USOO7454.212B2

(12) United States Patent (10) Patent No.: US 7.454.212 B2 Li et al. (45) Date of Patent: Nov. 18, 2008

(54) OFDMA WITH ADAPTIVE (56) References Cited SUBCARRIER-CLUSTER CONFIGURATION U.S. PATENT DOCUMENTS AND SELECTIVE LOADING (75) Inventors: Xiaodong Li, Bellevue, WA (US); Hui 4,670,889 A 6, 1987 Hewitt et al. Liu, Sammamish, WA (US); Kemin Li, Bellevue, WA (US); Wenzhong Zhang, Bellevue, WA (US) (Continued) (73) Assignee: Adaptix, Inc., Bellevue, WA (US) FOREIGN PATENT DOCUMENTS *) NotOt1Ce: Subjubject to anyy d1Sclaimer,disclai theh term off thithis patent is extended or adjusted under 35 DE 198 OO 953 7, 1999 U.S.C. 154(b) by 0 days. (21) Appl. No.: 11/199,586 (22) Filed: Aug. 8, 2005 (Continued) (65) Prior Publication Data OTHER PUBLICATIONS US 2006/OO83210 A1 Apr. 20, 2006 O O Wong et al. “Multiuser OFDM with Adaptive Subcarrier, Bit, and Related U.S. Application Data Power Allocation', IEEE Journal on Selected Areas in Communica (63) Continuation of application No. 09/738,086, filed on tions. IEEE. New York, US, 1999, vol. 17, NR. 10, pp. 1747-1758. Dec. 15, 2000, now Pat. No. 6,947,748. Mexican Office Action issued for PA/a/2003/005311 dated Mar. 31, (51) Int. Cl. 2006. H04B 700 (2006.01) (Continued) H04B 7/00 (2006.01) H04O 7/20 (2006.01) Primary Examiner Meless N Zewdu H04O 700 (2006.01) (74) Attorney, Agent, or Firm Fulbright & Jaworski L.L.P. H04Q 7/28 (2006.01) H04M I/00 (2006.01) (57) ABSTRACT H04M I/38 (2006.01) (52) U.S. Cl...... 455/450; 455/67.11:455/69; 455/452.2:455/464:455/509:455/55O1 A method and apparatus for Subcarrier selection for systems 455/556.2:455/561; 370/329; 370/341 is described. In one embodiment, the system employs (58) Field of Classification Search ...... 45.5/1791. Orthogonal frequency division multiple access (OFDMA). In 455/1881,422.1, 516 -517, 67.11561, 562 1. one embodiment, a method for Subcarrier selection comprises 455/132-135,456.5 456.6455. 423 425 each of multiple Subscribers measuring channel and interfer 455/63. 1632. 62–45, 41 24 3. 443 453. ence information for subcarriers based on pilot symbols 455/463-464 5095 10 553 512 5 13. 524 526. received from a base station, at least one of subscribers select 455,550. 1681 76.1 69 70. 266 403. ing a set of candidate Subcarriers, providing feedback infor 455/500,5562.370203210.311.346 347. mation on the set of candidate Subcarriers to the base station, 370/465,480. 31 2 31431 9 322. 328330. and the one Subscriber receiving an indication of Subcarriers 370/338, 341344, 395 .21, 395 .41, 430. of the set of subcarriers selected by the base station for use by 370/437,447,449,458,461462,913.375/31 the one subscriber. 375/240, 240.07, 240.11 See application file for complete search history. 33 Claims, 7 Drawing Sheets

Periodically Broadcast Pilot 101 OFMSymbolsta Subscribers

Subscribers) Continuously Monitors PilotSymbols. Measures.SINRandlor 102 Of Palaters

Each Subscriber Selects One or lore Clusters for each Base Station 13

Base Static Selects One or More Custers for Each Subscriber 104

Base Station Notifies the Subscriber regarding Cluster Allocation 15

US 7.454.212 B2 Page 2

U.S. PATENT DOCUMENTS 2005/0O25099 A1 2/2005 Heath et al. 5,280,630 1, 1994 Wang FOREIGN PATENT DOCUMENTS 5.437,054 7, 1995 Rappaport et al. 5,479.447 12, 1995 Chow et al. DE 198 OO 953 C1 7, 1999 4, 1996 DE 198OO953 * 7/1999 5,504,775 Chouly et al. DE O198OO953 C1 * T 1999 4, 1996 Bodin et al...... 455,34.1 5,507,034 EP O 869 647 A2 10, 1998 5,515,378 5, 1996 Roy, III et al. EP O882377 B1 4f1999 5,555,268 9, 1996 Fattouche et al. EP O 926912 A2 6, 1999 5,588,020 12, 1996 Schilling EP O929 202 A1 7, 1999 5,708,973 1, 1998 Ritter EP O999 658 5, 2000 5,726,978 3, 1998 Frodigh et al...... 370,252 EP O999658 5, 2000 5,734,967 3, 1998 Kotzin et al. FR 27774O7 A1 10, 1999 5,774,808 6, 1998 Sarkioja et al. JP 06029922 2, 1994 5,822,372 10, 1998 Emami KR 1999-28244 4f1999 5,839,074 11, 1998 Plehn et al. WO WO98, 16077 A2 4f1998 5,867.478 2, 1999 Baum et al. WO WO 98.30047 A1 7, 1998 5,886,988 3, 1999 Yun et al. WO WOO2493.05 A2 6, 2002 5,887,245 3, 1999 Lindroth et al. 5,909,436 6, 1999 Engstrom et al. OTHER PUBLICATIONS 5,914,933 6, 1999 Cimini et al. Ye Li et al.: "Clustered OFDM with Channel Estimation for High 5,933,421 8, 1999 Alamouti et al. Rate Wireless Data'; Mobile Multimedia Communications, 1999. 5,956,642 9, 1999 Larsson et al...... 455,449 (MoMuC99) 1999 IEEE International Workshop on Nov. 15-17, 5,973,642 10, 1999 Li et al. 1999: pp. 43-50. 5.991,273 11, 1999 Abu-Dayya Korean Office Action issued for 2003-7007962 dated Apr. 28, 2006. 6,005,876 12, 1999 Cimini, Jr. et al. Korean Office Action issued for 2003-7007963 dated Apr. 29, 2006. 6,009,553 12, 1999 Martinez et al. Bender et al., CDMA/HDR: A Bandwidth-Efficient High-Speed 6,023,622 2, 2000 Plaschke et al. Wireless Data Service for Nomadic Users, IEEE Communications 6,026,123 2, 2000 Williams Magazine, Jul. 2000, pp. 70-87. ** 6,041.237 3, 2000 Farsakh Frullone et al., PRMA Performance in Cellular Environments with 6,052,594 4, 2000 Chuang et al...... 455,450 Self-Adaptive Channel Allocation Strategies, IEEE Transactions on 6,061,568 5, 2000 Dent Vehicular Technology, Nov. 1996, pp. 657-665, vol. 45, No. 4. ** 5, 2000 Chow Xu, Guanghan and Li, San-Qi. Throughput Multiplication of Wire 6,064,692 less Lans for Multimedia Services: SDMA Protocol Design, 1994 6,064,694 5, 2000 Clark et al. IEEE, pp. 1326-1332. ** 6,067,290 5, 2000 Paulrajetal. Ward, James and Compton, R. Ted, Jr., High Throughput Slotted 6,091.955 T/2000 Aalto et al...... 455,447 Aloha Packet Radio Networks with Adaptive Arrays, IEEE Transac 6,108,374 8, 2000 Balachandran et al. tions on Communications, Mar. 1993, pp. 460-470, vol. 41, No. 3.* 6,111,919 8, 2000 Yonge, III Tsoulos, G.V. Smart Antennas for Mobile Communication Systems: 6,131,016 10, 2000 Greenstein et al. Benefits and Challenges, Electronics & Communication Engineering 6,141,565 10, 2000 Feuerstein et al...... 455,560 Journal, Apr. 1999, pp. 84-94.** 6,144,696 11, 2000 Shively et al. Shadet al., Indoor SDMA Capacity Using a Smart Antenna Basesta 6,226,320 5, 2001 Hakkinen et al. tion, 1997 IEEE, pp. 868-872.** 6,282,185 8, 2001 Hakkinen et al. Farsakh, Christof and Nossek, Josef A. On the Mobile Radio Capac 6,298,092 10, 2001 Heath, Jr. ity Increase Through SDMA, no date (after 1997).** 6,307,851 10, 2001 Jung et al. Farsakh, C. et al., “Maximizing the SDMA Mobile Radio Capacity 6,327,472 12, 2001 Westroos et al...... 455,450 Increase by DOA Sensitive Channel Allocation.” Wireless Personal 6,330,460 12, 2001 Wong et al...... 455,562 Communications, Kluwer Academic Publishers, NL, vol. 11, No. 1. 6,366,195 4, 2002 Harel et al. Oct. 1999, pp. 63-76, XP000835062, ISSN: 0929-6212.** 6,377,632 4, 2002 Paulrajetal. Wong, C.Y., et al., Multiuser OFDM With Adaptive Subcarrier, Bit, 6,377,636 4, 2002 Paulrajetal. and Power Allocation, IEEE Journal on Selected Areas in Commu 6,411, 186 6, 2002 Lilleberg et al. nications, Oct. 1999, IEEE Inc., New York, USA, vol. 17, Nr. 10, pp. T/2002 Liew 1747-1758, XPO00855475, ISSN: 0733-8716 Sections I and II 6,415,153 abstract. 6,449,246 9, 2002 Barton et al. Gruenheid, R. et al: "Adaptive Modulcation and Multiple Access for 6,473,467 10, 2002 Wallace et al...... 375,267 the OFDM Transmission Technique.” Wireless Personal Communi 6,477,158 11, 2002 Take cations, Kluwer Academic Publishers, NL, vol. 13, NR. 1/2, Year 6,526,281 2, 2003 Gorsuch et al...... 455,452.1 2000, pp. 5-13 XPO008.94156, ISSN: 0929-6212.** 6,545,997 4, 2003 Bohnke et al. Motegi, M. et al.: Optimum Band Allocation According to Subband 6,553,011 4, 2003 Yan et al...... 370,328 Condition for BST-OFDM 11th IEEE International Symposium on 6,567,383 5/2003 Bohnke Personal Indoor and Modile Radio Communications, vol. 2, Sep. 6,657,949 12, 2003 Jones, IV et al. 18-21, 2000, pp. 1236-1240, XP002213669, Piscataway, NJ, USA, 6,726.297 4, 2004 Uesugi ISBN: 0-78O3-6463-5** 6,904,283 6, 2005 Li et al...... 455,450 Kapoor, S. et al.: "Adaptive Interference Suppression in Multiuser 6,920,122 7/2005 Hanaoka et al. Wireless, OFDM Systems using Antenna Arrays.” IEEE Transactions 6,985.432 1, 2006 Hadad ...... 370,203 on Signal Processing, vol. 47, No. 12, Dec. 1999, pp. 3381-3391, 7,047,011 5, 2006 Wikman et al. XP000935422, IEEE, NY, USA, ISSN: 1053-587X.** 7,373,151 5/2008 Ahmed Ye Li, et al.: "Clustered OFDM with channel estimation for high rate 2002fO114269 8, 2002 Onggosanusi et al. wireless data.” Mobile Multimedia Communications, 1999. 2003, OO67890 4, 2003 Goel et al. (MOMUC 99). 1999 IEEE International Workshop on San Diego, 2003. O169681 9, 2003 Li et al. CA, USA, USA, IEEE, US, Nov. 15, 1999, p. 43-50, XPO 10370695, 2003. O169824 9, 2003 Chayat ISBN: 0-7803-5904-6.* US 7.454.212 B2 Page 3

Nogueroles, R. et al.: Improved Performance of a Random OFDMA Wong et al. “Multiuser OFDM with Adaptive Subcarrier, Bit, and Mobile Communications System: Vehicular Technology Confer Power Allocation', IEEE Journal on Selected Areas in Communica ence, 1998. VTC 98. 48th IEEE Ottawa, Ontario, Canada, May tions. IEEE. New York, US, 1999, vol. 17. NR. 10, pp. 1747-1758. 18-21, 1998, pp. 2502-2506, XPO10288.120, ISBN: 0-7803-4320-4. Mexican Office Action issued for PA/a/2003/005311 dated Mar. 31, ck : 2006. Kinugawa, Yet al.: “Frequency and Time Division Multiple Access Ye Li et al.: "Clustered OFDM with Channel Estimation for High with Demand-Assignment Using Multicarrier Modulation for Indoor Rate Wireless Data'; Mobile Multimedia Communications, 1999. Wireless Communications Systems.” IEICE Transactions on Com (MoMuC99) 1999 IEEE International Workshop on Nov. 15-17, munications, Institute of Electronics Information and Comm. Eng. 1999: pp. 43-50. Tokyo, Japan, vol. E77-B, NR. 3, Mar. 1994, pp. 396-402, Korean office Action issued for 2003-7007962 dated Apr. 28, 2006. XP00045 1014, ISSN: 0916-8516.** Korean Office Action issued for 2003-7007963 dated Apr. 29, 2006. Vittoria Mignone et al. “CD3-OFDM: A Novel Demodulation Office Action issued for Korean Patent Application No. 2003 Scheme for Fixed and Mobile Receivers.” IEEE Transactions on 7007961, dated Sep. 27, 2006. Communications, Sep. 1996, vol. 44, No. 9. * cited by examiner U.S. Patent Nov. 18, 2008 Sheet 1 of 7 US 7.454.212 B2

Subcarrier Custer 101 102

FIG. 1A f Cluster A Cluster B

Pilot OFDM Š S S Symbols SS N S 201 SS IN N Š S S t Occupied Clusters a. Cell A (A)

N N N 201 N N S N N S Š N N

2014N NS

V. R. Rae as were state sease saw sees as we as

t C. Cell C FIG. 2 (C) U.S. Patent Nov. 18, 2008 Sheet 2 of 7 US 7.454.212 B2

Periodically Broadcast Pilot 101 OFDM Symbols to Subscribers

Subscriber(s) Continuously Monitors Pilot Symbols/Measures SINR andlor 102 Other Parameters

Each Subscriber Selects One or More 103 Retraining Clusters for Each Base Station Needed

Base Station Selects One or More 104 Clusters for Each Subscriber

Base Station Notifies the SubSCriber Regarding Cluster Allocation 105

FIG. 1B U.S. Patent Nov. 18, 2008 Sheet 3 of 7 US 7.454.212 B2

Channel/Interference Estimation in Pilot Periods Cluster Ordering Request Selected

and Rate Clusters and Codingl

Traffic/Interference Prediction Modulation Rates Analysis in Date Periods

Per-Cluster SNR Estimation in 405

Pilot Periods Cluster Ordering? 404 Selection Based On Per-Cluster Request Selected

SINR and Power POWer Calculation Difference Clusters and Coding in Pilot Periods Modulation Rates

Per-Cluster Power Calculation in Data Periods 403 FIG. 4

501 502 503 504 504 504 Cluster Cluster Cluster O 8 O

Group 3 Group 4 U.S. Patent Nov. 18, 2008 Sheet 4 of 7 US 7.454.212 B2

AAAAA \AAA/V

FIG. 8 U.S. Patent Nov. 18, 2008 Sheet 5 Of 7 US 7.454.212 B2

1-8: Diverse Clusters 9-16: Plain Clusters 10 H H t s a. Cell A

H H :

FIG. 9

Subcarrier 1 Subcarrier 2 ??????????2323223372,2% a. Cell A

b. Cell B FIG. 10 U.S. Patent Nov. 18, 2008 Sheet 6 of 7 US 7.454.212 B2

Channel/Interference 1101 Variation Detection

1102 Any Yes Significant Variation Detected

1104 1103 Select Diversity Select Coherence Clusters Clusters

FIG. 11

f

t a. Cell A FIG. 12 U.S. Patent Nov. 18, 2008 Sheet 7 of 7 US 7.454.212 B2

User Data Buffer information 1311 User 1 at N

Multi-user Data 1302 Buffer

1301 Admission Control 1310

Cluster Allocation and

Load Scheduling Multiplexer 1303 Controller

Cluster 1 - M

Multi-cluster 1304 Transmission and SINRIRate Receiving Buffer Indices 1313

OFDM Transceiver 1305

Control Signal/ OFDM Signal Custer Allocation 1312 FIG. 13 US 7,454.212 B2 1. 2 OFDMAWITH ADAPTIVE the problem of intercell interference arises. It is clear that the SUBCARRIER-CLUSTER CONFIGURATION intercell interference in an OFDMA system is also frequency AND SELECTIVE LOADING selective and it is advantageous to adaptively allocate the subcarriers so as to mitigate the effect of intercell interfer CROSS REFERENCE TO RELATED 5 CCC. APPLICATION One approach to subcarrier allocation for OFDMA is a This is a continuing application of application Ser. No. joint optimization operation, not only requiring the activity 09/738,086, entitled “OFDMA WITH ADAPTIVE SUB and channel knowledge of all the subscribers in all the cells, CARRIER-CLUSTER CONFIGURATION AND SELEC 10 but also requiring frequent rescheduling every time an exist TIVE LOADING, filed Dec. 15, 2000, the disclosure of ing subscribers is dropped off the network or a new subscrib which is hereby incorporated herein by reference thereto. ers is added onto the network. This is often impractical in real wireless system, mainly due to the bandwidth cost for updat FIELD OF THE INVENTION ing the Subscriber information and the computation cost for 15 the joint optimization. The invention relates to the field of wireless communica tions; more particularly, the invention relates to multi-cell, SUMMARY OF THE INVENTION multi-subscriber wireless systems using orthogonal fre quency division multiplexing (OFDM). A method and apparatus for Subcarrier selection for sys tems is described. In one embodiment, the system employs BACKGROUND OF THE INVENTION orthogonal frequency division multiple access (OFDMA). In Orthogonal frequency division multiplexing (OFDM) is an one embodiment, a method for Subcarrier selection comprises efficient modulation scheme for signal transmission over fre a Subscriber measuring channel and interference information quency-selective channels. In OFDM, a wide bandwidth is 25 for subcarriers based on pilot symbols received from a base divided into multiple narrow-band subcarriers, which are station, the Subscriber selecting a set of candidate Subcarriers, arranged to be orthogonal with each other. The signals modu providing feedback information on the set of candidate sub lated on the subcarriers are transmitted in parallel. For more carriers to the base station, and receiving an indication of information, see Cimini, Jr., “Analysis and Simulation of a subcarriers of the set of subcarriers selected by the base Digital Mobile Channel Using Orthogonal Frequency Divi 30 station for use by the subscriber. sion Multiplexing.” IEEE Trans. Commun., vol. COM-33, no. 7, Jul. 1985, pp. 665-75; Chuang and Sollenberger, BRIEF DESCRIPTION OF THE DRAWINGS “Beyond 3G: Wideband Wireless Data Access Based on OFDM and Dynamic Packet Assignment.” IEEE Communi The present invention will be understood more fully from cations Magazine, Vol. 38, No. 7, pp. 78-87, July 2000. 35 the detailed description given below and from the accompa One way to use OFDM to support multiple access for nying drawings of various embodiments of the invention, multiple Subscribers is through time division multiple access which, however, should not be taken to limit the invention to (TDMA), in which each subscriber uses all the subcarriers the specific embodiments, but are for explanation and under within its assigned time slots. Orthogonal frequency division standing only. multiple access (OFDMA) is another method for multiple 40 access, using the basic format of OFDM. In OFDMA, mul FIG. 1A illustrates subcarriers and clusters. tiple subscribers simultaneously use different subcarriers, in a FIG. 1B is a flow diagram of one embodiment of a process fashion similar to frequency division multiple access for allocating Subcarriers. (FDMA). For more information, see Sari and Karam, FIG. 2 illustrates time and frequency grid of OFDM sym “Orthogonal Frequency-Division Multiple Access and its 45 bols, pilots and clusters. Application to CATV Networks. European Transactions on FIG. 3 illustrates subscriber processing. Telecommunications, Vol. 9(6), pp. 507-516, November/De FIG. 4 illustrates one example of FIG. 3. cember 1998 and Nogueroles, Bossert, Donder, and Zyablov, FIG. 5 illustrates one embodiment of a format for arbitrary “Improved Performance of a Random OFDMA Mobile Com cluster feedback. munication System.”. Proceedings of IEEE VTC 98, pp. 50 25O2-2506. FIG. 6 illustrates one embodiment of apartition the clusters Multipath causes frequency-selective fading. The channel into groups. gains are different for different subcarriers. Furthermore, the FIG. 7 illustrates one embodiment of a feedback format for channels are typically uncorrelated for: different subscribers. group-based cluster allocation. The subcarriers that are in deep fade for one subscriber may 55 FIG. 8 illustrates frequency reuse and interference in a provide high channel gains for another subscriber. Therefore, multi-cell, multi-sector network. it is advantageous in an OFDMA system to adaptively allo FIG. 9 illustrates different cluster formats for coherence cate the subcarriers to subscribers so that each subscriber clusters and diversity clusters. enjoys a high channel gain. For more information, see Wong FIG. 10 illustrates diversity clusters with subcarrier hop et al., “Multiuser OFDM with Adaptive Subcarrier, Bit and 60 ping. Power Allocation.” IEEE J. Select. Areas Commun., Vol. 17(10), pp. 1747-1758, October 1999. FIG. 11 illustrates intelligent switching between diversity Within one cell, the subscribers can be coordinated to have clusters and coherence clusters depending on Subscribers different subcarriers in OFDMA. The signals for different mobility. subscribers can be made orthogonal and there is little intracell 65 FIG. 12 illustrates one embodiment of a reconfiguration of interference. However, with aggressive frequency reuse plan, cluster classification. e.g., the same spectrum is used for multiple neighboring cells, FIG. 13 illustrates one embodiment of a base station. US 7,454.212 B2 3 4 DETAILED DESCRIPTION OF THE PRESENT feedback from the subscribers to the base station, and algo INVENTION rithms used by the base station for subcarrier selections. In the following description, numerous details are set forth A distributed, reduced-complexity approach for subcarrier to provide a thorough understanding of the present invention. allocation is described. The techniques disclosed herein are 5 It will be apparent, however, to one skilled in the art, that the described using OFDMA (clusters) as an example. However, present invention may be practiced without these specific they are not limited to OFDMA-based systems. The tech details. In other instances, well-known structures and devices niques apply to multi-carrier systems in general, where, for are shown in block diagram form, rather than in detail, in example, a carrier can be a cluster in OFDMA, a spreading order to avoid obscuring the present invention. code in CDMA, an antenna beam in SDMA (space-division 10 Some portions of the detailed descriptions which follow multiple access), etc. In one embodiment, Subcarrier alloca are presented in terms of algorithms and symbolic represen tion is performed in each cell separately. Within each cell, the tations of operations on data bits within a computer memory. allocation for individual Subscribers (e.g., mobiles) is also These algorithmic descriptions and representations are the made progressively as each new Subscriber is added to the means used by those skilled in the data processing arts to most system as opposed to joint allocation for Subscribers within 15 effectively convey the substance of their work to others each cell in which allocation decisions are made taking into skilled in the art. An algorithm is here, and generally, con account all Subscribers in a cell for each allocation. ceived to be a self-consistent sequence of steps leading to a For downlink channels, each subscriber first measures the desired result. The steps are those requiring physical manipu channel and interference information for all the subcarriers lations of physical quantities. Usually, though not necessarily, and then selects multiple Subcarriers with good performance these quantities take the form of electrical or magnetic signals (e.g., a high signal-to-interference plus noise ratio (SINR)) capable of being stored, transferred, combined, compared, and feeds back the information on these candidate subcarriers and otherwise manipulated. It has proven convenient at times, to the base station. The feedback may comprise channel and principally for reasons of common usage, to refer to these interference information (e.g., signal-to-interference-plus signals as bits, values, elements, symbols, characters, terms, noise-ratio information) on all Subcarriers or just a portion of 25 numbers, or the like. Subcarriers. In case of providing information on only a por It should be borne in mind, however, that all of these and tion of the subcarriers, a subscriber may provide a list of similar terms are to be associated with the appropriate physi subcarriers ordered starting with those subcarriers which the cal quantities and are merely convenient labels applied to Subscriber desires to use, usually because their performance these quantities. Unless specifically stated otherwise as is good or better than that of other subcarriers. 30 apparent from the following discussion, it is appreciated that Upon receiving the information from the subscriber, the throughout the description, discussions utilizing terms such base station further selects the Subcarriers among the candi as “processing or "computing” or "calculating” or “deter dates, utilizing additional information available at the base mining or “displaying or the like, refer to the action and station, e.g., the traffic load information on each Subcarrier, processes of a computer system, or similar electronic com amount of traffic requests queued at the base station for each 35 puting device, that manipulates and transforms data repre frequency band, whether frequency bands are overused, and/ sented as physical (electronic) quantities within the computer or how long a subscriber has been waiting to send informa system's registers and memories into other data similarly tion. In one embodiment, the Subcarrier loading information represented as physical quantities within the computer sys of neighboring cells can also be exchanged between base tem memories or registers or other Such information storage, 40 transmission or display devices. stations. The base stations can use this information in Subcar The present invention also relates to apparatus for perform rier allocation to reduce inter-cell interference. ing the operations herein. This apparatus may be specially In one embodiment, the selection by the base station of the constructed for the required purposes, or it may comprise a channels to allocate, based on the feedback, results in the selection of coding/modulation rates. Such coding/modula general purpose computer selectively activated or reconfig 45 ured by a computer program stored in the computer. Such a tion rates may be specified by the subscriber when specifying computer program may be stored in a computer readable subcarriers that it finds favorable to use. For example, if the SINR is less than a certain threshold (e.g., 12 dB), quadrature storage medium such as, but is not limited to, any type of disk phase shift keying (QPSK) modulation is used; otherwise, 16 including floppy disks, optical disks, CD-ROMs, and mag quadrature amplitude modulation (QAM) is used. Then the netic-optical disks, read-only memories (ROMs), random 50 access memories (RAMs), EPROMS, EEPROMs, magnetic base station informs the subscribers about the subcarrier allo or optical cards, or any type of media suitable for storing cation and the coding/modulation rates to use. electronic instructions, and each coupled to a computer sys In one embodiment, the feedback information for down tem bus. link subcarrier allocation is transmitted to the base station The algorithms and displays presented hereinare not inher through the uplink access channel, which occurs in a short 55 ently related to any particular computer or other apparatus. period every transmission timeslot, e.g., 400 microseconds in Various general purpose systems may be used with programs every 10-millisecond time slot. In one embodiment, the in accordance with the teachings herein, or it may prove access channel occupies the entire frequency bandwidth. convenient to construct more specialized apparatus to per Then the base station can collect the uplink SINR of each form the required method steps. The required structure for a subcarrier directly from the access channel. The SINRas well 60 variety of these systems will appear from the description as the traffic load information on the uplink subcarriers are below. In addition, the present invention is not described with used for uplink Subcarrier allocation. reference to any particular programming language. It will be For either direction, the base station makes the final deci appreciated that a variety of programming languages may be sion of subcarrier allocation for each subscriber. used to implement the teachings of the invention as described In the following description, a procedure of selective sub 65 herein. carrier allocation is also disclosed, including methods of A machine-readable medium includes any mechanism for channel and interference sensing, methods of information storing or transmitting information in a form readable by a US 7,454.212 B2 5 6 machine (e.g., a computer). For example, a machine readable may be particularly useful in diversity clusters where the medium includes read only memory (“ROM); random weighting applied to the Subcarriers may be different. access memory (RAM); magnetic disk storage media; opti The feedback of information from each subscriber to the cal storage media; flash memory devices; electrical, optical, base station contains a SINR value for each cluster and also acoustical or other form of propagated signals (e.g., carrier 5 indicates the coding/modulation rate that the subscriber waves, infrared signals, digital signals, etc.); etc. desires to use. No cluster index is needed to indicate which SINR value in the feedback corresponds to which cluster as Subcarrier Clustering long as the order of information in the feedback is known to The techniques described herein are directed to subcarrier the base station. In an alternative embodiment, the informa allocation for data traffic channels. In a cellular system, there 0 tion in the feedback is ordered according to which clusters are typically other channels, pre-allocated for the exchange of have the best performance relative to each other for the sub control information and other purposes. These channels often scriber. In Such a case, an index is needed to indicate to which include down link and up link control channels, uplink access cluster the accompanying SINR value corresponds. channels, and time and frequency synchronization channels. Upon receiving the feedback from a subscriber, the base FIG. 1A illustrates multiple subcarriers, such as subcarrier 15 station further selects one or more clusters for the subscriber 101, and cluster 102. A cluster, such as cluster 102, is defined among the candidates (processing block 104). The base sta as a logical unit that contains at least one physical Subcarrier, tion may utilize additional information available at the base as shown in FIG. 1A. A cluster can contain consecutive or station, e.g., the traffic load information on each Subcarrier, disjoint Subcarriers. The mapping between a cluster and its amount of traffic requests queued at the base station for each subcarriers can be fixed or reconfigurable. In the latter case, frequency band, whether frequency bands are overused, and the base station informs the subscribers when the clusters are how long a Subscriber has been waiting to send information. redefined. In one embodiment, the frequency spectrum The Subcarrier loading information of neighboring cells can includes 512 subcarriers and each cluster includes four con also be exchanged between base stations. The base stations secutive subcarriers, thereby resulting in 128 clusters. can use this information in Subcarrier allocation to reduce An Exemplary Subcarrier/Cluster Allocation Procedure 25 inter-cell interference. FIG. 1B is a flow diagram of one embodiment of a process After cluster selection, the base station notifies the sub for allocation clusters to subscribers. The process is per scriber about the cluster allocation through a downlink com formed by processing logic that may comprise hardware (e.g., mon control channel or through a dedicated downlink traffic dedicated logic, circuitry, etc.), software (such as that which channel if the connection to the subscriber has already been runs on, for example, a general purpose computer system or 30 established (processing block 105). In one embodiment, the dedicated machine), or a combination of both. base station also informs the subscriber about the appropriate Referring to FIG.1B, each base station periodically broad modulation/coding rates. casts pilot OFDM symbols to every subscriber within its cell Once the basic communication link is established, each (or sector) (processing block 101). The pilot symbols, often subscriber can continue to send the feedback to the base referred to as a Sounding sequence or signal, are known to 35 station using a dedicated traffic channel (e.g., one or more both the base station and the subscribers. In one embodiment, predefined uplink access channels). each pilot symbol covers the entire OFDM frequency band In one embodiment, the base station allocates all the clus width. The pilot symbols may be different for different cells ters to be used by a subscriber at once. In an alternative (or sectors). The pilot symbols can serve multiple purposes: embodiment, the base station first allocates multiple clusters, time and frequency synchronization, channel estimation and 40 referred to herein as the basic clusters, to establish a data link signal-to-interference/noise (SINR) ratio measurement for between the base station and the subscriber. The base station cluster allocation. then Subsequently allocates more clusters, referred to herein Next, each subscriber continuously monitors the reception as the auxiliary clusters, to the subscriber to increase the of the pilot symbols and measures the SINR and/or other communication bandwidth. Higher priorities can be given to parameters, including inter-cell interference and intra-cell 45 the assignment of basic clusters and lower priorities may be traffic, of each cluster (processing block 102). Based on this given to that of auxiliary clusters. For example, the base information, each subscriber selects one or more clusters with station first ensures the assignment of the basic clusters to the good performance (e.g., high SINR and low traffic loading) subscribers and then tries to satisfy further requests on the relative to each other and feeds back the information on these auxiliary clusters from the subscribers. Alternatively, the base candidate clusters to the base station through predefined 50 station may assign auxiliary clusters to one or more Subscrib uplink access channels (processing block 103). For example, ers before allocating basic clusters to other subscribers. For SINR values higher than 10 dB may indicate good perfor example, a base station may allocate basic and auxiliary clus mance. Likewise, a cluster utilization factor less than 50% ters to one subscriber before allocating any clusters to other may be indicative of good performance. Each Subscriber Subscribers. In one embodiment, the base station allocates selects the clusters with relatively better performance than 55 basic clusters to a new subscriber and then determines if there others. The selection results in each subscriber selecting clus are any other Subscribers requesting clusters. If not, then the ters they would prefer to use based on the measured param base station allocates the auxiliary clusters to that new Sub eters. scriber. In one embodiment, each subscriber measures the SIR of From time to time, processing logic performs retraining by each subcarrier cluster and reports these SINR measurements 60 repeating the process described above (processing block to their base station through an access channel. The SINR 106). The retraining may be performed periodically. This value may comprise the average of the SINR values of each of retraining compensates for Subscriber movement and any the subcarriers in the cluster. Alternatively, the SINR value for changes in interference. In one embodiment, each Subscriber the cluster may be the worst SINRamong the SINR values of reports to the base station its updated selection of clusters and the subcarriers in the cluster. In still another embodiment, a 65 their associated SINRs. Then the base station further per weighted averaging of SINR values of the subcarriers in the forms the reselection and informs the subscriber about the cluster is used to generate an SINR value for the cluster. This new cluster allocation. Retraining can be initiated by the base US 7,454.212 B2 7 8 station, and in which case, the base station requests a specific and modulation rate that a subscriber desires to use. Note that subscriber to report its updated cluster selection. even for the same subscribers, different clusters can have Retraining can also be initiated by the subscriber when it different modulation/coding rates. observes channel deterioration. Pilot symbols serve an additional purpose in determining interference among the cells. Since the pilots of multiple cells Adaptive Modulation and Coding are broadcast at the same time, they will interfere with each In one embodiment, different modulation and coding rates other (because they occupy the entire frequency band. This are used to Support reliable transmission over channels with collision of pilot symbols may be used to determine the different SINR. Signal spreading over multiple subcarriers amount of interference as a worst case scenario. Therefore, in may also be used to improve the reliability at very low SINR. 10 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 level Table 1. is the worst-case scenario, assuming that all the interference Sources are on. Thus, the structure of pilot symbols is such TABLE 1. that it occupies the entire frequency band and causes colli 15 sions among different cells for use in detecting the worst case Scheme Modulation Code Rate SINR in packet transmission systems. O QPSK, /8 Spreading A. During data traffic periods, the subscribers can determine 1 QPSK, 4 Spreading A. 2 QPSK, 1/2 Spreading A. the level of interference again. The data traffic periods are 3 QPSK A. used to estimate the intra-cell traffic as well as the inter-cell 4 8PSK 2/3 interference level. Specifically, the power difference during 5 16QAM 34 the pilot and traffic periods may be used to sense the (intra 6 64QAM 3/6 cell) traffic loading and inter-cell interference to select 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. 25 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 Athere is domain. For example, one QPSK symbol can be repeated less interference because cluster A is unused in cell B (while over four subcarriers of two OFDM symbols, resulting also it is used in cell C). Similarly, in cell A, cluster B will expe 1/8 spreading. rience lower interference from cell B because cluster B is The coding/modulation rate can be adaptively changed 30 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 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 sym 35 Sources are transmitting. bols simultaneously, and each pilot symbol occupies the In one embodiment, a subscriber utilizes the information entire OFDM frequency bandwidth, as shown in FIGS. 2A-C. available from both the pilot symbol periods and the data Referring to FIG. 2A-C, pilot symbols 201 are shown travers traffic periods to analyze the presence of both the intra-cell ing the entire OFDM frequency bandwidth for cells A, B and traffic load and inter-cell interference. The goal of the sub C, respectively. In one embodiment, each of the pilot symbols 40 scriber is to provide an indication to the base station as to have a length or duration of 128 microseconds with a guard those clusters that the subscriber desires to use. Ideally, the time, the combination of which is approximately 152 micro result of the selection by the subscriber is clusters with high seconds. After each pilot period, these are a predetermined channel gain, low interference from other cells, and high number of data periods followed by another set of pilot sym availability. The subscriber provides feedback information bols. In one embodiment, there are four data periods used to 45 that includes the results, listing desired clusters in order or not transmit data after each pilot, and each of the data periods is as described herein. 152 microseconds. FIG. 3 illustrates one embodiment of subscriber process A subscriber estimates the SINR for each cluster from the ing. The processing is performed by processing logic that pilot symbols. In one embodiment, the subscriber first esti may comprise hardware (e.g., dedicated logic, circuitry, etc.), mates the channel response, including the amplitude and 50 Software (such as that which runs on, for example, a general phase, as if there is no interference or noise. Once the channel purpose computer system or dedicated machine), or a com is estimated, the subscriber calculates the interference/noise bination of both. from the received signal. Referring to FIG. 3, channel/interference estimation pro The estimated SINR values may be ordered from largest to cessing block 301 performs channel and interference estima smallest SINRs and the clusters with large SINR values are 55 tion in pilot periods in response to pilot symbols. Traffic/ selected. In one embodiment, than 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 signal still allows a reliable (albeit low-rate) transmission supported information and information from channel/interference esti by the system. The number of clusters selected may depend mation block 301. on the feedback bandwidth and the request transmission rate. 60 Cluster ordering and rate prediction processing block 303 In one embodiment, the subscriber always tries to send the is coupled to outputs of channel/interference estimation pro information about as many clusters as possible from which cessing block 301 and traffic/interference analysis processing the base station chooses. block 302 to perform cluster ordering and selection along The estimated SINR values are also used to choose the with rate prediction. appropriate coding/modulation rate for each cluster as dis 65 The 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 US 7,454.212 B2 10 selections are sent to the base station. In one embodiment, the selected, the subscriber requests the selected clusters and the SINR on each cluster is reported to the base station through an coding/modulation rates with processing block 406. access channel. The information is used for cluster selection More specifically, in one embodiment, the signal power of to avoid clusters with heavy intra-cell traffic loading and/or each cluster during the pilot periods is compared with that strong interference from other cells. That is, a new subscriber 5 during the traffic periods, according to the following: 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 Pp = Ps + P + Pv, is so strong that the SINR only allows for low-rate transmis 10 PN, with no signal and interference sion or no reliable transmission at all. Ps + Pv, with signal only The channel/interference estimation by processing block Pld = P + Pw, with- interference only 301 is well-known in the art by monitoring the interference Ps + P + Pw, with both signal and interference that is generated due to full-bandwidth pilot symbols being simultaneously broadcast in multiple cells. The interface 15 Ps + Pi, with no signal and interference information is forwarded to processing block 302 which uses Pl, with signal only the information to Solve the following equation: Ps, with interference only 0, with both signal and interference where S, represents the signal for subcarrier (freq. band) i, I, is the interference for Subcarrier i, n, is the noise associated where P is the measured power corresponding to each cluster with subcarrieri, and y, is the observation for subcarrieri. In during pilot periods, P, is the measured power during the the case of 512 subcarriers, i may range from 0 to 511. The I, traffic periods, P is the signal power, P, is the interference and n, are not separated and may be considered one quantity. power, and P is the noise power. The interference/noise and channel gain H, are not know. 25 In one embodiment, the subscriber selects clusters with During pilot periods, the signal S, representing the pilot sym relatively large P/(P-P,) (e.g., larger than a threshold Such bols, and the observation y, are knowns, thereby allowing as 10 dB) and avoids clusters with low P/(P-P,) (e.g., determination of the channel gain H, for the case where there lower than a threshold such as 10 dB) when possible. is no interference or noise. Once this is known, it may be Alternatively, the difference may be based on the energy plugged back into the equation to determine the interference? 30 difference between observed samples during the pilot period noise during data periods since H, S and y, are all known. and during the data traffic period for each of the subcarriers in The interference information from processing blocks 301 a cluster Such as the following: and 302 are used by the subscriberto select desirable clusters. In one embodiment, using processing block 303, the sub 35 scriber orders clusters and also predicts the data rate that Thus, the subscriber sums the differences for all subcarriers. would be available using such clusters. The predicted data Depending on the actual implementation, a Subscriber may rate information may be obtained from a look up table with use the following metric, a combined function of both SINR precalculated data rate values. Such a look up table may store and P-P, to select the clusters: the pairs of each SINR and its associated desirable transmis 40 sion rate. Based on this information, the subscriber selects clusters that it desires to use based on predetermined perfor mance criteria. Using the ordered list of clusters, the sub where f is a function of the two inputs. One example of f is scriber requests the desired clusters along with coding and weighted averaging (e.g., equal weights). Alternatively, a modulation rates known to the subscriber to achieve desired subscriber selects a cluster based on its SINR and only uses 45 data rates. the power difference P-P, to distinguish clusters with simi lar SINR. The difference may be smaller than a threshold FIG. 4 is one embodiment of an apparatus for the selection (e.g., 1 dB). of clusters based on power difference. The approach uses Both the measurement of SINR and P-P, can be averaged information available during both pilot symbol periods and over time to reduce variance and improve accuracy. In one data traffic periods to perform energy detection. The process 50 ing of FIG. 4 may be implemented in hardware, (e.g., dedi embodiment, a moving-average time window is used that is cated logic, circuitry, etc.), software (such as is run on, for long enough to average out the statistical abnormity yet short example, a general purpose computer system or dedicated enough to capture the time-varying nature of channel and machine), or a combination of both. interference, e.g., 1 millisecond. Referring to FIG.4, a subscriber includes SINR estimation 55 Feedback Format for Downlink Cluster Allocation processing block 401 to perform SINR estimation for each In one embodiment, for the downlink, the feedback con cluster in pilot periods, power calculation processing block tains both the indices of selected clusters and their SINR. An 402 to perform power calculations for each cluster in pilot exemplary format for arbitrary cluster feedback is shown in periods, and power calculation processing block 403 to per FIG. 5. Referring to FIG. 5, the subscriber provides a cluster form power calculations in data periods for each cluster. 60 index (ID) to indicate the cluster and its associated SINR Subtractor 404 subtracts the power calculations for data peri value. For example, in the feedback, the subscriber provides ods from processing block 403 from those in pilot periods cluster. ID1 (501) and the SINR for the cluster, SINR1 (502), from processing block 402. The output of subtractor 404 is cluster ID2 (503) and the SINR for the cluster, SINR2 (504), input to power difference ordering (and group selection) pro and cluster ID3 (505), and the SINR for the cluster, SINR3 cessing block 405 that performs cluster ordering and selec 65 (506), etc. The SINR for the cluster may be created using an tion based on SINR and the power difference between pilot average of the SINRs of the subcarriers. Thus, multiple arbi periods and data periods. Once the clusters have been trary clusters can be selected as the candidates. As discussed US 7,454.212 B2 11 12 above, the selected clusters can also be ordered in the feed cluster allocation. Group-based cluster allocation may also be back to indicate priority. In one embodiment, the subscriber used to reduce inter-cell interference. may form a priority list of clusters and sends back the SINR After receiving the pilot signal from the base station, a information in a descending order of priority. Subscriber sends back the channel information on one or more Typically, an index to the SINR level, instead of the SINR cluster groups, simultaneously or sequentially. In one itself is sufficient to indicate the appropriate coding/modula embodiment, only the information on Some of the groups is tion for the cluster. For example, a 3-bit field can be used for sent back to the base station. Many criteria can be used to SINR indexing to indicate 8 different rates of adaptive cod choose and order the groups, based on the channel informa ing/modulation. tion, the inter-cell interference levels, and the intra-cell traffic 10 load on each cluster. An Exemplary Base Station In one embodiment, a subscriber first selects the group with The base station assigns desirable clusters to the subscriber the best overall performance and then feedbacks the SINR making the request. In one embodiment, the availability of the information for the clusters in that group. The subscriber may cluster for allocation to a subscriber depends on the total order the groups based on their number of clusters for which traffic load on the cluster. Therefore, the base station selects 15 the SINR is higher than a predefined threshold. By transmit the clusters not only with high SINR, but also with low traffic ting the SINR of all the clusters in the group sequentially, only load. the group index instead of all the cluster indices, needs to be FIG. 13 is a block diagram of one embodiment of a base transmitted. Thus, the feedback for each group generally station. Referring to FIG. 13, cluster allocation and load contains two types of information: the group index and the scheduling controller 1301 (cluster allocator) collects all the SINR value of each cluster within the group. FIG. 7 illustrates necessary information, including the downlink/uplink SINR an exemplary format for indicating a group based cluster of clusters specified for each subscriber (e.g., via SINR/rate allocation. Referring to FIG. 7, a group ID. ID1, is followed indices signals 1313 received from OFDM transceiver 1305) by the SINR values for each of the clusters in the group. This and user data, queue fullness/traffic load (e.g., via user data can significantly reduce the feedback overhead. buffer information 1311 from multi-user data buffer 1302). 25 Upon receiving the feedback information from the sub Using this information, controller 1301 makes the decision on scriber, the cluster allocator at the base station selects mul cluster allocation and load scheduling for each user, and tiple clusters from one or more groups, if available, and then stores the decision information in a memory (not shown). assigns the clusters to the Subscriber. This selection may be Controller 1301 informs the subscribers about the decisions performed by an allocation in a media access control portion through control signal channels (e.g., control signal/cluster 30 of the base station. allocation 1312 via OFDM transceiver 1305). Controller Furthermore, in a multi-cell environment, groups can have 1301 updates the decisions during retraining. different priorities associated with different cells. In one In one embodiment, controller 1301 also performs admis embodiment, the subscriber's selection of a group is biased sion control to user access since it knows the traffic load of the by the group priority, which means that certain Subscribers system. This may be performed by controlling user data buff 35 have higher priorities on the usage of Some groups than the ers 1302 using admission control signals 1310. other subscribers. The packet data of User 1-N are stored in the user data In one embodiment, there is no fixed association between buffers 1302. For downlink, with the control of controller one subscriber and one cluster group; however, in an alterna 1301, multiplexer 1303 loads the user data to cluster data tive embodiment there may be such a fixed association. In an buffers (for Cluster 1-M) waiting to be transmitted. For the 40 implementation having a fixed association between a Sub uplink, multiplexer 1303 sends the data in the cluster buffers scriber and one or more cluster groups, the group index in the to the corresponding user buffers. Cluster buffer 1304 stores feedback information can be omitted, because this informa the signal to be transmitted through OFDM transceiver 1305 tion is known to both subscriber and base station by default. (for downlink) and the signal received from transceiver 1305. In another embodiment, the pilot signal sent from the base In one embodiment, each user might occupy multiple clusters 45 station to the subscriberalso indicates the availability of each and each cluster might be shared by multiple users (in a cluster, e.g., the pilot signal shows which clusters have time-division-multiplexing fashion). already been allocated for other subscribers and which clus ters are available for new allocations. For example, the base Group-Based Cluster Allocation station can transmit a pilot sequence 1111 1111 on the Sub In another embodiment, for the downlink, the clusters are 50 carriers of a cluster to indicate that the cluster is available, and partitioned into groups. Each group can include multiple 1111-1-1-1-1 to indicate the cluster is not available. At the clusters. FIG. 6 illustrates an exemplary partitioning. Refer receiver, the Subscriber first distinguishes the two sequences ring to FIG. 6, groups 1-4 are shown with arrows pointing to using the signal processing methods which are well known in clusters that are in each group as a result of the partitioning. In the art, e.g., the correlation methods, and then estimates the one embodiment, the clusters within each group are spaced 55 channel and interference level. far apart over the entire bandwidth. In one embodiment, the With the combination of this information and the channel clusters within each group are spaced apart farther than the characteristics obtained by the subscriber, the subscriber can channel coherence bandwidth, i.e. the bandwidth within prioritize the groups to achieve both high SINR and good load which the channel response remains roughly the same. A balancing. typical value of coherence bandwidth is 100 kHz for many 60 In one embodiment, the subscriber protects the feedback cellular systems. This improves frequency diversity within information by using error correcting codes. In one embodi each group and increases the probability that at least some of ment, the SINR information in the feedback is first com the clusters within a group can provide high SINR. The clus pressed using source coding techniques, e.g., differential ters may be allocated in groups. Goals of group-based cluster encoding, and then encoded by the channel codes. allocation include reducing the data bits for cluster indexing 65 FIG. 8 shows one embodiment of a frequency reuse pattern thereby reducing the bandwidth requirements of the feedback for an exemplary cellular set up. Each cell has hexagonal channel (information) and control channel (information) for structure with six sectors using directional antennas at the US 7,454.212 B2 13 14 base stations. Between the cells, the frequency reuse factor is one. Within each cell, the frequency reuse factor is 2 where TABLE 3 the sectors use two frequencies alternatively. As shown in Cluster usage for the downlink of the shaded sectors with FIG.8, each shaded sector uses half of the available OFDMA less than 2/3 of the full load. clusters and each unshaded sector uses the other half of the clusters. Without loss of generality, the clusters used by the Cluster Usage Cell. A Cell B Cell C shaded sectors are referred to hereinas odd clusters and those 1 Group 1 Group 3 Group 2 2 Group 2 Group 1 Group 3 used by the unshaded sectors are referred to herein as even 3 clusters. 10 Consider the downlink signaling with omni-directional antennas at the subscribers. From FIG.8, it is clear that for the Table 4 shows the priority orders for the unshaded sectors, downlink in the shaded sectors, Cell A interferes with Cell B, which are different from those for the shaded sectors, since which in turn interferes with Cell C, which in turn interferes the interfering relationship is reversed. 15 with Cell A, namely, A->B->C->A. For the unshaded sec TABLE 4 tors Cell Ainterferes with Cell C, which in turninterferes with Cell B, which in turn interferes with Cell A, namely, A->C- Priority ordering for the downlink of the unshaded sectors. >B->A. Priority Ordering Cell. A Cell B Cell C Sector A1 receives interference from Sector C1, but its 1 Group 1 Group 2 Group 3 transmission interferes with Sector B1. Namely, its interfer 2 Group 2 Group 3 Group 1 ence source and the victims with which it interferes are not the 3 Group 3 Group 1 Group 2 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 25 Intelligent Switching between Coherence and Diversity Clus C1, the cluster may be assigned in A1 because it may be seen ters as clean in A1. However, the assignment of this cluster A1 can In one embodiment, there are two categories of clusters: cause interference problem to the existing assignment in B1. coherence clusters, containing multiple Subcarriers close to In one embodiment, different cluster groups are assigned 30 each other and diversity clusters, containing multiple Subcar different priorities for use in different cells to alleviate the riers with at least some of the subcarriers spread far apart over aforementioned problem when the traffic load is progres the spectrum. The closeness of the multiple subcarriers in sively added to a sector. The priority orders are jointly coherence clusters is preferably within the channel coherence designed such that a cluster can be selectively assigned to bandwidth, i.e. the bandwidth within which the channel 35 response remains roughly the same, which is typically within avoid interference from its interference source, while reduc 100 kHz for many cellular systems. On the other hand, the ing, and potentially minimizing, the probability of causing spread of subcarriers in diversity clusters is preferably larger interference problem to existing assignments in other cells. than the channel coherence bandwidth, typically within 100 Using the aforementioned example, the odd clusters (used kHz for many cellular systems. Of course, the larger the by the shaded sectors) are partitioned into 3 groups: Group 1. 40 spread, the better the diversity. Therefore, a general goal in 2, 3. The priority orders are listed in Table 2. Such cases is to maximize the spread. FIG. 9 illustrates exemplary cluster formats for coherence TABLE 2 clusters and diversity clusters for Cells A-C. Referring to FIG. 45 9, for cells A-C, the labeling of frequencies (subcarriers) Priority ordering for the downlink of the shaded sectors. 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 Priority Ordering Cell. A Cell B Cell C clusters. For example, all frequencies labeled 1 in a cell are 50 part of one diversity cluster; all frequencies labeled 2 in a cell 1 Group 1 Group 3 Group 2 are part of another diversity cluster, etc., while the group of 2 Group 2 Group 1 Group 3 frequencies labeled 9 are one coherence cluster, the group of 3 Group 3 Group 2 Group 1 frequencies labeled 10 are another coherence cluster, etc. The diversity clusters can be configured differently for different 55 cells to reduce the effect of inter-cell interference through Consider Sector A1. First, the clusters in Group 1 are interference averaging. selectively assigned. If there are still more subscribers FIG. 9 shows example cluster configurations for three demanding clusters, the clusters in Group 2 are selectively neighboring cells. The interference from a particular cluster assigned to subscribers, depending on the measured SINR in one cell are distributed to many clusters in other cells, e.g., (avoiding the clusters receiving strong interference from Sec 60 the interference from Cluster 1 in Cell A are distributed to Cluster 1, 8, 7, 6 in Cell B. This significantly reduces the tor C1). Note that the newly assigned clusters from Group 2 to interference power to any particular cluster in Cell B. Like Sector A1 shall not cause interference problem in Sector B1, wise, the interference to any particular cluster in one cell unless the load in Sector B1 is so heavy that the clusters in comes from many different clusters in other cells. Since not both Group 3 and 1 are used up and the clusters in Group 2 are 65 all cluster are strong interferers, diversity clusters, with chan also used. Table 3 shows the cluster usage when less than 2/3 nel coding across its Subcarriers, provide interference diver of all the available clusters are used in Sector A1, B1, and C1. sity gain. Therefore, it is advantageous to assign diversity US 7,454.212 B2 15 16 clusters to Subscribers that are close (e.g., within the coherent station. The channel/interference variation detector measures bandwidth) to the cell boundaries and are more subject to the channel (SINR) variation from time to time for each inter-cell interference. cluster. For example, in one embodiment, the channel/inter Since the Subcarriers in a coherence cluster are consecutive ference detector measures the power difference between pilot or close (e.g., within the coherent bandwidth) to each other, symbols for each cluster and averages the difference over a they are likely within the coherent bandwidth of the channel moving window (e.g., 4 time slots). A large difference indi fading. Therefore, the channel gain of a coherence cluster can cates that channel/interference changes frequently and Sub vary significantly and cluster selection can greatly improve carrier allocation may be not reliable. In Such a case, diversity the performance. On the other hand, the average channel gain clusters are more desirable for the subscriber. of a diversity cluster has less of a degree of variation due to the 10 FIG. 11 is a flow diagram of one embodiment of a process inherent frequency diversity among the multiple Subcarriers for intelligent selection between diversity clusters and coher spread over the spectrum. With channel coding across the ence clusters depending on Subscribers mobility. The process subcarriers within the cluster, diversity clusters are more is performed by processing logic that may comprise hardware robust to cluster mis-selection (by the nature of diversifica (e.g., circuitry, dedicated logic, etc.), Software (such as that tion itself), while yielding possibly less gain from cluster 15 which runs on, for example, a general purpose computer selection. Channel coding across the Subcarriers means that system or dedicated machine), or a combination of both. each codeword contains bits transmitted from multiple sub Referring to FIG. 11, processing logic in the base station carriers, and more specifically, the difference bits between performs channel/interference variation detection (process codewords (error vector) are distributed among multiple sub ing block 1101). Processing logic then tests whether the carriers. results of the channel/interference variation detection indi More frequency diversity can be obtained through subcar cate that the user is mobile or in a fixed position close to the rier hopping over time in which a Subscriber occupies a set of edge of the cell (processing block 1102). If the user is not subcarriers at one time slot and another different set of sub mobile or is not in a fixed position close to the edge of the cell, carriers at a different time slot. One coding unit (frame) processing transitions to processing block 1103 where pro contains multiple Such time slots and the transmitted bits are 25 cessing logic in the base station selects coherence clusters; encoded across the entire frame. otherwise, processing transitions to processing block 1104 in FIG. 10 illustrates diversity cluster with subcarrier hop which processing logic in the base station selects diversity ping. Referring to FIG. 10, there are four diversity clusters in clusters. each of cells A and B shown, with each subcarrier in indi The selection can be updated and intelligently switched vidual diversity clusters having the same label (1, 2, 3, or 4). 30 during retraining. There are four separate time slots shown and during each of The ratio allocation of the numbers of coherence and diver the timeslots, the subcarriers for each of the diversity clusters sity clusters in a cell depends on the ratio of the population of change. For example, in cell A. Subcarrier 1 is part of diversity mobile and fixed subscribers. When the population changes cluster 1 during timeslot1, is part of diversity cluster 2 during as the system evolves, the allocation of coherence and diver time slot 2, is part of diversity cluster 3 during time slot 3, and 35 sity clusters can be reconfigured to accommodate the new is part of diversity cluster 4 during time slot 4. Thus, more system needs. FIG. 12 illustrates a reconfiguration of cluster interference diversity can be obtained through subcarrier hop classification which can Support more mobile Subscribers ping overtime, with further interference diversity achieved by than that in FIG. 9. using different hopping patterns for different cells, as shown Whereas many alterations and modifications of the present in FIG. 10. 40 invention will no doubt become apparent to a person of ordi The manner in which the subscriber changes the subcarri nary skill in the art after having read the foregoing descrip ers (hopping sequences) can be different for different cells in tion, it is to be understood that any particular embodiment order to achieve betterinterference averaging through coding. shown and described by way of illustration is in no way For static Subscribers, such as in fixed wireless access, the intended to be considered limiting. Therefore, references to channels change very little over time. Selective cluster allo 45 details of various embodiments are not intended to limit the cation using the coherence clusters achieve good perfor scope of the claims which in themselves recite only those mance. On the other hand, for mobile subscribers, the channel features regarded as essential to the invention. time variance (the variance due to changes in the channel over We claim: time) can be very large. A high-gain cluster at one time can be 1. A method for subcarrier selection for a system employ in deep fade at another. Therefore, cluster allocation needs to 50 ing orthogonal frequency division multiple access (OFDMA) be updated at a rapid rate, causing significant control over comprising: head. In this case, diversity clusters can be used to provide a subscriber unit measuring channel and interference infor extra robustness and to alleviate the overhead of frequent mation for a plurality of Subcarriers based on pilot sym cluster reallocation. In one embodiment, cluster allocation is bols received from a base station; performed faster than the channel changing rate, which is 55 the Subscriber unit selecting a set of candidate Subcarriers; often measured by the channel Doppler rate (in Hz), i.e. how the subscriber unit providing feedback information on the many cycles the channel changes per second where the chan set of candidate subcarriers to the base station; nel is completely different after one cycle. Note that selective the subscriber unit receiving an indication of subcarriers of cluster allocation can be performed on both coherence and the set of subcarriers selected by the base station for use diversity clusters. 60 by the subscriber unit; and In one embodiment, for cells containing mixed mobile and the subscriber unit submitting updated feedback informa fixed subscribers, a channel/interference variation detector tion, after being allocated the set of subcarriers to be can be implemented at either the subscriber or the base sta allocated an updated set of subcarriers, and thereafter the tion, or both. Using the detection results, the subscriber and subscriber unit receiving another indication of the the base station intelligently selects diversity clusters to 65 updated set of subcarriers. mobile subscribers or fixed subscribers at cell boundaries, 2. The method defined in claim 1 further comprising the and coherence clusters to fixed subscribers close to the base Subscriber unit sending the indication to the base station. US 7,454.212 B2 17 18 3. The method defined in claim 2 further comprising send selected by the first base station for use by the at least one ing an indication of the set of subcarriers selected by the base Subscriber unit, and wherein the subscriber unit submits station for use by the subscriber unit. updated feedback information after being allocated the 4. The method defined in claim 3 further comprising the set of subscriber units to receive an updated set of sub base station selecting subcarriers for the subscriber unit based carriers and thereafter receives another indication of the on inter-cell interference avoidance. updated set of subcarriers. 5. The method defined in claim 1 further comprising the 19. The apparatus defined in claim 18 wherein each of the Subscriber unit using information from pilot symbol periods plurality of Subscriber units continuously monitors reception and data periods to measure channel and interference infor of the pilot symbols known to the base station and the plural mation. 10 ity of subscriber units and measures signal-plus-interference 6. The method defined in claim 1 wherein the pilot symbols to-noise ratio (SINR) of each cluster of subcarriers. occupy an entire OFDM frequency bandwidth. 20. The apparatus defined in claim 19 wherein each of the 7. The method defined in claim 6 wherein at least one other plurality of subscriber units measures inter-cell interference, pilot symbol from a different cell transmitted at the same time wherein the at least one subscriber unit selects candidate as the pilot symbols received from the base station collide 15 subcarriers based on the inter-cell interference. with each other. 21. The apparatus defined in claim 20 wherein the base 8. The method defined in claim 1 further comprising the station selects subcarriers for the one subscriber unit based on base station selecting the Subcarriers from the set of candidate inter-cell interference avoidance. subcarriers based on additional information available to the base station. 22. The apparatus defined in claim 18 wherein the at least 9. The method defined in claim 8 wherein the additional one subscriber unit uses information from pilot symbol peri information comprises traffic load information on each clus ods and data periods to measure channel and interference ter of subcarriers. information. 10. The method defined in claim 9 wherein the traffic load 23. The apparatus defined in claim 18 wherein the base information is provided by a data buffer in the base station. 25 station selects the subcarriers from the set of candidate sub 11. The method defined in claim 1 wherein the indication carriers based on additional information available to the base of Subcarriers is received via a downlink control channel. station. 12. The method defined in claim 1 wherein the plurality of 24. The apparatus defined in claim 23 wherein the addi Subcarriers comprises all Subcarriers allocable by a base sta tional information comprises traffic load information on each tion. 30 cluster of Subcarriers. 13. The method defined in claim 1 wherein providing feed 25. The apparatus defined in claim 24 wherein the traffic back information comprises arbitrarily ordering the set of load information is provided by a data buffer in the base candidate Subcarriers as clusters of Subcarriers. station. 14. The method defined in claim 13 wherein arbitrarily 26. The apparatus defined in claim 18 wherein the indica order candidate clusters comprise clusters in an order with 35 tion of subcarriers is received via a downlink control channel most desirable candidate clusters being listed first. between the base station and the at least one subscriber unit. 15. The method defined in claim 1 wherein providing feed back information comprises sequentially ordering candidate 27. The apparatus defined in claim 18 wherein the plurality clusters. of subcarriers comprises all subcarriers allocable by a base station. 16. The method defined in claim 1 further comprising: 40 the base station allocating a first portion of the Subcarriers 28. The apparatus defined in claim 18 wherein the plurality to establish a data link between the base station and the of subscriber units provide feedback information that com Subscriber unit; and then prises an arbitrarily ordered set of candidate subcarriers as the base station allocating a second portion of the Subcar clusters of subcarriers. riers to the Subscriber unit to increase communication 45 29. The apparatus defined in claim 18 wherein providing bandwidth. feedback information comprises sequentially ordering candi 17. The method defined in claim 16 wherein the base sta date clusters. tion allocates the second portion after allocating each Sub 30. The apparatus defined in claim 18 wherein the base scriber unit in the cell subcarriers to establish a data link station allocates a first portion of the subcarriers to establish between the base station and said each subscriber unit. 50 a data link between the base station and the subscriber unit; 18. An apparatus comprising: and then allocates a second portion of the Subcarriers to the a plurality of subscriber units in a first cell operable to Subscriber unit to increase communication bandwidth. generate feedback information indicating clusters of 31. The apparatus defined in claim 30 wherein the base subcarriers desired for use by the plurality of subscriber station allocates the second portion after allocating each Sub units; and 55 scriber unit in the cell subcarriers to establish a data link a first base station in the first cell, the first base station between the base station and said each subscriber unit. operable to allocate OFDMA subcarriers in clusters to the plurality of subscriber units: 32. A method comprising: each of said plurality of Subscriber units to measure chan a base station allocating a first portion of a plurality of nel and interference information for the plurality of sub 60 Subcarriers to establish a data link between the base carriers based on pilot symbols received from the first station and a subscriber unit; and base station and at least one of the plurality of subscriber the base stationallocating a second portion of said plurality units to select a set of candidate subcarriers from the of subcarriers to the subscriber unit to increase commu plurality of subcarriers, and said at least one subscriber nication bandwidth, wherein the base station allocates unit to provide feedback information on the set of can 65 the second portion after allocating each Subscriber unit didate subcarriers to the base station and to receive an in the cell subcarriers to establisha data link between the indication of Subcarriers from the set of subcarriers base station and said each Subscriber unit. US 7,454.212 B2 19 20 33. A base station comprising: cation bandwidth, wherein the base station allocates the means for allocating a first portion of a plurality of Subcar- second portion after allocating each Subscriber unit in riers to establish a data link between the base station and the cell Subcarriers to establish a data link between the a Subscriber unit; and base station and said each Subscriber unit. means for allocating a second portion of said plurality of 5 Subcarriers to the Subscriber unit to increase communi- k . . . .