a2) United States Patent
`US 10,965,512 B2
`(0) Patent No.:
`Mar.30, 2021
`(45) Date of Patent:
`Li et al.
`
`US010965512B2
`
`(54)
`
`METHOD AND APPARATUS USING
`CELL-SPECIFIC AND COMMONPILOT
`SUBCARRIERS IN MULTI-CARRIER, MULTI
`CELL WIRELESS COMMUNICATION
`NETWORKS
`
`(71)
`
`Applicant: Neo Wireless LLC, Wayne, PA (US)
`
`(72)
`
`Inventors: Xiaodong Li, Kirkland, WA (US);
`Titus Lo, Bellevue, WA (US); Kemin
`Li, Bellevue, WA (US); Haiming
`Huang, Bellevue, WA (US)
`
`CN
`CN
`
`(56)
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`5,825,807 A
`5,828,650 A
`
`10/1998 Kumar
`10/1998 Malkamaki et al.
`(Continued)
`
`FOREIGN PATENT DOCUMENTS
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`
`4/2003
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`
`(Continued)
`
`OTHER PUBLICATIONS
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`Broadcasting (DVB); Framingstructure, channel coding and modu-
`lation for digital terrestrial television, ETSI EN 300 744 V1.5.1
`(Jun. 2004).
`
`(Continued)
`
`Primary Examiner — Omoniyi Obayanyu
`(74) Attorney, Agent, or Firm — Volpe Koenig
`
`(57)
`
`ABSTRACT
`
`Amulti-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 somesubcarriers bear common network/system infor-
`mation.
`
`30 Claims, 13 Drawing Sheets
`
`(73)
`
`Assignee: NEO WIRELESS LLC, Wayne, PA
`(US)
`
`(*)
`
`Notice:
`
`Subject to any disclaimer, the term ofthis
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`(21)
`
`Appl. No.: 17/012,813
`
`(22)
`
`Filed:
`
`Sep. 4, 2020
`
`(65)
`
`(63)
`
`Prior Publication Data
`
`US 2020/0403838 Al
`
`Dec. 24, 2020
`
`Related U.S. Application Data
`
`Continuation of application No. 16/440,754, filed on
`Jun. 13, 2019, which is a continuation of application
`(Continued)
`
`(51)
`
`Int. Cl.
`
`HO4W 4/00
`HOAL 27/26
`
`(52)
`
`USS. Cl.
`
`(2018.01)
`(2006.01)
`(Continued)
`
`CPC wo. HOA4L 27/2637 (2013.01); HO4B 1/707
`(2013.01); HO4B 7/0413 (2013.01);
`(Continued)
`Field of Classification Search
`CPC .... HO4L 5/0053; HO4L 5/0007; H04L 5/0048
`See application file for complete search history.
`
`(58)
`
`GM 1001
`
`c
`
`§
`
`Subcarrier arrangement for Cell p
`ce
`
`S C
`
`ommon pilot
`
`subcarriers
`
`| Cell-specific pilot
`subcarriers
`
`1
`
`GM 1001
`
`

`

`US 10,965,512 B2
`Page 2
`
`Related U.S. Application Data
`
`No. 15/688,441, filed on Aug. 28, 2017, now Pat. No.
`10,326,631, which is a continuation of application
`No. 14/746,676, filed on Jun. 22, 2015, now Pat. No.
`9,749,168, which is a continuation of application No.
`14/595,132, filed on Jan. 12, 2015, now Pat. No.
`9,065,614, which is a 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)
`(2011.01)
`(2006.01)
`(2006.01)
`(2006.01)
`(2009.01)
`(2017.01)
`(2006.01)
`(2006.01)
`
`Int. CL
`HO4W 16/02
`HO4B 1/707
`HO4AL 5/00
`HO4L 25/03
`HOAL 27/00
`HOAW 72/04
`HO4B 7/0413
`HO4J 11/00
`HOAL 25/02
`(52) U.S. Cl.
`CPC oe H04J 11/005 (2013.01); HO4E 5/0007
`(2013.01); HO4E 5/0028 (2013.01); HO4L
`5/0048 (2013.01); HO4L 25/03834 (2013.01);
`HO4L 27/0008 (2013.01); HO4L 27/0012
`(2013.01); HO4E 27/2602 (2013.01); HO4E
`27/2613 (2013.01); HO4L 27/2626 (2013.01);
`HO4L 27/2646 (2013.01); HO4W 16/02
`(2013.01); HO4W 72/044 (2013.01); HO4W
`72/0446 (2013.01); HO4E 5/0016 (2013.01);
`HO4E 25/0228 (2013.01); HO4L 27/2607
`(2013.01); HO4E 27/2655 (2013.01); HO4L
`27/2657 (2013.01)
`
`(56)
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`
`2
`
`

`

`US 10,965,512 B2
`Page 3
`
`(56)
`
`References Cited
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`115-119.
`
`* cited by examiner
`
`3
`
`

`

`U.S. Patent
`
`Mar.30, 2021
`
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`US 10,965,512 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 U.S. patent applica-
`tion Ser. No. 16/440,754, filed Jun. 13, 2019, which is a
`continuation of U.S. patent application Ser. No. 15/688,441,
`filed Aug. 28, 2017, which issued as U.S. Pat. No. 10,326,
`631 on Jun. 18, 2019, which is a continuation of U.S. patent
`application Ser. No. 14/746,676, filed Jun. 22, 2015, which
`issued as U.S. Pat. No. 9,749,168 on Aug. 29, 2017, which
`is a continuation of U.S. patent application Ser. No. 14/595,
`132, filed Jan. 12, 2015, which issued as U.S. Pat. No.
`9,065,614 on Jun. 23, 2015, which is a continuation of U.S.
`patent application Ser. No. 13/874,278, filed Apr. 30, 2013,
`which issued as U.S. Pat. No. 8,934,473 on Jan. 13, 2015,
`which is a continuation of U.S. patent application Ser. No.
`13/212,116, filed Aug. 17, 2011, which issued as U.S. Pat.
`No. 8,432,891 on Apr. 30, 2013, which is a continuation of
`USS. patent application Ser. No. 10/583,530, filed May 30,
`2007, which issued as U.S. Pat. No. 8,009,660 on Aug. 30,
`2011, which is a U.S. National Stage of PCT Application No.
`PCT/US05/01939, filed Jan. 20, 2005, which claims the
`benefit of andpriority to U.S. Provisional Patent Application
`No. 60/540,032, filed on Jan. 29, 2004, the entire contents of
`all of which are hereby incorporated by reference herein.
`
`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 portion oftotal
`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 numberofbase stations,
`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 commoninformation that
`is commonto the entire network or to a portion of the 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
`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
`
`2
`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
`
`10
`
`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 frequencyoscillator.
`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
`
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`In the following description the invention is explained
`with respect to some ofits various embodiments, providing
`specific details for a thorough understanding and enable-
`ment. However, one skilled in the art will understandthat 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.
`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 opposed to 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
`singular number
`respectively. Additionally,
`the words
`“herein,” “above,” “below” and words of similar import,
`when used in 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
`
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`US 10,965,512 B2
`
`3
`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 bythe 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 each 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 by the receiver 104 to extract information
`unique to each individual cell. For example,
`these cell-
`specific pilot subcarriers can be used in channel estimation
`where it 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 values of 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
`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
`
`4
`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 channelthere 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 subcarriers, 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,
`showinga 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. The pilot 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
`domains is 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:
`1. 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
`
`AmBWD
`
`In a cellular wireless network, the geographical region to
`be serviced by the 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
`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 providesradio 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.
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`
`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
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`

`US 10,965,512 B2
`
`5
`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
`described as:
`
`6
`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 timet,, is
`given by
`
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`
`Mt
`Silt) = efter) Gin (Bera JerPimte)
`m=1
`
`(3)
`
`is much less than the coherence
`where At=t,,,-t,. If At
`period of the channel and
`
`Chiath=CiCiatier)
`
`and
`
`5y(G=P;yn (Cenr)+B;
`
`then the frequency can be determined by
`
`2np,At=arg{s,(k)s,(k+1)}-B;
`
`(4)
`
`(5)
`
`(6)
`
`where c,>0 and -1<{,sor 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 common pilot
`subcarriers, the received signal at f,, is given by
`
`;
`M
`|
`Sp(te) = elPRALT(ty s Qnm (t Jel¥nmtk )
`m=1
`
`ie)
`
`where Af=f,,-f, and T, denotes the sampling period.If Af is
`muchless than the coherence bandwidth of the channel and
`
`Obsyalby.)=C(Ep)Onmn (La)
`
`and
`
`40
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`5.0L)PnaE)
`
`then T, can be determined by
`
`2HAfT,(0,)=argt5;*(G)5.(G)}Vd)
`
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`50
`
`(8)
`
`(9)
`
`(10)
`
`M
`Sill) = ai,pte JerPiP(® + » Gin (te Joi)
`ml
`m#tp
`
`()
`
`where a,,,(t,) and @,,,,(t,) denote the signal amplitude and
`phase, respectively, associated with the i” subcarrier from
`the base station of the my, cell.
`Cell-Specific Pilot Subcarriers
`If the ith subcarrier is used as a pilot subcarrierat the pth
`cell for the cell-specific purposes, the cell-specific informa-
`tion carried by a,_,(t,) and @,,,(t,) will be of interest to the
`receiver at the pth cell and other signals described by the
`second term on the 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 @,_,(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.
`CommonPilot 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 set of attributes commonto all 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 subcarrierat 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
`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 case the cell
`in which the receiver is located becomes irrelevant and,
`consequently, the received signal can be rewritten as:
`
`wherec(t,)>0 and -m<y(t,)s0 are predetermined constants
`for all values of m.
`
`FIG.6 is an embodimentofpilot-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-
`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 smallthat the signalitself becomes unreliable. Phase
`diversity can help this adverse effect. In the example of
`
`/
`M
`Sil) = » Gin (ty elim)
`m=1
`
`60
`
`(2)
`
`The commonpilot subcarriers can be used for a numberof
`functionalities, such as frequency offset estimation and
`timing estimation.
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`US 10,965,512 B2
`
`7
`frequency estimation, a random phase 0,,,, can be added to
`another pilot subcarrier, say the Ith subcarrier, which results
`in
`
`Dint=Vimn(G)+97m
`
`and
`
`PirateViger)+8ym
`
`(11)
`
`(12)
`
`where 248 ,,, should be set differently for each cell, and
`provided that the following condition is met,
`
`10
`
`Oral)Pin Gerr)+B;, for all values of m
`
`(13)
`
`With the phase diversity, it is expected that the probability
`of both |s,(t,)| and Is,(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 diversity effect.
`Another embodimentis illustrated in FIG. 8, which effec-
`tively creates phase diversity by adding a random delay time
`duration 802, either in baseband or RF, to the time-domain
`signals.
`Power Control for Pilot Subcarriers
`
`In 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 ofcells.
`In another embodiment power control is implemented dif-
`ferently for cell-specific pilot subcarriers and commonpilot
`subcarriers. For example, stronger power is applied to
`commonpilot subcarriers than to the cell-specific subcarri-
`ers.
`
`Application to Multiple Antennas
`The methods and 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 are satisfied.
`
`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
`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 embodimentthe 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 common pilot sub-
`
`8
`carriers can be used along with the cell-specific subcarriers
`to determinethe cell-specific information in somescenarios,
`one of which is the operation at the edge of the network.
`Base Transmitters Synchronization
`Base stations at all cells are required to be synchronized
`in frequency and time. In one embodiment of the invention
`the collocated base station transmitt