Note: Descriptions are shown in the official language in which they were submitted.
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IMPROVED BEACON SIGNALS FACILITATING SIGNAL DETECTION AND
TIMING SYNCHRONIZATION
FIELD OF THE INVENTION
The present invention relates to methods and apparatus for providing signals
suitable for
identifying transmitter and/or making timing or other adjustments relative to
a transmitter and,
more particularly, to methods and apparatus for generating and using improved
beacon signals.
BACKGROUND
Narrow high powered signals may be transmitted periodically from a base
station
transmitter to allow a mobile device to identify a nearby transmitter and make
various signal
measurements. The signal measurements may be used to determine the relative
strength of
signals received from different transmitters and/or to make mobile
adjustments, e.g., timing
adjustments to facilitate communication with a base station from which a
beacon signal is
received.
In some systems beacon signals are transmitted periodically by each
transmitter in a
system. Normally neighboring transmitters transmit beacon signals at different
times. In most
cases wireless terminal receiving a beacon can identify the transmitter, e.g.,
base station or
sector of a base station, from the frequency, time and/or other beacon signal
related information.
In some known systems beacon signals are transmitted using a single tone
during a single
symbol transmission period with data being transmitted by the transmitter in
the following
symbol period.
The narrowband nature of such beacon signals makes them difficult to use for
timing
synchronization. To facilitate timing synchronization, wideband signals from
the transmitter
would be preferable.
Given that beacon signals tend to be very high power signals they are
relatively easy to
detect even if the receiver is not fully synchronized, in terms of symbol
timing, with the
transmitter. Unfortunately, if timing synchronization with the transmitting
station is not
accurate, the full energy of a beacon signal may not be detected within a
single symbol period.
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This makes measuring the energy of beacons from different
base station transmitters with which timing synchronization
may not exist difficult. In order for a mobile to make
accurate signal strength estimates, it is important that
accurate energy estimation be possible.
In view of the above discussion, it should be
appreciated that there is a need for improved beacon signal
transmission methods. It would be desirable if improved
beacon signaling and/or methods of transmitting and/or using
beacon signals were available which would facilitate both
accurate energy detection and/or facilitate timing
synchronization with the transmitting device, e.g., base
station or base station sector transmitter.
BRIEF DESCRIPTION OF THE FIGURES
Figure 1 is a drawing of an exemplary wireless
communications system implemented in accordance with the
present invention.
Figure 2 shows an example where timing for base
station C and base station D are offset by an amount which
is less than one symbol time period, where each beacon
signal occupies one Orthogonal Frequency Division
Multiplexing (OFDM) symbol time period, and where the
wireless terminal E receiver has been synchronized with
respect to base station C.
Figure 3 shows an example, in accordance with the
present invention, where timing for base station A and base
station B are offset, where each beacon signal occupies two
OFDM symbol time periods, and where an exemplary wireless
terminal receiver has been synchronized with respect to base
station A.
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Figures 4 and 5 illustrate an exemplary OFDM
beacon signal, in accordance with the present invention.
Figures 6 and 7 illustrate an exemplary OFDM
beacon signal/wideband synchronization signal combination in
accordance with the present invention.
Figure 8 illustrates an exemplary wireless
communications system implemented in accordance with the
invention.
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Figure 9 illustrates an exemplary base station, e.g., an access node (router),
implemented
in accordance with the invention.
Figure 10 illustrates an exemplary wireless terminal, e.g., mobile node,
implemented in
accordance with the present invention.
Figure 11 is a flowchart of an exemplary method of operating a base station in
accordance with the present invention.
Figure 12 is a flowchart of an exemplary method of operating a wireless
terminal, e.g.,
mobile node, in accordance with the present invention.
SUMMARY OF THE INVENTION
The present invention is directed to methods and apparatus for generating,
transmitting
and/or using improved narrowband beacon signals. In accordance with the
invention a
narrowband beacon signal is transmitted over a period of time corresponding to
multiple symbol
transmission time periods, e.g., two or more OFDM symbol transmission time
periods. A
beacon signal of the present invention will occupy the same tone for multiple
consecutive
symbol transmission time periods. Beacon signals transmitted in accordance
with the present
invention are transmitted at a high power level. The beacon signals may be
transmitted at a per
tone transmission power level that is 3db, 6db or more above the average per
tone transmission
power level used to transmit user data. In some embodiments the transmitter
energy placed on
the beacon signal includes 60% or more of the total transmitter transmission
power during a time
period in which the beacon signal is transmitted. However, this is not
mandatory and may not
occur in some implementations.
In addition to a beacon signal, a wideband signal, e.g., synchronization
signal, may be
transmitted in conjunction with a beacon signal. Tones in the wideband
synchronization signal
will remain the same for multiple symbol transmission time periods as do the
tones dedicated to
a beacon signal transmitted with the wideband signal.
Wideband signal transmission with a beacon signal is optional and may not
occur in all
cases where a beacon signal is transmitted.
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Tones allocated to the wideband signal will
normally be less than 500 of the tones used by the
transmitter. A relatively large number of tones are often
used as NULL tones when a beacon signal is transmitted.
This allows the power that would otherwise have been placed
on these tones to be allocated to the beacon signal while
also providing tones which can be used by a receiver in
determining signal interference levels since the NULL tones
are predictable and can be used by a receiver for
interference measurements.
A receiver can use the wideband signal for
implementing timing adjustments. It can also use the
wideband signal and measurements of the NULL tones to form a
channel estimate that can be used when communicating with
the base station which transmitted the received beacon
signal.
The beacon signal of the present invention having
a duration of multiple symbol transmission times facilitates
the use of energy detection techniques since the energy of
the signal will be present for more than a single symbol
transmission time period. Thus, a receiver which is not
perfectly synchronized with the transmitter should be able
to measure received signal energy for a period of time in
which a beacon signal is received, e.g., a symbol
transmission time period, without having to be perfectly
synchronized with the transmitter of the beacon signal.
According to one aspect of the present invention,
there is provided a communications method comprising:
operating a first transmitter in a first cell to transmit on
a recurring schedule, for at least two consecutive symbol
time periods, a narrowband beacon signal, said narrowband
beacon signal including at least 60 percent of the power
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transmitted by said first transmitter during said two
consecutive symbol time periods.
According to another aspect of the present
invention, there is provided a first base station
comprising: a first transmitter for transmitting on a
plurality of tones; stored transmission schedule
information; and a first transmitter control module for
controlling said first transmitter to transmit on a
recurring basis in accordance with said stored schedule
information, for at least two consecutive symbol time
periods, a narrowband beacon signal, said narrowband beacon
signal including at least 60 percent of the power
transmitted by said first transmitter during said two
consecutive symbol time periods.
According to still another aspect of the present
invention, there is provided a first base station
comprising: first transmitter means for transmitting on a
plurality of tones; memory means for storing transmission
schedule information; and first transmitter control means
for controlling said first transmitter means to transmit on
a recurring basis in accordance with said transmission
schedule information, for at least two consecutive symbol
time periods, a narrowband beacon signal, said narrowband
beacon signal including at least 60 percent of the power
transmitted by said first transmitter means during said two
consecutive symbol time periods.
According to yet another aspect of the present
invention, there is provided a computer readable medium
having computer executable instructions stored thereon for
execution by one or more computers for use in a first base
station, the computer readable medium comprising:
instructions for causing a first transmitter to transmit on
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. ,. .. . :....... . .. .. . . -. ., _.. . f .. ... . .. .. . ... ... ... . .
. . .
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a plurality of tones; instructions for causing the first
base station to store transmission schedule information; and
instructions for causing the first base station to control
said first transmitter to transmit on a recurring basis in
accordance with said transmission schedule information, for
at least two consecutive symbol time periods, a narrowband
beacon signal, said narrowband beacon signal including at
least 60 percent of the power transmitted by said first
transmitter during said two consecutive symbol time periods.
Numerous additional features, benefits and
embodiments of the invention are discussed and described in
the detailed description which follows.
DETAILED DESCRIPTION OF THE INVENTION
Figure 1 is a drawing of an exemplary wireless
communications system 100 implemented in accordance with the
present invention, the exemplary system 100 including two'
adjacent base stations, base station A (BS A) 102 and base
statibn B (BS B) 104. Cell A 106 represents the wireless
coverage area of BS A 102, while cell B 108 represents the
wireless coverage area of BS B 104. Wireless terminals
(WTs), e.g., mobile nodes, may move through the cells of the
system, and may communicate with peer nodes, e.g., other WTs
through the base stations. Exemplary WT 110, implemented in
accordance with the present invention, shown in Figure 1 is
currently using BS A 102 as its point of network attachment
and communicates with
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BS A 102 through wireless communication link 112. Each base station, (BS A
102, BS B 104),
transmits, e.g., periodically, a beacon signal, e.g., a relatively short
duration high power OFDM
signal with the base station transmission power concentrated primarily on one
or a few tones.
Base station A 102 transmits beacon signal 114, while base station B 104
transmits beacon
signal 116. The beacons for different base stations are normally transmitted
at different times.
The WTs, e.g. WT 110, monitors for and process the beacon signals from
multiple, e.g., adjacent
BSs.
In Figure 1, the exemplary WT's 110 point of attachment is BS A 102, and the
WT 110 is
communicating, e.g., as an active user receiving downlink traffic channel
data/information and
transmitting uplink traffic channel data/information, through BS A 102. The WT
110 is time
synchronized with respect to the timing cycle, e.g., OFDM symbol timing and
repetitive timing
structure upon which BS A 102 is operating. The WT 110 may or may not be
synchronized with
respect to the BS B 104 timing. In general, the BS A 102 and BS B 104 timing
cycles are not
synchronized, and the WT 110 in cell A 106, using BS A 102 as its current
point of network
attachment, will not be time aligned with respect to base station B 104.
Figure 2 shows an example where timing for BS C and BS D are offset by an
amount
which is less than one symbol time period, where each beacon signal occupies
one OFDM
symbol time period, and where the WT E receiver has been synchronized with
respect to BS C.
A symbol time period is the time used in the system to transmit a modulation
symbol. Multiple
modulation symbols may be transmitted in parallel using different tones during
a single symbol
time period, the combination of modulation symbols transmitted in a single
OFDM symbol
transmission time period is sometimes referred to as an OFDM symbol. The
single symbol time
period is sometimes called a symbol period or a symbol transmission time
period or an OFDM
symbol transmission time period. The first drawing 202 shows an exemplary BS C
transmitted
beacon signa1204 with respect to time 206, where each shown slot (208, 210,
212, 214, 216,
218, 220, 222, 224, 226, 228) represents one OFDM symbol transmission time
period. The
second drawing 242 shows an exemplary BS D transmitted beacon signal 244 with
respect to
time 206, where each shown slot (248, 250, 252, 254, 256, 258, 260, 262, 264,
266, 268)
represents one OFDM symbol transmission time period. Note that there is a
symbol timing
difference 270, e.g., an offset, between each BS C OFDM symbol timing slot and
each BS D
OFDM symbol timing slot. The third drawing 272 shows the WT E receiver beacon
signal
reception vs time 274. An FFT is used in a receiver to recover the symbols
transmitted on
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different tones during each symbol time. The WT E, as shown, has been
synchronized with
respect to the BS C; therefore the BS C beacon signal 276 is captured in its
entirety within one
FFT window 278 of the receiver. However, the BS D beacon signa1280, being out
of sync with
respect to the WT E receiver, is captured in portions over two successive FFT
windows (282,
284) of the receiver. The processing involved to reconstruct beacon signal D
from the
component FFT pieces and to obtain an accurate representation of beacon signal
D can be a
complex operation. The received energy of a beacon is used, e.g., to determine
which BS has
the stronger received signal.
In accordance with the invention, an OFDM beacon signal is generated and used
which
has a duration of at least 2 OFDM symbol transmission time periods. This
approach simplifies
the detection operation by the WT receiver, e.g., WT 110 receiver. The WT's
receiver FFT
window timing need not be synchronized to a base station. During at least one
FFT window, the
receiver should capture a clean symbol of the beacon signal. During that at
least one FFT
window, the receiver should observe a peak at the frequency of the beacon
signal. The WT 110
can measure the energy content of the beacon signal during that window, and
obtain an accurate
representation of the received beacon signal energy in one symbol period.
Beacon energy for
one symbol period can thus be compared in a reliable fashion.
Figure 3 shows an example where timing for BS A 102 and BS B 104 are offset,
where
each beacon signal occupies two OFDM symbol time, and where the WT 110
receiver has been
synchronized with respect to BS A 102. The first drawing 302 shows an
exemplary BS A
transmitted beacon signa1304 with respect to time 306, where each shown slot
(308, 310, 312,
314, 316, 318, 320, 322, 324, 326, 328) represents one OFDM symbol
transmission time period.
The second drawing 342 shows an exemplary BS B transmitted beacon signal 344
with respect
to time 306, where each shown slot (348, 350, 352, 354, 356, 358, 360, 362,
364, 366, 368)
represents one OFDM symbol transmission time period. Note that there is a
symbol timing
difference 370, e.g., an offset, between each BS A OFDM symbol timing slot and
each BS B
OFDM symbol timing slot. The third drawing 372 shows the WT receiver beacon
signal
reception vs time 374. The WT 110, as shown, has been synchronized with
respect to the BS A
102; therefore the BS A beacon signa1376 is captured in its entirety within
two FFT windows
(378, 380) of the receiver. However, the BS B beacon signal 381, being out of
sync with respect
to the WT receiver, is captured in portions over three successive FFT windows
(382, 384, 386)
of the receiver. In accordance with the invention, the WT's receiver detects
that the energy
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content of beacon signal B pealcs during the second of those three successive
OFDM FFT
windows, and therefore recognizes that the measured energy during the second
FFT window 384
is an accurate representation of received beacon signal B 381.
Figures 4 and 5 illustrate an exemplary OFDM beacon signal, in accordance with
the
present invention. Figure 4 is a drawing 400 of frequency on the vertical axis
402 vs time on the
horizontal axis 404. The available bandwidth 406, e.g., for the exemplary
communications
band, covers the range from frequency fo 408 to frequency f2 410. For example,
available
bandwidth 406 may correspond to a downlink tone block being used by the base
station, e.g., a
tone block of 113 evenly spaced contiguous tones. The exemplary beacon signal
412, e.g., a
single tone, is at frequency f1414, and has a duration of 2 OFDM symbol
transmission time
periods 416. Figure 5 is a drawing 500 of power on vertical axis 502 vs
frequency on horizontal
axis 504 during the time that the beacon signal 412 is transmitted. The
transmitter transmission
power is concentrated on the beacon signal 412 at frequency f1414. With the
beacon signal 412
of Figures 4 and 5, the beacon signal 414 can be easily detected by the WT
receiver, e.g., WT
110 receiver, and identified. When the WT detects and identifies the beacon
signal, associating
it with a base station, e.g., BS A 102 or BS B 104, the WT can figure out and
know the
approximate access time, e.g., for establishing communications with that base
station. However,
it would be beneficial if the WT could get more accurate timing information to
synchronize and
communicate with the base station than is available simply from the beacon.
The beacon signal
having a very small bandwidth is not as good a candidate from which to obtain
accurate timing
information as is a signal with a wide bandwidth. In accordance with a feature
of some
embodiments of the invention, the BS transmits in conjunction with the narrow
band high power
beacon signal, a wideband low power synchronization signal, which the WT may
use to
synchronize with the BS. The wideband signal in some embodiments has a
bandwidth at least 5
times wider than the beacon signal. In some embodiments, the wideband signal
includes at least
10 times, and in other embodiments at least 20 times the bandwidth of the
beacon signal. For
example, while the beacon signal is of one frequency tone, the wideband
synchronization signal
can have at least 10 or 20 tones. Those tones are not necessarily contiguous
in frequency.
Indeed, they may spread over a wide frequency range and leave some tones in-
between not
transmitted. The wideband synchronization signal is transmitted in the same
time interval as the
beacon signal. For example, if the beacon signal is transmitted in 2 OFDM
symbol periods, then
the wideband synchronization signal is transmitted in the same 2 OFDM symbol
periods. While
many times wider in terms of frequency than the beacon, the total transmitted
power of the
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wideband signal, excluding the beacon, is less than half the power of the
beacon signal. For
example, less than 40% total transmitted power may be assigned to the wideband
signal with the
beacon signal receiving at least 60% power.
Figures 6 and 7 illustrate an exemplary OFDM beacon signal 612 / wideband
synchronization signal 613 combination in accordance with the present
invention. Figure 6 is a
drawing 600 of frequency on vertical axis 602 vs time on horizontal axis 604.
The available
bandwidth 614, e.g., for the exemplary communications band, covers the range
from frequency
fo 608 to frequency f2 610. The exemplary beacon signal 612, e.g., a single
tone, is at frequency
f1614, and has a duration of 2 OFDM symbol transmission time periods 616. The
exemplary
wideband synchronization signal 613 may occupy a significant portion of the
frequency band
from fo 608 to f2 610 exclusive of the beacon signal tone or tones.
Preferably, the exemplary
wideband synchronization signal 613 is a multi-tone signal including multiple
tones transmitted
simultaneously. The number of tones is at least 10 or 20. In some case, the
number of tones can
be between 50 and 60, e.g., 56. The number of tones is preferably close to the
half of the total
number of tones. Note that those tones in the exemplary wideband
synchronization signal are not
necessarily contiguous. For example, suppose that all the available tones are
indexed as 0, 1, 2,
..., N-1, where N is the total number of tones. For example, N=1 13. Each tone
corresponds to a
tone frequency. Then the exemplary wideband synchronization signal may
includes tones 5, 6,
10, 11, 13, 15, 17, 20, 23, 30, 33, 42, 50, 59, 60, 67, 68, 74, 78, 80, 84,
92, 95, and 101, in which
case, the signal occupies the bandwidth from tone 5 to tone 101, but in-
between many tones,
e.g., tone 7, 8, 9, etc., are not transmitted.
Figure 7 is a drawing 700 of power on vertical axis 702 vs frequency on
horizontal axis
704 during the time that the beacon signal 612 and wideband synchronization
signal 613 are
transmitted. The base station transmitter transmission power is concentrated
on the high power
beacon signal 612 at frequency f1614; however, the wideband synchronization
signal 613 is
transmitted in parallel at a much lower power level. With the broadcast signal
of Figures 6 and
7, the beacon signal component 612 can be easily detected by the WT receiver,
e.g., WT 110
receiver, and identified, while the wideband synchronization signal 613 allows
for timing
synchronization to be accomplished by the WT so that the WT can communicate
with the
identified BS at the appropriate access time.
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Figure 8 illustrates an exemplary wireless communications system 10
implemented in
accordance with the invention. Exemplary wireless communications system TO is,
e.g., an
OFDM spread spectrum multiple access wireless communications system. Exemplary
system
includes a plurality of cells (cell 1 11, cell M 11'). Each cell (cell 1 11,
cell M 11') represents
5 the wireless coverage area for a base station (base station 1 12, base
station M 12'), respectively.
The base stations (12, 12') are coupled to a network node 21 via links (17,
17'), respectively.
Networlc node 21, e.g., a router, is coupled to the Internet and other network
nodes. In system
10, multiple mobile wireless terminals, shown as mobile nodes MN 1 (14)
through MN N (16)
communicate with the base station 12 in cell 1 11 through the use of
communication signals 13,
10 15 via wireless links. Each mobile wireless terminal may correspond to a
different mobile user
and are therefore sometimes referred to as user terminals. The signals 13, 15
may be, e.g.,
OFDM signals. The base station 12 and mobile stations 14, 16 each implement
the method of
the present invention. Thus, signals 13, 15 include signals of the type
discussed above, which
are transmitted in accordance with the invention. Similarly, in system 10,
multiple mobile
wireless terminals, shown as mobile nodes MN 1' (14') through MN N' (16')
communicate with
the base station 12' in cell M 11' through the use of communication signals
13', 15' via wireless
links. Each mobile wireless terminal may correspond to a different mobile user
and are
therefore sometimes referred to as user terminals. The signals 13', 15' may
be, e.g., OFDM
signals. The base station 12' and mobile stations 14', 16' each implement the
method of the
present invention. Thus, signals 13', 15' include signals of the type
discussed above, which are
transmitted in accordance with the invention.
Each base station (12, 12') transmits beacon signals (19, 19'), in accordance
with the
invention. Beacon signals 19, 19' may be received and processed by mobile
nodes within the
transmitting base stations cell and by mobile nodes within other, e.g.,
adjacent, cells within the
system. For example, beacon signal 19 may be received and processed by MNs 14,
16, 14', and
16'. In some embodiments of the invention, wideband synchronization signals
(20, 20') are
communicated at the same time as the beacon signals (19, 19'). For example,
with respect to
base station 1 12, in some embodiments, wideband synchronization signal 20 is
transmitted in
parallel with beacon signal 19. Similarly, with respect to BS M 12', in some
embodiments,
wideband synchronization signa120' is transmitted in parallel with beacon
signal 19'. These
wideband signals (20, 20') like the beacon signals (19, 19') will be detected.
The beacon signals
(19, 19') are used for power measurements and to identify the base station
which is the source of
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the signal, while the wideband portion ot the signal (20, 20') is used by a
receiving WT to
implement timing adjustments relative to the BS which transmitted the received
beacon.
Fig. 9 illustrates an exemplary base station 3000, e.g., an access node
(router),
implemented in accordance with the invention. Exemplary base station 3000 may
be any of the
exemplary base stations implemented in accordance with the present invention,
e.g., base station
A 102 of Figure 1, base station B 104 of Figure 1, base station 1 12 of Figure
8, or base station
M 12' of Figure 8. The base station 3000 includes antennas 2203, 2205 and
receiver/transmitter
modules 2202, 2204. The receiver module 2202 includes a decoder 2233 for
decoding received
uplink signals from wireless terminals, while the transmitter module 2204
includes an encoder
2235 for encoding downlink signals to be transmitted to wireless terminals.
The modules 2202,
2204 are coupled by a bus 2230 to an I/O interface 2208, processor (e.g., CPU)
2206 and
memory 2210. The IIO interface 2208 couples the base station 3000 to the
Internet and/or to
other network nodes, e.g., other base stations. The memory 2210 includes
routine 2221 and
data/information 2212. The processor 2206, e.g., a CPU, executes the routines
2211 and uses
the data/information 2212 in memory 2210 to control the operation of the base
station 3000 and
implement methods of the present invention. The memory 2210 includes routines
2211, which
when executed by the processor 2206, cause the base station 3000 to operate,
e.g., transmit
beacon and associated wideband signals, in accordance with the invention.
Routines 2211
includes communications routines 2223 used for controlling the base station
3000 to perform
various communications operations and implement various communications
protocols. The
routines 2211 also include a base station control routine 2225 used to control
the base station
3000 to implement the steps of the method of the present invention. The base
station control
routine 2225 includes a scheduling module 2222 used to control transmission
scheduling and/or
communication resource allocation. Thus, module 2222 may serve as a scheduler,
e.g.,
assigning uplink and downlink channel segments to wireless terminals using the
base station
3000 as their current point of network attachment. Base station control
routine 2225 also
includes a transmitter control module 2223, a beacon signaling module 2224,
and a wideband
synchronization signal generating module 2226. The transmitter control module
2223 controls
the transmitter 2204 to transmit on a recurring basis in accordance with
stored transmission
schedule information 2232, for two consecutive times OFDM symbol transmission
time periods,
a narrowband beacon signal, the narrowband beacon signal including at least
60% of the power
transmitted by the transmitter 2204 during said two consecutive OFDM symbol
transmission
time periods. The transmitter control module 2223 includes a transmission
power control
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module 2225. In some embodiments, the transmission power control module 2225
controls the
transmitter 2204 to supply at least 80% of the transmitter transmission power,
used during the
two consecutive symbol time periods in which a beacon signal is transmitted,
to the beacon
signal. Transmitter control module 2223 also controls the transmission of
generated wideband
synchronization signals, e.g., in parallel with the narrowband beacon signals.
Beacon signal
module 2224 generates beacon signals in accordance with the invention, e.g.,
having a high
concentration of power on a single tone and having a duration of at least two
OFDM symbol
transmission time periods, the same physical tone being used for the beacon
for the at least two
OFDM symbol transmission time periods. Wideband synchronization signal
generating module
2226 generating wideband synchronization signals in accordance with the
invention, e.g., using
less than 40% of the power transmitted during the time interval of the
wideband synchronization
signal and using at least 30% of the tones in the downlink tone block being
used by the
transmitter 2204. In some embodiment, the wideband synchronization signal uses
a plurality of
physical tones, said plurality of physical tones including the same physical
tones during each of
two consecutive symbol transmission time periods. In some embodiment, the
downlink tone
block comprises a set of contiguous evenly spaced tones 113 tones. In some
such embodiments,
the wideband synchronization signal includes at least 50 of the 113 tones. In
some
embodiments, the beacon signal and the wideband synchronization signal occupy
two
consecutive symbol transmission time periods, the same two consecutive symbol
transmission
time periods.
Memory 2210 also includes data/information 2212 used by communications
routines
2223, and control routines 2225. The data/information 2212 includes an entry
for each active
mobile station user 2213, 2213' which lists the active sessions being
conducted by the user and
includes information identifying the mobile station (MT) being used by a user
to conduct the
sessions, and information, e.g., user data related to the session.
Data/information 2212 also
includes beacon signal information 2228, e.g., tone information, power
information, time
duration information, e.g., two successive OFDM symbol time periods, time
position within a
recurring downlink timing structure, etc., associated with beacons to be
transmitted by BS 3000.
Wideband synchronization signal information 2230, e.g., tone information,
power level
information, time duration information, time position within a recurring
downlink timing
structure, e.g., in parallel with the beacon signal, etc., associated with
wideband synchronization
signals to be transmitted by BS 3000, is also included as part of
data/information 2212.
Data/information 2212 also includes stored transmission schedule information
2232, e.g., a
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recurring transmission schedule including information identifying where in the
schedule the
beacon and wideband synchronization signals should be transmitted, and stored
frequency
structure information 2234, e.g., information identifying the downlink and
uplink carrier
frequencies used by the base station, the number of tones in a tone block,
e.g., 113, and channel
segment structure information in relation to the tones of the tone block.
Servers and/or host devices may be implemented using circuitry which is the
same as, or
similar to, the circuitry of the exemplary access router shown in Fig. 9 but
with interfaces and/or
control routines suited to the particular server/host device's requirements.
The control routines
and/or hardware in such servers and/or hosts cause the devices to implement
the above described
methods.
Figure 10 illustrates an exemplary wireless terminal 4000, e.g., mobile node,
implemented in accordance with the present invention. Exemplary wireless
terminal 4000 may
be any of the exemplary wireless terminal implemented in accordance with the
present
invention, e.g. WT 110 of Figure 1, MN 1 14, MN N 16, MN 1' 14', or MN N' 16'
of Figure 8.
The mobile node 4000 may be used as a mobile terminal (MT). The wireless
termina14000
includes a receiver 2302, a transmitter 2304, a processor 2306, user I/O
devices 2307, and a
memory 2310 coupled together via a bus 2311 over which the various elements
can interchange
data and information.
The wireless termina14000 includes receiver and transmitter antennas 2303,
2305 which
are coupled to receiver and transmitter modules 2302, 2304 respectively. The
wireless terminal
receiver 2303 receives downlink signals including beacon signals and wideband
timing
synchronization signals via antenna 2302. In some embodiments a single antenna
is used for
receiver and transmitter, e.g., in combination with a duplex module. The
receiver module 2302
includes a decoder 2333, while the transmitter module 2304 includes an encoder
2335. User I/O
devices 2307, e.g., microphone, keypad, lceyboard, camera, mouse, switches,
speaker, display,
etc., allow the user of WT 4000 to input user data, output user data, control
applications, and
control at least some operations of the wireless terminal, e.g., initiate a
communications session.
Memory 2310 includes routines 2321 and data/information 2362. Processor 2306,
e.g., a
CPU, under control of one or more routines 2321 stored in memory 2310 uses the
data/information 2362 to cause the wireless terminal 4000 to operate in
accordance with the
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methods of the present invention. In order to control wireless terminal
operation, routines 2321
includes communications routine 2323, and wireless terminal control routines
2325. The
communications routine 2323 implements various communications protocols used
by the
wireless terminal 4000. The wireless terminal control routines 2325 are
responsible for insuring
that the wireless terminal operates in accordance with the methods of the
present invention.
Wireless terminal control routines 2325 include a beacon signal detection
module 2327, a
beacon signal measurement and evaluation module 2329, a wideband
synchronization signal
evaluation module 2331, a channel estimation module 2354, and a handoff
control module 2355.
Beacon signal detection module 2327 is used for detecting and identifying
beacon signals from a
plurality of cells and or sector base station transmitters. Beacon signal
measurement and
evaluation module 2329 measures the energy level and/or strength of the
received beacon
signals and evaluates beacon signals with respect to other received beacon
signals. Wideband
synchronization signal evaluation module 2331 processes received wideband
synchronization
signals and determines synchronization timing from the signals, e.g., used in
establishing
communications with a different base station as the mobile node's attachment
point. Wideband
synchronization signal evaluation module 2331 processes a received wideband
synchronization
signal to produce a timing adjustment control signal. Channel estimation
module 2354 performs
a channel estimate based on the received wideband synchronization signal and
Null tones
included in the wideband signal. Handoff control module 2355 is used for
changing attachment
points, e.g., from one base station to another base station, and the handoff
control module 2355
controls the adjustment of transmitter 2304 timing at the appropriate time in
the handoff process
using information supplied by the wideband signal evaluation module 2331. In
addition, the
handoff control module 2355 uses the channel estimate based on the wideband
signal 2351 to
initialize another channel estimate 2352 that is to be used when attaching to
the point from
which the wideband signal used to generate the channel estimate was
transmitted.
Data/information 2362 includes user/device/session /resource information 2312,
e.g.,
user information, device information, WT 4000 state information, peer node
info, addressing
information, routing information, session parameters, air link resource
information such as
information identifying uplink and downlink channel segments assigned to WT
4000.
User/device/session/resource information 2312 may be accessed and used to
implement the
methods of the present invention and/or data structures used to implement the
invention.
Data/information 2362 also includes system data/information 2333 which
includes a plurality of
sets of system base station information (BS 1 data/information 2360, ..., BS N
data/information
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2361). BS 1 data/information 2360 includes beacon information 2335,
synchronization signal
information 2337, timing information 2339, and frequency information 2341.
Data/information
2362 also includes a terminal ID 2343, e.g., a BS assigned identifier, timing
information 2345,
e.g., pertaining to the current point of attachment and also pertaining to
other base stations, base
station identification information 2347, e.g., the ID of the current
attachment point and the ID of
each BS associated with a received beacon signal. Data/information 2362 also
includes data
2349, e.g., user data such as voice data, image data, audio data, text data,
file data, etc., received
from and to be transmitted to a peer node of WT 4000 in a communications
session with WT
4000.
Data/information 2362 also includes timing adjustment control signal
information 2350,
channel estimate based on wideband signal/Null tones 2351, and channel
estimate for new
attachment point 2352. Timing adjustment control signal information 2350 is an
output of the
wideband signal evaluation module 2331 and is used as an input by the handoff
control module
2355. Channel estimate based on wideband signal/Null tones 2351 is an output
of the channel
estimation module 2354 and is used an input to the handoff control module
2355, which uses
channel estimate 2351 to initialization of another channel estimate, channel
estimate for new
attachment point 2352.
Figure 11 is a flowchart 1100 of an exemplary method of operating a base
station, e.g.,
exemplary base station 3000 of Figure 9, in accordance with the present
invention. The
exemplary method is started in step 1102, where the base station is powered on
and initialized.
Operation proceeds from start step 1102 to steps 1104 and step 1110. In step
1104, the base
station is operated to maintain current time index in a recurring transmission
structure being
used by the base station. Current time index 1106 is output from step 1104.
Step 1104 is
performed on an ongoing basis during base station operation. In step 1110, the
base station
compares the current time index 1106 to stored transmission schedule
information 1108. In step
1112, the base station proceeds based upon the result of the comparison. If
the comparison
indicates that a beacon signal should be transmitted operation proceeds to
step 1116; otherwise
operation proceeds to step 1114.
In step 1114, the base station is operated to transmit non-beacon signals,
e.g., an OFDM
symbol signal that does not include a beacon signal. Operation proceeds from
step 1114 via
connecting node A 1122 to step 1110.
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In step 1116, the base station is operated to transmit a narrowband beacon
signal and a
wideband synchronization signal in parallel. Step 1116 includes sub-steps
1118, 1120, and 1122
which are performed in parallel. In sub-step 1118, the base station operates
its transmitter to
transmit the beacon signal occupying one tone for two consecutive symbol
transmission time
periods at a higher power than any non-beacon signal transmitted during the
two consecutive
symbol time periods. In some embodiments, the narrowband beacon signal
corresponds to less
than 2% of the downlink tones used by the transmitter during and between at
least one
occurrence of the recurring beacon signal transmission time period. In sub-
step 1120, the base
station operates its transmitter to transmit null values on more than 40% of
the tones in the
downlink tone block being used by the transmitter. In some embodiment, in sub-
step 1120, the
base station operates its transmitter to transmit null tones on more than 50%
of the total number
of downlink tones in a downlink tone block corresponding to the base station
transmitter and
including the tone on which the single high power beacon tone is transmitted,
e.g., 57 Null tones
out of a downlink tone block of 113 tones. In sub-step 1122, the base station
operates its
transmitter to transmit the wideband synchronization signal including at least
50 non-zero signal
values, each non-zero signal value being transmitted on a different one of the
tones in the
downlink tone block. Operation proceeds from step 1116 via connecting node A
1122 to step
1110.
In some embodiments, the recurring transmission schedule is such that the
transmitter
will transmit signals for at least 50 symbol transmission time periods between
each of a
recurring beacon signal. In some embodiments, a narrowband beacon signal is
transmitted, with
a duration of two consecutive OFDM symbol transmission time periods, by a base
station sector
transmitter corresponding to a downlink tone block once for every beaconslot,
e.g., where a
beaconslot is 892 successive OFDM symbol transmission time periods in a
recurring
transmission schedule.
The flowchart 1100 of Figure 11 describes an exemplary method of operating a
base
station in accordance with the invention. The method of flowchart 1100 is
applicable to various
configurations including: a base station transmitter which covers an entire
cell acting as an
attachment point corresponding to the base station, a base station transmitter
which corresponds
to a base station sector acting as an attachment point corresponding to the
base station sector, a
base station cell transmitter associated with a downlink carrier and/or
downlink tone block
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acting as an attachment point corresponding to the cell and tone block/carrier
combination, and a
base station sector transmitter associated with a downlink carrier and/or
downlink tone block
acting as an attachment point corresponding to the base station sector and
tone block/carrier
combination.
An exemplary wireless communications system, in accordance with the present
invention
may include a plurality of base station transmitters each acting in accordance
with the methods
of the present invention. For example, a first transmitter in a first cell is
operated to transmit on
a recurring schedule, for at least two consecutive time periods, a narrowband
beacon signal
including at least 60% of the power transmitted by the first transmitter
during the two
consecutive time periods, and a second base station transmitter, located
adjacent the first
transmitter, is operated to transmit, for at least two consecutive time
periods a narrowband
beacon signal, said narrowband beacon signal including at least 60% of the
power transmitted by
said second transmitter during the two consecutive time periods. In some
embodiments, the first
and second transmitters are located in adjacent cells of a communications
system and the first
and second transmitters transmit beacon signals during different non-
overlapping time periods.
In various embodiments, the first transmitter is operated to transmit a
wideband signal during at
least one of the two consecutive time periods corresponding the beacon signal
from the first
transmitter. In some such embodiments, the wideband signal has the same
duration as the
beacon signal. In some embodiments, the wideband signal and the beacon signal
occupy two
consecutive symbol transmission time periods. In some embodiments, the beacon
signal uses a
single physical tone which is the same for each of the two consecutive time
periods of the
beacon signal transmission. In some embodiments, the wideband signal uses a
plurality of
physical tones, said plurality including the same physical tones during each
of said at least two
consecutive time periods. In various embodiments, the wideband signal uses at
least 30% of the
tones used by the first transmitter to transmit symbols in a symbol
transmission time period
immediately following the said at least two consecutive symbol time periods of
the beacon
signal transmission. In some embodiments at least 50 tones are used for the
wideband signal out
of a downlink tone block of 113 tones.
In various embodiments, the beacon signal uses at least 80% of the transmitter
power
during said at least two consecutive symbol time periods of the beacon
transmission interval. In
some embodiments, the wideband signal uses 20% or less of the transmitter
power during one of
said at least two consecutive symbol time periods of the beacon transmission
interval. In
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various embodiments, the wideband signal is at least 5 times wider than the
narrowband beacon
signal in terms of frequency width. In various embodiments, the wideband
signal is at least 10
times wider than the narrowband beacon signal in terms of frequency width. In
various
embodiments, the wideband signal is at least 20 times wider than the
narrowband beacon signal
in terms of frequency width.
In some embodiments, the beacon signal is less than 3 tone wide. In some such
embodiments, the beacons signal is a single tone wide and the transmitter
transmits using a
downlink tone block of at least 100 tones, e.g. 113 tones. In some
embodiments, the transmitter
is an OFDM transmitter and the symbol time is the time used to transmit a
single OFDM
symbol.
Figure 12 is a flowchart 1200 of an exemplary method of operating a wireless
terminal,
e.g., mobile node, in accordance with the present invention. The exemplary
wireless terminal is,
e.g., wireless terminal 4000 of Figure 10. The exemplary method starts in step
1202, where the
wireless terminal is powered on and initialized. Operation proceeds from start
step 1202 to steps
1204 and 1206. In step 1204, the wireless terminal is operated to receive
beacon signals, e.g.,
single tone beacon signals, and wideband signals, e.g., wideband
synchronization signals,
transmitted by a first base station transmitter in parallel. In step 1206, the
wireless terminal is
operated to receive beacon signals and wideband signals transmitted by a
second base station
transmitter in parallel. Operation proceeds from step 1204 to steps 1208 and
1210. Operation
proceeds from step 1206 to steps 1212 and 1214.
In step 1210, the wireless terminal measures the amount of received energy in
a first
beacon signal received from the first base station transmitter during a first
measurement time
interval in which the first beacon signal is received from the first
transmitter for the full duration
of the first measurement time interval to produce a first signal energy value,
measured energy 1
1220. In step 1212, the wireless terminal measures the amount of received
energy in a second
beacon signal received from the second base station transmitter during a
second measurement
time interval in which the second beacon signal is received from the second
transmitter for the
full duration of the second measurement time interval to produce a second
signal energy value,
measured energy 2 1224.
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In step 1208, the wireless terminal determines a transmitter timing
adjustment, timing
adjustment 1 1218, based on the received wideband signal from the first base
station transmitter.
Operation proceeds from step 1208 to step 1216. In step 1216, the wireless
terminal performs a
channel estimate operation on the received wideband signal from the first base
station
transmitter, obtaining channel estimate 1 1232.
In step 1214, the wireless terminal determines a transmitter timing
adjustment, timing
adjustment 2 1226, based on the received wideband signal from the second base
station
transmitter. Operation proceeds from step 1214 to step 1228. In step 1228, the
wireless
terminal performs a channel estimate operation on the received wideband signal
from the second
base station transmitter, obtaining channel estimate 2 1234.
Operation proceeds from steps 1210 and 1212 to step 1222, where the wireless
terminal
compares the first and second measured signal energy values (1220, 1224).
Operation proceeds
from step 1222 to step 1230. In step 1230, the wireless terminal selects an
attachment point
corresponding to the first base station transmitter or the second base station
transmitter based on
the result of the comparison of the first and second energy values. Operation
proceeds from step
1230 to step 1236. In step 1236, the wireless terminal determines if the
selected attachment
point of step 1230, is an attachment point with which the WT currently has
timing
synchronization, e.g., closed loop timing synchronization. If the selected
attachment point is an
attachment point at which the WT does not have timing synchronization,
operation proceeds to
step 1238; otherwise operation proceeds via connecting node A 1242 to steps
1204 and 1206.
In step 1238, the wireless terminal uses the channel estimation operation
result based on
the received wideband signal corresponding to the selected attachment point,
channel estimate 1
1232 or channel estimate 2 1234, to initialize another channel estimate, e.g.,
the channel
estimate used for subsequent non-beacon downlink signals. Operation proceeds
from step 1238
to step 1240. In step 1240, the wireless terminal malees a transmitter timing
signal adjustment
using the determined timing adjustment based on the received wideband signal
corresponding to
the selected attachment point, timing adjustment 11218 or timing adjustment 2
1226. Operation
proceeds from step 1240 via connecting node A 1242 to step 1204 and 1206 to
receive
additional beacon signals.
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In some embodiments, the first and second measurement time intervals are
different. In
some such embodiments, the first and second measurement time intervals are non-
overlapping
with each other. In some embodiments, the wideband signal includes multiples
tones spaced
over a frequency band at least 15 tones wide.
In some embodiments, the steps of determining a transmitter timing adjustment
and/or
performing a channel estimation operation based on a received wideband signal,
are performed
for a given attachment point when a selection has been made to use that
attachment point and
that selected attachment point corresponds to a new attachment point or a
handoff; however, the
steps of determining a transmitter timing adjustment and/or performing a
channel estimation
operation based on a received wideband signal are not performed for a given
attachment point
when a selection has been made not to use that attachment point or when that
attachment point is
an attachment point currently in use having an ongoing channel estimate and
being close loop
timing synchronized, e.g., the current in use active link attachment point.
In some embodiments the wireless terminal receives downlink signals in a
downlink tone
block, e.g., 113 contiguous evenly spaced tones, corresponding to a
transmitter. In some such
embodiments, the wideband signal includes at least 30% of the tones of the
downlink tone block.
In some embodiments, the wideband signal includes at least 50 tones
communicating a non-zero
value. In some embodiments, the beacon tone has been transmitted using at
least 60% of power
transmitted by the transmitter during an interval in which a beacon is
transmitted, while the
wideband signal during the same interval has been transmitted using less than
or equal to 40% of
the power transmitted by the transmitter during the interval in which a beacon
is transmitted.
In some embodiments, the first and second base station transmitter correspond
to
different base stations located at different location. In some embodiment, the
first and second
base station transmitters correspond to different base station sector
transmitters of the same base
station. In some embodiments, the first and second base station transmitters
correspond to
different downlink tone blocks and/or carriers. In some embodiments, the first
and second base
station transmitters correspond to different tone blocks and/or carriers of
the same sector of the
same base station.
In some embodiments, the base station transmitters transmit intentional nulls
on at least
some of tone block tones during the beacon/wideband signaling transmission
time periods.
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In some embodiments of the invention the beacon signal rides on top of one of
the tones
used to transmit the wideband signal during the same symbol time as the beacon
signal. In such
an implementation the wideband signal may occupy the same tone as the beacon
signal. In other
embodiments the beacon and wideband signal do not use the same tone. The
wideband signal
need not occupy each tone in the band over which the signal is spread but may
be implemented
using a plurality of spaced tones. The spacing of the wideband signal tones
may be preselected
and thus know to wireless terminals.
The techniques of the present invention may be implemented using software,
hardware
and/or a combination of software and hardware. The present invention is
directed to apparatus,
e.g., mobile nodes such as mobile terminals, base stations, communications
system which
implement the present invention. It is also directed to methods, e.g., method
of controlling
and/or operating mobile nodes, base stations and/or communications systems,
e.g., hosts, in
accordance with the present invention. The present invention is also directed
to machine
readable medium, e.g., ROM, RAM, CDs, hard discs, etc., which include machine
readable
instructions for controlling a machine to implement one or more steps in
accordance with the
present invention.
In various embodiments nodes described herein are implemented using one or
more
modules to perform the steps corresponding to one or more methods of the
present invention, for
example, signal processing, message generation and/or transmission steps.
Thus, in some
embodiments various features of the present invention are implemented using
modules. Such
modules may be implemented using software, hardware or a combination of
software and
hardware. Many of the above described methods or method steps can be
implemented using
machine executable instructions, such as software, included in a machine
readable medium such
as a memory device, e.g., RAM, floppy disk, etc. to control a machine, e.g.,
general purpose
computer with or without additional hardware, to implement all or portions of
the above
described methods, e.g., in one or more nodes. Accordingly, among other
things, the present
invention is directed to a machine-readable medium including machine
executable instructions
for causing a machine, e.g., processor and associated hardware, to perform one
or more of the
steps of the above-described method(s)
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Whi1e ctescrlbeca in the context of an OFDM system, at least some of the
methods and
apparatus of the present invention, are applicable to a wide range of
communications systems
including many other frequency division multiplexed systems and non-OFDM
and/or non-
cellular systems. Many of the methods and apparatus of the present invention
are also
applicable in the context of a multi-sector multi-cell wireless communications
system.
Numerous additional variations on the methods and apparatus of the present
invention
described above will be apparent to those skilled in the art in view of the
above description of
the invention. Such variations are to be considered within the scope of the
invention. The
methods and apparatus of the present invention may be, and in variou,_~~,<
ients are, used
with CDMA, orthogonal frequency division multiplexing (OFDM), and/or various
other types of
communications techniques which may be used to provide wireless communications
links
between access nodes and mobile nodes. In some embodiments the access nodes
are
implemented as base stations which establish communications links with mobile
nodes using
OFDM and/or CDMA. In various embodiments the mobile nodes are implemented as
notebook
computers, personal data assistants (PDAs), or other portable devices
including
receiver/transmitter circuits and logic and/or routines, for implementing the
methods of the
present invention.
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