U.S. patent application number 14/621700 was filed with the patent office on 2015-06-11 for downlink network synchronization mechanism for femtocell in cellular ofdm systems.
The applicant listed for this patent is MEDIATEK INC.. Invention is credited to I-Kang Fu.
Application Number | 20150163762 14/621700 |
Document ID | / |
Family ID | 42128275 |
Filed Date | 2015-06-11 |
United States Patent
Application |
20150163762 |
Kind Code |
A1 |
Fu; I-Kang |
June 11, 2015 |
Downlink Network Synchronization Mechanism for Femtocell in
Cellular OFDM Systems
Abstract
A method of downlink synchronization for a femto base station in
a cellular orthogonal frequency division multiplexing (OFDM) system
is provided. The femto base station first scans one or more
received reference signals transmitted from a plurality of
neighboring macro base stations. The femto base station then
determines a desired reference signal from the received one or more
reference signals based on the scanning result. Finally, the femto
base station configures its downlink radio signal transmission time
based on the desired reference signal such that the femto base
station is synchronized with the plurality of neighboring macro
base stations.
Inventors: |
Fu; I-Kang; (Taipei City,
TW) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
MEDIATEK INC. |
Hsinchu |
|
TW |
|
|
Family ID: |
42128275 |
Appl. No.: |
14/621700 |
Filed: |
February 13, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12589974 |
Oct 30, 2009 |
8989085 |
|
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14621700 |
|
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61109999 |
Oct 31, 2008 |
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Current U.S.
Class: |
370/350 |
Current CPC
Class: |
H04J 11/0069 20130101;
H04J 3/0641 20130101; H04L 27/2655 20130101; H04J 11/0093 20130101;
H04W 56/0015 20130101; H04J 11/0056 20130101; H04W 72/042 20130101;
H04L 27/2627 20130101 |
International
Class: |
H04W 56/00 20060101
H04W056/00; H04W 72/04 20060101 H04W072/04; H04L 27/26 20060101
H04L027/26 |
Claims
1. A method for downlink transmission synchronization in a cellular
orthogonal frequency division multiplexing (OFDM) system, the
method comprising: (a) scanning and receiving one or more reference
signals transmitted from a plurality of neighboring macro base
stations by a femto base station; (b) determining a desired
reference signal from the received one or more reference signals,
wherein the desired reference signal is a reference signal first
received by the femto base station during an observation window;
and (c) configuring downlink radio signal transmission of the femto
base station based on the desired reference signal such that the
femto base station is synchronized with the plurality of
neighboring macro base stations.
2. The method of claim 1, wherein downlink radio signals
transmitted by the plurality of neighboring macro base stations are
synchronized before the scanning the one or more reference signals
by the femto base station.
3. The method of claim 1, wherein two base stations are
synchronized when arrival time difference for radio signals
transmitted from the two base stations is smaller than a guard
interval of an OFDM symbol.
4. The method of claim 1, wherein the observation window length is
smaller than half of a frame length but substantially larger than
propagation delay among the plurality of neighboring macro base
stations.
5. The method of claim 1, wherein the desired reference signal is a
downlink preamble transmitted at a starting boundary of a downlink
frame, wherein the preamble is received by the femto base station
at a certain timing, and wherein the femto base station transmits a
downlink frame at the same certain timing.
6. The method of claim 1, wherein the desired reference signal is
transmitted during a downlink frame with an offset, wherein the
desired reference signal is received by the femto base station at a
certain timing, and wherein the femto base station transmits a
downlink frame at the same certain timing with the same offset.
7. The method of claim 1, wherein the one or more reference signals
comprise a reference signal transmitted by a base station with
multipath effect.
8. The method of claim 1, wherein the femto base station is an
access-point base station used for indoor coverage in the cellular
OFDM system, and wherein the femto base station is connected to a
backhaul server through a normal broadband physical link.
9. The method of claim 1, wherein the femto base station and each
of the plurality of neighboring macro base stations have
overlapping cell coverage.
10. A femto base station in a cellular orthogonal frequency
division multiplexing (OFDM) system, the femto base station
comprising: a radio frequency (RF) module that receives one or more
reference signals transmitted by a plurality of neighboring macro
base stations; a timing detector that detects a corresponding
arrival time of each of the reference signals; a timing abstractor
that determines a desired timing reference based on the detected
arrival time of a desired reference signal, wherein the desired
reference signal is a reference signal first received by the femto
base station during an observation window; and a timing
configuration module that configures downlink transmission timing
of the femto base station based on the desired timing reference
such that the femto base station is synchronized with the plurality
of neighboring macro base stations.
11. The femto base station of claim 10, wherein two base stations
are synchronized when arrival time difference for radio signals
transmitted from the two base stations is smaller than a guard
interval of an OFDM symbol.
12. The femto base station of claim 10, wherein the observation
window length is smaller than an OFDM frame length but
substantially larger than propagation delay among the plurality of
neighboring macro base stations.
13. The femto base station of claim 10, wherein the desired
reference signal is a downlink preamble transmitted at a starting
boundary of a downlink frame, wherein the preamble is received by
the femto base station at the reference timing, and wherein the
femto base station transmits a downlink frame at the same reference
timing.
14. The femto base station of claim 10, wherein the desired
reference signal is transmitted during a downlink frame with an
offset, wherein the desired reference signal is received by the
femto base station at the reference timing, and wherein the femto
base station transmits a downlink frame at the same reference
timing with the same offset.
15. The femto base station of claim 10, wherein the one or more
reference signals comprise a reference signal transmitted by a base
station with multipath effect.
16. The femto base station of claim 10, further comprising: a
correlation module that correlates one or more received sequences
in frequency domain such that the timing detector detects the
corresponding arrival times of the one or more received reference
signals in time domain.
17. The femto base station of claim 10, wherein the femto base
station is an access-point base station used for indoor coverage in
the cellular OFDM system, and wherein the femto base station is
connected to a backhaul server through a normal broadband physical
link.
18. A femto base statoin in a cellular orthogonal frequency
division multiplexing (OFDM) system, the femto base station
comprising: a radio freuqency module that receives one or more
reference signals transmitted by a plurality of neighboring macro
base stations; and means for detecting a corresponding arrival time
of each of the reference signals and thereby determining a desired
timing reference of a desired reference signal, wherein the desired
reference signal is a reference signal first received by the femto
base station during an observation window, and wherein the femto
base station configures downlink transmission timing based on the
desired timing reference such that the femto base station is
synchronized with the plurality of neighboring macro base
stations.
19. The femto base station of claim 18, wherein two base stations
are synchronized when arrival time difference for radio signals
transmitted from the two base stations is smaller than a predefined
value, and wherein the predefine value is smaller than a guard
interval of an OFDM symbol.
20. The femto base station of claim 19, wherein the observation
window length is smaller than half of a frame length but
substantially larger than propagation delay among the plurality of
neighboring macro base stations.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation, and claims priority
under 35 U.S.C. .sctn.120 from nonprovisional U.S. patent
application Ser. No. 12/589,974, entitled "Downlink Network
Synchronization Mechanism for Femtocell in Cellular OFDM Systems,"
filed on Oct. 30, 2009, the subject matter of which is incorporated
herein by reference. Application Ser. No. 12/589,974, in turn,
claims priority under 35 U.S.C. .sctn.119 from U.S. Provisional
Application No. 61/109,999, entitled "Network Synchronization
Mechanism to Support Femtocell in Wireless OFDM Systems," filed on
Oct. 31, 2008, the subject matter of which is incorporated herein
by reference.
TECHNICAL FIELD
[0002] The present invention relates generally to cellular OFDM
systems and, more particularly, to downlink network synchronization
for Femto base stations in cellular OFDM systems.
BACKGROUND
[0003] As bandwidth demand for indoor wireless users continues to
grow, cellular operators are trying to explore bandwidth
provisioned from indoor, in addition to providing bandwidth from
outdoor. Because of the physical nature of radio signals, however,
cellular operators have faced difficulties to provide full coverage
for indoor users. Cellular repeater is one of the common solutions
for current systems, but it may degrade received signal quality and
has no intelligence for signal processing. Relay station is another
solution developed to resolve this problem, but there is no
commercial relay station available yet and it is still under
development. Femtocell is yet another solution that is developed to
enhance indoor coverage by reusing the licensed spectrum as part of
the cellular network infrastructure.
[0004] FIG. 1 (Prior Art) illustrates a simplified cellular network
10 that comprises a macro base station BS11 and a femto base
station BS12. Cellular network 10 also comprises an outdoor mobile
station MS14 and an indoor mobile station MS15. As illustrated in
FIG. 1, macro BS11 provides strong signal strength to outdoor MS14,
while provides relatively weak signal strength to indoor MS15
because of physical obstruction and/or reflection caused by
building 13. On the other hand, femto BS12, an access-point base
station (e.g., a small indoor base station), is able to provide
strong signal strength and good signal quality to indoor MS15
because femto BS12 is located inside building 13.
[0005] Femtocell is anticipated to be an important feature to
support extreme high-speed transmission for 4G systems. Both IEEE
802.16m and 3GPP RAN1&RAN2 are currently developing femtocell
technology as part of the standards for WiMAX 2.0 and LTE-Advanced
systems. Extreme high-speed transmission will result in very high
power consumption and is usually used to support multimedia
services, which are more possible be requested by users at indoor
environment. By using femto base stations, more radio resources can
be saved by using shorter range and lower transmission power. FIG.
2 (Prior Art) illustrates system architecture of a WiMAX femtocell
system 20.
[0006] Network synchronization of downlink transmission timing in a
cellular network is usually performed by Global Positioning System
(GPS). GPS is a global navigation satellite system that provides
reliable positioning, navigation, and timing service. However, a
femto BS may not be able to receive GPS signals and obtain timing
reference. FIG. 3 (Prior Art) illustrates a cellular network 30
that comprises a GPS31. Cellular network 30 also comprises macro
BS32 and BS33, as well as a femto BS34. As illustrated in FIG. 3,
BS32 and BS33 are able to receive GPS signals from GPS31, while
BS34 is not able to receive GPS signals and obtain timing reference
because it is located inside building 35.
[0007] In addition to GPS, backhaul signaling may also help to
achieve network synchronization among difference BSs. However,
backhaul connection of a femto BS is not reliable for obtaining
timing reference. FIG. 4 (Prior Art) illustrates a backhaul
connection of a femto BS in a WiMAX femtocell system 40. As
illustrated in FIG. 4, Femto BS backhaul is expected to be low-cost
xDSL or DOCSIS link. It is not as robust and reliable as dedicated
connections used in Macro-/Micro-/Pico-BS. In addition, the round
trip delay may be time variant and result in difficulty on precise
timing refinement. Thus, downlink network synchronization for
femtocell in a cellular orthogonal frequency division multiplexing
(OFDM) and/or orthogonal frequency division multiple access (OFDMA)
systems remains a challenge.
SUMMARY
[0008] A method of downlink synchronization for a femto base
station in a cellular orthogonal frequency division multiplexing
(OFDM) system is provided. The femto base station first scans one
or more received reference signals transmitted from a plurality of
neighboring macro base stations. Before the scanning, downlink
transmission time among the neighboring macro base stations are
already well synchronized. From a mobile station perspective, the
arrival time difference between radio signals transmitted by the
macro base stations is smaller than the Guard Interval Duration
(Tg) of an OFDM symbol.
[0009] The femto base station then determines a desired reference
signal from the received one or more reference signals based on the
scanning result. In one novel aspect, the desired reference signal
is a reference signal first received by the femto base station
during an observation window. In one embodiment, the observation
window length is smaller than a half of a frame length but
substantially larger than propagation delay among the plurality of
base stations. The actual arrival time of the desired reference
signal can be detected by the femto base station using digital
signal processing plus a timing detector and a timing
abstractor.
[0010] Finally, the femto base station configures its downlink
radio signal transmission time based on the desired reference
signal such that the femto base station is synchronized with the
plurality of neighboring macro base stations. In one embodiment,
the reference signal is transmitted at the starting boundary of the
downlink frame by a neighboring BS. The femto BS sets its downlink
starting frame boundary to be the same as the timing when the
reference signal is first received by the femto BS. In another
embodiment, the reference signal is transmitted at the starting
boundary plus an offset of the downlink frame by a neighboring BS.
The femto BS sets its downlink starting frame boundary to be the
same as the timing when the reference signal is first received by
the femto BS plus the same offset.
[0011] Other embodiments and advantages are described in the
detailed description below. This summary does not purport to define
the invention. The invention is defined by the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] The accompanying drawings, where like numerals indicate like
components, illustrate embodiments of the invention.
[0013] FIG. 1 (Prior Art) FIG. 1 illustrates a simplified cellular
network that comprises a macro base station and a femto base
station.
[0014] FIG. 2 (Prior Art) illustrates system architecture of a
WiMAX femtocell system.
[0015] FIG. 3 (Prior Art) illustrates a cellular network that
comprises a global positioning system.
[0016] FIG. 4 (Prior Art) illustrates a backhaul connection of a
femto BS in a WiMAX femtocell system.
[0017] FIG. 5 illustrates a cellular OFDM network 50 with a femto
base station in accordance with one novel aspect.
[0018] FIG. 6 is a simplified block diagram of a femto base station
in accordance with one novel aspect.
[0019] FIG. 7 is a flow chart of a method of activating a femto
base station in a cellular OFDM network.
[0020] FIG. 8 illustrates a cellular OFDM network with both an
isolated femtocell and an overlapped femtocell.
[0021] FIG. 9 illustrates a cellular OFDM network with a femtocell
overlapping with three macro cells.
[0022] FIG. 10 illustrates multiple reference signals received by a
femtocell in a cellular OFDM network.
[0023] FIG. 11 is a generalized network topology of a cellular OFDM
network that illustrates a novel network synchronization
approach.
[0024] FIG. 12 is a special network topology of a cellular OFDM
network that illustrates a novel network synchronization
approach.
[0025] FIG. 13 illustrates a general method of detecting the
arrival time of the desired reference signal from multiple received
reference signals.
[0026] FIG. 14 illustrates a first embodiment of detecting the
arrival time of a reference signal.
[0027] FIG. 15 illustrates a second embodiment of detecting the
arrival time of a reference signal with multipath effect.
[0028] FIG. 16 illustrates a third embodiment of detecting the
arrival time of multiple reference signals with multipath
effect.
[0029] FIG. 17 illustrates a first embodiment of configuring
downlink transmission time by a femto base station.
[0030] FIG. 18 illustrates a second embodiment of configuring
downlink transmission time by a femto base station.
DETAILED DESCRIPTION
[0031] Reference will now be made in detail to some embodiments of
the invention, examples of which are illustrated in the
accompanying drawings.
[0032] Femtocell is developed to enhance indoor coverage by reusing
the licensed spectrum as part of cellular network infrastructure.
In a femtocell system such as a WiMAX femtocell system, indoor
services are mainly served by WiMAX femtocell access point (WFAP).
Femtocells typically provide very small cell coverage (e.g. <35
meters) with extreme high-speed transmission for indoor
applications such as multimedia services. By reusing the same air
interface and operating at the same licensed spectrum as macro
cells, network operators benefit from reduced development cost on
macro cells for indoor coverage and increased revenue from indoor
wireless communication. However, downlink network synchronization
among femtocells and macro cells is critical to maintain
orthogonality between sub-carriers and prevent inter-carrier
interference (ICI) in cellular Orthogonal Frequency Division
Multiplexing (OFDM) and/or Orthogonal Frequency Division Multiple
Access (OFDMA) networks.
[0033] OFDMA has been adopted as the downlink transmission scheme
for candidate 4G technologies. OFDMA has been considered for both
WiMAX 2.0 and LTE-Advanced downlink transmission. Inter-carrier
interference (ICI), however, is a special and significant problem
in OFDM networks, which is caused mainly by frequency offset and
time variation. To prevent ICI, OFDM network requires synchronized
transmission by each base station to maintain orthogonality between
subcarriers. Thus, OFDM symbol timing transmitted by each base
station should be well aligned. In general, the difference among
the arrival times of a radio signal from different base stations
shall be smaller than the guard interval of an OFDM symbol to
prevent undesired ICI.
[0034] FIG. 5 illustrates a cellular OFDM network 50 with a femto
base station in accordance with one novel aspect. Cellular OFDM
network 50 comprises a plurality of macro base stations BS51-BS57,
a femto base station BS58, and a global positioning system GPS59.
Macro base stations BS51-BS57 receive GPS signals from GPS59 and
thereby obtain reliable and precise timing reference. As a result,
downlink transmission time for BS51-BS57 is well synchronized. From
a mobile station perspective, this means that the arrival time
difference between radio signals transmitted by BS51-BS57 is
smaller than the Guard Interval Duration (Tg) of an OFDM symbol. On
the other hand, femto BS58 is not able to receive GPS signals to
obtain timing reference. In one novel aspect, femto BS58 receives
and scans a plurality of reference signals transmitted from the
plurality of macro base stations BS51-BS57, and determines a
desired reference signal. Based on the desired reference signal,
BS58 is then able to obtain timing reference such that downlink
transmission time for femto BS58 and other macro base stations
BS51-BS57 is also well synchronized.
[0035] FIG. 6 is a simplified block diagram of femto base station
BS58 in cellular OFDM network 50 in accordance with one novel
aspect. BS58 comprises a storage device 61, a CPU 62, a radio
frequency (RF) module 64 coupled with an antenna 63, a signal
processing module 65, a timing detector and abstractor 66, and a
timing configuration module 67. In the embodiment of FIG. 6, RF
module 64 receives a reference signal 68 (analog signal) via
antenna 63. Signal processing module 65 converts the analog signal
into a digital signal and outputs corresponding sequence code of
reference signal 68. Timing detector and abstractor 66 detects the
arrival time of reference signal 68 based on the sequence code and
then determines a desired timing reference based on the detected
arrival time of reference signal 68. Based on the desired timing
reference, timing configuration module 67 configures downlink
transmission time for femto BS58 to achieve network
synchronization.
[0036] FIG. 7 is a flow chart of a method of activating a femto
base station in a cellular OFDM network. In step 71, the femto BS
first communicates with its backhaul server and registers to the
cellular network after being powered on. The femto BS cannot
transmit any radio signal before authorized by the backhaul server.
In step 72, the femto BS communicates with the backhaul server to
exchange its service capability such as supportable channel
bandwidth, protocol version, power class, and support of
multi-carrier. In addition, the backhaul server may inform a set of
parameters for femtocell operation such as center frequency and
bandwidth of the assigned frequency channel(s), transmit power,
permutation scheme, and supportable throughput. In step 73, the
femto BS scans a plurality of reference signals over the assigned
frequency channel. The plurality of reference signals are
transmitted from a plurality of neighboring macro base stations.
Based on the scanning result, the femto BS detects the arrival time
of each of the plurality of reference signals and thereby
determines a desired reference signal (step 74). Finally, the femto
BS configures its downlink frame boundary based on the timing of
the desired reference signal such that downlink network
synchronization is achieved among the femto BS and other
neighboring macro BSs (step 75). After proper timing configuration,
the femto BS is ready to activate downlink transmission (step
76).
[0037] A challenging problem in activating a femtocell in a
cellular OFDM network is how to achieve downlink network
synchronization. There are several issues to be considered in
addressing this problem. First, the scope of downlink network
synchronization needs to be identified. That is, the femtocell
needs to determine which macro base stations to synchronize to in
the OFDM network. Second, a desired reference signal needs to be
determined from a plurality of reference signals transmitted from a
plurality of macro base stations in the OFDM network. By
configuring downlink transmission time based on the desired
reference signal, network synchronization among the femtocell and
other macro base stations can be achieved. Third, the actual
arrival time of the desired reference signal needs to be detected.
The femtocell can use the exact arrival time of the desired
reference signal for its downlink transmission to achieve network
synchronization. Each issue is now described below with more
detail.
[0038] FIG. 8 illustrates a cellular OFDM network 80 with both an
isolated femtocell and an overlapped femtocell. Cellular OFDM
network 80 comprises a macro base station BS81, a first femto base
station BS82, a second femto base station BS83, and a mobile
station MS84. Macro BS81 provides signal coverage for cell 85 and
is the serving base station for MS84, femto BS82 provides signal
coverage for cell 86, and femto BS83 provides signal coverage for
cell 87. The cell coverage of a base station is the longest
distance within which a mobile station can establish connection. As
illustrated in FIG. 8, cell 86 is isolated from cell 85, while cell
87 is overlapped with cell 85. For example, since MS84 is located
outside the cell boundary of cell 86, it receives relatively weak
interference from femto BS82. On the other hand, since MS84 is
located inside the cell boundary of cell 87, it receives relatively
strong interference from femto BS83. As a result, the interference
problem of a mobile station served by a macro cell caused by a
femtocell will be significant only when the femtocell coverage is
overlapped with the macro cell coverage, and interference due to
unsynchronized transmission can be ignored if the femtocell is an
isolated cell. Therefore, in a cellular OFDM network, a femtocell
needs to synchronize to only nearby macro cells with overlapping
cell coverage. That is, the femtocell needs to scan only reference
signals transmitted from overlapping macro cells.
[0039] After determining the scope of downlink network
synchronization, the femtocell needs to determine a desired
reference signal from one or more reference signals transmitted by
the nearby macro base stations. FIG. 9 illustrates a cellular OFDM
network 90 with a femtocell overlapping with three macro cells.
Cellular OFDM network 90 comprises a first macro base station BS91
providing cell coverage for cell 96, a second macro base station
BS92 providing cell coverage for cell 97, a third macro base
station BS93 providing cell coverage for cell 98, and a femto base
station BS94 providing cell coverage for cell 99. In the example of
FIG. 9, cell 99 overlaps with all three macro cells 96-97, and
interference due to unsynchronized transmission between femto BS94
and other macro BS91-93 cannot be ignored. In order to synchronize
with all three macro BS91-BS93, femto BS94 scans multiple reference
signals transmitted by all three macro BS91-BS93 and then
determines which reference signal is a desired reference signal to
be used such that downlink synchronization can be achieved between
femto BS94 and all three macro BS91-BS93.
[0040] FIG. 10 illustrates multiple reference signals received by
femto BS94 in cellular OFDM network 90. Because macro BS91-BS93 are
overlapping macro cells with femto BS94, femto BS94 scans the
reference signals (e.g., either preambles or synchronization
signals) transmitted by BS91-93. A preamble is a predefined
sequence that is modulated over sub-carriers in frequency domain
and transmitted as the first OFDM symbol in time domain. In the
example of FIG. 10, the arrival times of each preamble signal
transmitted from BS91-93 are different, caused by both multipath
effect and propagation delay. Among the different arrival times,
inter-BS arrival time difference is caused by propagation delay,
while intra-BS arrival time difference is caused by multipath
effect. Femto BS94 needs to determine which reference signal should
be used as the desired reference signal for its downlink
transmission to achieve downlink network synchronization. In one
novel aspect, femto BS94 always selects a reference signal with the
earliest arrival time as the desired reference signal, regardless
of which macro BS transmits (normally a macro BS that is closest to
the femto BS) the reference signal. Mathematical analysis of
selecting the desired reference signal is now described below with
more details.
[0041] FIG. 11 is a generalized network topology of a cellular OFDM
network 100 that illustrates the novel network synchronization
approach. In cellular OFDM network 100, BS1 is a macro base station
that has the shortest propagation delay when transmitting signals
to a femto BS3. BS2 is an arbitrary located macro base station with
propagation delay longer than BS1 when transmitting signals to the
femto BS3. The time for BS1 to transmit a reference signal to femto
BS3 is t1, and the time for BS2 to transmit a reference signal to a
mobile station MS4 is t2. The distance between MS4 and femto BS3 is
assumed to be less than 150 m, where the femtocell coverage is
generally less than 35 m. Other time variables .tau., .tau.1, and
.tau.2 are depicted in FIG. 11, where |.tau.|>=|.tau.1| and
|.tau.|>=|.tau.2|. Because macro base stations BS1 and BS2 are
already synchronized through (e.g. by GPS or by backhaul network),
another important assumption is that T.sub.DIFF(BS1,
BS2)=|t1+.tau.1-t2|<=T.sub.SYNC, where T.sub.DIFF(A,B is the
arrival time difference between the signal transmitted by A and B
from MS4 perspective, and where T.sub.SYNC is the maximum OFDM
symbol arrival time difference between two difference base stations
to satisfy network synchronization condition. Based on the
definition of network synchronization, T.sub.SYNC is always smaller
than the Guard Interval Duration T.sub.G (e.g. 11 .mu.s for IEEE
802.16 m system or 8 .mu.s in 3GPP LTE system), but can be assumed
to be larger than 1 .mu.s (e.g., 3 .mu.s in 3GPP LTE system). It is
observed that if femto BS3 set its transmission time to be t1
(e.g., the same time as femto BS3 receives the reference signal
from the closest macro BS1), then downlink network synchronization
conditions can be met. That is, if femto BS3 is synchronized with a
closest macro BS1, then femto BS3 is also synchronized with any
arbitrary located macro BS2 (e.g., T.sub.DIFF(BS1,
BS3)<=T.sub.SYNC and T.sub.TIFF(BS2, BS3)<=T.sub.SYNC).
[0042] FIG. 12 is a special network topology of cellular OFDM
network 100 that illustrates the above observed network
synchronization approach. In the example of FIG. 12, Macro BS1 and
BS2, femto BS3, and MS4 are all physically located on the same
line. Suppose femto BS3 sets its downlink transmission time to be
t1. As a result, T.sub.DIFF(BS1, BS3)=|(t1+.tau.1)-(t1+.tau.1)|=0,
and T.sub.DIFF(BS2, BS3)=|(t2-(t1+.tau.1)|=T.sub.DIFF(BS2,
BS1)<=T.sub.SYNC. Therefore, femto BS3 is synchronized to both
macro BS1 and BS2 if it is synchronized to BS1.
[0043] Now referring back to the generalized network topology of
network 100 in FIG. 11, where femto BS3 also sets its downlink
transmission time to be t1. It can be shown that network
synchronization conditions T.sub.DIFF(BS1, BS3)<=T.sub.SYNC and
T.sub.DIFF(BS2, BS3)<=T.sub.SYNC are satisfied if femto BS3 is
synced to macro BS1. First, it can be shown that T.sub.DIFF(BS1,
BS3)=|(t1+.tau.)-(t1+.tau.1)|=|.tau.-.tau.1|<=2 .tau.(because
.tau.>=.tau.1)=2*(Femtocell coverage/propagation
speed)<=2.times.150/(3.times.10.sup.8)=10.sup.-6 sec=1
.mu.s<=T.sub.SYNC. Thus, network synchronization condition
between macro BS1 and femto BS3 is satisfied.
[0044] Second, it can be shown that T.sub.DIFF(BS2,
BS3)=|(t1+.tau.)-t2|. This equation can be further expanded under
two different scenarios. In a first scenario, if (t1+.tau.)>=t2,
then T.sub.DIFF(BS2, BS3)=t1+.tau.-t2<=(t2+.tau.2)+.tau.-t2
(because 0<t1<=t2+.tau.2)=.tau.+.tau.2<=2 .tau. (because
.tau.>=.tau.2)=2*(Femtocell coverage/propagation
speed)<=2.times.150/(3.times.10.sup.8)=10.sup.-6 sec=1
.mu.s<=T.sub.SYNC. In a second scenario, if (t1+.tau.)<t2,
then T.sub.DIFF(BS2,
BS3)=t2-t1-.tau.=(t2-t1-.tau.1)+(.tau.1-.tau.)=T.sub.DIFF(BS1,
BS2)+(.tau.1-.tau.)<=T.sub.SYNC+(.tau.1-.tau.)<=T.sub.SYNC
(because T.sub.DIFF(BS1, BS2)<=T.sub.SYNC, and
.tau.>=.tau.1). Thus, network synchronization condition between
macro BS2 and femto BS3 is also satisfied under both scenarios.
Therefore, the network synchronization conditions are satisfied if
femto BS3 is synchronized with the reference signal transmitted
from its closest macro BS1.
[0045] Having determined the scope of downlink network
synchronization and also determined the first received reference
signal transmitted from its closest macro base station as the
desired reference signal, the femtocell still needs to be able to
detect the actual arrival time of the desired reference signal and
then configure its downlink transmission time based on the arrival
time of the desired reference signal.
[0046] FIG. 13 illustrates a general method of detecting the
arrival time of the desired reference signal from multiple received
reference signals. Reference signals are usually transmitted by
neighboring macro BSs periodically (e.g., every 5 ms for each
frame), and the first received reference signal can thus be
identified by using a predefined observation window. For example,
the observation window can be of the length of smaller than one
half of the reference signal periodicity length (e.g., a half of
frame duration). As illustrated in FIG. 13, because propagation
delay and multipath effect are typically much smaller than a half
of frame duration, the femto BS is able to capture the arrival
times of all the reference signals transmitted by different BSs
within one observation window. Thus, the femto BS is able to
identify and detect the first arrived reference signal as the
desired reference signal within one observation window.
[0047] FIG. 14 illustrates a first embodiment of detecting the
arrival time of a reference signal by a femto BS. The femto BS
comprises an RF module, an A/D converter, a FFT module, a
correlation module coupled to an antenna, a timing detector, a
timing abstractor, and a timing configuration module. In the
example of FIG. 14, only one reference signal is transmitted by a
macro base station without multipath effect. The RF module first
receives the analog reference signal via the antenna in time
domain. The analog reference signal is then digitized by the A/D
converter to a digital signal, which is transferred by the FFT
module into a received sequence at time instance t0 in frequency
domain. The received sequence is then correlated with different
sequence input by the correlation module. Finally, the timing
detector detects the reference signal with a correlated sequence
code at time instance t0. The timing abstractor then selects t0 as
the arrival time of the reference signal.
[0048] FIG. 15 illustrates a second embodiment of detecting the
arrival time of a reference signal with multipath effect. In the
example of FIG. 15, only one reference signal is transmitted by a
macro base station with multipath effect. Thus, the femto BS may
detect the same reference signal received at different time
instance by the timing detector. Based on the inputs from the
timing detector, the timing abstractor then determines the best
timing reference by selecting the first one. The timing
configuration module then uses the timing reference to configure
downlink transmission time for the femto BS to achieve network
synchronization.
[0049] FIG. 16 illustrates a third embodiment of detecting the
arrival time of multiple reference signals with multipath effect.
In the example of FIG. 16, multiple reference signals are
transmitted by multiple macro base stations and each of the
reference signal experiences multipath fading channel. Thus, the
femto BS may detect the different reference signals received at
different time instance by the timing detector. Based on the inputs
from the timing detector, the timing abstractor then determines the
best timing reference by selecting the first one, regardless of
which BS transmits the selected reference signal. The timing
configuration module then uses the timing reference to configure
downlink transmission time for the femto BS to achieve network
synchronization.
[0050] FIG. 17 illustrates a first embodiment of configuring
downlink transmission time by a femto BS. In the example of FIG.
17, the reference signal (e.g., preamble or synchronization signal)
is transmitted at the starting boundary of the downlink frame by a
neighboring BS. The femto BS simply sets its downlink starting
frame boundary to be the same as the timing when the reference
signal is first received by the femto BS.
[0051] FIG. 18 illustrates a second embodiment of configuring
downlink transmission time by a femto BS. In the example of FIG.
18, the reference signal (e.g., mid-amble or post-amble) is not
transmitted at the starting boundary of the downlink frame by a
neighboring BS. In such a case, the femto BS first estimates the
offset between the reference signal and preamble. The femto BS then
sets its downlink starting frame boundary to be the same as the
timing when the reference signal is first received by the femto BS
plus the same offset.
[0052] Although the present invention has been described in
connection with certain specific embodiments for instructional
purposes, the present invention is not limited thereto. For
example, there are different ways to detect the timing of a
received reference signal, using a correlation module to match the
received sequence in frequency domain is just one example.
Accordingly, various modifications, adaptations, and combinations
of various features of the described embodiments can be practiced
without departing from the scope of the invention as set forth in
the claims.
* * * * *