US008340232B2
`
`(12; United States Patent
`Ding et al.
`
`(10) Patent No.:
`(45) Date of Patent:
`
`US 8,340,232 B2
`Dec. 25, 2012
`
`(54 APPARATUS AND METHOD FOR CHANNEL
`ESTIMATION USING TRAINING SIGNALS
`WITH REDUCED SIGNAL OVERHEAD
`
`(75
`
`Inventors: Yinong Ding, Plano, TX (US); Farooq
`Khan, Allen, TX (US); Cornelius Van
`Rensburg, Dallas, TX (US)
`
`(56)
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`2002/0122382 Al *
`9/2002 Ma et al.
`..................... .. 370/208
`2003/0058926 A1 *
`3/2003 Balakrishnan et al.
`..... .. 375/146
`2004/0100939 A1 *
`5/2004 Kriedte et al.
`.............. .. 370/347
`. . . . .. 370/344
`2006/0013186 Al *
`l/2006 Agrawal et al.
`. . . . .
`
`9/2006 Hayase ....................... .. 375/267
`2006/0198461 Al *
`
`(73 Assignee: Samsung Electronics Co., Ltd.,
`Suwon-si (KR)
`
`* cited by examiner
`
`( *
`
`Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. l54(b) by 2142 days.
`
`Primary Examiner — David C. Payne
`Assistant Examiner — Tanmay Shah
`
`(57)
`
`ABSTRACT
`
`(21 Appl.No.: 11/297,879
`
`(22
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`Filed:
`
`Dec. 9, 2005
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`(65
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`(51
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`(52
`(58
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`Prior Publication Data
`
`US 2007/0165726 A1
`
`Jul. 19,2007
`
`Int. Cl.
`(2006.01)
`H04B 7/10
`(2006.01)
`H04L 1/02
`
`U.S. Cl.
`........................................... .. 375/347
`Field of Classification Search ................. .. 375/347
`See application file for complete search history.
`
`A base station comprising an antenna array including M
`antennas for transmitting data to a plurality of subscriber
`stations. The base station generates a first pilot signal pre-
`amble by adding a first cyclic prefix to a first pilot signal
`sequence and generates a second pilot signal preamble by
`adding a second cyclic prefix to a second pilot signal
`sequence. The second pilot signal sequence is a circularly
`shifted copy of the first pilot signal sequence. The first pilot
`signal preamble is transmitted from a first antenna and the
`second pilot signal preamble is transmitted from a second
`antenna concurrently with transmission of the first preamble.
`
`20 Claims, 5 Drawing Sheets
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`31 0a
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`320b
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`330b
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`1 02
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`APPLE 1006
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`U.S. Patent
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`Dec. 25, 2012
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`Sheet 1 of5
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`FIG. 1
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`1
`APPARATUS AND METHOD FOR CHANNEL
`ESTIMATION USING TRAINING SIGNALS
`WITH REDUCED SIGNAL OVERHEAD
`
`TECHNICAL FIELD OF THE INVENTION
`
`The present application relates generally to wireless com-
`munications and, more specifically, to a channel estimation
`technique having reduced signaling overhead for use in mul-
`tiple-input, multiple-output (MIMO) systems.
`
`BACKGROUND OF TI IE INVENTION
`
`An increasing number of wireless networks are imple-
`mented as multiple-input, multiple-output (MIMO) systems
`that use multiple antennas to communicate with subscriber
`stations (also called mobile stations, mobile terminals, and
`the like). For example, MIMO antenna systems are used in
`code division multiple access (CDMA) networks, time divi-
`sion multiplexing (TDM) networks, time division multiple
`access (TDMA) networks, orthogonal frequency division
`multiplexing (OFDM) networks, orthogonal frequency divi-
`sion multiple access (OFDMA) networks, and others. In
`order to maximize throughput, MIMO networks use a variety
`of channel estimation techniques to measure the transmission
`channels between base stations of the wireless networks and
`mobile devices.
`
`For example, a MIMO base station may transmit a first
`pilot signal from a first antenna and a second pilot signal from
`a second antenna. A subscriber station receives both pilot
`signals and uses each pilot signal to perform channel estima-
`tion for each antenna. The symbols of the pilot signals are
`transmitted on orthogonal subcarriers to prevent the pilot
`signals from interfering with each other. No data symbols are
`transmitted on the subcarriers ofthe pilot signals. Ifmore than
`two antennas are used, each antenna transmits a separate pilot
`signal on a dedicated subcarrier that is orthogonal to the other
`pilot signal subcarriers. The drawback to this method is that a
`large amount of signaling overhead is used for channel esti-
`mation. This wastes bandwidth and reduces system capacity.
`Besides achieving pilot orthogonality in the dimension of
`the subcarriers, the pilot orthogonality may also achieved in
`the dimension of time. A MIMO base station may transmit
`separate pilot signals from different antennas using time mul-
`tiplexing to separate the pilot signals. For example, during a
`first transmit time, the base station may transmit a first pilot
`signal from a first antenna. Then, during a second transmit
`time, the base station may transmit a second pilot signal from
`a second antenna. If more than two antennas are used, each
`antenna transmits a separate pilot signal during a separate
`transmit time. The drawback to this method is that more time
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`slots are needed for pilot signals, thereby reducing the num-
`ber of time slots available for transmitting user data. As
`before, this reduces system capacity.
`In some CDMA systems, multiple pilot signals are trans-
`mitted simultaneously from multiple antennas, but they are
`separated in the dimension of “code”. The pilots from all
`antennas are transmitted with preambles that use different
`pseudo-random noise (PN) codes. This reduces the number of 60
`time slots required forpilot signals. However, the drawback to
`this method is that the receiver circuitry in the mobile device
`must use complicated interference cancellation techniques to
`recover the MIMO pilot signals. This method reduces pilot
`signaling overhead at the cost of a more complicated pilot
`recovery scheme. This method also decreases the reliability
`of the channel estimates.
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`Therefore, there is a need in the art for an improved appa-
`ratus and method for performing channel estimation in a
`multiple-input, multiple-output (MIMO) wireless network.
`
`SUMMARY OF THE INVENTION
`
`In one embodiment, a base station is disclosed comprising
`an antenna array including M antennas for transmitting data
`to a plurality of subscriber stations. The channel estimation
`scheme is described in the context of an OFDM system. The
`base station generates a first pilot signal preamble by adding
`a first cyclic prefix to a first pilot signal sequence and gener-
`ates a second pilot signal preamble by adding a second cyclic
`prefix to a second pilot signal sequence, wherein the second
`pilot signal sequence is a circularly shifted copy of the first
`pilot signal sequence. The first pilot signal preamble is trans-
`mitted from a first one of the M antennas and the second pilot
`signal preamble is transmitted from a second one of the M
`antennas. The first and second pilot signal preambles are
`transmitted concurrently.
`In another embodiment, a method is disclosed for trans-
`mitting data from M antennas capable of transmitting to a
`plurality of subscriber stations. The disclosed method com-
`prises the steps of: generating a first pilot signal preamble by
`adding a first cyclic prefix to a first pilot signal sequence;
`generating a second pilot signal preamble by adding a second
`cyclic prefix to a second pilot signal sequence, wherein the
`second pilot signal sequence is a circularly shifted copy ofthe
`first pilot signal sequence; transmitting the first pilot signal
`preamble from a first one of the M antennas; and transmitting
`the second pilot signal preamble from a second one of the M
`antennas. The first and second pilot signal preambles are
`transmitted concurrently.
`Before undertaking the DETAILED DESCRIPTION OF
`THE INVENTION below, it may be advantageous to set forth
`definitions of certain words and phrases used throughout this
`patent document: the terms “include” and “comprise,” as well
`as derivatives thereof, mean inclusion without limitation; the
`term “or,” is inclusive, meaning and/or; the phrases “associ-
`ated with” and “associated therewit
`,” as well as derivatives
`thereof, may mean to include, be included within, intercon-
`nect with, contain, be contained within, connect to or with,
`couple to or with, be communicable with, cooperate with,
`interleave, juxtapose, be proximate to, be bound to or with,
`have, have a property of, or the like; and the term “controller”
`means any device, system or part thereofthat controls at least
`one operation, such a device may be implemented in hard-
`ware, firmware or software, or some combination of at least
`two of the same. It should be noted that the functionality
`associated with any particular controller may be centralized
`or distributed, whether locally or remotely. Definitions for
`certain words and phrases are provided throughout this patent
`document, those of ordinary skill in the art should understand
`that in many, if not most instances, such definitions apply to
`prior, as well as future uses of such defined words and
`phrases.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`For a more complete understanding of the present disclo-
`sure and its advantages, reference is now made to the follow-
`ing description taken in conjunction with the accompanying
`drawings, in which like reference numerals represent like
`parts:
`FIG. 1 illustrates a wireless network that implements chan-
`nel estimation according to the principles of the disclosure;
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`US 8,340,232 B2
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`3
`FIG. 2 illustrates a base station that implements channel
`estimation according to the principles of the disclosure;
`FIG. 3 illustrates selected portions of the transmitter cir-
`cuitry in a base station according to an exemplary embodi-
`ment of the present disclosure;
`FIG. 4 illustrates four exemplary preambles transmitted by
`a base station using four antennas according to an exemplary
`embodiment;
`FIG. 5 illustrates a cross-correlation circuit in a subscriber
`station; and
`FIG. 6 illustrates the estimation of the channel responses
`h1(t), h2(t), h3(t), and h4(t) from the four (4) base station
`antennas to the receiving subscriber station according to one
`embodiment.
`
`DETAILED DESCRIPTION OF THE INVENTION
`
`FIGS. 1 through 6, discussed below, and the various
`embodiments used to describe the principles of the present
`disclosure in this patent document are by way of illustration
`only and should not be construed in any way to limit the scope
`of the disclosure. Those skilled in the art will understand that
`
`the principles of the present disclosure may be implemented
`in any suitably arranged wireless network.
`The disclosed apparatus and method for estimating wire-
`less communication channels use pilot (or training) signals
`that reduce signaling overhead and require only a simple
`channel response estimator in the receiver of the mobile
`device (i.e., subscriber station, mobile station, mobile tenni-
`nal, etc.). In the description that follows, the charmel estima-
`tion apparatus is embodied in an orthogonal frequency divi-
`sion multiple access (OFDMA) wireless network using a
`multiple-input, multiple-output (MIMO) antenna system.
`However, this is by way of illustration only and should not be
`construed to limit the scope of the disclosure and the claims
`herein. Those skilled in the art will understand that the dis-
`
`closed apparatus and method may be easily adapted for use in
`any type of wireless network that uses a multiple-input, mul-
`tiple-output (MIMO) antenna system.
`FIG. 1 illustrates exemplary wireless network 100, which
`implements channel estimation according to the principles of
`the present disclosure. In the illustrated embodiment, wire-
`less network 100 includes base station (BS) 101, base station
`(BS) 102, and base station (BS) 103. Base station 101 com-
`municates with base station 102 and base station 103. Base
`
`station 101 also communicates with Internet protocol (IP)
`network 13 0, such as the Internet, a proprietary IP network, or
`other data network.
`
`Base station 102 provides wireless broadband access to
`network 130, via base station 101, to a first plurality of sub-
`scriber stations within coverage area 120 of base station 102.
`The first plurality of subscriber stations includes subscriber
`station (SS) 111, subscriber station (SS) 112, subscriber sta-
`tion (SS) 113, subscriber station (SS) 114, subscriber station
`(SS) 115 and subscriber station (SS) 116. In an exemplary
`embodiment, SS 111 may be located in a small business (SB),
`SS 112 may be located in an enterprise (E), SS 113 may be
`located in a WiFi hotspot (HS), SS 114 may be located in a
`first residence, SS 115 may be located in a second residence,
`and SS 116 may be a mobile (M) device. Base station 103 also
`provides wireless broadband access to network 130, via base
`station 101, to a second plurality of subscriber stations within
`coverage area 125 ofbase station 103. The second plurality of
`subscriber stations includes SS 115 and SS 116.
`
`In order to maximize throughput, wireless network 100
`estimates the communication channels between base stations
`
`101-103 and subscriber stations 111-116. The apparatus and
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`method for estimating channels disclosed herein use pilot (or
`training) signals that reduce signaling overhead and require
`only a simple channel response estimator in the receivers of
`subscriber stations 111-116.
`FIG. 2 illustrates in greater detail exemplary base station
`102, which implements channel estimation according to one
`embodiment of the present disclosure. Base station 102 is
`illustrated by way of example only. However, it will be under-
`stood that the components illustrated and described with
`respect to base station 102 are also part of base stations 101
`and 103. In one embodiment, base station 102 comprises
`controller 225. charmel controller 235. transceiver interface
`(IF) 245, radio frequency (RF) transceiver unit 250, and
`antenna array 255. According to the principles of the present
`disclosure, antenna array 255 is a multiple-input, multiple
`output (MIMO) antenna system.
`Controller 225 comprises processing circuitry and
`memory capable of executing an operating program that con-
`trols the overall operation of base station 102. In an embodi-
`ment, the controller 225 may be operable to communicate
`with the network 130. Under normal conditions, controller
`225 directs the operation of channel controller 235, which
`comprises a number of cha1mel elements, such as exemplary
`channel element 240, each of which performs bidirectional
`communication in the forward channel and the reverse chan-
`
`nel. A forward charmel (or downlink) refers to outbound
`signals from base station 102 to subscriber stations 111-116.
`A reverse channel (or uplink) refers to inbound signals from
`subscriber stations 111-116 to base station 102. Channel ele-
`
`ment 240 also preferably performs all baseband processing,
`including processing any digitized received signal to extract
`the information or data bits conveyed in the received signal,
`typically including demodulation, decoding, and error cor-
`rection operations, as known to those of skill in the art. Trans-
`ceiver IF 245 transfers bidirectional charmel signals between
`channel controller 235 and RF transceiver unit 250.
`
`Furthermore, according to the principles of the present
`disclosure, antenna array 255 is a multiple-input, multiple-
`output (MIMO) antenna system that uses multiple antennas to
`transmit pilot signals and user data to subscriber stations
`111-116. Each antenna transmits its own pilot signal. Sub-
`scriber stations 111-116 use the pilot signals to estimate the
`channel response for each antenna in antenna array 255 using
`well-known channel estimation techniques.
`FIG. 3 illustrates selected portions of the transmitter cir-
`cuitry in base station 102 according to an exemplary embodi-
`ment of the present disclosure. Base station 102 comprises N
`modulation blocks 310,
`including exemplary modulation
`blocks 310a-310c, serial-to-parallel (S/P) converters 320,
`including exemplary S/P converters 320a-320c, inverse Fast
`Fourier Transform (IFFT) blocks 330, including exemplary
`IFFT blocks 330a-330c, parallel-to-serial (P/S) converters
`340, including exemplary parallel-to-serial (P/S) converters
`340a-340c, cyclic prefix insertion (CPI) blocks 350, includ-
`ing exemplary CPI blocks 350a-350c, up-co11version blocks
`360, including exemplary up-conversion blocks 36011-3600,
`and antenna array 255. Antenna array 255 comprises M
`antennas, including exemplary antennas 371, 372 and 373.
`In FIG. 3, individual transmit paths are shown for indi-
`vidual user data inputs, such as Data 1, Data 2 and Data N.
`Each transmit path includes its own components, such as
`modulator block 310, IFFT block 330, and so forth. However,
`this is done for the purposes of simplicity and clarity in
`explaining the operation of base station 102. In practice,
`many of the separate components in each transmit path may
`be implemented by a single functional block. For example,
`IFFT blocks 330a, 330b and 330c may all be part ofa single
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`5
`IFFT component in base station 102. Similarly, up-conver-
`sion blocks 360a, 360b and 360c may be implemented by a
`single up-converter inbase station 102. Additionally, some of
`the components in each transmit path are distributed across
`different ones of channel controller 235, transceiver IF 245
`and RF transceiver 250 in FIG. 2.
`
`In each transmit path, modulation block 310 receives an
`input data stream (e.g., Data 1) and modulates the input data
`to produce modulated symbols according to any known
`modulation technique. The modulated symbols are converted
`to parallel format by S/P converter 320 and mapped onto a
`plurality of orthogonal subcarriers in the frequency domain as
`inputs to the IFFT block 330. The outputs of IFFT block 330
`are converted into serial data by P/S converter 340. Cyclic
`prefix insertion block 350 then adds a cyclic prefix to the
`output of the P/S converter 340. The output of CPI block 350
`is up-converted by up-conversion block 360 for transmission
`via an antenna (e.g., antenna 371) in antenna array 255.
`In each transmission time interval (TTI), base station 102
`transmits a sequence of OFDM data symbols that is preceded
`by a pilot symbol (i.e., preamble) that trains the receiver and
`is used for channel estimation. CPI block 350 adds a cyclic
`prefix to each OFDM data symbol and to each pilot signal
`symbol. Thus, the first symbol of each TTI is a pilot signal
`preamble formed from an N-sarnple long pseudo-random
`noise (PN) code. The remaining symbols in each TTI are
`OFDM symbols carrying the information data.
`As is well known,
`the result of the preamble passing
`through a wireless charmel with a channel response, h(t), is
`the N-point circular convolution of p(t),
`the pilot signal
`sequence (i.e., the PN sequence/code), and h(t), the impulse
`response of the wireless channel. This is expressed as p(t)®h
`(t), where 6) represents the N-point circular convolution
`operation.
`For a MIMO system, such as base station 102, with M
`transmit antennas, CPI block 350 receives a pilot signal
`sequence, namely a PN code, p(t), and appends a cyclic prefix
`to the start of the pilot signal sequence. Thus, a first pilot
`signal preamble transmitted from a first antenna (e.g., antenna
`371) is formed directly from the original PN code. A second
`preamble transmitted by a second antem1a (e.g., antenna 372)
`is generated by circularly shifting the same PN code by L
`samples to produce a second pilot signal sequence and adding
`a second cyclic prefix. A third preamble transmitted by a third
`antenna (e.g., antenna 373) is generated by circularly shifting
`the secondpilot signal sequence by an additional L samples to
`produce a third pilot signal sequence and adding a third cyclic
`prefix.
`FIG. 4 illustrates four exemplary pilot signal preambles
`transmitted by base station 102 using four antennas according
`to an exemplary embodiment. The antennas in antenna array
`255 are labeled Antenna 1, Antenna 2, Antenna 3, and
`Antenna 4 in this example. Base station 101 sends preamble
`410 to Antenna 1, preamble 420 to Antenna 2, preamble 430
`to Antenna 3, and preamble 440 to Antenna 4. In the exem-
`plary embodiment, each pilot signal preamble is generated
`from an original pilot signal sequence that is a pseudo-ran-
`dom noise (PN) code that is 512 samples in length. The choice
`of 512 samples is by way of example only. In other embodi-
`ments, the PN code may contain more than 512 samples or
`less than 512 samples.
`The 512 sample PN code is logically divided into eight (8)
`blocks, where eachblock contains 64 samples. The blocks are
`sequentially labeled [B1 B2 B3 B4 B5 B6 B7 B8].As is well
`known, a cyclic prefix for a block ofN data samples is created
`by copying the last L samples of the block of N data samples
`and appending them to the front of the block of N data
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`samples. This increases the length of the block to N+L data
`samples, where the first L samples and the last L samples of
`the block ofN+L data samples are the same. Ideally, the value
`of L is at least as long as the charmel impulse response length.
`In base station 102, CPI block 350 generates preamble 410
`by copying the last L:64 samples (i.e., block B8) of the
`original pilot signal sequence, namely the 512-sample PN
`code [B1 B2 B3 B4 B5 B6 B7 B8]. CPI block 350 then
`appends block B8 to the start of the original pilot signal
`sequence to form preamble 410. Thus, the blocks ofpreamble
`410 are transmitted in the following order: [B8 B1 B2 B3 B4
`B5 B6 B7 B8].
`CPI block 350 generates preamble 420 in parallel with
`preamble 410. CPI block 350 generates preamble 420 by
`circularly shifting the 512-sample PN code [B1 B2 B3 B4 B5
`B6 B7 B8] by 64 samples to the right to generate a second
`pilot signal sequence, namely a new 512-sample PN code:
`[B8 B1 B2 B3 B4 B5 B6 B7]. Next, CPI block 350 copies the
`last 64 samples of the new 512-sample PN code, namely the
`64 samples in block B7, and appends block B7 to the start of
`the second pilot signal sequence. Thus, the blocks of pre-
`amble 420 are transmitted in the following order: [B7 B8 B1
`B2 B3 B4 B5 B6 B7].
`CPI block 350 generates preamble 430 in parallel with
`preambles 410 and 420. CPI block 350 generates preamble
`430 by circularly shifting a second time the second pilot
`signal sequence, namely the 512-sample PN code [B8 B1 B2
`B3 B4 B5 B6 B7], by an additional 64 samples to the right to
`generate a third pilot signal sequence, namely the new 512-
`sarnple PN code: [B7 B8 B1 B2 B3 B4 B5 B6]. Next, CPI
`block 350 copies the last 64 samples ofthe new 512-sample
`PN code, namely the 64 samples in block B6, and appends
`block B6 to the start of the third pilot signal sequence. Thus,
`the blocks of preamble 430 are transmitted in the following
`order: [B6 B7 B8 B1 B2 B3 B4 B5 B6].
`CPI block 350 generates preamble 440 in parallel with
`preambles 410, 420 and 430. CPI block 350 generates pre-
`amble 440 by circularly shifting a third time the second pilot
`signal sequence, namely the 512-sample PN code [B7 B8 B1
`B2 B3 B4 B5 B6], by an additional 64 samples to the right to
`generate a fourth pilot signal sequence, namely the new 512-
`sarnple PN code: [B6 B7 B8 B1 B2 B3 B4 B5]. Next, CPI
`block 350 copies the last 64 samples of the new 512 sample
`PN code, namely the 64 samples in block B5, and appends
`block B5 to the start of the fourth pilot signal sequence. Thus,
`the blocks of preamble 440 are transmitted in the following
`order: [B5 B6 B7 B8 B1 B2 B3 B4 B5].
`It is noted that the circular shift of 64 samples for each one
`of preambles 420, 430 and 440 is by way of example only. In
`alternate embodiments, a circular shift of more than 64
`samples or less than 64 samples may be used.
`After transmission from Antennas 1, 2, 3 and 4, preambles
`410, 420, 430 and 440 are received at a subscriber station at
`the same time. At the receiving subscriber station,
`it is
`assumed that the channel response for Antenna 1 is h1(t), the
`channel response for Antenna 2 is h2(t), the charmel response
`for Antenna 3 is h3(t), and the channel response for Antenna
`4 is h4(t). Thus, the baseband signal, r(t), in the receiving
`subscriber station may be represented as:
`
`V(l):p(l)®h1(l)+p[l-64lN®h2(l)+p[l-128lN®h3(l)+P
`[z—192]N®h4(l)+n(l)
`
`In the above equation, p[t—l]N is the L-sa1nple circularly
`shifted version of the N-sarnple long sequence p(t) and n(t) is
`the white Gaussian noise. I11 this example, L:64 and N:5 l 2.
`In the absence of noise, the channel responses h1(t), h2(t),
`h3(t), and h4(t) may be ideally identified by the circular
`
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`US 8,340,232 B2
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`7
`(cross) correlation of r(t) and p(t). FIG. 5 illustrates cross-
`correlation circuit 500 in a subscriber station. Cross-correla-
`
`tion circuit 500 receives the baseband signal, r(t), and the
`original 512 sample PN code, p(t), of the pilot signal and
`generates the channel responses h1(t), h2(t), h3(t), and h4(t).
`FIG. 6 depicts timing diagram 600, which illustrates the
`estimation of the charmel responses h1(t), h2(t), h3(t), and
`h4(t) in the receiving subscriber station according to one
`embodiment. Because ofthe circular shifts ofpreambles 420,
`430 and 440 with respect to preamble 410, the receiver in the
`subscriber station sequentially detects the channel responses
`h1(t), h2(t), h3(t), and h4(t). Thus, it is not necessary to include
`complex cancellation circuitry in the receiver to cancel, for
`example,
`the channel
`response hl (t)
`from the channel
`response h2(t).
`The disclosed apparatus and method employ the circular
`shift orthogonal property of a pre-designed pseudo-random
`(PN) code. Each antenna transmits a circularly shifted version
`of the PN code, and at the receiver, one cross-correlation
`operation enables the identification of all charmel responses,
`as long as the product (L><M) ofthe charmel impulse response
`length, L, and the number ofantennas, M, does not exceed the
`length N of the PN code.
`Although the present disclosure has been described with an
`exemplary embodiment, various changes and modifications
`may be suggested to one skilled in the art. It is intended that
`the present disclosure encompass such changes and modifi-
`cations as fall within the scope of the appended claims.
`
`What is claimed is:
`
`1. A base station configured to communicate with a plural-
`ity of subscriber stations of a wireless network, the base
`station comprising:
`an antenna array comprising a plurality of antennas con-
`figured to transmit data to the plurality of subscriber
`stations; and
`a cyclic prefix insertion block configured to:
`generate a first pilot signal preamble by adding a first
`cyclic prefix to a first pilot signal sequence;
`generate a second pilot signal preamble by adding a
`second cyclic prefix to a second pilot signal sequence;
`and
`
`transmit the first and second pilot signal preambles
`through the plurality of antennas,
`wherein the second pilot signal sequence is a circularly
`shifted copy of the first pilot signal sequence,
`wherein the first cyclic prefix comprises the last one or
`more samples of the first pilot signal sequence, and
`wherein a product of the length of the first and second
`cyclic prefix and the number of antennas does not
`exceed a length of the first and second pilot signal
`sequence, respectively.
`2. The base station as set forth in claim 1, wherein the cyclic
`prefix insertion block is configured to transmit the first pilot
`signal preamble from a first one of the plurality of antennas
`and the second pilot signal preamble from a second one ofthe
`plurality of antennas, wherein the first and second pilot signal
`preambles are transmitted concurrently.
`3. The base station as set forth in claim 2, wherein the first
`pilot signal sequence comprises a pseudo-random noise code.
`4. The base station as set forth in claim 3, wherein the cyclic
`prefix insertion block is configured to generate the second
`pilot signal sequence by circularly shifting the first pilot sig-
`nal sequence by one or more samples to the right.
`5. The base station as set forth in claim 4, wherein the
`second cyclic prefix comprises the last one or more samples
`of the second pilot signal sequence.
`
`5
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`10
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`15
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`20
`
`25
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`30
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`35
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`40
`
`45
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`50
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`55
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`60
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`65
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`8
`6. The base station as set forth in claim 2, wherein the cyclic
`prefix insertion block is configured to generate a third pilot
`signal preamble by adding a third cyclic prefix to a third pilot
`signal sequence, wherein the third pilot signal sequence com-
`prises a circularly shifted copy of the second pilot signal
`sequence.
`7. The base station as set forth in claim 6, wherein the cyclic
`prefix insertion block is configured to transmit the third pilot
`signal preamble from a third one of the plurality of antennas,
`wherein the first, second and third pilot signal preambles are
`transmitted concurrently.
`8. A wireless network comprising a plurality of base sta-
`tions configured to communicate with a plurality of sub-
`scriber stations, each one of the plurality of base stations
`comprising:
`an antenna array comprising a plurality of antennas con-
`figured to transmit data to the plurality of subscriber
`stations; and
`a cyclic prefix insertion block configured to:
`generate a first pilot signal preamble by adding a first
`cyclic prefix to a first pilot signal sequence;
`generate a second pilot signal preamble by adding a
`second cyclic prefix to a second pilot signal sequence;
`and
`
`transmit the first and second pilot signal preambles
`through the plurality of antennas,
`wherein the second pilot signal sequence is a circularly
`shifted copy ofthe first pilot signal sequence, wherein
`the second cyclic prefix comprises the last one or
`more samples of the second pilot signal sequence,
`wherein a product of the length of the first and second
`cyclic prefix and the number of antennas does not
`exceed a length of the first and second pilot signal
`sequence, respectively.
`9. The wireless network as set forth in claim 8, wherein the
`cyclic prefix insertion block is configured to transmit the first
`pilot signal preamble fron1 a first one of the plurality of
`antennas and the second pilot signal preamble from a second
`one of the plurality of antennas, wherein the first and second
`pilot signal preambles are transmitted concurrently.
`10. The wireless network as set forth in claim 9, wherein
`the first pilot signal sequence comprises a pseudo-random
`noise code.
`11. The wireless network as set forth in claim 10, wherein
`the first cyclic prefix comprises the last one or more samples
`of the first pilot signal sequence.
`12. The wireless network as set forth in claim 11, wherein
`the cyclic prefix insertion block is configured to generate the
`second pilot signal sequence by circularly shifting the first
`pilot signal sequence by one or more samples to the right.
`13. The wireless network as set forth in claim 12, wherein
`the cyclic prefix insertion block is configured to generate a
`third pilot signal preamble by adding a third cyclic prefix to a
`third pilot signal sequence, wherein the third pilot signal
`sequence comprises a circularly shifted copy of the second
`pilot signal sequence.
`14. The wireless network as set forth in claim 13, wherein
`the cyclic prefix insertion block is configured to transmit the
`third pilot signal preamble from a third one of the plurality of
`antennas, wherein the first, second and third pilot signal pre-
`ambles are transmitted concurrently.
`15. A method for transmitting from an antenna array com-
`prising a plurality of antennas that transmit data to a plurality
`of subscriber stations, the method comprising:
`generating a first pilot signal preamble by adding a first
`cyclic prefix to a first pilot signal sequence;
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`9
`generating a second pilot signal preamble by adding a
`second cyclic prefix to a second pilot signal sequence,
`wherein the second pilot signal sequence is a circularly
`shifted copy of the first pilot signal sequence;
`transmitting the first pilot signal preamble from a first one
`of the plurality of antennas; and
`transmitting the second pilot signal preamble from a sec-
`ond one ofthe plurality of antennas, wherein the first and
`second pilot signal preambles are transmitted concur-
`rently,
`wherein the first cyclic prefix comprises the last one or
`more samples of the first pilot signal sequence, and
`wherein a product of the length of the first and second
`cyclic prefix and the number ofantennas does not exceed
`a length of the first and s