`
`
`a2, United States Patent
`US 9,749,168 B2
`(10) Patent No.:
`
` Li et al. (45) Date of Patent: *Aug. 29, 2017
`
`
`US009749168B2
`
`(54) METHOD AND APPARATUS USING
`CELL-SPECIFIC AND COMMONPILOT
`SUBCARRIERS IN MULITI-CARRIER,
`MULTI-CELL WIRELESS COMMUNICATION
`NETWORKS
`,
`7
`.
`(71) Applicant: Neocitic, Inc., Bellevue, WA (US)
`(72)
`Inventors: Xiaodong Li, Kirkland, WA (US);
`Titus Lo, Bellevue, WA (US); Kemin
`2
`2
`2
`Li, Bellevue, WA (US); Haiming
`Huang, Bellevue, WA (US)
`
`(73) Assignee: Neocific, Inc., Bellevue, WA (US)
`
`(*) Notice:
`
`Subject to any disclaimer, the term ofthis
`patent is extended or adjusted under 35
`US.C. 154(b) by 0 days.
`This patent is subject to a terminal dis-
`claimer.
`
`(21) Appl. No.: 14/746,676
`“yg,
`Filed:
`
`(22)
`(65)
`
`Jun. 22, 2015
`Prior Publication Data
`US 2015/0312076 Al
`Oct. 29, 2015
`
`Related U.S. Application Data
`.
`sas
`.
`(63) onuinvatonof application Ne beseoatne on
`ane
`4s
`» HOW at
`On AMON WME
`IS a
`(Continued)
`(2006.01)
`.
`(2006.01)
`(Continued)
`
`(1) ee6
`HOAL 27/26
`
`(52) U.S. Cl.
`CPC wa HOAL 27/2637 (2013.01); HO4B 1/707
`(2013.01); HO4B 7/0413 (2013.01);
`(Continued)
`(58) Field of Classification Search
`CPC ..... HO04B 1/707; H04B 7/0413; HO4J 11/005;
`H04L 25/0228; HO4L 25/03834;
`(Continued)
`:
`‘
`“ite
`References Cited
`U.S. PATENT DOCUMENTS
`
`5
`(56)
`
`6,515,960 BL*
`
`6,480,558 B1* 11/2002 Ottosson ..........0... HO04B 1/7083
`375/350
`2/2003 USUi ween HO4L 5/023
`.
`370/203
`(Continued)
`Primary Examiner — Babar Sarwar
`(74) Attorney, Agent, or Firm — Perkins Coie LLP
`
`ABSTRACT
`(57)
`A multi-carrier cellular wireless network (400) employs base
`stations (404) that transmit two different groups of pilot
`subcarriers:
`(1) cell-specific pilot subcarriers, which are
`used by a receiver to extract information unique to each
`individual cell (402), and (2) common pilots subcarriers,
`which are designed to possess a set of characteristics com-
`monto all the base stations (404) of the system. The design
`criteria and transmission formats of the cell-specific and
`commonpilot subcarriers are specified to enable a receiver
`to perform different system functions. The methods and
`processes can be extended to other systems, such as those
`with multiple antennas in an individual sector and those
`where some subcarriers bear common network/system infor-
`tion.
`man
`
`10 Claims, 13 Drawing Sheets
`
`Subcarrler arrangement for Cell ¢
`
` lu
`
`
`
`
`Subcarrier arrangament for Call g
`
`C
`
`wo
`
`Common pilot
`subcarriers
`
`Cell-specific pilot
`subcarriers
`
`& Subcarriers
`} for data
`
`VWGOoA EX1040
`VWGOAV. Neo Wireless
`
`IPR2022-01539
`
`VWGoA EX1040
`VWGoA V. Neo Wireless
`IPR2022-01539
`
`

`

`US 9,749,168 B2
`Page 2
`
`Related U.S. Application Data
`
`continuation of application No. 13/874,278, filed on
`Apr. 30, 2013, now Pat. No. 8,934,473, which is a
`continuation of application No. 13/212,116, filed on
`Aug. 17, 2011, now Pat. No. 8,432,891, which is a
`continuation of application No. 10/583,530, filed as
`application No. PCT/US2005/001939 on Jan. 20,
`2005, now Pat. No. 8,009,660.
`
`(60) Provisional application No. 60/540,032, filed on Jan.
`29, 2004.
`
`(51)
`
`(2009.01)
`(2009.01)
`(2011.01)
`(2006.01)
`(2006.01)
`(2006.01)
`(2017.01)
`(2006.01)
`(2006.01)
`
`Int. Cl.
`HO4W 16/02
`HO4W 72/04
`HO4B 1/707
`HOAL 5/00
`HOAL 25/03
`HO4L 27/00
`HO4B 7/0413
`HO04J 11/00
`HOAL 25/02
`(52) U.S. CL.
`CPC oeeseesen HO4J 11/005 (2013.01); HO4L 5/0007
`(2013.01); HO4E 5/0028 (2013.01); HO4L
`5/0048 (2013.01); HO4E 25/03834 (2013.01);
`HO4L 27/0008 (2013.01); HO4L 27/0012
`(2013.01); HO4L 27/2602 (2013.01); HO4L
`27/2613 (2013.01); HO4E 27/2626 (2013.01);
`HO4L 27/2646 (2013.01); HO4W 16/02
`(2013.01); HO4W 72/044 (2013.01); HO4W
`72/0446 (2013.01); HO4L 5/0016 (2013.01);
`HO4L 25/0228 (2013.01); HO4L 27/2607
`(2013.01); HO4L 27/2655 (2013.01); HO4L
`27/2657 (2013.01)
`
`(58) Field of Classification Search
`CPC vee HO4L 27/0008; HO4L 27/0012; HO4L
`27/2602; HO4L 27/2607; HO4L 27/2613;
`HO4L 27/2626; HO4L 27/2637; HO4L
`27/2646; HO4L 27/02
`
`USPC wee 455/404.2, 412.1-414.2, 418-4221],
`455/67.11-67.13, 115.1, 423, 436-444,
`455/502, 552.1, 562.1, 434, 525;
`370/324, 332, 319, 344, 350, 329, 338,
`370/328, 203, 280, 342, 335
`See application file for complete search history.
`
`(56)
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`6,567,383 BL*
`
`6,731,673 BL*
`
`6,741,578 BL*
`
`6,922,388 BL*
`
`7,039,001 B2*
`
`7,062,002 BL*
`
`6/2006 Michel
`
`5/2003 Bohnke .......cccessee.: HO4L 5/005
`370/280
`6,643,281 BL* 11/2003 Ryan ween HO4B 7/2668
`370/208
`5/2004 Kotov oes HO4B 1/7083
`375/145
`5/2004 Moon on... HO4B 1/7083
`370/335
`7/2005 Laroia o.....ccce HO04J 3/0682
`370/208
`5/2006 Krishnan ............. HO4L 25/023
`370/203
`........ccc0. HO04B 1/7073
`375/145
`3/2008 Chiou oo... HO4L 25/0232
`375/231
`7,443,829 B2* 10/2008 RiZVi ween HO4B 1/7105
`370/336
`6/2010 Ma ccc H04J 11/0069
`370/208
`3/2011 Han eeseccnn H04B 1/70735
`370/350
`2002/0159422 AL* 10/2002 Li vvccseeeeen HO4B 1/707
`370/342
`5/2003 Walton wc... HO4B 1/692
`370/206
`5/2004 Morita wc... H04B 1/70735
`370/342
`6/2006 Hasegawa ............. HO4B 1/707
`370/208
`2006/0245409 AL* 11/2006 Korpela o..cceecce HO4L 1/06
`370/345
`
`7,342,974 B2*
`
`7,738,437 B2*
`
`7,907,592 B2*
`
`2003/0081538 AL*
`
`2004/0085946 AL*
`
`2006/0114815 AL*
`
`* cited by examiner
`
`

`

`  
`
`U.S. Patent
`
`Aug. 29, 2017
`
`Sheet 1 of 13
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`US 9,749,168 B2
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`Aug. 29, 2017
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`Aug. 29, 2017
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`US 9,749,168 B2
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`Aug. 29, 2017
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`Aug. 29, 2017
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`US 9,749,168 B2
`
`1
`METHOD AND APPARATUS USING
`CELL-SPECIFIC AND COMMONPILOT
`SUBCARRIERS IN MULTI-CARRIER,
`MULTI-CELL WIRELESS COMMUNICATION
`NETWORKS
`
`
`
`CROSS-REFERENCE TO RELATED
`
`
`
`APPLICATION(S)
`
`This application is a continuation of, and incorporates
`herein by reference in its entirety, U.S. patent application
`Ser. No. 14/595,132, now granted U.S. Pat. No. 9,065,614,
`entitled “METHODS AND APPARATUS USING CELL-
`SPECIFIC AND COMMON PILOT SUBCARRIERS IN
`MULTI-CARRIER, MULTI-CELL WIRELESS COMMU-
`NICATION NETWORKS,”filed Jan. 12, 2015, which is a
`continuation of, and incorporates herein by reference in its
`entirety, U.S. patent application Ser. No. 13/874,278, now
`granted US. Pat. No. 8,934,473, entitled “METHODS AND
`APPARATUS USING CELL-SPECIFIC AND COMMON
`PILOT SUBCARRIERS IN MULTI-CARRIER, MULTI-
`CELL WIRELESS COMMUNICATION NETWORKS,”
`filed Apr. 30, 2013, which is a continuation of, and incor-
`porates herein by reference in its entirety, U.S. patent
`application Ser. No. 13/212,116, now granted U.S. Pat. No.
`8,432,891,
`entitled “METHODS AND APPARAIUS
`USING CELL-SPECIFIC AND COMMONPILOT SUB-
`
`CARRIERS IN MULTI-CARRIER, MULTI-CELL WIRE-
`LESS COMMUNICATION NETWORKS,”filed Aug. 17,
`2011, which is a continuation of, and incorporates herein by
`reference in its entirety, U.S. patent application Ser. No.
`10/583,530, now granted U.S. Pat. No. 8,009,660, entitled
`“METHODS AND APPARATUS USING CELL-SPECIFIC
`AND COMMON PILOT SUBCARRIERS IN MULTI-
`
`CARRIER, MULTI-CELL WIRELESS COMMUNICA-
`TION NETWORKS,”filed May 30, 2007 which is a U.S.
`National Stage of PCT Application No. PCT/US05/01939,
`entitled “METHODS AND APPARATUS FOR MULTI-
`CARRIER, MULTI-CELL WIRELESS COMMUNICA-
`TION NETWORKS,”filed Jan. 20, 2005, which claims the
`benefit of and priority to U.S. Provisional Patent Application
`No. 60/540,032, entitled “METHODS AND APPARATUS
`FOR MULTI-CARRIER, MULTI-CELL WIRELESS
`
`COMMUNICATION NETWORKS,”filed on Jan. 29, 2004.
`BACKGROUND
`
`
`
`
`
`In multi-carrier wireless communications, many impor-
`tant system functions such as frequency synchronization and
`channel estimation, depicted in FIG. 1, are facilitated by
`using the network information provided by a portionoftotal
`subcarriers such as pilot subcarriers. The fidelity level of the
`received subcarriers dictates how well these functions can be
`
`achieved, which in turn affect the efficiency and capacity of
`the entire network.
`Ina wireless network, there are a numberofbasestations,
`each of which provides coverage to designated areas, nor-
`mally called a cell. If a cell is divided into sectors, from a
`system engineering point of view each sector can be con-
`sidered a cell. In this context, the terms “cell” and “sector”
`are interchangeable. The network information can be cat-
`egorized into two types: the cell-specific information thatis
`unique to a particular cell, and the common information that
`is commonto the entire network or to a portion ofthe entire
`networks such as a group ofcells.
`In a multi-cell environment, for example, the base station
`transmitter of each cell transmits its own pilot subcarriers, in
`
`2
`addition to data carriers, to be used by the receivers within
`the cell. In such an environment, carrying out the pilot-
`dependent functions becomes a challenging task in that, in
`addition to the degradation due to multipath propagation
`channels, signals originated from the basestations at differ-
`ent cells interfere with each other.
`
`One approach to deal with the interference problem has
`been to have each cell transmit a particular pattern of pilot
`subcarriers based on a certain type of cell-dependent random
`process. This approach, to a certain degree, has mitigated the
`impact of the mutual interference between the pilot subcar-
`riers from adjacent cells; however, it has not provided for a
`careful and systematic consideration of the unique require-
`ments of the pilot subcarriers.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 depicts a basic multi-carrier wireless communica-
`tion system consisting of a transmitter and a receiver.
`FIG. 2 showsbasic structure of a multi-carrier signal in
`the frequency domain, which is made up of subcarriers.
`FIG. 3 showsa radio resource divided into small units in
`
`both the frequency and time domains: subchannels and time
`slots.
`
`FIG. 4 depicts a cellular wireless network comprised of
`multiple cells, in each of which coverage is provided by a
`base station (BS).
`FIG. 5 shows pilot subcarriers divided into two groups:
`cell-specific pilot subcarriers and commonpilot subcarriers.
`FIG.6 is an embodimentofpilot-generation-and-insertion
`functional block shown in FIG. 1, which employs a micro-
`processor to generate pilot subcarriers and insert them into
`a frequency sequence contained in the electronic memory.
`FIG. 7 showsthat commonpilot subcarriers are generated
`by a microprocessor of FIG. 6 to realize phase diversity.
`FIG.8 is an embodimentof delay diversity, which effec-
`tively creates phase diversity by adding a random delay time
`duration, either in baseband or RF,
`to the time-domain
`signals.
`FIG. 9 shows two examples for extension to multiple
`antenna applications.
`FIG. 10 is an embodiment of synchronization in fre-
`quency and time domains of two collocated base stations
`sharing a common frequency oscillator.
`FIG. 11 is an embodiment of synchronization in fre-
`quency and time domains with base stations at different
`locations sharing a common frequency reference signal
`generated from the GPSsignals.
`FIG. 12 is an embodiment depicting a wireless network
`consisting of three groups of cells (or sectors) and base
`stations in each group that share their own set of common
`pilot subcarriers.
`FIG. 13 showsall base stations within a network transmit,
`along with a common pilot subcarrier, a data subcarrier
`carrying data information commonto all cells in the net-
`work.
`
`DETAILED DESCRIPTION
`
`In the following description the invention is explained
`with respect to some of its various embodiments, providing
`specific details for a thorough understanding and enable-
`ment. However, one skilled in the art will understand that the
`invention may be practiced without such details. In other
`instances, well-known structures and functions have not
`been shown or described in detail to avoid obscuring the
`depiction of the embodiments.
`
`3
`
`45
`
`55
`
`65
`
`

`

`US 9,749,168 B2
`
`3
`Unless the context clearly requires otherwise, throughout
`the description and the claims, the words “comprise,” “com-
`prising,” and the like are to be construed in an inclusive
`sense as opposedto an exclusive or exhaustive sense; thatis
`to say, in the sense of “including, but not limited to.” Words
`using the singular or plural numberalso includethe plural or
`2
`singular number
`respectively. Additionally,
`the words
`“herein,” “above,”
`“below” and words of similar import,
`when usedin this application, shall refer to this application
`as a whole and not
`to any particular portions of this
`application. When the claims use the word “or” in reference
`to a list of two or more items, that word covers all of the
`following interpretations of the word: any of the items in the
`list, all of the items in the list and any combination of the
`items in the list.
`
`FIG. 1 depicts a basic multi-carrier wireless communica-
`tion system consisting of a transmitter 102 and a receiver
`104. A functional block 106 at the transmitter, called Pilot
`generation and insertion, generates pilot subcarriers and
`inserts them into predetermined frequency locations. These
`pilot subcarriers are used by the receiver to carry out certain
`functions. In aspects of this invention, pilot subcarriers are
`divided into two different groups according to their func-
`tionalities, and hence their distinct requirements. The trans-
`mission format of cach group of pilot subcarriers will be
`designed so that it optimizes the performance of the system
`functions such as frequency synchronization and channel
`estimation.
`The first group is called “cell-specific pilot subcarriers,”
`and will be used bythe receiver 104 to extract information
`unique to each individual cell. For example,
`these cell-
`specific pilot subcarriers can be used in channel estimation
`whereit is necessary for a particular receiver to be able to
`differentiate the pilot subcarriers that are intended forits use
`from those of other cells. For these pilot subcarriers, coun-
`ter-interference methods are necessary.
`The second group is termed “commonpilot sub-carriers,”
`and are designed to possess a set of characteristics common
`to all base stations of the system. Thus, every receiver 104
`within the system is able to exploit these common pilot
`subcarriers to perform necessary functions without interfer-
`ence problem. For instance, these commonpilot subcarriers
`can be used in the frequency synchronization process, where
`it is not necessary to discriminate pilot subcarriers of dif-
`ferent cells, but it is desirable for the receiver to combine
`coherently the energy of commonpilot subcarriers with the
`same carrier index from different cells, so as to achieve
`relatively accurate frequency estimation.
`Aspects of this invention provide methods to define the
`transmission formats of the cell-specific and common pilot
`subcarriers that enable a receiver to perform different system
`functions. In particular, a set of design criteria are provided
`for pilot subcarriers. Other features of this invention further
`provide apparatus or means to implement the above design
`processes and methods. In particular, signal reception can be
`improved by manipulating phase valuesof the pilot subcar-
`riers and by using powercontrol.
`The methods and processes can also be extended to other
`cases, such as where multiple antennas are used within an
`individual sector and where some subcarriers are used to
`carry common network/system information. Base stations
`can be synchronized in frequency and time by sharing a
`common frequency oscillator or a common frequencyref-
`erence signal, such as the one generated from the signals
`provided by the Global Positioning System (GPS).
`Multi-Carrier Communication System
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`4
`In a multi-carrier communication system such as multi-
`carrier code division multiple access (MC-CDMA) and
`orthogonal frequency division multiple access (OFDMA),
`information data are multiplexed on subcarriers that are
`mutually orthogonal in the frequency domain. In effect, a
`frequency selective channel
`is broken into a number of
`parallel but small segments in frequency that can be treated
`as flat fading channels and hence can be easily dealt with
`using simple one-tap equalizers. The modulation/demodu-
`lation can be performed using the fast Fourier transform
`(FFT).
`In a multi-carrier communication system the physical
`media resource (e.g., radio or cable) can be divided in both
`the frequency and the time domains. This canonical division
`provides a high flexibility and fine granularity for resource
`sharing. The basic structure of a multi-carrier signal in the
`frequency domain is made up of subcarriers, and within a
`particular spectral band or channel there are a fixed number
`of subcarriers. There are three types of subcarriers:
`1. Data subcarriers, which carry information data;
`2. Pilot subcarriers, whose phases and amplitudes are
`predetermined and made knownto all receivers and which
`are used for assisting system functions such as estimation of
`system parameters; and
`3. Silent subcarricrs, which have no energy and are used
`for guard bands and DCcarriers.
`The data subcarriers can be arranged into groups called
`subchannels to support multiple access and scalability. The
`subcarriers forming one subchannel are not necessarily
`adjacent to each other. This conceptis illustrated in FIG. 2,
`showing a basic structure of a multi-carrier signal 200 in the
`frequency domain, which is made up of subcarriers. Data
`subcarriers can be grouped into subchannels in a particular
`way. Thepilot subcarriers are also distributed over the entire
`channel in a particular way.
`The basic structure of a multi-carrier signal in the time
`domain is made up of time slots to support multiple-access.
`The resource division in both the frequency and time
`domainsis depicted in FIG. 3 which showsa radio resource
`divided into small units in both the frequency and time
`domains: subchannels and timeslots, 300. The basic struc-
`ture of a multi-carrier signal in the time domain is made up
`of time slots.
`
`As depicted in FIG. 1, in a multi-carrier communication
`system, a generic transmitter may consist of the following
`functional blocks:
`1. Encoding and modulation 108
`2. Pilot generation and insertion 106
`3. Inverse fast Fourier transform (IFFT) 110
`4. Transmission 112
`
`And a generic receiver may consist of the following func-
`tional blocks:
`
`RDOBWNPR
`
`. Reception 114
`. Frame synchronization 116
`. Frequency and timing compensation 118
`. Fast Fourier transform (FFT) 120
`. Frequency, timing, and channel estimation 122
`. Channel compensation 124
`~]
`. Decoding 126
`Cellular Wireless Networks
`In a cellular wireless network, the geographical region to
`be serviced bythe network is normally divided into smaller
`areas called cells. In each cell the coverage is provided by
`a base station. Thus,
`this type of structure is normally
`referred to as the cellular structure depicted in FIG. 4, which
`illustrates a cellular wireless network 400 comprised of
`
`

`

`US 9,749,168 B2
`
`5
`multiple cells 402, in each of which coverage is provided by
`a base station (BS) 404. Mobile stations are distributed
`within each coverage area.
`A base station 404 is connected to the backbone of the
`
`network via a dedicated link and also provides radio links to
`mobile stations within its coverage. A base station 404 also
`serves as a focal point to distribute information to and collect
`information from its mobile stations by radio signals. The
`mobile stations within each coverage area are used as the
`interface between the users and the network.
`
`In an M-cell wireless network arrangement, with one-way
`or two-way communication and time division or frequency
`division duplexing,
`the transmitters at all
`the cells are
`synchronized via a particular means and are transmitting
`simultaneously. In a specific cell 402 of this arrangement,
`the pth cell, a receiver receives a signal at a specific
`subcarrier, the ith subcarrier, at the time t,, which can be
`describedas:
`
`.
`us
`;
`Silla) = dip(tettir®? + » aim (te eto)
`m=
`mtp
`
`()
`
`where a,,,(t,) and 9,,,,(t,) denote the signal amplitude and
`phase, respectively, associated with the i” subcarrier from
`the base station of the m,, cell.
`Cell-Specific Pilot Subcarriers
`If the ith subcarrier is used as a pilot subcarrier at the pth
`cell for the cell-specific purposes, the cell-specific informa-
`tion carried by a,_,,(t,) and 9,,, (t,) will be of interest to the
`receiver at the pth cell and other signals described by the
`second term onthe right hand side of equation (1) will be
`interference, which is an incoherent sum of signals from
`other cells. In this case, a sufficient level of the carrier-to-
`interference ratio (CIR) is required to obtain the estimates of
`a,p(t,) and 9,,, (t,) with desirable accuracy.
`There are many waysto boost the CIR. For examples, the
`amplitude of a pilot subcarrier can be set larger than that of
`a data subcarrier; power control can be applied to the pilot
`subcarriers; and cells adjacent to the pth cell may avoid
`using the ith subcarrier as pilot subcarrier. All these can be
`achieved with coordination between the cells based on
`certain processes, described below.
`Common Pilot Subcarriers
`The common pilot subcarriers for different cells are
`normally aligned in the frequency index at the time of
`transmission, as depicted in FIG. 5, which shows pilot
`subcarriers divided into two groups: cell-specific pilot sub-
`carriers and common pilot subcarriers. The cell-specific
`pilot subcarriers for different cells are not necessarily
`aligned in frequency. They can be used by the receiver to
`extract cell-specific information. The commonpilot subcar-
`riers for different cells may be aligned in frequency, and
`possess a setofattributes commontoall base stations within
`the network. Thus, every receiver within the system is able
`to exploit these common pilot subcarriers without interfer-
`ence problem. The power of the pilot subcarriers can be
`varied through a particular power control scheme and based
`on a specific application.
`If the ith subcarrier is used as a pilot subcarrier at the pth
`cell for the commonpurposes, it is not necessary to consider
`the second term on the right hand side of equation (1) to be
`interference. Instead, this term can be turned into a coherent
`componentof the desirable signal by designing the common
`
`6
`pilot carriers to meet the criteria specified in the aspects of
`this invention, provided that base stations at all cells are
`synchronized in frequency and time. In such a casethe cell
`in which the receiver is located becomes irrelevant and,
`consequently, the received signal can be rewritten as:
`
`.
`M
`Silt) = » Aim (etme)
`m=1
`
`2)
`
`The commonpilot subcarriers can be used for a number of
`functionalities, such as frequency offset estimation and
`timing estimation.
`To estimate the frequency, normally signals at different
`times are utilized. In an example with two commonpilot
`subcarriers of the same frequency index, the received signal
`at time t,,,, with respect to the received signal at time t,, is
`given by
`
`M
`mt
`IP}
`ja fjA
`silt) =e? s Gm (eeeink)
`m=1
`
`is much less than the coherence
`where At=t,,,-t,. If At
`period of the channel and
`Chall)=COiy(t)
`
`4)
`
`and
`
`Dig (t“Dion CetB:
`
`then the frequency can be determined by
`2afArmarg{s,(K)s(k+1)}-B;
`
`(5)
`
`(6)
`
`where c,>0 and -m<f,< or are predetermined constants for
`all values of m. And from all the frequency estimates{f,},
`a frequency offset can be derived based on a certain crite-
`rion.
`For timing estimation, normally multiple common pilot
`carriers are required. In an example of two commonpilot
`subcarriers, the received signal at f,, is given by
`
`/
`M
`Sn(t,) = elt) Onn (te JelPnmlte)
`m=1
`
`@
`
`where Af=f,-f, and T, denotes the sampling period.If Af is
`muchless than the coherence bandwidth of the channel and
`
`10
`
`15
`
`20
`
`25
`
`30
`
`40
`
`45
`
`Chalti=C(th)Ongnltn)
`
`and
`
`Diya (teOnn GAVE)
`
`then T, can be determined by
`2MAfT(6,)-arels" (G.)SGd 3h)
`
`(8)
`
`9%)
`
`(10)
`
`where c(t,)>0 and -=y(t,)s are predetermined constants
`for all values of m.
`FIG.6 is an embodimentof pilot-generation-and-insertion
`functional block 106 shown in FIG. 1, which employs a
`microprocessor 602 to generate pilot subcarriers and insert
`them into a frequency sequence contained in electronic
`memory 604. In one embodimentof the invention illustrated
`in FIG. 6, a microprocessor 602 embedded in the pilot-
`
`

`

`US 9,749,168 B2
`
`7
`generation-and-insertion functional block 106 computes the
`attributes of the pilot subcarriers such as their frequency
`indices and complex values specified by their requirements,
`and insert them into the frequency sequence contained in the
`electronic memory 604, such as a RAM, ready for the
`application of IFFT.
`Diversity for Common Pilot Subcarriers
`Considering equation (2), which is the sum of a number
`of complex signals,
`it is possible for these signals to be
`destructively superimposed on each other and cause the
`amplitude of the receiver signal at this particular subcarrier
`to be so small that the signal itself becomes unreliable. Phase
`diversity can help this adverse effect. In the example of
`frequency estimation, a random phase ¥ ,,,, can be added to
`anotherpilot subcarrier, say the Ith subcarrier, which results
`in
`
`PrynG=PiEt v Lm
`and
`
`(11)
`
`enerVignGer)+ v Lm
`where ¥,,, should be set differently for each cell, and
`provided that the following condition is met,
`Drya(tDryer)+By, for all values of m
`
`(13)
`
`(a2)
`
`With the phase diversity, it is expected that the probability
`of both |s,(t,)l and |s,(t,)| diminishing at the same time is
`relatively small. The embodiment of phase diversity is
`depicted in FIG. 7, which shows commonpilot subcarriers
`generated by a microprocessor of FIG. 6 to realize phase
`diversity. It should be noted that time delay will achieve the
`equivalent diversityeffect.
`Another embodimentis illustrated in FIG. 8, which effec-
`tively creates phase diversity by adding a random delaytime
`duration 802, either in baseband or RF, to the time-domain
`signals.
`Power Control for Pilot Subcarriers
`
`Tn one embodimentof the invention, power control can be
`applied to the pilot subcarriers. The power of the pilot
`subcarriers can be adjusted individually or as a subgroup to
`1. meet the needs of their functionalities;
`2. adapt to the operation environments (e.g., propagation
`channels); and
`3. reduce interference between cells or groups of cells.
`In another embodiment power control is implemented dif-
`ferently for cell-specific pilot subcarriers and commonpilot
`subcarriers. For example, stronger power is applied to
`common pilot subcarriers than to the cell-specific subcarri-
`ers.
`
`Application to Multiple Antennas
`The methodsand processes providedby this invention can
`also be implemented in applications where multiple anten-
`nas are used within an individual sector, provided that the
`criteria specified either by equations (4) and (5) for fre-
`quency estimation or by equations (8) and (9) for timing
`estimation aresatisfied.
`FIG. 9 shows two examples for extension to multiple
`antenna applications. In case (a) where there is only one
`transmission branchthat is connected to an array of antennas
`902 through a transformer 904 (e.g., a beam-forming
`matrix), the implementation is exactly the same as in the
`case of single antenna. In case (b) of multiple transmission
`branches connected to different antennas 906 (e.g.,
`in a
`transmission diversity scheme or a multiple-input multiple-
`output scheme), the cell-specific pilot subcarriers for trans-
`mission branchesare usually defined by a multiple-antenna
`scheme whereas the common pilot subcarriers for each
`
`8
`transmission branch are generated to meet the requirements
`of (4) and (5) for frequency estimation or (8) and (9) for
`timing estimation.
`Joint-Use of Cell-Specific and Common Pilot Subcarriers
`In one embodiment the cell-specific and common pilot
`subcarriers can be used jointly in the same process based on
`certain information theoretic criteria, such as the optimiza-
`tion of the signal-to-noise ratio. For example, in the estima-
`tion of a system parameter (e.g. frequency), some orall
`cell-specific subcarriers, if they satisfy a certain criterion,
`such as to exceed a CIR threshold, may be selected to be
`used together with the commonpilot subcarriers to improve
`estimation accuracy. Furthermore, the commonpilot sub-
`carriers can be used along with the cell-specific subcarriers
`to determinethe cell-specific information in some scenarios,
`one of which is the operation at the edge of the network.
`Base Transmitters Synchronization
`Basestations at all cells are required to be synchronized
`in frequency andtime. In one embodimentof the invention
`the collocated base station transmitters are locked to a single
`frequency oscillator, as in the case where a cell is divided
`into sectors and the base stations of these sectors are
`
`physically placed at the same location.
`FIG. 10 is an embodiment of synchronization in fre-
`quency and time domains of two collocated base stations
`sharing a common frequency oscillator 1002. Mobile sta-
`tions 1004 covered by these two base stations do not
`experience interference when receiving the commonpilot
`subcarriers. The base station transmitters that are located at
`
`different areas are locked to a commonreference frequency
`source, such as the GPSsignal. FIG. 11 depicts an embodi-
`ment of synchronization in frequency and time domains with
`base stations 1102 and 1104 at different locations sharing a
`commonfrequency reference signal generated from the GPS
`1106 signals. Mobile stations 1108 covered by these two
`base stations 1102 and 1104 do not experience interference
`when receiving the commonpilot subcarriers.
`In some applications, the entire wireless network may
`consist of multiple groups of cells (or sectors) and each
`group may haveits own set of commonpilot subcarriers. In
`such scenarios, only those base stations within their group
`are required to synchronize to a common reference. While
`the common pilot
`subcarriers within each group are
`designed to meetthe criteria defined by equations (4) and (5)
`or by (8) and (9) forthe use byits base stations, a particular
`counter-interference process (e.g., randomization in fre-
`quency or powercontrol) will be applied to different sets of
`commonpilot subcarriers. This will cause the signals from
`the cells within the same group to add coherently while the
`signals from the cells in other groups are treated as random-
`ized interference.
`
`One embodimentof such implementationis illustrated in
`FIG. 12, where a wireless network consists of three groups
`(A, B, and C) of cells (or sectors). The base stations within
`their own group share the same set of common pilot sub-
`carriers. In this scenario, only those base stations within
`their group are required to synchronize to a commonrefer-
`ence. While the commonpilot subcarriers within each group
`are designed to meet the criteria defined in this invention, a
`particular counter-interference process (e.g., randomization
`in frequency) will be applied to different sets of common
`pilot subcarriers. For example, the base stations at Cells Al,
`A2, and A3 in Group A synchronize to their own common
`reference source and transmit the same set of commonpilot
`subcarriers; an