(19) United States
`(12) Patent Application Publication (10) Pub. No.: US 2004/0131007 A1
`(43) Pub. Date:
`Jul. 8, 2004
`Smee et al.
`
`US 2004O131007A1
`
`(54) PILOT TRANSMISSION SCHEMES FOR
`WIRELESS MULTI-CARRIER
`COMMUNICATION SYSTEMS
`(76) Inventors: John Smee, San Diego, CA (US); Jay
`Rod Walton, Carlisle, MA (US); Durga
`Prasad Malladi, San Diego, CA (US)
`Correspondence Address:
`Qualcomm Incorporated
`Patents Department
`5775 Morehouse Drive
`San Diego, CA 92.121-1714 (US)
`(21) Appl. No.:
`10/359,811
`(22) Filed:
`Feb. 7, 2003
`Related U.S. Application Data
`(60) Provisional application No. 60/438,601, filed on Jan.
`7, 2003.
`
`Publication Classification
`
`(51) Int. Cl. ................................................... H04J 11/00
`
`(52) U.S. Cl. ............................................ 370/208; 370/203
`
`(57)
`
`ABSTRACT
`
`Pilot transmission Schemes Suitable for use in wireless
`multi-carrier (e.g., OFDM) communication systems. These
`pilot transmission Schemes may utilize frequency, time, or
`both frequency and time orthogonality to achieve orthogo
`nality among the pilots transmitted by multiple base Stations
`on the downlink. Frequency orthogonality is achieved by
`transmitting pilots on disjoint Sets of Subbands. Time
`orthogonality is achieved by transmitting pilots using dif
`ferent orthogonal codes (e.g., Walsh codes). The pilots may
`also be scrambled with different scrambling codes, which
`are used to randomize pilot interference and to enable
`identification of the transmitters of these pilots. Pilot inter
`ference cancellation may be performed to improve perfor
`mance Since Subbands used for data transmission by one
`transmitter may also be used for pilot transmission by
`another transmitter. Pilot interference is estimated and then
`Subtracted from received symbols to obtain pilot-canceled
`Symbols having improved quality.
`
`
`
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`US 2004/O131007 A1
`
`Jul. 8, 2004
`
`PILOT TRANSMISSION SCHEMES FOR
`WIRELESS MULTI-CARRIER COMMUNICATION
`SYSTEMS
`0001. This application claims the benefit of provisional
`U.S. Application Serial No. 60/438,601, entitled “Pilot
`Transmission Schemes for Wireless Multi-Carrier Commu
`nication Systems, filed on Jan. 7, 2003, assigned to the
`assignee of the present application, and incorporated herein
`by reference in its entirety for all purposes.
`
`BACKGROUND
`
`0002)
`I. Field
`0003. The present invention relates generally to commu
`nication, and more specifically to pilot transmission Schemes
`for wireleSS multi-carrier communication Systems.
`II. Background
`0004)
`0005. A multi-carrier communication system employs
`multiple carriers for data transmission to a single end-point.
`These multiple carriers may be employed, for example, in
`the context of orthogonal frequency division multiplexing
`(OFDM) or some other multi-carrier modulation techniques.
`OFDM effectively partitions the overall system bandwidth
`into a number of (N) orthogonal Subbands, which are also
`referred to as tones, frequency bins, and frequency Subchan
`nels. With OFDM, each Subband is associated with a respec
`tive carrier upon which data may be modulated.
`0006. In a wireless communication system, data to be
`transmitted is processed (e.g., coded and modulated) at a
`transmitter and upconverted onto a radio frequency (RP)
`carrier Signal to generate an RF modulated Signal. The RF
`modulated Signal is then transmitted over a wireleSS channel
`and may reach a receiver via a number of propagation paths.
`The characteristics of the propagation paths typically vary
`over time due to a number of factorS Such as, for example,
`fading, multipath, and external interference. Consequently,
`the transmitted RF modulated Signal may experience differ
`ent channel conditions (e.g., different fading and multipath
`effects) and may be associated with different complex gains
`and Signal-to-noise ratios (SNRs) over time.
`0007. In a wireless communication system, a pilot is
`often transmitted from a transmitter (e.g., a base station) to
`a receiver (e.g., a terminal) to assist the receiver in perform
`ing a number of functions. The pilot is typically generated
`based on known Symbols and processed in a known manner.
`The pilot may be used by the receiver for channel estima
`tion, timing and frequency acquisition, coherent data
`demodulation, received signal Strength measurements, and
`SO O.
`0008 Various challenges are encountered in the design of
`a pilot transmission Scheme for a multi-carrier communica
`tion System. AS one consideration, Since pilot transmission
`represents overhead in the System, it is desirable to minimize
`pilot transmission to the extent possible while Still providing
`the desired performance. AS another consideration, pilots
`needs to be transmitted in a manner Such that the receivers
`in the System are able to detect and distinguish the pilots
`transmitted by the individual transmitters in the system.
`Moreover, the pilot transmission Scheme needs to address
`the additional dimensionality created by the multiple carriers
`of the multi-carrier System.
`
`0009. There is therefore a need in the art for pilot
`transmission Schemes for multi-carrier communication SyS
`temS.
`
`SUMMARY
`0010 Pilot transmission schemes suitable for use in wire
`less multi-carrier communication Systems (e.g., OFDM Sys
`tems) are provided herein. These pilot transmission Schemes
`may utilize frequency orthogonality, time orthogonality, or
`both frequency and time orthogonality to achieve orthogo
`nality among pilots transmitted by multiple base Stations on
`the downlink. Frequency orthogonality may be achieved by
`transmitting pilots from different base Stations on disjoint
`sets of Subbands. Time orthogonality may be achieved by
`transmitting pilots using different orthogonal codes (e.g.,
`Walsh codes). The pilots may also be scrambled with
`different Scrambling codes, which are used to randomize
`pilot interference and to enable identification of the trans
`mitters of these pilots.
`0011. The pilot transmission schemes described herein
`efficiently facilitate both channel estimation and pilot detec
`tion. These Schemes allow terminals in the System to obtain
`high quality wideband channel estimates and pilot Strength
`estimates for base Stations in the System, which may be used
`to perform coherent data demodulation, Soft handoff, and
`hard handoff, as described below.
`0012 Techniques to estimate and cancel pilot interfer
`ence are also provided herein. Pilot interference cancellation
`may be performed to improve performance Since Subbands
`used for data or pilot transmission by one transmitter may
`also be used for pilot transmission by another transmitter
`(i.e., an “interfering” transmitter). Pilot interference may be
`estimated by obtaining an estimate of the channel to the
`interfering Source, generating the pilot in the Same manner
`performed by the interfering transmitter, and multiplying the
`generated pilot with the channel estimate. The pilot inter
`ference is then subtracted from received symbols to obtain
`pilot-canceled Symbols having improved quality.
`0013 Various aspects and embodiments of the invention
`are also described in further detail below.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`0014. The features, nature, and advantages of the present
`invention will become more apparent from the detailed
`description Set forth below when taken in conjunction with
`the drawings in which like reference characters identify
`correspondingly throughout and wherein:
`0015 FIG. 1 shows a wireless multiple-access multi
`carrier communication System;
`0016 FIG. 2A shows an OFDM Subband structure;
`0017 FIG. 2B shows T disjoint sets of Subbands based
`on the OFDM Subband structure shown in FIG. 2A;
`0018 FIGS. 3A and 3B show exemplary assignments of
`Subbands for a 9-sector 3-cell cluster and a 21-sector 7-cell
`cluster, respectively, to achieve frequency orthogonality;
`0019 FIGS. 4A and 4B show exemplary assignments of
`orthogonal codes to achieve time orthogonality for a 3-Sec
`tor 1-cell cluster with one antenna and two antennas per
`Sector, respectively;
`
`17
`
`

`

`US 2004/O131007 A1
`
`Jul. 8, 2004
`
`0020 FIGS. 4C and 4D show exemplary assignments of
`Subbands and orthogonal codes for a 9-Sector 3-cell cluster
`and a 21-Sector 7-cell cluster, respectively, to achieve fre
`quency and time orthogonality;
`0021
`FIG. 5 shows an exemplary system layout whereby
`a different Scrambling code is assigned to each 7-cell cluster;
`0022 FIGS. 6A and 6B show the transmission of pilots
`from multiple Sectors for a Synchronous burst pilot trans
`mission Scheme and a Synchronous continuous pilot trans
`mission Scheme, respectively;
`0023 FIG. 7 shows a block diagram of a base station and
`a terminal;
`0024 FIG. 8 shows a block diagram of a modulator
`within the base station;
`0025 FIGS. 9A and 9B shows block diagrams of two
`embodiments of a demodulator within the terminal; and
`0.026
`FIG. 10 shows a block diagram of a pilot interfer
`ence canceller within the demodulator.
`
`DETAILED DESCRIPTION
`0027. The word “exemplary” is used herein to mean
`“Serving as an example, instance, or illustration.” Any
`embodiment or design described herein as “exemplary' is
`not necessarily to be construed as preferred or advantageous
`over other embodiments or designs.
`0028 FIG. 1 shows a wireless multiple-access multi
`carrier communication system 100 that supports a number of
`users and is capable of implementing the pilot transmission
`schemes described herein. System 100 includes a number of
`base Stations 110 that Support communication for a number
`of terminals 120. Abase station is a fixed station that is used
`for communicating with the terminals and may also be
`referred to as an access point, a Node B, or Some other
`terminology.
`0029. As shown in FIG. 1, various terminals 120 may be
`dispersed throughout the System, and each terminal may be
`fixed (i.e., stationary) or mobile. A terminal may also be
`referred to as a mobile Station, a remote Station, a user
`equipment (UE), a wireless communication device, an
`access terminal, or Some other terminology. Each terminal
`may communicate with one or possibly multiple base Sta
`tions on the downlink and/or uplink at any given moment.
`The downlink (i.e., forward link) refers to the communica
`tion link from the base Station to the terminal, and the uplink
`(i.e., reverse link) refers to the communication link from the
`terminal to the base station. In FIG. 1, terminals 120a
`through 120O receive pilots, Signaling, and possibly user
`Specific data transmission from base Stations 110a through
`110 g.
`0030) A system controller (not shown in FIG. 1) typically
`couples to base Stations 110 and may be designed to perform
`a number of functions Such as (1) coordination and control
`for the base stations coupled to it, (2) routing of data among
`these base Stations, and (3) access and control of the
`terminals Served by these base Stations.
`0.031) System 100 may be a cellular system or some other
`type of wireless system. System 100 may also be designed
`to implement any of the Standards and designs for code
`division multiple-access (CDMA), time division multiple
`
`access (TDMA), frequency division multiple-access
`(FDMA), and so on. The CDMA standards include IS-95,
`cdma2000, IS-856, W-CDMA, and TS-CDMA, and the
`TDMA standards include GSM. These standards are well
`known in the art.
`0032 Each base station 110 in the system provides cov
`erage for a particular geographic area 102. The coverage
`area of each base Station may be defined, for example, as the
`area over which the terminals can achieve a particular grade
`of service (GoS). The size and shape of each base station's
`coverage area are typically dependent on various factors
`Such as terrain, obstructions, and So on. For Simplicity, the
`coverage area of each base Station is often represented by an
`ideal hexagon. The base Station and/or its coverage area are
`also often referred to as a “cell', depending on the context
`in which the term is used.
`0033. In a typical System deployment, to increase capac
`ity, the coverage area of each base Station may be partitioned
`into multiple Sectors. If each cell is partitioned into three
`Sectors, then each Sector of a Sectorized cell is often repre
`sented by an ideal 120 wedge that is /3 of the cell. In an
`actual deployment, the coverage area of each base Station
`often has a shape that is different from the ideal hexagon,
`and the shape of each sector is often different from the ideal
`120 wedge. Moreover, the sectors of a sectorized cell
`typically overlap at the edges. Each Sector may be served by
`a corresponding base transceiver Subsystem (BTS). For a
`Sectorized cell, the base Station for that cell often includes all
`of the BTSS that serve the sectors of that cell. The term
`“sector” is also often used to refer to a BTS and/or its
`coverage area, depending on the context in which the term
`is used.
`0034) For simplicity, the following description assumes
`that each cell is partitioned into three sectors and their BTSs
`are located within the base station for the cell. This base
`Station is located in the center of the cell. Also for Simplicity,
`in the following description, the term “base Station' is used
`generically for both a fixed Station that Serves a cell and a
`fixed Station that Serves a Sector.
`0035). For a CDMA system, the pilot transmitted by each
`base Station is spectrally spread acroSS the entire System
`bandwidth prior to transmission over the wireleSS channel.
`At a terminal, the pilot transmitted by each base Station may
`be received with a low signal-to-noise ratio (SNR). How
`ever, the complementary despreading operation performed
`by the terminal provides processing gain that is relied upon
`to recover the pilot in the presence of a large amount of noise
`and interference. For a multi-carrier System, it is typically
`not feasible to perform direct Sequence spread spectrum
`processing for the pilot, as used in CDMA. Other means
`must then be used to transmit the pilot from each base Station
`such that it can be readily detected by the terminals in the
`System.
`0036 Pilot transmission schemes suitable for use in
`multi-carrier communication Systems, Such as the one
`shown in FIG. 1, are provided herein. As noted above, pilots
`are transmitted to Support various functions that may be
`needed for proper System operation, Such as timing and
`frequency acquisition, channel estimation, coherent data
`demodulation, and So on. The multiple carriers may be
`provided by OFDM or some other multi-carrier modulation
`
`18
`
`

`

`US 2004/O131007 A1
`
`Jul. 8, 2004
`
`technique. The pilot transmission Schemes described herein
`are well Suited for use on the downlink but may also be used
`for the uplink.
`0037 For clarity, the pilot transmission schemes are
`specifically described for the downlink of an OFDM system.
`This OFDM system has Northogonal Subbands. Each base
`station can transmit one OFDM symbol in each OFDM
`symbol period, as described below.
`0038
`I. Pilot Transmission Constructs
`0039 Table 1 lists three “constructs” that may be utilize
`for pilot transmission Schemes.
`
`TABLE 1.
`
`Constructs
`
`Description
`
`Time
`Orthogonality
`
`Transmission of pilots on different disjoint sets of
`Frequency
`Orthogonality subbands by different base stations to achieve
`orthogonality in the frequency domain for the pilot
`transmissions.
`Use of different orthogonal codes (e.g., Walsh codes) for
`the pilots by different base stations to achieve
`orthogonality in the time domain for the pilot
`transmissions.
`Use of different scrambling codes for the pilots by
`different base stations for pilot interference
`randomization and base station identification.
`
`Scrambling
`Codes
`
`0040. The orthogonal and scrambling “codes” are also
`referred to as “sequences” in the following description. Each
`of the constructs listed in Table 1 is described in further
`detail below. The processing at the base Station and the
`terminal for these constructs is also described below.
`0041
`Various pilot transmission schemes may be devised
`based on any one or any combination of these constructs.
`For example, a pilot transmission Scheme may employ (1)
`frequency and time orthogonality, (2) frequency orthogo
`nality and Scrambling codes, (3) frequency orthogonal, time
`orthogonality, and Scrambling codes, or (4) Some other
`combination.
`0.042
`1. Frequency Orthogonality
`0.043
`Frequency orthogonality may be used to avoid
`interference resulting from Simultaneous transmission of
`pilots by multiple base Stations. For frequency orthogonality,
`pilots are transmitted by multiple base Stations on different
`sets of Subbands that are “disjoint” (where disjoint is
`described below) so that interference is avoided. Frequency
`orthogonality may be achieved in various manners, Some of
`which are described below.
`0044 FIG. 2A shows an OFDM Subband structure 200
`that may be used for multi-carrier system 100. The system
`has an overall system bandwidth of W MHz, which is
`partitioned into Northogonal Subbands using OFDM. In a
`typical OFDM system, only M of the N total Subbands are
`used for pilot and data transmission, where M-N. The
`remaining N-M Subbands are not used for pilot/data trans
`mission and Serve as guard Subbands to allow the System to
`meet Spectral mask requirements. The MuSable Subbands
`include Subbands F through F+M-1, where F is an integer
`typically selected such that the M usable Subbands are
`centered in the middle of the operating band.
`004.5
`FIG. 2A also shows an embodiment of the parti
`tioning of the Musable Subbands for pilot transmission. In
`
`this embodiment, the M usable Subbands are initially
`divided into K groups, with each group including T con
`secutive Subbands. In general, K, T, and M may each be any
`integer greater than one and K-Ts M. The T Subbands in
`each group are then assigned to T sets Such that the i-th
`Subband in each group is assigned to the i-th Set.
`0046 FIG. 2B shows the Tsets of Subbands generated
`based on the partitioning shown in FIG. 2A. The K Sub
`bands in each of the Tsets are shown by the shaded boxes.
`For this embodiment, the K Subbands in each set are
`uniformly/evenly distributed across the Musable Subbands,
`and consecutive Subbands in the Set are spaced apart by T
`Subbands. The T Subband sets may be assigned to T cells or
`T Sectors for pilot transmission. Each cell or Sector only
`transmits pilot on the Subbands in the Set assigned to that
`cell/Sector.
`0047 As a specific example, the multi-carrier system
`may have 512 Subbands that are assigned indices of 1
`through 512. Of these 512 Subbands, 50 Subbands may be
`allocated for pilot transmission in each sector. The 512
`Subbands may then be used to form 9 sets of 50 Subbands
`(i.e., T=9 and K=50), as shown in Table 2.
`
`TABLE 2
`
`Set
`
`Subbands
`
`10, 20, 30, ... 500
`11, 21, 31, ... 501
`12, 22, 32, ... 502
`13, 23, 33, ... 503
`14, 24, 34,... 504
`15, 25, 35, ... 505
`16, 26, 36, ... 506
`17, 27, 37, ... 507
`18, 28, 38, ... 508
`
`0048. The 9 subband sets may then be assigned to 9
`different Sectors for pilot transmission.
`0049. In general, the Musable Subbands may be allocated
`to the Tsets in various manners, and this is within the Scope
`of the invention. The T sets may include the same or
`different numbers of Subbands. Moreover, the Subbands in
`each set may be uniformly or non-uniformly distributed
`across the M usable Subbands. The T Subband sets are
`“disjoint” from one another so that interference is avoided.
`The Subband sets are disjoint in that each of the Musable
`Subbands is assigned to at most one Set. Each Set further
`includes a Sufficient number of Subbands to enable the
`terminals to characterize the channel based on the pilot
`transmission on only these Subbands. In general, the number
`of sets to form and the number of Subbands to be included
`in each set (i.e., the specific values for T and K) may be
`dependent on various factorS Such as:
`the number of usable Subbands in the system;
`0050
`0051 the delay spread or coherence bandwidth of
`the System, which determines the maximum spacing
`between consecutive pilot Subbands in each Set to
`avoid performance degradation;
`0052 the size of the cluster for which frequency
`orthogonality is to be achieved; and
`0053 whether or not time orthogonality is also used
`for pilot transmission.
`
`19
`
`

`

`US 2004/O131007 A1
`
`Jul. 8, 2004
`
`0054) The cyclic prefix for OFDM symbols (described
`below) may be defined to include C. Samples, where C is
`properly Selected based on the delay spread of the System
`Such that the cyclic prefix contains a significant portion of all
`multipath energies. To avoid performance degradation, the
`number of Subbands in each set (K) may be selected such
`that K2C, and these subbands may be evenly distributed
`acroSS the System operating bandwidth. In this case, the
`maximum number of disjoint Sets that may be formed is
`N/C. For example, if N=256 and C=16, then up to 16 sets
`of Subbands may be formed. Fewer number of disjoint sets
`may also be formed, with each set including more than C,
`Subbands. In this case, the inclusion of more than the
`minimum required number of Subbands can allow the pilot
`to be received with higher Signal quality, and thus improved
`channel estimate and pilot Strength estimate may be
`obtained. Alternatively, greater number of disjoint Sets may
`also be formed, with each set including less than C. Sub
`bands. In this case, the inclusion of fewer than the minimum
`required number of Subbands may result in inadequate
`characterization of the frequency Selectivity of the operating
`band, and Some performance degradation may occur.
`0.055
`For simplicity, the following description assumes
`that each of the T Subband sets include K Subbands, the
`Subbands in each Set are uniformly distributed and are
`spaced apart by T Subbands (as shown in FIG. 2B), and
`K-T=M. The number of sets to form is dependent on the size
`of the cluster for which frequency orthogonality is desired,
`as described below.
`0056 FIG. 3A shows an exemplary Subband assignment
`to achieve frequency orthogonality for a cluster with 3 cells,
`where each cell includes 3 Sectors (i.e., a 9-sector 3-cell
`cluster). Each of the 9 Sectors in the cluster is assigned one
`of 9 subband sets (which may be formed, for example, as
`shown in Table 2). The Subband set assigned to each sector
`is indicated by the numeric reference next to the arrow in
`FIG. 3A. Each sector would then transmit its pilot on only
`the Subbands in its assigned Set. The 9 Sectors in the cluster
`may simultaneously transmit their pilots on 9 disjoint Sets of
`Subbands while achieving orthogonality in the frequency
`domain and avoiding interference.
`0057 FIG. 3B shows an exemplary Subband assignment
`to achieve frequency orthogonality for a cluster with 7 cells,
`where each cell includes 3 Sectors (i.e., a 21-sector 7-cell
`cluster). Each of the 21 Sectors in the cluster is assigned one
`of 21 Subband sets. The 21 sectors in the cluster may
`Simultaneously transmit their pilots on 21 disjoint Sets of
`Subbands while achieving orthogonality in the frequency
`domain and avoiding interference.
`0.058. In general, a cluster may be defined to include any
`number of cells, and each cell may comprise any number of
`Sectors. AS examples, a cluster may be defined to include 1,
`2, 3, 7, or 19 cells. The size of the cluster may be dependent
`on various factors, Such as those enumerated above.
`0059 Frequency orthogonality may also be achieved for
`a System that employs multiple: antennas at each Sector for
`pilot and data transmission to achieve Spatial diversity and
`improve reliability. For example, each Sector may transmit
`data from two antennas using a Space-time transmit diversity
`(STTD) scheme or an Alamoutischeme. The STTD scheme
`is described in 3GTS 25.211 and in provisional U.S. Patent
`Application Serial No. 60/421,309, entitled “MIMO WLAN
`
`System,” filed Oct. 25, 2002, assigned to the assignee of the
`present application, and incorporated herein by reference.
`The Alamouti scheme is described by S. M. Alamouti in a
`paper entitled “A Simple Transmit Diversity Technique for
`Wireless Communications.” IEEE JSAC, October 1998,
`which is also incorporated herein by reference. For a System
`with Sectors having multiple antennas, each antenna may be
`assigned a different Subband Set.
`0060 2. Time Orthogonality
`0061 Time orthogonality may be achieved by “covering”
`the pilot of each cell or sector with a different orthogonal
`code. At a terminal, the pilot from each cell/Sector may be
`recovered by “decovering the received signal with the same
`orthogonal code used by that cell/Sector. Covering is a
`process whereby a given pilot or data Symbol (or a set of Q
`pilot/data symbols with known values) to be transmitted is
`multiplied by all Q chips of a Q-chip orthogonal Sequence to
`obtain Q covered Symbols, which are further processed and
`then transmitted. Decovering is a complementary process
`whereby received symbols are multiplied by (a) the Q chips
`of the same Q-chip orthogonal sequence and (b) the complex
`conjugate of the pilot or data symbol (or the complex
`conjugate of the Q pilot/data Symbols) to obtain Q decov
`ered Symbols, which are then accumulated to obtain an
`estimate of the transmitted pilot or data Symbol. Covering
`and decreeing are known in the art and also described below.
`The decovering removes or cancels the pilots transmitted by
`other cells/sectors that use different orthogonal codes for
`their pilots. In this way, orthogonality among the pilot
`transmissions from multiple cells/Sectors may be achieved.
`0062) The effectiveness of the pilot orthogonalization
`through covering is dependent on having the knowledge of
`the timing for the base Stations. Time orthogonality may be
`achieved for Sectors of the same cell Since these Sectors may
`be operated Synchronously. The cells in each cluster or all
`cells in the System may also be operated Synchronously to
`allow time orthogonality to be achieved for the pilots
`transmitted by these cells.
`0063 Time orthogonality may be achieved with various
`types of orthogonal codes, Such as Walsh codes and orthogo
`nal variable spreading factor (OVSF) codes. The length of
`the orthogonal codes used for pilot covering is dependent on
`the number of orthogonal codes required, which in turn is
`dependent on the Size of the cluster for which time orthogo
`nality is to be achieved. For example, if time orthogonality
`is desired for a cell with 3 Sectors, then 3 Orthogonal codes
`are needed (i.e., one code for each Sector) and each orthogo
`nal code would then have a length of 4 chips.
`0064) Table 3 lists four 4-chip Walsh codes that may be
`assigned to up to four different Sectors, cells, or antennas.
`
`TABLE 3
`
`Walsh Codes
`
`Values
`
`W (n)
`W., (n)
`Ws (n)
`W (n)
`
`1.
`1.
`1.
`1.
`
`1.
`1.
`-1
`-1
`
`1.
`-1
`1.
`-1
`
`1.
`-1
`-1
`1.
`
`0065. A specific Walsh code may be assigned to each
`Sector or each antenna of a given cell. A value of "-1" for
`
`20
`
`

`

`US 2004/O131007 A1
`
`Jul. 8, 2004
`
`the Walsh code may indicate an inversion of the pilot symbol
`(i.e., p(n) >-p(n)) and a value of “1” may indicate no
`inversion. The same Walsh code may be applied to each of
`the Subbands used for pilot transmission. For each pilot
`Subband, the four chips of the Walsh code are applied to four
`pilot symbols to be transmitted in four consecutive OFDM
`symbol periods. The length of the Walsh code is thus
`Tw=4T, where T, denotes one OFDM symbol period.
`If the pilot transmission is longer than four OFDM symbol
`periods, then the same Walsh code may be repeated as many
`times as needed. A Walsh code is also referred to as a Walsh
`Sequence or a Walsh Symbol, and Tw denotes one Walsh
`Symbol period.
`0.066
`FIG. 4A shows an exemplary orthogonal code
`assignment to achieve time orthogonality for a cell with
`three sectors (i.e., a 3-sector 1-cell cluster). Each of the three
`Sectors in the cell is assigned a different orthogonal code.
`The three orthogonal codes assigned to the 3 Sectors are
`labeled as A, B, and C. AS indicated in FIG. 4A, the same
`Subband set may be used by all three sectors in the cell.
`Orthogonality is then achieved in the time domain for the
`pilot transmissions from these three Sectors via the use of
`different orthogonal codes.
`0067 FIG. 4B shows an exemplary orthogonal code
`assignment to achieve time orthogonality for a cell with
`three Sectors, with each Sector employing two antennas for
`pilot and data transmission. Each of the three Sectors in the
`cell is assigned two Orthogonal codes, one code for each
`antenna. The three pairs of orthogonal codes assigned to the
`three sectors are labeled as A/B, C/D, and E/F. The 3-sector
`cell would then require a total of six orthogonal codes, and
`each orthogonal code may then have a length of 8 chips.
`0068 The time orthogonality property may be degraded
`by temporal variations in the propagation paths between the
`base Stations and the terminal. Thus, it is desirable to use
`Short orthogonal codes So that the propagation paths are
`essentially constant over the duration of the orthogonal
`codes.
`0069. 3. Combined Frequency and Time Orthogonality
`0070 A combination of frequency and time orthogonality
`may be used for pilot transmission. In one embodiment,
`frequency orthogonality is achieved for multiple cells in a
`cluster, and time orthogonality is achieved for multiple
`Sectors within each cell.
`0071
`FIG. 4C shows an exemplary Subband and code
`assignment to achieve frequency and time orthogonality for
`a 9-sector 3-cell cluster. Each of the three cells in the cluster
`is assigned a different Subband Set to achieve frequency
`orthogonality among the three cells. The three Sectors of
`each cell are also assigned three different orthogonal codes
`to achieve time orthogonality among the three Sectors. Each
`Sector of each cell would then transmit its pilot using its
`assigned orthog