IJS007599327B2
`
`(12) Ulllted States Patent
`Zhuang
`
`(10) Patent No.:
`(45) Date of Patent:
`
`US 7,599,327 B2
`Oct. 6, 2009
`
`(54) METHOD AND APPARATUS FOR ACCESSING
`A WIRELESS COMMUNICATION SYSTEM
`
`(56)
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`75
`
`(
`
`)
`
`.
`.
`I“"em°r' Xlangyang Zh“a“g= H°ffma“ ES‘a‘eS=
`IL (US)
`
`(73) Assignee: Motorola, Inc., Schaumburg, IL (US)
`
`( >i< ) Notice:
`
`Subject to any disclaimer, the term Ofthjs
`patent is extended or adjusted under 35
`U.S.C. l54(b) by 313 days.
`
`(21) Appl. No.: 11/070,061
`
`5,930,299 A
`6,178,158 B1 *
`6,178,197 B1
`6,788,728 B1
`6,947,476 B2 *
`2004/0114504 Al *
`
`............. .. 370/203
`
`7/1999 Vannatta et al.
`1/2001 Suzuki et a1.
`1/2001 Froelich et al.
`9/2004 Prasad et al.
`9/2005 Song ........................ .. 375/149
`
`6/2004 Jung et al.
`. . . . . . .
`. . . . . .. 370/203
`
`............ .. 370/210
`7/2004 Sandell et al.
`2004/0131011 Al *
`OTHER PUBLICATIONS
`Huy, Vu: The International Search Report or the Declaration, ISM
`US, Alexandria Virginia, completed: Nov. 17, 2005, mailed: Feb. 22,
`2006.
`* cited by examiner
`
`(22) Filed,
`
`Man 2, 2005
`
`Pri/nary Examiner—George Eng
`Assistant Examiner—Marcos L Torres
`
`(65)
`
`Prior Publication Data
`
`US 2005/0286465 A1
`
`Dec‘ 29: 2005
`
`_
`_
`Related U-S- APP11°3t1011 Data
`(60) Provisional application No. 60/582,602, filed on Jun.
`24 2004.
`’
`
`(51) 4/00
`
`2006 01
`‘
`
`(
`
`)
`
`...................... .. 370/329; 370/203; 370/210
`(52) U.S. Cl.
`(58) Field of Classification Search ............... .. 370/329,
`370/203, 208, 210, 330, 336, 394
`See application file for complete search history.
`
`ABSTRACT
`(57)
`Access to a wireless communication system (100) by a sub-
`Y
`g
`scriber station 101-103 is facilitated b selectin
`705 an
`access sequence from a set of sequences that have been iden-
`tified to have a low average of peak-to-average-power-ratios
`of access signals generated by the set of sequences and also
`base;10: atfood CrOSS'COr1r.e1aU:n Ofthe a7CeSS Slgnalsi form-
`mg(
`)
`e access wave orm y generating an access signal
`using the access sequence and appending in the time domain
`a cyclic prefix to the access signal; and transmitting (715) the
`access waveform. In some implementations, the access wave-
`form is cyclically shifted (820) before the cyclic prefix is
`appended, and in some implementations, the signal is trans-
`mitted (710, 810) in a randomly selected sub-band of an
`2100655 interval.
`
`21 Claims, 6 Drawing Sheets
`
`70
`
`
`
`
`
`
`
`SELECT AN ACCESS SEQUENCE FROM A SET OF Nc ACCESS SEOUENCES
`THAT HAVE BEEN IDENTIFIED TO HAVE A LOW AVERAGE OF PEAK-TO-
`AVERAGE—POVlER-RATIOS OF ACCESS SIGNALS GENERATED BY THE SET OF
`Nc ACCESS SEOUENCES AND TO HAVE A GOOD CROSS-CORRELATION OF
`THE ACCESS SIGNALS GENERATED BY THE SET OF Nc ACCESS
`SEOUENCES, AND VIHEREIN THE SET OF Nc ACCESS SEOUENCES HAS
`BEEN GENERATED BY A CORRESPONDING SET OF SEOUENCES OF LENGTH
`K, WHEREIN K IS A OUANTITY OF CARRIERS IDENTIFIED FOR TRANSMITTING
`AN ACCESS WAVEFORM
`
`
`
`7T0
`
`714
`
`SELECT A SUB—BAND
`
`FORM THE ACCESS WAVEFORM BY GENERATING AN ACCESS SIGNAL USING
`THE ACCESS SEQUENCE AND APPENDING IN THE TIME DOMAIN A CYCLIC
`PREFIX TO THE ACCESS SIGNAL
`
`
`
`
`
`715
`
`
`
`TRANSMIT THE ACCESS WAVEFORM
`
`
`
`APPLE 1007
`
`APPLE 1007
`
`1
`
`

`
`U.S. Patent
`
`Oct. 6, 2009
`
`Sheet 1 of 6
`
`US 7,599,327 B2
`
`xx“
`vs-““<I‘e»‘»°°‘
`BANDWIDTH
`REQUEST(2
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`FIG. 1
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`U.S. Patent
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`Oct. 6, 2009
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`U.S. Patent
`
`Oct. 6, 2009
`
`Sheet 4 of6
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`US 7,599,327 B2
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`502
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`
`SELECT AN ACCESS SEQUENCE FROM A SET OF Nc ACCESS SEOUENCES
`THAT HAVE BEEN IDENTIFIED TO HAVE A LOW AVERAGE OF PEAK-TO-
`
`AVERAGE—POWER-RATIOS OF ACCESS SIGNALS GENERATED BY THE SET OF
`
`Nc ACCESS SEOUENCES AND TO HAVE A GOOD CROSS-CORRELATION OF
`
`THE ACCESS SIGNALS GENERATED BY THE SET OF Nc ACCESS
`
`SEOUENCES, AND WHEREIN THE SET OF Nc ACCESS SEOUENCES HAS
`
`
`BEEN GENERATED BY A CORRESPONDING SET OF SEQUENCES OF LENGTH
`K. WHEREIN K IS A QUANTITY OF CARRIERS IDENTIFIED FOR TRANSMITTING
`
`AN ACCESS WAVEFORM
`
`
`
`
`
`7M
`
`7M
`
`7%
`
`SELECT A SUB-BAND
`
`FORM THE ACCESS WAVEFORM BY GENERATING AN ACCESS SIGNAL USING
`THE ACCESS SEQUENCE AND APPENDING IN THE TIME DOMAIN A CYCLIC
`PREFIX TO THE ACCESS SIGNAL
`
`TRANSMIT THE ACCESS WAVEFORM
`
`01
`
`5
`
`

`
`U.S. Patent
`
`Oct. 6, 2009
`
`Sheet 5 of6
`
`US 7,599,327 B2
`
`SELECTING AN ACCESS SEQUENCE FROM A SET OF Nc ACCESS SEOUENCES
`
`SELECTING A SUB-BAND
`
`GENERATING AN ACCESS SIGNAL USING THE SELECTED ACCESS SEQUENCE
`
`CYCLICALLY TIME SHIFTING THE GENERATED ACCESS SIGNAL BY A SHIFT
`VALUE THAT IS ONE OF A DEFINED SET OF A PLURALITY OF Nsh SHIFT VALUES
`
`FORMING AN ACCESS WAVEFORM BY APPENDING A CYCLIC PREFIX TO THE
`SELECTED ACCESS WAVEFORM
`
`TRANSMITTING THE ACCESS WAVEFORM
`
`FIG. 8
`
`805
`
`BM
`
`8L5
`
`824
`
`825
`
`905
`
`IDENTIFYING IN A TRANSMITTED CONTROL SIGNAL ONE OR MORE SUB—BANDS
`IN AN ACCESS INTERVAL, EACH OF WHICH COMPRISES K SUB—CARRIERS AND
`FOR WHICH EACH OF THE SUB-BANDS IS AVAILABLE FOR A SUBSCRIBER
`STATION TO USE TO TRANSMIT AN ACCESS SIGNAL
`
`DECODING THE ACCESS SIGNAL
`
`RECEIVING AN ACCESS SIGNAL FROM THE SUBSCRIBER STATION IN ONE OF
`THE ONE OR MORE SUB—BANDS DURING THE ACCESS INTERVAL
`
`9m
`
`Ofi
`
`FIG. 9
`
`6
`
`

`
`U.S. Patent
`
`Oct. 6, 2009
`
`Sheet 6 of6
`
`US 7,599,327 B2
`
`
`
`1005
`
`I010
`
`I015
`
`RECEIVING AN ACCESS SIGNAL FROM A SUBSCRIBER STATION DURING
`AN ACCESS INTERVAL
`
`
`
`ANALYZING THE ACCESS SIGNAL TO IDENTIFY AT LEAST ONE OF A CYCLIC
`SHIFT OF A DEFINED SET OF CYCLIC SHIFTS AND AN ACCESS SEQUENCE OF
`
`A SET OF ACCESS SEQUENCES
`W
`
`
`
`
`
`PROCESSING THE ACCESS SIGNAL T0 EXTRACT SUBSCRIBER STATION
`SYNCHRONIZATION INFORMATION
`
`FIG. 10
`
`7
`
`

`
`1
`METHOD AND APPARATUS FOR ACCESSING
`A WIRELESS COMMUNICATION SYSTEM
`
`2
`for an efficient and flexible air interface mechanism that
`enables fast and reliable user access to the network.
`
`US 7,599,327 B2
`
`FIELD OF THE INVENTION
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`Ihe present invention relates generally to communication
`systems, and in particular, to a method and apparatus for
`randomly accessing a wireless communication system by a
`subscriber station in order to obtain or maintain such param-
`eters as uplink timing, power control, channel estimation, and
`frequency alignment of the subscriber station.
`
`BACKGROUND OF THE INVENTION
`
`In a wireless communication system, it is critical to design
`a mechanism for allowing a remote subscriber station (SS) to
`access the network by sending an access signal to a Base
`Station (BS). The access signal fulfills important functions
`such as requesting resource allocation from the BS, alerting
`the BS of the existence of the SS that is trying to enter the
`network, and initiating a process that allows the BS to mea-
`sure some parameters of the SS (e.g., timing offset caused by
`propagation, transmit power, etc.) that must be maintained
`and adjusted constantly in order to ensure a non-interfering
`sharing of the uplink resource. Unlike ordinary data trafiic
`that is sent using scheduled resources that are allocated by the
`BS to the SS, such an access signal is often transmitted in an
`unsolicited manner. Therefore, this process is often referred
`to as a random access. Sometimes the process is also referred
`to as “ranging”, as used in the Institute of Electrical and
`Electronic Engineers (IEEE) 802.16 standards, because the
`access signal can help the BS to measure the propagation
`distance from the SS (thus, its range). A parameter known as
`a timing advance offset is used by the SS to advance its
`transmission rclativc to thc rcfcrcncc timing at thc BS so that
`the signals from all the SS’s appear synchronized at the BS
`(i.e., uplink timing synchronization). Once uplink timing syn-
`chronization is achieved, the SS orthogonality is ensured (i.e.,
`each SS occupies its own allocated sub-carriers without inter-
`fering with other SS). In this specification, the terms “access”,
`“random access”, and “ranging” will be used interchangeably
`to describe these processes and also to describe the signal
`transmitted by the SS to initiate the access process.
`Ihe random access or ranging process includes an initial/
`handover ranging function for synchronizing an SS with a BS
`during the initial network entry or re-entry and during cell
`handoff, a periodic ranging function for maintaining SS syn-
`chronization, and a bandwidth request function that allows
`each SS to request uplink bandwidth allocation. These uplink
`ranging functions fulfill very important tasks that can signifi-
`cantly influence the user experience. For example, the band-
`width request ranging performance directly impacts the
`access latency perceived by a user, especially during commu-
`nication sessions (e.g., HTTP) that consist of sporadic packet
`tralfic that requires fast response, in which case high detection
`and low collision probabilities of the access request are very
`desirable. In another example, robust detection of an initial
`ranging signal is essential in order to allow a user to quickly
`enter the network or to be handed over to a new serving sector.
`Reliable extraction of the accurate timing offsets from the
`initial ranging signals is also critical for achieving uplink
`synchronization that ensures user orthogonality (i.e., to make
`sure that each SS occupies its own allocated sub-carriers
`without interfering with other SS). Other important informa-
`tion that the BS needs to extract from ranging includes power
`measurement,
`frequency synchronization,
`and channel
`impulse response estimation, etc. Therefore, there is a need
`
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`FIG. 1 is a block diagram of a communication system, in
`accordance with some embodiments of the present invention.
`FIG. 2 is a time-domain diagram of a “basic” dedicated
`basic ranging interval, in accordance with some embodiments
`of the present invention.
`FIG. 3 is a time-domain diagram of an extended dedicated
`ranging interval, in accordance with some embodiments of
`the present invention.
`FIG. 4 is a frequency-domain diagram of a variation of the
`extended dedicated ranging interval, in accordance with some
`embodiments of the present invention.
`FIG. 5 is a time-domain diagram of an example design for
`an OFDM system such as the one defined by the IEEE 802.16
`standard.
`
`FIG. 6 is a block diagram of the division of ranging oppor-
`tunities in frequency, time, and code domains, in accordance
`with some embodiments of the present invention.
`FIGS. 7 and 8 are flow charts of methods of accessing a
`communication system, in accordance with some embodi-
`ments of the present invention.
`FIGS. 9 and 10 are methods used by a base station in a
`wireless communication system for facilitating an access of
`tl1e cor11r11ur1icatior1 system by a subscriber station, ir1 accor-
`dance with some embodiments of the present invention.
`
`DETAILED DESCRIPTION OF THE INVENTION
`
`Before describing in detail the particular communication
`system accessing technology in accordance with the present
`invention, it should be ob served that the present invention
`resides primarily in combinations of method steps and appa-
`ratus components related to accessing a communication sys-
`tem by a subscriber station. Accordingly, the apparatus com-
`ponents and method steps have been represented where
`appropriate by conventional symbols in the drawings, show-
`ing only those specific details that are pertinent to understand-
`ing the present invention so as not to obscure the disclosure
`with details that will be readily apparent to those of ordinary
`skill in the art having the benefit of the description herein.
`Turning now to the drawings, wherein like numerals des-
`ignate like components, FIG. 1 is a block diagram of com-
`munication system 100. Communication system 100 com-
`prises a plurality of cells 106 and 107 (only two shown) each
`having a base station (BS) 104, 105. The service area of the
`BS 104 covers a plurality of subscriber stations (SSs) 101-
`103, each at a time may be performing some type of ranging
`function, which is also called herein a random access func-
`tion. For example, SS 101 may move out ofthe service area of
`BS 104 and enter into the service area of BS 105, in which
`case a handover occurs that often involves a handover access.
`
`In other examples, SS 102 makes a bandwidth request and/or
`SS 103 makes an initial entry access when it is first activated
`within the communication system. In one embodiment of the
`present invention, communication system 100 utilizes an
`Orthogonal Frequency Division Multiplexed (OFDM) modu-
`lation or other variants of OFDM such as multi-carrier
`
`CDMA (MC-CDMA), n1ulti-carrier direct sequence CDMA
`(MC-DS-CDMA). In other embodiments of the present
`invention, the multi-channel communication system 100 can
`use any arbitrary technology such as TDMA, FDMA, and
`CDMA.
`
`8
`
`

`
`US 7,599,327 B2
`
`3
`Definition of Dedicated Ranging Zone
`Referring to FIG. 2, a time-domain diagram shows a
`“basic” dedicated basic ranging zone 201 defined for an
`OFDM example system (the term “zone” is interchangeable
`herein with the term “interval” used in the figure), in accor-
`dance with some embodiments of the present invention. The
`duration of the dedicated basic ranging interval 201 consists
`of an interval of a special OFDM symbol 202 (denoted as
`“extended-CP” OFDM symbol) and a “dead interval” 204
`that is a no-transmission interval equal to the maximum tim-
`ing delay to be accommodated in the cell. The special OFDM
`symbol 202 has a duration equal to the sum of the duration of
`a special Fast Fourier Transform (FFT) window 209 and the
`duration ofan extended cyclic prefix (CP) 203 wherein the CP
`represents the repeat of a portion of the signal as commonly
`known in OFDM. Hence, the special OFDM symbol is also
`referred to as an “extended-CP” OFDM symbol in FIG. 2.
`The special Fast Fourier Transform (FFT) window 209 may
`be chosen conveniently to be the same as a “regular” OFDM
`symbol period in an example deployment of an OFDM sys-
`tem, or other designed value (discussed later). The duration of
`the extended CP 203 equals to the sum of the duration of a
`“regular” CP 205 and the maximum timing delay 206 to be
`accommodated. The maximum timing delay is chosen based
`on the possible timing differences among all possible sub-
`scriber locations. This value directly relates to the round-trip
`propagation delay and the cell size. Meanwhile, the duration
`of a “regular” CP 205 within the extended CP 203 is the same
`as the CP length defined for regular data transmissions if the
`invention is used for an OFDM system. For other systems, the
`time duration of a regular CP is often chosen based on the
`excessive delay spread of the channels encountered in a
`deployment environment, which is also how the CP length is
`determined for OFDM systems. Lastly, as described above,
`the appended “dead” interval is chosen according to the maxi-
`mal timing delay,
`A ranging signal is allowed to be transmitted only in the
`defined ranging interval. The ranging waveform itself is con-
`structed as an OFDM symbol, i.e., by appending a CP of a
`certain length to a ranging signal. For convenience, we will
`use the term “waveform” to refer to the CP-included signal
`and the term “signal” for the CP-excluded portion only. The
`ranging waveform transmission starts from what the SS deter-
`mines to be the right timing. For initial ranging users, that
`transmission point (i.e., the transmission start time) will be
`the beginning of the dedicated ranging interval according to
`the base reference plus the one-way propagation delay. The
`initial ranging SS should send at that point a waveform whose
`CP portion is of the length of an extended CP. For other
`ranging SS’ s that have already synchronized with the BS, the
`SS should have known the timing advance and transmit in
`advance to some reference point so that all the SS signals
`arrive at the BS at roughly the same time. In one embodiment,
`the non-initial ranging SS can either transmit a waveform
`with a regular CP at a timing point in advance to the start of
`205 within 203 of FIG. 2, or transmit a waveform with an
`extended CP at a timing point in advance to the start of 203.
`With the above definition of ranging interval, all types of
`ranging signals will not interfere with any transmission that
`precedes and follows the ranging interval, such as OFDM
`symbols 207 and 208 in an OFDM-based example system.
`The maximum timing delay should be large enough to accom-
`modate the maximum propagation delay for SSs that have not
`adjusted their timing (i.e., initial ranging users). The maxi-
`mum timing delay is a parameter detennined based on the cell
`size. For the receiver processing at the BS, since the BS
`predefines the maximum timing delay and thus the extended
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`CP length, the BS should know how to adjust the sampling
`position accordingly in order to extract the special FFT win-
`dow 209. The special FFT window can be any size in theory.
`A large special FFT window can reduce the proportion of the
`extended CP to the special FFT size (i.e., reducing the over-
`head) and provide more ranging opportunities to reduce col-
`lision. Also, the time span of the transmission can also be
`extended so that there will be more signal power arriving at
`the BS for the same average transmit power. However, with a
`large special FFT window, the overall overhead of a ranging
`signal as a portion of the uplink sub-frame increases and the
`ranging signal also becomes more susceptible to charmel time
`variations (e.g. mobility) that results in inter-carrier interfer-
`ence caused by Doppler shift. The choice of the special FFT
`size should also consider practical
`implementation. For
`example, in an OFDM system, making it an integer multiple
`of the regular FFT size may simplify the BS processing.
`The total ranging overhead, which is the ratio of the dura-
`tion ofthe dedicated basic ranging interval to the entire uplink
`sub-frame, depends only on the uplink sub-frame. The longer
`the uplink, the lower is the overhead. If the overhead due to
`the “dead interval” 204 delay becomes too excessive, the
`“dead interval” 204 can be omitted at the price of generating
`inevitable interference to the next symbol.
`Referring to FIG. 3, a time-domain diagram shows an
`“extended” dedicated ranging interval 303 that is built upon
`the “basic” dedicated ranging interval 201, in accordance
`with some embodiments of the present invention. If more
`ranging opportunities are needed than what a basic ranging
`interval can provide, an extended ranging interval 303 can be
`defined where one or more regular OFDM symbols 301 and
`3 02 with only a regular CP length may be added in front ofthe
`extended-CP special symbol. Initial ranging transmission is
`allowed only during the extended-CP interval, but other rang-
`ing transmissions are allowed everywhere. This design is an
`alternative to the case in which the special FFT size is
`enlarged, as described with reference to FIG. 2.
`Referring to FIG. 4, a frequency-don1ain diagram of a
`variation ofthe extended dedicated ranging interval is shown,
`in accordance with some embodiments of the present inven-
`tion. In these embodiments, the ranging signal is allowed to
`occupy only a portion of the system bandwidth instead of the
`entire bandwidth as before. For example, for the extended-CP
`symbol 401 (that is the same as extended CP symbol 202 in
`FIG. 2), a portion of the bandwidth 402 is dedicated to rang-
`ing, and the remaining bandwidth 403 is for data traffic. In
`fact, such a design in which the ranging and data trafiic are
`multiplexed can be done using different data/ranging ratios
`for each symbol in the extended ranging interval such as that
`illustrated in FIG. 4, where the additional regular OFDM
`symbols 404 and 405 are used. The generic term “frequency-
`time ranging zone” is used for these cases.
`Referring to FIG. 5, a time-domain diagram shows an
`exemplary ranging interval for an OFDM system similar to
`OFDM systems described by drafts and the published version
`of the IEEE 802.16 standard The ranging interval 501 is
`composed of one special OFDMA symbol with an extended
`CP that may be preceded by up to four regular OFDMA
`symbols each having a regular CP for providing more ranging
`opportunities if needed. The duration of the extended CP is
`signaled by the base in a control message sent from the BS
`(e.g., the UL-MAP message defined in draft and published
`versions of IEEE 802.16 standards) as an integer multiple of
`the regular CP. Similarly,
`the special FFT size of the
`extended-CP symbol, which may also be an integer multiple
`of the regular FFT size, is signaled in the control message as
`well. Immediately after the special OFDMA symbol, there is
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`US 7,599,327 B2
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`5
`a “dead” interval that equals to the largest maximum timing
`difference. But it may be omitted to trade performance deg-
`radation for overhead reduction. The control message may
`indicate whether the dead interval is included. The duration of
`
`the dead interval is implicitly known to the SS and is equal to
`the difference between the extended-CP symbol duration and
`the regular CP duration.
`
`Division of Ranging Opportunities in Frequency, Time, and
`Code Domains
`
`Referring to FIG. 6, a block diagram shows the division of
`ranging opportunities in frequency, time, and code domains,
`in accordance with some embodiments of the present inven-
`tion. Each random access signal is generated based on a
`ranging sequence (interchangeable with “access sequence”
`and “ranging code” and “access code”) that is randomly cho-
`sen from a code group 601 allocated to the sector (the code
`group size is denoted herein as NC, an integer). The access
`sequences used in a code group and the allocation of code
`groups to different sectors are specified later. The ranging
`sequence may be used to generate an access signal by directly
`modulating the contiguous sub-carriers in a frequency block
`(sub-band) that is randomly chosen among NM sub-bands
`602, wherein NM is an integer including the value “ l ” known
`to both BS and SS. NM may be determined based on the
`system bandwidth and be made known to the BS and the SS.
`The time-domain access signal is generated by performing an
`IFFT on the ranging sequence after modulating the chosen
`sub-bar1d. Before a CP is inserted ir1 front of tlie access signal
`to form a complete access waveform, the access signal may be
`cyclically (circularly) shifted in time domain, where the shift
`is chosen randomly among Nsh allowed values 603 that are
`known to the BS and SS, wherein Nsh is an integer. Lastly, a
`CP is added to form the final ranging waveform where the
`length of the CP is that of the extended CP for initial ranging
`and for other ranging, either the extended CP or the regular
`CP depending on the transmission point (discussed above).
`The duration of the waveform corresponds to the duration of
`one OFDM symbol
`in the extended ranging interval,
`in
`embodiments such as those described with reference to FIG.
`3 and FIG. 5 In embodiments such as those described with
`
`reference to FIG. 5, the ranging sequence may be used to
`generate an access waveform by appending data symbols to
`the ranging sequence and directly modulating the contiguous
`sub-carriers in the frequency block (sub-band) that is ran-
`domly chosen, using terms of the appended ranging
`sequence.
`
`More detail on the division of ranging opportunities in
`frequency, time, and code domains is as follows. Firstly, in the
`frequency domain, an entire frequency band is divided into
`NM frequency blocks 602 (NM sub-ba11ds with K sub-carriers
`in each sub-band). A ranging signal may occupy only one
`sub-band. The reason for dividing the bandwidth into
`orthogonal blocks is for better flexibility. First, the number of
`ranging opportunities can be made adjustable to the band-
`width: larger bandwidth systems need to provide more oppor-
`tunities than narrower bandwidth systems for a similar colli-
`sion rate. Second, transmitting on a narrow sub-band allows
`power boost on that band to achieve a decent uplink SNR,
`even though narrowband transmission has lower timing reso-
`lution than wider bandwidth transmission (NM channel taps
`will collapse into one charmel tap when only l/NM of the
`bandwidth is excited). On the other hand, the number of
`sub-carriers in each sub-band, which equals to the length of
`the ranging sequence code, affects the cross-correlation char-
`acteristics. For example, halving the number of sub-carriers
`in a sub-band allows a 3 dB transmit power boost on that band,
`
`5
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`10
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`15
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`20
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`25
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`30
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`35
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`40
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`45
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`55
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`60
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`6
`but the potential interference from other co-channel ranging
`codes also increases by 3 dB. So the number of sub-carriers in
`a sub-band involves a tradeoff between SNR boost and inter-
`
`ference sacrifice. In summary, the parameter NM is specified
`by the BS based on the bandwidth (FFT size), uplink SNR
`requirement,
`timing precision requirement,
`suppression
`capability to potential co-channel interferences, and the num-
`ber of ranging opportunities that needs to be provided. It
`should also be specifiedjointly with the other two parameters
`NC and NM, described in more detail below.
`Secondly, in each sub-band, a number ofranging codes 601
`(i.e., NC sequences) may be allowed. Since these ranging
`codes occupy the same band, they may interfere with each
`other even without any code collision. Sequences with good
`cross-correlation are much desired for better code detection
`and channel estimation. In addition, a low PAPRi of the
`time-domain ranging waveform is also much desirable in
`order to be able to boost the transmission power to improve
`the uplink SNR. The details of the sequences that have these
`desirable properties will be discussed in the next section.
`Additionally, for cellular deployment, a number of sequence
`groups (each having NC access sequences) are also required
`for allocating to different neighboring sectors. So when those
`codes are generated and grouped, any pair of codes from
`distinct groups needs to l1ave good cross correlation, just like
`any pair of codes in the same group. In summary, the param-
`eter NC is determined by the BS based on the access needs and
`the maximally tolerable interference level at which the suc-
`cessful detection rate is still good.
`Thirdly, for each ranging code, Nsh cyclic time shifts 603 of
`the time-domain ranging signal (phase rotation in frequency
`domain) can be used to further increase the number ofranging
`opportunities. Mathematically,
`the
`frequency
`domain
`sequence, after the j”’ shift is
`
`sj(k):s(k)e*f2W*1>L/NF”,
`
`(1)
`
`where s(k) is the original (or 0”’ shift) sequence, L is the CP
`length (regular or extended CP, depending on the type of
`ranging) and NFFTis the FFT size. In essence, code separa-
`bility is achieved by the fact that the estimated channel is
`shifted in time domain by some multiples of L. If L is large
`enough to cover most ofthe charmel length, the access signals
`using distinct cyclic shifts will allow their corresponding
`channels to be separated reasonably well.
`The initial ranging transmission may be used by any SS
`that wants to synchronize to the system charmel for the first
`time. In one embodiment of the invention, a control message
`from the BS may specify the sub-bands that an initial ranging
`signal can use. All sub-bands or, for example, a specified
`number of the sub-bands starting from the lowest frequency
`offset may be allowed for initial ranging. Maximally, only
`NS,,:[NSP/Lcpej shifts are preferred for interference-free code
`separation among different shifts of a ranging signal where
`[x] denotes the flooring function (i.e., the maximum integer
`that is not greater than x), LCpe is the length ofan extended CP
`and N5? is the FFT size ofthe special FFT symbol (209 of FIG.
`2) that may be multiples of the regular FFT size N. If some
`interference between estimated charmels can be tolerated,
`that maximal number may be even increased. In general,
`more shifts can be used at the expense of increased interfer-
`ence. But a good practice is setting the number of shifts to
`NSh':[NSP/LCPej—l so that a good estimation of the noise and
`interference level can be obtained from the “channel-free”
`
`IFFT samples. Since LC” can be significantly larger than the
`regular CP length (denoted as LCP) used in an OFDM system
`(for no11-OFDM that does not define a CP length, the duration
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`US 7,599,327 B2
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`7
`of a regular CP, or LCP, is often chosenbased on the excessive
`delay spread of the channels encountered in a deployment, as
`discussed before),
`the allowed Nsh can be significantly
`reduced. To improve the number of shifts available for other
`non-initial ranging functions, the initial ranging can be con-
`fined to a certain number of (say Nb 1') sub-bands on which the
`allowed number of shifts is, only for example, NS,,':[NSP/
`LCPej—l. But on the remaining NM-NM‘ sub-bands, where
`only non-initial ranging is allowed, the number of shifts can
`be increased to NS,,:[NSP/LCPj—l. Often, the total ranging
`opportunities increases. If initial ranging is allowed on only
`Nb1'(<Nb1) sub-bands, the number of initial ranging opportu-
`nity is then NS,,'*Nc*Nb1'. If initial ranging is allowed on all
`sub-bands, the total number of all ranging opportunities is
`NS,1'*Nc*Nb1, of which a portion may be assigned to initial
`ranging.
`Periodic-ranging transmissions are sent periodically for
`system periodic ranging. Bandwidth-requests transmissions
`are for requesting uplink allocations from the BS. These
`non-initial ranging transmissions may be sent only by SSs
`that have already synchronized to the system. These trans-
`missions can also use the additional OFDM symbols if these
`symbols are allocated for ranging in a control message from
`the BS.
`
`Ranging Codes
`It is desirable to use ranging sequences that have low PAPR
`(peak to average power ratio) and good cross-correlation. A
`large PAPR requires more power backoff in order to avoid
`signal distortion. A reduced average transmit power resulting
`from using such power backoff causes a decrease of the
`uplink SNR, which can be problematic for the BS to detect the
`ranging signals from mobile devices with limited power. In
`OFDM, the PAPR is usually much higher than that in the
`traditional “single-carrier” transmission when the OFDM
`sub-carriers are modulated with random PSK/QAM symbols.
`For example, the PAPR for the access signals described in
`drafts and a published version of the IEEE 802.16 standard
`are in the range of 6.5 to 12 dB.
`In terms of the other important sequence characteristic—
`the cross correlation, since distinct ranging signals can inter-
`fere with each other, a good cross correlation among them can
`mitigate the interference, which results in improved detection
`rate and reduced false alarm. The presence of other ranging
`codes on the same set of sub-carriers and at the same cyclic
`shift may severely distort the estimation of the desired chan-
`nel if the cross correlation property is unsatisfactory. This
`results in low detection rate and high false alarm rate evenjust
`for the purpose of detecting the presence of a ranging code,
`needless to say the goal of obtaining accurate channel knowl-
`edge. The performance becomes more and more unaccept-
`able as the channel conditions become worse (for example,
`under larger delay spread) or the number of ranging users
`increases.
`
`In some embodiments ofthe present invention, the ranging
`signal uses access sequences that have good PAPR and cross
`correlation. In one embodiment of the invention, the set of
`sequences can come from a search of a special type of
`sequences such as random PSK or Golay PSK sequences so
`that the resulting set has good PAPR and cross correlation. In
`an