`A first branch is a low IF receiver having I and Q channels,
`each of which has a bandpass filters 814, 816 with a first
`bandwidth. A second branch is also configured as a low IF
`receiver as shown in FIG. 10, and also has I and Q channels,
`each of which has a bandpass filters 814, 816 with a second
`bandwidth, different from the first bandwidth. A first sub set of
`downconverted radio signals may be received using the first
`branch, and a second subset of downconverted radio signals
`may be received using the second branch.
`FIG. 11 is a schematic diagram showing an alternative
`receiver having two branches each having different bandpass
`filters in an embodiment ofthe invention, in which a single set
`of quadrature mixers is shared between the two branches.
`Embodiments of the invention will now be described in
`more detail. Embodiments of the invention relate to multi-
`
`carrier wireless systems, using carrier aggregation. Operators
`may own non-contiguous allocation of spectrum; this may
`come about, for example, if an operator buys another opera-
`tor’s businesses. If the spectrums happen to be non-adjacent
`then the allocation is non-contiguous. Operators typically
`wish to exploit their spectrum as effectively as possible, so the
`need for non-contiguous multi-carrier systems is increasing.
`An example of such scenario is presented in FIG. 2. In a
`scenario such as that illustrated in FIG. 2, there may be a
`problem with single receiver cha