U.S. patent application number 13/595815 was filed with the patent office on 2013-02-28 for femtocell channel assignment and power control for improved femtocell coverage and efficient cell search.
This patent application is currently assigned to NTT DOCOMO Inc.. The applicant listed for this patent is Ismail Guvenc, Hiroshi Inamura, Moo Ryong Jeong, Fujio Watanabe. Invention is credited to Ismail Guvenc, Hiroshi Inamura, Moo Ryong Jeong, Fujio Watanabe.
Application Number | 20130051365 13/595815 |
Document ID | / |
Family ID | 41340477 |
Filed Date | 2013-02-28 |
United States Patent
Application |
20130051365 |
Kind Code |
A1 |
Guvenc; Ismail ; et
al. |
February 28, 2013 |
FEMTOCELL CHANNEL ASSIGNMENT AND POWER CONTROL FOR IMPROVED
FEMTOCELL COVERAGE AND EFFICIENT CELL SEARCH
Abstract
A method and a communication system including femtocells within
a macrocell efficiently manage interference between the different
femtocells, and between each femtocell and a macrocell. An
efficient frequency assignment scheme for the femtocells minimizes
interference between a femtocell and a macrocell and among
different femtocells using a spectrum-sensing technique carried out
by the femtocells. The frequency assignment scheme selects a
suitable channel from a set of candidate channels and ensures that
the femtocell has an acceptable coverage area even when it is close
to the macrocell base station (BS). The frequency assignment scheme
favors a co-channel implementation to take advantage of the
hand-off and cell search characteristics of the co-channel
implementation. In one embodiment, a joint power control and
frequency band assignment technique is used, which partitions the
coverage area of the macrocell into an inner region, a power
control region, and an outer region.
Inventors: |
Guvenc; Ismail; (Santa
Clara, CA) ; Jeong; Moo Ryong; (Saratoga, CA)
; Watanabe; Fujio; (Union City, CA) ; Inamura;
Hiroshi; (Cupertino, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Guvenc; Ismail
Jeong; Moo Ryong
Watanabe; Fujio
Inamura; Hiroshi |
Santa Clara
Saratoga
Union City
Cupertino |
CA
CA
CA
CA |
US
US
US
US |
|
|
Assignee: |
NTT DOCOMO Inc.
Tokyo
JP
|
Family ID: |
41340477 |
Appl. No.: |
13/595815 |
Filed: |
August 27, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12463307 |
May 8, 2009 |
8280387 |
|
|
13595815 |
|
|
|
|
61055345 |
May 22, 2008 |
|
|
|
61073276 |
Jun 17, 2008 |
|
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Current U.S.
Class: |
370/331 |
Current CPC
Class: |
H04W 16/30 20130101;
H04W 72/02 20130101; H04W 52/343 20130101; H04W 52/244 20130101;
H04W 52/243 20130101; H04W 72/082 20130101; H04W 16/10 20130101;
H04W 84/045 20130101; H04W 52/40 20130101; H04W 72/0453
20130101 |
Class at
Publication: |
370/331 |
International
Class: |
H04W 36/00 20090101
H04W036/00 |
Claims
1. A method for selecting a frequency band for use in a femtocell
located within a coverage area of a macrocell, comprising:
partitioning the coverage area of the macrocell into an inner
region and an outer region; selecting for the femtocell a frequency
band used in the macrocell, when the femtocell is located within
the outer region, and selecting for the femtocell a different
frequency band than a frequency band used in the macrocell, when
the femtocell is located within the inner region, wherein a mobile
station switches between the macrocell and the femtocell after a
cell-search step.
2. A method as in claim 1, wherein the macrocell neighbors a
plurality of other macrocells, and wherein the different frequency
band selected for the femtocell is also different from any
frequency band used in such other macrocells.
3. A method as in claim 1, wherein the different frequency band is
selected using a spectrum-sensing technique.
4. A method as in claim 3, wherein the different frequency band is
selected in a manner that reduces inter-femtocell interference.
5. A method as in claim 1, wherein the different frequency band is
selected so as to maintain a femtocell coverage area greater than a
threshold value.
6. A method as in claim 1, further comprising determining both a
coverage area and a coverage region for the femtocell, and based on
the determined coverage area and the determined coverage region,
positioning a base station of the femtocell within the coverage
region.
7. A method as in claim 6, wherein the coverage region covers a
predetermined premise.
8. A method as in claim 1, wherein the selected frequency band is
selected for a downlink, and wherein a frequency assignment for an
uplink is based on interference from one or more mobile stations in
the macrocell.
9. A method as in claim 1, wherein a base station in the femtocell
measures a signal quality in a received signal and reports the
signal quality to a base station in the macrocell, and wherein the
base station of the macrocell determines if the femtocell is within
the inner region or the outer region.
10. A method as in claim 1, wherein the partitioning further
partitions the coverage area of the macrocell into a power control
region.
11. A communication system, comprising: a macrocell having a
coverage area partitioned into an inner region and an outer region;
a first femtocell within the outer region, the first femtocell
communicating using a frequency band used in the macrocell; and a
second femtocell within the inner region, the second femtocell
communicating using a different frequency band than a frequency
band used in the macrocell.
12. A communication system as in claim 11, wherein the coverage
area is partitioned using a distance from a base station of the
macrocell.
13. A communication system as in claim 11, further comprising a
mobile station that switches between the macrocell and one of the
femtocells after a cell-search step.
14. A communication system as in claim 11, further comprising a
mobile station that switches between the macrocell and one of the
femtocells after a hand-off step.
15. A communication system as in claim 11, further comprising a
plurality of other macrocells neighboring the macrocell, and
wherein the different frequency band selected for the femtocell is
also different from any frequency band used in such other
macrocells.
16. A communication system as in claim 11, wherein the different
frequency band is selected using a spectrum-sensing technique.
17. A communication system as in claim 16, wherein the different
frequency band is selected in a manner that reduces inter-femtocell
interference.
18. A communication system as in claim 16, wherein the different
frequency band is selected so as to maintain a femtocell coverage
area greater than a threshold value.
19. A communication system as in claim 16, further comprising a
base station of one of the femtocells that is placed at a position
determined based on a coverage area and a coverage region
determined for the femtocell of the base station.
20. A communication system as in claim 19, wherein the coverage
region covers a predetermined premise.
21. A communication system as in claim 11, wherein the selected
frequency band is selected for a downlink, and wherein a frequency
assignment for an uplink is based on interference from one or more
mobile stations in the macrocell.
22. A communication system as in claim 11, wherein a base station
in one of the femtocells measures a signal quality in a received
signal and reports the signal quality to a base station in the
macrocell, and wherein the base station of the macrocell determines
if the femtocell is within the inner region or the outer region.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional application of U.S. patent
application Ser. No. 12/463,307, filed on May 8, 2009, which in
turn claims priority to U.S. Provisional Patent Application No.
61/055,345, filed on May 22, 2008 and to U.S. Provisional Patent
Application No. 61/073,276, filed on Jun. 17, 2008.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to wireless communication.
More specifically, the present invention relates to methods for
efficiently assigning channels to femtocells, taking into account
hand-off, interference, coverage area, and power control
considerations.
[0004] 2. Discussion of the Related Art
[0005] Because mobile telephones may be used practically
everywhere, they are replacing fixed wired telephones. The article,
"UMA and Femtocells: Making FMC Happen" ("Choudhury"), by Partho
Choudhury and Deepak Dahuja, "White Paper, December 2007.
(available: at http://www.parthochoudhurv.com/UMAFemto.doc),
discloses (a) that approximately 30-35% of all voice calls made
over a mobile network are made by mobile subscribers at their
homes, and (b) about 35% of video streaming and broadcasting
service uses over cellular wireless networks in 2006 took place
while the mobile subscribers are at their homes.
[0006] The trend, therefore, is for the mobile telephone to become
the primary or only telephone for an individual subscriber.
Furthermore, the article, "Femto Cells: Personal Base StationA"
("Airvand"), published by Airvana Inc., White Paper, 2007
(available
http://www.airvana.com/files/Femto_Overview_Whitepaper_FINAL.sub.--12-Jul-
y-07.pdf), reveals that those 24 years of age or younger make up to
80% of their long distance calls on wireless networks rather than
over wired networks. However, there is still much to be improved in
reliability, voice quality, and cost of today's mobile telephone
networks in indoor environments. Typically, the mobile telephone
service is more costly than a wired telephone service, and there
are dead spots and poor coverage. These deficiencies result in poor
customer experience, thus preventing the mobile telephone to
successfully replace the wired telephone as the primary or only
telephone for most subscribers.
[0007] Choudhury, Airvana, and the article "The Case for Home Base
Stations" ("PicoChip"), published by PicoChip Designs Ltd., White
Paper, April 2007 (available
http://www.picochip.com/downloads/27c85c984cd0d348edcffe7413f6ff79/femtoc-
ell_wp.pdf) all disclose a new class of base stations (BSs)
designed for indoor and personal uses, The cells served by these
personal BSs have come to be known as "femtocells." A femtocell
(e.g., the home e-node B (HeNB) defined in the 3GPP standard)
enables indoor wireless connectivity through existing broadband
Internet connections. As described in Choudhury, femtocells are
also featured in fixed-mobile convergence (FMC), where the
subscribers are provided the ability to switch an active data/voice
call session between home wireless network (e.g., femtocell) and a
mobile network (e.g., a cellular network). As reported by
Choudhury, Airvana and PicoChip, the benefits of femtocells include
improved indoor coverage, reduced capital and operational
expenditure, reduced bandwidth load, reduced power requirements,
additional high-end revenue streams, improved customer royalty,
increased average revenue per user, compatibility with existing
handsets (without requiring dual-mode terminals), deployment in an
operator-owned spectrum, and enhanced emergency services (since the
femtocells are location-aware).
[0008] Despite these benefits, femtocell technology is still at its
infancy. As identified in Airvana, the technical issues to be
solved include those related to interference management (both
between different femtocells and between the femtocell and the
macrocell), efficient hand-off mechanisms, security, scalability,
and access control. For example, co-channel implementations of
femtocells--where the macrocell network and the femtocell network
share the same frequency band--introduce serious challenges.
Co-channel deployment of femtocells has desirable hand-off
characteristics, as a mobile station (MS) may more efficiently scan
the cells using the same frequency band compared to identifying the
cells using other frequency bands, which require band-switching to
accomplish the scanning. However, for distances that are close to
the macrocell base station (mBS), severe interference from the mBS
may prevent co-channel deployment.
[0009] The article, "Effects of User-Deployed, Co-Channel
Femtocells on the Call Drop Probability in a Residential Scenario"
("Lester"), by Lester T. W. Ho and Holger Claussen, published in
Proc. of IEEE Int. Symp. on Personal, Indoor and Mobile Radio
Communications (PIMRC), pp. 1-5, September 2007, shows that the
received signals from the femtocell and the macrocell in such an
implementation have identical power levels at the border of the
macrocell. Thus, without adequate power control, the femtocell
coverage area decreases for those femtocells that are closer to the
macrocell BS (mBS). However, when the femtocell coverage area falls
below a certain size, the femtocell does not completely cover a
user's premise, which is the preferred coverage area. A different
solution is desired, under such circumstances.
[0010] The article, "Uplink Capacity and Interference Avoidance for
Two-Tier Cellular Network" ("Chandrasekhar"), by Vikram
Chandrasekhar and Jeffrey G. Andrews, published in Proc. IEEE
Global Telecommunications Conference (GLOBECOM), pp. 3322-3326,
November 2007, derived and analyzed the uplink (UL) capacity of a
co-channel femtocell network coexisting with a macrocell network
(i.e., a shared-spectrum network). In a split spectrum network, the
femtocell users and the macrocell users use orthogonal
sub-channels. While the split spectrum network avoids interference
between the macrocell and the different femtocells, the total
number of users that can be supported is less than a shared
spectrum network. In a shared spectrum network, a femtocell may use
a sub-channel that is already used in the macrocell, so long as
there is little interference between the femtocell and the portion
of the macrocell network where the common sub-channel is used. In a
co-channel femtocell deployment, an MS need not scan through
multiple frequency bands to search for the cell.
[0011] Chadrasekhar suggests using interference avoidance methods
to reduce the outage probability. For example, each macrocell user
and each femtocell may employ time-hopping in order to decrease
interference. Further, the macrocell and femtocell may both use a
sectored antenna reception for improving the capacity.
Chandrasekhafs analytical/simulation results show that, by using
interference avoidance (specifically, time-hopped code-division
multiple access (TH-CDMA) and sectorized antennas), up to seven
times higher femtocell BS (fBS) density can be supported in a
shared spectrum network, relative to to a split spectrum network
with omnidirectional femtocell antennas. However, sectored antennas
may be difficult to implement at the femtocells (which are
necessarily, for practical considerations, simpler devices than
regular BSs). Further, a time-hoping approach increases symbol
duration (and hence, decreases data rate).
[0012] Lester, discussed above, analyzed hand-off probabilities for
different power configurations at a femtocell. Since the manual
cell-planning used in macrocell networks is not economically
practical for femtocells, femtocells typically require
auto-configuration capabilities (e.g., automatic power and cell
size configuration). Lester's simulations show that call drop
probabilities can be significantly decreased in a residential
co-channel femtocell deployment through simple pilot power
adaptation mechanisms.
[0013] The article, "Performance of Macro- and Co-Channel
Femtocells in a Hierarchical Cell Cell Structure" ("Clausseri"), by
Holger Claussen, published in Proc. of IEEE Int. Symp. on Personal,
Indoor and Mobile Radio Communications (PIMRC), pp. 1-5, September
2007, discloses a simple power control algorithm for pilots and
data signals in femtocells. Simulation results show that the
interference to the macrocell network can be minimized through
intelligent power control techniques.
[0014] In Lester, Chandrasekhar and Clausen, relatively simple
power control mechanisms are proposed for femtocells, so that the
signal-to-interference ratio (SINR) is equal to 0 dB at the cell
edge. However, depending on the distance between the mBS and the
fBS, such power control strategies may not be effective. For
example, as mentioned above, the maximum transmission power of a
co-channel femtocell may not be sufficient to provide satsifactory
coverage when the fBS is close to the mBS.
[0015] The article, "Home NodeB Output Power," published by
Ericsson, 3GPP TSG Working Working Group 4 meeting (available at
http://www.3gpp.org/ftp/tsg_ran/WG4_Radio/TSGR4.sub.--43bis/Docs/),
provides a power control scheme which reduces the femtocell
transmit power as the distance between the macrocell BS and the
femtocell BS increases. Under such an arrangement, the macrocell
MSs experience better coverage as a result of reduced interference
from the femtocell. However, this approach is questionable when a
femtocell is either very close to or very far away from the
macrocell BS.
[0016] The article, "Uplink User Capacity in a Multicell CDMA
System with Hotspot Microcells," by S. Kishore, L. J. Greenstein,
H. V. Poor, and S. C. Schwartz, published in IEEE Trans. On
Wireless Communications, vol. 5, no. 6, pp. 1333-1342, June 2006,
overcomes the near-far effect by increasing femtocell coverage.
Increased femtocell coverage is achieved by allowing an MS close to
a femtocell to communicate with the macrocell BS only when the
signal quality from the macrocell BS is significantly better. This
approach increases interference at neighboring femtocells.
[0017] The following patent application publications disclose
femtocell implementations: (a) U.S. Patent Application Publication
2007/0183427, "Access Control in Radio Access Network Having Pico
Base Stations," by T. Nylander et al., filed Oct. 3, 2006; (b) U.S.
Patent Application Publication 2007/0254620, entitled "Dynamic
Building of Monitored Set", by T. L. E. Lindqvist et al., filed
Apr. 28, 2006; and (c) Internation Patent Application Publication
WO2006/0139460, entitled "Method and Apparatus for Remote
Monitoring of Femto Radio Base Stationg", by J. Vikeberg et al.,
May 30, 2006. However, none of these patent applications offers an
efficient frequency assignment scheme for a femtocell
deployment.
SUMMARY OF THE INVENTION
[0018] Femtocells may increase the efficiency and coverage of macro
cellular networks. Successful femtocell deployment depends on
efficiently managing both interference among different femtocells
and interference between a femtocell and a macrocell.
[0019] According to one embodiment of the present invention, an
efficient frequency assignment scheme for femtocells is provided.
The frequency assignment scheme reduces the interference between a
femtocell and a macrocell and among different femtocells. Under
this method, based on its location and where required, the
femtocells perform spectrum-sensing and select suitable channels
from a set of candidate channels. The method also provides an
acceptable femtocell coverage area, even when the fBS is close to
the mBS (i.e., as determined using a threshold). The frequency
assignment scheme prefers sharing the frequency band of the
macrocell network with the femtocells to take advantage of the
desirable hand-off characteristics (i.e., the MS need not scan
different frequency bands to search for cells). However, when the
interference from the mBS exceeds a threshold, the assignment
scheme requires the femtocell to use a different frequency band
selected from a set of appropriate frequencies to ensure an
acceptable femtocell coverage area. The frequency assignment scheme
is applicable to various types of femtocells, such as the Home
eNodeB and other personal BSs.
[0020] According to another embodiment of the present invention,
the frequency assignment scheme may also provide joint power
control and frequency allocation. Under this embodiment, the
frequency assignment scheme is priority-based, such that the
candidate frequency bands are selected depending on various
parameters, such as the relative locations of the fBS and the mBS,
path loss exponents and a frequency reuse factor (N). When the fBS
is far away from the mBS, the fBS selects a frequency band from
among those used by the mBS, and when the fBS is close to the mBS,
a different frequency band is assigned to the fBS to achieve an
acceptable coverage area. When the femtocell is located within a
power control region, the transmission power of the femtocell is
controlled to maintain a fixed coverage area for the femtocell.
Among the femtocells, spectrum-sensing is used to select candidate
frequency bands for use, in order to reduce interference among
different femtocells.
[0021] The present invention is better understood upon
consideration of the detailed description below in conjunction with
the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 illustrates a macrocell network that has a frequency
reuse factor N>1.
[0023] FIG. 2 illustrates a frequency band assignment for a network
having a reuse factor N=3, where no neighboring macrocells use the
same frequency band.
[0024] FIG. 3 shows a partition of the macrocell into two regions
(i.e., an inner region and an outer region), in accordance with one
embodiment of the present invention.
[0025] FIG. 4 is a flow chart illustrating in greater detail a
frequency assignment framework according to one embodiment of the
present invention.
[0026] FIG. 5 is a flow chart illustrating another femtocell
frequency assignment framework, according to a second embodiment of
the present invention.
[0027] FIG. 6 illustrates a spectrum scanning technique, according
to one embodiment of the present invention.
[0028] FIG. 7 shows yet another frequency band assignment
framework, in accordance with one embodiment of the present
invention.
[0029] FIG. 8 shows, for N=1, another frequency assignment scheme
which assigns a separate frequency band to each femtocell,
according to one embodiment of the present invention.
[0030] FIG. 9 shows a frequency division duplex (FDD) frequency
assignment scheme for a macrocell network, in accordance with one
embodiment of the present invention.
[0031] FIG. 10 shows an example of an actual FDD frequency
assignment in a network of the type discussed with respect to FIG.
9, according to one embodiment of the present invention.
[0032] FIG. 11 illustrates a network implementing joint power
control and frequency assignment, in accordance with one embodiment
of the present invention.
[0033] FIG. 12 provides an example of frequency band assignments in
the regions of FIG. 11, for both downlink and uplink frequency
assignments, in accordance with one embodiment of the present
invention.
[0034] FIG. 13 is a block diagram illustrating joint power control
and frequency band assignment in the regions of FIG. 11, in
accordance with one embodiment of the present invention.
[0035] FIG. 14 is a block diagram illustrating a first approach the
fBS may use to determine the region of its location, in accordance
with one embodiment of the present invention.
[0036] FIG. 15 is a block diagram illustrating a second approach
the fBS may use to determine the region of its location, in
accordance with one embodiment of the present invention.
[0037] FIG. 16 shows one implementation of a method for refining
the calculation of transmit power, in accordance with one
embodiment of the present invention.
[0038] FIG. 17 shows an examplary message exchange between an fBS
and an MS, in accordance with one embodiment of the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0039] FIG. 1 illustrates a macrocell network 0 that has a
frequency reuse factor N>1. In such a network, N frequency bands
are assigned to the macrocells of the network, such that each
macrocell is assigned a different frequency band than any of its
neighboring macrocells. In the example of FIG. 1, without loss of
generality, frequency band F.sub.i is assigned to macrocell 0. FIG.
2 illustrates a frequency band assignment for a network having
reuse factor N=3, where no neighboring macrocells use the same
frequency band. As shown in FIG. 2, macrocell 4, for example, is
assigned frequency band F.sub.1. Within each frequency band, the
users may be further separated in time, frequency, or code domains.
For example, in a wideband code division multiple access (WCDMA)
network, each user in a macrocell is assigned a different CDMA
code, selected from a set of CDMA codes, to minimize interference
among users in the macrocell cell. There may be tens or even more
femtocells within a macrocell.
[0040] As discussed above, a co-channel implementation is
preferable for a femtocell, from hand-off and cell search points of
view. However, co-channel operation may not always be possible, due
to interference from the mBS. FIG. 3 shows a partition of the
macrocell into two regions (i.e., inner region 6 and outer region
7), in accordance with one embodiment of the present invention.
Femtocells that are located in outer region 7 may use the same
frequency band as one that is used in the macrocell network. For
example, fBSs in outer region 7 and the mBS may both use frequency
band F.sub.1. Under such an arrangement, an MS can experience
easier hand-off and cell search, when switching between the
femtocell and the macrocell, as both cells use the same frequency
band. Also, since the fBS is far away from the mBS, there is little
interference from the mBS. Consequently, the IBS has a sufficiently
large.sup.1 coverage area. .sup.1 The coverage area is particularly
sensitive to an interference-limited environment. If the
interference is weak, i.e., a noise-limited environment, the
coverage area would depend on the noise-floor. Interference is weak
when the fBS is far away from the mBS. Where interference is
strong, it is best to use a frequency band that is different from
that of the macrocell.
[0041] For this detailed description, the coverage area of a fBS is
defined for convenience sake by a contour along which the levels of
the received power from the fBS and from the mBS are the same. The
present invention is, however, is not limited by this convention.
The present invention is applicable to situations where the
coverage area may be defined in other ways. For example, the
coverage area may be defined by a hand-off parameter, such as the
cell search initiation threshold or the hand-off execution
threshold. In such a case, the received power levels from the fBS
and from the mBS may not be the same. Also, in such other cases,
there may be more than one contour that defines the coverage area
(e.g., a contour for an incoming user to the fBS, and another
contour for an outgoing user from the fBS).
[0042] Referring to FIG. 3, for fBSs that are located within inner
region 6, the power level received from the mBS is high. Therefore,
if these fBSs use the same frequency band as the mBS, their
respective coverage areas are small, such as illustrated by
fBS.sub.2 in femtocell coverage area 8. For a reuse factor N>1,
the femtocell may use one of the other N-1 bands, that are used by
other macrocells and not the current macrocell. For example,
fBS.sub.3 uses frequency band F.sub.2, which is an orthogonal
channel to frequency band F.sub.1. As interference from other
macrocells is expected to be minimal, the coverage area for the
femtocell fBS.sub.3 may be kept sufficiently large.
[0043] FIG. 4 is a flow chart that illustrates in greater detail a
frequency assignment framework according to one embodiment of the
present invention. In FIG. 4, n.sub.1 denotes the path loss
exponent (PLE) between an MS and the mBS, n.sub.2 denote the PLE
between an MS and the fBS, P.sub.1 denote the received signal power
from the mBS at an MS, and P.sub.2 denote the received signal power
from the fBS at an MS. Also, (x.sub.mBS, y.sub.mBS) and (x.sub.fBS,
y.sub.fBS) denote the geographical locations of the mBS and the
fBS, respectively.sup.2. The values of these parameters are
measured at step 15. At step 20, as the SINR is 0 dB at the border
of a femtocell coverage area (under co-channel implementation), the
femtocell coverage area may be calculated algebraically. One method
for calculating femtocell coverage area is discussed in further
detail below.sup.3,4. Coverage area A.sub.fBS of a co-channel
femtocell preferably covers completely a user's premises. At step
30, coverage area A.sub.fBS is compared to a threshold value. If
the coverage area is greater than the threshold value, the
femtocell can operate in a co-channel manner with the macrocell.
Otherwise, i.e., if the coverage area A.sub.fBS is less than the
threshold value, severe interference is experienced, and the
femtocell should select a different frequency band. While the
threshold value may be based only on coverage area considerations,
it can also take into account other parameter values, such as the
monthly subscription fee charged to the user pays, or a QoS
requirement, and other interference considerations. .sup.2 The
relative locations of the two BSs with respect to each other
provide the distance between them..sup.3 The path loss exponents
may be obtained from prior measurements performed in the same
environment. Alternatively, it may be possible to obtain them on
the fly. For example, the mBS may inform the fBS about its
transmission power during femtocell initialization. The fBS may
then estimate the path loss exponent from the mBS using the known
distance between itself and the mBS. A position estimate of the
location of the fBS may be obtained using any of several ways, such
as GPS, triangulation, utilization of TV signals for position
finding, or the a-priori information regarding the home's
geographic location..sup.4 See, also, the article "Co-Channel
Femtocell Coverage Area Analysis", by Ismail Guvenc, Moo-Ryong
Jeong, and Fujio Watanabe, IEEE Commun Lett., Volume 12, Issue 12,
December 2008, Page(s): 880-882.
[0044] At step 40, when coverage area A.sub.fBS is less than the
threshold value, the fBS should choose one of the N-1 frequency
bands that is different than F.sub.i. To minimize interference with
other femtocells, the femtocell preferably first scans the N-1
frequency bands. FIG. 6 illustrates a spectrum scanning technique
that may be used to implement step 40, according to one embodiment
of the present invention. As shown in FIG. 6, at step 42, the power
levels in the N-1 frequency bands are determined using, for
example, an energy detection technique. Then, at step 44, the
frequency band that has the least noise level is chosen. The chosen
frequency band is then used for communication in the femtocell
(step 46). A large N provides a lesser inter-femtocell
interference, as there are more candidate frequency bands to choose
from.
[0045] In addition to the coverage area A.sub.fBS of a fBS, an
approximate region of coverage R.sub.fBS may also be determined
algebraically, as described in further detail below. Frequently,
region R.sub.fBS may be approximated by a circle, with the center
of this circle being collinear with the locations of the fBS and
the mBS, and further away from the mBS relative to the fBS. The
distance between the center of the circle of region R.sub.fBS and
the fBS location depends on such parameters as the path loss
exponent and the transmission powers of the fBS and the mBS.
Referring back to FIG. 4, the coverage region R.sub.fBS for a given
placement of an fBS provides a rough yardstick for determining if
coverage area A.sub.fBS completely covers the user's premises (step
60). If not, i.e., coverage area A.sub.fBS does not completely
covers the user's premises, the placement of the fBS may be
adjusted to improve coverage (step 50). Steps 50 and 60 may be
iterated to achieve an optimal placement of the fBS.
[0046] FIG. 5 is a flow chart illustrating another femtocell
frequency assignment method, according to a second embodiment of
the present invention. At step 16, the respective geographical
locations (x.sub.mBS, y.sub.mBS) and (x.sub.fBS, y.sub.fBS) of the
mBS and the fBS are determined. At step 25, the distance d.sub.fBS
between the fBS and the mBS is determined from these geographical
locations. Under this framework, knowledge of the path-loss
exponents and the transmission powers of the mBS and fBS are not
required. Instead, the distance d.sub.fBS is used to determine if
the fBS lies within or outside of the border (a threshold value)
between inner region 6 and outer region 7 of the macrocell (step
35). If distance d.sub.fBS is greater than the threshold value, the
femtocell uses frequency band F.sub.i (step 70). Otherwise, at step
40 (see details in FIG. 6), the femtocell selects the one of the
other N-1 frequency bands having the minimum noise level.
[0047] FIG. 7 shows yet another frequency band assignment
framework, in accordance with one embodiment of the present
invention. As shown in FIG. 7, in a network where the macrocells
have a reuse factor of N, a separate frequency band F.sub.N+1 is
assigned for use by all of the fBSs within a macrocell. In this
way, interference from the current mBS and any of the surrounding
mBSs are avoided. However, because the fBSs uses a different
frequency band from the serving mBS, hand-off process is less
efficient.
[0048] According to yet another frequency band assignment
framework, during an initialization step of the femtocell, an MS or
the femtocell fBS may measure the received signal strength and
report the received signal strength to the mBS. The mBS may then
determine if the fBS belongs to inner region 6 or outer region 7,
and report the determination to the fBS. Frequency band assignment
may then proceed along any of the schemes discussed above based on
the geographical region determination.
[0049] For an mBS located at x.sub.1=(x.sub.1; y.sub.1), with
transmit power P.sub.1, and an fBS located at x.sub.2=(x.sub.2;
y.sub.2), with transmit power P.sub.2, under an empirical path loss
model, the power of a signal transmitted from an mBS and received
at an MS is given by
P.sub.r,1=P.sub.1P.sub.o(d.sub.o/d).sup.n, (1)
where P.sub.o is the measured path loss at a reference distance
d.sub.o (typically, P.sub.o may be approximated by
(4.pi./.lamda.).sup.2 for d.sub.o=1, with .lamda. denoting the
wavelength of the signal) and n is the path loss exponent. Because
the signal-to-interference ratio (SIR) is assumed to be 0 dB at the
border of a co-channel femtocell, without loss of generality, Po is
the same for both the fBS and the mBS at a distance d.sub.o=1, so
that:
P 1 d 1 n = P 2 d 2 n . ( 2 ) ##EQU00001##
Thus, in a system in which n=2, the coordinates x=(x; y) of the
points on the border of the femtocell satisfy:
d.sub.j.sup.2=(x-x.sub.j).sup.2+(y-y.sub.j).sup.2, j.epsilon.{1,
2}. (3)
Combining equations (3) and (2), one obtains:
x 2 + y 2 + B 1 A x + B 2 A y + C A = 0 , ( 4 ) ##EQU00002##
where
A=P.sub.1-P.sub.2, (5)
B.sub.1=2P.sub.2x.sub.1-2P.sub.1x.sub.2, (6)
B.sub.2=2P.sub.2y.sub.1-2P.sub.1y.sub.2, (7)
C=+P.sub.1x.sub.2.sup.2+P.sub.1y.sub.2.sup.2-P.sub.2x.sub.1.sup.2-P.sub.-
2y.sub.1.sup.2. (8)
Equation (4) is in the form of a circle of radius r which is
centered at k=(k.sub.1; k.sub.2), i.e.,
x.sup.2+y.sup.2-2k.sub.1x-2k.sub.2y+k.sub.1.sup.2+k.sub.2.sup.2-r.sup.2=-
0. (9)
Therefore, comparing equations (9) and (4), the center point
k=(k.sub.1; k.sub.2), representing the center of femtocell coverage
area, is
k 1 = - B 1 2 A , k 2 = - B 2 2 A , ##EQU00003##
which may also be expressed in vector form as:
k = P 2 P 1 - P 2 x 1 - P 1 P 1 - P 2 x 2 . ( 10 ) ##EQU00004##
The radius r is given by:
r = k 1 2 + k 2 2 - C A = B 1 2 4 A 2 + B 2 2 4 A 2 - C A . ( 11 )
##EQU00005##
Then, the femtocell coverage area is:
A fem = .pi. r 2 = .pi. ( B 1 2 4 A 2 + B 2 2 4 A 2 - C A ) . ( 12
) ##EQU00006##
Equation (12) shows that the femtocell coverage area depends on
both the locations of the mBS and the fBS, and their respective
transmit powers.
[0050] For n.noteq.2 (i.e., for any arbitrary path loss exponent),
taking the (2/n)-th root of both sides of equation (2)
provides:
P 1 2 n d 1 2 = P 2 2 n d 2 2 , ( 13 ) ##EQU00007##
which is equivalent to the scenario with n=2 above, but
characterized by a different set of transmit powers. The center of
coverage and the area of coverage may be derived using the same
procedure illustrated above with respect to equations (10) and
(12), substituting (P.sub.1).sup.2/n for P.sub.1 and
(P.sub.2).sup.2/n for P.sub.2.
[0051] In general, the path loss exponents at the fBS and mBS are
different (i.e., Po is not the same for both the fBS and the mBS at
a distance d.sub.o=1). In that case, rather than equation (2):
P 1 d 1 n 1 = P 2 d 2 n 2 . ( 14 ) ##EQU00008##
Because the mBS is typically located at a higher altitude (e.g., on
a cell tower), the power loss exponents may satisfy
n.sub.1<n.sub.2. Taking the (2/n.sub.2)-th root of both sides of
equation (14), one obtains:
P 1 2 n 2 d 1 2 1 / n 2 = P 2 2 n 2 d 2 2 = 0 , ( 15 )
##EQU00009##
The assumption that the signal-to-interference ratio (SIR) is 0 dB
at the border of a co-channel femtocell results in:
P.sub.2.sup.2/n.sup.2(x.sup.2+y.sup.2-2xx.sub.1-2yy.sub.1+x.sub.1.sup.2+-
y.sub.1.sup.2).sup.n.sup.1.sup./n.sup.2-P.sub.1.sup.2/n.sup.2(x.sup.2+y.su-
p.2-2xx.sub.2-2yy.sub.2+x.sub.2.sup.2+y.sub.2.sup.2)=0, (16).
where 0<n.sub.1/n.sub.2<1. In equation (16), the highest
power of both the x and y terms is 2. Equation (16) may be
simplified by approximating d.sup.2n1/n2 and evaluating equation
(16) at a point (a; b), using a second-order Taylor series (Point
(a;b) may be approximated by (x.sub.1;x.sub.2) in most cases):
T ( x , y ) = f ( a , b ) + ( x - a ) f x ( a , b ) + ( y - b ) f y
( a , b ) + 0.5 [ ( x - a ) 2 f xx ( a , b ) + 22 ( x - a ) ( y - b
) f xy ( a , b ) + ( y - b ) 2 f yy ( a , b ) ] , ( 17 ) f ( a , b
) = ( a 2 + b 2 - 2 ax 1 - 2 by 1 + x 1 2 + y 1 2 ) n 1 n 2 ( 18 )
f x ( a , b ) = 2 n 1 n 2 ( a - x 1 ) [ f ( a , b ) ] n 1 n 2 - 1 (
19 ) f y ( a , b ) = 2 n 1 n 2 ( b - y 1 ) [ f ( a , b ) ] n 1 n 2
- 1 ( 20 ) f xx ( a , b ) = 2 n 1 n 2 [ f ( a , b ) ] n 1 n 2 - 1 +
4 n 1 n 2 ( n 1 n 2 - 1 ) ( a - x 1 ) 2 [ f ( a , b ) ] n 1 n 2 - 2
( 21 ) f yy ( a , b ) = 2 n 1 n 2 [ f ( a , b ) ] n 1 n 2 - 1 + 4 n
1 n 2 ( n 1 n 2 - 1 ) ( b - y 1 ) 2 [ f ( a , b ) ] n 1 n 2 - 2 (
22 ) f xy ( a , b ) = 4 n 1 n 2 ( n 1 n 2 - 1 ) ( a - x 1 ) ( b - y
1 ) [ f ( a , b ) ] n 1 n 2 - 2 . ( 23 ) ##EQU00010##
Then, using the result of equation (17) in equation (16), one
obtains:
A ^ 1 x 2 + A ^ 2 y 2 + B ^ 1 x + B ^ 2 y + B ^ 3 xy + C ^ = 0 ( 24
) A ^ 1 = 1 2 P 2 2 n 2 f xx ( a , b ) - P 1 2 n 1 ( 25 ) A ^ 2 = 1
2 P 2 2 n 2 f yy ( a , b ) - P 1 2 n 1 ( 26 ) B ~ 1 = P 2 2 n 2 [ f
x ( a , b ) - af xx ( a , b ) - bf xy ( a , b ) ] - 2 P 2 2 n 1 x 2
( 27 ) B ~ 2 = P 2 2 n 2 [ f y ( a , b ) - bf yy ( a , b ) - af xy
( a , b ) ] - 2 P 1 2 n 1 y 2 ( 28 ) B ~ 3 = 2 P 2 2 n 2 f xy ( a ,
b ) ( 29 ) C ~ = P 2 2 n 2 [ f ( a , b ) - af x ( a , b ) - bf y (
a , b ) + 0.5 a 2 f xx ( a , b ) + 0.5 b 2 f yy ( a , b ) + abf xy
( a , b ) ] - P 1 2 n 1 ( x 2 2 + y 2 2 ) . ( 30 ) ##EQU00011##
Equation (24) resembles the circle of equation (4), except for: 1)
the xy cross-terms, so that the coverage area is an ellipse, rather
than a circle, and 2) the coefficients for x.sub.2 and y.sub.2 are
different. If P.sub.2<<P.sub.1 one may set .sub.1.apprxeq.
.sub.2, and {tilde over (B)}.sub.3.apprxeq.0 (i.e., once again
approximating the coverage area by a circle). With this
simplification, the coordinates at the center of the coverage area,
and the area of the coverage area, can be obtained using the
procedures discussed above with respect to equations (10) and (12),
using equations (25)-(30).
[0052] Because spectrum resources are scarce, small frequency reuse
factors are preferable. Thus, a frequency reuse factor of N=1 may
be preferable in many future wireless systems. In such a system,
femtocells may have to use the same frequency band as the macrocell
in all locations within the macrocell coverage area. However, as
discussed above, when an lBS is very close to the mBS, the
femtocell experiences severe interference from the mBS.
Interference averaging techniques, such as those discussed in
Chandrasekhar (above) may be used to mitigate the interference.
Such techniques have inherent disadvantages, and may also not be
sufficient to overcome the interference in femtocells at close
proximities to the mBS.
[0053] FIG. 8 shows, for N=1, another frequency assignment scheme
which assigns a separate frequency band F.sub.2 to all the
femtocells in a macrocell network using frequency band F.sub.1,
according to one embodiment of the present invention. The system of
FIG. 8 ensures interference-free operation among the femtocells and
the macrocell, at the expense of additional spectrum resources.
Also, when available, more than one frequency band may be assigned
for femtocell operations to reduce inter-femtocell interference,
using a spectrum-sensing approach, such as described for step 40 of
FIGS. 4-6.
[0054] The scenarios discussed above are applicable to downlink
channel assignments. The interference for the uplink may be
different, and a different channel assignment scheme may be needed
for duplex operation. For example, a macrocell MS (mMS) may need a
larger transmit power to reach the mBS, when the MS is far away
from the mBS. Hence, when the femtocell is located within outer
region 7, as far as the uplink operation is concerned, the
interference from an mMS to the femtocell may be more significant,
when the femtocell and the mMS both use the same channels for
communication. Thus, to avoid interference between the mMS and the
femtocell, different channels are preferably assigned to the mMS
and femtocell in outer region 7. Within inner region 6, because the
mMS uses weaker signals to communicate with the mBS, co-channel
operation with the femtocell may be possible.
[0055] FIG. 9 shows a frequency division duplex (FDD) frequency
assignment scheme for a macrocell network, in accordance with one
embodiment of the present invention. As shown in FIG. 9, the
macrocell uses frequency band 0 (i.e., frequency band F.sub.i) in
the downlink, and frequency band 12 (i.e., frequency band
F.sub.N+i) in the uplink. In such a network, the frequency band
assigned to the femtocell depends on both 1) whether or not the
femtocell is within inner region 6 or outer region 7, and 2)
whether or not the communication is downlink or uplink. FIG. 10
shows an example of an actual FDD frequency assignment in a network
of the type discussed with respect to FIG. 9, according to one
embodiment of the present invention. The downlink frequency
assignment may be achieved using the methods in FIGS. 4-8. For the
uplink, when the femtocell is within inner region 6, the same
frequency band is used for the femtocell as the macrocell (i.e.,
frequency band F.sub.N+i). Otherwise, i.e., if the femtocell is in
outer region 7, a different frequency band than frequency band
F.sub.N+i is used to avoid interference to or from the mMS that are
far away from the mBS.
[0056] According to another aspect of the present invention, power
control and frequency assignment may be carried out simultaneously
("joint power control and frequency assignment"). The macrocell
network of FIG. 9 above may be used to illustrate such an approach,
for the case where the frequency reuse factor of N>1. In such a
network, N downlink frequency bands and N uplink frequency bands
are used, with each frequency band being assigned to a different
macrocell. In the following detailed description, downlink
operation is first discussed. Without loss of generality, frequency
band 0 (i.e., frequency band F.sub.i) is assigned to the macrocell
of interest. For N=3, which is illustrated above in FIG. 2, where
no neighboring macrocells use the same frequency band, and the
macrocell of interest uses frequency band 4 (i.e., frequency band
F.sub.1). As explained above, within each frequency band, users may
also be further separated in time, frequency, or code domains. For
example, in a wideband code division multiple access (WCDMA)
system, multiple CDMA codes are used to minimize interference among
the different users within a macrocell.
[0057] To provide joint power control and frequency assignment, a
method according to the present invention follows the following
criteria: (a) co-channel operation of the femtocell is preferable
from cell-search and hand-off points of view, subject to the
interference conditions from the mBS; (b) in all cases, the
femtocell guarantees a minimum coverage area A.sub.fem through
power control; and (c) the femtocell has a maximum transmission
power limit P.sub.max and a minimum transmission power limit
P.sub.min. Power limit P.sub.max may represent hardware constraints
or interference constraints among femtocells, while power limit
P.sub.min may be the minimum transmission power needed for good
coverage, in the absence of any interference. FIG. 11 illustrates a
network implementing joint power control and frequency assignment,
in accordance with one embodiment of the present invention. As
shown in FIG. 11, the macrocell is partitioned into inner region
111, power control region 112, and outer region 113. While no power
control is applied in inner and outer regions, the fBS uses power
control in power-control region 112 to provide a pre-determined
femtocell coverage area A.sub.fem.
[0058] As discussed above, the coverage area of the IBS is
determined by the area within a contour along which the received
power levels (typically, for the pilot signals, rather than the
data signals) from the fBS and mBS are the same. The present
invention, however, is equally applicable to systems in which the
coverage areas are defined in other ways, such as hand-off
parameters. Such hand-off parameters may include, for example, cell
search initiation threshold and handoff execuation threshold. In
such cases, the received power levels from the fBS and mBS may not
be the same, and there may be more than one contour that defines
the coverage area (e.g., one contour for an incoming user to the
fBS and another contour for an outgoing user from the IBS).
[0059] FIG. 12 provides an example of frequency band assignments in
the regions of FIG. 11, for both downlink and uplink frequency
assignments, in accordance with one embodiment of the present
invention. As shown in FIG. 12, during downlink transmission,
macrocell uses frequency band F.sub.i in all three regions. The
frequency band used for downlink by a femtocell, however, depends
on which region the fBS is located: (a) when the femtocell is in
power control region 112, or in outer region 113, the femtocell
uses the same frequency band as the macrocell (i.e., F.sub.i.); and
(b) when the femtocell is within inner region 111 (i.e., when
interference from the mBS is high), the fBS uses a frequency band
different than that used by the macrocell (e.g., a frequency band
used by one of the neighboring macrocells, or a frequency band
specifically reserved for femtocells).
[0060] FIG. 13 is a block diagram illustrating joint power control
and frequency band assignment in the regions of FIG. 11, in
accordance with one embodiment of the present invention. In FIG.
13, if the locations of the fBS and the mBS are available (step
124), a fBS obtains at step 110 (a) n.sub.1 and n.sub.2, which
denote the path loss exponents for the mBS and the fBS,
respectively; (b) P.sub.1 and P.sub.2, which denote the
transmission powers for the mBS and fBS, respectively; and (c)
(x.sub.mBS,y.sub.mBS) and (x.sub.fBS,y.sub.fBS), which denote the
locations of the mBS and the fBS, respectively. Otherwise, i.e.,
the locations of the fBS and the mBS are not available, distance
d.sub.fm between an fBS and mBS is obtained at step 126. At step
120, using the information obtained in either step 126 or step 110,
the fBS determines if it is in inner region 111, power control
region 112, or outer region 113. If the fBS is in power control
region 112, at step 60, the femtocell is assigned the same
frequency band as the macrocell (i.e., frequency band F.sub.i for
downlink operations). In power control region 112, at step 190, the
fBS selects a power level P.sub.2 which ensures that coverage area
A.sub.fem is achieved, in the presence of macrocell interference.
If the fBS determines that it is in outer region 113, at step 170,
the same frequency band as the macrocell is assigned to femtocell
(i.e., frequency band F.sub.i for downlink operations) (i.e.,
frequency band F.sub.i for downlink operations). In outer region
113, even though interference by the mBS to the femtocell is low, a
low transmission power may result in unsatisfactory performance.
Therefore, at step 1100, a fixed minimum transmission power
P.sub.min is selected for the fBS. If the femtocell is determined
to be in neither power control region 112, nor outer region 113
(i.e., fBS is in inner region 111), at step 150, a frequency band
which is orthogonal to the macrocell frequency band is assigned to
the femtocell. Under such an assignment, interference from the
macrocell is insignificant. Thus, in inner region 111, the
femtocell may also select at step 1110 a fixed transmission power
P.sub.min for communication. A larger fixed transmission power may
be used to overcome interference from neighboring macrocells or
other femtocells.
[0061] FIGS. 14 and 15 are block diagrams illustrating different
approaches the fBS may use to decide the region of its location
(i.e., step 120 of FIG. 13), in accordance with one embodiment of
the present invention. In FIG. 14, at step 131, a femtocell
calculates a transmission power P.sub.2 required to obtain a
coverage area A.sub.fem. Some examples of such a calculation is
discussed in further detail below. Then, at step 132, the
calculated power level P.sub.2 is compared to power thresholds
P.sub.min and P.sub.max discussed above. If
P.sub.min<P.sub.2<P.sub.max (step 135), the femtocell is in
power control region 112. Otherwise, if P.sub.2>P.sub.max (step
133), the femtocell is in inner region 111 (step 136), and if
P.sub.2<P.sub.min (step 134), the femtocell is in outer region
113 (step 137). In the method of FIG. 14, the power level at step
131 may be calculated using the parameters n.sub.1, n.sub.2,
P.sub.1, A.sub.fem, and the locations of the fBS and mBS (i.e.,
step 110 of FIG. 13), or the distance between the fBS and the mBS
(i.e., step 126 of FIG. 13) and other remaining parameters.
[0062] FIG. 15 shows a simpler approach by which the femtocell can
determine the region of its location. In the method of FIG. 15,
only distance d.sub.fin to the mBS, calculated at step 141, is used
to decide whether it is in inner region 111, power control region
112, or outer region 113 (i.e., the parameters n.sub.1, n.sub.2,
P.sub.1, A.sub.fem are not utilized). At step 142, if
d.sub.in<d.sub.fin<d.sub.out, where d.sub.in and d.sub.out,
are the radii of inner region 111 and outer region 13,
respectively, the femtocell is determined to be in power control
region 112 (step 145). Otherwise, if d.sub.fin>d.sub.out (step
143), the femtocell is determined to be in outer region 113, while
if d.sub.fin<d.sub.in (step 144), the femtocell is determined to
be in inner region 111 (step 147).
[0063] In one implementation of a method of the present invention,
where the coverage area A.sub.fem may not be available, but the
total coverage area of a uses premises may be classified into K
premise types (e.g., a studio, small apartment, large apartment,
house, or office), a predetermined coverage area may be assigned
for the premises for the purpose of calculating the transmit power,
according to the premise type.
[0064] FIG. 16 shows one implementation of a method for refining
the calculation of transmit power, in accordance with one
embodiment of the present invention. As shown in FIG. 16, at step
152, an initial transmission power level P.sub.2 of the fBS is
determined using any method, such as the method of FIG. 14. Then,
transmission power level P.sub.2 is updated at step 154 by the
responses to periodic control signals sent to the MSs. By taking
short-term and long-term averages of the received signal powers at
the fBS and at the MSs, average signal-to-noise ratios (SNRs) of
different MSs may be obtained (step 156). Then, at step 158, the
fBS may tune the transmission power level P.sub.2 to achieve an
acceptable receiver SNR. In other words, based on received signals,
the fBS may increase or decrease its transmission power around the
initial power estimate to satisfy different SNR metrics.
[0065] According to the present invention, various criteria based
on SNR may be used to set the power level at the fBS. These
criteria may include (a) average SNR at the fBS, (b) average SNR at
the MSs, (c) average SNR at the fBS and the MSs, (d) minimum SNRs
at the fBS (for different time scales), (e) minimum SNRs at the MSs
(at different time scales), and (f) minimum SNRs at the fBS and
minimum SNRs at the MSs (at different time scales). The received
signals may show smaller variations for a small apartment, so that
short-term averages would provide information for necessary power
levels, while typically much larger variations are expected for a
large house, thereby necessitating long-term averages.
[0066] Apart from the received SNR, the fBS may utilize some other
metrics for setting its transmission power at step 156 of FIG. 16.
For example, if no response is received from any MS to the control
messages transmitted from the fBS for a certain period of time, the
fBS may conclude that there is no MS to communicate with the fBS at
that time. Under such condition, the fBS may set its (pilot)
transmission power to a minimum level (i.e., P.sub.min) to minimize
interference with other femtocells and/or the macrocell. Also, the
fBS may record the activities of the mMSs at different time scales
to adjust its transmission power. For example, between the morning
and evening, there may be little connection from MSs to a
femtocell, and considerably more activities may occur in the
evening. By monitoring such usage patterns, an fBS may decide to
minimize its transmission power during the daytime.
[0067] FIG. 17 shows an example message exchange between an fBS and
an MS, in accordance with one embodiment of the present invention.
As shown in FIG. 17, the fBS sends control message 162 initially.
If no response is received from any MS to control message 162 after
a time period, at step 172, the fBS may set its transmission power
to a minimum level to minimize interference to other femtocells and
the macrocell. However, if the fBS receives a response 164 from an
MS, the fBS may utilize this signal to estimate the SNR of the MS.
Once having SNRs calculated from communications with different MSs,
the fBS may update at step 174 its transmission power based on the
SNRs for future communications (step 176). The fBS may transmit
control messages periodically (step 178) to adaptively update its
transmission power, in response to changes in the environment
(e.g., number of users, locations of users, or channel qualities of
the users).
[0068] The transmission power level may be determined using, for
example, the following method. From equations (4)-(8) above, in a
femtocell interference-limited coverage area (ILCA) environment
with equal path loss exponents, circular coverage area A.sub.fem is
given by
A fem = .pi. r 2 = .pi. ( B 1 2 4 A 2 + B 2 2 4 A 2 - C A ) ( 31 )
##EQU00012##
[0069] In a power-controlled femtocell environment, the femtocell
provides a coverage area A.sub.fem at all times by adjusting its
transmission power P.sub.2 (which is calculated for a given
A.sub.fem). Equation (31) may be rearranged to:
B 1 2 + B 2 2 - 4 AC - 4 A 2 A fem .pi. = 0 ( 32 ) ##EQU00013##
which may be simplified to:
aP 2 2 + bP 2 + c = 0 Where ( 33 ) a = - 4 A fem .pi. b = 4 P 1 ( (
x 1 - x 2 ) 2 + ( y 1 - y 2 ) 2 + 2 A fem .pi. ) c = - rP 1 2 A fem
.pi. ( 33 ) ##EQU00014##
The required power level P.sub.2 that provides coverage area
A.sub.fem may be solved by finding the roots of the second order
polynomial of equation (32). Equation (13) above relates the
transmit power levels P.sub.1 and P.sub.2 to the coverage areas of
the mBS and the fBS, respective, for a network in which n.gtoreq.2,
n being any arbitrary path loss exponent. As discussed above,
equation (13) represents the case where n=2, but with a different
pair of transmit powers. The techniques above may be used to
calculate the required transmit power P.sub.1.
[0070] The above detailed description is provided to illustrate the
specific embodiments of the present invention, and is not intended
to be limiting. Numerous variations and modifications are possible
within the scope of the present invention. The present invention is
set forth in the following claims.
* * * * *
References