U.S. patent application number 12/245670 was filed with the patent office on 2009-01-29 for broadcast superframe with variable reuse and interference levels for a radio communications system.
Invention is credited to Tibor Boros, Mitchell D. Trott.
Application Number | 20090028128 12/245670 |
Document ID | / |
Family ID | 39797322 |
Filed Date | 2009-01-29 |
United States Patent
Application |
20090028128 |
Kind Code |
A1 |
Trott; Mitchell D. ; et
al. |
January 29, 2009 |
BROADCAST SUPERFRAME WITH VARIABLE REUSE AND INTERFERENCE LEVELS
FOR A RADIO COMMUNICATIONS SYSTEM
Abstract
A method and apparatus are provided to enhance the efficiency of
reuse of a broadcast channel. In one embodiment, the invention
includes sending a first burst of a broadcast channel from a
broadcast channel radio subject to a first reuse factor, and
sending a second burst of the broadcast channel from the broadcast
channel radio subject to a second reuse factor. In another
embodiment, the invention includes broadcast channel structure with
a primary segment having a first reuse factor, a plurality of
secondary segments having a second reuse factor greater than the
first reuse factor.
Inventors: |
Trott; Mitchell D.;
(Mountain View, CA) ; Boros; Tibor; (San
Francisco, CA) |
Correspondence
Address: |
Gordon R. Lindeen III;BLAKELY, SOKOLOFF, TAYLOR & ZAFMAN LLP
12400 Wilshire Boulevard 7th Floor
Los Angeles
CA
90025
US
|
Family ID: |
39797322 |
Appl. No.: |
12/245670 |
Filed: |
October 3, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10187062 |
Jun 28, 2002 |
7433347 |
|
|
12245670 |
|
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Current U.S.
Class: |
370/350 ;
370/345; 455/561 |
Current CPC
Class: |
H04B 7/26 20130101 |
Class at
Publication: |
370/350 ;
370/345; 455/561 |
International
Class: |
H04J 3/06 20060101
H04J003/06; H04J 3/00 20060101 H04J003/00; H04B 1/38 20060101
H04B001/38 |
Claims
1. A method comprising: sending a first burst of a broadcast
channel from a broadcast channel radio subject to a first reuse
factor; and sending a second burst of the broadcast channel from
the broadcast channel radio subject to a second reuse factor.
2. The method of claim 1, wherein the first reuse factor is
associated with a first level of interference from other broadcast
channel radios and the second reuse factor is associated with a
second lower level of interference from other broadcast channel
radios.
3. The method of claim 1, wherein the first burst is sent in a
first slot of a repeating frame and the second burst is sent in a
second slot of the repeating frame.
4. The method of claim 3, wherein the repeating frame comprises a
third slot, the method further comprising not sending a burst of
the broadcast channel during the third slot.
5. The method of claim 3, wherein the slots are synchronized
between other broadcast channel radios after a common timing
reference.
6. The method of claim 5, wherein the common timing reference
comprises a satellite timing signal.
7. The method of claim 1, wherein the second burst comprises a
first segment using a first set of spatial parameters, a second
segment using a second set of spatial parameters, and a third
segment using the first set of spatial parameters.
8. The method of claim 7, wherein the first, second and third
segments of the burst are transmitted consecutively.
9. The method of claim 7, wherein the spatial parameters comprise
relative phases and amplitudes across elements of an antenna
array.
10. The method of claim 1, wherein the first burst is for signal
acquisition by receiving terminals and the second burst is for
transmitting data regarding a particular transmitting terminal.
11. The method of claim 1, wherein the first burst contains timing
data and the second burst contains data regarding a particular
terminal.
12. The method of claim 1, further comprising sending a third burst
having a reuse of one and wherein the first burst contains
frequency synchronization bits and the third burst contains timing
synchronization bits.
13. The method of claim 1, wherein the first and second bursts are
sent on the same channel with a time interval in between the
bursts.
14. A cellular base station comprising: a timing reference to
synchronize transmissions with other base stations; a processor to
generate a broadcast channel frame, the frame comprising at least
two bursts; and a transmitter to send a first burst of the
broadcast channel subject to a first reuse factor, and a second
burst of the broadcast channel subject to a second reuse
factor.
15. The base station of claim 14, wherein the timing reference
comprises a satellite timing signal receiver.
16. The base station of claim 14, wherein the first reuse factor is
associated with a first level of interference from other base
stations and the second reuse factor is associated with a second
lower level of interference from other base stations.
17. The base station of claim 14, wherein the primary segment is
for signal acquisition by receiving terminals and the secondary
segments are for transmitting data regarding a particular
transmitting terminal.
18. The base station of claim 14, wherein the primary segment
contains timing data and the secondary segments contain data
regarding a particular terminal.
19. The base station of claim 14, wherein the processor generates a
frame comprising a third burst and wherein the transmitter sends
the third burst subject to a reuse of one, the first burst
containing frequency synchronization bits and the third burst
containing timing synchronization bits.
20. The base station of claim 14, wherein the transmitter sends the
first and second bursts on the same channel with a time interval in
between the bursts.
Description
[0001] This U.S. continuation application claims priority to
pending application U.S. application Ser. No. 10/187,062, filed
Jun. 28, 2002, entitled "Broadcast Superframe with Variable Reuse
and Interference levels for a Radio Communications System".
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The invention relates generally to the field of digital
radio signal communications. More particularly, the invention
relates to transmitting different bursts of a broadcast channel
with different amounts of reuse and to the repeating superframe
structure used to transmit the broadcast burst.
[0004] 2. Description of the Related Art
[0005] Mobile radio communications systems such as cellular voice
radio systems typically have several base stations in different
locations available for use by mobile remote terminals, such as
cellular telephones or wireless web devices. Each base station
typically is assigned a set of frequencies or channels to use for
communications with the remote terminals. The channels are
different from those of neighboring base stations in order to avoid
interference between neighboring base stations. As a result, the
remote terminals can easily distinguish the transmissions received
from one base station from the signals received from another. In
addition, each base station can act independently in allocating and
using the channel resources assigned to it.
[0006] Such radio communications systems typically include a
broadcast channel (BCH). The BCH is broadcast to all remote
terminals whether they are registered on the network or not and
informs the remote terminals about the network. In order to access
the network, a remote terminal normally tunes to and listens to the
BCH before accessing the network. A remote terminal will typically
scan a range of likely frequencies when it wants to access the
network until it finds the strongest or clearest BCH. It will then
use the BCH signal for synchronization and use information in the
BCH to request access to the network.
[0007] Because the BCH is transmitted to all potential remote
terminals within the range of a particular base station, it is
typically broadcast omni-directionally or across a wide
simultaneous directional range. This causes a great amount of
interference and noise. It also consumes resources that might
otherwise be used to carry traffic. Existing wireless systems
employ a fixed reuse on the broadcast channel. The more resources
that are used for traffic, the fewer resources there are left for
traffic.
[0008] For example, a GSM system typically reuses the broadcast
carrier every nine cells. An advantage of the sparse reuse pattern
in GSM is the improved reliability of the broadcast channel.
Because neighboring cells do not use the channel, the coverage area
of the broadcast channel is much improved. This makes handover and
network access more reliable. On the other hand, to find the
broadcast channel in a GSM system, the user terminal must scan over
at least nine frequencies, delaying handover and network access.
Furthermore, the sparse reuse pattern in GSM requires that at least
nine frequency resources be allocated to the broadcast channel,
wasting valuable frequency resources. Recognizing the need to
conserve resources, the traffic channels in GSM have a typical
reuse pattern of three.
[0009] Conversely, IS-95 CDMA reuses the broadcast frequency in
every cell. An advantage of this tight reuse pattern is that a user
terminal can listen to just one carrier frequency and quickly
determine a list of all base stations in communication range.
However, in IS-95, as a cell gets loaded, the broadcast channel
becomes unreliable and it is much harder for new terminals to even
find the broadcast channel from the loaded cell.
BRIEF SUMMARY OF THE INVENTION
[0010] A method and apparatus are provided to enhance the
efficiency of reuse of a broadcast channel. In one embodiment, the
invention includes sending a first burst of a broadcast channel
from a broadcast channel radio subject to a first reuse factor, and
sending a second burst of the broadcast channel from the broadcast
channel radio subject to a second reuse factor. In another
embodiment, the invention includes broadcast channel structure with
a primary segment having a first reuse factor, a plurality of
secondary segments having a second reuse factor greater than the
first reuse factor.
[0011] Other features of the present invention will be apparent
from the accompanying drawings and from the detailed description
that follows.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0012] The present invention is illustrated by way of example, and
not by way of limitation, in the figures of the accompanying
drawings in which like reference numerals refer to similar elements
and in which:
[0013] FIG. 1 is a flow chart showing one embodiment of the present
invention;
[0014] FIG. 2 is a simplified block diagram of a base station on
which an embodiment of the invention can be implemented;
[0015] FIG. 3 is a block diagram of a remote terminal on which an
embodiment of the invention can be implemented; and
[0016] FIG. 4 is a table of superframe transmission showing the
superframe of Table 1 and the corresponding BS groups, BS0 through
BS7 that transmit for each frame of the superframe according to an
embodiment of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0017] Introduction
[0018] The present invention, according to one embodiment, allows
for changes in reuse and interference levels between bursts in the
broadcast channel. In other words, the BCH is divided into
different bursts within its superframe that are each sent with
different levels of reuse. Different bursts can also be sent with
different spatial parameters, for example, phase and amplitude
between elements of a spatial diversity array. An F burst is sent
by all base stations omni-directionally. A T burst is sent by all
base stations using changes in the spatial parameters. The B bursts
are sent by subsets of the base station with changes in the spatial
parameters. This progression enhances user terminal reception while
minimizing interference.
[0019] Process Flow
[0020] FIG. 1 shows a process flow diagram for building and sending
a sequence of broadcast channel (BCH) bursts in accordance with the
present invention. This process is described in the context of
several base stations (BS), in a network that includes many base
stations, sending a BCH superframe to any subscriber stations or
remote user terminals that may be within range of the base
stations. The superframe includes bursts that are designated for
use by different groups of the base stations. The designation of
superframes, frames, slots and subslots used here is for convenient
reference purposes only. Each burst described below could be
characterized as a subslot of a slot, a slot of a frame or some
portion of a superframe as in the example embodiment described.
[0021] Initially, all of the base stations simultaneously send an F
burst of the BCH 303. In one embodiment, the F burst is an
identical burst at every base station and is selected to allow user
terminals to perform frequency correction on their BCH reception.
In one embodiment, the F burst is transmitted as a pair of pure
tones. This allows the tones to be slightly out of phase as they
are received from two different base station and still be mixed by
the receiver to obtain a reliable frequency reference. The base
stations act almost as repeater stations so that the F burst
penetrates the entire network. The particular content of the F
burst is not significant but can relate to aspects of the BCH that
are common to all base stations. In order to simultaneously send
the F burst, the base stations can be synchronized in any of a
variety of ways well-known in the art. A GPS signal can be used for
example. The GPS signal has a benefit that it does not consume any
frequency resources that could otherwise be used for traffic. Since
the F burst is transmitted by all base stations at the same time,
it corresponds to the highest level of interference added to the
air waves, the communication resources of the system.
[0022] Following the F burst, the base stations all send a T burst
305. The base stations can be synchronized in the same way as with
the F burst. In one embodiment, the T burst contains information
regarding the base station color code (BSCC) of the transmitting
base station, however, as with the F burst, information common to
all base stations can be transmitted. The T burst can also be sent
with varying spatial parameters. This may allow any user terminal
that can receive the T burst simultaneously from different base
stations to distinguish the different received bursts from each
other. It also reduces the overall level of interference and noise
injected into the system as compared to the F burst. In other
embodiments, one of either the F burst or the T burst can be
eliminated. While using both bursts configured as described helps
the user terminal receive the next bursts, eliminating one or the
other will further conserve frequency resources.
[0023] Following the T burst, the first B burst of the BCH is sent
307. The B burst is also sent using varying spatial parameters.
There are several B bursts and they are divided up between the base
stations so that only a portion of the base stations use each B
burst. In one embodiment, each base station only uses one B burst.
This means that the reuse factor on the B bursts is equivalent to
the number of B bursts. In the present embodiment there are eight B
bursts so the reuse factor is eight. Any other number of B bursts
can be used depending on the needs of a particular system
configuration, the number of base station and the tendency of the
base stations to interfere with one another.
[0024] The B bursts can be allocated in any desired way. In order
to minimize the level of interference added by the B burst, the B
bursts can be allocated so that simultaneously transmitting base
stations are as far apart from each other as possible. In one
embodiment, this corresponds to the allocation of BSCC's. As a
result, all of the BSCC's can be divided into eight groups and one
of the eight B bursts can be assigned to each group of BSCC's. This
approach has an added advantage that the BSCC can be used to
identify B bursts from different base stations. Base stations
simultaneously transmitting on the same B burst with the same BSCC
will be a maximum distance apart.
[0025] Because B bursts from different base stations are easier to
distinguish, they can better carry information that is specific to
a particular base station. This information can include a
particular base station's BSCC, transmit power, traffic load,
available frequency resources and allocations, hopping sequences,
and any other desired information.
[0026] After the first B burst, the second group of base stations
send the second B burst 309. Then the third B burst of the BCH is
sent from third BS group 311. The fourth B burst of the BCH is sent
from the fourth BS group 313. The fifth B burst of the BCH is sent
from the fifth BS group 315. The sixth B burst of the BCH is sent
from the sixth BS group 317. The seventh B burst of the BCH is sent
from the seventh BS group 319, and the eighth B burst of the BCH is
sent from the eighth BS group 321. This completes the superframe.
The superframe can then be repeated by sending the F, T and B
bursts again 323 as long as the system is in operation.
[0027] The bursts are described as being of a certain number and
being sent in a certain order, however, these factors can be
modified to meet any particular system requirements. Since the
frame is repeated, the precise ordering is less important, any type
of burst can be sent first, B bursts can be interspersed between F
and T bursts, and additional high or low reuse bursts can be added
or removed from the superframe. The bursts may be sent immediately
one after the other, or other bursts may be interspersed between
the bursts. The bursts can be on a channel that is exclusively used
for transmissions from base stations or it can be a duplexed
channel used for both uplink and downlink. The number of B bursts
can be varied to suit system needs as well. In addition, the
particular designations, F, T, and B used here are not important.
In the described example, the F burst is a shared burst with a high
level of interference. The T burst is a shared burst with a lower
level of interference, and the B burst is a base station-specific
burst with the lowest level of interference. However, the
variations in reuse, interference generation and transmitted
information can be varied to suit any particular application.
[0028] Broadcast Channel Superframe
[0029] According to one embodiment of the present invention,
communication sessions are initiated for each user terminal or
remote terminal from the broadcast channel BCH which is transmitted
as a burst from the base station to all potential user terminals.
The BCH burst, unlike the TCH (traffic channel) bursts, is
transmitted in segments in many different directions where user
terminals are likely to be, the specific beam pattern parameters
will depend on the network. The BCH communicates enough basic
information to enable the UT to gain access to the network by
transmitting a message of its own, for example a subsequent
exchange of a CR (configuration request) and a CM (configuration
message) between the base station and the user terminal.
[0030] The BCH also provides good frequency offset and timing
update information to all user terminals, even when the BCH is not
specifically directed toward any one user terminal in particular.
The presently described embodiment has been selected in order to
minimize the amount of information transmitted in the BCH as well
as to minimize the bit rate. The broadcast channel information
symbols provide the information needed for a user terminal to
request a configuration message from the base station. They also
provide information to guide user terminal handover decisions.
[0031] The broadcast logical channel (BCH) provides information
that can be used by a UT (user terminal) to open a configuration
channel (CCH) to the BS (base station). It also provides
information to guide UT handover decisions for handovers to other
base stations. The BCH logical channel can be located on a fixed RF
(radio frequency) resource, e.g. a particular time slot and
frequency, throughout the network of base stations. This fixed RF
resource is, in one embodiment, dedicated to BCH and CCH, and is
not used for RACH (random access channels) TCH (traffic channels)
or other traffic. The other slots which are not dedicated to BCH
and CCH can be used for RACH, TCH or any other purpose. In the
present example, downlink slot 1 on an RF channel near the middle
of the RF allocation is used for BCH and CCH functions. The
particular choice of allocations will depend on the available
resources and the requirements for overhead traffic.
[0032] In one embodiment, an RF allocation of 5 MHz is divided in
frequency into 8 RF channels each of width 625 kHz. Each RF channel
is divided in time into 5 ms frames. Each frame has 6 slots, 3 for
receive and 3 for transmit, in a paired TDD (time-division duplex)
arrangement. This particular specific structure has been found to
be useful in a paired TDD system, however it can be adapted as
desired for TDMA, FDD, and CDMA systems. For high noise
environments or for increased robustness, the BCH can hop
frequencies according to a predetermined scheme or be repeated on
several different frequencies. In another embodiment, the BCH is on
its own channel and RACH and other overhead are on a separate
control channel. Alternately, one BCH can be provided on a constant
frequency and a secondary BCH can be provided on another channel
with hopping frequency. The particular details described here are
not necessary to obtain the benefit of the invention and many
variations are possible.
[0033] In detail, each 5 ms frame has the following sequence of
fields, where the uplink slots are receive slots used for
communication from a user terminal (UT) to a base station (BS) and
the downlink slots are transmit slots used for communication from
the BS to the UT.:
[0034] 545 .mu.s for uplink slot 1
[0035] 545 .mu.s for uplink slot 2
[0036] 545 .mu.s for uplink slot 3
[0037] 10 .mu.s guard time
[0038] 1090 .mu.s for downlink slot 1
[0039] 1090 .mu.s for downlink slot 2
[0040] 1090 .mu.s for downlink slot 3
[0041] 85 .mu.s guard time
[0042] The BCH logical channel, the first downlink slot in the
example above, carries three burst types, called F, T, and B. The F
and T bursts in the present example have a reuse of one. All base
stations transmit them on the same carrier at the same time. The B
burst has a reuse of eight. The superframe structure can be
simplified as the following sequence: F T B1 B2 B3 B4 B5 B6 B7 B8.
One eighth of the base stations use broadcast slot B5, for example,
while all the base stations use F and T.
[0043] The repeating superframe structure is shown in more detail
is shown in Table 1 below. The superframe has a period of 20
frames.
TABLE-US-00001 TABLE 1 Frame 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
17 18 19 20 Uplink C C C C C C C C C C Down F C T C B0 C B1 C B2 C
B3 C B4 C B5 C B6 C B7 C link
[0044] As shown in Table 1, even-numbered frames in the superframe
are labeled "C" and carry CCH (configuration channel) bursts.
Odd-numbered frames carry F, T, or B bursts. There are eight
different versions of the B bursts indicated by the symbols B0-B7.
The F, T and B bursts are used differently by the base stations of
the system. All base stations transmit F and T bursts at the same
time in the appropriate frame, once every superframe. The base
stations are all synchronized to a GPS (Global Positioning System)
receiver reference timing so that they can all transmit at almost
exactly the same time.
[0045] The frame timing is established by the base stations that
are in the area and transmitting on the RF carrier designated for
the BCH. The carrier can be searched for or pre-programmed into the
user terminals. The base stations, or base station if there is only
one, can employ GPS or some other precise common timing reference
to establish the frame timing. GPS timing offers the advantage that
it is accurately synchronized and inexpensively available to all
base stations. This allows the BCH to be shared by all the base
stations with only a minimal guard time in the BCH between base
stations. Precise timing also allows the remote terminals to make
distance-based comparisons of the base stations for selection
purposes.
[0046] The F and T frames, as described above, are occupied by all
of the base stations on each repetition of the superframe. This is
shown in the columns corresponding to frames 1 and 3 in the
superframe of FIG. 4. This corresponds to a reuse factor of 1, as
can be seen in FIG. 4 in which each BS, BS0 through BS7 transmits
the F and T bursts. The B frames, labeled B0-B7, however, have a
reuse factor of 8. The B frames are assigned to each base station
based on its BSCC (Base Station Color Code), although any other
assignment mechanism can be used, as can any other number of
different B frames. This is shown in FIG. 4 in which the column for
each frame 5, 7, 9, 11, 13, 15, 17, and 19 shows only transmitting
BS. One benefit of assigning the B bursts is that the base stations
that transmit simultaneously on the same burst will be separated
from each other by at least one other base station. Accordingly,
the identifiers BS0 through BS7 in FIG. 4 may each correspond to a
single base station or to different groups of the base
stations.
[0047] In one embodiment, during a frame labeled Bn, only those
base stations whose BSCC satisfy the equation, BSCC=n (mod 8),
transmit a B burst at the same time. There are 64 base station
color codes, numbered 0-63, leading to eight different color codes
being simultaneously transmitted by the base stations in any one
burst. So, for example, the base stations in group BS5 in FIG. 4
are those that have any of the eight different BSCC's corresponding
to 5 (mod 8) or 5, 13, 21, 29, 37, 45, 53, 61. These base stations
transmit their B burst in frame 15 or frame B5 of the
superframe.
[0048] A base station can be considered a collection of base
station modems serving a group of RF carriers. Alternatively, a
base station can be an installation with a set of modems at a
single site. For other system configurations each modem
modulator/demodulator set can be considered a base station. Each
base station is assigned a unique 32-bit base station identifier,
BSID. The BSID is used to derive a base station color code which is
defined in one embodiment as follows: BSCC=BSID mod 64.
[0049] As a function of the BSCC, a base station frequency hops,
broadcasts BCH, listens for uplink CR, and sends downlink CM in
response to CR. The BSCC can also be used by base stations and
terminals to ensure that messages transmitted to and from one base
station are not confused with messages transmitted to and from a
neighboring base station. Within a geographical region where radio
transmissions overlap, the BSCC is uniquely assigned. No base
station should be able to routinely see user terminals that are
communicating with a base station of the same color code. Likewise,
no user terminal should be able to see two base stations that are
assigned the same BSCC. A UT should never be in simultaneous
communication range of two base stations that have the same BSCC.
To help ensure that this occurs, BSCC's that differ by a multiple
of 8 can be assigned to non-adjacent base stations. This ensures
that the frames labeled B0-B7 in the superframe are received with
minimal interference at a UT. The total number of base stations as
well as the number of frames in a superframe, the number of slots
in a frame and the particular slots used for transmitting BCH
bursts, CRs and CMs can be modified to suit particular
applications.
[0050] Broadcast Channel F Burst
[0051] In the present example embodiment, the F burst contains:
[0052] 10 .mu.s of ramp-up, followed by
[0053] 1056 .mu.s of symbols f(1)-f(528), followed by
[0054] 10 .mu.s of ramp-down, followed by
[0055] 14 .mu.s of guard time.
[0056] The symbol period for all bursts (F, T and B) is 2 .mu.s
(500,000 symbols per second). Bursts can be transmitted in QPSK
(Quarternary Phase Shift Keying), so that the nominal occupied
bandwidth is 625 kHz.
[0057] The 1056 .mu.s of symbols contains frequency correction
symbols that follow a known predictable pattern. As observed at a
user terminal, the frequency correction symbols can be a mixture of
two complex tones.
[0058] Broadcast Channel T Burst
[0059] The T burst in this example consists of a short preamble
followed by 8 consecutive QPSK signals of length 64 symbols each.
Each repeated signal is generated from a code word, such as a
Walsh-Hadamard code word, determined as a function of the base
station color code (BSCC). The 8 repetitions are scrambled using a
scrambling sequence that does not depend on the BSCC. Any
scrambling code can be used. A pseudorandom sequence provides
overall consistent waveform properties when modulated and can be
generated using any of a variety of ways well-known in the art. In
one embodiment, a scrambling sequence is generated from a
congruential pseudorandom sequence generator.
[0060] The T burst in this example is made up of:
[0061] 10 .mu.s of ramp-up, followed by
[0062] 32 .mu.s of preamble r(1)-r(16), followed by
[0063] 1024 .mu.s of symbols t(1)-t(512), followed by
[0064] 10 .mu.s of ramp-down, followed by
[0065] 14 .mu.s of guard time.
[0066] For the T burst, the preamble is a known sequence of symbols
that are adjacent in the QPSK modulation format. A variety of
different sequences can be used. The preamble sequence provides
some additional ramp-up and guard time. The particular sequence
will depend on the modulation format, the quality of the RF
channels and other possible intended uses.
[0067] The symbols t(1)-t(512) are a function of the BSCC (base
station color code). The symbols t(1)-t(512) consist of 8 scrambled
repetitions of the selected 64-bit Walsh-Hadamard or other type of
code word using QPSK modulation. In one embodiment, each of the 8
scrambled repetitions is transmitted from the base station using a
different beam pattern.
[0068] Broadcast Channel B Burst
[0069] Like the T burst, the B burst in this example consists of a
short preamble followed by 8 consecutive QPSK signals of length 64
symbols each. The signals are also modulated code words, such as
Walsh-Hadamard code words. A single code word of length 64 is
selected as a function of the BCH payload, and is repeated 8 times.
Each repetition is scrambled using a linear feedback shift register
initialized using a function of the base station color code.
[0070] The spatial parameters, for example transmit weights, used
to transmit the last four segments of the B burst are the same as
the spatial parameters used to transmit the first four segments.
Thus any phase change between, e.g., the 1st and 5th segments may
be attributed to frequency offset. It's useful that there be a gap
in time before the weights are repeated; a longer gap gives a more
accurate frequency measurement but reduces the acquisition range of
the frequency measurement. While, for example, the 1st and 5th
segments of the B burst are transmitted with the same spatial
parameters, they are not identical signals. They contain the same
Walsh-Hadamard code word scrambled differently.
[0071] The B burst consists of:
[0072] 10 .mu.s of ramp-up, followed by
[0073] 32 .mu.s of preamble r(1)-r(16), followed by
[0074] 1024 .mu.s of symbols b(1)-b(512), followed by
[0075] 10 .mu.s of ramp-down, followed by
[0076] 14 .mu.s of guard time.
[0077] As with the T burst, the preamble for the B burst is a known
sequence of symbols that are adjacent in the QPSK modulation
format. A variety of different sequences can be used. This sequence
helps the UT establish timing. The particular sequence will depend
on the modulation format, the quality of the RF channels and other
possible intended uses.
[0078] The symbols b(1)-b(512) are a function of the base station
color code BSCC, the base station transmit power bsTxPwr, and the
base station load bsLoad. These symbols can be derived in a variety
of different ways. In one example, a six-bit message is defined as
p(1)-p(6). The first four bits p(1)-p(4) carry the base station
transmit power field bsTxPwr. The last two bits carry the bsLoad
field.
[0079] The bsTxPwr field is interpreted by the UT as the
per-antenna transmit power of the B burst. It can be encoded using
the formula:
bsTXPwr=3(p(1)+2p(2)+4p(3)+8p(4))dBm. i.
[0080] Thus the bsTxPwr fields encodes a power from 0-45 dBm in 3
dB steps. The BStxPwr can be the effective isotropic radiated power
of the broadcast message. This number indicates the power
transmitted by the base station taking into account the number of
amplifiers and diversity antennas available at the base
station.
[0081] The bsLoad field, encoded in p(5) and p(6), gives an
indication of the current traffic load of the base station. The
four possible values {p(5),p(6)}={00,10,01, 11} indicate light,
medium, heavy, and very heavy loading, respectively. BSload is the
load on the base station, used by the user terminal to determine
how frequently to send random access messages and whether to
attempt access. BSload is an indication of the amount of unused
capacity the base station has. It can be different from the number
of active registered subscribers because subscribers can require
different amounts of traffic capacity. BSload represents the
transmit and receive bit rates of each modem of the base station
over a period of a few minutes measured against maximum possible
loading.
[0082] To minimize, the data rate of BCH bursts still further, the
BSCC and BSload can be removed from the BCH burst. The BCH burst
then contains only training or synchronization and BStxPwr, the
only information directly related to handover decisions. The user
terminal can still distinguish and compare different base stations
for selection and handover decisions based on timing of the
received BCH bursts. The user terminal can also direct its message
requesting access based on timing.
[0083] If there are only very few possible power levels for base
stations in the system, for example two different transmit power
levels, then the two power levels can be distinguished by providing
two different training sequences. This allows the BSTxPwr bits to
be eliminated. For a single base station system, or if all base
stations transmit with the same power, the BSTxPwr bits can also be
deleted. If there is only one base station, it is not necessary to
evaluate path loss but only whether the signal can be received. The
rest of the network information can be learned upon
registration.
[0084] Having derived the six-bit sequence, this can be used to
select a code word, such as a Walsh-Hadamard code word, that will
be used to encode the information together with the BSCC as a
64-bit sequence, h(1)-h(64). The code word, h(1)-h(64), is
scrambled and transmitted as the symbols b(1)-b(512).
[0085] The symbols b(1)-b(512) consist of 8 scrambled repetitions
of the code word h(1)-h(64) with QPSK modulation. As with the T
burst, the first four repetitions are transmitted from the base
station using different beam patterns, and the last four
repetitions use the same beam patterns as the first four in the
same order. That is, repetitions 0 and 4 use the same beam pattern,
repetitions 1 and 5 use the same beam pattern, and so on.
[0086] Noncoherent Modulation
[0087] The discontinuity in the BCH described above allows the BCH
to be transmitted throughout the BS coverage area with minimal
interference. However, it causes difficulties for the receiver. The
modulation and coding structure of the broadcast burst therefore
can be selected to aid the receiver in tolerating the phase and
amplitude changes that occur between segments of the burst. This
may be achieved in a number of ways.
[0088] One way is to include training or pilot data symbols in each
segment of the BCH. By comparing the received signal to the known
training or pilot symbols, the receiver can estimate the gain and
phase of each segment. The gain and phase can then be corrected to
some nominal value (such as 1) across the entire received burst.
The corrected burst can then be processed ignoring the phase or
gain changes.
[0089] A disadvantage of pilot symbols is that they expend signal
energy that could otherwise be used to transmit information
signals. The required number of symbols can be a very large
fraction of the burst at the low SNRs (signal to noise ratios) at
which a broadcast channel may operate.
[0090] Another way to facilitate phase changes between segments is
differentially coherent signaling. DPSK (differential phase shift
keying), for example, encodes information in the phase changes
between successive symbols. If the receiver uses a differential
receiver, the sudden phase changes that occur at segment boundaries
will result in one or two errors, but the remainder of the segment
will be processed correctly. Error correction encoding at the
transmitter and error correction at the receiver can be used to
repair these errors. However, differential signaling, such as DPSK,
has several disadvantages. One disadvantage is the errors that will
occur at segment boundaries, described above. Another disadvantage
is that a differential receiver has lower performance.
[0091] The differential receiver attempts to track phase changes
during a segment. In the BCH burst described above, the transmitter
transmits each segment using a fixed beam pattern. As a result, the
phase does not change significantly during the segment. The use of
the differential receiver to receive a message that does not
significantly change in phase is wasteful since it degrades
performance during the reception of each burst segment.
[0092] Another way, which avoids pilot symbols and differential
modulation, is to employ noncoherent modulation and coding. This
approach does not require phase recovery at the receiver. One type
of noncoherent modulation is orthogonal signaling, in which one
from a set of M equal-energy orthogonal signals {x_i(t): i=1-M} is
selected for transmission in each segment. To transmit a 6-bit
message, for example, requires M=2.sup.6=64 orthogonal signals.
[0093] The orthogonal signals may, for convenience, be defined by
the rows of an orthogonal matrix H, where orthogonality means that
H times its conjugate transpose HA* is a scaled identity matrix. If
the entries of H further take values +1 or -1, the signals may be
transmitted using binary phase shift keying, which simplifies the
transmitter and receiver. One such matrix H is the Walsh-Hadamard
mentioned above.
[0094] There is a great variety of different noncoherent orthogonal
signaling formats. Some formats also includes noncoherent signaling
as a special case, such as PPM (pulse-position modulation) and FSK
(frequency shift keying). While code word sets that are orthogonal
and equal amplitude work very well for noncoherent signaling, these
constraints can be greatly eased. The code words need only be
sufficiently uncorrelated that they easily can be distinguished at
the receiver. This will depend on the quality of the channel, of
the transmitter and of the receiver.
[0095] Base Station Structure
[0096] In one embodiment as discussed above, the present invention
is implemented in an SDMA (Spatial Division Multiple Access) radio
data communications system. In such a spatial division system, each
terminal is associated with a set of spatial parameters that relate
to the radio communications channel between, for example, the base
station and a user terminal. The spatial parameters comprise a
spatial signature for each terminal. Using the spatial signature
and arrayed antennas, the RF energy from the base station can be
more precisely directed at a single user terminal, reducing
interference with and lowering the noise threshold for other user
terminals. Conversely, data received from several different user
terminals at the same time can be resolved at lower receive energy
levels. With spatial division antennas at the user terminals, the
RF energy required for communications can be even less. The
benefits are even greater for subscribers that are spatially
separated from one another. The spatial signatures can include such
things as the spatial location of the transmitters, the
directions-of-arrival (DOAs), times-of-arrival (TOAs) and the
distance from the base station.
[0097] Estimates of parameters such as signal power levels, DOAs,
and TOAs can be determined using known training sequences placed in
digital data streams for the purpose of channel equalization in
conjunction with sensor (antenna) array information. This
information is then used to calculate appropriate weights for
spatial demultiplexers, multiplexers, and combiners. Techniques
well known in the art, can be used to exploit the properties of the
training sequences in determining spatial parameters. Further
details regarding the use of spatial division and SDMA systems are
described, for example, in U.S. Pat. Nos. 5,828,658, issued Oct.
27, 1998 to Ottersten et al. and 5,642,353, issued Jun. 24, 1997 to
Roy, III et al.
[0098] (SDMA) technology can be combined with other multiple access
systems, such as time division multiple access (TDMA), frequency
division multiple access (FDMA) and code division multiple access
(CDMA). Multiple access can be combined with frequency division
duplexing (FDD) or time division duplexing (TDD).
[0099] FIG. 2 shows an example of a base station of a wireless
communications system or network suitable for implementing the
present invention. The base station uses SDMA technology which can
be combined with other multiple access systems, such as time
division multiple access (TDMA), frequency division multiple access
(FDMA) and code division multiple access (CDMA). Multiple access
can be combined with frequency division duplexing (FDD) or time
division duplexing (TDD). The system or network includes a number
of subscriber stations, also referred to as remote terminals or
user terminals, such as that shown in FIG. 3. The base station may
be connected to a wide area network (WAN) through its host DSP 31
for providing any required data services and connections external
to the immediate wireless system.
[0100] To support spatial diversity, a plurality of antennas 3 is
used to form an antenna array 4, for example four antennas,
although other numbers of antennas may be selected. Each antenna is
an element of a four-element array 4. And a plurality of arrays are
provided 4-1, 4-2, 4-3. The antenna elements may have a spacing of
from one-quarter to four wavelengths of a typical carrier frequency
while the arrays may be separated by ten or twenty wavelengths. The
best spacing for spatial diversity will depend upon the particular
frequencies involved, the physical installation and other aspects
of the system. In many applications, the spacing between antenna
elements of each array can be less than two wavelengths of the
received signal. The spacing between antenna arrays can be more
than two wavelengths of the received signal. In general, the
spacing between elements in an array is selected to minimize
grating lobes when transmissions from each element are coherently
combined. In an alternative approach, the arrays are spaced apart
so as to form a uniform array of elements. The distance between
nearest elements in different arrays is the same as the spacing
between elements within an array. As mentioned above, it is also
possible for each array to have only a single element.
[0101] A set of spatial multiplexing weights for each subscriber
station are applied to the respective modulated signals to produce
spatially multiplexed signals to be transmitted by the bank of four
antennas. The host DSP 31 produces and maintains spatial signatures
for each subscriber station for each conventional channel and
calculates spatial multiplexing and demultiplexing weights using
received signal measurements. In this manner, the signals from the
current active subscriber stations, some of which may be active on
the same conventional channel, are separated and interference and
noise suppressed. When communicating from the base station to the
subscriber stations, an optimized multi-lobe antenna radiation
pattern tailored to the current active subscriber station
connections and interference situation is created. The channels
used may be partitioned in any manner. In one embodiment the
channels used may be partitioned as defined in the GSM (Global
System for Mobile Communications) air interface, or any other time
division air interface protocol, such as Digital Cellular, PCS
(Personal Communication System), PHS (Personal Handyphone System)
or WLL (Wireless Local Loop). Alternatively, continuous analog or
CDMA channels can be used.
[0102] The outputs of the antennas are connected to a duplexer
switch 7, which in a TDD embodiment, may be a time switch. Two
possible implementations of the duplexer switch are as a frequency
duplexer in a frequency division duplex (FDD) system, and as a time
switch in a time division duplex (TDD) system. When receiving, the
antenna outputs are connected via the duplexer switch to a receiver
5, and are converted down in analog by RF receiver ("RX") modules 5
from the carrier frequency to an FM intermediate frequency ("IF").
This signal then is digitized (sampled) by analog to digital
converters ("ADCs") 9. Final down-converting to baseband is carried
out digitally. Digital filters can be used to implement the
down-converting and the digital filtering, the latter using finite
impulse response (FIR) filtering techniques. This is shown as block
13. The invention can be adapted to suit a wide variety of RF and
IF carrier frequencies and bands.
[0103] There are, in the example of GSM, eight down-converted
outputs from each antenna's digital filter 13, one per receive
timeslot. The particular number of timeslots can be varied to suit
network needs. While GSM uses eight uplink and eight downlink
timeslots for each TDMA frame, desirable results can also be
achieved with any number of TDMA timeslots for the uplink and
downlink in each frame. For each of the eight receive timeslots,
the four down-converted outputs from the four antennas are fed to a
digital signal processor (DSP) 31 an ASIC (Application Specific
Integrated Circuit) or FPGA (Field Programmable Gate Array)
(hereinafter "timeslot processor") for further processing,
including calibration, according to one aspect of this invention.
For TDMA signals, eight Motorola DSP56300 Family DSPs can be used
as timeslot processors, one per receive timeslot. The timeslot
processors 17 monitor the received signal power and estimate the
frequency offset and time alignment. They also determine smart
antenna weights for each antenna element. These are used in the
SDMA scheme to determine a signal from a particular remote user and
to demodulate the determined signal. In a WCDMA system, the
channels may be separated using codes in an FPGA and then further
processed separately perhaps using separate DSPs for different
users. Instead of being timeslot processors the processors are
channel processors.
[0104] The output of the timeslot processors 17 is demodulated
burst data for each of the eight receive timeslots. This data is
sent to the host DSP processor 31 whose main function is to control
all elements of the system and interface with the higher level
processing, which is the processing which deals with what signals
are required for communications in all the different control and
service communication channels defined in the system's
communication protocol. The host DSP 31 can be a Motorola DSP56300
Family DSP. In addition, timeslot processors send the determined
receive weights for each user terminal to the host DSP 31. The host
DSP 31 maintains state and timing information, receives uplink
burst data from the timeslot processors 17, and programs the
timeslot processors 17. In addition it decrypts, descrambles,
checks error correcting code, and deconstructs bursts of the uplink
signals, then formats the uplink signals to be sent for higher
level processing in other parts of the base station.
[0105] Furthermore DSP 31 may include a memory element to store
data, instructions, or hopping functions or sequences.
Alternatively, the base station may have a separate memory element
or have access to an auxiliary memory element. With respect to the
other parts of the base station it formats service data and traffic
data for further higher processing in the base station, receives
downlink messages and traffic data from the other parts of the base
station, processes the downlink bursts and formats and sends the
downlink bursts to a transmit controller/modulator, shown as 37.
The host DSP also manages programming of other components of the
base station including the transmit controller/modulator 37 and the
RF timing controller shown as 33. The RF controller 33 reads and
transmits power monitoring and control values, controls the
duplexer 7 and receives timing parameters and other settings for
each burst from the host DSP 31.
[0106] The transmit controller/modulator 37, receives transmit data
from the host DSP 31. The transmit controller uses this data to
produce analog IF outputs which are sent to the RF transmitter (TX)
modules 39. Specifically, the received data bits are converted into
a complex modulated signal, up-converted to an IF frequency,
sampled, multiplied by transmit weights obtained from host DSP 31,
and converted via digital to analog converters ("DACs") which are
part of transmit controller/modulator 37 to analog transmit
waveforms. The analog waveforms are sent to the transmit modules
39. The transmit modules 39 up-convert the signals to the
transmission frequency and amplify the signals. The amplified
transmission signal outputs are sent to antennas 3 via the
duplexer/time switch 7. In a CDMA system, the signals may also be
spread and scrambled using appropriate codes.
[0107] User Terminal Structure
[0108] FIG. 3 depicts an example component arrangement in a remote
terminal that provides data or voice communication. The remote
terminal's antenna 45 is connected to a duplexer 46 to permit the
antenna 45 to be used for both transmission and reception. The
antenna can be omni-directional or directional. For optimal
performance, the antenna can be made up of multiple elements and
employ spatial processing as discussed above for the base station.
In an alternate embodiment, separate receive and transmit antennas
are used eliminating the need for the duplexer 46. In another
alternate embodiment, where time division duplexing is used, a
transmit/receive (TR) switch can be used instead of a duplexer as
is well known in the art. The duplexer output 47 serves as input to
a receiver 48. The receiver 48 produces a down-converted signal 49,
which is the input to a demodulator 51. A demodulated received
sound or voice signal 67 is input to a speaker 66.
[0109] The remote terminal has a corresponding transmit chain in
which data or voice to be transmitted is modulated in a modulator
57. The modulated signal to be transmitted 59, output by the
modulator 57, is up-converted and amplified by a transmitter 60,
producing a transmitter output signal 61. The transmitter output 61
is then input to the duplexer 46 for transmission by the antenna
45.
[0110] The demodulated received data 52 is supplied to a remote
terminal central processing unit 68 (CPU) as is received data
before demodulation 50. The remote terminal CPU 68 can be
implemented with a standard DSP (digital signal processor) device
such as a Motorola series 56300 Family DSP. This DSP can also
perform the functions of the demodulator 51 and the modulator 57.
The remote terminal CPU 68 controls the receiver through line 63,
the transmitter through line 62, the demodulator through line 52
and the modulator through line 58. It also communicates with a
keyboard 53 through line 54 and a display 56 through line 55. A
microphone 64 and speaker 66 are connected through the modulator 57
and the demodulator 51 through lines 65 and 67, respectively for a
voice communications remote terminal. In another embodiment, the
microphone and speaker are also in direct communication with the
CPU to provide voice or data communications. Furthermore remote
terminal CPU 68 may also include a memory element to store data,
instructions, and hopping functions or sequences. Alternatively,
the remote terminal may have a separate memory element or have
access to an auxiliary memory element.
[0111] In one embodiment, the speaker 66, and the microphone 64 are
replaced or augmented by digital interfaces well-known in the art
that allow data to be transmitted to and from an external data
processing device (for example, a computer). In one embodiment, the
remote terminal's CPU is coupled to a standard digital interface
such as a PCMCIA interface to an external computer and the display,
keyboard, microphone and speaker are a part of the external
computer. The remote terminal's CPU 68 communicates with these
components through the digital interface and the external
computer's controller. For data only communications, the microphone
and speaker can be deleted. For voice only communications, the
keyboard and display can be deleted.
[0112] General Matters
[0113] In the description above, for the purposes of explanation,
numerous specific details are set forth in order to provide a
thorough understanding of the present invention. It will be
apparent, however, to one skilled in the art that the present
invention may be practiced without some of these specific details.
In other instances, well-known circuits, structures, devices, and
techniques have been shown in block diagram form or without detail
in order not to obscure the understanding of this description.
[0114] The present invention includes various steps. The steps of
the present invention may be performed by hardware components, such
as those shown in FIGS. 2 and 3, or may be embodied in
machine-executable instructions, which may be used to cause a
general-purpose or special-purpose processor or logic circuits
programmed with the instructions to perform the steps.
Alternatively, the steps may be performed by a combination of
hardware and software. The steps have been described as being
performed by either the base station or the user terminal. However,
many of the steps described as being performed by the base station
may be performed by the user terminal and vice versa. Furthermore,
the invention is equally applicable to systems in which terminals
communicate with each other without either one being designated as
a base station, a user terminal, a remote terminal or a subscriber
station. Thus, the present invention is equally applicable and
useful in a peer-to-peer wireless network of communications devices
using spatial processing. These devices may be cellular phones,
PDA's, laptop computers, or any other wireless devices. Generally,
since both the base stations and the terminals use radio waves,
these communications devices of wireless communications networks
may be generally referred to as radios.
[0115] In portions of the description above, only the base station
is described as performing spatial processing using adaptive
antenna arrays. However, the user terminals can also contain
antenna arrays, and can also perform spatial processing both on
receiving and transmitting (uplink and downlink) within the scope
of the present invention.
[0116] Furthermore, in portions of the description above, certain
functions performed by a base station could be coordinated across
the network, to be performed cooperatively with a number of base
stations. For example, each base station antenna array could be a
part of a different base station. The base station's could share
processing and transceiving functions. Alternatively, a central
base station controller could perform many of the functions
described above and use the antenna arrays of one or more base
stations to transmit and receive signals.
[0117] The present invention may be provided as a computer program
product, which may include a machine-readable medium having stored
thereon instructions, which may be used to program a computer (or
other electronic devices) to perform a process according to the
present invention. The machine-readable medium may include, but is
not limited to, floppy diskettes, optical disks, CD-ROMs, and
magneto-optical disks, ROMs, RAMs, EPROMs, EEPROMs, magnet or
optical cards, flash memory, or other type of
media/machine-readable medium suitable for storing electronic
instructions. Moreover, the present invention may also be
downloaded as a computer program product, wherein the program may
be transferred from a remote computer to a requesting computer by
way of data signals embodied in a carrier wave or other propagation
medium via a communication link (e.g., a modem or network
connection).
[0118] Many of the methods are described in their most basic form,
but steps can be added to or deleted from any of the methods and
information can be added or subtracted from any of the described
messages without departing from the basic scope of the present
invention. It will be apparent to those skilled in the art that
many further modifications and adaptations can be made. The
particular embodiments are not provided to limit the invention but
to illustrate it. The scope of the present invention is not to be
determined by the specific examples provided above but only by the
claims below.
[0119] It should also be appreciated that reference throughout this
specification to "one embodiment" or "an embodiment" means that a
particular feature may be included in the practice of the
invention. Similarly, it should be appreciated that in the
foregoing description of exemplary embodiments of the invention,
various features of the invention are sometimes grouped together in
a single embodiment, figure, or description thereof for the purpose
of streamlining the disclosure and aiding in the understanding of
one or more of the various inventive aspects. This method of
disclosure, however, is not to be interpreted as reflecting an
intention that the claimed invention requires more features than
are expressly recited in each claim. Rather, as the following
claims reflect, inventive aspects lie in less than all features of
a single foregoing disclosed embodiment. Thus, the claims following
the Detailed Description are hereby expressly incorporated into
this Detailed Description, with each claim standing on its own as a
separate embodiment of this invention.
* * * * *