Adaptive pilot insertion for a mimo-ofdm system

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WO 2006/019579 PCT/US2005/023979

ADAPTIVE PILOT INSERTION FOE A MMO-OWM SYSTEM

BACKGROUND)

I. Field

[0001] The present invention relates generally to communication, and more specifically to techniques for transmitting pilot and signaling in a multiple-input multiple-output (ΜΙΜΟ) communication system.

II_e Background

[0002] A ΜΙΜΟ system employs multiple (T) transmit antennas at a transmitting entity and multiple (R) receive antennas at a receiving entity for data transmission. A ΜΙΜΟ channel formed by the T transmit antennas and R receive antennas may be decomposed into S spatial channels, where S < min {T, R}. The S spatial channels may be used to transmit data in parallel to achieve higher throughput and/or redundantly to achieve greater reliability.

[0003] Orthogonal frequency division multiplexing (OFDM) is a multi-carrier modulation technique that effectively partitions the overall system bandwidth into multiple (K) orthogonal frequency subbands. These subbands are also referred to as tones, subcarriers, bins, and frequency channels. With OFDM, each subband is associated with a respective subcarrier that may be modulated with data. Up to K modulation symbols may be sent on the K subbands in each symbol period.

[0004] A MIMO-OFDM system is a MEMO system that utilizes OFDM. The MIMOOFDM system has S spatial channels for each of the K subbands. Each spatial channel of each subband may be called a “transmission channel” and may be used to transmit one modulation symbol in each symbol period. Each transmission channel may experience various deleterious channel conditions such as, e.g., fading, multipath, and interference effects. The S · K transmission channels of the MEMO channel may also experience different channel conditions and may be associated with different complex gains and signal-to-noise-and-interference ratios (SNRs).

[0005] To achieve high performance, it is often necessary to characterize the ΜΙΜΟ channel. For example, the transmitting entity may need an estimate of the MEMO channel response to perform spatial processing (described below) in order to transmit

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74769-1614 data to the receiving entity. The receiving entity typically needs an estimate of the M1M0 channel response to perform receiver spatial processing on signals received from the transmitting entity in order to recover the transmitted data.

[0006] The transmitting entity normally transmits a pilot to assist the receiving entity in performing a number of functions. The pilot is typically composed of known modulation symbols that are transmitted in a known manner. The receiving entity may use the pilot for channel estimation, timing and frequency acquisition, data detection, and so on. Since the pilot represents overhead in the system, it is desirable to minimize the amount of system resources used to transmit the pilot. The system may thus employ a pilot structure that provides an adequate amount of pilot for most receiving entities under normal (or most) channel conditions. However, this pilot structure may be inadequate for certain receiving entities observing adverse channel conditions.

[0007] There is therefore a need in the art for techniques to transmit pilot for various channel conditions.

SUMMARY

[0007a] According to one embodiment of the present invention, there is provided a method of transmitting signaling in a multiple-input multiple-output (ΜΙΜΟ) communication system utilizing orthogonal frequency division multiplexing (OFDM), comprising: selecting a signaling value from among a plurality of signaling values; selecting a set of pilot symbols from among a plurality of sets of pilot symbols, wherein each of the plurality of sets of pilot symbols corresponds to a different one of the plurality of signaling values, and wherein the selected set of pilot symbols corresponds to the selected signaling value, and multiplexing the selected set of pilot symbols on a first set of frequency subbands used for a carrier pilot, wherein the plurality of signaling values includes a signaling value that indicates that one or more additional pilot symbols for deriving an improved channel response estimation are being transmitted on a second set of frequency subbands.

[0007b] According to another embodiment of the present invention, there is provided an apparatus in a multiple-input multiple-output (ΜΙΜΟ) communication system utilizing

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74769-1614 orthogonal frequency division multiplexing (OFDM), comprising: a controller configured to select a signaling value from among a plurality of signaling values; a processor configured to select a set of pilot symbols from among a plurality of sets of pilot symbols, wherein each of the plurality of sets of pilot symbols corresponds to a different one of the plurality of signaling values, and wherein the selected set of pilot symbols corresponds to the selected signaling value; and a multiplexer configured to multiplex the selected set of pilot symbols on a first set of frequency subbands used for a carrier pilot, wherein the plurality of signaling values includes a signaling value that indicates the presence or absence of one or more additional pilot symbols for deriving an improved channel response estimation being transmitted on a second set of frequency subbands.

[0007c] According to still another embodiment of the present invention, there is provided an apparatus in a multiple-input multiple-output (ΜΙΜΟ) communication system utilizing orthogonal frequency division multiplexing (OFDM), comprising: means for selecting a signaling value from among a plurality of signaling values; means for selecting a set of pilot symbols from among a plurality of sets of pilot symbols wherein each of the plurality of sets of pilot symbols corresponds to a different one of the plurality of signaling values, and wherein the selected set of pilot symbols corresponds to the selected signaling value; and means for multiplexing the selected set of pilot symbols on a first set of frequency subbands used for carrier pilot, wherein the plurality of signaling values includes a signaling value that indicates the presence or absence of one or more additional pilot symbols for deriving an improved channel response estimation being transmitted on a second set of frequency subbands.

[0007d] According to yet another embodiment of the present invention, there is provided a machine-readable medium having stored thereon instructions for

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3a transmitting signaling in a multiple-input multiple-output (ΜΙΜΟ) communication system utilizing orthogonal frequency division multiplexing (OFDM), the instructions, when executed, cause a machine to perform operations comprising: selecting a signaling value from among a plurality of signaling values; selecting a set of pilot symbols from among a plurality of sets of pilot symbols, wherein each of the plurality of sets of pilot symbols corresponds to a different one of the plurality of signaling values, and wherein the selected set of pilot symbols corresponds to the selected signaling value; and multiplexing the selected set of pilot symbols on a first set of frequency subbands used for a carrier pilot, wherein the plurality of signaling values includes a signaling value that indicates the presence or absence of one or more additional pilot symbols for deriving an improved channel response estimation being transmitted on a second set of frequency subbands.

[0008] In some embodiments, techniques to adaptively and flexibly transmit additional pilot, e.g., based on channel conditions and/or other factors, in order to achieve good performance are described herein. A transmitting entity transmits a base pilot in each protocol data unit (PDU). A receiving entity is able to derive a sufficiently accurate channel response estimate of a ΜΙΜΟ channel between the transmitting and receiving entities with the base pilot under nominal (or most) channel conditions. The transmitting entity selectively transmits an additional pilot if and as needed, e.g., based on the channel conditions and/or other factors. The additional pilot may be adaptively inserted in any symbol period in the PDU, except for symbol periods with other designated transmissions. The receiving entity is able to derive an improved channel response estimate with the additional pilot. The base pilot represents a fixed overhead and is selected to provide good performance under nominal (or most) channel conditions. The additional pilot may be sent when needed and may provide good performance for adverse channel conditions, without having to incur a fixed and high overhead for the pilot.

[0009] The transmitting entity sends signaling to indicate that additional pilot is being sent. This signaling may be conveniently embedded within a carrier pilot that is transmitted on a designated set of P subbands across most of the PDU (e.g., P = 4). A set of P pilot symbols is sent on the set of P subbands in each symbol period in which the carrier pilot is transmitted.

Different sets of P pilot symbols may be formed for different signaling values, e.g., one

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3b signaling value to indicate that data symbols are being transmitted on the remaining usable subbands, another signaling value to indicate that additional pilot symbols are being transmitted, and so on. The signaling for the additional pilot may be sent by selecting the proper set of P pilot symbols and sending these P pilot symbols on the P subbands used for the carrier pilot. The additional pilot and its signaling may thus be selectively and concurrently sent in almost any symbol period in the PDU. The signaling for the additional pilot may also be sent in some other manners.

[0010] Various aspects and embodiments of the invention are described in further detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] FIG 1 shows an OFDM subband structure used by IEEE 802.1 la;

[0012] FIG 2 shows an exemplary PDU format suitable for a ΜΙΜΟ system;

[0013] FIG 3 shows a process to transmit an additional pilot;

[0014] FIG 4 shows a process to receive and utilize the additional pilot;

[0015] FIG 5 shows a block diagram of a transmitting entity and a receiving entity;

[0016] FIG 6 shows a block diagram of a transmit (TX) spatial processor; and

[0017] FIG 7 shows a block diagram of a TX pilot signaling processor.

DETAILED DESCRIPTION

[0018] The word “exemplaiy” is used herein to mean “serving as an example, instance, or illustration”. Any embodiment described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments.

[0019] The pilot transmission and signaling techniques described herein may be used for a single-input single-output (SISO) system, a single-input multiple-output (SIMO) system, a multipleinput single-output (MISO) system, and a ΜΙΜΟ system. These techniques may be used for an OFDMbased system and for other multi-carrier communication systems. These techniques may also be used with various OFDM

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WO 2006/019579 PCT/US2005/023979 subband structures. For clarity, these techniques are specifically described below for a MWO-OFDM system utilizing the OFDM subband structure defined by IEEE 802.1 la.

[0020] The IEEE 802.11 OFDM subband structure partitions the overall system bandwidth into 64 orthogonal subbands (i.e., K = 64), which are assigned indices of --32 to +31. Of these 64 subbands, 48 subbands with indices of ±{1,..., 6, 8,..., 20,22, ..., 26} may be used for data and pilot transmission and are called “data” subbands, 4 subbands with indices of ± {7, 21} may be used for a carrier pilot and possibly signaling and are called “pilot” subbands, the DC subband with index of 0 is not used, and the 11 remaining subbands are also not used and serve as guard subbands. Thus, the 64 total subbands include 52 “usable” subbands composed of the 48 data subbands and the 4 pilot subbands and 12 “unused” subbands. This OFDM subband structure is described in a document for IEEE Standard 802.11a entitled “Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications: High-speed Physical Layer in the 5 GHz Band,” September 1999, which is publicly available. In general, an OFDM-based system may utilize any OFDM subband structure with any number of data, pilot, and guard subbands.

[0021] FIG. 1 shows a PDU format 100 defined by IEEE 802.11 and suitable for use for various communication systems. At a physical (PHY) layer in the protocol stack for IEEE 802.11, data is processed and transmitted in PHY protocol data units (PPDUs), which are also called “PDUs” herein for simplicity. Each PDU 110 for IEEE 802.11 includes a preamble section 120, a signal section 130, and a data section 150. Preamble section 120 carries short and long training symbols that are described below. Signal section 130 carries one OFDM symbol for signaling for the PDU. Data section 150 carries a variable number of OFDM symbols for traffic/packet data for the PDU. The length of data section 150 is indicated by the signaling in signal section 130.

[0022] Preamble section 120 carries ten short training symbols sent in two OFDM symbol periods followed by two long training symbols sent in two OFDM symbol periods. Four short training symbols are formed by performing an inverse discrete Fourier transform (IDFT) on a specific set of 12 pilot symbols sent on 12 subbands with indices of {-24, -20, -16, -12, -8, -4, 4, 8, 12, 16, 20, and 24}. A “pilot symbol” is a modulation symbol for pilot and is typically known a priori by both the transmitting and receiving entities. The same set of 12 pilot symbols is used for all short training symbols. Each long training symbol is formed by performing an IDFT on a specific set

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WO 2006/019579 PCT/US2005/023979 of 52 pilot symbols sent on the 52 usable subbands. The same set of 52 pilot symbols is also used for both long training symbols. A receiving entity may use the short training symbols for signal detection, coarse frequency offset estimation, timing synchronization, automatic gain control (AGC), and so on. The receiving entity may use the long training symbols for channel estimation, fine frequency offset estimation, and so on.

[0023] Signaling and data are sent on the 48 data subbands in signal section 130 and data section 150, respectively. A carrier pilot is sent on the four pilot subbands in the signal and data sections. The carrier pilot is composed of four pilot symbols that are sent on the four pilot subbands across the signal and data sections. Prior to transmission, the pilot symbol for each pilot subband is multiplied with a 127-chip circularly extended pseudo-random number (PN) sequence to generate a predetermined symbol sequence for that pilot subband. The receiving entity may use the carrier pilot to track the phase of a carrier signal across the signal and data sections.

[0024] The pilot structure shown in FIG. 1 comprises ten short training symbols, two long training symbols, and the carrier pilot. This pilot structure is generally suitable for a SISO system.

[0025] A ΜΙΜΟ system may utilize different types of pilot to support various functions needed for proper system operation, such as timing and frequency acquisition, channel estimation, calibration, and so on. Table 1 lists four types of pilot and their short description. A pilot is also called a “reference”, and these two terms are often used interchangeably.

Table 1 - Pilot Types

<td> Pilot Type</td><td> Description</td>

<td> Beacon Pilot</td><td> A pilot transmitted from all transmit antennas and used for timing and frequency acquisition.</td>

<td> Unsteered ΜΙΜΟ Pilot</td><td> A pilot transmitted from all transmit antennas and used for channel estimation, with the pilot transmission from each transmit antenna being identifiable by a receiving entity.</td>

<td> Steered ΜΙΜΟ Pilot</td><td> A pilot transmitted on “eigenmodes” of a ΜΙΜΟ channel and used for channel estimation and possibly rate control.</td>

<td> Carrier Pilot</td><td> A pilot used for phase tracking of a carrier signal.</td>

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The unsteered and steered ΜΙΜΟ pilots are described in detail below.

[0026] FIG. 2 shows an exemplary PDU format 200 suitable for the ΜΙΜΟ system. A

PDU 210 for this format includes a preamble section 220, a signal section 230, a ΜΙΜΟ pilot section 240, and a data section 250. Preamble section 220 carries the beacon pilot. For the embodiment shown in FIG. 2, the beacon pilot is composed of ten short training symbols and two long training symbols. Preamble section 220 is thus similar to preamble section 120 in FIG. 1. Signal section 230 carries signaling for PDU 210 and may include (1) a field that indicates whether the PDU has format 200 or some other format (e.g., format 100) and (2) a field that indicates the length of ΜΙΜΟ pilot section 240. ΜΙΜΟ pilot section 240 carries a “base” ΜΙΜΟ pilot, which may be unsteered or steered. The base MEMO pilot is typically sent in each PDU and may be transmitted in the same manner as the data in the PDU. Data section 250 carries the data for PDU 210. A carrier pilot is sent on the four pilot subbands in signal section 230, ΜΙΜΟ pilot section 240, and data section 250. A PDU may also be called a packet, a data unit, a frame, a slot, a block, or some other terminology.

[0027] PDU format 200 includes an exemplary pilot structure for the ΜΙΜΟ system.

To minimize overhead, the pilot structure may include a minimal (or nominal) amount of pilot (the base pilot) needed for proper system operation under normal channel conditions. For example, ΜΙΜΟ pilot section 240 may carry T OFDM symbols for the ΜΙΜΟ pilot for T transmit antennas. Additional pilot may be adaptively inserted and sent if and as needed in order to achieve improved performance. The additional pilot may be beneficial under certain adverse channel conditions such as increased fade rates due to Doppler effect, changing interference and/or jamming characteristics, and so on. The additional pilot may also be sent based on other factors, e.g., if the PDU is for a retransmission because an acknowledgment (ACK) was not received for a prior transmission of the PDU. The additional pilot may be inserted in the data section of the PDU. Signaling to indicate transmission of the additional pilot may be efficiently embedded within the carrier pilot, as described below, or sent in signal section 230.

[0028] A ΜΙΜΟ channel between a transmitting entity and a receiving entity may be characterized by an RxT channel response matrix H(&amp;) for each subband k, which maybe expressed as:

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<td></td><td></td><td></td><td></td><td></td><td></td>

<td rowspan="2"> aw=</td><td></td><td rowspan="2"> h₂,₂(k) ·</td><td> • /_TW</td><td rowspan="2"> , for k = 1 ... K,</td><td rowspan="2"> Eq(l)</td>

<td></td><td></td>

<td></td><td> A.i(^/c)</td><td> M*) ·</td><td> ^R,T O</td><td></td><td></td>

where entry for z = l ... R and / = 1 ... T, denotes the coupling or complex channel gain between transmit antenna J and receive antenna i for subband k. For simplicity, the ΜΙΜΟ channel is assumed to be full rank with S = T &lt; R.

[0029] The receiving entity may obtain an estimate of H(Æ) for each subband k based on an unsteered ΜΙΜΟ pilot sent by the transmitting entity. The unsteered ΜΙΜΟ pilot comprises T pilot transmissions sent from T transmit antennas, where the pilot transmission from each transmit antenna is identifiable by the receiving entity. This may be achieved by sending the pilot transmission for each transmit antenna with a different orthogonal (e.g., Walsh) sequence using code multiplexing, on a different subband using subband multiplexing, in a different symbol period using time multiplexing, and so on. An unsteered ΜΙΜΟ pilot sent using code multiplexing may be expressed as:

x“_;/oi (k, n) = W(n) · p (fc, n) , for k e K_u, Eq (2) where p(k,n) is a vector of T pilot symbols to be sent from the T transmit antennas on subband k in symbol period n;

W(n) is a diagonal Walsh matrix for the T transmit antennas in symbol period n; x^u_piIot(k,n) is a vector of transmit symbols for the unsteered ΜΙΜΟ pilot for subband k in symbol period n; and

K_u is a set of subbands on which the unsteered ΜΙΜΟ pilot is sent.

A “transmit symbol” is a symbol to be sent from a transmit antenna. The same Walsh matrix W(«) may be used for all subbands and may thus not be a function of subband index k.

[0030] As an example, if T = 4, then the four transmit antennas may be assigned four

Walsh sequences of ^=(1,1,1,1), W₂ = (1,-1, 1,-1), W₃ = (1, 1,-1,-1), and W₄ = {1, -1, -1, 1}. Walsh matrix W(l) then contains the first element of the four

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Walsh sequences along its diagonal, W(2) contains the second element of the four Walsh sequences, W(3) contains the third element of the four Walsh sequences, and W(4) contains the fourth element of the four Walsh sequences. The four Walsh matrices W(l) through W(4) may be used in four symbol periods to transmit the unsteered ΜΙΜΟ pilot. In general, a complete unsteered ΜΙΜΟ pilot may be sent in T (consecutive or non-consecutive) symbol periods with code multiplexing, or one symbol period for each chip of the orthogonal sequence. Upon receiving the complete unsteered ΜΙΜΟ pilot, the receiving entity may perform the complementary processing on the received pilot to estimate H(£).

[0031] The transmitting entity may transmit data on S eigenmodes of the channel response matrix H(7c) for each subband k to achieve improved performance. The channel response matrix H(&amp;) for each subband k may be “diagonalized” to obtain the S eigenmodes of the ΜΙΜΟ channel for that subband. This diagonalization may be achieved by performing either singular value decomposition of H(7c) or eigenvalue decomposition of a correlation matrix of H(&amp;), which is R(£) = (/&lt;) · H(k), where

H⁷⁷ denotes the conjugate transpose of H. The singular value decomposition of H(fc) maybe expressed as:

H(£) = U(fc) · Σ(£) · N^h (k) , Eq (3) where U(A?) is an R x R unitary matrix of left eigenvectors of H(Ar) ;

Σ(&amp;) is an R x T diagonal matrix of singular values of H(Æ) ; and

V(/c) is a T x T unitary matrix of right eigenvectors of H(/c).

A unitary matrix M is characterized by the property M^ffM = 1, where I is the identity matrix. The columns of a unitary matrix are orthogonal to one another. The transmitting entity may use the right eigenvectors in V(k) for spatial processing to transmit data on the S eigenmodes of H(&amp;). The receiving entity may use the left eigenvectors in U(/c) for receiver spatial processing to recover the data transmitted on the S eigenmodes of H(/c). The diagonal matrix Z(k) contains non-negative real values along the diagonal and zeros elsewhere. These diagonal entries are referred to as

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WO 2006/019579 PCT/US2005/023979 singular values of H(/c) and represent the channel gains for the S eigenmodes of H(/c). Singular value decomposition is described by Gilbert Strang in “Linear Algebra and Its Applications,” Second Edition, Academic Press, 1980.

[0032] The transmitting entity may transmit a steered ΜΙΜΟ pilot as follows:

= for/ce^, Eq(4) where v_m(k) is the zzz-th eigenvector/column of V(/c) ;

p_m (k) is a pilot symbol to be transmitted on the zzz-th eigenmode of H(/&lt;) ;

^ptiot, is ^a transmit vector for the steered ΜΙΜΟ pilot for the rn-th eigenmode of H(&amp;) ; and

K_s is a set of subbands on which the steered ΜΙΜΟ pilot is sent.

[0033] The received steered MEMO pilot at the receiving entity may be expressed as:

W*) =H(i).x^)+n(t) , ----------------------------------------- — = U(k)-E(/c)-V^/i(k).v_nI(fc).p_BI(/t)+_B(k) , forked, Eq(5) = u_m(fc)-a_m(k)-p_m(k) + n(k) , where r^s_piIot(k) is a vector of received symbols for the steered ΜΙΜΟ pilot for the zzz-th eigenmode of H(fc) ;

σ_η (k) is the zzz-th diagonal element of Z(k) ; and u_m(k) is the zzz-th eigenvector/column of U(/c).

A “received symbol” is a symbol obtained from a receive antenna. The transmitting entity may transmit a complete steered ΜΙΜΟ pilot on all S eigenmodes of H(A) in S symbol periods, e.g., on one eigenmode per symbol period using time multiplexing as shown in equation (4). The receiving entity may obtain an estimate of U(Â:), one column at a time, based on the steered ΜΙΜΟ pilot sent using time multiplexing, as shown in equation (5).

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[0034] The transmitting entity may also transmit the steered ΜΙΜΟ pilot on all S eigenmodes of H(/c) concurrently in S symbol periods using coding multiplexing. The steered ΜΙΜΟ pilot with code multiplexing may be expressed as:

(k, n) = V(/c, n) W(n) · p(k, n) , for k ¢= K₅, Eq (6) where V(7c,u) is a matrix of right eigenvectors of H(7c,?z) for subband k in symbol period n. The receiving entity may obtain an estimate of U(/c,u) after receiving the complete steered ΜΙΜΟ pilot.

[0035] The transmitting entity may also transmit the complete steered ΜΙΜΟ pilot for all S eigenmodes of H(fc) on S subbands k through k + S -1 in one symbol period using subband multiplexing. The transmitting entity may also transmit the steered ΜΙΜΟ pilot on less than S eigenmodes. For example, the transmitting entity may transmit the steered ΜΙΜΟ pilot on the best or principal eigenmode in one symbol period, on the two best eigenmodes in two symbol periods, and so on.

[0036] In general, the transmitting entity may transmit the unsteered and steered ΜΙΜΟ pilots in various manners using code, subband, and/or time multiplexing. Code multiplexing allows the transmitting entity to use the maximum transmit power available for each transmit antenna for pilot transmission, which may improve channel estimation performance.

[0037] The additional pilot may be a ΜΙΜΟ pilot, as described above. The additional pilot may also be some other type of pilot. For example, the transmitting entity may transmit a single stream of pilot symbols on a single eigenmode or may beam steer a single stream of pilot symbols in some other manner. This additional pilot may be used, for example, to drive the timing offset, correct residual frequency offset, and so on.

[0038] The pilot structure includes the base pilot (e.g., ΜΙΜΟ pilot section 240 in FIG.

2) that provides good performance under nominal channel conditions. This results in low overhead for the pilot. Additional pilot may be transmitted if and as needed. The amount of additional pilot to be sent as well as the placement of the additional pilot within a PDU may be flexibly selected based on the channel conditions and/or other factors. For example, a larger amount of additional pilot may be sent under more severe channel conditions. The additional pilot may be sent at or near the start of a PDU, which may simplify channel estimation and data detection and may further reduce

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WO 2006/019579 PCT/US2005/023979 buffering requirement. The additional pilot may also be dispersed throughout a PDU, which may improve performance for a time-varying channel.

[0039] Referring to PIG. 2, four pilot symbols may be sent on the four pilot subbands in each symbol period in data section 250. These pilot symbols may be used to indicate/ signal the content being sent on the 48 data subbands. If each pilot symbol is formed with B bits, then up to 2^4B different signaling values may be defined with the four pilot symbols sent on the four pilot subbands. For example, using binary phase shift keying (BPSK), each pilot symbol is formed with one bit, and up to 2⁴ = 16 different signaling values may be defined with the four pilot symbols.

[0040] In general, detection performance for the signaling embedded in the four pilot symbols degrades in proportion to the number of signaling values defined for these pilot symbols. The receiving entity receives noisy versions of the four pilot symbols and needs to ascertain the specific signaling value sent by the transmitting entity based on these noisy received pilot symbols. The receiving entity may compute a metric (e.g., a distance) between the received pilot symbols and the set of pilot symbols for each valid signaling value. The receiving entity then selects the signaling value with the best metric (e.g., the shortest distance) as the value sent by the transmitting entity. Detection error is more likely when there are more valid signaling values from which to choose.

[0041] In an embodiment, the four pilot symbols are used to indicate whether data or additional pilot is being sent in the OFDM symbol. Table 2 shows an exemplary signaling set for this embodiment with four bits hi, b₃, b₃, and 6₄ carried by the four pilot symbols with BPSK.

Table 2

<td> Bits</td><td> Value</td><td> Definition</td>

<td rowspan="2"></td><td> ‘0000’</td><td> Data is being sent in the OFDM symbol</td>

<td> ‘1111’</td><td> ΜΙΜΟ pilot is being sent in the OFDM symbol</td>

The additional ΜΙΜΟ pilot may be steered or unsteered, e.g., may be sent in the same manner as data symbols in the PDU. A “data symbol” is a modulation symbol for data.

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[0042] Tn another embodiment, the 4B bits are used to indicate whether additional pilot is being sent in the OFDM symbol and, if yes, specific information for the additional pilot. Table 3 shows an exemplary signaling set for this embodiment with four bits 61, b₂, bz, and Z&gt;4 carried by the four pilot symbols with BPSK.

Table 3

<td> Bite</td><td> Value</td><td> Definition</td>

<td rowspan="4"> bib₂</td><td> W</td><td> Data is being sent in the OFDM symbol</td>

<td> ‘01’</td><td> Steered ΜΙΜΟ pilot is being sent in the OFDM symbol</td>

<td> ‘10’</td><td> Unsteered ΜΙΜΟ pilot is being sent in the OFDM symbol</td>

<td> ‘11’</td><td> Reserved</td>

<td rowspan="2"> bz</td><td> ‘0’</td><td> Additional pilot is being sent with code multiplexing</td>

<td> ‘1’</td><td> Additional pilot is being sent with subband multiplexing</td>

<td rowspan="2"> 64</td><td> ‘0’</td><td> Additional pilot is being sent on 48 data subbands</td>

<td> ‘1’</td><td> Additional pilot is being sent on 24 data subbands</td>

[0043] For the embodiment shown in Table 3, bits b\ and b₂ indicate whether an unsteered ΜΙΜΟ pilot, a steered ΜΙΜΟ pilot, or no additional pilot is being sent in the OFDM symbol. Bit bz indicates whether the ΜΙΜΟ pilot is being sent using code/time multiplexing or subband multiplexing. For code multiplexing, the ΜΙΜΟ pilot is sent over multiple symbol periods using orthogonal sequences. For example, an unsteered ΜΙΜΟ pilot may be sent from four transmit antennas in four symbol periods using 4chip Walsh sequences, as shown in equation (2). A steered ΜΙΜΟ pilot may be sent on all four eigenmodes concurrently in four symbol periods using 4-chip Walsh sequences, as shown in equation (6). For subband multiplexing, the ΜΙΜΟ pilot is sent on multiple subbands in one symbol period. For example, an unsteered ΜΙΜΟ pilot may be sent from four transmit antennas on four different subbands in one symbol period (e.g., from transmit antenna 1 on subband k, from transmit antenna 2 on subband k + 1, from transmit antenna 3 on subband k + 2, and from transmit antenna 4 on subband k + 3). A steered ΜΙΜΟ pilot may be sent on four eigenmodes using four different subbands in one symbol period (e.g., on eigenmode 1 using subband k, on eigenmode 2 using subband fc + 1, on eigenmode 3 using subband k + 2, and on eigenmode 4 using subband k + 3). Bit b^ indicates the number of subbands used for the additional pilot.

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For example, additional pilot symbols may be sent on all 48 data subbands or on only 24 data subbands (e.g,, every other data subband).

[0044] Tables 2 and 3 show two specific embodiments of the signaling embedded in the four pilot subbands with four bits using BPSK. In general, the 4B bits for the carrier pilot may be used to convey any type of information for the additional pilot such as (1) whether or not the additional pilot is being sent, (2) the type of additional pilot being sent (e.g., unsteered ΜΙΜΟ pilot, steered ΜΙΜΟ pilot, and so on), (3) the manner in which the pilot is being sent (e.g., code multiplexing, subband multiplexing, time multiplexing, and so on), (4) the number of subbands used for the additional pilot (e.g., all, half, quarter, or some other number of data subbands), and (5) possibly other pertinent information. More signaling values provide more flexibility in the transmission of the additional pilot. However, detection performance is also worse with more signaling values. A tradeoff may be made between detection performance and pilot insertion flexibility.

[0045] The signaling for the additional pilot in a given PDU may also be sent in signal section 230 of the PDU. This signaling may indicate any or all of the possible information noted above for the additional pilot. Furthermore, this signaling may indicate the specific symbol periods in which the additional pilot will be sent (e.g., in the middle of data section 250, in every quarter of the data section, in every L-th symbol period, and so on).

[0046] The carrier pilot may be used to send signaling for the additional pilot, as described above. The carrier pilot may also be used to send other types of signaling such as, for example, the rate (e.g., coding and modulation scheme) used for a PDU being sent, the rate to be used for the other link (e.g., downlink or uplink), power control information (e.g., UP and DOWN power control commands used to adjust transmit power), transmission parameters (e.g., the allocated traffic channels, frequency subbands, and so on), an acknowledgment (ACK) or a negative acknowledgment (NAK) for a PDU received via the other link, a set of base station(s) to use for communication, and so on. Different types of signaling may have different reliability requirements and may employ different encoding schemes and/or different signaling sets. Regardless of the type of signaling to be sent, the transmitting entity may conveniently send this signaling on the pilot subbands, and the receiving entity may quickly detect this signaling.

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[0047] ÏIG. 3 shows a flow diagram of a process 300 performed by the transmitting entity to send additional pilot. Process 300 may be performed for each PDU. The transmitting entity multiplexes and transmits the base pilot in the PDU (block 310). The transmitting entity also determines whether or not to transmit additional pilot in the PDU, e.g., based on channel conditions and/or other factors (block 312). If additional pilot is not to be sent in the PDU, as determined in block 314, then the transmitting entity processes and transmits the PDU in the normal manner without any additional pilot (block 316). Otherwise, if additional pilot is to be sent, then the transmitting entity determines the amount, type, location, and so on, of the additional pilot to be sent in the PDU, e.g., based on the channel conditions and/or other factors (block 318). The transmitting entity then sends signaling for the additional pilot in the PDU, e.g., embedded in the pilot symbols sent on the four pilot subbands (block 320). The transmitting entity also multiplexes and transmits the additional pilot as indicated by the signaling (block 322). The transmitting entity also processes and transmits the PDU in light of the additional pilot (block 324). For example, the length of the PDU may be extended by the amount of additional pilot being sent in the PDU.

[0048] MG. 4 shows a flow diagram of a process 400 performed by the receiving entity to receive and utilize the additional pilot. Process 400 may also be performed for each PDU. The receiving entity receives the base pilot (e.g., the ΜΙΜΟ pilot sent in ΜΙΜΟ pilot section 240) and derives a ΜΙΜΟ channel response estimate based on the received base pilot (block 410). The receiving entity receives signaling for the additional pilot, e.g., from the pilot symbols sent on the four pilot subbands (block 412). The receiving entity determines whether or not additional pilot is being sent based on the received signaling (block 414). If additional pilot is not being sent, then the process proceeds to block 420. Otherwise, the receiving entity receives and demultiplexes the additional pilot as indicated by the received signaling (block 416). The receiving entity then derives an improved ΜΙΜΟ channel response estimate with the additional pilot (block 418). The receiving entity uses the channel response estimate to perform data detection on received data symbols for the PDU (block 420).

[0049] MG. 5 shows a block diagram of a transmitting entity 510 and a receiving entity

550 in a ΜΙΜΟ system 500. Transmitting entity 510 may be an access point or a user terminal. Receiving entity 550 may also be an access point or a user terminal.

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[0050] At transmitting entity 510, a TX data processor 512 processes (e.g., encodes, interleaves, and symbol maps) traffic/packet data to obtain data symbols. A TX spatial processor 520 receives and demultiplexes pilot and data symbols onto the proper subbands, performs spatial processing as appropriate, and provides T streams of transmit symbols for the T transmit antennas to T OFDM modulators (Mod) 530a through 530t. Each OFDM modulator 530 performs OFDM modulation on a respective transmit symbol stream and provides a stream of samples to an associated transmitter unit (TMTR) 532. Each transmitter unit 532 processes (e.g., converts to analog, amplifies, filters, and frequency upconverts) its sample stream to generate a modulated signal. Transmitter units 532a through 532t provide T modulated signals for transmission from T antennas 534a through 534t, respectively.

[0051] At receiving entity 550, R antennas 552a through 552r receive the T transmitted signals, and each antenna 552 provides a received signal to a respective receiver unit (RCVR) 554. Each receiver unit 554 processes its received signal and provides a corresponding sample stream to an associated OFDM demodulator (Demod) 560. Each OFDM demodulator 560 performs OFDM demodulation on its sample stream and provides received data symbols to a receive (RX) spatial processor 570 and received pilot symbols to a channel estimator 584 within a controller 580. Channel estimator 584 derives channel response estimates for the ΜΙΜΟ channel between transmitting entity 510 and receiving entity 550 for subbands used for data transmission. The channel response estimates may be derived with the base pilot and/or the additional pilot sent by transmitting entity 510. Controller 580 also derives spatial filter matrices based on the ΜΙΜΟ channel response estimates. RX spatial processor 570 performs receiver spatial processing (or spatial matched filtering) on the received data symbols for each subband with the spatial filter matrix derived for that subband and provides detected data symbols for the subband. Each detected data symbol is an estimate of a data symbol sent by transmitting entity 510. An RX data processor 572 then processes the detected data symbols for all subbands and provides decoded data.

[0052] Controllers 540 and 580 control the operation of the processing units at transmitting entity 510 and receiving entity 550, respectively. Memory units 542 and

582 store data and/or program codes used by controllers 540 and 580, respectively.

[0053] FIG. 6 shows a block diagram of an embodiment of TX spatial processor 520 at transmitting entity 510. Within processor 520, a TX data spatial processor 610 receives

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WO 2006/019579 PCT/US2005/023979 and performs spatial processing on the data symbols for transmission via the T transmit antennas or the S eigenmodes of each data subband. TX data spatial processor 610 provides T streams of spatially processed data symbols for the T transmit antennas to T symbol multiplexers (Mux) 640a through 640t A TX pilot spatial processor 620 performs spatial processing on pilot symbols and provides (1) an unsteered MEMO pilot for transmission from the T transmit antennas or (2) a steered MEMO pilot for transmission on up to S eigenmodes of each subband used for pilot transmission. TX pilot spatial processor 620 provides spatially processed pilot symbols for the T transmit antennas to T symbol multiplexers 640a through 640t.

[0054] A TX pilot signaling processor 630 generates signaling for the additional pilot, if any, being sent. For the embodiment shown in FIG. 6, the signaling for the additional pilot is embedded within the pilot symbols sent on the four pilot subbands for the carrier pilot. TX pilot signaling processor 630 provides carrier pilot symbols, with the signaling embedded therein, to symbol multiplexers 640a through 640t. Each symbol multiplexer 640 receives and multiplexes the spatially processed data symbols, the spatially processed pilot symbols, and the carrier pilot symbols for its transmit antenna onto the proper subband and symbol period. T symbol multiplexers 640a through 640t provide T streams for transmit symbols for the T transmit antennas to T OFDM modulators 530a through 530t.

[0055] Each OFDM modulator 530 performs OFDM modulation on a respective transmit symbol stream and provides a corresponding OFDM symbol stream. For each symbol period, each OFDM modulator 530 obtains K frequency-domain values, e.g., for 48 data and/or pilot symbols to be sent on the 48 data subbands, four carrier pilot symbols to be sent on the four pilot subbands, and 12 signal values of zero for the 12 unused subbands. An inverse fast Fourier transform (IFFT) unit 650 transforms the K frequency-domain values to the time domain with a K-point IFFT and provides a “transformed” symbol that contains K time-domain chips. To combat intersymbol interference (ISI), which is caused by frequency selective fading, a cyclic prefix generator 652 repeats a portion of each transformed symbol to form a corresponding OFDM symbol. The repeated portion is often called a cyclic prefix or guard interval. An OFDM symbol period (or simply, a symbol period) is the duration of one OFDM symbol.

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[0056] FIG. 7 shows a block diagram of an embodiment of TX pilot signaling processor 630. Controller 540 provides a signaling value for the additional pilot to a signaling look-up table (LUT) 710, which then provides four pilot symbols corresponding to this signaling value to four multipliers 712a through 712d. Each multiplier 712 also receive a PN sequence from a PN generator 714 and, for each symbol period, multiplies the pilot symbol for that symbol period with the PN value for that symbol period to generate a scrambled pilot symbol. Multipliers 712a through 712d provide four scrambled pilot symbols for the four pilot subbands to T symbol multiplexers 640a through 640t. Each symbol multiplexer 640i, for / = 1 ... T, multiplexes the scrambled pilot symbols onto the four pilot subbands used for the carrier pilot and further multiplexes spatially processed data and pilot symbols for transmit antenna i onto the data subbands.

[0057] The pilot transmission and signaling techniques described herein may be implemented by various means. For example, these techniques may be implemented in hardware, software, or a combination thereof. For a hardware implementation, the processing units used to transmit additional pilot and signaling may be implemented within one or more application specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), field programmable gate arrays (FPGAs), processors, controllers, micro-controllers, microprocessors, other electronic units designed to perform the functions described herein, or a combination thereof. The processing units used to receive the additional pilot and signaling may also be implemented within one or more ASICs, DSPs, and so on.

[0058] For a software implementation, the techniques described herein may be implemented with modules (e.g., procedures, functions, and so on) that perform the functions described herein. The software codes may be stored in a memory unit (e.g., memory units 542 and/or 582 in FIG. 5) and executed by a processor (e.g., controller 540 and/or 580 in FIG. 5). The memory unit may be implemented within the processor or external to the processor, in which case it can be communicatively coupled to the processor via various means as is known in the art.

[0059] The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic

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74769-1614 principles defined herein may be applied to other embodiments without departing from the scope of the claims. Thus, the present invention is not intended to be limited to the embodiments shown herein.