Adaptive pilot insertion for a mimo-ofdm system

ES 2 389 488 T3

DESCRIPTION

Adaptive Driver Insertion for a MIMO-OFDM System

BACKGROUND

I. Field

The present invention relates generally to communication, and more specifically to techniques for transmitting pilots and signaling in a multiple input multiple output (MIMO) communication system.

II. Background

A MIMO system employs multiple transmitting antennas (T) in a transmitting entity and multiple receiving antennas (R) in a receiving entity for data transmission. A MIMO channel formed by the T transmitting antennas and the R receiving antennas can be decomposed into S spatial channels, where S <min {T, R}. The S spatial channels can be used to transmit data in parallel for higher performance and / or redundantly for higher reliability.

Orthogonal Frequency Division Multiplexing (OFDM) is a multi-carrier modulation technique that effectively divides the total system bandwidth into multiple (K) orthogonal frequency subbands. These subbands are also called tones, subcarriers, bins, and frequency channels. With OFDM, each subband is associated with a respective subcarrier that can be modulated with data. Up to K modulation symbols can be sent on the K subbands in each symbol period.

A MIMO-OFDM system is a MIMO system that uses OFDM. The MIMO-OFDM system has S spatial channels for each of the K subbands. Each spatial channel in each subband can be called a transmission channel and can be used to transmit one modulation symbol in each symbol period. Each transmission channel can experience different harmful channel conditions such as, for example, fading, multipath, and the effects of interference. The S · K transmission channels of the MIMO channel may also experience different channel conditions and may be associated with different complex gains and signal-to-noise and interference ratios (SNRs).

To achieve high performance, it is often necessary to characterize the MIMO channel. For example, the transmitting entity may need an estimate of the MIME channel response to perform spatial processing (described below) for transmitting data to the receiving entity. The receiving entity typically needs an estimate of the MIMO channel response to perform receiver spatial processing of the signals received from the transmitting entity in order to recover the transmitted data.

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

Therefore, there is a need in the art for techniques to transmit pilots for different channel conditions.

ABSTRACT

The invention is defined by the independent claims. Techniques for adaptively and flexibly transmitting additional pilots, for example 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 obtain a sufficiently accurate channel response estimate of a MIMO 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 when necessary, eg based on channel conditions and / or other factors. The additional pilot can be adaptively inserted at 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 data overhead and is selected to provide good performance under nominal (or most) channel conditions. The additional pilot can be sent when required and can provide good performance for adverse channel conditions, without incurring a fixed and high data overhead for the pilot.

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The transmitting entity sends signaling to indicate that an additional pilot is being sent. This signaling can be conveniently embedded within a carrier pilot that is transmitted over a designated set of P subbands through most PDUs (eg, P = 4). A set of P pilot symbols is sent in the set of P subbands in each symbol period in which the carrier pilot is transmitted. Different sets of P pilot symbols may be made up of different signaling values, for example one signaling value to indicate that the data symbols are transmitted in the remaining usable subbands, another signaling value to indicate that additional pilot symbols they are being broadcast, and so on. The signaling for the additional pilot can be sent by selecting an appropriate set of P pilot symbols and sending these P pilot symbols in the P subbands used for the carrier pilot. The additional pilot and its signaling can be sent selectively and simultaneously at almost any symbol period in the PDU. The signaling for the additional pilot can also be sent in some other way.

Various aspects and embodiments of the invention are described in more detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows an OFDM subband structure used by IEEE 802.11a;

Figure 2 shows an example of a PDU format suitable for a MIMO system;

Figure 3 shows a process for transmitting an additional pilot;

Figure 4 shows a process for receiving and using the additional pilot;

Figure 5 shows a block diagram of a transmitting entity and a receiving entity;

Figure 6 shows a block diagram of a transmit spatial processor (TX); and

Figure 7 shows a block diagram of a TX pilot signaling processor.

DETAILED DESCRIPTION

The word exemplary is used herein with meaning that serves as an example, case, or illustration. Any embodiment described herein as exemplary should not necessarily be construed as preferred or advantageous over other embodiments.

The pilot transmission and signaling techniques described in this document can be used for a single input single output (SISO) system, a single input multiple output (SIMO) system, a multiple input single output system ( MISO) and a MIMO system. These techniques can be used for an OFDM-based system and for other multi-carrier communication systems. These techniques can also be used with various OFDM subband structures. For clarity, these techniques are specifically described below for a MIMO-OFDM system using the OFDM subband structure defined in IEEE 802.1 Ia.

OFDM's IEEE 802.11 subband structure divides the total system bandwidth into 64 orthogonal subbands (that is, K = 64), which are assigned indices from -32 to 31. Of these 64 subbands, 48 ​​subbands with indices of ± {1, ..., 6, 8, ..., 20, 22, ... , 26} can be used for data transmission and pilot and are called data subbands, 4 subbands with indices of ± {7, 21} can be used for a carrier pilot and possibly signaling and are called pilot subbands, the DC subband with index 0 is not used, and the remaining 11 subbands are also not used, and serve as guard subbands. Therefore, the 64 total subbands include 52 usable subbands made up of 48 data subbands and 4 pilot subbands and 12 unused subbands. This OFDM subband structure is described in a document for the IEEE 802.11a standard titled 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 can use any OFDM subband structure with any number of data, pilots, and guard subbands.

Figure 1 shows a PDU 100 format defined by IEEE 802.11 and suitable for use in various communication systems. At a physical layer (PHY) in the protocol stack for IEEE 802.11, data is processed and transmitted in PHY Protocol Data Units (PPDUs), which are also called thus in this document for simplicity. Each PDU 110 for IEEE 802.11 includes a preamble section 120, a signal section 130, and a data section 150. The preamble section 120 carries short and long training symbols which are described below. Signal section 130 carries an OFDM symbol for signaling for the PDU. Data section 150 carries a variable number of OFDM symbols for data traffic / packets for the PDU. The length of data section 150 is indicated by marking in signal section 130.

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. The four short training symbols are formed by performing an inverse discrete Fourier transform (IDFT) on a specific set of 12 pilot symbols sent in 12 subbands with indices {-24, -20, -16, -12,

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-8, -4, 4, 8, 12, 16, 20, and 24}. A pilot symbol is a modulation symbol for the pilot and is normally known a priori by both the transmitting and receiving entity. 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 of 52 pilot symbols sent on the 52 usable subbands. The same set of 52 pilot symbols is also used for the two long training symbols. A receiving entity may use the short training symbols for signal detection, imprecise estimation of frequency deviation, timing synchronization, automatic gain control (AGC), and so on. The receiving entity can use the long training symbols for channel estimation, precise estimation of frequency deviation, and so on.

Signaling and data are sent in 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 made up of four pilot symbols that are sent on the four pilot subbands through the signal and data sections. Prior to transmission, the pilot symbol for each pilot subband is multiplied by a circularly extended pseudo-random number (PN) sequence of 127 chips to generate a predetermined symbol sequence for that pilot subband. The receiving entity may use the carrier pilot to in-phase track a carrier signal along the signal and data sections.

The pilot structure shown in Figure 1 comprises ten short training symbols, two long training symbols, and the carrier pilot. This pilot structure is generally suitable for an SISO system.

A MIMO system can use different types of pilots to support various functions necessary for the proper operation of the system, such as timing and frequency acquisition, channel estimation, calibration, etc. Table 1 lists four types of pilot and a short description. A pilot is also called a reference, and these two terms are often used interchangeably.

Table 1 - Types of Pilot

<td>Rider type</td><td>Description</td>

<td>Beacon Pilot</td><td>One pilot transmitted from all transmitting antennas and used for timing and frequency acquisition</td>

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

<td>Pilot MEMO Directed</td><td>A pilot transmitted in eigenmodes of a MIMO 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>

Directed and undirected MIMO pilots are described in detail below.

Figure 2 shows an example of a PDU 200 format suitable for the MIMO system. A UDP 210 for this format includes a preamble section 220, a signal section 230, a MIMO pilot section 240, and a data section 250. The preamble section 220 carries the beacon pilot. For the embodiment shown in Figure 2, the beacon pilot is made up of ten short training symbols and two long training symbols. Preamble section 220 is therefore similar to preamble section 120 in Figure 1. Signal section 230 carries the signaling for PDU 210 and may include (1) a field that indicates whether the PDU is formatted 200 or some other format (eg format 100) and (2) a field indicating the length of the MIMO pilot section 240. The MIMO pilot section 240 carries a base MIMO pilot, which may or may not be directed. The base MIMO pilot is normally sent on each PDU and can be transmitted in the same way as the data on the PDU. Data section 250 carries the data PDU for 210. A carrier pilot is sent on the four pilot subbands in signal section 230, MIMO pilot section 240, and data section 250. A PDU can also be called. packet, data drive, frame, slot, block, or some other terminology.

The PDU 200 format includes an example pilot structure for the MIMO system. To reduce data overhead, the pilot structure may include a minimum (or nominal) number of pilots (the base pilot) necessary for proper system operation under normal channel conditions. For example, the MIMO pilot section 240 may carry T OFDM symbols for the MIMO pilot for T transmit antennas. The additional pilot can be adaptively inserted and dispatched if and when needed to achieve improved performance. Additional pilot can be beneficial in certain adverse channel conditions, such as increased fading rates due to the Doppler effect, changing interference characteristics, and so on. The additional pilot may also be sent based on other factors, for example, if the PDU is from a retransmission because an acknowledgment (ACK) has not been received from a previous transmission of the PDU. The additional pilot can be inserted into the data section of the PDU. Can be embedded effectively

ES 2 389 488 T3 signaling to indicate transmission of the additional pilot within the carrier pilot, as described below, or to be sent in signal section 230.

A MIMO channel between a transmitting entity and a receiving entity may be characterized by a H channel response matrix R x T (k) for each subband k, which may be expressed as:

<td></td><td>XM</td><td>W</td><td>- WO</td>

<td>H (A) =</td><td>WO</td><td></td><td>'·· WO</td>

<td></td><td>TO#)</td><td>W *)</td><td></td>

for fc = l ... K,

Eq (l)

Where the input h¡j (k), for / = 1 ... R and j = 1 ... T, denotes the coupling or complex channel gain between the transmitting antenna j and the receiving antenna / for subband k . For simplicity, the MIMO channel is assumed to be full-range with S = T &lt;R.

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

Eq (2) where j, (k, n) is a vector of T pilot symbols to be sent from the T transmitting antennas in subband k in symbol period n;

W (n) is a Walsh diagonal matrix for T transmitting antennas in symbol period n;

is a transmit symbol vector for the undirected MIMO pilot for subband k in symbol period n; and

K_or it is a set of subbands in which the undirected MIMO pilot is sent.

A transmit symbol is a symbol that is sent from a transmitting antenna. The same Walsh matrix W (n) can be used for all subbands and therefore may not be a function of the subband index k.

As an example, if T = 4, then the four transmitting antennas can be assigned four Walsh sequences W1 = {1, 1, 1, 1}, W2 = {1, -1, 1, -1}, W3 = {1, 1, -1, 1}, and W4 = {1, -1, -1, 1}. The Walsh matrix W (1) then contains the first element of the four 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 sequences Walsh, and W (4) contains the fourth element of the four Walsh sequences. The four Walsh matrices W (1) through W (4) can be used in four symbol periods to transmit the undirected MIMO pilot. In general, a full MIMO undirected pilot can be sent in T (consecutive or non-consecutive) symbol periods with code multiplexing, or a symbol period for each chip in the orthogonal sequence. Upon receiving the full MIMO undirected pilot, the receiving entity can perform supplementary processing on the received pilot to estimate H (k).

The transmitting entity may transmit data in the S eigenmodes of the channel response matrix H (k) for each subband k to achieve improved performance. The channel response matrix H (k) for each subband k can be diagonalized to obtain the S eigenmodes of the MIMO channel for that subband. This diagonalization can be achieved by performing either singular value decomposition of H (k) or eigenvalue decomposition of a correlation matrix of H (k), which is R (k) = H^H(k) H (k), where H^Hdenotes the conjugate transpose of H. The singular value decomposition of H (/ r) can be expressed as:

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H (*) = U (¿) S (fr) · V^ff(¿) ,

Eq {3) where

U (/ r) is a unitary matrix R χ R of left eigenvectors of H (/ r); Z (/ r) is a diagonal matrix R χ T of singular values ​​of H (/ r); and V (/ r) is a right-hand eigenvector matrix T χ T of H (/ r).

A unitary matrix M is characterized by the property M^HM_ =], where I is the identity matrix. The columns of a unit matrix are orthogonal to each other. The transmitting entity may use the right eigenvalues ​​in V (/ r) for spatial processing to transmit data on the S eigenmodes of H (/ r). The receiving entity will be able to use the left eigenvalues ​​in U (/ r) for the spatial processing of the receiver to recover the data transmitted in the S eigenmodes of H (/ r). The diagonal matrix Z (/ r) contains real nonnegative values ​​along the diagonal and zeros elsewhere. These diagonal inputs are called singular values ​​of H (/ r) and represent the channel gains for the S eigenmodes of H (/ r). Singular value decomposition is described by Gilbert Strang in Linear Algebra and Applications, Second Edition, Academic Press, 1980.

The transmitting entity may transmit a directed MIMO pilot as follows:

<img file="ES2389488T3_D0001.tif" />

Eq (4) where

Vm (/ f) is the mth eigenvector / column of V (/ r);

p_m(k) is a pilot symbol to be transmitted in the m-th eigenmode of H (/ r);

—_a transmission vector for the directed pilot for the directed MIMO for the mth eigenmode of H (/ r); and

K_s it is a set of subbands in which the directed MIMO pilot is sent.

The directed MIMO pilot received at the receiving entity can be expressed as:

= U (i).? (A) .Vⁱ'(¿) -V „(fc) -p_to(t) + Q (fr), forteK ,, Eq (5) where i-_Fito¡ \) _{it's a} vector of received symbols for the MIMO pilot directed for the m-th eigenmode of H (/ r); or_m(/ r) is the m-th diagonal element of Z (/ r); And u_m(/ r) is the m-th eigenvector / column of U (/ r).

A received symbol is a symbol obtained from a receiving antenna. The transmitting entity may transmit a complete directed MIMO pilot in all S eigenmodes of H (/ r) in S symbol periods, for example, in an eigenmode per symbol period using time multiplexing as shown in equation (4). The receiving entity can obtain an estimate of U (/ r), one column at a time, based on the directed MIMO pilot sent using time multiplexing, as shown in equation (5).

The transmitting entity may also transmit the directed MIMO pilot in all S eigenmodes of H (/ r) simultaneously in S symbol periods using code multiplexing. The code multiplexing directed MIMO pilot can be expressed as:

. &amp; r keK_t,

Eq (6) where V (/ r, n) is a matrix of right eigenvalues ​​of H (/ r, n) for subband k in symbol period n. The receiving entity can obtain an estimate of U (/ r, n) after receiving the complete directed MIMO pilot.

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The transmitting entity may also transmit the complete directed MIMO pilot for all S eigenmodes of H (k) in S subbands k to k + S - 1 in one symbol period using subband multiplexing. The transmitting entity may also transmit the directed MIMO pilot in less than S eigenmodes. For example, the transmitting entity may transmit the MIMO pilot directed in the best or main eigenmode in one symbol period, in the two best eigenmodes in two symbol periods, and so on.

In general, the transmitting entity can transmit the directed and undirected MIMO pilots in various ways using multiplexing of, code, subband, and / or time. Code multiplexing allows the transmitting entity to use the maximum transmit power available to each transmitting antenna for pilot transmission, which can improve channel estimation performance.

The additional pilot can be a MIMO pilot, as described above. The additional pilot can also be some other type of pilot. For example, the transmitting entity may transmit a single pilot symbol stream in a single eigenmode or direct a single pilot symbol stream in some other way. This additional pilot can be used, for example, to drive timing deviation, correct residual frequency deviation, and so on.

The pilot structure includes the base pilot (eg MIMO pilot section 240 in Figure 2) which provides good performance under nominal channel conditions. This results in low data overhead for the pilot. Additional pilots can be transmitted if and when necessary. The number of additional pilots to send, as well as the placement of the additional pilot within a PDU can be flexibly adapted based on channel conditions and / or other factors. For example, a larger number of additional pilots can be dispatched in more adverse channel conditions. The additional pilot can be sent at or near the beginning of a PDU, which can simplify channel estimation and data discovery and can further reduce temporary storage requirements. The additional pilot can also be spread across a PDU, which can improve the performance of a time-changing channel.

Referring to Figure 2, four pilot symbols can be sent in the four pilot subbands in each symbol period in data section 250. These pilot symbols can be used to indicate / signal the content that is sent in the 48 data subbands. If each pilot symbol consists of B bits, then up to 2^4B different signaling values ​​can be determined with the four pilot symbols sent in the four pilot subbands. For example, using binary phase shift keying (BPSK), each pilot symbol is made up of one bit, and up to 2⁴ = 16 different signaling values ​​can be determined with the four pilot symbols.

In general, the 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 determine the specific signaling value sent by the transmitting entity based on these noisy received pilot symbols. The receiving entity may calculate a metric (eg, 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 (eg, shortest distance) as the value sent by the transmitting entity. Error detection is more likely when there are more valid signaling values ​​to choose from.

In one embodiment, the four pilot symbols are used to indicate whether additional data or pilots are sent in the OFDM symbol. Table 2 shows an example signaling set for this embodiment with four bits b1, b2, b3 and b4 carried by the four pilot symbols with BPSK.

Table 2

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

<td rowspan="2">bib₂b₃ba</td><td> '0000'</td><td>Data is being sent in OFDM symbol</td>

<td> '1111'</td><td>A MIMO pilot is being sent on the OFDM symbol</td>

The additional MIMO pilot can be directed or undirected, for example, it can be sent in the same way as the data symbols in the PDU. A data symbol is a modulation symbol for data.

In another embodiment, bits 4B are used to indicate whether an additional pilot is being sent in the OFDM symbol and, if so, information specific to the additional pilot. Table 3 shows an example signaling set for this embodiment with four bits b1, b2, b3 and b4 carried by the four pilot symbols with BPSK.

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Table 3

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

<td rowspan="4">b1b2</td><td> '00'</td><td>Data is being sent in OFDM symbol</td>

<td> '01'</td><td>A directed MIMO pilot is being sent on the OFDM symbol</td>

<td> '10'</td><td>An undirected MIMO pilot is being sent on the OFDM symbol</td>

<td> '11'</td><td>Reserved</td>

<td rowspan="2">b3</td><td> '0'</td><td>An additional pilot with code multiplexing is being sent</td>

<td> '1'</td><td>An additional pilot is being sent with subband multiplexing</td>

<td rowspan="2">b4</td><td> '0'</td><td>An additional pilot is being sent on 48 data subbands</td>

<td> '1'</td><td>An additional pilot is being sent on 24 data subbands</td>

For the embodiment shown in Table 3, bits bi and b2 indicate whether an undirected MIMO pilot, a directed MIMO pilot, or no additional pilots are being sent on the OFDM symbol. Bit b3 indicates whether the MIMO pilot is sent using code / time multiplexing or subband multiplexing. For code multiplexing, the MIMO pilot is sent over multiple symbol periods using multiple orthogonal sequences. For example, an undirected MIMO pilot can be sent from four transmit antennas in four symbol periods using 4-chip Walsh sequences, as shown in equation (2). A directed MIMO pilot can be sent by all four eigenmodes simultaneously in four symbol periods using 4-chip Walsh sequences, as shown in equation (6). For subband multiplexing, the MIMO pilot is sent on multiple subbands in one symbol period. For example, an undirected MIMO pilot can be sent from all four transmitting antennas on four different subbands in one symbol period (e.g., from transmitting antenna 1 in subband k, from transmitting antenna 2 in subband k + 1, from transmitting antenna 3 in subband k + 2, and from transmitting antenna 4 in subband k + 3). A directed MIMO pilot can be sent in four eigenmodes using four different subbands in one symbol period (for example, in eigenmode 1 using subband k, in eigenmode 2 using subband k + 1, in eigenmode 3 using the subband k + 2 and in eigenmode 4 using subband k + 3). Bit b4 indicates the number of subbands used for the additional pilot. For example, additional pilot symbols may be sent on all 48 data subbands or only 24 data subbands (eg, each data subband).

Tables 2 and 3 show two specific embodiments of the signaling embedded in the four pilot subbands with four bits using BPSK. In general, bits 4B for the carrier pilot can be used to transmit any type of information for the additional pilot, such as (1) if the additional pilot is being sent, (2) what type of additional pilot is being sent ( e.g. Undirected MIMO pilot, Directed MIMO pilot and so on), (3) the way 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 (eg, all, half, a quarter, or some other number of data subbands), and (5) possibly other pertinent information. More signaling values ​​provide more flexibility in transmitting the additional pilot. However, the detection performance is also worse with more signaling values. A trade-off could be reached between detection performance and pilot insertion flexibility.

Additional signaling for the pilot in a given PDU can also be sent in signal section 230 of the PDU. This signaling may indicate some or all of the possible information indicated above for the additional pilot. On the other hand, this signaling may indicate the specific symbol periods in which the additional pilot has been sent (for example, in the middle of the data section 250, in all rooms of the data section, in each period of L-th symbol, and so on).

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

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Figure 3 shows a flow chart of a process 300 performed by the transmitting entity to send additional pilots. Process 300 can 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 additional pilots are transmitted in the PDU, for example, based on channel conditions and / or other factors (block 312). If the 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 way, without any additional pilots (block 316). On the contrary, if the additional pilot is to be sent, then the transmitting entity determines the amount, type, location, and so on, of the additional pilot to send in the PDU, for example, based on the conditions of the channel and / or other factors (block 318). The transmitting entity then sends the additional signaling for the pilot in the PDU, eg, embedded in the pilot symbols sent in the four pilot subbands (block 320). The transmitting entity also multiplexes and transmits the additional pilot as indicated by signaling (block 322). The transmitting entity also processes and transmits the PDU in view of the additional pilot (block 324). For example, the length of the PDU can be extended by the number of additional pilots that are sent in the PDU.

Figure 4 shows a flow diagram of a process 400 performed by the receiving entity to receive and use the additional pilot. Process 400 can also be performed for each PDU. The receiving entity receives the base pilot (eg, the MIMO pilot sent in the MIMO pilot section 240) and derives a MIMO channel response estimate based on the received base pilot (block 410). The receiving entity receives the additional signaling for the pilot, for example, from the pilot symbols sent in the four pilot subbands (block 412). The receiving entity determines whether or not the additional pilot is being sent based on the received signaling (block 414). If no additional pilot is 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 the improved MIMO 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).

Figure 5 shows a block diagram of a transmitting entity 510 and a receiving entity 550 in a MIMO system 500. Transmitting entity 510 may be an access point or a user terminal. The receiving entity 550 can also be an access point or a user terminal.

At transmitting entity 510, a TX data processor 512 processes (eg, encodes, interleaves, and assigns symbols) data traffic / packets to obtain data symbols. A TX spatial processor 520 receives and demultiplexes the pilot and data symbols into the appropriate subbands, performs the appropriate spatial processing, and provides T transmit symbol streams for the T transmit antennas to T OFDM (Mod) modulators 530a through 530T. Each OFDM modulator 530 performs OFDM modulation of a respective stream of transmit symbols and provides a stream of samples to an associated transmitter unit (TMTR) 532. Each transmitter unit 532 processes (eg, converts to analog, amplifies, filters, and converts frequency up) its sample stream to generate a modulated signal. Transmitter units 532a through 532t provide T modulated signals for T transmit antennas 534a through 534t, respectively.

At receiver entity 550, R antennas 552a through 552r receive the T transmission signals, and each antenna 552 provides a received signal to a respective receiver unit (RCVR) 554. Each receiver unit 554 processes the received signal and provides a stream of samples. corresponding 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 spatial processor (RX) 570 and the received pilot symbols to a channel estimator 584 within a controller 580. The channel estimator 584 derives channel response estimates for the MIMO channel between transmitting entity 510 and receiving entity 550 for subbands used for data transmission. The channel response estimates can be derived with the base pilot and / or the additional pilot sent to the transmitting entity 510. The controller 580 also derives spatial filter matrices based on the MIMO channel response estimates. The RX 570 spatial processor performs receiver spatial processing (or adapted spatial filtering) on ​​the received data symbols for each subband with the derived spatial filtering matrix 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 572 data processor then processes the detected data symbols for all subbands and provides decoded data.

Controllers 540 and 580 control the operation of the processing units in 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.

Figure 6 shows a block diagram of one embodiment of spatial processor TX 520 at transmitter entity 510. Within processor 520, a spatial data processor TX 610 receives and performs spatial processing on data symbols for transmission through of the T transmitting antennas or the S eigenmodes of each data subband. The TX spatial data processor 610 provides T spatially processed data symbol streams for the T transmit antennas to the T symbol multiplexers (Mux) 640a

ES 2 389 488 T3 up to 640t. A pilot spatial processor TX 620 performs spatial processing on the pilot symbols and provides (1) an undirected MIMO pilot for transmission over the T transmitting antennas or (2) a directed MIMO pilot for transmission at a maximum of S own modes of each sub-band used for the transmission of pilots. A pilot spatial processor TX 620 provides pilot spatially processed symbols for the T symbol transmitting antennas to T symbol multiplexers 640a through 640T.

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

Each OFDM modulator 530 performs OFDM modulation of a respective stream of transmit symbols and provides a corresponding stream of OFDM symbols. For each symbol period, each OFDM modulator 530 obtains K frequency domain values, for example, for 48 data and / or pilot symbols to send in the 48 data subbands, four carrier pilot symbols to send in all four pilot subbands, and 12 zero signal values ​​for the 12 unused subbands. An inverse fast Fourier transform (IFFT) unit 650 transforms the K values ​​from the frequency domain to the time domain with an IFFT of K points and provides a transformed symbol containing K chips in the time domain. To combat inter-symbol 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 repeating part is often referred to as a cyclic prefix or guard interval. An OFDM symbol period (or simply, a symbol period) is the duration of an OFDM symbol.

Figure 7 shows a block diagram of one embodiment of the TX pilot signaling processor 630. The controller 540 provides a value for the additional signaling pilot for a signaling look-up table (LUT) 710, which then provides four signaling symbols. pilot corresponding to that signaling value to four multipliers 712a through 712d. Each multiplier 712 also receives 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 mixed pilot symbol. The multipliers 712a through 712d provide four mixed pilot symbols for the four experimental subbands to T symbol multiplexers 640a through 640t. Each 640i symbol multiplexer, for i = 1 ... T, multiplexes the mixed pilot symbols into the four pilot subbands used for the carrier pilot and multiplexes new spatially processed data and pilot symbols for the transmitting antenna i into the data subbands.

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

For a software implementation, the techniques described in this document can be implemented with modules (eg, procedures, functions, and so on) that perform the functions described in this document. The software codes can be stored in a memory unit (for example, memory units 542 and / or 582 in Figure 5) and be executed by a processor (for example, controller 540 and / or 580 in Figure 5 ). The memory unit may be implemented within the processor or external to the processor, in which case it may be communicatively coupled to the processor through various means as is known in the art.

The foregoing description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention.