US007400573B2
`
`(12) United States Patent
`Sundstrom et al.
`
`(10) Patent No.:
`(45) Date of Patent:
`
`US 7,400,573 B2
`Jul. 15, 2008
`
`(54) DYNAMIC ALLOCATION OF CYCLIC
`EXTENSION IN ORTHOGONAL
`I§3I§§3EU1\I/IIEICY DIVISION MULTIPLEXING
`
`6,088,327 A *
`6,577,603 B1*
`6,674,855 B1*
`6,956,812 132*
`
`....... .. 370/210
`7/2000 Muschalliketal.
`6/2003 Hakalin et al.
`............ .. 370/252
`1/2004 Karelic et al.
`............. .. 379/386
`10/2005 Okada etal.
`.............. .. 370/208
`
`(75)
`
`Inventors, KurtE_ Sundstmm:WOOdinVfl1e,WA
`(US); Ernest Tsui, Cupertinoa CA (US);
`Richard B. Nicholls, Banks, OR (US)
`
`............... .. 375/259
`3/2006 Jones et al.
`7,010,049 B1*
`2003/0231582 A1* 12/2003 Logvinov et al.
`......... .. 370/208
`2004/0081131 A1*
`4/2004 Walton et al.
`............. .. 370/344
`2004/0213145 A1* 10/2004 Nakamura ................ .. 370/208
`
`(73 Assignee:
`
`Intel Corporation, Santa Clara, CA
`(US)
`
`( *
`
`Notice:
`
`Subject to any disclaimer, the term of this
`.
`.
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 915 days.
`
`* cited by examiner
`
`Primary Examiner—Ricky Ngo
`Assistant Examiner—Pao Sinkantarakorn
`
`(21 APPI N05 10/423429
`
`(22
`(65
`
`(51
`
`(56)
`
`Filed:
`
`Apr. 29, 2003
`Prior Publication Data
`US 2004/0218522 A1
`Nov. 4, 2004
`
`I11t- CL
`(200601)
`H041 11/00
`U.S. Cl.
`...................... ..
`Of Classification Search ............... ..
`370/208, 210, 252, 259, 386, 528; 375/250,
`375/285, 345, 348
`See aPPIICaII0II fiIe IOI COIIIPIBIB Search IIISIOIY
`References Cited
`U.S. PATENT DOCUMENTS
`
`(74) Attorney, Agent, or Firm—Schwegma11, Lundberg &
`W
`, P.A.
`Oessner
`57
`)
`
`ABSTRACT
`
`(
`
`Briefly, in accordance with one embodiment of the invention,
`an orthogonal frequency division multiplexing system may
`provide a dynamically calculated cyclic extension, the length
`may be based at least in part Qn a delay spread due
`to an experienced environmental condition. The length Qfthe
`cyclic extension may be calculated by determining a channel
`impulse response, and then computing the energy distribution
`of the channel impulse response. The length of the cyclic
`extension may then be set according to the energy distribution
`of the charmel impulse response.
`
`5,442,696 A *
`
`8/1995 Lindberg et al.
`
`.......... .. 379/386
`
`16 Claims, 2 Drawing Sheets
`
`220
`
`
`
`CHANNEL
`ESTIMATIGN
`
`
`
`
`
`OPTIMIZE
`
`CYCUC
`
`EXIENSION
`
`
`COMPUTE ENERGY
`DISTRIBUTION OF
`CHANNEL IMPULSE
`RESPONSE
`
`PREAMBLE
`
`PROCESSING
`
`214
`
`REMOVE
`CYCUC
`EXI
`
` 210
`
`
`
`218
`
`232
`
`1
`
`APPLE 1027
`
`APPLE 1027
`
`1
`
`

`
`U.S. Patent
`
`Jul. 15,2008
`
`Sheet 1 of2
`
`US 7,400,573 B2
`
`110
`
`112
`
`/- 100
`
`OFDM Data (IFFT output)
`
`i
`
`:
`
`
`,
`
`
`112
`
`112
`
`
`
`110
`
`174
`
`OFDM Data
`
`
`
`OVERHEAD
`
`DATA
`
`TRANSMITIED omm SYMBOL
`
`FIG.1
`
`,—222
`
`220
`
`§"""2'274_ """‘E56’;-----------"iii;_____
`
`230
`
`Ogéfif
`W %%Wr:%f'5g5Rg:!
`0HA£Igg;$tL4§gLSE
`EXTENSION
`
`
`
`
`CHANNEL
`ESTIMATION
`
`
`
`
`
`
`
`214
`
`216
`
`212
`
`2
`
`

`
`U.S. Patent
`
`Jul. 15,2008
`
`Sheet 2 of2
`
`US 7,400,573 B2
`
`310
`
`312
`
`316
`
` RECEIVE
`SYMBOL FROM
`MOBILE UNIT
`
` SUPPORT DYNAMIC
`NO
`CYCLIC EXTENSION
`?
`
`USE PREDETERMINED
`
`
`
`
`
`
`STORE CYCLIC
`EXTENSION FOR
`THAT MOBILE UNIT
`
`
`
`COMMUNICATE
`CYCLIC EXTENSION
`TO MOBILE UNIT
`
`FIG. 3
`
` BASE
`
`STATION
`
`& MAC
`
`WIRELESS
`TRANCEIVER
`
`BASEBANO
`PROCESSOR
`
`FIG. 4
`
`CYCLIC EXTENSION
`
`
`Dggggxglglcuc
`
`
`
`
`
`COMPUTE ENERGY
`ESTIMATE
`msmjgunom 0}-'
`CHANNEL IMPULSE
`RESPONSE
`CHANNEL IMPULSE
`
`RESPONSE
`
`324
`
`3
`
`

`
`US 7,400,573 B2
`
`1
`DYNAMIC ALLOCATION OF CYCLIC
`EXTENSION IN ORTHOGONAL
`FREQUENCY DIVISION MULTIPLEXING
`SYSTEMS
`
`BACKGROUND OF THE INVENTION
`
`One common component of an orthogonal frequency divi-
`sion multiplexing (OFDM) system is the cyclic extension of
`each OFDM symbol. A cyclic extension, or guard interval,
`may be produced by replicating part of the OFDM symbol
`and either prepending or appending the replicated segment
`onto the original symbol. When attached to the frontal portion
`of the symbol, the cyclic extension may be referred to as a
`cyclic prefix extension. When attached to the terminal portion
`of the symbol, the cyclic extension may be referred to as a
`cyclic postfix extension. For example, given a set of 64
`samples, the cyclic extension may be 16 samples. In the case
`of a prepend, the last 16 samples of the original 64 samples
`may be prepended to the front of the original 64 samples to
`result in an 80 sample symbol. In the case of an append, the
`first 16 samples of the original 64 samples may be appended
`to the end of the original 64 samples to result in an 80 sample
`symbol. In either case, taking 64 samples of the 80 sample
`symbol will result in the original 64 samples by cyclically
`wrap at the end back to the front if needed. The cyclic exten-
`sion of an OFDM symbol converts the linear channel convo-
`lution into a cyclic convolution. An OFDM receiver exploits
`the properties of the cyclic convolution to mitigate inter-
`symbol interference and inter-carrier interference.
`While the cyclic extension helps to mitigate channel dis-
`tortion in an OFDM system, using a cyclic extension involves
`the transmission of duplicate information, and as such repre-
`sents an overhead associated with an OFDM transmission
`
`scheme. Furthermore, to mitigate the effects of the channel
`temporal dispersion, the cyclic extension typically is required
`to be of a duration greater than or equal to the channel
`memory. For example,
`in a wireless local area network
`(WLAN) compliant with an Institute of Electrical and Elec-
`tronics Engineers (IEEE) standard, such as the IEEE 802.1 la
`standard, the cyclic extension is a predetermined length, for
`example 800 ns,
`in order to provide immunity to delay
`spreads up to a maximum of 800 ns. However, under typical
`conditions, the actual delay spread experienced is often much
`lower than the predetermined delay spread of 800 ns, thereby
`resulting in an unnecessary overhead represented by the dif-
`ference between the predetermined delay spread and the actu-
`ally experienced delay spread. It would therefore be desirable
`to provide an OFDM system in which the length of the cyclic
`extension may be dynamically allocated according to the
`actually experienced delay spread, rather than being based on
`the predetermined, maximum, worst case scenario delay
`spread. Furthermore, since the actually experienced delay
`spread may be different for different users, it would in addi-
`tion be desirable to provide a cyclic extension length for each
`user based upon the actual delay spread experienced by each
`respective user.
`
`DESCRIPTION OF THE DRAWING FIGURES
`
`The subject matter regarded as the invention is particularly
`pointed out and distinctly claimed in the concluding portion
`of the specification. The invention, however, both as to orga-
`nization and method of operation, together with objects, fea-
`tures, and advantages thereof, may best be understood by
`reference to the following detailed description when read
`with the accompanying drawings in which:
`
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`FIG. 1 is a diagram of an orthogonal frequency division
`multiplexing scheme utilizing a cyclic extension in accor-
`dance with one embodiment of the present invention;
`FIG. 2 is a block diagram of an orthogonal frequency
`division multiplexing receiver that dynamically allocates the
`length of a cyclic extension in accordance with one embodi-
`ment of the present invention;
`FIG. 3 is a flow diagram of a method to dynamically deter-
`mine and allocate a length of a cyclic extension in an orthogo-
`nal frequency division multiplexing system in accordance
`with an embodiment of the present invention; and
`FIG. 4 is a block diagram of a wireless communication
`system in accordance with one embodiment of the present
`invention.
`
`It will be appreciated that for simplicity and clarity of
`illustration, elements illustrated in the figures have not nec-
`essarily been drawn to scale. For example, the dimensions of
`some of the elements are exaggerated relative to other ele-
`ments for clarity. Further, where considered appropriate, ref-
`erence numerals have been repeated among the figures to
`indicate corresponding or analogous elements.
`
`DETAILED DESCRIPTION
`
`In the following detailed description, numerous specific
`details are set forth in order to provide a thorough understand-
`ing of the invention. However, it will be understood by those
`skilled in the art that the present invention may be practiced
`without these specific details. In other instances, well-known
`methods, procedures, components and circuits have not been
`described in detail so as not to obscure the present invention.
`Some portions of the detailed description that follows are
`presented in terms of algorithms and symbolic representa-
`tions ofoperations on data bits or binary digital signals within
`a computer memory. These algorithmic descriptions and rep-
`resentations may be the techniques used by those skilled in
`the data processing arts to convey the substance of their work
`to others skilled in the art.
`
`An algorithm is here, and generally, considered to be a
`self-consistent sequence of acts or operations leading to a
`desired result. These include physical manipulations of
`physical quantities. Usually, though not necessarily, these
`quantities take the form of electrical or magnetic signals
`capable of being stored, transferred, combined, compared,
`and otherwise manipulated. It has proven convenient at times,
`principally for reasons of common usage, to refer to these
`signals as bits, values, elements, symbols, characters, terms,
`numbers or the like. It should be understood, however, that all
`ofthese and similar terms are to be associated with the appro-
`priate physical quantities and are merely convenient labels
`applied to these quantities.
`Unless specifically stated otherwise, as apparent from the
`following discussions, it is appreciated that throughout the
`specification, discussions utilizing terms such as processing,
`computing, calculating, determining, or the like, refer to the
`action or processes of a computer or computing system, or
`similar electronic computing device, that manipulate or trans-
`form data represented as physical, such as electronic, quan-
`tities within the registers or memories of the computing sys-
`tem into other data similarly represented as physical
`quantities within the memories, registers or other such infor-
`mation storage, transmission or display devices of the com-
`puting system.
`Embodiments of the present invention may include appa-
`ratuses for performing the operations herein. This apparatus
`may be specially constructed for the desired purposes, or it
`may comprise a general purpose computing device selec-
`
`4
`
`

`
`US 7,400,573 B2
`
`3
`tively activated or reconfigured by a program stored in the
`device. Such a program may be stored on a storage medium,
`such as, but is not limited to, any type ofdisk including floppy
`disks, optical disks, CD-ROMs, magnetic-optical disks, read-
`only memories (ROMs), random access memories (RAMs),
`electrically programmable read-only memories (EPROMs),
`electrically erasable and programmable read only memories
`(EEPROMs), flash memory, magnetic or optical cards, or any
`other type of media suitable for storing electronic instruc-
`tions, and capable of being coupled to a system bus for a
`computing device.
`The processes and displays presented herein are not inher-
`ently related to any particular computing device or other
`apparatus. Various general purpose systems may be used with
`programs in accordance with the teachings herein, or it may
`prove convenient to construct a more specialized apparatus to
`perform the desired method. The desired structure for a vari-
`ety of these systems will appear from the description below.
`In addition, embodiments of the present invention are not
`described with reference to any particular programming lan-
`guage. It will be appreciated that a variety of programming
`languages may be used to implement the teachings of the
`invention as described herein.
`
`In the following description and claims, the terms coupled
`and connected, along with their derivatives, may be used. In
`particular embodiments, connected may be used to indicate
`that two or more elements are in direct physical or electrical
`contact with each other. Coupled may mean that two or more
`elements are in direct physical or electrical contact. However,
`coupled may also mean that two or more elements may not be
`in direct contact with each other, but yet may still cooperate or
`interact with each other.
`
`It should be understood that embodiments of the present
`invention may be used in a variety of applications. Although
`the present invention is not limited in this respect, the circuits
`disclosed herein may be used in many apparatuses such as in
`the transmitters and receivers of a radio system. Radio sys-
`tems intended to be included within the scope of the present
`invention include, by way ofexample only, wireless local area
`networks (WLAN) devices and wireless wide area network
`(WWAN) devices including wireless network interface
`devices and network interface cards (NICs), base stations,
`access points (APs), gateways, bridges, hubs, cellular radio-
`telephone communication systems, satellite communication
`systems, two-way radio communication systems, one-way
`pagers, two-way pagers, personal communication systems
`(PCS), personal computers (PCs), personal digital assistants
`(PDAs), and the like, although the scope of the invention is
`not limited in this respect.
`Types of wireless communication systems intended to be
`within the scope of the present
`invention may include,
`although are not limited to, Wireless Local Area Network
`(WLAN), Wireless Wide Area Network (WWAN), Code
`Division Multiple Access (CDMA) cellular radiotelephone
`communication systems, Global System for Mobile Commu-
`nications (GSM) cellular radiotelephone systems, North
`American Digital Cellular (NADC) cellular radiotelephone
`systems, Time Division Multiple Access (TDMA) systems,
`Extended—TDMA (E—TDMA) cellular radiotelephone sys-
`tems, third generation (3G) systems like Wide-band CDMA
`(WCDMA), CDMA-2000, Time Division Synchronization
`Code Division Multiple Access (TDS-CDMA), and the like,
`although the scope of the invention is not limited in this
`respect.
`Referring now to FIG. 1, a diagram of an orthogonal fre-
`quency division multiplexing scheme utilizing a cyclic exten-
`sion in accordance with one embodiment ofthe present inven-
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`tion will be discussed. In the orthogonal frequency division
`multiplexing (OFDM) scheme 100 as shown in FIG. 1,
`OFDM data 110 may be provided at the output of an inverse
`fast-Fourier transform (IFFT) block such as utilized in an
`OFDM transceiver, although the scope of the invention is not
`limited in this respect. A portion of the of OFDM data 110,
`typically a terminal portion thereof, may be duplicated and
`appended to the front of OFDM data 110 as a cyclic extension
`112 as shown in FIG. 1. In one embodiment of the invention,
`cyclic extension 112 may be prepended to the front of OFDM
`data 110 to provide a cyclic prefix extension. In an alternative
`embodiment, cyclic extension may be appended to the termi-
`nal end of OFDM data 110 to provide a cyclic postfix exten-
`sion, although the scope of the invention is not limited in this
`respect. Thus, although FIG. 1 illustrates a prepend case, the
`invention is not limited in this respect, and the invention may
`include an append case using a cyclic extension of a first
`portion of OFDM data 110. The combination of OFDM data
`110 and cyclic extension 112 may comprise an entire OFDM
`symbol 114 to be transmitted by an OFDM transmitter,
`although the scope of the invention is not limited in this
`respect.
`Referring now to FIG. 2, a block diagram of an orthogonal
`frequency division multiplexing receiver that dynamically
`allocates the length of a cyclic extension in accordance with
`one embodiment of the present invention. OFDM receiver
`200 may be utilized in a wireless transceiver 410 as shown in
`FIG. 4, although the scope of the invention is not limited in
`this respect. As shown in FIG. 2, sample data provided by an
`analog-to-digital converter in a wireless transceiver may be
`provided to receiver 200 at an input 210 ofpreamble process-
`ing block 212. After preamble processing, the data may be
`passed to block 214 at which cyclic extension 112 may be
`removed from symbol data 114 to provide OFDM data 110 to
`a fast-Fourier transform (FFT) block 216. The output of FFT
`block 216 may be passed to an OFDM demodulator block 218
`to demodulate OFDM data 100 to extract baseband data that
`
`may be provided at output 232 to a baseband processor such
`as baseband processor 412 of FIG. 4, or example for media
`access control (MAC) processing, although the scope of the
`invention is not limited in this respect.
`Receiver 200 may include block 222 to determine a desired
`length of cyclic extension 112. In one embodiment of the
`invention, block 222 may be a circuit of receiver 200,
`although the scope of the invention is not limited in this
`respect. The output of FFT block 216 may be provided to
`channel estimation block 220, which may provide its output
`to block 222 to dynamically allocate the duration of cyclic
`extension 112 when OFDM data 110 is transmitted by wire-
`less transceiver 410. In block 222, the output of channel
`estimation block 220 may be passed to an inverse fast-Fourier
`transform (IFFT) block 224, which may in turn provide an
`output to block 226. Block 226 may compute an energy
`distribution of the channel impulse response, and then pro-
`vide an output to block 228 to calculate a new, updated dura-
`tion of cyclic extension 228 for subsequent transmissions
`based on the energy distribution of the channel
`impulse
`response computed at block 226. The calculated new, updated
`duration of cyclic extension 228 may be provided at output
`230 to store the new duration of cyclic extension 228 in a
`MAC database via baseband processor 412 so that the new,
`updated duration of cyclic extension 228 may be utilized for
`subsequent OFDM transmissions. Block 222 may continu-
`ally update the duration of cyclic extension 228 as described
`for subsequent OFDM data 110 received by receiver 200 so
`that the duration of cyclic extension may be dynamically
`computed and changed in accordance with changing delay
`
`5
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`

`
`US 7,400,573 B2
`
`5
`spreads experienced in the physical environment in which
`receiver 200 is utilized, although the scope of the invention is
`not limited in this respect.
`Referring now to FIG. 3, a flow diagram of a method to
`dynamically determine and allocate a length ofa cyclic exten-
`sion in an orthogonal frequency division multiplexing system
`in accordance with an embodiment of the present invention
`will be discussed. In a wireless LAN system in which there
`are one or more devices operating, to dynamically allocate the
`cyclic extension for each device, several functions may be
`performed utilizing method 300. For example, in a basic
`services set (BSS) mode, the access point, which may be for
`example base station 420 as shown in FIG. 4, and mobile unit
`402 of FIG. 4, may have a mechanism to identify whether or
`not mobile unit 402 supports dynamic cyclic extension.
`Although a BSS mode is discussed herein, the scope of the
`invention is not limited in this respect and may be applied to
`other types of WLAN systems, for example a WLAN system
`operating in an infrastructure basic services set (IBSS) mode.
`In many WLAN systems, control frames such as beacons,
`association request, and association response currently exist
`and may be utilized to provide mechanisms for conveying
`capability information between the mobile unit and the access
`point. For example, the information elements within such
`frames may have unused fields that could be employed, dur-
`ing association or re-association, to convey that the mobile
`unit and the access point both support dynamic cyclic exten-
`sion. Thus, such information elements and a transmitted sym-
`bol may be received from a mobile unit at block 310, and a
`determination may be made at block 312 whether the mobile
`unit supports dynamic cyclic extension. In the event either the
`mobile unit or the access point does 11ot support dynamic
`cyclic extension, a predetermined cyclic extension may be
`utilized at block 314.
`
`In the event botl1 the access point and the mobile ur1it
`support dynamic cyclic extension, the access point may esti-
`mate the channel impulse response at block 316 as observed
`by a given mobile unit using the preamble or packet data
`information from that mobile unit. The access point may
`estimate the charmel impulse response of the mobile unit by
`performing an inverse fast-Fourier transform (IFFT) on the
`channel estimates obtained by the access point from one or
`more incoming packets received from the mobile unit. For
`example, such estimation may be performed during an asso-
`ciation phase or some equivalent phase when the mobile unit
`initiates a connection to the access poi11t. A packet received
`from the mobile unit may be assumed to have employed a
`maximum length cyclic extension during this phase, although
`the scope of the invention is not limited in this respect.
`From the estimate of the channel impulse response as
`determined at block 316, the access point then may estimate
`the energy distribution ofthe channel at block 3 18 and use the
`energy distribution to set the length of the cyclic extension
`112 for that mobile unit. An algorithm may be performed in
`the access point to determine the energy distribution in the
`impulse response, for example a ratio ofthe total energy in the
`impulse response to truncated subsets of the impulse
`response, although the scope of the invention is no limited in
`this respect. The energy distribution for the charmel of that
`particular mobile unit may then be employed to determine an
`appropriate new length for cyclic extension 112 at block 320.
`For example, if the access point determines that the majority
`of the energy is contained in the first half of the estimated
`impulse response, the access point may set the new length of
`cyclic extension 112 to be half the maximum length of cyclic
`extension.
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`The new cyclic extension 112 for that particular mobile
`unit may be stored at block 322. The access point may have a
`mechanism to store and access the required cyclic extension
`length one or more mobile units within the local BSS. In
`present day OFDM WLAN systems, the media access con-
`troller (MAC) in the access point may already maintain a
`database for each address ofthe mobile units to track existing
`mobile dependent parameters, for example per user data rate
`in an IEEE 802.lla compliant system. Additional informa-
`tion regarding the required cyclic extension for one or more
`mobile units may be stored within the same or a similar
`database and accessed in the same or a similar manner as the
`
`mobile dependent information of existing WLAN systems.
`The new, calculated cyclic extension may be communicated
`to the respective mobile unit at step 324 for subsequent com-
`munications. The access point may have a mechanism to
`communicate the calculated cyclic extension 112 to the
`respective mobile unit 402. Communicating the cyclic exten-
`sion to the mobile could be performed in a like manner as
`other mobile dependent parameters used in today’s systems.
`For example, in an IEEE 802.1 la compliant system the data
`rate may be communicated in the signal field of each OFDM
`packet. In accordance with one embodiment of the present
`invention, the signal field additionally may be used to com-
`municate the cyclic extension length to the mobile unit,
`although the scope of the invention is not limited in this
`respect.
`Referring now to FIG. 4, a block diagram of a wireless
`communication system in accordance with one embodiment
`of the present invention will be discussed. In the communi-
`cations system 400 shown in FIG. 4, a mobile unit 402 may
`include a wireless transceiver 410 to couple to an antenna 416
`and to a processor 412. Processor 412 in one embodiment
`may comprise a single processor, or alternatively may com-
`prise a baseband processor and an applications processor, and
`may implement one or more functions of the MAC layer,
`although the scope of the invention is not limited in this
`respect. Processor 412 may couple to a memory 414 which
`may include volatile memory such as DRAM, non-volatile
`memory such as flash memory or EEPROM, or alternatively
`may include other types of storage such as a hard disk drive,
`although the scope of the invention is not limited in this
`respect. Some portion or all of memory 414 may be included
`on the same integrated circuit as processor 412, or altema-
`tively some portion or all ofmemory 414 may be disposed on
`an integrated circuit or other medium, for example a hard disk
`drive, that is external to the integrated circuit of processor
`412, although the scope of the invention is not limited in this
`respect.
`Mobile unit 402 may communicate with base station (or
`access point) 420 via wireless communication link 424,
`where base station 420 may include at least one antenna 418.
`Base station 420 may couple with network 422 so that mobile
`unit 402 may communicate with network 422,
`including
`devices coupled to network 422, by communicating with base
`station 420 via wireless communication link 424. Network
`
`422 may include a public network such as a telephone net-
`work or the Internet, or alternatively network 422 may
`include a private network such as an intranet, or a combina-
`tion of a public and a private network, although the scope of
`the invention is not limited in this respect. Communication
`between mobile unit 402 and base station 420 may be imple-
`mented via a wireless local area network (WLAN), for
`example a network compliant with a an Institute of Electrical
`and Electronics Engineers (IEEE) standard such as IEEE
`802.1 la, IEEE 802.1 lb, and so on, although the scope of the
`invention is not limited in this respect. In another embodi-
`
`6
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`

`
`US 7,400,573 B2
`
`8
`7. An apparatus as claimed in claim 5, said circuit including
`a cyclic extension circuit to calculate an updated cyclic exten-
`sion based on the energy distribution.
`8.An apparatus as claimed in claim 5, said circuit including
`an inverse fast-Fourier transform circuit
`to compute an
`impulse response of a channel of the transceiver, an energy
`distribution circuit to detennine an energy distribution in an
`impulse response of a channel of the receiver, and a cyclic
`extension circuit to calculate an updated cyclic extension
`based on the energy distribution.
`9. A method, comprising:
`estimating a charmel impulse response of a received sym-
`bol;
`computing an energy distribution of the channel impulse
`response including computing a ratio of a total energy of
`the charmel impulse response to at least one or more
`truncated subsets of the charmel impulse response; and
`determining a length of a cyclic extension based at least in
`part on the energy distribution of the channel impulse
`response.
`10. A method as claimed in claim 9, further comprising
`storing the determined length of a cyclic extension for a
`device from which the received symbol was transmitted, and
`communicating with the device using the cyclic extension
`having the determined length.
`11. A method as claimed in claim 9, further comprising
`receiving a subsequent symbol, and executing said estimat-
`ing, said computing, and said detennining based on the sub-
`sequent symbol.
`12. A method as claimed in claim 9, further comprising
`communicating the determined length of the cyclic extension
`to the device from which the received symbol was transmit-
`ted.
`
`7
`ment, communication between mobile unit 402 and base
`station 420 may be implemented via a cellular communica-
`tion network compliant with a 3"’ Generation Partnership
`Project (3GPP) standard, although the scope of the invention
`is not limited in this respect.
`Although the invention has been described with a certain
`degree of particularity, it should be recognized that elements
`thereof may be altered by persons skilled in the art without
`departing from the spirit and scope of the invention. It is
`believed that the dynamic allocation of cyclic extension in
`orthogonal frequency division multiplexing systems of the
`present invention and many ofits attendant advantages will be
`understood by the forgoing description, and it will be appar-
`ent that various changes may be made in the form, construc-
`tion and arrangement of the components thereof without
`departing from the scope and spirit ofthe invention or without
`sacrificing all of its material advantages, the form herein
`before described being merely an explanatory embodiment
`thereof, and further without providing substantial change
`thereto. It is the intention of the claims to encompass and
`include such changes.
`What is claimed is:
`
`1. An apparatus, comprising:
`a circuit to determine a cyclic extension based at least in
`part on an energy distribution of an impulse response of
`a symbol received by a transceiver; and
`an energy distribut'on circuit to determine an energy dis-
`tribution in an impulse response ofa channel wherein the
`energy distribution circuit determines the energy distri-
`bution of the impulse response by calculating a ratio of a
`total energy in I16 impulse response to at least one or
`more truncated subsets of the impulse response.
`2. An apparatus as claimed in claim 1, further comprising
`an inverse fast-Four'er transform circuit
`to compute an
`impulse response of a channel of the transceiver.
`3. An apparatus as claimed in claim 1, further comprising a
`cyclic extension circuit to calculate an updated cyclic exten-
`sion based on the ene*gy distribution.
`4. An apparatus as claimed in claim 1, further comprising
`an inverse fast-Four'er transform circuit
`to compute an
`impulse response of a channel of the transceiver, an energy
`distribution circuit to determine an energy distribution in an
`impulse response of a channel of the receiver, and a cyclic
`extension circuit to calculate an updated cyclic extension
`based on the energy distribution.
`5. An apparatus, comprising:
`a transceiver;
`an omnidirectional antenna to couple to said transceiver;
`and
`
`a circuit to determine a cyclic extension based at least in
`part on an energy distribution of an impulse response of
`a symbol received by the transceiver, said circuit includ-
`ing an energy distribution circuit to determine an energy
`distribution in an impulse response of a channel wherein
`the energy distribution circuit determines the energy
`distribution of the impulse response by calculating a
`ratio of a total energy in the impulse response to at least
`one or more truncated subsets of the impulse response.
`6. An apparatus as claimed in claim 5, said circuit including
`an inverse fast-Fourier transform circuit
`to compute an
`impulse response of a channel of the transceiver.
`
`5
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`13. A computer-readable medium embodied with com-
`puter executable instructions comprising:
`estimating a charmel impulse response of a received sym-
`bol;
`computing an energy distribution of the channel impulse
`response;
`determining a length of a cyclic extension based at least in
`part on the energy distribution of the channel impulse
`response; and
`computing a ratio of a total energy of the charmel impulse
`response to at least one or more truncated subsets of the
`channel impulse response.
`14. A machine readable computer-readable medium as
`claimed in claim 13, further comprising storing the deter-
`mined length ofa cyclic extension for a device from which the
`received symbol was transmitted, and communicating with
`the device using the cyclic extension having the determined
`length.
`15. A computer-readable medium as claimed in claim 13,
`further comprising receiving a subsequent symbol, and
`executing said estimating, said computing, and said deter-
`mining based on the subsequent symbol.
`16. A computer-readable medium as claimed in claim 13,
`further comprising communicating the determined length of
`the cyclic extension to the device from which the received
`symbol was transmitted.
`*
`
`*
`
`*
`
`*
`
`*
`
`7
`
`

`
`UNITED STATES PATENT AND TRADEMARK OFFICE
`
`CERTIFICATE OF CORRECTION
`
`PATENT NO.
`APPLICATION NO.
`
`: 7,400,573 B2
`: 10/425429
`
`DATED
`INVENTOR(S)
`
`: July 15, 2008
`: Sundstrom et a1.
`
`It is certified that error appears in the above—identified patent and that said Letters Patent is
`hereby corrected as shown below:
`
`In column 8, line 46, in Claim 14, before “computer-readable” delete “machine
`readable”.
`
`Signed and Sealed this
`
`Seventh Day of October, 2008
`
`AWQDMA
`
`JON W. DUDAS
`Director ofthe United States Patent and Trademark Oflice
`
`8