(12) United States Patent
`Li et al.
`
`(io) Patent No.: US 8,432,891 B2
`(45) Date of Patent:
`Apr. 30, 2013
`
`US008432891B2
`
`(54) METHOD, APPARATUS, AND SYSTEM FOR
`MITIGATING PILOT SIGNAL
`DEGRADATION BY EMPLOYING
`CELL-SPECIFIC PILOT SUBCARRIER AND
`COMMON PILOT SUBCARRIER
`TECHNIQUES IN A MULTI-CARRIER
`CELLULAR NETWORK
`
`(75) Inventors: Xiaodong Li, Kirkland, WA (US); Titus
`Lo, Bellevue, WA (US); Kemin Li,
`Bellevue, WA (US); Haiming Huang,
`Bellevue, WA (US)
`
`(73) Assignee: Neociflc, Inc., Bellevue, WA (US)
`
`( * ) Notice: Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`(21) Appl. No.: 13/212,116
`
`(22) Filed:
`
`Aug. 17, 2011
`
`(65)
`
`Prior Publication Data
`US 2011/0299474 Al Dec. 8, 2011
`
`Related U.S. Application Data
`(63) 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)
`
`(2006.01)
`
`Int. Cl.
`H04J3/06
`(52) U.S. Cl.
`USPC ........... 370/350; 370/331; 370/332; 370/324;
`370/344; 370/319; 455/422.1; 455/552.1;
`455/502; 455/561; 455/434
`(58) Field of Classification Search ................... 370/350,
`370/324, 331-332, 319, 344, 329, 338, 328,
`
`370/203, 280, 342, 335; 455/67.11-67.13,
`455/423, 503, 115.1, 436-444, 502, 552.1,
`455/562.1, 422.1, 434, 525
`See application file for complete search history.
`
`(56)
`
`References Cited
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`JP
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`(Continued)
`FOREIGN PATENT DOCUMENTS
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`OTHER PUBLICATIONS
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`(Continued)
`
`Primary Examiner — Vladimir Magloire
`Assistant Examiner — Babar Sarwar
`(74) Attorney, Agent, or Firm — Perkins Coie LLP
`
`(57)
`ABSTRACT
`A multi-carrier cellular wireless network (400) employs base
`stations (404) that transmit two different groups of pilot sub­
`carriers: (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 common to all the base
`stations (404) of the system. The design criteria and transmis­
`sion formats of the cell-specific and common pilot 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 com­
`mon network/system information.
`
`38 Claims, 13 Drawing Sheets
`
`Subcarrier arrangement for Cell p
`
`Subcarrier arrangement for Cell q
`
`Common pilot
`subcarriers
`
`Cell-specific pilot 4 Subcarriers
`I for data
`subcarriers
`
`VWGoA EX1036
`VWGoA V. Neo Wireless
`IPR2022-01539
`
`

`

`US 8,432,891 B2
`Page 2
`
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`
`* cited by examiner
`
`

`

`U.S. Patent
`U.S. Patent
`
`Apr. 30, 2013
`Apr. 30, 2013
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`Sheet 1 of 13
`Sheet 1 of 13
`
`US 8,432,891 B2
`US 8,432,891 B2
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`U.S. Patent
`U.S. Patent
`
`Apr. 30, 2013
`Apr. 30, 2013
`
`Sheet 2 of 13
`Sheet 2 of 13
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`US 8,432,891 B2
`US 8,432,891 B2
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`

`

`U.S. Patent
`U.S. Patent
`
`Apr.30, 2013
`Apr. 30, 2013
`
`Sheet 3 of 13
`Sheet 3 of 13
`
`US 8,432,891 B2
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`

`

`U.S. Patent
`U.S. Patent
`
`Apr.30, 2013
`Apr. 30, 2013
`
`Sheet 4 of 13
`Sheet 4 of 13
`
`US 8,432,891 B2
`US 8,432,891 B2
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`
`
`FIG. 4
`
`400
`
`

`

`U.S. Patent
`
`Apr. 30, 2013
`
`Sheet 5 of 13
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`US 8,432,891 B2
`
`Subcarrier arrangement for Cell p
`
`

`

`U.S. Patent
`U.S. Patent
`
`Apr.30, 2013
`Apr. 30, 2013
`
`Sheet 6 of 13
`Sheet 6 of 13
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`US 8,432,891 B2
`US 8,432,891 B2
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`U.S. Patent
`U.S. Patent
`
`Apr. 30, 2013
`Apr. 30, 2013
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`Sheet 7 of 13
`Sheet 7 of 13
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`US 8,432,891 B2
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`U.S. Patent
`U.S. Patent
`
`Apr. 30, 2013
`Apr. 30, 2013
`
`Sheet 8 of 13
`Sheet 8 of 13
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`US 8,432,891 B2
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`U.S. Patent
`
`Apr. 30, 2013 Sheet 9 of 13
`
`US 8,432,891 B2
`
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`
`FIG. 9
`
`

`

`U.S. Patent
`U.S. Patent
`
`Apr. 30, 2013
`Apr. 30, 2013
`
`Sheet 10 of 13
`Sheet 10 of 13
`
`US 8,432,891 B2
`US 8,432,891 B2
`
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`

`

`U.S. Patent
`U.S. Patent
`
`Apr. 30, 2013
`Apr. 30, 2013
`
`Sheet 11 of 13
`Sheet 11 of 13
`
`US 8,432,891 B2
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`U.S. Patent
`U.S. Patent
`
`Apr. 30, 2013
`Apr. 30, 2013
`
`Sheet 12 of 13
`Sheet 12 of 13
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`US 8,432,891 B2
`US 8,432,891 B2
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`
`GroupC
`
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`
`GroupA
`
`

`

`U.S. Patent
`
`Apr. 30, 2013
`
`Sheet 13 of 13
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`US 8,432,891 B2
`
`FIG. 13
`
`Common data
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`

`

`US 8,432,891 B2
`
`1
`METHOD, APPARATUS, AND SYSTEM FOR
`MITIGATING PILOT SIGNAL
`DEGRADATION BY EMPLOYING
`CELL-SPECIFIC PILOT SUBCARRIER AND
`COMMON PILOT SUBCARRIER
`TECHNIQUES IN A MULTI-CARRIER
`CELLULAR NETWORK
`
`CROSS-REFERENCE TO RELATED
`APPLICATION(S)
`
`This application is a continuation of U.S. patent applica­
`tion Ser. No. 10/583,530, entitled “METHODS AND APPA­
`RATUS USING CELL-SPECIFIC AND COMMON PILOT
`SUBCARRIERS IN MULTI-CARRIER, MULTI-CELL
`WIRELESS COMMUNICATION NETWORKS,” filed May
`30, 2007 and hereby incorporated herein by reference in its
`entirety, which is a U.S. National Stage of PCT Application
`No. PCT/US05/01939, entitled “METHODS AND APPA­
`RATUS FOR MULTI-CARRIER, MULTI-CELL WIRE­
`LESS COMMUNICATION NETWORKS,” filed Jan. 20,
`2005, which claims the benefit of and priority to U.S. Provi­
`sional Patent Application No. 60/540,032, entitled “METH­
`ODSANDAPPARATUS FOR MULTI-CARRIER, MULTI­
`CELL WIRELESS COMMUNICATION NETWORKS,”
`filed on Jan. 29, 2004.
`
`BACKGROUND
`
`In multi-carrier wireless communications, many important
`system functions such as frequency synchronization and
`channel estimation, depicted in FIG. 1, are facilitated by
`using the network information provided by a portion of total
`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.
`In a wireless network, there are a number of base 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 consid­
`ered a cell. In this context, the terms “cell” and “sector” are
`interchangeable. The network information can be categorized
`into two types: the cell-specific information that is unique to
`a particular cell, and the common information that is common
`to the entire network or to a portion of the entire networks
`such as a group of cells.
`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, sig­
`nals originated from the base stations at different cells inter­
`fere 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 subcarri­
`ers 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.
`
`2
`FIG. 2 shows basic structure of a multi-carrier signal in the
`frequency domain, which is made up of subcarriers.
`FIG. 3 shows a 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 common pilot subcarriers.
`FIG. 6 is an embodiment of pilot-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 shows that common pilot subcarriers are generated
`by a microprocessor ofFIG. 6 to realize phase diversity.
`FIG. 8 is an embodiment of delay diversity, which effec­
`tively creates phase diversity by adding a random delay time
`duration, either in baseband or RF, to the time-domain sig­
`nals.
`FIG. 9 shows two examples for extension to multiple
`antenna applications.
`FIG. 10 is an embodiment of synchronization in frequency
`and time domains of two collocated base stations sharing a
`common frequency oscillator.
`FIG. 11 is an embodiment of synchronization in frequency
`and time domains with base stations at different locations
`sharing a common frequency reference signal generated from
`the GPS signals.
`FIG. 12 is an embodiment depicting a wireless network
`consisting of three groups of cells (or sectors) and base sta­
`tions in each group that share their own set of common pilot
`subcarriers.
`FIG. 13 shows all base stations within a network transmit,
`along with a common pilot subcarrier, a data subcarrier car­
`rying data information common to all cells in the network.
`
`DETAILED DESCRIPTION
`
`In the following description the invention is explained with
`respect to some of its various embodiments, providing spe­
`cific details for a thorough understanding and enablement.
`However, one skilled in the art will understand that the inven­
`tion 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; that is to say,
`in the sense of “including, but not limited to.” Words using the
`singular or plural number also include the 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 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 genera­
`tion and insertion, generates pilot subcarriers and inserts them
`into predetermined frequency locations. These pilot subcar­
`riers are used by the receiver to carry out certain functions. In
`
`5
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`US 8,432,891 B2
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`3
`aspects of this invention, pilot subcarriers are divided into two
`different groups according to their functionalities, and hence
`their distinct requirements. The transmission 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-spe­
`cific pilot subcarriers can be used in channel estimation where
`it is necessary for a particular receiver to be able to differen­
`tiate the pilot subcarriers that are intended for its use from
`those of other cells. For these pilot subcarriers, counter-inter­
`ference methods are necessary.
`The second group is termed “common pilot 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 sub­
`carriers to perform necessary functions without interference
`problem. For instance, these common pilot subcarriers can be
`used in the frequency synchronization process, where it is not
`necessary to discriminate pilot subcarriers of different cells,
`but it is desirable for the receiver to combine coherently the
`energy of common pilot 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 power control.
`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 frequency refer­
`ence signal, such as the one generated from the signals pro­
`vided 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 par­
`allel 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/demodulation 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 pro­
`vides a high flexibility and fine granularity for resource shar­
`ing. The basic structure of a multi-carrier signal in the fre­
`quency 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 pre­
`determined and made known to all receivers and which are
`used for assisting system functions such as estimation of
`system parameters; and
`
`5
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`4
`3. Silent subcarriers, which have no energy and are used for
`guard bands and DC carriers.
`The data subcarriers can be arranged into groups called
`subchannels to support multiple access and scalability. The
`subcarriers forming one subchannel are not necessarily adja­
`cent to each other. This concept is illustrated in FIG. 2, show­
`ing a basic structure of a multi-carrier signal 200 in the fre­
`quency 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 shows a radio resource divided
`into small units in both the frequency and time domains:
`subchannels and time slots, 300. The basic structure 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
`2. Frame synchronization 116
`3. Frequency and timing compensation 118
`4. Fast Fourier transform (FFT) 120
`5. Frequency, timing, and channel estimation 122
`6. Channel compensation 124
`7. Decoding 126
`Cellular Wireless Networks
`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 cover­
`age 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 syn­
`chronized via a particular means and are transmitting simul­
`taneously. 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 Q, which can be described as:
`
`-5/(¾) =
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`
`M
`
`m=l
`m±p
`
`(1)
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`US 8,432,891 B2
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`6
`where At=ti+1-tt. If At is much less than the coherence period
`of the channel and
`
`and
`
`5
`
`(4)
`
`5
`where a,jm(tt) and φ,denote the signal amplitude and
`phase, respectively, associated with the\th 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, (fy) and 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-interfer-
`ence ratio (CIR) is required to obtain the estimates of a, (fy)
`and with desirable accuracy.
`There are many ways to 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 pro­
`cesses, described below.
`Common Pilot Subcarriers
`The common pilot subcarriers for different cells are nor­
`mally aligned in the frequency index at the time of transmis­
`sion, 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 infor­
`mation. The common pilot subcarriers for different cells may
`be aligned in frequency, and possess a set of attributes com­
`mon to all base stations within the network. Thus, every
`receiver within the system is able to exploit these common
`pilot subcarriers without interference 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 common purposes, 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
`component of 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 synchro­
`nized 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:
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`Φ.>,(Φ)=(Φ.·,™(Φ+ι)+β.·
`then the frequency can be determined by
`2πφ ,Ar=arg{si(i)si(i+1)}- β,·
`(6)
`where c >0 and -jtS β,=π or are predetermined constants for
`all values of m. And from all the frequency estimates (1,). a
`frequency offset can be derived based on a certain criterion.
`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
`
`(7)
`
`and fy denotes the sampling period. If A.f is
`where
`much less than the coherence bandwidth of the channel and
`(8)
`
`and
`
`(9)
`
`then fy can be determined by
`2πΑφϊ;(4)=ΗΓΒ{5,.*(4)5„(4)}-γ(4)
`(10)
`where c(fy)>0 and
`are predetermined constants
`for all values of m.
`FIG. 6 is an embodiment of 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 embodiment of the invention illustrated
`in FIG. 6, a microprocessor 602 embedded in the pilot-gen-
`eration-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 appli­
`cation 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 destruc­
`tively 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 estima­
`tion, a random phase θζ can be added to another pilot sub­
`carrier, say the Ith subcarrier, which results in
`Φ/_™(4)=Φ.>,&)+θ/,„,
`
`(ii)
`
`and
`
`ΦΙζ„(Φ+ι)=Φ.>,&+ι)+θ/,™
`(12)
`where θζ should be set differently for each cell, and provided
`that the following condition is met,
`Φ/,™(Φ)=Φ/,™&+ι)+βΛ for all values of m
`(13)
`With the phase diversity, it is expected that the probability
`of both ls,(tt)l and lsz(t*)l diminishing at the same time is
`relatively small. The embodiment of phase diversity is
`depicted in FIG. 7, which shows common pilot subcarriers
`
`M
`
`(2)
`
`50
`
`The common pilot subcarriers can be used for a number of
`functionalities, such as frequency offset estimation and tim­
`ing estimation.
`To estimate the frequency, normally signals at different
`times are utilized. In an example with two common pilot
`subcarriers of the same frequency index, the received signal at
`time ti+1, with respect to the received signal at time tfo is given
`by
`
`M
`
`(3)
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`7
`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 embodiment is 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 embodiment of the invention, power control can be
`applied to the pilot subcarriers. The power of the pilot sub­
`carriers 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 differ­
`ently for cell-specific pilot subcarriers and common pilot
`subcarriers. For example, stronger power is applied to com­
`mon pilot subcarriers than to the cell-specific subcarriers.
`Application to Multiple Antennas
`The methods and processes provided by this invention can
`also be implemented in applications where multiple antennas
`are used within an individual sector, provided that the criteria
`specified either by equations (4) and (5) for frequency esti­
`mation 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 branch that 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 con­
`nected to different antennas 906 (e.g., in a transmission diver­
`sity scheme or a multiple-input multiple-output scheme), the
`cell-specific pilot subcarriers for transmission branches are
`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 fre­
`quency 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 or all 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 common pilot subcarriers to improve estimation
`accuracy. Furthermore, the common pilot sub-carriers can be
`used along with the cell-specific subcarriers to determine the
`cell-specific information in some scenarios, 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 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 frequency
`and time domains of two collocated base stations sharing a
`common frequency oscillator 1002. Mobile stations 1004
`covered by these two base stations do not experience inter­
`ference when receiving the common pilot subcarriers. The
`base station transmitters that are located at different areas are
`locked to a common reference frequency source, such as the
`GPS signal. FIG. 11 depicts an embodiment of synchroniza­
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`US 8,432,891 B2
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`8
`tion in frequency and time domains with base stations 1102
`and 1104 at different locations sharing a common frequency
`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
`common pilot subcarriers.
`In some applications, the entire wireless network may con­
`sist of multiple groups of cells (or sectors) and each group
`may have its own set of common pilot 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
`meet the criteria defined by equations (4) and (5) or by (8) and
`(9) for the use by its base stations, a particular counter-inter­
`ference process (e.g., randomization in frequency or power
`control) will be applied to different sets of common pilot
`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 randomized interference.
`One embodiment of such implementation is 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 subcar­
`riers. In this scenario, 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 meet the criteria defined in this invention, a par­
`ticular 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 refer­
`ence source and transmit the same set of common pilot sub­
`carriers; and the ba