Adaptix, Inc. v. T-Mobile USA, Inc.
Filing
1
COMPLAINT against T-Mobile USA, Inc. ( Filing fee $ 350 receipt number 0540-3620802.), filed by Adaptix, Inc.. (Attachments: # 1 Civil Cover Sheet, # 2 Exhibit A - U.S. PATENT NO. 7,146,172, # 3 Exhibit B - U.S. PATENT NO. 6,870,808, # 4 Exhibit C - U.S. PATENT NO. 7,573,851, # 5 Exhibit D - U..S. PATENT NO. 6,904,283, # 6 Exhibit E - U.S. PATENT NO. 7,072,315)(Hill, Jack)
EXHIBIT A
111111111111111111111111111111111!I I I,11,1111111111111111111111111
(12)
United States Patent
(10) Patent No.:
US 7,146,172 B2
(45) Date of Patent:
*Dec. 5, 2006
Li et al.
(54)
MULTI-CARRIER COMMUNICATIONS
ADAPTIVE CLUSTER
CONFIGURATION AND SWITCHING
5,507,034 A
5,515,378 A
5,555,268 A
5,588,020 A
5,708,973 A
5,726,978 A
5,734,967 A
5,774,808 A
5,822,372 A
5,867,478 A
5,886,988 A
5,887,245 A
5,909,436 A
5,914,933 A
5,933,421 A
5,956,642 A
WITH
(75)
Inventors: Xiaodong Li, Bellevue, WA (US); Hui
Liu, Sammamish, WA (US); Wenzhong
Zhang, Bellevue, WA (US); Kemin Li,
Bellevue, WA (US)
Assignee: Adaptix, Inc., Seattle, WA (US)
Notice:
Subject to any disclaimer, the term of this
patent is extended or adjusted under 35
U.S.C. 154(b) by 758 days.
This patent is subject to a terminal disclaimer.
DE
198 00 953
Prior Publication Data
US 2002/0147017 Al
*
Bodin et al.
Roy et al.
Fattouche et al.
Schilling
Ritter
Frodigh et al.
Kotzin et al.
Sarkioja et al.
Emami
Baum et al.
Yun et al.
Lindroth et al.
Engstrom et al.
Cimini et al.
Alamouti et al.
Larsson et al.
455/452.2
370/334
375/141
370/337
455/62
370/252
455/63.1
455/436
375/260
370/203
370/208
455/449
FOREIGN PATENT DOCUMENTS
Apr. 17, 2001
(65)
*
*
*
*
*
*
*
4/1996
5/1996
9/1996
12/1996
1/1998
3/1998
3/1998
6/1998
10/1998
2/1999
3/1999
3/1999
6/1999
6/1999
8/1999
9/1999
(Continued)
(21) Appl. No.: 09/837,701
(22) Filed:
*
*
7/1999
(Continued)
Oct. 10, 2002
OTHER PUBLICATIONS
Related U.S. Application Data
(63) Continuation-in-part of application No. 09/738,086,
filed on Dec. 15, 2000.
Mignone et al., CD3-OFDM: A Novel Demodulation Scheme for
Fixed and Mobile REceivers, IEEE Trnasations on Communications, vol. 44, No. 9, Sep. 1996.*
(Continued)
(51) Int. Cl.
HO4Q 7/20
(52) U.S. Cl.
(58)
(56)
(2006.01)
455/452.1; 455/450; 455/452.2;
455/447; 455/448
Field of Classification Search
455/447,
455/450, 452.1, 452.2, 453; 310/203, 208,
310/343, 480, 205, 439, 210, 299, 347; 375/276
See application file for complete search history.
References Cited
U.S. PATENT DOCUMENTS
5,280,630 A * 1/1994 Wang
5,479,447 A * 12/1995 Chow et al.
5,504,775 A * 4/1996 Chouly et al.
455/452.2
375/260
370/210
Primary Examiner—Temica Beamer
Assistant Examiner Joy Contee
(74) Attorney, Agent, or Firm—Fulbright & Jaworski LLP
(57)
ABSTRACT
A method and apparatus for allocating subcarriers in an
orthogonal frequency division multiple access (OFDMA)
system is described. In one embodiment, the method comprises allocating at least one diversity cluster of subcarriers
to a first subscriber and allocating at least one coherence
cluster to a second subscriber.
13 Claims, 7 Drawing Sheets
Cl.v.
Detection
o)
/101_
ASH
Yes
Select Diversity
Clusters
—110/
No
Select Coherence
Clusters
I 1 0.1
US 7,146,172 B2
Page 2
U.S. PATENT DOCUMENTS
5,973,642 A
10/1999 Li et al
5,991,273 A
11/1999 Abu-Dayya
6,005,876 A
12/1999 Cimini, Jr. et al.
6,009,553 A
12/1999 Martinez et al.
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2/2000 Williams
6,041,237 A
3/2000 Farsakh
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11/2000 Shively et al.
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9/2003 Chayat
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342/378
370/525
455/450
455/450
375/219
375/224
370/329
370/342
370/342
455/562.1
370/205
375/260
370/203
370/334
FOREIGN PATENT DOCUMENTS
DE
EP
EP
EP
EP
FR
GB
JP
WO
WO
WO
198 00 953 Cl
7/1999
0 869 647 A2 10/1998
0 926 912 A2
6/1999
0 929 202 Al
7/1999
0 999 658 A2
5/2000
2 777 407 Al 10/1999
2 209 858 A
8/1997
06029922
2/1994
WO 98/16077 A2
4/1998
WO 98/30047 Al
7/1998
WO 02 49305 A2 6/2002
OTHER PUBLICATIONS
Farsakh, C. et al.. "Maximizing the SDMA Mobile Radio Capacity
Increase by DOA Sensitive Channel Allocation". Wireless Personal
Communications, Kluwer Academic Publishers, NL, vol. 11, No. 1,
Oct. 1999, pp. 63-76, XP000835062, ISSN: 0929-6212.
Wong, C.Y., et al., Multiuser OFDM With Adaptive Subcarrier, Bit,
and Power Allocation. IEEE Journal on Selected Areas in Communications, Oct. 1999, IEEE Inc., New York, USA, vol. 17, Nr. 10, pp.
1747-1758, XP000854075, ISSN: 0733-8716 Sections I and II
abstract.
Gruenheid, R. et al: "Adaptive Modulation and Multiple Access for
the OFDM Transmission Technique", Wireless Personal Communications, Kluwer Academic Publishers, NL, vol. 13, NR. 1/2, Year
2000, pp. 5-13 XP000894156, ISSN: 0929-6212, no month listed.
Motegi, M. et al.: "Optimum Band Allocation According to Subband Condition for BST-OFDM" 11e IEEE International Symposium on Personal Indoor and Mobile Radio Communications, vol.
2, Sep. 18-21, 2000, pp. 1236-1240, XP002213669, Piscataway, NJ,
USA, ISBN: 0-7803-6463-5.
Kapoor, S. et al.: "Adaptive Interference Suppression in Multiuser
Wireless OFDM Systems Using Antenna Arrays" IEEE Transactions on Signal Processing, vol. 47, No. 12, Dec. 1999, pp. 33813391, XP000935422, IEEE, New York, USA, ISSN: 1053-587X.
Ye Li, et al.: "Clustered OFDM with channel estimation for high
rate wireless data", Mobile Multimedia Communications, 1999.
(MOMUC '99). 1999 IEEE International Workshop on San Diego,
CA, USA, IEEE, US, Nov. 15, 1999, pp. 43-50, XP010370695,
ISBN: 0-7803-5904-6.
Nogueroles, R. et al.: "Improved Performance of a Random
OFDMA Mobile Communication System" Vehicular Technology
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May 18-21, 1998, pp. 2502-2506, XP010288120, ISBN: 0-78034320-4.
Kinugawa, Y. et al.: -Frequency and Time Division Multiple Access
with Demand-Assignment Using Multicarrier Modulation for
Indoor Wireless Communications Systems", IEICE Transactions on
Communications, Institute of Electronics Information and Comm.
Eng. Tokyo, Japan, vol. E77-B, NR. 3, Mar. 1994, pp. 396-402,
XP000451014, ISSN: 0916-8516.
Bender et al., CDMA/HDR: A Bandwidth-Efficient High-Speed
Wireless Data Service for Nomadic Users, IEEE Communications
Magazine, Jul. 2000, pp. 70-87.
Tsoulos, G.V., Smart Antennas For Mobile Communication Systems: Benefits And Challenges, Electronics & Communication
Engineering Journal, Apr. 1999, pp. 84-94.
Shad et al., Indoor SDMA Capacity Using a Smart Antenna Basestation, 1997 IEEE, pp. 868-872, no month listed.
Farsakh, Christof and Nossek, Josef A., On the Mobile Radio
Capacity Increase Through SDMA, no date (after 1997).
Frullone et al., PRMA Performance in Cellular Environments with
Self-Adaptive Channel Allocation Strategies, IEEE Transactions on
Vehicular Technology, Nov. 1996, pp. 657-665 vol. 45, No. 4.
Xu, Guanghan and Li, San-Qi, Throughput Multiplication of Wireless Lans for Multimedia Services: SDMA Protocol Design, 1994
IEEE, pp. 1326-1332, no month listed.
Ward, James and Compton, R. Ted, Jr., High Throughput Slotted
ALOHA Packet Radio Networks with Adaptive Arrays, IEEE Transactions on Communications, Mar. 1993, pp. 460-470, vol. 41, No.
3.
Wong et al. "Multiuser OFDM with Adaptive Subcarrier, Bit, and
Power Allocation", IEEE Journal on Selected Areas in Communications. IEEE. New York, US, 1999, vol. 17. NR. 10, pp. 17471758, no month listed.
Mexican Office Action issued for PA/a/2003/005311 dated Mar. 31,
2006.
* cited by examiner
U.S. Patent
US 7,146,172 B2
Sheet 1 of 7
Dec. 5, 2006
Subcarrier
Cluster
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Figure lA
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Sheet 2 of 7
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US 7,146,172 B2
Sheet 3 of 7
Channel/Interference
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Periods
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US 7,146,172 B2
Sheet 4 of 7
Dec. 5, 2006
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US 7,146,172 B2
Sheet 5 of 7
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US 7,146,172 B2
Sheet 6 of 7
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US 7,146,172 B2
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US 7,146,172 B2
1
2
MULTI-CARRIER COMMUNICATIONS
WITH ADAPTIVE CLUSTER
CONFIGURATION AND SWITCHING
One approach to subcarrier allocation for OFDMA is a
joint optimization operation, not only requiring the activity
and channel knowledge of all the subscribers in all the cells,
but also requiring frequent rescheduling every time an
existing subscribers is dropped off the network or a new
subscribers is added onto the network. This is often impractical in real wireless system, mainly due to the bandwidth
cost for updating the subscriber information and the computation cost for the joint optimization.
This patent application is a Continuation-in-part (CIP) of
patent application Ser. No. 09/738,086 filed Dec. 15, 2000,
entitled "OFDMA with Adaptive Subcarrier-Cluster Configuration and Selective Loading."
FIELD OF THE INVENTION
The invention relates to the field of wireless communications; more particularly, the invention relates to multi-cell,
multi-subscriber wireless systems using orthogonal frequency division multiplexing (OFDM).
5
10
SUMMARY OF THE INVENTION
15
BACKGROUND OF THE INVENTION
Orthogonal frequency division multiplexing (OFDM) is
an efficient modulation scheme for signal transmission over
frequency-selective channels. In OFDM, a wide bandwidth 20
is divided into multiple narrow-band subcarriers, which are
arranged to be orthogonal with each other. The signals
modulated on the subcarriers are transmitted in parallel. For
more information, see Cimini, Jr., "Analysis and Simulation
of a Digital Mobile Channel Using Orthogonal Frequency 25
Division Multiplexing," IEEE Trans. Commun., vol. COM33, no. 7, July 1985, pp. 665 75; Chuang and Sollenberger,
"Beyond 3G: Wideband Wireless Data Access Based on
OFDM and Dynamic Packet Assignment," IEEE Communications Magazine, Vol. 38, No. 7, pp. 78-87, July 2000. 30
One way to use OFDM to support multiple access for
multiple subscribers is through time division multiple access
(TDMA), in which each subscriber uses all the subcarriers
within its assigned time slots. Orthogonal frequency division
multiple access (OFDMA) is another method for multiple 35
access, using the basic format of OFDM. In OFDMA,
multiple subscribers simultaneously use different subcarriers, in a fashion similar to frequency division multiple
access (FDMA). For more information, see Sari and Karam,
"Orthogonal Frequency-Division Multiple Access and its 40
Application to CATV Networks," European Transactions on
Telecommunications, Vol. 9 (6), pp. 507-516, November/
December 1998 and Nogueroles, Bossed, Donder, and
Zyablov, "Improved Performance of a Random OFDMA
Mobile Communication System,", Proceedings of IEEE 45
VTC'98, pp. 2502-2506.
Multipath causes frequency-selective fading. The channel
gains are different for different subcarriers. Furthermore. the
channels are typically uncorrelated for different subscribers.
The subcarriers that are in deep fade for one subscriber may 50
provide high channel gains for another subscriber. Therefore, it is advantageous in an OFDMA system to adaptively
allocate the subcarriers to subscribers so that each subscriber
enjoys a high channel gain. For more information, see Wong
et al., "Multiuser OFDM with Adaptive Subcarrier, Bit and 55
Power Allocation," IEEE J. Select. Areas Commun., Vol.
17(10), pp. 1747-1758, October 1999.
Within one cell, the subscribers can be coordinated to
have different subcarriers in OFDMA. The signals for different subscribers can be made orthogonal and there is little so
intracell interference. However, with aggressive frequency
reuse plan, e.g., the same spectrum is used for multiple
neighboring cells, the problem of intercell interference
arises. It is clear that the intercell interference in an OFDMA
system is also frequency selective and it is advantageous to 65
adaptively allocate the subcarriers so as to mitigate the effect
of intercell interference.
A method and apparatus for allocating subcarriers in an
orthogonal frequency division multiple access (OFDMA)
system is described. In one embodiment, the method comprises allocating at least one diversity cluster of subcarriers
to a first subscriber and allocating at least one coherence
cluster to a second subscriber.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be understood more fully from
the detailed description given below and from the accompanying drawings of various embodiments of the invention,
which, however, should not be taken to limit the invention
to the specific embodiments, but are for explanation and
understanding only.
FIG. 1A illustrates subcarriers and clusters.
FIG. 1B is a flow diagram of one embodiment of a process
for allocating subcarriers.
FIG. 2 illustrates time and frequency grid of OFDM
symbols, pilots and clusters.
FIG. 3 illustrates subscriber processing.
FIG. 4 illustrates one example of FIG. 3.
FIG. 5 illustrates one embodiment of a format for arbitrary cluster feedback.
FIG. 6 illustrates one embodiment of a partition the
clusters into groups.
FIG. 7 illustrates one embodiment of a feedback format
for group-based cluster allocation.
FIG. 8 illustrates frequency reuse and interference in a
multi-cell, multi-sector network.
FIG. 9 illustrates different cluster formats for coherence
clusters and diversity clusters.
FIG. 10 illustrates diversity clusters with subcarrier hopping.
FIG. 11 illustrates intelligent switching between diversity
clusters and coherence clusters depending on subscribers
mobility.
FIG. 12 illustrates one embodiment of a reconfiguration
of cluster classification.
FIG. 13 illustrates one embodiment of a base station.
DETAILED DESCRIPTION OF THE PRESENT
INVENTION
A method and apparatus for allocating subcarriers in an
orthogonal frequency division multiple access (OFDMA)
system is described. In one embodiment, the method comprises allocating at least one diversity cluster of subcarriers
to a first subscriber and allocating at least one coherence
cluster to a second subscriber.
The techniques disclosed herein are described using
OFDMA (clusters) as an example. However, they are not
limited to OFDMA-based systems. The techniques apply to
multi-carrier systems in general, where, for example, a
carrier can be a cluster in OFDMA, a spreading code in
US 7,146,172 B2
3
4
CDMA, an antenna beam in SDMA (space-division multiple
Some portions of the detailed descriptions which follow
access), etc. In one embodiment, subcarrier allocation is
are presented in terms of algorithms and symbolic repreperformed in each cell separately. Within each cell, the
sentations of operations on data bits within a computer
allocation for individual subscribers (e.g., mobiles) is also
memory. These algorithmic descriptions and representations
made progressively as each new subscriber is added to the 5 are the means used by those skilled in the data processing
system as opposed to joint allocation for subscribers within
arts to most effectively convey the substance of their work
each cell in which allocation decisions are made taking into
to others skilled in the art. An algorithm is here, and
account all subscribers in a cell for each allocation.
generally, conceived to be a self-consistent sequence of steps
For downlink channels, each subscriber first measures the
leading to a desired result. The steps are those requiring
channel and interference information for all the subcarriers 0
physical manipulations of physical quantities. Usually,
and then selects multiple subcarriers with good performance
though not necessarily, these quantities take the form of
(e.g.. a high signal-to-interference plus noise ratio (SINR))
electrical or magnetic signals capable of being stored, transand feeds back the information on these candidate subcarferred, combined, compared, and otherwise manipulated. It
riers to the base station. The feedback may comprise channel
has proven convenient at times, principally for reasons of
and interference information (e.g., signal-to-interference- 15
common usage, to refer to these signals as bits, values,
plus-noise-ratio information) on all subcarriers or just a
elements, symbols, characters, terms, numbers, or the like.
portion of subcarriers. In case of providing information on
It should be borne in mind, however, that all of these and
only a portion of the subcarriers, a subscriber may provide
a list of subcarriers ordered starting with those subcarriers
similar terms are to be associated with the appropriate
which the subscriber desires to use, usually because their 20 physical quantities and are merely convenient labels applied
performance is good or better than that of other subcarriers.
to these quantities. Unless specifically stated otherwise as
Upon receiving the information from the subscriber, the
apparent from the following discussion, it is appreciated that
base station further selects the subcarriers among the canthroughout the description, discussions utilizing terms such
didates, utilizing additional information available at the base
as "processing" or "computing" or "calculating" or "deterstation, e.g., the traffic load information on each subcarrier, 25 mining" or "displaying" or the like, refer to the action and
amount of traffic requests queued at the base station for each
processes of a computer system, or similar electronic comfrequency band, whether frequency bands are overused,
puting device, that manipulates and transforms data repreand/or how long a subscriber has been waiting to send
sented as physical (electronic) quantities within the cominformation. In one embodiment, the subcarrier loading
puter system's registers and memories into other data
information of neighboring cells can also be exchanged 30
similarly represented as physical quantities within the combetween base stations. The base stations can use this inforputer system memories or registers or other such informamation in subcarrier allocation to reduce inter-cell interfertion storage, transmission or display devices.
ence.
The present invention also relates to apparatus for perIn one embodiment, the selection by the base station of
the channels to allocate, based on the feedback, results in the 35 forming the operations herein. This apparatus may be speselection of coding/modulation rates. Such coding/modulacially constructed for the required purposes, or it may
tion rates may be specified by the subscriber when specifycomprise a general purpose computer selectively activated
ing subcarriers that it finds favorable to use. For example, if
or reconfigured by a computer program stored in the comthe SINR is less than a certain threshold (e.g., 12 dB),
puter. Such a computer program may be stored in a computer
quadrature phase shift keying (QPSK) modulation is used; 40 readable storage medium, such as, but is not limited to, any
otherwise, 16 quadrature amplitude modulation (QAM) is
type of disk including floppy disks, optical disks, CDused. Then the base station informs the subscribers about the
ROMs, and magnetic-optical disks, read-only memories
subcarrier allocation and the coding/modulation rates to use.
(ROMs), random access memories (RAMs), EPROMs,
In one embodiment, the feedback information for downEEPROMs, magnetic or optical cards. or any type of media
link subcarrier allocation is transmitted to the base station 4 5
suitable for storing electronic instructions, and each coupled
through the uplink access channel, which occurs in a short
to a computer system bus.
period every transmission time slot, e.g., 400 microseconds
The algorithms and displays presented herein are not
in every 10-millisecond time slot. In one embodiment, the
inherently related to any particular computer or other appaaccess channel occupies the entire frequency bandwidth.
Then the base station can collect the uplink SINR of each so ratus. Various general purpose systems may be used with
programs in accordance with the teachings herein, or it may
subcarrier directly from the access channel The SINR as
prove convenient to construct more specialized apparatus to
well as the traffic load information on the uplink subcarriers
perform the required method steps. The required structure
are used for uplink subcarrier allocation.
for a variety of these systems will appear from the descripFor either direction, the base station makes the final
55 tion below. In addition, the present invention is not
decision of subcarrier allocation for each subscriber.
described with reference to any particular programming
In the following description, a procedure of selective
language. It will be appreciated that a variety of programsubcarrier allocation is also disclosed, including methods of
ming languages may be used to implement the teachings of
channel and interference sensing, methods of information
the invention as described herein.
feedback from the subscribers to the base station, and
algorithms used by the base station for subcarrier selections. so A machine-readable medium includes any mechanism for
storing or transmitting information in a form readable by a
In the following description, numerous details are set
machine (e.g., a computer). For example, a machine-readforth to provide a thorough understanding of the present
able medium includes read only memory ("ROM"); random
invention. It will be apparent, however, to one skilled in the
access memory ("RAM"); magnetic disk storage media;
art, that the present invention may be practiced without these
specific details. In other instances, well-known structures 65 optical storage media; flash memory devices; electrical,
optical, acoustical or other form of propagated signals (e.g.,
and devices are shown in block diagram form, rather than in
carrier waves, infrared signals, digital signals. etc.); etc.
detail, in order to avoid obscuring the present invention.
US 7,146,172 B2
5
6
Subcarrier Clustering
indicates the coding/modulation rate that the subscriber
desires to use. No cluster index is needed to indicate which
The techniques described herein are directed to subcarrier
SINR value in the feedback corresponds to which cluster as
allocation for data traffic channels In a cellular system, there
long as the order of information in the feedback is known to
are typically other channels, pre-allocated for the exchange
of control information and other purposes. These channels 5 the base station. In an alternative embodiment, the information in the feedback is ordered according to which clusters
often include down link and up link control channels, uplink
have the best performance relative to each other for the
access channels, and time and frequency synchronization
subscriber. In such a case, an index is needed to indicate to
channels.
which cluster the accompanying SINR value corresponds.
FIG. lA illustrates multiple subcarriers, such as subcarrier
Upon receiving the feedback from a subscriber, the base
101, and cluster 102. A cluster, such as cluster 102. is defined 10
station further selects one or more clusters for the subscriber
as a logical unit that contains at least one physical subcarrier,
among the candidates (processing block 104). The base
as shown in FIG. 1A. A cluster can contain consecutive or
station may utilize additional information available at the
disjoint subcarriers. The mapping between a cluster and its
base station, e.g., the traffic load information on each subsubcarriers can be fixed or reconfigurable. In the latter case,
the base station informs the subscribers when the clusters are 15 carrier, amount of traffic requests queued at the base station
for each frequency band, whether frequency bands are
redefined. In one embodiment, the frequency spectrum
overused, and how long a subscriber has been waiting to
includes 512 subcarriers and each cluster includes four
send information. The subcarrier loading information of
consecutive subcarriers, thereby resulting in 128 clusters.
neighboring cells can also be exchanged between base
An Exemplary Subcarrier/Cluster Allocation Procedure
20 stations. The base stations can use this information in
FIG. 1B is a flow diagram of one embodiment of a process
subcarrier allocation to reduce inter-cell interference.
for allocation clusters to subscribers. The process is perAfter cluster selection, the base station notifies the subformed by processing logic that may comprise hardware
scriber about the cluster allocation through a downlink
(e.g.. dedicated logic, circuitry, etc.). software (such as that
common control channel or through a dedicated downlink
which runs on, for example, a general purpose computer 25 traffic channel if the connection to the subscriber has already
system or dedicated machine), or a combination of both.
been established (processing block 105). In one embodiReferring to FIG. 1B, each base station periodically
ment, the base station also informs the subscriber about the
broadcasts pilot OFDM symbols to every subscriber within
appropriate modulation/coding rates.
its cell (or sector) (processing block 101). The pilot symbols,
Once the basic communication link is established, each
often referred to as a sounding sequence or signal, are 30 subscriber can continue to send the feedback to the base
known to both the base station and the subscribers. In one
station using a dedicated traffic channel (e.g., one or more
embodiment, each pilot symbol covers the entire OFDM
predefined uplink access channels).
frequency bandwidth. The pilot symbols may be different for
In one embodiment, the base station allocates all the
different cells (or sectors). The pilot symbols can serve
clusters to be used by a subscriber at once. In an alternative
multiple purposes: time and frequency synchronization, 35 embodiment, the base station first allocates multiple clusters,
channel estimation and signal-to-interference/noise (SINR)
referred to herein as the basic clusters, to establish a data link
ratio measurement for cluster allocation.
between the base station and the subscriber. The base station
Next, each subscriber continuously monitors the reception
then subsequently allocates more clusters, referred to herein
of the pilot symbols and measures the SINR and/or other
as the auxiliary clusters, to the subscriber to increase the
parameters, including inter-cell interference and intra-cell 40 communication bandwidth. Higher priorities can be given to
traffic, of each cluster (processing block 102). Based on this
the assignment of basic clusters and lower priorities may be
information, each subscriber selects one or more clusters
given to that of auxiliary clusters. For example, the base
with good performance (e.g.. high SINR and low traffic
station first ensures the assignment of the basic clusters to
loading) relative to each other and feeds hack the informathe subscribers and then tries to satisfy further requests on
tion on these candidate clusters to the base station through 45 the auxiliary clusters from the subscribers. Alternatively, the
predefined uplink access channels (processing block 103).
base station may assign auxiliary clusters to one or more
For example, SINR values higher than 10 dB may indicate
subscribers before allocating basic clusters to other subscribgood performance. Likewise, a cluster utilization factor less
ers. For example, a base station may allocate basic and
than 50% may be indicative of good performance. Each
auxiliary clusters to one subscriber before allocating any
subscriber selects the clusters with relatively better perfor- 50 clusters to other subscribers. In one embodiment, the base
mance than others. The selection results in each subscriber
station allocates basic clusters to a new subscriber and then
selecting clusters they would prefer to use based on the
determines if there are any other subscribers requesting
measured parameters.
clusters. If not, then the base station allocates the auxiliary
In one embodiment, each subscriber measures the SINR
clusters to that new subscriber.
of each subcarrier cluster and reports these SINR measure- 55
From time to time, processing logic performs retraining
ments to their base station through an access channel The
by repeating the process described above (processing block
SINR value may comprise the average of the SINR values
106). The retraining may be performed periodically. This
of each of the subcaniers in the cluster. Alternatively, the
retraining compensates for subscriber movement and any
SINR value for the cluster may be the worst SINR among
changes in interference. In one embodiment, each subscriber
the SINR values of the subcarriers in the cluster. In still so reports to the base station its updated selection of clusters
another embodiment, a weighted averaging of SINR values
and their associated SINRs. Then the base station further
of the subcarriers in the cluster is used to generate an SINK
performs the reselection and informs the subscriber about
value for the cluster. This may be particularly useful in
the new cluster allocation. Retraining can be initiated by the
diversity clusters where the weighting applied to the subbase station, and in which case, the base station requests a
carriers may be different.
65 specific subscriber to report its updated cluster selection.
The feedback of information from each subscriber to the
Retraining can also be initiated by the subscriber when it
base station contains a SINR value for each cluster and also
observes channel deterioration.
US 7,146,172 B2
7
8
Adaptive Modulation and Coding
Pilot symbols serve an additional purpose in determining
interference among the cells. Since the pilots of multiple
In on embodiment, different modulation and coding rates
cells are broadcast at the same time, they will interfere with
are used to support reliable transmission over channels with
each other (because they occupy the entire frequency band).
different SINR. Signal spreading over multiple subcarriers
may also be used to improve the reliability at very low 5 This collision of pilot symbols may be used to determine the
amount of interference as a worst case scenario. Therefore,
SINR.
in one embodiment, the above SINR estimation using this
An example coding/modulation table is given below in
method is conservative in that the measured interference
Table 1.
level is the worst-case scenario, assuming that all the intern) ference sources are on. Thus, the structure of pilot symbols
TABLE 1
is such that it occupies the entire frequency band and causes
Scheme
Modulation
Code Rate
collisions among different cells for use in detecting the worst
case SINR in packet transmission systems.
0
QPSK, 1/2 Spreading
1/2
During data traffic periods, the subscribers can determine
QPSK, V4 Spreading
1/2
2
QPSK, 1 Spreading
1/2
is the level of interference again. The data traffic periods are
1/2
3
QPSK
1/2
used to estimate the intra-cell traffic as well as the inter-cell
2h
4
8 PSK
interference level. Specifically, the power difference during
5
16 QAM
3 '4
/
the pilot and traffic periods may be used to sense the
6
64 QAM
5
/6
(intra-cell) traffic loading and inter-cell interference to select
20 the desirable clusters.
In the example above, 1/8 spreading indicates that one
The interference level on certain clusters may be lower,
QPSK modulation symbol is repeated over eight subcarriers.
because these clusters may be unused in the neighboring
The repetition/spreading may also be extended to the time
cells. For example. in cell A, with respect to cluster A there
domain. For example, one QPSK symbol can be repeated
is less interference because cluster A is unused in cell B
over four subcarriers of two OFDM symbols, resulting also 25 (while it is used in cell C). Similarly, in cell A, cluster B will
1/8 spreading.
experience lower interference from cell B because cluster B
The coding/modulation rate can be adaptively changed
is used in cell B but not in cell C.
according to the channel conditions observed at the receiver
The modulation/coding rate based on this estimation is
after the initial cluster allocation and rate selection.
robust to frequent interference changes resulted from bursty
30 packet transmission. This is because the rate prediction is
Pilot Symbols and SINR Measurement
based on the worst case situation in which all interference
In one embodiment, each base station transmits pilot
sources are transmitting.
symbols simultaneously, and each pilot symbol occupies the
In one embodiment, a subscriber utilizes the information
entire OFDM frequency bandwidth, as shown in FIGS.
available from both the pilot symbol periods and the data
2A—C. Referring to FIG. 2A-C, pilot symbols 201 are shown
35 traffic periods to analyze the presence of both the intra-cell
traversing the entire OFDM frequency bandwidth for cells
traffic load and inter-cell interference. The goal of the
A, B and C, respectively. In one embodiment, each of the
subscriber is to provide an indication to the base station as
pilot symbols have a length or duration of 128 microseconds
to those clusters that the subscriber desires to use. Ideally,
with a guard time, the combination of which is approxithe result of the selection by the subscriber is clusters with
mately 152 microseconds. After each pilot period, there are
40 high channel gain, low interference from other cells, and
a predetermined number of data periods followed by another
high availability. The subscriber provides feedback inforset of pilot symbols. In one embodiment, there are four data
mation that includes the results, listing desired clusters in
periods used to transmit data after each pilot, and each of the
order or not as described herein.
data periods is 152 microseconds.
FIG. 3 illustrates one embodiment of subscriber processA subscriber estimates the SINR for each cluster from the 45 ing. The processing is performed by processing logic that
pilot symbols. In one embodiment, the subscriber first
may comprise hardware (e.g., dedicated logic, circuitry,
estimates the channel response, including the amplitude and
etc.), software (such as that which runs on, for example, a
phase, as if there is no interference or noise. Once the
general purpose computer system or dedicated machine), or
channel is estimated, the subscriber calculates the interfera combination of both.
ence/noise from the received signal.
so
Referring to FIG. 3, channel/interference estimation proThe estimated SINR values may be ordered from largest
cessing block 301 performs channel and interference estito smallest SINRs and the clusters with large SINR values
mation in pilot periods in response to pilot symbols. Traffic/
are selected. In one embodiment, the selected clusters have
interference analysis processing block 302 performs traffic
SINR values that are larger than the minimum SINR which
and interference analysis in data periods in response to
still allows a reliable (albeit low-rate) transmission sup- 55 signal information and information from channel/interferported by the system. The number of clusters selected may
ence estimation block 301.
depend on the feedback bandwidth and the request transCluster ordering and rate prediction processing block 303
mission rate. In one embodiment, the subscriber always tries
is coupled to outputs of channel/interference estimation
to send the information about as many clusters as possible
processing block 301 and traffic/interference analysis profrom which the base station chooses.
6c) cessing block 302 to perform cluster ordering and selection
The estimated SINR values are also used to choose the
along with rate prediction.
appropriate coding/modulation rate for each cluster as disThe output of cluster ordering processing block 303 is
cussed above. By using an appropriate SINR indexing
input to cluster request processing block 304, which requests
scheme, an SINR index may also indicate a particular coding
clusters and modulation/coding rates. Indications of these
and modulation rate that a subscriber desires to use. Note 65 selections are sent to the base station. In one embodiment,
that even for the same subscribers, different clusters can
the SINR on each cluster is reported to the base station
have different modulation/coding rates.
through an access channel. The information is used for
US 7,146,172 B2
9
cluster selection to avoid clusters with heavy intra-cell traffic
loading and/or strong interference from other cells. That is,
a new subscriber may not be allocated use of a particular
cluster if heavy intra-cell traffic loading already exists with
respect to that cluster. Also, clusters may not be allocated if
the interference is so strong that the SINR only allows for
low-rate transmission or no reliable transmission at all.
The channel/interference estimation by processing block
301 is well-known in the art by monitoring the interference
that is generated due to full-bandwidth pilot symbols being
simultaneously broadcast in multiple cells. The interface
information is forwarded to processing block 302 which
uses the information to solve the following equation:
10
More specifically, in one embodiment. the signal power of
each cluster during the pilot periods is compared with that
during the traffic periods, according to the following:
5
PP = PS +PI + PN,
PN,
with no signal and interference
Ps + PN, with signal only
PD =
+ PN,
10
with interference only
Ps + Pi + PN, with both signal and interference
Ps + PI, with no signal and interference
P1, with signal only
PP = PD =
15
where S, represents the signal for subcarrier (freq. band) i,
is the interference for subcarrier i, n, is the noise associated
with subcarrier i, and y, is the observation for subcarrier i. In
the case of 512 subcarriers, i may range from 0 to 511. The
20
I, and ff are not separated and may be considered one
quantity. The interference/noise and channel gain H, are not
know. During pilot periods, the signal S, representing the
pilot symbols, and the observation y, are knowns, thereby
allowing determination of the channel gain H, for the case
25
where there is no interference or noise. Once this is known,
it may be plugged back into the equation to determine the
interference/noise during data periods since H, S, and y, are
all known.
The interference information from processing blocks 301 30
and 302 are used by the subscriber to select desirable
clusters. In one embodiment, using processing block 303,
the subscriber orders clusters and also predicts the data rate
that would be available using such clusters. The predicted
data rate information may be obtained from a look up table 35
with precalculated data rate values. Such a look up table may
store the pairs of each SINR and its associated desirable
transmission rate. Based on this information, the subscriber
selects clusters that it desires to use based on predetermined
performance criteria. Using the ordered list of clusters, the 40
subscriber requests the desired clusters along with coding
and modulation rates known to the subscriber to achieve
desired data rates.
FIG. 4 is one embodiment of an apparatus for the selection of clusters based on power difference. The approach 45
uses information available during both pilot symbol periods
and data traffic periods to perform energy detection. The
processing of FIG. 4 may be implemented in hardware, (e.g.,
dedicated logic, circuitry, etc.), software (such as is run on,
for example, a general purpose computer system or dedi- 50
cated machine), or a combination of both.
Referring to FIG. 4, a subscriber includes SINR estimation processing block 401 to perform SINR estimation for
each cluster in pilot periods, power calculation processing
block 402 to perform power calculations for each cluster in 55
pilot periods, and power calculation processing block 403 to
perform power calculations in data periods for each cluster.
Subtractor 404 subtracts the power calculations for data
periods from processing block 403 from those in pilot
periods from processing block 402. The output of subtractor 60
404 is input to power difference ordering (and group selection) processing block 405 that performs cluster ordering
and selection based on SINR and the power difference
between pilot periods and data periods. Once the clusters
have been selected, the subscriber requests the selected 65
clusters and the coding/modulation rates with processing
block 406.
Ps, with interference only
0, with both signal and interference
where Pp is the measured power corresponding to each
cluster during pilot periods, PD is the measured power during
the traffic periods, P c is the signal power, P1 is the interference power, and P„ is the noise power.
In one embodiment, the subscriber selects clusters with
relatively large Pp/(Pp—PD) (e.g., larger than a threshold
such as 10 dB) and avoids clusters with low PAPF—PD)
(e.g., lower than a threshold such as 10 dB) when possible.
Alternatively, the difference may be based on the energy
difference between observed samples during the pilot period
and during the data traffic period for each of the subcarriers
in a cluster such as the following:
o
=1YrI — IYT)I
Thus, the subscriber sums the differences for all subcarriers.
Depending on the actual implementation, a subscriber
may use the following metric, a combined function of both
SINK and pp—PD, to select the clusters:
fi=ftsINR, pit/(Pp-PD)
where f is a function of the two inputs. One example of f
is weighted averaging (e.g., equal weights). Alternatively, a
subscriber selects a cluster based on its SINR and only uses
the power difference PP PD to distinguish clusters with
similar SINR. The difference may be smaller than a threshold (e.g., 1 dB).
Both the measurement of SINR and Pp—PE, can be averaged over time to reduce variance and improve accuracy. In
one embodiment, a moving-average time window is used
that is long enough to average out the statistical abnormity
yet short enough to capture the time-varying nature of
channel and interference. e.g., 1 millisecond.
Feedback Format for Downlink Cluster Allocation
In one embodiment, for the downlink, the feedback contains both the indices of selected clusters and their SINR. An
exemplary format for arbitrary cluster feedback is shown in
FIG. 5. Referring to FIG. 5, the subscriber provides a cluster
index (ID) to indicate the cluster and its associated SINR
value. For example, in the feedback, the subscriber provides
cluster ID1 (501) and the SINR for the cluster, SINR1 (502),
cluster ID2 (503) and the SINR for the cluster, SINR2 (504),
and cluster ID3 (505), and the SINR for the cluster, SINR3
(506), etc. The SINR for the cluster may be created using an
average of the SINRs of the subcarriers. Thus, multiple
arbitrary clusters can be selected as the candidates. As
US 7,146,172 B2
11
discussed above, the selected clusters can also be ordered in
the feedback to indicate priority. In one embodiment, the
subscriber may form a priority list of clusters and sends back
the SINR information in a descending order of priority.
Typically, an index to the SINR level, instead of the SINR
itself is sufficient to indicate the appropriate coding/modulation for the cluster. For example, a 3-bit field can be used
for SINR indexing to indicate 8 different rates of adaptive
coding/modulation.
An Exemplary Base Station
The base station assigns desirable clusters to the subscriber making the request. In one embodiment, the availability of the cluster for allocation to a subscriber depends on
the total traffic load on the cluster. Therefore, the base station
selects the clusters not only with high SINR, but also with
low traffic load.
FIG. 13 is a block diagram of one embodiment of a base
station. Referring to FIG. 13, cluster allocation and load
scheduling controller 1301 (cluster allocator) collects all the
necessary information, including the downlink/uplink SINR
of clusters specified for each subscriber (e.g., via SINR/rate
indices signals 1313 received from OFDM transceiver 1305)
and user data, queue fullness/traffic load (e.g., via user data
buffer information 1311 from multi-user data buffer 1302).
Using this information, controller 1301 makes the decision
on cluster allocation and load scheduling for each user, and
stores the decision information in a memory (not shown).
Controller 1301 informs the subscribers about the decisions
through control signal channels (e.g., control signal/cluster
allocation 1312 via OFDM transceiver 1305). Controller
1301 updates the decisions during retraining.
In one embodiment, controller 1301 also performs admission control to user access since it knows the traffic load of
the system. This may be performed by controlling user data
buffers 1302 using admission control signals 1310.
The packet data of User 1—N are stored in the user data
buffers 1302. For downlink, with the control of controller
1301, multiplexer 1303 loads the user data to cluster data
buffers (for Cluster 1-4\4) waiting to be transmitted. For the
uplink, multiplexer 1303 sends the data in the cluster buffers
to the corresponding user buffers. Cluster buffer 1304 stores
the signal to be transmitted through OFDM transceiver 1305
(for downlink) and the signal received from transceiver
1305. In one embodiment. each user might occupy multiple
clusters and each cluster might be shared by multiple users
(in a time-division-multiplexing fashion).
Group-Based Cluster Allocation
In another embodiment. for the downlink, the clusters are
partitioned into groups. Each group can include multiple
clusters. FIG. 6 illustrates an exemplary partitioning. Referring to FIG. 6, groups 1-4 are shown with arrows pointing
to clusters that are in each group as a result of the partitioning. In one embodiment, the clusters within each group are
spaced far apart over the entire bandwidth. In one embodiment, the clusters within each group are spaced apart farther
than the channel coherence bandwidth, i.e. the bandwidth
within which the channel response remains roughly the
same. A typical value of coherence bandwidth is 100 kHz for
many cellular systems. This improves frequency diversity
within each group and increases the probability that at least
some of the clusters within a group can provide high SINR.
The clusters may be allocated in groups.
Goals of group-based cluster allocation include reducing
the data bits for cluster indexing, thereby reducing the
bandwidth requirements of the feedback channel (informa-
12
tion) and control channel (information) for cluster allocation. Group-based cluster allocation may also be used to
reduce inter-cell interference.
After receiving the pilot signal from the base station, a
5 subscriber sends back the channel information on one or
more cluster groups, simultaneously or sequentially. In one
embodiment, only the information on some of the groups is
sent back to the base station. Many criteria can be used to
choose and order the groups, based on the channel inform mation, the inter-cell interference levels, and the intra-cell
traffic load on each cluster.
In one embodiment, a subscriber first selects the group
with the best overall performance and then feedbacks the
SINR information for the clusters in that group. The subis scriber may order the groups based on their number of
clusters for which the SINR is higher than a predefined
threshold. By transmitting the SINR of all the clusters in the
group sequentially. only the group index, instead of all the
cluster indices, needs to be transmitted. Thus, the feedback
20 for each group generally contains two types of information:
the group index and the SINR value of each cluster within
the group. FIG. 7 illustrates an exemplary format for indicating a group-based cluster allocation. Referring to FIG. 7,
a group ID, IDE is followed by the SINR values for each of
25 the clusters in the group. This can significantly reduce the
feedback overhead.
Upon receiving the feedback information from the subscriber, the cluster allocator at the base station selects
multiple clusters from one or more groups, if available, and
30 then assigns the clusters to the subscriber. This selection
may be performed by an allocation in a media access control
portion of the base station.
Furthermore, in a multi-cell environment, groups can
have different priorities associated with different cells. In
35 one embodiment, the subscriber's selection of a group is
biased by the group priority, which means that certain
subscribers have higher priorities on the usage of some
groups than the other subscribers.
In one embodiment, there is no fixed association between
40 one subscriber and one cluster group; however, in an alternative embodiment there may be such a fixed association. In
an implementation having a fixed association between a
subscriber and one or more cluster groups, the group index
in the feedback information can be omitted, because this
45 information is known to both subscriber and base station by
default.
In another embodiment, the pilot signal sent from the base
station to the subscriber also indicates the availability of
each cluster, e.g., the pilot signal shows which clusters have
so already been allocated for other subscribers and which
clusters are available for new allocations. For example, the
base station can transmit a pilot sequence 1111 1111 on the
subcarriers of a cluster to indicate that the cluster is available, and 1111-1-1-1-1 to indicate the cluster is not avail55 able. At the receiver, the subscriber first distinguishes the
two sequences using the signal processing methods which
are well known in the art, e.g., the correlation methods, and
then estimates the channel and interference level.
With the combination of this information and the channel
so characteristics obtained by the subscriber, the subscriber can
prioritize the groups to achieve both high SINR and good
load balancing.
In one embodiment, the subscriber protects the feedback
information by using error correcting codes. In one embodi65 ment, the SINR information in the feedback is first compressed using source coding techniques, e.g., differential
encoding, and then encoded by the channel codes.
US 7,146,172 B2
14
13
FIG. 8 shows one embodiment of a frequency reuse
pattern for an exemplary cellular set up. Each cell has
hexagonal structure with six sectors using directional antennas at the base stations. Between the cells, the frequency
reuse factor is one. Within each cell, the frequency reuse
factor is 2 where the sectors use two frequencies alternatively. As shown in FIG. 8, each shaded sector uses half of
the available OFDMA clusters and each unshaded sector
uses the other half of the clusters. Without loss of generality,
the clusters used by the shaded sectors are referred to herein
as odd clusters and those used by the unshaded sectors are
referred to herein as even clusters.
Consider the downlink signaling with omni-directional
antennas at the subscribers. From FIG. 8, it is clear that for
the downlink in the shaded sectors, Cell A interferes with
Cell B, which in turn interferes with Cell C, which in turn
interferes with Cell A, namely, A->B->C->A. For the
unshaded sectors, Cell A interferes with Cell C. which in
turn interferes with Cell B, which in turn interferes with Cell
A, namely, A->C->B->A.
Sector Al receives interference from Sector Cl, but its
transmission interferes with Sector Bl. Namely, its interference source and the victims with which it interferes are not
the same. This might cause a stability problem in a distributed cluster-allocation system using interference avoidance:
if a frequency cluster is assigned in Sector B1 but not in
Sector Cl, the cluster may be assigned in Al because it may
be seen as clean in Al. However, the assignment of this
cluster Al can cause interference problem to the existing
assignment in Bl.
In one embodiment, different cluster groups are assigned
different priorities for use in different cells to alleviate the
aforementioned problem when the traffic load is progressively added to a sector. The priority orders are jointly
designed such that a cluster can be selectively assigned to
avoid interference from its interference source, while reducing, and potentially minimizing, the probability of causing
interference problem to existing assignments in other cells.
Using the aforementioned example, the odd clusters (used
by the shaded sectors) are partitioned into 3 groups: Group
1, 2, 3. The priority orders are listed in Table 2.
TABLE 2
Priority ordering for the downlink of the shaded sectors.
Priority Ordering
1
2
3
Cell A
Cell B
Cell C
Group 1
Group 2
Group 3
Group 3
Group 1
Group 2
Group 2
Group 3
Group 1
Consider Sector Al. First, the clusters in Group 1 are
selectively assigned. If there are still more subscribers
demanding clusters, the clusters in Group 2 are selectively
assigned to subscribers, depending on the measured SINR
(avoiding the clusters receiving strong interference from
Sector Cl). Note that the newly assigned clusters from
Group 2 to Sector Al shall not cause interference problem
in Sector Bl, unless the load in Sector Bl is so heavy that
the clusters in both Group 3 and 1 are used up and the
clusters in Group 2 are also used. Table 3 shows the cluster
usage when less than 2/3 all the available clusters are used in
Sector Al, Bl, and Cl.
TABLE 3
5
Cluster usage for the downlink of the shaded sectors
with less than 2/3 of the full load.
Cluster Usage
1
2
3
Cell A
Cell B
Cell C
Group 1
Group 2
Group 3
Group 1
Group 2
Group 3
10
Table 4 shows the priority orders for the unshaded sectors,
which are different from those for the shaded sectors, since
the interfering relationship is reversed.
15
TABLE 4
Priority ordering for the downlink of the unshaded sectors.
Priority Ordering
Cell A
Cell B
Cell C
Group 1
Group 2
Group 3
Group 2
Group 3
Group 1
Group 3
Group 1
Group 2
20
1
2
3
25
Intelligent Switching between Coherence and Diversity
Clusters
In one embodiment, there are two categories of clusters:
coherence clusters. containing multiple subcarriers close to
each other and diversity clusters, containing multiple sub30
carriers with at least some of the subcarriers spread far apart
over the spectrum. The closeness of the multiple subcarriers
in coherence clusters is preferably within the channel coherence bandwidth, i.e. the bandwidth within which the channel
response remains roughly the same, which is typically
35
within 100 kIIz for many cellular systems. On the other
hand, the spread of subcarriers in diversity clusters is
preferably larger than the channel coherence bandwidth,
typically within 100 kHz for many cellular systems. Of
course, the larger the spread, the better the diversity. There40
fore, a general goal in such cases is to maximize the spread.
FIG. 9 illustrates exemplary cluster formats for coherence
clusters and diversity clusters for Cells A—C. Referring to
FIG. 9, for cells A—C, the labeling of frequencies (subcar45 riers) indicates whether the frequencies are part of coherence
or diversity clusters. For example. those frequencies labeled
1-8 are diversity clusters and those labeled 9-16 are coherence clusters. For example, all frequencies labeled 1 in a cell
are part of one diversity cluster, all frequencies labeled 2 in
sr) a cell are part of another diversity cluster, etc., while the
group of frequencies labeled 9 are one coherence cluster, the
group of frequencies labeled 10 are another coherence
cluster, etc. The diversity clusters can be configured differently for different cells to reduce the effect of inter-cell
55 interference through interference averaging.
FIG. 9 shows example cluster configurations for three
neighboring cells. The interference from a particular cluster
in one cell are distributed to many clusters in other cells,
e.g.. the interference from Cluster 1 in Cell A are distributed
so to Cluster 1, 8. 7, 6 in Cell B. This significantly reduces the
interference power to any particular cluster in Cell B.
Likewise, the interference to any particular cluster in one
cell comes from many different clusters in other cells. Since
not all cluster are strong interferers. diversity clusters, with
65 channel coding across its subcarriers, provide interference
diversity gain. Therefore, it is advantageous to assign diversity clusters to subscribers that are close (e.g., within the
US 7,146,172 B2
15
16
coherent bandwidth) to the cell boundaries and are more
station. The channel/interference variation detector measubject to inter-cell interference.
sures the channel (SINR) variation from time to time for
Since the subcarriers in a coherence cluster are consecueach cluster. For example, in one embodiment, the channel;
tive or close (e.g., within the coherent bandwidth) to each
interference detector measures the power difference between
other, they are likely within the coherent bandwidth of the 5 pilot symbols for each cluster and averages the difference
channel fading. Therefore, the channel gain of a coherence
over a moving window (e.g., 4 time slots). A large difference
cluster can vary significantly and cluster selection can
indicates that channel/interference changes frequently and
greatly improve the performance. On the other hand, the
subcarrier allocation may be not reliable. In such a case,
average channel gain of a diversity cluster has less of a
diversity clusters are more desirable for the subscriber.
degree of variation due to the inherent frequency diversity m
FIG. 11 is a flow diagram of one embodiment of a process
among the multiple subcarriers spread over the spectrum.
for intelligent selection between diversity clusters and
With channel coding across the subcarriers within the cluscoherence clusters depending on subscribers mobility. The
ter, diversity clusters are more robust to cluster mis-selection
process is performed by processing logic that may comprise
(by the nature of diversification itself), while yielding poshardware (e.g., circuitry, dedicated logic, etc.), software
sibly less gain from cluster selection Channel coding across 15 (such as that which runs on, for example, a general purpose
the subcarriers means that each codeword contains bits
computer system or dedicated machine), or a combination of
transmitted from multiple subcarriers, and more specifically,
both.
the difference bits between codewords (error vector) are
Referring to FIG. 11, processing logic in the base station
distributed among multiple subcarriers.
performs channel/interference variation detection (processMore frequency diversity can be obtained through sub- 20 ing block 1101). Processing logic then tests whether the
carrier hopping over time in which a subscriber occupies a
results of the channel/interference variation detection indiset of subcarriers at one time slot and another different set of
cate that the user is mobile or in a fixed position close to the
subcarriers at a different time slot. One coding unit (frame)
edge of the cell (processing block 1102). If the user is not
contains multiple such time slots and the transmitted bits are
mobile or is not in a fixed position close to the edge of the
encoded across the entire frame.
25 cell, processing transitions to processing block 1103 where
FIG. 10 illustrates diversity cluster with subcarrier hopprocessing logic in the base station selects coherence clusping. Referring to FIG. 10, there are four diversity clusters
ters; otherwise, processing transitions to processing block
in each of cells A and B shown, with each subcarrier in
1104 in which processing logic in the base station selects
individual diversity clusters having the same label (1, 2, 3,
diversity clusters.
or 4). There are four separate time slots shown and during 30
In one embodiment, the base station determines whether
each of the time slots, the subcarriers for each of the
a subscriber is mobile or fixed by detecting a rate of change
diversity clusters change. For example, in cell A, subcarrier
of pilot signals, or the normalized channel variation, and
1 is part of diversity cluster 1 during time slot 1. is part of
determining that the rate of change is greater than a predediversity cluster 2 during time slot 2, is part of diversity
termined threshold. The normalized instantaneous difference
cluster 3 during time slot 3, and is part of diversity cluster 35 between channels may be represented as
4 during time slot 4. Thus, more interference diversity can
be obtained through subcarrier hopping over time. with
further interference diversity achieved by using different
11 - 112
-li
hopping patterns for different cells, as shown in FIG. 10.
The manner in which the subscriber changes the subcar- 40
riers (hopping sequences) can be different for different cells
in order to achieve better interference averaging through
where 1-1, represents the channel and i is the index to
coding.
represent the individual channels.
For static subscribers, such as in fixed wireless access. the
The threshold is system dependent. For example, the rate
channels change very little over time. Selective cluster 45
of change is greater than 10% (although any precentage
allocation using the coherence clusters achieves good per(e.g., 20%) could be used), then the base station concludes
formance. On the other hand, for mobile subscribers, the
that the subscriber is mobile. In one embodiment, if the
channel time variance (the variance due to changes in the
constant period in signaling is not greater than a multiple of
channel over time) can be very large. A high-gain cluster at
one time can be in deep fade at another. Therefore, cluster 50 the round trip delay (e.g., 5 times the round trip delay), then
the base station determines that the subscriber is mobile and
allocation needs to be updated at a rapid rate, causing
allocates diversity clusters; otherwise, the base station allosignificant control overhead. In this case, diversity clusters
cates coherence clusters.
can be used to provide extra robustness and to alleviate the
The selection can be updated and intelligently switched
overhead of frequent cluster reallocation. In one embodiment, cluster allocation is performed faster than the channel 55 during retraining.
The ratio/allocation of the numbers of coherence and
changing rate, which is often measured by the channel
diversity clusters in a cell depends on the ratio of the
Doppler rate (in Hz), i.e. how many cycles the channel
population of mobile and fixed subscribers. When the popuchanges per second where the channel is completely differlation changes as the system evolves, the allocation of
ent after one cycle. Note that selective cluster allocation can
60 coherence and diversity clusters can be reconfigured to
be performed on both coherence and diversity clusters.
accommodate the new system needs. FIG. 12 illustrates a
In one embodiment, for cells containing mixed mobile
reconfiguration of cluster classification which can support
and fixed subscribers, a channel/interference variation detecmore mobile subscribers than that in FIG. 9.
tor can be implemented at either the subscriber or the base
station, or both. Using the detection results, the subscriber
Whereas many alterations and modifications of the
and the base station intelligently selects diversity clusters to 65 present invention will no doubt become apparent to a person
mobile subscribers or fixed subscribers at cell boundaries,
of ordinary skill in the art after having read the foregoing
and coherence clusters to fixed subscribers close to the base
description, it is to be understood that any particular embodi-
US 7,146,172 B2
17
18
ment shown and described by way of illustration is in no
7. The method defined in claim 1 wherein subcarriers of
way intended to be considered limiting. Therefore, referone coherence cluster are within the coherent bandwidth of
ences to details of various embodiments are not intended to
a channel between a base station and a subscriber.
limit the scope of the claims which in themselves recite only
8. The method defined in claim I further comprising
5
those features regarded as essential to the invention.
updating allocation of clusters to the subscriber.
We claim:
9. The method defined in claim 1 further comprising
1. A method for use in allocating subcarriers in an
reconfiguring cluster classification when population of
OFDMA system comprising
mobile and fixed subscribers in a cell changes.
allocating at least one diversity cluster of subcarriers to a
first subscriber; and
10
10. The method defined in claim 1 wherein the at least one
allocating at least one coherence cluster to a second
diversity cluster is configured to reduce the effect of intersubscriber. such that communication with the first and
cell interference.
second subscribers is able to occur by simultaneously
11. The method defined in claim 1 further comprising
using the at least one diversity cluster and the at least
determining characteristics of the first subscriber and the
one coherence cluster, respectively.
15 second subscriber, wherein allocating the at least one diver2. The method defined in claim 1 wherein the first
sity cluster of subcarriers to the first subscriber and allocatsubscriber comprises a mobile subscriber and the second
ing the at least one coherence cluster to the second subsubscriber comprises a fixed subscriber.
scriber are based on determined characteristics of the first
3. The method defined in claim 1 wherein the first
and second subscribers.
subscriber comprises a fixed subscriber located at a cell 2 0
12. The method defined in claim 11 wherein the characedge.
teristics comprise whether the first and second subscribers
4. The method defined in claim 1 further comprising
are fixed or mobile subscribers.
transmitting information using one diversity cluster while
13. The method defined in claim 1 further comprising
performing frequency hopping.
5. The method defined in claim 1 wherein using one 25 adaptively switching allocation of clusters to either the first
or second subscriber so that the first subscriber is allocated
diversity cluster includes channel coding across subcarriers
at least one coherence cluster of subcarriers or the second
of the one diversity cluster.
subscriber is allocated at least one diversity cluster of
6. The method defined in claim 1 further comprising
subscribers based on channel conditions and subscriber
transmitting codewords in which each codeword contains
bits transmitted from multiple subcarriers and with differ- 30 characteristics.
ence bits between codewords being distributed among multiple subcarriers.
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