PATENT DOCUMENT

Publication Number: US-8472399-B2
Application Number: US-83098110-A
Country: US
Kind Code: B2

Title: Ranging channel structures and methods

Abstract:
To facilitate ranging between mobile terminals and a base station in a wireless communication network employing orthogonal frequency division multiplexing (OFDM) for uplink data communications, a periodic ranging channel for use by a mobile terminal is defined. The channel specifies a plurality N of blocks of sub-carrier frequencies of an OFDM frequency band which are non-contiguous within the OFDM frequency band. The channel also specifies a time slot within an OFDM subframe which spans one or more OFDM symbol periods. A ranging transmission is periodically sent as a spread signal across the specified N blocks of sub-carrier frequencies within the specified time slot. The duration of the ranging transmission may be less than a duration of the OFDM subframe. A notional grid of tiles representing time and frequency resources associated with the subframe may facilitate channel definition. A similar approach may be used to define an initial access channel for initial access transmissions.

Claims:
What is claimed is: 
     
       1. In a wireless communication network employing orthogonal frequency division multiplexing (OFDM) for uplink data communications between mobile terminals and a base station, a method of performing periodic ranging between a mobile terminal and said base station, the method comprising:
 defining a periodic ranging channel for use by said mobile terminal, said periodic ranging channel comprising a plurality N of blocks of sub-carrier frequencies of an OFDM frequency band, said N blocks of sub-carrier frequencies being non-contiguous within said OFDM frequency band, said channel further comprising a time slot, within a particular OFDM subframe, within which ranging transmissions shall be sent from said mobile terminal to said base station using said N blocks of sub-carrier frequencies, said time slot spanning one or more OFDM symbol periods but being less that a duration of said OFDM subframe; and 
 periodically sending a ranging transmission over said periodic ranging channel from said mobile terminal to said base station, said sending comprising transmitting said ranging transmission within said time slot as a spread signal, said spread signal being spread across the sub-carrier frequencies of said N blocks, wherein a duration of said ranging transmission is less than the duration of said OFDM subframe, 
 wherein the number of OFDM symbol periods comprising said time slot is configured based on an estimated or determined maximum ranging delay between said mobile terminal and said base station. 
 
     
     
       2. The method of  claim 1  wherein the number of OFDM symbol periods comprising said time slot increases as said estimated or determined maximum ranging delay increases. 
     
     
       3. The method of  claim 1  wherein said ranging transmission comprising a message or signal and wherein said transmitting comprises transmitting constituent elements of said message or signal over respective sub-carrier frequencies of said N blocks within said time slot of said OFDM subframe. 
     
     
       4. The method of  claim 3  wherein said time slot spans multiple adjacent OFDM symbol periods of said subframe, wherein said ranging transmission comprises a leading buffer interval within said time slot and a trailing buffer interval within said time slot, and wherein the mobile terminal refrains from said transmitting of said constituent elements of said message or signal during said leading buffer interval and said trailing buffer interval. 
     
     
       5. The method of  claim 1  wherein said mobile terminal is a first mobile terminal, said ranging transmission is a first ranging transmission, and wherein said periodic ranging channel is also for use by a second mobile terminal for sending a second ranging transmission, and wherein each of said first and second ranging transmissions comprise sequences that are orthogonal to one another. 
     
     
       6. The method of  claim 1  wherein said periodic ranging channel is associated with a sector of a cell of said base station, wherein said N blocks of sub-carrier frequencies and said time slot are selected from a predetermined subset of sub-carrier frequency blocks of said OFDM frequency range and a predetermined subset of OFDM symbol periods of said subframe collectively referred to as a periodic ranging region for said sector, and wherein said periodic ranging region for said sector is aligned, in time and frequency, with a periodic ranging region of a proximate sector. 
     
     
       7. The method of  claim 1  wherein said periodic ranging channel is associated with a sector of a cell of said base station, wherein said N blocks of sub-carrier frequencies and said time slot are selected from a predetermined subset of sub-carrier frequency blocks and a predetermined subset of OFDM symbol periods of said subframe collectively referred to as a periodic ranging region for said sector, and wherein either one or both of said predetermined subset of sub-carrier frequency blocks and said predetermined subset of OFDM symbol periods differs from the subset of sub-carrier frequency blocks and subset of OFDM symbol periods, respectively, of a periodic ranging region of a proximate sector. 
     
     
       8. The method of  claim 7  wherein a section of said periodic ranging region for said sector is intended for defining periodic ranging channels for cell edge users and wherein said section is aligned, in time and frequency, with a section of the periodic ranging region of said proximate sector that is intended for defining periodic ranging channels for cell edge users. 
     
     
       9. The method of  claim 1  wherein either one or both of said N blocks of sub-carrier frequencies and said time slot defining said periodic ranging channel are dynamically selected by the mobile terminal based on an average power estimate of a synchronization channel used for initial acquisition of a base station signal and for initial download timing. 
     
     
       10. The method of  claim 1  wherein the number of OFDM symbol periods comprising said time slot is configured based on a size of a cell that is served by the base station. 
     
     
       11. In a wireless communication network employing orthogonal frequency division multiplexing (OFDM) for uplink data communications between mobile terminals and a base station, a method of performing periodic ranging between a mobile terminal and said base station, the method comprising:
 defining a periodic ranging channel for said mobile terminal, said periodic ranging channel being represented as a plurality N of tiles in a notional grid of tiles representing OFDM time and frequency resources, said notional grid having a time dimension comprising a plurality of OFDM symbol periods of an OFDM subframe and a frequency dimension comprising a plurality of blocks of sub-carriers of an OFDM frequency band, each of said N tiles representing an allocation of one of the blocks of sub-carriers for use by at least said mobile terminal during one or more of said OFDM symbol periods, said N tiles being non-contiguous in the frequency dimension of said notional grid, said N tiles each spanning the same time slot in the time dimension of said notional grid, said time slot having a duration that is one or more OFDM symbol periods but is less than a duration of said OFDM subframe; 
 periodically sending a ranging transmission from said mobile terminal to said base station over said periodic ranging channel, said sending comprising transmitting said ranging transmission within said time slot as a spread signal, said spread signal being spread across the sub-carrier frequencies of said N tiles, with a duration of said ranging transmission being less than the duration of said OFDM subframe, 
 wherein the number of OFDM symbol periods comprising said time slot is configured based on an estimated or determined maximum ranging delay between said mobile terminal and said base station. 
 
     
     
       12. The method of  claim 11  wherein the number of OFDM symbols comprising said ranging transmission increases as said estimated or determined maximum ranging delay increases. 
     
     
       13. The method of  claim 11  wherein said periodic ranging channel is associated with a sector of a cell of said base station, wherein said N tiles are selected from a predetermined subset of tiles of said grid referred to as periodic ranging region for said sector, and wherein said periodic ranging region for said sector is aligned, in the time dimension and the frequency dimension, with a periodic ranging region of a proximate sector. 
     
     
       14. The method of  claim 11  wherein said periodic ranging channel is associated with a sector of a cell of said base station, wherein said N tiles are selected from a predetermined subset of tiles of said grid referred to as periodic ranging region for said sector, and wherein said periodic ranging region for said sector is different in size from a periodic ranging region for a proximate sector. 
     
     
       15. The method of  claim 14  wherein a section of said periodic ranging region for said sector is for defining periodic ranging channels for cell edge users and wherein said section is aligned, in the time dimension and the frequency dimension, with a section of the periodic ranging region for said proximate sector. 
     
     
       16. The method of  claim 11  wherein said N tiles defining said periodic ranging channel are dynamically selected by the mobile terminal based on an average power estimate of a synchronization channel used for initial acquisition of a base station signal and for initial download timing. 
     
     
       17. The method of  claim 11  wherein said notional grid comprises six OFDM symbol periods in said time dimension, wherein said plurality N of tiles is three tiles, and wherein each of said three tiles spans two adjacent OFDM symbol periods. 
     
     
       18. The method of  claim 17  wherein said periodic ranging channel is a first periodic ranging channel, said ranging transmission is a first ranging transmission and further comprising:
 defining a second periodic ranging channel and a third periodic ranging channel, each of said second and third periodic ranging channels being represented as a distinct set of three tiles of said notional grid, each set of three tiles being non-contiguous in the frequency dimension of said grid and spanning two adjacent OFDM symbol periods in the time dimension of said grid, each said set of three tiles comprising the same sub-carriers in the frequency dimension of said grid as the corresponding three tiles of said first periodic ranging channel; and 
 periodically sending second and third ranging transmissions from said mobile terminal to said base station over said second and third periodic ranging channels respectively, said sending comprising transmitting said ranging transmission as a spread signal, said spread signal being spread across the sub-carrier frequencies of said N tiles as two OFDM symbols during said two OFDM symbol periods, with a duration of said ranging transmission being less than the duration of said OFDM subframe. 
 
     
     
       19. The method of  claim 11  wherein the number of OFDM symbol periods comprising said time slot is configured based on a size of a cell that is served by the base station. 
     
     
       20. In a wireless communication network employing orthogonal frequency division multiplexing (OFDM) for uplink data communications between mobile terminals and a base station, a method of performing initial access from a mobile terminal to said base station, the method comprising:
 defining an initial access channel for use by said mobile terminal, said initial access channel comprising a plurality N of blocks of sub-carrier frequencies of an OFDM frequency band, said N blocks of sub-carrier frequencies being non-contiguous within said OFDM frequency band, said initial access channel further comprising a time slot, within a particular OFDM subframe, within which initial access transmissions shall be sent from said mobile terminal to said base station using said N blocks of sub-carrier frequencies, said time slot spanning one or more OFDM symbol periods but being less that a duration of said OFDM subframe; and 
 sending an initial access transmission over said initial access channel from said mobile terminal to said base station, said sending comprising transmitting said initial access transmission within said time slot as a spread signal, said spread signal being spread across the sub-carrier frequencies of said N blocks, wherein a duration of said initial access transmission is less than the duration of said OFDM subframe, 
 wherein the number of OFDM symbol periods comprising said time slot is configured based on a size of a cell that is served by the base station and an estimated or determined maximum ranging delay between said mobile terminal and said base station.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of the non-provisional application no. 12/806,182, resulting from conversion under 37 C.F.R. §1.53(c)(3) of U.S. provisional patent application no. 61/223,108 filed on Jul. 6, 2009, which claims the benefit of U.S. provisional patent application No. 61/078,574 filed on Jul. 7, 2008, the contents of each of which are hereby incorporated by reference, as if fully and completely set forth herein. 
     TECHNICAL FIELD 
     The present disclosure relates to wireless communication techniques in general, and to techniques relating to orthogonal frequency division multiplexing (OFDM) in particular. 
     BACKGROUND 
     In a wireless communication network, such as a cellular network, a plurality of mobile stations or mobile terminals (e.g. cellular telephones, smartphones, or other forms of wireless communication devices) may communicate with a base station by way orthogonal frequency division multiplexing for uplink and/or downlink data communications. Orthogonal frequency division multiplexed networks may facilitate cell-based high speed services such as those provided under the IEEE 802.16 standards, which may be referred to as WiMAX or less commonly as WirelessMAN or the Air Interface Standard. 
     In a cellular network using OFDM, a base station of a cell may be responsible for allocating OFDM frequency band sub-carrier frequencies to mobile terminals within the cell for use in particular time slots. If the distance between the mobile terminals and the base station varies over time, the transmission delay of wireless data communications between the mobile terminals and the base station may also vary. This may disadvantageously result in misalignment of data communications received at the base station with respect to the particular time slots. Similar problems may arise when a mobile terminal communicates with a base station for the first time, e.g. upon entry of a mobile terminal into cell or upon awakening of the mobile terminal from a period of idleness, because the distance to the base station may not yet have been established. 
     ART RELATED TO THE APPLICATION 
     In draft IEEE 802.16m System Description Document, IEEE 802.16m-08/003r1, dated Apr. 15, 2008, it is stated that: 
     This [802.16m] standard amends the IEEE 802.16 WirelessMAN-OFDMA specification to provide an advanced air interface for operation in licensed bands. It meets the cellular layer requirements of IMT-Advanced next generation mobile networks. This amendment provides continuing support for legacy WirelessMAN-OFDMA equipment. 
     And the standard will address the following purpose: 
     i. The purpose of this standard is to provide performance improvements necessary to support future advanced services and applications, such as those described by the ITU in Report ITU-R M.2072. 
     More generally, the below embodiments could be applied in any communication system which employs multi-carrier or OFDM-type technology on the uplink. 
     SUMMARY 
     In one aspect, there is provided in a wireless communication network employing orthogonal frequency division multiplexing (OFDM) for uplink data communications between mobile terminals and a base station, a method of performing periodic ranging between a mobile terminal and the base station, the method comprising: defining a periodic ranging channel for use by the mobile terminal, the periodic ranging channel comprising a plurality N of blocks of sub-carrier frequencies of an OFDM frequency band, the N blocks of sub-carrier frequencies being non-contiguous within the OFDM frequency band, the channel further comprising a time slot, within a particular OFDM subframe, within which ranging transmissions shall be sent from the mobile terminal to the base station using the N blocks of sub-carrier frequencies, the time slot spanning one or more OFDM symbol periods but being less that a duration of the OFDM subframe; and periodically sending a ranging transmission over the periodic ranging channel from the mobile terminal to the base station, the sending comprising transmitting the ranging transmission within the time slot as a spread signal, the spread signal being spread across the sub-carrier frequencies of the N blocks, wherein a duration of the ranging transmission is less than the duration of the OFDM subframe. 
     In another aspect, there is provided in a wireless communication network employing orthogonal frequency division multiplexing (OFDM) for uplink data communications between mobile terminals and a base station, a method of performing periodic ranging between a mobile terminal and the base station, the method comprising: defining a periodic ranging channel for the mobile terminal, the periodic ranging channel being represented as a plurality N of tiles in a notional grid of tiles representing OFDM time and frequency resources, the notional grid having a time dimension comprising a plurality of OFDM symbol periods of an OFDM subframe and a frequency dimension comprising a plurality of blocks of sub-carriers of an OFDM frequency band, each of the N tiles representing an allocation of one of the blocks of sub-carriers for use by at least the mobile terminal during one or more of the OFDM symbol periods, the N tiles being non-contiguous in the frequency dimension of the notional grid, the N tiles each spanning the same time slot in the time dimension of the notional grid, the time slot having a duration that is one or more OFDM symbol periods but is less than a duration of the OFDM subframe; periodically sending a ranging transmission from the mobile terminal to the base station over the periodic ranging channel, the sending comprising transmitting the ranging transmission within the time slot as a spread signal, the spread signal being spread across the sub-carrier frequencies of the N tiles, with a duration of the ranging transmission being less than the duration of the OFDM subframe. 
     In a further aspect, there is provided in a wireless communication network employing orthogonal frequency division multiplexing (OFDM) for uplink data communications between mobile terminals and a base station, a method of performing initial access from a mobile terminal to the base station, the method comprising: defining an initial access channel for use by the mobile terminal, the initial access channel comprising a plurality N of blocks of sub-carrier frequencies of an OFDM frequency band, the N blocks of sub-carrier frequencies being non-contiguous within the OFDM frequency band, the initial access channel further comprising a time slot, within a particular OFDM subframe, within which initial access transmissions shall be sent from the mobile terminal to the base station using the N blocks of sub-carrier frequencies, the time slot spanning one or more OFDM symbol periods but being less that a duration of the OFDM subframe; and sending an initial access transmission over the periodic ranging channel from the mobile terminal to the base station, the sending comprising transmitting the initial access transmission within the time slot as a spread signal, the spread signal being spread across the sub-carrier frequencies of the N blocks, wherein a duration of the initial access transmission is less than the duration of the OFDM subframe, wherein the number of OFDM symbol periods comprising the time slot is configurable based on an estimated or determined maximum ranging delay between the mobile terminal and the base station. 
     Aspects and features of the present disclosure will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of a disclosure in conjunction with the accompanying drawing figures and appendices. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present disclosure will now be described, by way of example only, with reference to the accompanying drawing figures, wherein: 
         FIG. 1  is a block diagram of a cellular communication system; 
         FIG. 2  is a block diagram of an example base station that might be used to implement some embodiments of the present disclosure; 
         FIG. 3  is a block diagram of an example wireless terminal that might be used to implement some embodiments of the present disclosure; 
         FIG. 4  is a block diagram of an example relay station that might be used to implement some embodiments of the present disclosure; 
         FIG. 5  is a block diagram of a logical breakdown of an example OFDM transmitter architecture that might be used to implement some embodiments of the present disclosure; 
         FIG. 6  is a block diagram of a logical breakdown of an example OFDM receiver architecture that might be used to implement some embodiments of the present disclosure; and 
         FIGS. 7 and 8  illustrate notional grids representing time and frequency resources associated with an OFDM subframe, which may facilitate the definition of ranging channels. 
     
    
    
     Like reference numerals are used in different figures to denote similar elements. 
     DETAILED DESCRIPTION OF THE DRAWINGS 
     Wireless System Overview 
     Referring to the drawings,  FIG. 1  shows a base station controller (BSC)  10  which controls wireless communications within multiple cells  12 , which cells are served by corresponding base stations (BSs)  14 . In some configurations, each cell is further divided into multiple sectors  13  or zones (not shown). In general, each base station  14  facilitates communications using OFDM with mobile and/or wireless terminals  16 , which are within the cell  12  associated with the corresponding base station  14 . The movement of the mobile terminals  16  in relation to the base stations  14  results in significant fluctuation in channel conditions. As illustrated, the base stations  14  and mobile terminals  16  may include multiple antennas to provide spatial diversity for communications. In some configurations, relay stations  15  may assist in communications between base stations  14  and wireless terminals  16 . Wireless terminals  16  can be handed off  18  from any cell  12 , sector  13 , zone (not shown), base station  14  or relay  15  to an other cell  12 , sector  13 , zone (not shown), base station  14  or relay  15 . In some configurations, base stations  14  communicate with each and with another network (such as a core network or the internet, both not shown) over a backhaul network  11 . In some configurations, a base station controller  10  is not needed. 
     With reference to  FIG. 2 , an example of a base station  14  is illustrated. The base station  14  generally includes a control system  20 , a baseband processor  22 , transmit circuitry  24 , receive circuitry  26 , multiple antennas  28 , and a network interface  30 . The receive circuitry  26  receives radio frequency signals bearing information from one or more remote transmitters provided by mobile terminals  16  (illustrated in  FIG. 3 ) and relay stations  15  (illustrated in  FIG. 4 ). A low noise amplifier and a filter (not shown) may cooperate to amplify and remove broadband interference from the signal for processing. Downconversion and digitization circuitry (not shown) will then downconvert the filtered, received signal to an intermediate or baseband frequency signal, which is then digitized into one or more digital streams. 
     The baseband processor  22  processes the digitized received signal to extract the information or data bits conveyed in the received signal. This processing typically comprises demodulation, decoding, and error correction operations. As such, the baseband processor  22  is generally implemented in one or more digital signal processors (DSPs) or application-specific integrated circuits (ASICs). The received information is then sent across a wireless network via the network interface  30  or transmitted to another mobile terminal  16  serviced by the base station  14 , either directly or with the assistance of a relay  15 . 
     On the transmit side, the baseband processor  22  receives digitized data, which may represent voice, data, or control information, from the network interface  30  under the control of control system  20 , and encodes the data for transmission. The encoded data is output to the transmit circuitry  24 , where it is modulated by one or more carrier signals having a desired transmit frequency or frequencies. A power amplifier (not shown) will amplify the modulated carrier signals to a level appropriate for transmission, and deliver the modulated carrier signals to the antennas  28  through a matching network (not shown). Modulation and processing details are described in greater detail below. 
     With reference to  FIG. 3 , an example of a mobile terminal  16  is illustrated. Similarly to the base station  14 , the mobile terminal  16  will include a control system  32 , a baseband processor  34 , transmit circuitry  36 , receive circuitry  38 , multiple antennas  40 , and user interface circuitry  42 . The receive circuitry  38  receives radio frequency signals bearing information from one or more base stations  14  and relays  15 . A low noise amplifier and a filter (not shown) may cooperate to amplify and remove broadband interference from the signal for processing. Downconversion and digitization circuitry (not shown) will then downconvert the filtered, received signal to an intermediate or baseband frequency signal, which is then digitized into one or more digital streams. 
     The baseband processor  34  processes the digitized received signal to extract the information or data bits conveyed in the received signal. This processing typically comprises demodulation, decoding, and error correction operations. The baseband processor  34  is generally implemented in one or more digital signal processors (DSPs) and application specific integrated circuits (ASICs). 
     For transmission, the baseband processor  34  receives digitized data, which may represent voice, video, data, or control information, from the control system  32 , which it encodes for transmission. The encoded data is output to the transmit circuitry  36 , where it is used by a modulator to modulate one or more carrier signals that is at a desired transmit frequency or frequencies. A power amplifier (not shown) will amplify the modulated carrier signals to a level appropriate for transmission, and deliver the modulated carrier signal to the antennas  40  through a matching network (not shown). Various modulation and processing techniques available to those skilled in the art are used for signal transmission between the mobile terminal and the base station, either directly or via the relay station. 
     In OFDM modulation, the transmission band is divided into multiple, orthogonal carrier waves. Each carrier wave is modulated according to the digital data to be transmitted. Because OFDM divides the transmission band into multiple carriers, the bandwidth per carrier decreases and the modulation time per carrier increases. Since the multiple carriers are transmitted in parallel, the transmission rate for the digital data, or symbols, on any given carrier is lower than when a single carrier is used. 
     OFDM modulation utilizes the performance of an Inverse Fast Fourier Transform (IFFT) on the information to be transmitted. For demodulation, the performance of a Fast Fourier Transform (FFT) on the received signal recovers the transmitted information. In practice, the IFFT and FFT are provided by digital signal processing carrying out an Inverse Discrete Fourier Transform (IDFT) and Discrete Fourier Transform (DFT), respectively. Accordingly, the characterizing feature of OFDM modulation is that orthogonal carrier waves are generated for multiple bands within a transmission channel. The modulated signals are digital signals having a relatively low transmission rate and capable of staying within their respective bands. The individual carrier waves are not modulated directly by the digital signals. Instead, all carrier waves are modulated at once by IFFT processing. 
     In operation, OFDM may be used for at least downlink transmission from the base stations  14  to the mobile terminals  16 . Each base station  14  is equipped with “n” transmit antennas  28  (n&gt;=1), and each mobile terminal  16  is equipped with “m” receive antennas  40  (m&gt;=1). Notably, the respective antennas can be used for reception and transmission using appropriate duplexers or switches and are so labelled only for clarity. 
     When relay stations  15  are used, OFDM may be used for downlink transmission from the base stations  14  to the relays  15  and from relay stations  15  to the mobile terminals  16 . 
     With reference to  FIG. 4 , an example of a relay station  15  is illustrated. Similarly to the base station  14 , and the mobile terminal  16 , the relay station  15  will include a control system  132 , a baseband processor  134 , transmit circuitry  136 , receive circuitry  138 , multiple antennas  130 , and relay circuitry  142 . The relay circuitry  142  enables the relay  14  to assist in communications between a base station  16  and mobile terminals  16 . The receive circuitry  138  receives radio frequency signals bearing information from one or more base stations  14  and mobile terminals  16 . A low noise amplifier and a filter (not shown) may cooperate to amplify and remove broadband interference from the signal for processing. Downconversion and digitization circuitry (not shown) will then downconvert the filtered, received signal to an intermediate or baseband frequency signal, which is then digitized into one or more digital streams. 
     The baseband processor  134  processes the digitized received signal to extract the information or data bits conveyed in the received signal. This processing typically comprises demodulation, decoding, and error correction operations. The baseband processor  134  is generally implemented in one or more digital signal processors (DSPs) and application specific integrated circuits (ASICs). 
     For transmission, the baseband processor  134  receives digitized data, which may represent voice, video, data, or control information, from the control system  132 , which it encodes for transmission. The encoded data is output to the transmit circuitry  136 , where it is used by a modulator to modulate one or more carrier signals that is at a desired transmit frequency or frequencies. A power amplifier (not shown) will amplify the modulated carrier signals to a level appropriate for transmission, and deliver the modulated carrier signal to the antennas  130  through a matching network (not shown). Various modulation and processing techniques available to those skilled in the art are used for signal transmission between the mobile terminal and the base station, either directly or indirectly via a relay station, as described above. 
     With reference to  FIG. 5 , a logical OFDM transmission architecture will be described. Initially, the base station controller  10  will send data to be transmitted to various mobile terminals  16  to the base station  14 , either directly or with the assistance of a relay station  15 . The base station  14  may use the channel quality indicators (CQIs) associated with the mobile terminals to schedule the data for transmission as well as select appropriate coding and modulation for transmitting the scheduled data. The CQIs may be directly from the mobile terminals  16  or determined at the base station  14  based on information provided by the mobile terminals  16 . In either case, the CQI for each mobile terminal  16  is a function of the degree to which the channel amplitude (or response) varies across the OFDM frequency band. 
     Scheduled data  44 , which is a stream of bits, is scrambled in a manner reducing the peak-to-average power ratio associated with the data using data scrambling logic  46 . A cyclic redundancy check (CRC) for the scrambled data is determined and appended to the scrambled data using CRC adding logic  48 . Next, channel coding is performed using channel encoder logic  50  to effectively add redundancy to the data to facilitate recovery and error correction at the mobile terminal  16 . Again, the channel coding for a particular mobile terminal  16  is based on the CQI. In some implementations, the channel encoder logic  50  uses known Turbo encoding techniques. The encoded data is then processed by rate matching logic  52  to compensate for the data expansion associated with encoding. 
     Bit interleaver logic  54  systematically reorders the bits in the encoded data to minimize the loss of consecutive data bits. The resultant data bits are systematically mapped into corresponding symbols depending on the chosen baseband modulation by mapping logic  56 . Preferably, Quadrature Amplitude Modulation (QAM) or Quadrature Phase Shift Key (QPSK) modulation is used. The degree of modulation may be chosen based on the CQI for the particular mobile terminal. The symbols may be systematically reordered to further bolster the immunity of the transmitted signal to periodic data loss caused by frequency selective fading using symbol interleaver logic  58 . 
     At this point, groups of bits have been mapped into symbols representing locations in an amplitude and phase constellation. When spatial diversity is desired, blocks of symbols are then processed by space-time block code (STC) encoder logic  60 , which modifies the symbols in a fashion making the transmitted signals more resistant to interference and more readily decoded at a mobile terminal  16 . The STC encoder logic  60  will process the incoming symbols and provide “n” outputs corresponding to the number of transmit antennas  28  for the base station  14 . The control system  20  and/or baseband processor  22  as described above with respect to  FIG. 5  will provide a mapping control signal to control STC encoding. At this point, assume the symbols for the “n” outputs are representative of the data to be transmitted and capable of being recovered by the mobile terminal  16 . 
     For the present example, assume the base station  14  has two antennas  28  (n=2) and the STC encoder logic  60  provides two output streams of symbols. Accordingly, each of the symbol streams output by the STC encoder logic  60  is sent to a corresponding IFFT processor  62 , illustrated separately for ease of understanding. Those skilled in the art will recognize that one or more processors may be used to provide such digital signal processing, alone or in combination with other processing described herein. The IFFT processors  62  will preferably operate on the respective symbols to provide an inverse Fourier Transform. The output of the IFFT processors  62  provides symbols in the time domain. The time domain symbols are grouped into frames, which are associated with a prefix by prefix insertion logic  64 . Each of the resultant signals is up-converted in the digital domain to an intermediate frequency and converted to an analog signal via the corresponding digital up-conversion (DUC) and digital-to-analog (D/A) conversion circuitry  66 . The resultant (analog) signals are then simultaneously modulated at the desired RF frequency, amplified, and transmitted via the RF circuitry  68  and antennas  28 . Notably, pilot signals known by the intended mobile terminal  16  are scattered among the sub-carriers. The mobile terminal  16 , which is discussed in detail below, will use the pilot signals for channel estimation. 
     Reference is now made to  FIG. 6  to illustrate reception of the transmitted signals by a mobile terminal  16 , either directly from base station  14  or with the assistance of relay  15 . Upon arrival of the transmitted signals at each of the antennas  40  of the mobile terminal  16 , the respective signals are demodulated and amplified by corresponding RF circuitry  70 . For the sake of conciseness and clarity, only one of the two receive paths is described and illustrated in detail. Analog-to-digital (A/D) converter and down-conversion circuitry  72  digitizes and downconverts the analog signal for digital processing. The resultant digitized signal may be used by automatic gain control circuitry (AGC)  74  to control the gain of the amplifiers in the RF circuitry  70  based on the received signal level. 
     Initially, the digitized signal is provided to synchronization logic  76 , which includes coarse synchronization logic  78 , which buffers several OFDM symbols and calculates an auto-correlation between the two successive OFDM symbols. A resultant time index corresponding to the maximum of the correlation result determines a fine synchronization search window, which is used by fine synchronization logic  80  to determine a precise framing starting position based on the headers. The output of the fine synchronization logic  80  facilitates frame acquisition by frame alignment logic  84 . Proper framing alignment is important so that subsequent FFT processing provides an accurate conversion from the time domain to the frequency domain. The fine synchronization algorithm is based on the correlation between the received pilot signals carried by the headers and a local copy of the known pilot data. Once frame alignment acquisition occurs, the prefix of the OFDM symbol is removed with prefix removal logic  86  and resultant samples are sent to frequency offset correction logic  88 , which compensates for the system frequency offset caused by the unmatched local oscillators in the transmitter and the receiver. Preferably, the synchronization logic  76  includes frequency offset and clock estimation logic  82 , which is based on the headers to help estimate such effects on the transmitted signal and provide those estimations to the correction logic  88  to properly process OFDM symbols. 
     At this point, the OFDM symbols in the time domain are ready for conversion to the frequency domain using FFT processing logic  90 . The results are frequency domain symbols, which are sent to processing logic  92 . The processing logic  92  extracts the scattered pilot signal using scattered pilot extraction logic  94 , determines a channel estimate based on the extracted pilot signal using channel estimation logic  96 , and provides channel responses for all sub-carriers using channel reconstruction logic  98 . In order to determine a channel response for each of the sub-carriers, the pilot signal is essentially multiple pilot symbols that are scattered among the data symbols throughout the OFDM sub-carriers in a known pattern in both time and frequency. Continuing with  FIG. 6 , the processing logic compares the received pilot symbols with the pilot symbols that are expected in certain sub-carriers at certain times to determine a channel response for the sub-carriers in which pilot symbols were transmitted. The results are interpolated to estimate a channel response for most, if not all, of the remaining sub-carriers for which pilot symbols were not provided. The actual and interpolated channel responses are used to estimate an overall channel response, which includes the channel responses for most, if not all, of the sub-carriers in the OFDM channel. 
     The frequency domain symbols and channel reconstruction information, which are derived from the channel responses for each receive path are provided to an STC decoder  100 , which provides STC decoding on both received paths to recover the transmitted symbols. The channel reconstruction information provides equalization information to the STC decoder  100  sufficient to remove the effects of the transmission channel when processing the respective frequency domain symbols. 
     The recovered symbols are placed back in order using symbol de-interleaver logic  102 , which corresponds to the symbol interleaver logic  58  of the transmitter. The de-interleaved symbols are then demodulated or de-mapped to a corresponding bitstream using de-mapping logic  104 . The bits are then de-interleaved using bit de-interleaver logic  106 , which corresponds to the bit interleaver logic  54  of the transmitter architecture. The de-interleaved bits are then processed by rate de-matching logic  108  and presented to channel decoder logic  110  to recover the initially scrambled data and the CRC checksum. Accordingly, CRC logic  112  removes the CRC checksum, checks the scrambled data in traditional fashion, and provides it to the de-scrambling logic  114  for de-scrambling using the known base station de-scrambling code to recover the originally transmitted data  116 . 
     In parallel to recovering the data  116 , a CQI, or at least information sufficient to create a CQI at the base station  14 , is determined and transmitted to the base station  14 . As noted above, the CQI may be a function of the carrier-to-interference ratio (CR), as well as the degree to which the channel response varies across the various sub-carriers in the OFDM frequency band. For this embodiment, the channel gain for each sub-carrier in the OFDM frequency band being used to transmit information is compared relative to one another to determine the degree to which the channel gain varies across the OFDM frequency band. Although numerous techniques are available to measure the degree of variation, one technique is to calculate the standard deviation of the channel gain for each sub-carrier throughout the OFDM frequency band being used to transmit data. 
     In some embodiments, a relay station may operate in a time division manner using only one radio, or alternatively include multiple radios. 
       FIGS. 1 to 6  provide one specific example of a communication system that could be used to implement embodiments of the application. It is to be understood that embodiments of the application can be implemented with communications systems having architectures that are different than the specific example, but that operate in a manner consistent with the implementation of the embodiments as described herein. 
       FIG. 7  illustrates a notional grid  700  representing allocatable time and frequency resources for a single OFDM subframe for an exemplary sector  18  of  FIG. 1 . The grid  700  may alternatively be referred to as a “resource block.” In some embodiments, the notional grid  700  may be represented as a corresponding data structure, such as a table or two-dimensional array, in memory (not expressly shown) of a base station  16  of the relevant exemplary sector  18  or cell  14  ( FIG. 1 ). The base station may be responsible for allocating the represented time and frequency resources among any mobile terminals operating within the sector. The OFDM subframe whose time and frequency resource allocation is represented by notional grid  700  may be one of F subframes comprising an OFDM frame in uplink data communications with the base station  16  originating from various mobile terminals within the cell or sector, where F is an integer greater than one. 
     As illustrated, notional grid  700  has a time dimension and a frequency dimension, which are shown on the horizontal axis and the vertical axis respectively. Other representations of grid  700  are possible. It will be appreciated that the grid  700  is notional and may not actually be logically or physically represented (whether as a grid per se or otherwise) in some embodiments. 
     Each column in the time dimension of grid  700  represents a single OFDM symbol period. The OFDM symbol period may be a predetermined duration, dictated by operative standards (e.g. IEEE 802.16m), for transmitting an OFDM symbol. In the illustrated example, the time dimension of grid  700  comprises six columns labelled a-f. The number of columns (six) reflects the fact that the represented OFDM subframe has a duration of six OFDM symbol periods. In alternative embodiments, the number of OFDM symbols periods in the time dimension may be less than or greater than six. 
     The frequency dimension of exemplary notional grid  700  comprises sixteen rows labelled  701 - 716 . The number of rows (sixteen) reflects the fact that the sub-carrier frequencies (or simply “sub-carriers”) of the operative OFDM frequency band have been partitioned into sixteen blocks. Each of the sixteen blocks of sub-carriers is capable of being allocated to a different mobile terminal; moreover, more than one block of sub-carriers may be assigned to a single mobile terminal. In alternative embodiments, the number of blocks in the frequency dimension may be less than or greater than sixteen. 
     Each block in the frequency dimension contains (spans) a plurality of orthogonal sub-carrier frequencies, which may for example be 6, 9 or 18 in number. The range of sub-carrier frequencies in each block may be such that a variation in gain from the highest sub-carrier to the lowest sub-carrier frequency within the sub-range is minimal. In other words, the range of sub-carrier frequencies in each block may span a frequency that is less than the coherence bandwidth of the channel. Each sub-carrier frequency can be used to modulate one modulation symbol. Each sub-carrier can therefore represent one bit during a single OFDM time period, or more than one bit if modulated appropriately. 
     Each intersection between a row and column of grid  700  forms a tile. A tile represents an allocation of the block of sub-carriers represented by the row for use during the OFDM symbol period that is represented by the column. In  FIG. 7 , a tile containing an alphanumeric identifier signifies that the time and frequency resources have been allocated to a channel identified by that alphanumeric identifier. An empty tile may signify either that the time and frequency resources associated with that tile have not been allocated or that they have been allocated for use unrelated to the present disclosure (i.e. unrelated to periodic ranging). 
     Based on the above-described conventions, it can be seen that  FIG. 7  shows twenty-one allocated tiles of grid  700 . Specifically, tiles  701   a ,  708   a  and  715   a  have been collectively allocated as a first channel whose identifier is “A”; tiles  701   b ,  708   b  and  715   b  have been collectively allocated as a first channel whose identifier is “B”; tiles  701   c ,  708   c  and  715   c  have been collectively allocated as a first channel whose identifier is “C”; tiles  701   d ,  708   d  and  715   d  have been collectively allocated as a first channel whose identifier is “D”; tiles  701   e ,  708   e  and  715   e  have been collectively allocated as a first channel whose identifier is “E”; tiles  701   f ,  708   f  and  715   f  have been collectively allocated as a first channel whose identifier is “F”; and tiles  702   a ,  709   a  and  716  have been collectively allocated as a first channel whose identifier is “G.” As will be described, the allocations may be centrally orchestrated by a base station  14  or base station controller  10 . The number of possible distinct channels within a grid is referred to as M. For example, if the entire grid of 96 tiles of  FIG. 7  were used for periodic ranging, and if it is assumed that N=3 (i.e. that there are three tiles per channel), then M would be 96/3 or 32. In general, M is configurable. 
     As noted above, the plurality of tiles that are allocated to a particular are collectively referred to as a “channel” (or, alternatively, as a “location” or “transmission opportunity”) within the grid  700 . In the illustrated embodiment, the number of tiles defining each channel is three. That is, each set of three tiles having a common channel identifier in  FIG. 7  defines a single channel for use by a particular mobile terminal  16  to which the channel is allocated. As such, a total of seven periodic ranging channels are defined in  FIG. 7 , i.e. the number of channels shown in the subframe is seven (A-G). In general, the number of tiles allocated per channel is N, where N is an integer greater than one. The number of tiles per channel may differ as between different mobile terminals, even in the same cell  12  or sector  13  ( FIG. 1 ), and may possibly be dynamically configurable, as will be described. 
     The relationship between channels and mobile terminals is either one-to-one or one-to-many. In the latter case, each mobile terminal using the same channel may employs a different one of a plurality of orthogonal or low correlation sequences, as described below, to allow its signal to be ascertained at the base station from among multiple signals transmitted simultaneously by multiple mobile terminals on that channel. 
     For clarity, it should be appreciated that the identifiers A-G used for identifying distinct mobile terminals in  FIG. 7  bear no relationship in the identifiers a-f used to identify the columns of grid  700 . It should also be appreciated that the mobile terminals identified using identifiers A-G are not necessarily expressly illustrated in  FIG. 1 . Further it should be appreciated that the allocations illustrated in  FIG. 7  are as of a particular moment in time. As mobile terminals  16  are activated or deactivated or as they enter or exit a relevant sector  18 , the allocated channels may change. 
     The channels defined in the grid of  FIG. 7  are periodic ranging channels (or simply “ranging channels”). Such channels are intended for carrying transmissions specifically for use in ranging. Ranging refers to the estimating or determining of a degree of timing offset of wireless transmissions from a predetermined uplink subframe timing due to transmission delay between the mobile terminal and the base station. Periodic may for example allow data communications between the mobile terminal and the base station to be adjusted as necessary in order to account for movement of the mobile terminal within the relevant cell, sector or zone. For example, if it is determined that a mobile terminal  16  is currently far from the base station  14 , the mobile terminal  16  may be configured to send its transmissions slightly earlier in time (forward time shift), in order to account for comparatively large MS-to-BS transmission delays. In contrast, if it is determined that a mobile terminal  16  is currently is proximate to the base station  14 , the mobile terminal  16  may be configured to send its transmissions slightly later in time (backward time shift), in order to account for minimal MS-to-BS transmission delays in that case. This may permit transmissions originating from various mobile terminals within a cell, sector or zone to substantially coincide with one another upon arrival at the base station. Interference between transmissions originating from different mobile terminals in adjacent OFDM symbol periods of the subframe may thus be minimized. 
     In general, each ranging transmission that is periodically sent over a periodic ranging channel from the mobile terminal to the base station may comprise a known message or one a set of known signals. The message or signal is for the purpose of estimating or determining the timing of the arrival of a mobile transmission relative to a timing of the uplink subframes as defined at the base station. For example, the message or signal may allow a timing offset between a received wireless transmission originating from the mobile terminal and an OFDM subframe time slot (e.g. one or more OFDM symbol periods) to be estimated or determined. The message or signal may for example be an orthogonal or low correlation sequence, such as Walsh sequence, gold sequence, or Zadoff-chu sequences. In cases where the sequence is assigned by the base station, the base station may assign a unique sequence to each mobile terminal that uses the same channel (e.g. channel A of  FIG. 7 ). For example, the sequence assigned to a first mobile terminal using channel A may be distinct from, and may be orthogonal or have a low correlation to, a sequence assigned to a second mobile terminal using channel A. The base station may re-use sequences between channels. The content of a ranging transmissions may be defined, e.g., in either or both of standards IEEE 802.16e and/or IEEE 802.16m, which are hereby incorporated by reference hereinto. 
     For clarity, a ranging transmission is distinct from a pilot symbol that may be used in OFDM data communications. A ranging transmission refers to a known message or signal, such as a sequence, that is sent over the plurality of sub-carriers that define a channel and is used for ranging purposes. A pilot symbol is a known symbol that is transmitted for channel estimation purposes. 
     A periodic ranging channel may be defined as follows. Firstly, one of the F subframes comprising an OFDM frame is initially selected for definition of a periodic ranging channel therewithin. The selection of a subframe is typically made by the mobile terminal  16 , although this is not necessarily true in all embodiments (e.g. it may be made by the base station  14 ). In some embodiments, the selection of an OFDM subframe is unnecessary because the subframe to be used for periodic ranging is predetermined. 
     A notional grid representing the time and frequency resources of the selected or predetermined OFDM subframe may be used to facilitate channel definition. An exemplary grid  700  is shown in  FIG. 7 , as noted above. 
     Thereafter, time and frequency resources of the relevant subframe are chosen for the channel. This may be achieved through selection of a plurality N of tiles from the notional grid  700 . The selection is typically made by the base station and then communicated to the mobile terminal  16 , e.g. in accordance with mechanisms defined in IEEE standards 802.16e and/or 802.16m, although that is not necessarily true in all embodiments. The selection of the OFDM subframe and/or the N tiles may, optionally, be randomized in whole or in part, with a view to minimizing contention between the periodic ranging channels that are defined for different mobile terminals within the same cell, sector or zone. In some embodiments, the tiles from which the N tiles are be selected are limited a subset of the total number of tiles within the grid, e.g. if certain grid tiles are reserved or already used for other purposes. 
     In some embodiments, either one or both of the selected N blocks of frequency sub-carriers and the selected OFDM symbol periods may be dynamically selected by the mobile terminal  16  based on an average power estimate of a synchronization channel. For clarity, a synchronization channel is a channel that the mobile terminal uses for initial acquisition of a base station&#39;s signal and for initial downlink timing. The mobile station may estimate the received power of the synchronization channel from the base station. If the estimate suggests higher power, which may indicate the user is proximate to the base station, a ranging channel of short duration is used (for example, a duration of one OFDM symbol period as in  FIG. 7 ). If the estimate is of lower power, which may suggests that the mobile is not proximate to the base station, a longer channel duration may be used (for example, a duration of two OFDM symbol periods as in  FIG. 8 ). The estimate of the received power can come from numerous downlink message transmitted by the base station such as broadcast message, signalling channels, etc. 
     Each of the N tiles that is selected during definition of a single periodic ranging channel spans the same time period of the subframe, i.e. is situated within the same column, of the notional grid  700 . This reflects the fact that the ranging transmission will be sent using the sub-carriers of each of the selected sub-carrier blocks simultaneously. The selected time period (i.e. the selected column(s) in the example) may be referred to herein as the “time slot” of the channel. For clarity, if it is assumed that each tile of grid  700  has a fixed width of one OFDM symbol period, then a channel spanning multiple OFDM symbol periods may be considered to have a number of tiles that is a multiple of N (e.g. if the number of spanned OFDM symbol periods is two, then the number of tiles may be considered to be 2N). Alternatively, if it is considered that a plurality of adjacent OFDM symbol periods of a single sub-carrier frequency block forms a single “wide” tile, the number of tiles comprising a multi-column-wide channel may be considered to be N. Regardless of these semantics, the selected tiles will be non-contiguous in the frequency dimension of notional grid  700 . In other words, no two selected tiles will be adjacent to one another within the relevant column(s) of grid  700 . This is with a view to introducing frequency diversity into each ranging transmission, to combat frequency-selective fading. 
     For example, the three tiles  701   a ,  708   a  and  715   a  of  FIG. 7  that are selected for channel “A” are all within the same column (column a) of grid  700 , indicating that they all occur within the same OFDM symbol period of the relevant OFDM subframe. As well, they are non-contiguous within column a. 
     As a further aspect of channel definition in at least some embodiments, a sequence for use as a ranging transmission over the channel is assigned (e.g. by the base station) or otherwise selected. As noted above, the sequence for a given channel will be mobile terminal specific. That is, to the extent that a particular channel (e.g. channel “A” of  FIG. 7 ) is used for ranging transmissions by more than one mobile terminal, each mobile terminal using that channel will use a different sequence (e.g. one of a plurality of orthogonal or low correlation sequences). However, a sequence that is used for one channel may also be used for another channel comprising a distinct set of N tiles. In the present embodiment, the sequence has a length L which is greater than one. In some embodiments, the ranging transmission may be a predetermined message or signal that is not a sequence (e.g. if each channel will be used by only one mobile terminal). 
     In some embodiments, the base station may apprise the mobile terminal, via a communication over a downlink connection from the base station, of which N tiles shall define the channel and/or of the sequence to be used on that channel and/or of the sequence that the mobile terminal is to use over that channel. The mechanism for this communication may be defined in standards IEEE 802.16e and/or 802.16m. 
     Once the mobile terminal is aware of the tiles defining the channel and of the sequence to be used over that channel, the mobile terminal may thereafter periodically send a ranging transmission, e.g. as defined above, to the base station over the channel. In the present embodiment, sending of the ranging transmission comprises spreading the assigned or selected sequence of length L across the sub-carrier frequencies of the N blocks within the selected time slot of the OFDM subframe, wherein the spreading results in a spread signal. In general, the sending comprises transmitting constituent elements of the message or signal (e.g. portions or bits of the sequence) over respective sub-carrier frequencies of the N blocks within the selected time slot. If the channel duration spans more than one OFDM symbol period, the OFDM symbol may be transmitted for only a portion of the channel duration, with a view to reducing the likelihood of interference with data transmitted over the same sub-carrier frequencies in adjacent time slots. Each ranging transmission may then be used at the base station  14  for estimating or determining a current timing offset of transmissions originating at the mobile terminal relative to a timing of the OFDM subframe. 
     In some embodiments, the number of OFDM symbol periods used by each ranging transmission (e.g. the number of columns spanned by a channel) is configurable, e.g. by the base station or mobile terminal, based on an estimated or determined maximum ranging delay between the mobile terminal and the base station. For example, the number of OFDM symbol periods comprising a periodic ranging channel may be increased as the estimated or determined maximum ranging delay increases. The duration of the ranging transmission will generally be less than the duration of an OFDM subframe. However, it is possible that, in certain cases, the duration of the ranging transmission match the duration of the OFDM subframe, e.g. if the range between the mobile station and the base station is large. 
     In some embodiments, a subset of tiles in the notional grid for the subframe in which the periodic ranging channel is defined may be reserved for use in defining periodic ranging channels. The N tiles that are selected for the periodic ranging channel may be selected from only that subset of tiles. For example, in  FIG. 7 , the reserved subset of tiles may be all of the tiles in rows  701 ,  702 ,  708 ,  709 ,  715  and  716  of grid  700 . This reserved subset of tiles is referred to as a “periodic ranging region.” The base station may apprise each mobile terminal in the relevant sector or cell as to the bounds of the periodic ranging region by way of a downlink communication. The region may be indicated by way of identifiers of the encompassed tiles, or by way of identifiers of the encompassed sub-carrier frequency blocks and/or OFDM symbol periods. 
     In some embodiments, the periodic ranging region defined for a particular sector may be aligned with a periodic ranging region of a proximate sector, in both time and frequency. For example, if the notional grids for a particular OFDM subframe is the same for two sectors that are proximate to one another, then the subset of reserved tiles within those notional grids may be intentionally made the same. In other embodiments, the periodic ranging region defined for a particular sector may not be aligned with a periodic ranging region of other sectors in time or frequency. 
     If a periodic ranging region has been defined, then the size of the periodic ranging region, i.e. the extent of the subset of reserved tiles in the grid, may be sector-specific or cell-specific. If the sizes of the periodic ranging regions are different for different sectors, then at least a section of the region that is reserved for possible use by mobile terminals of cell edge users should be aligned in frequency and in time as between proximate sectors or cells. As is known in the art, cell edge users are users whose mobile terminals are far from their respective base stations and are approaching the boundary of another cell. A mobile terminal of a cell edge user generally transmits at a high power to in order to be received at the base station. This could interfere with the reception of desired signals at other base stations. Alignment of the periodic ranging regions, e.g. for cell edge users, allows for the transmission of these high power signals at the same time in different cells. This means that data or other sensitive signals, which may be utilizing time and frequency resources other than those used for ranging, may be protected against the high-power ranging transmissions of cell edge users. 
       FIG. 8  illustrates the definition of three periodic ranging channels for an exemplary cell or sector of  FIG. 1 , which channels are different from any of the seven periodic ranging channels defined in  FIG. 7 . The conventions used in  FIG. 8  are the same as those used in  FIG. 7 . Like notional grid  700 , the notional grid  800  comprises sixteen rows  801 - 816  and six columns a-f. The sixteen rows reflect the same partitioning of the operative OFDM frequency band into sixteen blocks as in  FIG. 7 , and the six columns a-f reflect that the duration of the represented OFDM subframe is six OFDM symbol periods. 
     As shown in  FIG. 8 , three periodic ranging channels are defined. The first channel, denoted “A,” is defined by tiles  801   a - 801   b ,  808   a - 808   b , and  815   a - 815   b . The second channel, denoted “B,” is defined by tiles  801   c - 801   d ,  808   c - 808   d , and  815   c - 815   d . The third channel, denoted “C,” is defined by tiles  801   e - 801   f ,  808   e - 808   f , and  815   e - 815   f . For clarity, the identifiers “A”, “B” and “C” in the  FIG. 8  do not refer to the same channels as are referred to by the identifiers “A”, “B” and “C” in  FIG. 7 . The three periodic ranging channels defined in  FIG. 8  differ from those of  FIG. 7  primarily in that each channel spans two OFDM periods rather than one. The ranging transmissions (e.g. sequences) that are sent over these channels are not necessarily transmitted for the full duration of the ranging channel. For example, each may be transmitted for the duration of a single OFDM symbol period, timed so as to occupy the “middle” of the time slot defined by the two columns which define the channel (i.e. with a leading buffer interval preceding the substance of the ranging transmission within the time slot and a trailing buffer interval following the substance of the ranging transmission within the time slot, during which no data is sent by the mobile terminal over the relevant sub-carriers). The duration of the buffer intervals (i.e. the sum of their durations) may be substantially equal to a duration of the message, signal or sequence comprising the substance of the ranging transmission. By sending the sequence with a leading and trailing buffer, it may be possible to prevent or limit interference with any data that may be sent in the preceding or subsequent time slot of the subframe. Given that the time/frequency resources of the channels do not overlap, the same sequence may be used for the ranging transmission of each channel. 
     It is generally noted that the various methods and techniques relating to the definition and use of periodic ranging channels, as described above, may also be used for the definition and use of initial access channels. Initial access channels are used for sending initial access transmissions, which are initial communications from a mobile terminal to a base station upon entry of the mobile terminal into the relevant cell or sector or upon its awakening from a period of idleness. An initial access is generally comprised of a transmission of a known message, or one of a set of known signals, from a mobile station to a base station for the purpose of estimating the timing of the arrival of mobile transmission relative to the uplink subframe timing defined at the base station. As with ranging transmissions above, the message or signal may for example be a sequence, such as Walsh sequence, gold sequence, or Zadoff-chu sequences. The sequence assigned to a first mobile terminal using a particular channel may be distinct from, and may be orthogonal or have a low correlation to, a sequence assigned to a second mobile terminal using the same channel. In initial access, a timing offset of a mobile transmission relative to a subframe may be significantly larger than in the case of periodic ranging. For initial access, the channel to be used may be randomly selected from among a predetermined set of time/frequency resources available or reserved generally for this purpose. 
     Appendix A describes aspects of the embodiments described above. 
     The various methods and techniques described above may be effected in hardware, firmware, software, or combinations of these. In the case of firmware and/or software, processor-executable instructions may be loaded into the memory of a computing device, such as a mobile terminal, base station or a base station controller for example, from a computer-readable or machine-readable medium, such as a magnetic storage medium or optical disk, and may be executed by one or more processors at that device in order to effect the relevant method or technique. 
     The above-described embodiments of the present disclosure are intended to be examples only. Those of skill in the art may effect alterations, modifications and variations to the particular embodiments without departing from the scope of the disclosure. 
     APPENDIX A 
     Introduction 
     In 19442RO, details regarding time-frequency allocation of initial access channels is documented. 
     This document presents details, including on the symbol structure, of the initial access/ranging and periodic channel. In this document, we assume that the initial access region can be used for ranging, or/and ranging can also used for initial access. Initial access and ranging can also be preformed separately, but the design here applies to either. 
     UL Initial Access Channel 
     As in 1994RO, the initial access channel can occupy a set of distributed tile. The set maybe 1 or more tiles in size. 
     UL random access channel is a contention based channel for multiple MSs to request initial access. A designated resource is allocated for these initial access request. 
     The request is spread across the N resources tiles (e.g. N=3) used for Initial access. Spread in frequency domain over OFDM resources allows for frequency diversity. Spreading length over N resources is L. 
     Selection of access channel signaling ID: The MS randomly selects from one sequence L sequences. The MS randomly selects from M locations within the subframe, and F subframes (eg. A, B, C etc). 
     The number of distinct codes/resource/subframes per subframe is N=LMF. M and F are configurable by the base station. 
     In some embodiments, the region for periodic ranging also occupies a set of distributed tile. The set maybe 1 or more tiles in size. 
     Each user is assigned a sequence, and a set of tiles, to periodically transmit a ranging channel. 
     The signal is spread across the N resources tiles (e.g. N=3) used for periodic ranging. Spread in frequency domain over OFDM resources allows for frequency diversity. Spreading length over N resources is L. 
     Initial access regions 
     In some embodiments, the initial access region for different sectors should be aligned. 
     In some embodiments, there is a section of the region for initial access or region for cell edge users. This section for cell edge users should be aligned between adjacent sectors. In cases where the initial access/ranging regions are different size for different sectors, at least the section cell edge user should be aligned. 
     Periodic ranging regions 
     In some embodiments, the periodic ranging region for different sectors should be aligned. 
     In some embodiments, there is a section of the region for periodic ranging or region for cell edge users. This section for cell edge users should be aligned between adjacent sectors. In cases where the periodic ranging regions are different size for different sectors, at least the section cell edge user should be aligned. 
     In some embodiments, the base station can schedule periodic ranging from users in a manner that considers interference from other sector&#39;s raging channels. In some embodiments, this scheduling my be coordinated 
     Symbol structure 
     The initial access or ranging channel is sent over N tiles. Each tile occupies the time duration of subframe, as indicated in the figure, which in some embodiments is 6 OFDM symbols. The signal can be sent as one OFDM symbol that spans a duration less than 6 OFDM symbols, with an extended cyclic prefix. 
     In some embodiments, the number of OFDM symbols used by each ranging transmission is configurable. It is configured based of the size of the cell and estimated (or determined) maximum ranging delay. 
     In some embodiment the ranging channel can be configured to durations of one or more OFDM symbols; specifically including 2 and 3 symbols durations. In the case where the duration of a channel is less than a sub frame duration, multiple regions can be concatenated in time to span the sub frame duration. 
     In some embodiments, the initial access /ranging regions of different sizes and/or durations can co-exist in the same system, or possibly same frame or sub-frame. The users would select a region based an estimate, or inferred estimate from another parameter such as received power, etc., of their relative delay and therefore, the size of ranging channel that is appropriate. For example, based on an average power estimate of the synchronization channel, the mobile may determine whether its very close, or quite far from terminal Based on this determination, the mobile selects one of the ranging regions indicated in the broadcast information of the system. Additional formation to assist the mobile with this determination can be transmitted in the superframe header or other broadcast channel. The types and duration of ranging channels are indicated in the broadcast channel. 
     In some embodiments, the periodic ranging channel is sent over N tiles. Each tile occupies the time duration of subframe, as indicated in the figure, which in some embodiments is 6 OFDM symbols. The signal can be sent using the 6 OFDM symbols as used for other traffic and signaling. 
     Tile structure: 
     In some embodiments, for either periodic ranging or initial access/ranging regions: The each of the N tiles that make of the N channels span 6, 9 or 18 subcarriers. The total number of subcarriers used for a channel is a multiple (k) of 18 subcarriers (i.e. N x {6, 9, or 18}=18k).

Metadata:
Filing Date: 20100706
Publication Date: 20130625
Grant Date: 20130625
Priority Date: 20100706
Inventors: NOVAK ROBERT
FONG MO-HAN
Assignee: APPLE INC
CPC Classifications: [{"code": "H04L27/2626", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04L27/2655", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04W72/0446", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04J3/0682", "inventive": true, "first": true, "tree": "[]"}, {"code": "H04L5/0026", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04L5/0048", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04L5/0041", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04L25/0226", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04L27/2626", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04W72/0446", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04L5/0048", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04L5/0026", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04J3/0682", "inventive": true, "first": true, "tree": "[]"}, {"code": "H04L25/0226", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04L5/0041", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04L27/2655", "inventive": true, "first": false, "tree": "[]"}]
Family ID: 45438542