U.S. patent application number 16/440754 was filed with the patent office on 2020-04-30 for method and apparatus using cell-specific and common pilot subcarriers in multi-carrier, multi-cell wireless communication networ.
The applicant listed for this patent is Neocific, Inc.. Invention is credited to Haiming Huang, Kemin Li, Xiaodong Li, Titus Lo.
Application Number | 20200136876 16/440754 |
Document ID | / |
Family ID | 34826174 |
Filed Date | 2020-04-30 |
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United States Patent
Application |
20200136876 |
Kind Code |
A1 |
Li; Xiaodong ; et
al. |
April 30, 2020 |
METHOD AND APPARATUS USING CELL-SPECIFIC AND COMMON PILOT
SUBCARRIERS IN MULTI-CARRIER, MULTI-CELL WIRELESS COMMUNICATION
NETWORKS
Abstract
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 common to all the base stations
(404) of the system. The design criteria and transmission 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 common network/system information.
Inventors: |
Li; Xiaodong; (Kirkland,
WA) ; Lo; Titus; (Bellevue, WA) ; Li;
Kemin; (Bellevue, WA) ; Huang; Haiming;
(Bellevue, WA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Neocific, Inc. |
Bellevue |
WA |
US |
|
|
Family ID: |
34826174 |
Appl. No.: |
16/440754 |
Filed: |
June 13, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15688441 |
Aug 28, 2017 |
10326631 |
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16440754 |
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14746676 |
Jun 22, 2015 |
9749168 |
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15688441 |
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14595132 |
Jan 12, 2015 |
9065614 |
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14746676 |
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13874278 |
Apr 30, 2013 |
8934473 |
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14595132 |
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13212116 |
Aug 17, 2011 |
8432891 |
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13874278 |
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10583530 |
May 30, 2007 |
8009660 |
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PCT/US05/01939 |
Jan 20, 2005 |
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13212116 |
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60540032 |
Jan 29, 2004 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04J 11/005 20130101;
H04L 5/0007 20130101; H04L 27/0012 20130101; H04L 27/2657 20130101;
H04L 27/2637 20130101; H04L 5/0048 20130101; H04L 27/0008 20130101;
H04L 5/0028 20130101; H04L 27/2607 20130101; H04W 72/044 20130101;
H04L 5/0016 20130101; H04L 27/2613 20130101; H04L 27/2602 20130101;
H04L 27/2646 20130101; H04B 1/707 20130101; H04L 25/03834 20130101;
H04W 72/0446 20130101; H04W 16/02 20130101; H04L 27/2626 20130101;
H04B 7/0413 20130101; H04L 27/2655 20130101; H04L 25/0228
20130101 |
International
Class: |
H04L 27/26 20060101
H04L027/26; H04W 16/02 20060101 H04W016/02; H04B 1/707 20060101
H04B001/707; H04L 5/00 20060101 H04L005/00; H04L 25/03 20060101
H04L025/03; H04L 27/00 20060101 H04L027/00; H04W 72/04 20060101
H04W072/04; H04B 7/0413 20060101 H04B007/0413; H04J 11/00 20060101
H04J011/00 |
Claims
1. A transmitting method for a base station in a cell within a
group of cells in an orthogonal frequency division multiplexing
system, the base station having multiple transmission branches, the
method comprising: transmitting cell-specific pilot subcarriers, in
accordance with a transmission diversity scheme or a multiple-input
multiple-output scheme, wherein some of the cell-specific pilot
subcarriers are not aligned in frequency subcarrier index with
cell-specific pilot subcarriers transmitted by another base station
in the group of cells; transmitting common pilot subcarriers in
accordance with a common pilot transmission scheme, wherein: the
common pilot subcarriers are aligned in frequency subcarrier index
with common pilot subcarriers transmitted by other base stations in
the group of cells; a ratio of an amplitude (a.sub.i,m and
a.sub.n,m) of two common pilot subcarriers having a same time index
(t.sub.k) that are transmitted via a common transmission branch is
a constant for the group of cells; and a difference of a phase
(.phi..sub.i,m and .phi..sub.n,m) of two common pilot subcarriers
having a same time index (t.sub.k) that are transmitted via a
common transmission branch is a constant for the group of cells;
and controlling independently a transmission power of the
cell-specific pilot subcarriers and a transmission power of the
common pilot subcarriers.
2. The method of claim 1, wherein the transmission power of the
cell-specific pilot subcarriers is higher than the transmission
power of the common pilot subcarriers.
3. The method of claim 1, wherein the transmission power of the
cell-specific pilot subcarriers is lower than the transmission
power of the common pilot subcarriers.
4. The method of claim 1, wherein the cell-specific pilot
subcarriers enable a mobile station in the cell to determine
channel coefficients for a channel from the base station to the
mobile station.
5. The method of claim 1, wherein the specific amplitudes, phases,
and frequency subcarrier indices of the cell-specific pilot
subcarriers enable a mobile station in the cell to differentiate
the cell-specific pilot subcarriers from pilot subcarriers
transmitted by base stations in other cells.
6. The method of claim 1, wherein the common pilot subcarriers
enable a mobile station in the cell to determine composite channel
coefficients for a composite channel, the composite channel
corresponding to an aggregate of different channels from base
stations in the group of cells to the mobile station.
7. In a cell within a group of cells in an orthogonal frequency
division multiplexing system, a base station having multiple
transmission branches, the base station comprising: a first
transmitter component that is configured to transmit cell-specific
pilot subcarriers, in accordance with a transmission diversity
scheme or a multiple-input multiple-output scheme, wherein some of
the cell-specific pilot subcarriers are not aligned in frequency
subcarrier index with cell-specific pilot subcarriers transmitted
by another base station in the group of cells; a second transmitter
component that is configured to transmit common pilot subcarriers
in accordance with a common pilot transmission scheme, wherein: the
common pilot subcarriers are aligned in frequency subcarrier index
with common pilot subcarriers transmitted by other base stations in
the group of cells; a ratio of an amplitude (a.sub.i,m and
a.sub.n,m) of two common pilot subcarriers having a same time index
(t.sub.k) that are transmitted via a common transmission branch is
a constant for the group of cells; and a difference of a phase
(.phi..sub.i,m and .phi..sub.n,m) of two common pilot subcarriers
having a same time index (t.sub.k) that are transmitted via a
common transmission branch is a constant for the group of cells;
and a controller component that is configured to control
independently a transmission power of the cell-specific pilot
subcarriers and a transmission power of the common pilot
subcarriers.
8. The base station of claim 7, wherein the transmission power of
the cell-specific pilot subcarriers is higher than the transmission
power of the common pilot subcarriers.
9. The base station of claim 7, wherein the transmission power of
the cell-specific pilot subcarriers is lower than the transmission
power of the common pilot subcarriers.
10. The base station of claim 7, wherein the cell-specific pilot
subcarriers enable a mobile station in the cell to determine
channel coefficients for a channel from the base station to the
mobile station.
11. The base station of claim 7, wherein the specific amplitudes,
phases, and frequency subcarrier indices of the cell-specific pilot
subcarriers enable a mobile station in the cell to differentiate
the cell-specific pilot subcarriers from pilot subcarriers
transmitted by base stations in other cells.
12. The base station of claim 7, wherein the common pilot
subcarriers enable a mobile station in the cell to determine
composite channel coefficients for a composite channel, the
composite channel corresponding to an aggregate of different
channels from base stations in the group of cells to the mobile
station.
Description
CROSS-REFERENCE TO RELATED APPLICATION(S)
[0001] This application is a continuation of, and incorporates
herein by reference in its entirety, U.S. patent application Ser.
No. 15/688,441, entitled "METHODS AND APPARATUS USING CELL-SPECIFIC
AND COMMON PILOT SUBCARRIERS IN MULTI-CARRIER, MULTI-CELL WIRELESS
COMMUNICATION NETWORKS," filed Aug. 28, 2017, which is a
continuation of, and incorporates by reference in its entirety,
U.S. patent application Ser. No. 14/746,676, now granted U.S. Pat.
No. 9,749,168 entitled "METHODS AND APPARATUS USING CELL-SPECIFIC
AND COMMON PILOT SUBCARRIERS IN MULTI-CARRIER, MULTI-CELL WIRELESS
COMMUNICATION NETWORKS," filed Jun. 22, 2015, which is a
continuation of, and incorporates 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
COMMUNICATION NETWORKS," filed Jan. 12, 2015, which is a
continuation of, and incorporates by reference in its entirety,
U.S. patent application Ser. No. 13/874,278, now granted U.S. 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 incorporates 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 APPARATUS USING CELL-SPECIFIC
AND COMMON PILOT SUBCARRIERS IN MULTI-CARRIER, MULTI-CELL WIRELESS
COMMUNICATION NETWORKS," filed Aug. 17, 2011, which is a
continuation of, and incorporates 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
COMMUNICATION 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
COMMUNICATION 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
[0002] 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.
[0003] In a wireless network, there are a number of base stations,
each of which provides coverage to designated areas, normally
called a cell. If a cell is divided into sectors, from a system
engineering point of view each sector can be considered 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.
[0004] 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 base stations at different cells interfere with
each other.
[0005] 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 subcarriers
from adjacent cells; however, it has not provided for a careful and
systematic consideration of the unique requirements of the pilot
subcarriers.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] FIG. 1 depicts a basic multi-carrier wireless communication
system consisting of a transmitter and a receiver.
[0007] FIG. 2 shows basic structure of a multi-carrier signal in
the frequency domain, which is made up of subcarriers.
[0008] FIG. 3 shows a radio resource divided into small units in
both the frequency and time domains: subchannels and time
slots.
[0009] FIG. 4 depicts a cellular wireless network comprised of
multiple cells, in each of which coverage is provided by a base
station (BS).
[0010] FIG. 5 shows pilot subcarriers divided into two groups:
cell-specific pilot subcarriers and common pilot subcarriers.
[0011] FIG. 6 is an embodiment of pilot-generation-and-insertion
functional block shown in FIG. 1, which employs a microprocessor to
generate pilot subcarriers and insert them into a frequency
sequence contained in the electronic memory.
[0012] FIG. 7 shows that common pilot subcarriers are generated by
a microprocessor of FIG. 6 to realize phase diversity.
[0013] FIG. 8 is an embodiment of delay diversity, which
effectively creates phase diversity by adding a random delay time
duration, either in baseband or RF, to the time-domain signals.
[0014] FIG. 9 shows two examples for extension to multiple antenna
applications.
[0015] FIG. 10 is an embodiment of synchronization in frequency and
time domains of two collocated base stations sharing a common
frequency oscillator.
[0016] 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.
[0017] 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.
[0018] FIG. 13 shows all base stations within a network transmit,
along with a common pilot subcarrier, a data subcarrier carrying
data information common to all cells in the network.
DETAILED DESCRIPTION
[0019] In the following description the invention is explained with
respect to some of its various embodiments, providing specific
details for a thorough understanding and enablement. 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.
[0020] Unless the context clearly requires otherwise, throughout
the description and the claims, the words "comprise," "comprising,"
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.
[0021] FIG. 1 depicts a basic multi-carrier wireless communication
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 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.
[0022] 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 for its use from those of other
cells. For these pilot subcarriers, counter-interference methods
are necessary.
[0023] 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 subcarriers 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.
[0024] 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 subcarriers and by using
power control.
[0025] 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 reference signal, such as the one
generated from the signals provided by the Global Positioning
System (GPS).
Mufti-Carrier Communication System
[0026] 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/demodulation can be performed using the fast Fourier
transform (FFT).
[0027] 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: [0028] 1. Data subcarriers, which carry
information data; [0029] 2. Pilot subcarriers, whose phases and
amplitudes are predetermined and made known to all receivers and
which are used for assisting system functions such as estimation of
system parameters; and [0030] 3. Silent subcarriers, which have no
energy and are used for guard bands and DC carriers.
[0031] 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 concept is 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. The pilot subcarriers are
also distributed over the entire channel in a particular way.
[0032] 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.
[0033] As depicted in FIG. 1, in a multi-carrier communication
system, a generic transmitter may consist of the following
functional blocks: [0034] 1. Encoding and modulation 108 [0035] 2.
Pilot generation and insertion 106 [0036] 3. Inverse fast Fourier
transform (IFFT) 110 [0037] 4. Transmission 112
[0038] And a generic receiver may consist of the following
functional blocks: [0039] 1. Reception 114 [0040] 2. Frame
synchronization 116 [0041] 3. Frequency and timing compensation 118
[0042] 4. Fast Fourier transform (FFT) 120 [0043] 5. Frequency,
timing, and channel estimation 122 [0044] 6. Channel compensation
124 [0045] 7. Decoding 126
Cellular Wireless Networks
[0046] 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.
[0047] 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.
[0048] 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.sub.k, which can be described as:
s i ( t k ) = a i , p ( t k ) e j .PHI. i , p ( t k ) + m = 1 m
.noteq. p M a i , m ( t k ) e j .PHI. i , m ( t k ) ( 1 )
##EQU00001##
[0049] where .alpha..sub.i,m(t.sub.k) (and .phi..sub.i,m(t.sub.k)
denote the signal amplitude and phase, respectively, associated
with the i.sup.th subcarrier from the base station of the m.sub.th
cell.
Cell-Specific Pilot Subcarriers
[0050] If the ith subcarrier is used as a pilot subcarrier at the
pth cell for the cell-specific purposes, the cell-specific
information carried by .alpha..sub.i,p(t.sub.k) (and
.phi..sub.i,p(t.sub.k) 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 .alpha..sub.i,p(t.sub.k) (and
.phi..sub.i,p(t.sub.k) with desirable accuracy.
[0051] 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 processes,
described below.
Common Pilot Subcarriers
[0052] 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
common pilot subcarriers for different cells may be aligned in
frequency, and possess a set of attributes common 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.
[0053] 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
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:
s i ( t k ) = m = 1 M a i , m ( t k ) e j .PHI. i , m ( t k ) ( 2 )
##EQU00002##
The common pilot subcarriers can be used for a number of
functionalities, such as frequency offset estimation and timing
estimation.
[0054] 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 t.sub.k+1
, with respect to the received signal at time t.sub.k, is given
by
s i ( t k + 1 ) = e j 2 .pi. f i .DELTA. t m = 1 M a i , m ( t k +
1 ) e j .PHI. i , m ( t k + 1 ) ( 3 ) ##EQU00003##
where .DELTA.t=.sub.t-1-t.sub.k. If .DELTA.t is much less than the
coherence period of the channel and
.alpha..sub.i,m(t.sub.k)=c.sub.i.alpha..sub.i,m(t.sub.k+1) (4)
and
.phi..sub.i,m(t.sub.k)=.phi..sub.i,m(t.sub.k+1)+.beta..sub.i
(5)
then the frequency can be determined by
2.pi.f.sub.i.DELTA.t=arg{s.sub.i(k)s.sub.i(k+1)}-.beta..sub.i
(6)
where c.sub.i>0 and -.pi..ltoreq..beta..sub.i.ltoreq..pi. or are
predetermined constants for all values of m. And from all the
frequency estimates {f.sub.i}, a frequency offset can be derived
based on a certain criterion.
[0055] For timing estimation, normally multiple common pilot
carriers are required. In an example of two common pilot
subcarriers, the received signal at f.sub.n, is given by
s n ( t k ) = e j 2 .pi. f T s ( t k ) m = 1 M a n , m ( t k ) e j
.PHI. n , m ( t k ) ( 7 ) ##EQU00004##
where .DELTA.f=f.sub.n-f.sub.i and T.sub.s denotes the sampling
period. If .DELTA.f is much less than the coherence bandwidth of
the channel and
.alpha..sub.i,m(t.sub.k)=c(t.sub.k).alpha..sub.n,m(t.sub.k) (8)
and
.phi..sub.i,m(t.sub.k)=.phi..sub.n,m(t.sub.k)+.gamma.(t.sub.k)
(9)
then T.sub.s can be determined by
2.pi..DELTA.fT.sub.s(t.sub.k)=arg
{s.sub.i*(t.sub.k)s.sub.n(t.sub.k)}-.gamma.(t.sub.k) (10)
where c(t.sub.k)>0 and .pi..ltoreq..gamma.(t.sub.k).ltoreq..pi.
are predetermined constants for all values of m.
[0056] 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-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
[0057] 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 .differential..sub.l,m can be added to another pilot
subcarrier, say the Ith subcarrier, which results in
.phi..sub.l,m(t.sub.k)=.phi..sub.i,m(t.sub.k)+.differential..sub.l,m
(11)
and
.phi..sub.l,m(t.sub.k+1)=.phi..sub.i,m(t.sub.k+1)+.differential..sub.l,m
(12)
where .differential..sub.l,m should be set differently for each
cell, and provided that the following condition is met,
.phi..sub.l,m(t.sub.k)=.phi..sub.l,m(t.sub.k+1)+.beta..sub.l, for
all values of m (13)
[0058] With the phase diversity, it is expected that the
probability of both |s.sub.i(t.sub.k)| and |s.sub.i(t.sub.k)|
diminishing at the same time is relatively small. The embodiment of
phase diversity is depicted in FIG. 7, which shows common pilot
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.
[0059] Another embodiment is illustrated in FIG. 8, which
effectively 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
[0060] In one embodiment of 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 [0061]
1. meet the needs of their functionalities; [0062] 2. adapt to the
operation environments (e.g., propagation channels); and [0063] 3.
reduce interference between cells or groups of cells. In another
embodiment power control is implemented differently for
cell-specific pilot subcarriers and common pilot subcarriers. For
example, stronger power is applied to common pilot subcarriers than
to the cell-specific subcarriers.
Application to Multiple Antennas
[0064] 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 estimation
or by equations (8) and (9) for timing estimation are
satisfied.
[0065] 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 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
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
frequency estimation or (8) and (9) for timing estimation.
Joint-Use of Cell-Specific and Common Pilot Subcarriers
[0066] 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 optimization of
the signal-to-noise ratio. For example, in the estimation 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
[0067] 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.
[0068] 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 interference 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 synchronization 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.
[0069] In some applications, the entire wireless network may
consist 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-interference 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.
[0070] 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 subcarriers. 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 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 A1, A2, and A3 in Group A synchronize to their
own common reference source and transmit the same set of common
pilot subcarriers; and the base stations at Cells B1, B2, and B3 in
Group B synchronize to their own reference source and transmit
another set of common pilot subcarriers that are located at
different places in the frequency domain.
Extension to Transmission of Data Information
[0071] All design processes, criteria, and methods described in the
embodiments of this invention can be extended to applications where
common network information is required to be distributed to all
receivers within the network. In one example, all the base stations
within the network transmit, along with some common pilot
subcarriers, an identical set of data subcarriers in which the data
information common to all the cells in the network is imbedded.
[0072] FIG. 13 shows all base stations within a network transmit,
along with a common pilot subcarrier, a data subcarrier carrying
data information common to all cells in the network. A receiver
within the network can determine the composite channel coefficient
based on the common pilot subcarrier and apply it to the data
subcarrier to compensate for the channel effect, thereby recovering
the data information.
[0073] The above detailed descriptions of embodiments of the
invention are not intended to be exhaustive or to limit the
invention to the precise form disclosed above. While specific
embodiments of, and examples for, the invention are described above
for illustrative purposes, various equivalent modifications are
possible within the scope of the invention, as those skilled in the
relevant art will recognize. For example, while steps are presented
in a given order, alternative embodiments may perform routines
having steps in a different order. The teachings of the invention
provided herein can be applied to other systems, not necessarily
the system described herein. These and other changes can be made to
the invention in light of the detailed description.
[0074] The elements and acts of the various embodiments described
above can be combined to provide further embodiments.
[0075] These and other All of the above U.S. patents and
applications and other references are incorporated herein by
reference. Aspects of the invention can be modified, if necessary,
to employ the systems, functions and concepts of the various
references described above to provide yet further embodiments of
the invention.
[0076] Changes can be made to the invention in light of the above
detailed description, In general, the terms used in the following
claims should not be construed to limit the invention to the
specific embodiments disclosed in the specification, unless the
above detailed description explicitly defines such terms.
Accordingly, the actual scope of the invention encompasses the
disclosed embodiments and all equivalent ways of practicing or
implementing the invention under the claims.
[0077] While certain aspects of the invention are presented below
in certain claim forms, the inventors contemplate the various
aspects of the invention in any number of claim forms. For example,
while only one aspect of the invention is recited as embodied in a
computer-readable medium, other aspects may likewise be embodied in
a computer-readable medium. Accordingly, the inventors reserve the
right to add additional claims after filing the application to
pursue such additional claim forms for other aspects of the
invention.
* * * * *