WO2007084988A2

WO2007084988A2 - Method and apparatus for pilot multiplexing in a wireless communication system

Info

Publication number: WO2007084988A2
Application number: PCT/US2007/060776
Authority: WO
Inventors: Byoung-Hoon Kim; Yongbin Wei; Amir Dabbagh
Original assignee: Qualcomm Incorporated
Priority date: 2006-01-20
Filing date: 2007-01-19
Publication date: 2007-07-26
Also published as: BRPI0706579A2; WO2007084988A3; EP2026518A2; EP1989850A2; US20160020865A1; RU2404529C2; KR101077386B1; HK1165124A1; US20070195906A1; EP2053816A1; CN102281115B; EP2053816B1; KR101025073B1; CA2636201A1; KR20080086934A; JP6026459B2; US20100142490A1; CN102394681A; KR20110005748A; BRPI0706579B1

Abstract

Techniques for multiplexing pilots in a wireless transmission are described. In one aspect, a transmitter station generates multiple pilot sequences for multiple transmit antennas, with each pilot sequence comprising pilot symbols sent in the time domain on a different set of subcarriers. The transmitter station further generates multiple pilot transmissions for the transmit antennas based on the pilot sequences. In another aspect, a transmitter station generates multiple pilot sequences for multiple transmit antennas based on frequency-domain code division multiplexing (FD-CDM) of a Chu sequence defined by a transmitter-specific value. The transmitter station further generates multiple pilot transmissions for the transmit antennas based on the pilot sequences. In yet another aspect, a transmitter station generates multiple pilot transmissions for multiple transmit antennas based on a first multiplexing scheme and generates multiple data transmissions based on a second multiplexing scheme that is different from the first multiplexing scheme.

Description

METHOD AND APPARATUS FOR PILOT MULTIPLEXING IN A

WIRELESS COMMUNICATION SYSTEM

Claim of Priority under 35 U.S.C §119

(000l| The present Application for Patent claims priority to Provisional Application

Serial No 60/760,482, entitled "METHOD AND APPARATUS FOR PILOT MULTIPLEXING IN A WIRELESS COMMUNICATIONS SYSTEM," filed January 20, 2006, assigned to the assignee hereof, and expressly incorporated herein by reference.

BACKGROUND Field

[0002J The present disclosure relates generally to communication, and more specifically to techniques for transmitting pilots in a wireless communication system

Background

|0003J In a wireless communication system, a transmitter station (e g,, a base station or a terminal) may utilize multiple (T) transmit antennas for a multiple-input multfple- output (MlMO) transmission to a receiver station eqtilpped with multiple (R) receive antennas. The multiple transmit and receive antennas form a MlMO channel that may he used to increase throughput anci/or improve reliability. For example, the transmitter station may transmit up to T data streams simultaneously from the T transmit antennas to improve throughput. Alternatively, the transmitter station may transmit a single data stream from up to T transmit antennas to improve reception by the receiver station. j 0004| Good performance may be achieved if the MlλlO channel response can be accurately estimated. For example, the receiver station may use the MIMO channel response to perform data detection for the MIMO transmission, to select a .spatial mapping matrix to be applied by the transmitter station for the MlMO transmission, etc. Channel estimation is typically supported by transmitting pilot symbols that are known a priori by the receiver station. The receiver station may then estimate the MlMO channel response based on the received pilot symbols and the known pilot symbols. fOOΘ5| Channel estimates obtained based on pilot are typically impaired by both noise and interference. The noise may come from various sources such as the wireless channel, receiver electronics, etc. The interference may include mter-antenna interference and inter- transmitter interference. Inter-antenna interference is interference due to transmissions from other transmit antennas. Tnter-antenna pilot interference may be present If multiple pilot transmissions are sent simultaneously from all T transmit amentias and the pilot transmission from each antenna interferes with the pilot transmissions from the other antennas, Inter-transmitter interference is interference due to transmissions from other transmitter stations Inter-transmitter interference may also be referred to as inter-sector interference, inter-cell interference, inter-terminal interference, etc. The bUer-antenna interference ami hUer-transmitter interference may adversely impact channel estimation, which may then degrade data performance. (0006) There is therefore a need in the art for techniques to transmit pilot in a wireless communication system

SUMMARY

}8OΘ7| According to an aspect, an apparatus is described which generates a plurality of pilot sequences for a plurality of transmit antennas, with each pilot sequence comprising a plurality of pilot sy mbols sent in the time domain on a different set of subcarriers, The apparatus further geneiates a plurality of pilot transmissions for the plurality of transmit antennas based on the plurality of pilot sequences.

[OOΘSj According to another aspect, an apparatus is described which generates a plurality of pilot sequences for a plurality of transmit antennas based on frequency- domain code division multiplexing (FD-CDM) of a constant amplitude zero autocorrelation <CΛZΛC) sequence such as a Chu sequence defined by a transmitter- specific value. The apparatus further generates a plurality of pilot transmissions for the plurality of transmit antennas based on the plurality of pilot sequences

[0009} According to yet another aspect, an apparatus is described which receives a plurality of pilot transmissions via a plurality of receive antennas, with each pilot transmission comprising a plurality of pilot symbols sent in the time domain on a different set of subcatriers. The apparatus processes the plurality of received pilot transmissions to obtain channel estimates.

(OOtOJ According to yet another aspect, an apparatus is described which receives a plurality of pilot transmissions via a plurality of receive antennas, with the pilot ttansmisskms being generated based on FD-CDM of a CAZAC sequence such as a Ch« sequence defined by & transmitter-specific value. The apparatus processes the plurality of received pilot transmissions to obtain channel estimates,

[001.1.! According to yet another aspect, an apparatus is described which generates a. plurality of pilot transmissions for a plurality of transmit antennas based on a first multiplexing scheme. The apparatus further generates a plurality of data transmissions for the plurality of transmit antennas based on a second multiplexing scheme that is different from the first, multiplexing scheme.

|0ϋi2| According to yet another aspect, an apparatus is described which receives a plurality of pilot transmissions generated based on a first multiplexing scheme. The apparatus further receives a plurality of data transmissions generated based on a second multiplexing scheme that is different from the first multiplexing scheme. The plurality of pilot transmissions and the plurality of data transmissions are for a MIMO transmission sent from multiple transmit antennas to multiple receive antennas. The multiple transmit antennas may be located at a single transmitter station or multiple transmitter stations.

J0013J Various aspects and features of the disclosure are described in further detail below.

BRIEF DESCRIPTION OF THE PRAWIKGS i_.0014j FIG-. I shows a wireless multiple-access communication system.

{0015! FIG. 2 shows a block diagram of a base station and a terminal.

[0036j FIGS. 3 A and 3B show two interleaved frequency division multiplexing (IFDM) pilot subcamer structures.

(0017J FIGS. 4 and 5 show two processes for generating an ΪFDM pilot.

[001$! FIG. 6 shows pilot transmissions from 4 transmit antennas for an FD-CDM pilot.

[001 ?! FIGS. 7 and 8 show two processes for generating the FD-CDM pilot.

[0020| HG. 9 shows a process for sending pilot and data with different multiplexing schemes.

DETAILED DESCRIPTION

[0021 j The techniques described herein may be used for various wireless communication systems such as multiple-access communication systems, broadcast systems, wireless local area networks (WLANs). etc. The terms "systems" and "networks" are often used interchangeably. The multiple-access systems may be code division multiple access (CDMA) systems, time division multiple access (TDIVlA) systems, frequency division multiple access (FDMA.) systems, orthogonal FDMA (OFDMA) systems, single-carrier FDlSlA (SC-FDMA) systems., spatial division multiple access (SDMA) systems, etc. The techniques may also be used for systems that employ different multiple access schemes for the downlink and uplink, e.g., OFDMA for the downlink and SC-FDMA for the uplink, The downlink (or forward link.) refers to the communication link from the base sfatloRS to the terminals, and the uplink (or reverse link) refers to the communication link from the terminals to the base stations.

{OO22| An OFDlVlA system utilises orthogonal frequency division multiplexing

(OFDM). An S€-FDMA system utilizes single-carrier frequency division multiplexing (SC-FDM), OFDM and SC-FDM partition the system bandwidth into multiple (K) orthogonal subcarπers, which are also commonly referred to as tones, bins, etc, Each subearrier may be modulated with data. In general, symbols are seat m the frequency domain with OFDM and m the time domain with SC-FDMl SC-FDM includes (a) IFDM. which transmits information on subcarriers that are uniformly distributed across a given frequency allocation and (b) localized frequency division multiplexing (LFDy) which transmits information on adjacent subcamers.

J0G23J FJC I shows a wireless multiple-access communication system 100 with multiple base stations 110. A base station is generally a fixed station that communicates with the terminals and may also be referred to as a Node B, an enhanced Node B (eNbde B)₁ an access point, etc. Each base station 110 provides communication coverage for a particular geographic area. The term "cell" can refer to a base station and/or its coverage area depending on the context, in which the term is used. To improve system capacity, a base station cox'erage area may be partitioned into multiple smaller areas, e.g., three smaller areas. Each smaller area may be served by a respective base transceiver station (BTS)₁ The term "sector" can refer to a BTS and/or its coverage area depending on the context m which the term is used. For a sectorized cell, the BTSs for all sectors of that cell are typically co-located within the base station for the cell.

[0024J Terminals 120 may be dispersed throughout, the system. A termmal may be stationary or mobile and may also be referred to as a user equipment, a mobile station,, a mobile equipment, an access terminal, a station, etc. A terminal may be a cellular phone, a personal digital assistant (FDA)₅ a wireless modem, a wireless communication device, a handheld device, a subscriber unit, a laptop computer, a cordless phone, etc.

JOO25| A system controller 130 may couple to base stations N O and provide coordination and control for these base stations. System controller 130 may be a single network entity or a collection of network entities.

{0026} FlG_* 2 shows a block, diagram of a design of a base station 1 10 and a terminal

120 in. system 100. Base station H O is equipped with multiple (U) antennas 220a through 22Ou₃ which may be used for data transmission on the downlink and data reception on the uplink. Terminal .120 is equipped with .multiple (V) antennas 152a through 152v, which may be used for data transmission on the uplink and data reception on the downlink. Each antenna may be a. physical antenna or an antenna array.

J0G27J On the downlink, at base station 110, a transmit ( TX) data and pilot processor

21.4 receives data from a data source 212, processes (e,g., formats, encodes, interleaves, and symbol maps) the data, and generates data symbols. Processor 214 also generates pilot symbols as described below and provides the pilot and data symbols to a TX spatial processor 216. As used herein, a data symbol is a symbol for data, a pilot symbol is a symbol for pilot, a zero symbol is a signal value of zero, and a symbol is typically a complex value. The data symbols may be .modulation symbols from a modulation, scheme mch as FSK or QAM. Pilot is data that is known a priori by both the transmitter and receiver stations. Processor 216 multiplexes the pilot and data symbols, performs transmitter spatial mapping (if applicable), and provides U output symbol streams to U modulators (MOD) 218a through 218u. Each modulator 218 performs modulation (e.g., for OFDM₅ SC-FDM₅ etc.) on its output symbol stream to generate output chips and further processes (e.g., converts from digital to analog, analog Biters, amplifies, and upcoiϊverts) the output chips to generate a downlink signal, ϋ downlink signals from modulators 218a through 21Su are transmitted via U antennas 220a through 22Ou₅ respectively.

(002Sj At terminal 120, V antennas 252a through 252v receive the U downlink signals, and each antenna 252 provides a received signal to a respective demodulator (DEMOD) 254, Each demodulator 254 processes (e.g., filters, amplifies, downconverts, and digitizes) its received signal to obtain samples and further performs demodulation (e.g., for OFDM₅ SC-FDM₅ etc.) on the samples to obtain received symbols. Each demodulator 254 provides received data symbols to a MIMO detector 256 and provides received pilot symbols to a channel processor 284« Channel processor 284 estimates the downlink MIMO channel response based on the received pilot symbols arid provides channel estimates to MΪMO detector 256. MIMO detector 256 performs MIMO detection on the received data, symbols with the channel estimates and provides data, symbol estimates. Au RX data, processor 258 further processes (e.g., defo.terieaves and decodes) the data symbol estimates and provides decoded data to a data sink 260.

J0G29J On the uplink, at terminal 120, data from a data source 272 and pilot are processed by a TX data and pilot processor 274, further processed by a TX spatial processor 276, and modulated and processed by modulators 254a through 254v to generate V uplink signals, which are transmitted via V antennas 252a through 252 v\ At base station 110, the uplink signals are received by U antennas 220a through 22Ou, processed and demodulated by demodulators 218a through 218u_? detected by a MIMO detector 232, and further processed by an RX data processor 234 to recover the data sent by terminal 120. A channel processor 244 estimates the uplink MIMO channel response based on the received pilot symbols and provides channel estimates to MIlVlO detector 232 for MIMO detection.

J0O30J Controllers/processors 240 and 280 control the operation at base station i 10 and terminal 1.20, respectively. Memories 242 and 2S2 store data aad program codes for base station 110 and terminal 120, respectively.

{Θ031.J The techniques described herein may be used with various subcarrier structures.

The following description assumes that K total subcarriers SZQ available for transmission and are assigned indices of 0 through K-I.

{0032] FϊC 3A. shows an IFDM pilot subcarrier structure 300 that may be used for

JFDM or distributed OFDM data transmission. In subcamer structure 300.. the K total subcarriers are arranged ύtto T disjoint or αoix-ovedappitig sets such that each set contains L' subcarriers that are uniformly distributed across the K total subcarriers, where T and L' are suitably selected integer values, Consecutive subcarriers in each set are spaced apart by T subcarriers, where K ~ T - L' . Hence, set i contains subcarriers i, T -ri, 2T + 7, ..„, (L' -O- T + Λ for * <~ {G, ..., T-I ) .

J0033J FlG_* 3B shows an IFDM pilot subcarrier structure 310 lhat may be used for

LFDM or localized OFDM data transmission. In subcarrier structure 310, the K total subcarriers are arranged into G disjoint groups such that each group contains N^* - K /O consecutive subcarriers, where N" and Q are suitably selected integer values. Thus, group 0 includes subcarrier 0 through N" ~- i, group .1 includes subcamers N" through 2N" — 1L and so on. and group G-- 1 includes subcarriers K-N" through K - I. . f0034J The N^* subcamers in each group may be arranged into T disjoint sets such, that each set contains L" subcarriers that are uniformly distributed across the N" subcamers in that group, where N* — LΛT . The N" subcarriers in each group may thus be arranged in similar manner as described above in. FIXl 3 A. FiQ. 3B shows the T sets of subcarriers for subcarrier group 1.

|0035i *n general, any subcarrier structure may be used for pilot and data transmission on the downlink and uplink. For example, subcarrier structure 300 may be used for the downlink, and subcarrier structure 310 may be used for the uplink. Other subcarrier structures may also be used. On each HnIc, pilot and data may be sent using the same or different subcarrier structures.

}^'0036| A transmitter station may transmit pilot via multiple (T) transmit antennas using various multiplexing schemes such as time division multiplexing (TDMX time-domain code division multiplexing (TD-CDM), OFDM, IFDM, FD-CDM, etc. A .receiver station may receive the pilot via multiple (R) receive antennas and may estimate the MIMO channel response as well as background noise and interference based on the received pilot. For the downlink, the transmitter station may be base station 110, the receiver staiiorx may be terminal .120, T .may be equal to U, and R may be equal to V. For the uplink, the transmitter station may be terminal 120, the receiver station may be base station 110, T may be equal to V, and R may be equal to U. Pilot for a MIMO transmission may comprise a different pilot sequence for each of the T transmit antennas. A pilot sequence is a sequence of known symbols that may be sent in the time domain or frequency domain depending on the .multiplexing scheme used for the pilot.

J0037) For TDM pilot, a time interval designated for pilot may be partitioned into T time segments that may be assigned to the T transmit antennas. The transmitter station may send a pilot transmission from each antenna in the time segment assigned to that, antenna. The piiot. transmission from each antenna may be any pilot, sequence and may be appended with a cyclic prefix, to combat frequency selective fading caused, by delay spread in a multipara channel. A cyclic prefix is also referred to as a guard interval, a preamble, etc. The cyclic prefix length may be selected based on the expected delay spread. A unique word may also be used in place of the cyclic prefix. The receiver station may estimate the MtMO channel response and noise using time-domain RAKE processing (which is commonly used in CDMA systems) or frequency-domain processing. Noise estimation may be trivial since pilot, is sent from only one transmit antenna in any given time segment and there is no interference from other transmit antennas. Inter-transmitter pilot interference from other transmitter stations may be suppressed by using different pilot scrambling sequences for different transmitter stations.

J003SJ For TD-CDM pilot, T different orthogonal sequences may be assigned to the T transmit antennas and used to achieve orthogonality in the time domain. The transmitter station may generate a time-domain pilot sequence for each transmit antenna by multiplying a time-domain base sequence with the orthogonal sequence for that antenna. The transmitter station may then generate a piiot transmission for each transmit antenna based on its time-domain pilot sequence. The pilot transmission from each transmit antenna may not observe multipath interference due to data streams but may observe imiliipaih interference due to pilot transmissions from other transmit antennas. The receiver station may estimate the MIMO channel response using time-domain RAKE processing, which can exploit the orthogonality among the T pilot transmissions due to use of the orthogonal sequences assigned to the T transmit antennas. The receiver station may estimate noise without interference from the observed data streams. Inter- transϊϊiitter pilot interference may be suppressed by using different piiot. scrambling sequences for different transmitter stations.

J0039J For OFDM and IFDM pilots. N subcaπiers may be used for pilot transmission and may be arranged into T disjoint sets, e.g., as shown in FIG. 3 A or 3B, with each set including L subcarriers, where N = T- L < K . In FlG. 3 A, N may be equal to K, and L may be equal to V . in FIG. 3B, N may be equal to N'', and L may be equal to L*. In any case, the L subcarriers in each set may be uniformly distributed across the TSf sub carriers to allow the receiver station to sample the frequency spectrum across all of the N subcarriers, which may improve channel and noise estimation performance. Each of the T transmit antennas may be assigned a different one of the T subcarrier sets.

{0040} For OFDM pilot, the transmitter station may send a pilot transmission from each transmit antenna in the frequency domain on a set of L suhc&rriers assigned to that antenna. For each transmit antenna, the transmitter station may map L pilot symbols to L subcarriers in the assigned set, map zero symbols to the remaining K ~ L subcarriers, and generate an OFDM symbol based on the mapped pilot and zero symbols. The T pilot transmissions from the T transmit antennas occupy different subcarriers and are thus orthogonal in frequency. The receiver station may perform channel and noise estimation based on received pilot symbols using frequency-domain processing. The channel and noise estimation does not suffer from inter-antenna interference since orthogonality is achieved among the T pilot transmissions, However, a drawback with OFDM, is high peak-to-average power ratio (PAPR), which means thai the ratio of the peak power to the average power of an OFDM waveform can be high in the time domain. The pilot symbols used for each transmit antenna, may be generated or selected such that the PAPR is as low as possible. Inter-transmitter interference may be mitigated by proper pilot planning, frequency hopping, etc.

|004:i.| For IFDM. pilot, the transmitter station may send a pilot transmission from each transmit antenna in the time domain on a set of L subcarriers assigned to that antenna. For each transmit, antenna, the transmitter station may transform L pilot symbols from the time domain to the frequency domain, map the L transformed symbols to L subcarriers in the assigned set₆ map zero symbols to the remaining K ^•- L- subcarriers. and generate an ΪFDM symbol based on the mapped transformed and zero symbols. The T pilot transmissions from the T transmit antennas occupy different subcarriers and are thus orthogonal in frequency. The receiver station may perform channel and noise estimation based on received pilot symbols using frequency-domain processing. The channel and noise estimation does not suffer from inter-antenna interference since orthogonality is achieved among the T pilot, transmissions. Furthermore, high PAPR may be avoided by using pilot symbols with constant amplitude in the time domain. Good channel estimation performance may be achieved by properly generating the pilot symbols as described below. Inter-sector interference may be mitigated by proper pilot planning, frequency hopping, etc.

J0042J For FD-CDM pilot, T different orthogonal sequences may be assigned Io the T transmit antennas and used to achieve orthogonality in the frequency domain. The transmitter station may generate a frequency-domain pilot sequence for each transmit antenna by multiplying a frequency-domain base sequence with the orthogonal sequence for that antenna. The transmitter station may then generate a pilot transmission, for each transmit antenna based on Its frequency-domain pilot sequence. The T pilot transmissions from the T transmit antennas may be nearly orthogonal in a multipath channel due to use of different orthogonal sequences. The receiver station may perform channel and noise estimation based on received pilot symbols using frequency-domain processing, e.g., in similar manner as for the OFDM and IFDM pilots. [0043} Several multiplexing schemes for pilot, are described in further detail below.

1. IFDM pilot

(0044J An IFDM pilot may be sent from the T transmit antennas on T disjoint sets of subcarriers, e.g., as shown in FIG. 3A or 3B. one set of L subcamers for each transmit antenna. The IFDM piiot may be generated with a base sequence having good properties. For example the base sequence may be selected to have good temporal characteristics (e.g., a constant time-domain envelope) and good spectral characteristics (e.g., a fiat frequency spectrum). These good temporal and spectral characteristics may be obtained with various CAZAC (constant amplitude zero auto-correlation) sequences. Some example CAZAC sequences include a Chu sequence, a Frank sequence, a generalized chirp-like (GCI.,) sequence, a Golomb sequence, PL P3_S P4 and Px sequences, etc.

[004Sj ϊn one design, a Chu sequence C₁ (n) of length L is used as the base sequence for the WDM. pilot This Chu sequence may be expressed as:

C₁. (») ™ e ^>'- , for ft ~ 0, .,.» L- I with even L ₅ Eq (1)

c_L (n) - e ^l , for « ~ 0, ..., L - 1 with, odd L , Eq (2)

where λ is a frequency increment index that is selected such, that λ and L are relatively prime and have a greatest common denominator of L L is the base sequence length and may correspond to the number of subcarriers assigned to each transmit antenna for pilot transmission, L may be a prime number (e.g., L - 257 ), which may provide good cross-correlation properties for Chu sequences generated with L- J different values of λ. L may also be selected based on the number of subcarriers used for pilot transmission by each transmit, antenna (e.g., L ~ 256). π

J00461 .In equations (i) and (2), X may be used as a transmitter-specific value or code to distinguish the pilots from different transmitter stations, as described below. A set of values may be determined for X based on the sequence length. L. For example, the set may include X values of .1 , 2, 3,. 4, 5, and 6 for sequence length of L - 7. Different X values .may be assigned to different transmitter stations, e.g., different base stations oκ the downlink or different terminals on the uplink. Since two base sequences generated with different X values have minimal cross-correlation if the difference between the two X values are relatively prime to L, pilots sent by different transmitter stations with different. X values minimally interfere with one another in such a case.

JOO47| The Chυ sequence has a. constant time-domain envelope, which results Jn low

PAPR for the pilot. The Chu sequence also has & flat frequency spectrum, which may improve channel estimation performance especially wtien the distribution, of the channel spectral density is not known. l^'004S| In another design, an L-pomt inverse discrete Fourier transform (IDFT) is performed on the Chu. sequence c_L(«) to obtain a transformed sequence C_L(k) with L symbols. The transformed sequence is then used as the base sequence.

J0049J In yet another design, a pseudo-random number (PN) sequence /?«(») having good auto-correlation and cross-correlation properties and low PAPR properties in the time domain Is used as the base sequence. The PN sequence may be derived in any manner known in the art, e.g., based on a polynomial generator or with an exhaustive search of all possible sequences of length L. Other sequences may also be used as the base sequence. jOOSβj The IFDM pilot for the T transmit antennas may be generated in various manners. In one scheme, the base sequence is replicated T times and concatenated to obtain an extended base sequence, as follows:

T" i

M«⁾ -!>,(* -^) Eq O)

where b_L(μ —hϊS) is abase sequence that is delayed by ? - L samples, and δ_ήff(«) is an extended base sequence of length N.

} 005.11 The base sequence &_L(n) of length L may be equal to (a) the Chu sequence so that 6_J O?) ~ C_J (», (b) the PN sequence so that b_L(^ή) - pn(ri) , or (c) some ofner sequence. In equation (3), the T copies of the base sequence b_L(n) are delayed and arranged such that the start of the /-th sequence follows immediately after the end of the (M)-th sequence. The T delayed sequences are summed to obtain the extended base sequence h_m(n) of length N.

[00521 A pilot sequence may be generated for each transmit antenna as follows:

,2x i »

Mn) _^ e * . *.„,(») , for « - 0, ... , N - I , Eq (4)

where P((n) is the pilot, sequence for transmit antenna /. Equation (4) applies a linear phase ramp to the N samples in the extended base sequence. The slope of the phase ramp Is different for different transmit, antennas.

}^'θøS3J The base sequence &_h(n) contains L time-domain samples and occupies L consecutive subcarriers. The replication of the base sequence T times results in the extended base sequence £_ΛW(») occupying every T-th subcarriers in the frequency domain, with zeros for the T - ] subcarriers between consecutive occupied subcarriers. The multiplication with _e&*^miH _m - _eq_Uaϊj_on ^ effectively shifts the pilot sequence for transmit antenna / by / subcarriers in the frequency domain. The T pilot sequences for the T antennas are shifted by different numbers of subcarriers and are thus orthogonal in the frequency domain, with each pilot sequence occupying a different set of L subcarriers, e.g., as shown m FlG. 3 A or 3B.

{ΘO54| FKJ_* 4 shows a process 400 for generating the LFDM pilot. A plurality of pilot sequences are generated for a plurality of transmit antennas, with each pilot sequence comprising a plurality of pilot symbols sent in the lime domain on a different set of subcarriers (block 410). The plurality of pilot sequences may be generated based on a Chu sequence with Ji—^'l, a Chu sequence defined by a transmitter-specific λ value, some other CAZAC sequence, a FN sequence, etc. A plurality of pilot transmissions are generated based on the plurality of pilot sequences (block 420). fOOSSi FIG. 5 shows a process 500 for generating the IFDM pilot. Process 500 includes blocks 5.10 and 520 that correspond to blocks 410 &nύ 420, respectively, in FIG. 4. A base sequence of length L (e.g., a Chu sequence, an IDFI" of the Chu sequence, a PN sequence, etc.) is initially generated (block 512). An extended base sequence of length N is then generated by replicating and concatenating .multiple (T) copies of the base sequence (block 514). A pilot sequence is generated for each transmit antenna by applying a different phase ramp to the extended base sequence, e.g., as shown in equation (4) (block 516). A pilot transmission of length N+C may be generated .for each transmit antenna by appending a cyclic prefix of length C to the pilot, sequence for tliat antenna (block 520), The cyclic prefix insertion is achieved by copying the last C samples of the pilot sequence axid appending these C samples to the start of the pilot sequence. The pilot transmission may also be generated in other .manners based on the pilot seqiietice, e.g., the pilot sequence may be provided directly as the pilot transmission without any cyclic prefix.

[0056] In another scheme for generating the IFDM pilot for the T transmit antennas, which may be used for any sυhcarrier structure including the ones shown in FIGS. 3A and 3S₅ a time-domain base sequence with .L pilot symbols (e.g., a Chu sequence) is initially generated. An L-point discrete Fourier transform (DFT) is then performed on the time-domain base sequence to obtain a frequency -domain base sequence with L transformed symbols. For each transmit antenna, the L transformed symbols are mapped to the L sυbcarrkrs assigned to that antenna, and N - L zero symbols are mapped to the remaining subcarøers. An N-poiut IDPT Is then performed on the N transformed symbols and zero symbols to obtain a time-domain pilot, sequence with N samples. A cyclic prefix may be appended to this pilot sequence to obtain the pilot transmission for the transmit antenna. The IFDM pilot for the T transmit antennas may also be generated in other manners.

|00571 In general, a pilot sequence or a pilot transmission may be generated by determining the symbols or samples for the pilot sequence/transmission based on appropriate equations, e.g., as described above. A pilot sequence or a pilot transmission may also be pre-corøputed and stored in memory, ϊn this case, the pilot sequence or piϊot transmission may he generated, whenever needed, by simply reading from the memory. Thus, the term "generate" may include any action (e.g.. computation, memory retrieval, etc.) to obtain the pilot sequence or pilot transmission. fOOSSJ For the IFDM pilot, the T pilot transmissions from the T transmit antennas are disjoint in frequency sad are thus orthogonal in a multipath channel. The PAPE. is low if a pilot sequence with constant envelope in the time domain is used. Furthermore, the pilot energy is uniformly distributed in frequency if a CAZAC sequence such as a Chu sequence is used., which may simplify channel and noise estimation while providing good performance.

2- FP-CPMi pilot fϋO59j An FD-CDM pilot, may be sent from the T transmit antennas on the same set of

N sυbcarriers, However, the pilot transmission from each antenna is multiplied in the frequency domain with a different orthogonal sequence. The FD-CDM pilot .may be generated with a base sequence having good characteristics. f006ø| In one design, a Chu sequence c^(ri) of length N is used as a time-domain base sequence for the FD-CDM pilot. This Chu sequence, for even N, may be expressed as:

£0061 | An N-point ΪDFT m&y be performed on the Chu sequence %(#) to obtain a transformed Chu sequence C_H(k) with N symbols. The transformed Chu sequence may be used as a frequency-domain base sequence B^(k) . In another design, the Chu sequence c-_$(n) is used directly as the frequency-domain base sequence. En yet another design, a PN sequence PN (k) of length N is used as the frequency-domain base sequence. Other sequences may also be used as the base sequence. Jn general, the frequency-domain base sequence B_%(k) of length N may be equal to (a) the Chu sequence so that B_$(k) - <%(«} where n - k „ (b) the transformed Chu sequence so that J^_Jf(Ar) = C₈(A-), (c) the PN sequence so that B_n(k) - PN (k) , or (d) some other sequence.

[0062] The ED-CDM pilot for the T transmit, antennas may be generated m various manners. In one scheme, a frequency-domain pilot sequence may be generated for each transmit antenna as follows;

where W.{k) is an orthogonal sequence for transmit antenna, i, and

Pi(Ic) is a frequency-domain pilot sequence for transmit antenna L }ϋø<>3| la general, various orthogonal sequences may be used for fV._:(k) in equation (6).

For example, the orthogonal sequences may be Walsh sequences from a Hadamard matrix, sequences from a Fourier matrix, etc. The orthogonal sequences may also have any length, that ϊ$ equal to or longer than T and is aα integer divisor of N. In one design, the orthogonal sequences are defined as follows:

»;<*) «^■. «^{' "■"} , for & .^ 0, ..,, N~ i and / - 0, .,.,T-I . Eq (7)

j0064{ T orthogonal sequences may be generated based on equation (7) for

?^' ~ G, ...,T - I . These orthogonal sequences have length of N but are periodic over T and thus repeat every T symbols. The use of these orthogonal sequences does not increase the time-domain PAPR nor the frequency-domain PAPR, which is desirable.

}0065| The frequency-domain pilot sequence for each transmit antenna may then be generated as follows:

- B,_;(k) , for k ~ 0, . „, N - i . Eq (S)

J0066J Equation (8) essentially modulates the frequency-domain base sequence with the orthogonal sequence, which is at. a different frequency for each transmit antenna. It can be shown that modulating the frequency-domain base sequence by eJ^ZπU:rx is equivalent to cyclically shifting the corresponding time-domain base sequence by L -/ samples. The time-domain pilot sequence for each transmit antenna may then be generated as follows:

P&tϊ- bvdn -hTL) mod N) , for « ^ 0,..., ^'N - 1 , Eq (9)

where £_κ(ff) is a time-domain base sequence of length M, and

P₁(Tt) h a time-domain pilot sequence for transmit antenna /.

10067| The time-domain base sequence k^ζn) may he equal to (a) tlie Chu sequence so that fr_H (h) - %(«) _> (b) *.e FN sequence so that b_n(») ~ pn(fi) _^ or (c) some other sequence. The cyclic shifting in equation (9) is achieved by taking the last L-i samples of the time-domain base sequence and appending these L-j samples to the start of the base sequence. Different numbers of samples are cyclically shifted for different transmit antennas. 1« particular, O samples are cyclically shifted .for transmit antenna 0, L samples are cyclically shifted tor transmit antenna I- and so on, and (T - J) - L samples are cyclicaHy shifted for transmit antenna T - 1 .

{0068} FIG. 6 shows example pilot sequences and pilot transmissions for T ^■■■■^■■ 4 transmit antennas for the FD-CDM pilot. The pilot sequence for transmit antenna 0 is equal to the base sequence b_u («) . The pilot sequence for transmit antenna ϊ is equai to the base sequence cyclically shifted by L samples. The pilot sequence for transmit antenna 2 is equal to the base sequence cyclically shifted by 2L samples. The pilot sequence for transmit, antenna 3 is equal to the base sequence cyclically shifted by 3L samples. The pilot transmission for each transmit antenna is generated by appending a cyclic prefix to the pilot sequence for that transmit antenna, iW$9\ FfG_* 7 shows a process 700 for generating the FD-CDM pilot. A plurality of pilot sequences are generated for a plurality of transmit antennas based on FD-CDM of a h&&& sequence,, e.g., a CAZAC sequence such as a Chu sequence defined by a transmitter-specific & value (block 710). A plurality of pilot transmissions are generated based on the plurality of pilot sequences (block 720), The pilot transmissions may be sent on the downlink, and neighboring base stations may be assigned different transmitter-specific values. The pilot transmissions may also be sent on the uplink, and ditYerent terminals may be assigned different transmitter-specific values.

(007«{ FiG. 8 shows a process SOO for generating the FD-CDM pilot, Process 800 includes blocks 810 and 820 that correspond to blocks 710 and 720_> respectively, m FlG. 7, A. time-domain base sequence of length N (e,g., a Chu sequence defined by a transmitter-specific value, a PN sequence, etc.) is initially generated (block S12). A time-domain pilot sequence for each transmit antenna / is then generated by cyclically shifting the time-domain base sequence by Iw samples (block S 1.4). The cyclic shifting in the time domain achieves the frequency -domain multiplication with the orthogonal sequence shown m equation (7). A. pilot transmission of length N rC may be generated for each transmit antenna by appending a cyclic prefix of length C to the time-domain pilot sequence for that antenna (block 820).

{0071 { In another scheme for generating the FD-CDM pilot for the T transmit antennas, which may be used with any orthogonal sequences and for any subcarrier structure, a time-domaio base sequence of length N (e.g.., a Chu sequence defined by a transmitter- specific value) is initially generated aτtd transformed with an H-point DFT to ob&m a frequency-domain base sequence_* For each transmit antenna, the frequency-domain base sequence is multiplied with an orthogonal sequence assigned to that antenna to obtain an intermediate sequence. An N-point IDFT is then performed on the intermediate sequence to obtain a time-domain pilot sequence of length N, A cyclic prefix may be appended to the time-domain pilot sequence to obtain the pilot transmission for the transmit antenna. The FD-CDM pilot for the T transmit antennas may also be generated in other manners.

[00721 For both the IFDM and FD-CDM pilots with the Chu sequences, different values of X may be assigned to different transmitter stations to reduce pilot interference and assist the receiver stations acquire pilots from the different transmitter stations. On the downlinks different λ values may be assigned to neighboring base stations or BTSs, one X value for each base station or BTS. Each base station or BTS may generate U pilot transmissions for its U antennas with the assigned Λ value, e,g., as described above. A terminal may receive pilot transmissions from multiple base stations and may be able to detect and distinguish the pilot transmissions from each base station based on the λ value assigned to that base station or BTS. On the uplink, different, λ values may be assigned to different terminals that may simultaneously sent pilot transmissions to the same base station or BTS, one X value for each terminal. Each terminal niay generate V pilot transmissions for its V antennas with the assigned λ value, e.g., as described above. The base station may receive pilot transmissions from multiple terminals and may be able to detect and distinguish the pilot transmissions from each terminal based on the X value assigned to that terminal.

|0073| Lt is desirable for the pilot sequences from different transmitter stations (e.g., different base stations on the downlink or different terminals on the uplink) to have as low cross-correlation as possible. Pilot sequences of length L for the IFDM pilot or length N for the FD-CDM pilot .may be generated with different X values. The cross- correlation between these pilot sequences may be determined for different time shifts, A set of X values with small cross-correlation between, their pilot sequences may be selected for use,

(0074} Different λ values may also be used to support, spatial division multiplexing

(SDM) on the uplink, For example, multiple terminals transmitting simultaneously to a given base station may be assigned different λ values. Each terminal may generate its pilot transmissions based oo its assigned λ value. Alternatively, the multiple terminals transmitting simultaneously to the base station may be assigned the same λ value but different orthogonal sequences or cyclic shifts. Each terminal may generate its pilot transmissions based on the common λ value and its assigned orthogonal sequences or cyclic shifts,

3. Pilot ami Data Multifitextng Schemes

[Q075J In general, the transmitter station may achieve orthogonality between pilot and date using TDM, FDM, etc. For TDM, the transmitter station may send pilot in some time intervals and send data in other time intervals. For F.DM, the transmitter station may sead pilot on some subcaoiers and send data on other sυbcarriers, The transmitter station, may achieve orthogonality between the pilot transmissions from the T transmit antennas using any of the multiplexing schemes described above. The transmitter station may send pilot from T transmit antennas using a first multiplexing scheme and may send data from the T antennas using a second multiplexing .scheme, In general, the first multiplexing scheme may be the same as or different from the second multiplexing scheme.

}0076{ FKx. 9 shows a process 900 for sending pilot and data with different

.multiplexing schemes. A plurality of pilot transmissions for a plurality of transmit antennas are generated based on a .first multiplexing scheme (block 912). A plurality of data transmissions for the plurality of transmit antennas are generated based on a second multiplexing scheme that, is different from the first multiplexing scheme (block 914). The plurality of pilot transmissions may be sent hi a first time interval, and the plurality of data transmissions may he sent in a second time interval with TDM (block 916). The plurality of pilot transmissions .may also be sent on a first set of subcaπiers, and the plurality of data transmissions may be sent on a second set of subcarriers with FDM,

(0077} The first multiplexing scheme may be OFDM, and the second multiplexing scheme may be SC-FDM (e.g., IFDM or LFDM), TD-CDM, SDM₅ etc. The first multiplexing scheme may be SC-FDM (e.g., IFDM)₁ and the second multiplexing scheme may be OFDM, TD-CDM, SDM, etc. The first multiplexing scheme may be FD-CDM, and the second multiplexing scheme may be OFDM, SC-FDM, TD-CDM. SDM, etc. The first and second multiplexing schemes may also be other combinations of multiplexing schemes. f0078J The first multiplexing scheme may be selected to reduce pilot overhead while achieving good channel and noise estimation performance for MIMO transmission. The second multiplexing scheme may be selected to achieve good performance for data transmission among different streams of a single terminal or among different terminals. Different multiplexing schemes for pilot and data may be readily supported using frequency-domain processing for channel estimation and data detection, as described below.

4. Clhmiael_nEstlniatjo«

J0079] The receiver station may receive pilot transmissions from the transmitter station and may perform channel estimation in various manners based on the received pilot transmissions. Channel estimation may be performed in different manners for different pilot multiplexing schemes. Several example channel estimation techniques are described below.

[00801 For the IFDM pilot, the receiver station may obtain R received pilot transmissions via the R receive antennas and may remove the cyclic prefix in each received pilot transmission to obtain N time-domain samples. The receiver station may then transform the N time-domain samples for each receive antenna, with an M-poiπt DFT to obtain N received symbols for the N subcarriers used for the IFDM pilot The received symbols from each receive antenna may be expressed as:

0, ..., N -- i , Eq (IO)

where /J{&) is a transmitted symbol from transmit antenna * on subcarrier k_t

H^". .(A-) is a complex channel gain from transmit antenna / to receive antenna. J on subcarrier k, R,(k) is a received symbol from receive antenna/ on subcarrier k, and

N ,.(A-) is noise for receive antenna j on subcarrier L

P₁Ik) is a frequency-domain pilot sequence that, may be obtained by performing an N- pomt DFT on the time-domain pilot sequence /?,(«) for transmit antenna i. |øG81J As shown ϊn equation (J.0), the received symbol R_j(k) from receive antenna ,/ is composed of the sum of the T transmitted symbols P_f(k) weighted by the channel gains H_KJ(k) between die T transmit antennas and receive antenna_,/. The received symbol /?,(A) is further degraded by noise N ,.(k) . For the 3FDM pilot, each transmit antenna i is assigned a different subset of the N subcaπϊers. Hence, the transmitted symbols P_f(k) from transmit antenna i are non-zero for only L subcamers assigned to antenna /.

{0OS2J In one design, the channel gains axe estimated based on least-squares technique, as follows:

where H_1. ,(A--T i- i) is a channel, gain estimate between transmit antenna / and receive antenna / for subcarrier fe -T-f/, which is an estimate of J7_; ,.(# - T 4- Ϊ) . Since each transmit antenna is assigned a different set of L siibcarriers, equation (11) derives the channel gain estimates for each transmit antenna / by dividing the received symbols from the L subcarriens assigned to antenna / by the transmitted symbols from antenna /. |00S3| In another design., the channel gains are estimated based on minimum mean- square error (MMBB) technique, as follows:

where <τjl (A- -T + /) is tϊie variance of the noise N_f{k - T + f) for stibcarrier k-T + i . For the €hu sequence, \ P_f(k-Υ + i) \ ² ~ ϊ and the denominator in equation. (12) may be replaced with .1 + <r£(& - T-+ /) .

|0084| A channel gain estimate may be derived based on equation (I l ) or (J 2) or some other equation for each subcarπεr k of each pair of transmit antenna / and receive antenna/ T - R. sets of channel gam estimates may be obtained for a.U T Itammit antennas arid R receive antennas, one set for each transmit-receive antenna pair, with each set including L channel gain estimates for L subcarriers. Each set of channel gain estimates may be transformed with an L-ρoint IDFX to obtain a corresponding channel impulse response estimate with L taps, as follows:

k._>;ii) ~ lOΨΥ {H_u .(k)} , Bq (13)

where \_?{f) is a channel impulse response estimate between transmit, antenna / and receive antenna_,/. The channel impulse response estimate may also be obtained from the channel gain estimates using least-squares, MMSE, robust MMSE. or some other technique known in the art.

JOOSSj Various types of post-processing such as truncation, thresholding, tap selection, etc, may be performed on the L channel taps of each channel impulse response estimate. For truncation, the first Q channel taps are retained and the remaining L ^■-^■ Q channel laps are zeroed out, where Q may be selected based on the expected delay spread of the wireless channel For thresholding, channel taps with magnitude below a threshold are .zeroed out, where the threshold may be a fixed vaiue or a particular percentage of the total energy of ail L channel taps. For tap selection, B best channel taps are retained and all other channel taps are zeroed out, where B may be a fixed value or a configurable value determined based on SNR, etc.

}0086| After completing the post-processing, the L-iap channel impulse response estimate for each transniit-receive antenna pair may be padded with N ^■- L zeros. An N-pomt DFT may then be performed on the zero-padded channel impulse response estimate to obtain N channel gain estimates for the M sufacarriers of the transmit-receive antenna pair. The channel gain estimates may be used for MIMO detection of received data symbols and/or other purposes

JQ0S7J For the FD-CDM pilot, the received symbols from each receive antenna may be expressed as".

fl(k) - 2 P₁(Jc) Η_u (JC) + N,{ky . for * = 0, .... N - 1. Eq (14)

;~0

where M Jk) is a received symbol from receive antenna./ on subc&rrier k,

|008S| In one design, tlie channel gains are estimated based on the leastrsquares technique, as follows: #.#) ^< 0,.../N ~.k Bq (IS)

T--t where #^/*) « JΪ£SΛ≤L_ , and Eq (16)

N_f(k) ~ N_/(k)/P_t(k) is processed noise.

[0089j #■ _{w\ f}{#) ϊs the interference observed by the channel gain estimates H_Uf(k) for transmit antenna / due to pilot transmissions from the other T - 1 transmit antennas. For the orthogonal sequence shown in. equation (7), the interference from each transmit antenna m to transmit antenna / may be expressed as:

{0090J An N-pomi IDFT of equation (37) may be expressed as;

j.0091 j Equations (17) and. (18) indicate that the interference from transmit antenna m to transmit antenna / is tlie channel impwise response h,_{K !}(£) for transmit antenna //? shifted by (m ~ f) - L taps. The amount, of shift in k_mJ{£) is equal to the difference in the cyclic shifts for transmit antennas m and /. L should thus be greater than the expected delay spread of the wireless channel. An N-poim IDFT of equation (15) may then be expressed as:

^^ ^AO - ∑KΛ* - (>» ~ 0 ' L) . Eq (20)

[0092| Equations (.19) and (20) indicate that, the channel impulse response estimate h_(J(£) between transmit antenna / and receive aiUennay includes the desired channel impulse response h_u{€) plus T ~l time shifted channel impulse responses for the other

T - 1 transmit antennas. The removal of the other pilot sequences in equation (6) may thus be performed in the lime domain by retaining the Gut L channel taps, which contain ^ .(i) for transmit antenna L and discarding the remaining N - L channel taps, which contain. h_{m J}(£) for the other T- I. transmit antennas.

|00.93| For the least-squares technique with a Gh u sequence having a ^"flat frequency spectrum, an N-point IDFT may be performed on the K received symbols R.(k) for N subcarriers, after removing the phase of the transformed Can sequence, to obtain N channel taps. For other base sequences without a flat frequency spectrum (e.g., a PN sequence), the received symbols i?₇{&) tnay be divided by the frequency-domain base sequence #_N(A) and then transformed with an N-point. IDFT to obtain N channel taps. For the orthogonal sequence shown in equation (7), the first L channel taps may be provided as the channel impulse response estimate /?_α,(Q for transmit antenna 0, the next L channel taps may be provided as the channel impulse response estimate h_{t i}(i) for transmit antenna .1, and so on, and the last L channel taps may be provided as the channel impulse response estimate h ^._Ki(i) for transmit antenna. T - 1.

J0094] In another design, the channel gains are estimated based on the MMSE technique, as follows;

{0O95| An N-point EDFT may be performed on the M channel gain estimates from equation (21) to obtain. N channel taps for T channel impulse response estimates for the T transmit antennas, as described above.

[00961 In general, N received symbols M_f{k) from N streamers for each receive antenna/ may be processed with the frequency-domain base sequence #>.,(£) based on the least-squares technique, the MMSE technique, or some other technique to obtain N initial channel gain estimates H_{t f}(k} . The N initial channel gain estimates may be multiplied in the frequency domain with the orthogonal sequence W.{k) for each transmit antenna to obtain L channel gain estimates for that transmit antenna. The L channel gain estimates for each transmit antenna may be transformed with an L-point IDFT to obtain an L-tap channel impulse response estimate A_M(.<?) for that transmit antenna. Alternatively, the removal of the other pilot sequences may be performed in the time domain, as described above. In any case, post-processing (e.g., truncation, thresholding, tap selection, zero padding, etc.) may be performed on the L-tap channel impulse response estimate for each transmit antenna to obtain an N-tap zero-padded channel impulse response estimate, which may then be transformed with an N-point. DFT to obtain M final channel gain estimates for the N subearriers of thai transmit' antenna. The processing may be performed in different manners depending on the frequency-domain base sequence B_N(k) and the orthogonal sequences W_g(k) used for the FD-CDM pilot. Channel estimatioii may also be performed in other manners. (00971 TUe background noise and interference for each subcarrier may be estimated based on the received symbols and the channel gain estimates. For the ΪFDM pilot, the noise and interference for eacϊi subcarrier k may be estimated as follows:

T-i σ *:,,<*⁾ = |Λ/(*)~ ^■Σ∑ W⁾-A^)P - Eq (22⁾

where er_v^(£)the estimated variance of the noise and interference for receive antenna j on subcarrier k. The noise and interference may be estimated m ^" similar manner for the FD-CDM pilot, albeit with /?.,(£) replaced witii 1,(1") and Iffi) replaced with ϊ]{k) .

The noise and interference estimate <r^^(k) may be averaged across R receive antennas to obtain a noise and interference estimate <τ^. (k) for each subcarrier A-, which may be used for MIMO detection and/or other purposes. The noise and interference estimate σl,{k) may also be averaged over all subearriers and across time to obtain a long-term noise and interference estimate, which may be «sed to estimate operating conditions and/or for other purposes.

5, MIMO Detection

}0G9S| The receiver station may recover data symbols sent by the transmitter station based on various MlMO detection techniques such as au MMSB technique, a zero- forcing (ZF) technique, a .maximal ratio combining (MRC) technique, a. space-frequency equalization technique, etc. The received data symbols from the R receive antennas for each subcarrier /f may he expressed as:

where

JGU99J The transmitted symbols X(A') may be data symbols sent, in the frequency domain with OFDM or the DFT of data symbols sent in the time domain with SC-FDM.

The channel gains in h,(k) and H(&) may be estimated based on the received pilot transmissions, as described above. |Qøϊδffj Equalizer coefficients may be derived based on the MMSE, ZF, and MRC techniques, as follows:

where

H^(Λ') is an R, Ά R noise and interference covariance matrix for antenna if, and " -^lf " denotes a conjugate transpose

1.00101 j The noise and interference covariance matrix may be expressed as:

where ■!R(k) ™ E {n(k}- -n^h (k) } is an R x R noise covariance matrix, and E{ } is an expectation operation. {00102] The noise covariance matrix, may be approximated as R(&) — σ^, (k) - ϊ for spatially and spectrally uneorreiaied noise, -where | is an identity matrix. R(k) may also be estimated based on equation (22), J00t03j MIMO detection for each transmit antenna / may be performed as follows:

where K(^) is a biased estimate of X, (Ar) sent from transmit antenna /^"„

is a. scaling factor for X_t(k) , and V_jiik) is post-detection noise and interference for .X^A) .

[00104] The detected symbols for each transmit antenna i may then be expressed as:

}(M>305} The detected symbols £'){&) may be provided directly as data symbol estimates ϊf the data symbols axe sent in the frequency domain with OFDM The detected symbols may be transformed with an IDFT to obtain the data symbol estimates if the data symbols are sent, in the time domain with SC-FDM.

{00306] Those of skill in the art would understand that information and signals may be represented using any of a variety of different technologies and techniques. For example, data, instructions, commands, iftforraation_^ signals, bits, symbols, and chips that may be referenced throughout the above description may be represented by voltages, currents,, electromagnetic waves, magnetic fields or particles, optica] fields of particles, or any combination thereof.

J00107) Those of skill would further appreciate that th$ various illustrative logical blocks, modules, circuits, and algorithm steps described in connection with the disclosure herein may be implemented as electronic hardware, computer software, or combinations of both, To clearly illustrate this interehangeability of hardware and software,, various illustrative components, blocks, modules, circuits, and steps have beers described above generally m terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints Imposed on the overall system. Skilled artisans may implement the described functionality in varying ways for each particular application,, but such implementation decisions should not be interpreted as causing a departure from the scope of the present disclosure.

(0OiOS] The various illustrative logical blocks, modules, and circuits described m connection with, the disclosure herein may be implemented or performed with a general- purpose processor, a digital signal processor (DSF), an application specific integrated circuit (ASIC), 8 field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein, A general- purpose processor may be a microprocessor, but in the alternative, the processor may be any conventional processor, controller, .microcontroller, or state machine. A processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any otϊier such configuration.

{00109} The steps of a method or algorithm described in connection with the disclosure herein may be embodied directly in hardware, in a software module executed by a processor, or in a combination of the two. A software module may reside in RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, hard disk, a removable disk, a CD-ROM, or any other form of storage medium known in the art. An exemplary storage medium is coupled to the processor such that the processor can read mfomiation from, and write information to, the storage medium. TE the alternative, the storage medium may be integral to the processor. The processor and the storage medium may reside in an ASIC. The ASTC may reside in a user terminal. 1« the alternative, the processor and the storage medium may reside as discrete components in a user terminal. jøϋHOj Headings axe included herein for reference and to aid in locating certain sections. These headings are not Intended to limit the scope of the concepts described therein under, and these concepts may have applicability in other sections throughout the entire specification,

(001. JJ J The previous description of the disclosure is provided to enable any person skilled in the art to make or use the disclosure. Various modifications to the disclosure will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other variations without departing from the spύii or scope of the disclosure. IhUS₁₁ the disclosure is not intended to be limited to the examples described herein but is to be accorded the widest, scope consistent with the principles and novel features disclosed herein.

JOOi i 2 j WHAT IS CLAIMED IS:

Claims

<.XAIMS

L An apparatus comprising: at least one processor to generate a. plurality of pilot sequences for a plurality of transmit, antennas, each pilot sequence comprising a plurality of pilot symbols sent, in time domain on a different set of subcarriers, and to generate a plurality of pilot transmissions based on the plurality of pilot sequences; and a memory coupled to the at least one processor.

2. The apparatus of claim 1, wherein the at least one processor generates the plurality of pilot sequences based on a Ch α sequence.

3. The apparatus of claim I₅ wherein the at least one processor generates the plurality of pilot sequences based on a Chu sequence defined by a transmitter- specific value.

4. The apparatus of claim I₅ wherein the at least one processor generates the plurality of pilot sequences based on a, CAZAC (constant amplitude zero autocorrelation) sequence or a pseudo-random number (PN) sequence.

5. The apparatus of claim 1, wherein the at least one processor generates an extended base sequence by replicating a base sequence a plurality of ti.mes_s and generates the plurality of pilot sequences by applying a plurality of different phase ramps to the extended base sequence.

6. The apparatus of claim 1 , wherein the at least one processor generates a. time-domain base sequence comprising a plurality of time-domain symbols, transforms the time-domain base sequence to obtain a frequency-domain base sequence comprising a plurality of transformed symbols, maps the plurality of transformed symbols to a plurality of sets of subcaniers for the plurality of transmit antennas, one set of subcarriers for each transmit antenna, and generates a pilot sequence for each transmit antenna based on transformed symbols on a set of subearπers- for the transmit a&tetma.

7. A method comprising; generating a plurality of pilot sequences for a plurality of transmit antennas, each pilot sequence comprising a plurality of pilot symbols sent in time domain on a different set of subcarriers; and generating a plurality of pilot transmissions based on the plurality of pilot sequences.

8. The method of claim 7, wherein the generating the plurality of pilot sequences compri ses generating the plurality of pilot sequences based on a Chu sequence, a CAZAC (constant, amplitude zero auto-correlation) sequence, or a pseudo-random number (PN) sequence.

9. The method of claim 7, wherein the generating the plurality of pilot sequences comprises generating a« extended base sequence by replicating a base sequence a plurality of times, and generating the plurality of pilot sequences by applying a plurality of different phase ramps to the extended base sequence.

10. An apparatus comprising; at least one processor to receive a plurality of pilot, transmissions via a plurality of receive antennas, each pilot transmission comprising a plurality of pilot symbols sent in time domain on a different set of subcatriers, and to process the plurality of received pilot transmissions to obtain channel estimates; and a memory coupled to the at least one processor,

1 1. The apparatus of claim 10, wherein the at least one processor obtains received symbols based on the plurality of received pilot transmissions, obtains a plurality of pilot sequences for a plurality of transmit antennas, and derives the channel estimates based on the received symbols and the plurality of pilot, sequences.

12. The apparatus of claim 1 I, wherein, for each received pilot transmission the at least one processor obtains a plurality of sets of received symbols for a plurality of sets of subcaπiers based on the received pilot transmission, scales each set of received symbols based on a corresponding pilot sequence to obtain a set. of scaled symbols, and. derives channel estimates for each transmit antenna based on a corresponding set of scaled symbols,

13. The apparatus of claim 10, wherein for each received pilot transmission the at least one processor derives a plurality of channel impulse response estimates for a plurality of transmit antennas based on the received pilot transmission, and derives channel gain estimates for the plurality of transmit antennas based on the plurality of channel impulse response estimates.

14. The apparatus of claim 13, wherein the at least one processor performs thresholding on each channel impulse response estimate to zero out channel taps with magnitude below a threshold, and derives channel gain estimates for each transmit antenna based on a corresponding channel impulse response estimate after thresholding.

15. A method comprising; receiving a plurality of pilot transmissions via a plurality of receive antennas, each pilot transmission comprising a plurality of pilot symbols sent in time domain on a different set of subcarriers; and processing the plurality of received pilot transmissions to obtain channel estimates.

16. The method of claim 15, wherein the processing the plurality of received pilot transmissions comprises obtaining received symbols based on the plurality of received pilot transmissions, obtaining a plurality of pilot sequences for a plurality of transmit antennas, and deriving the channel estimates based on the received symbols and the plurality of pilot sequences.

17. The method of claim 15, wherein the processing the plurality of received pilot transmissions comprises, for each received pilot transmission, deriving a plurality of channel impulse response estimates for a plurality of transmit antennas based, on the received pilot transmission, and deriving channel gain estimates for the plurality of transmit, antennas based on the plurality of channel impulse response estimates.

18. Aτι apparatus comprising: at least one processor to generate a plurality of pilot sequences for a plurality of transmit antennas based on frequency-domain code division multiplexing (FD-CDM) of a Chu sequence defined by a transmitter-specific value, and to generate a plurality of pilot transmissions based on the plurality of pilot sequences; and a memory coupled to the at least one processor,

19. The apparatus of claim IS, wherein the at least one processor generates the Chu sequence with the transmitter-specific value, and generates the plurality of pilot sequences based on the Chu sequence.

20. The apparatus of claim 19, wherein the at least one processor generates the Chu sequence as follows:

c(n) - e *

where e(n) is the Chu sequence_^ N is length of the Chu sequence, λ ϊs the- transmitter- specific value_* and v Is time index.

21. The apparatus of claim 19, wherein the at least one processor generates each of the plurality of pilot sequences based on a different cyclic shift of the Chu sequence.

22. The apparatus of claim IS, wherein the at least one processor generates a frequency-domain base sequence based on the Chu sequence, multiplies the frequency- domain base sequence with a plurality of orthogonal sequences to obtain a plurality of intermediate sequences, and generates the plurality of pilot sequences based on the plurality of intermediate sequences,

23. The apparatus of claim 18, wherein the plurality of pilot transmissions are sent on downlink, and wherein neighboring base stations are assigned different transmitter-specific values.

24. The apparatus of claim IS, wherein the plurality of pilot transmissions are sent on uplink, and wherein different terminals are assigned different transmitter- specific values.

25. A method comprising: generating a plurality of pilot sequences for a plurality of transmit antennas based on frequency-domain, code division multiplexing (FD-CDM) of a Cbu sequence defined by a transmitter- specific value; and generating a plurality of pilot transmissions based on the plurality of pilot sequences.

26. The method of claim 25, wherein the generating the plurality of pilot sequences comprises generating the Chu sequence with the transmitter-specific value, and generating the plurality of pilot sequences based on the Chu sequence.

27. The method of claim 25, wherein the generating the plurality of pilot sequences comprises generating each of the plurality of pilot sequences based on a different cyclic shift of the Chu sequence.

28. An apparatus comprising: means for generating a plurality of pilot sequences for a plurality of transmit anaemias based on frequency-domain code division multiplexing (FD-CDM) of a Chu sequence defined by a transmitter-specific value; and means for generating a. plurality of pi lot transmissions based on the plurality of pilot sequences.

29. The apparatus of claim 28, wherein the means for generating the plurality of pilot sequences comprises means for generating the Chu sequence with the transmitter-specific value, and means for generating the plurality of pilot sequences based on the Chu sequence

30. The apparatus of claim 28, wherein the means for generating the plurality of pilot sequences comprises means for generating each of the plurality of pilot sequences based on a different cyclic shift of tli e Chu sequence.

3 i . An apparatus comprising: at least one processor to receive, via a plurality of receive antennas, a plurality of pilot transmissions generated based on frequency-domain code division multiplexing (FD-CDM) of a Chu sequence defined by a transmitter-specific value, and to process the plurality of received pilot πanstniϋsions to obtain channel estimates; and a memory coupled to the at least one processor,

32. The apparatus of claim 31. wherein for each received pilot transmission the at least one processor obtains received symbols based on the received pilot transmission and derives channel estimates for a plurality of transmit antennas based on the received symbols.

33. The apparatus of claim 32, wherein for each received pilot transmission the at least one processor derives a plurality of channel taps comprising a plurality of non-overlapping sets of channel taps based on the received symbols, and provides each of the plurality of non-overlapping sets of channel taps as a channel impulse response estimate for a different one of the plurality of transmit antennas.

34. The apparatus of claim 32. wherein for each received pilot transmission the at least one processor multiplies the received symbols with a plurality of orthogonal sequences to obtain a plurality of sets of symbols for the plurality of transmit antennas, and derives channel estimates for each transmit antenna based on a corresponding set of symbols.

35. The apparatus of claim 3 L wherein for each received pilot transmission the a.t least one processor obtains received symbols based on the received pilot transmission, scales the received symbols based on a plurality of pilot, sequences determined based on the Chu sequence to obtain a plurality of sets of scaled symbols, and derives channel estimates for a plurality of transmit antennas based on the plurality of sets of scaled symbols.

36. The apparatus of claim 3 L wherein the plurality of received pilot transmissions are obtained via downlink, and wherein neighboring base stations are assigned different transmitter-specific values.

37. The apparatus of claim 3 L wherein the plurality of received pilot transmissions are obtained via uplink, and wherein different terminals are assigned different transmitter-specific values.

38. The apparatus of ciaira 3 L wherein the plurality of received pilot transmissions are obtained via uplink, and wherein different terminals are assigned a common transmilter-specitic value and different cyclic shifts of a pilot sequence generated by FD-CDM of the Chu sequence defined by the common transmitter-specific value.

39. A method comprising: receiving, via a plurality of receive antennas, a plurality of pilot transmissions generated based on frequency-domain code division multiplexing (FD-CDM) of a Ch« sequence defined by a transmitter-specific value; and processing the plurality of received pilot transmissions to obtain channel estimates.

40. The method of claim 39, wherein the processing the plurality of received pilot transmissions comprises, for each received pilot transmission, obtaining received symbols based on the received pilot transmission, deriving a plurality of channel taps comprising a plurality of non-overlapping sets of channel taps based on the received symbols, and providing each of the plurality of non-overlapping sets of channel taps as a channel impulse response estimate for a different, one of a plurality of transmit, antennas.

41. The method of claim 39, wherein the processing the plurality of received pilot transmissions comprises, for each received pilot transmission, obtaining received symbols based on the received pilot transmission, scaling the received symbols based on a plurality of pilot sequences determined based on the Cfeu sequence to obtain a plurality of sets of scaled symbols, and deriving channel estimates for a plurality of transmit antennas based on the plurality of sets of scaled symbols.

42. An apparatus comprising: means for receiving, via. a plurality of receive antennas, a plurality of pilot transmissions generated based on frequency-domain code division multiplexing (FD- CDM) of a Chu sequence defined by a transmitter-specific value; and means for processing the plurality of received pilot transmissions to obtain channel estimates.

43. The apparatus of claim 42, wherein the means for processing the plurality of received pilot transmissions comprises, for each received pilot transmission, means for obtaining received symbols based on the received pilot transmission, means for deriving a plurality of channel taps comprising a plurality of non- overlapping sets of channel taps based on the received symbols, and means for providing each of the plurality of non-overlapping sets of channel taps as a channel impulse response estimate for a different one of a plurality of transmit antennas.

AA. The apparatus of claim 42_S wherein the mesas for processing the plurality of received pilot transmissions comprises, for each received pilot transmission, means for obtaining received symbols based on the received pilot transmission, means for scaling the received symbols based on a plurality of pilot sequences determined based on the Chu sequence to obtain a plurality of sets of scaled symbols, and meaas for deriving channel estimates for a plurality of transmit antennas based on the plurality of sets of scaled symbols.

45. An apparatus comprising: at least one processor to generate a plurality of pilot transmissions for a plurality of transmit antennas based on. a first multiplexing scheme, and to generate a plurality of data transmissions for the plurality of transmit antennas based on a second multiplexing scheme different from the first multiplexing scheme; and a memory coupled to the at least one processor.

46. The apparatus of claim 45, wherein the first multiplexing scheme is orthogonal frequency division multiplexing (OFDM) and the second multiplexing scheme is single-carrier frequency division multiplexing (SC-FDM) or code division multiplexing (CDM).

47. The apparatus of claim 45_; wherein the .first, multiplexing scheme is single-carrier frequency division multiplexing (SC-KDM) and the second multiplexing scheme is orthogonal frequency division multiplexing (QFDM) or code division multiplexing (CDM),

48. The apparatus of claim 45,, wherein the first multiplexing scheme is frequency-domain code division multiplexing (FD-CDM) and the second multiplexing scheme is orthogonal frequency division multiplexing (OFDM) or single-carrier frequency division multiplexing (SC-FDM).

49. The apparatus of claim 45, wherein the .first multiplexing scheme is interleaved frequency division multiplexing (IFDM) and the second multiplexing scheme is localized frequency division multiplexing (LFDM).

50. The apparatus of claim 45, wherein the at least one processor sends the plurality of pilot, transmissions Ln a first time interval and. sends the plurality of data transmissions in a second time interval with time division multiplexing (TDM).

51. The apparatus of claim 45, wherein the at. least one processor sends the plurality of pilot transmissions on a first set of subcarriers and sends the plurality of data transmissions on a second set of subcarriers with frequency division multiplexing (FDM).

52. A method comprising: generating a plurality of pilot transmissions for a plurality of transmit antennas based on a first, multiplexing scheme; and generating a plurality of data transmissions for the plurality of transmit antennas based on a. second multiplexing scheme different from the first multiplexing scheme.

53. The method of claim 52, further comprising: sending the plurality of pi lot transmissions in a first time interval.; and sending the plurality of data transmissions in a second lime interval with time division multiplexing (TDM).

54. An apparatus comprising: at least one processor to receive a plurality of pilot transmissions generated based on a first multiplexing scheme, and to receive a plurality of data transmissions generated based on a second multiplexing scheme different from the first multiplexing scheme, the plurality of pilot transmissions and the plurality of data transmissions being for a multiple-input multiple-output (IVlIMO) transmission sent from a plurality of transmit antennas to a plurality of receive antennas; and a memory coupled to the at least one processor.

55. The apparatus of claim 54, wherein the at least one processor derives channel estimates for the plurality of transmit antennas and trie plurality of receive antennas based on the plurality of received pilot transmissions, and performs data detection for the plurality of received data transmissions based on the channel estimates,

56. A method comprising: receiving a plurality of pilot transmissions generated based on a first multiplexing scheme; and receiving a plurality of data transmissions generated based on a second multiplexing scheme different from the first multiplexing scheme, the plurality of pilot transmissions and the plurality of data transmissions being for a multiple-input multiple- output (IVIIMO) transmission sent from a plurality of transmit antennas to a plurality of receive antennas.

57. The method of claim 56, further comprising: deriving channel estimates for the plurality of transmit antennas and the plurality of receive antennas based on the plurality of received pilot transmissions; and performing data detection for the plurality of received data transmissions based on the channel estimates.