# Patent

## Title
Quality control scheme for multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems

## Abstract

A method and apparatus for optimizing the system capacity of an Orthogonal Frequency Division Multiplexing (OFDM) system that uses with Multiple-Input Multiple-Output (MIMO) antennas. In a receiver, a target quality of service (QoS) metric and reference data rate are set. The target QoS metric may be set to a predetermined value and/or may be adjusted dynamically with respect to packet error rate (PER) by a slow outer-loop control processor. The QoS of received signals are compared to the target QoS. Depending on the comparison, the receiver generates a channel quality indicator (CQI) which is sent to the transmitter. The CQI is a one or two bit indicator which indicates to the transmitter to disable, adjust or maintain data transmission rates of particular sub-carriers, groups of sub-carriers per transmit antenna, or groups of sub-carriers across all transmit antennas. At the transmitter, the transmitted data rate is disabled, adjusted or maintained.

## Background
CROSS REFERENCE TO RELATED APPLICATION
This application is a continuation of U.S. patent application Ser. No. 14/224,195, filed on Mar. 25, 2014, which issued as U.S. Pat. No. 9,509,378 on Nov. 29, 2016, which is a continuation of U.S. patent application Ser. No. 12/983,491, filed on Jan. 3, 2011, which issued as U.S. Pat. No. 8,705,389 on Apr. 22, 2014, which is a continuation of application Ser. No. 11/118,867, filed on Apr. 29, 2005, which issued as U.S. Pat. No. 7,864,659 on Jan. 4, 2011, which claims the benefit of U.S. Provisional Application Ser. No. 60/598,183, filed on Aug. 2, 2004, the contents of which are hereby incorporated by reference herein.


FIELD OF INVENTION
The present invention relates to wireless communications. More particularly, the present invention relates to a method and apparatus for optimizing the system capacity of an Orthogonal Frequency Division Multiplexing (OFDM) system that uses Multiple-Input Multiple-Output (MIMO) antennas.
BACKGROUND
Orthogonal Frequency Division Multiplexing (OFDM) is an efficient data transmission scheme where the data is split into smaller streams and each stream is transmitted using a sub-carrier with a smaller bandwidth than the total available transmission bandwidth. The efficiency of OFDM results from selecting sub-carriers that are mathematically orthogonal to each other. This orthogonality prevents closely situated sub-carriers from interfering with each other while each is carrying a portion of the total user data.
For practical reasons, OFDM may be preferred over other transmission schemes such as Code Division Multiple Access (CDMA). When user data is split into streams carried by different sub-carriers, for example, the effective data rate on each sub-carrier is less than the total transmitted data rate. As a result, the symbol duration of data transmitted with an OFDM scheme is much larger than the symbol duration of data transmitted with other schemes. Larger symbol durations are preferable as they can tolerate larger delay spreads. For instance, data that is transmitted with large symbol duration is less affected by multi-path than data that is transmitted with shorter symbol duration. Accordingly, OFDM symbols can overcome delay spreads that are typical in wireless communications without the use of a complicated receiver for recovering from such multi-path delay.
Multiple-Input Multiple-Output (MIMO) refers to a type of wireless transmission and reception scheme wherein both a transmitter and a receiver employ more than one antenna. A MIMO system takes advantage of the spatial diversity or spatial multiplexing options created by the presence of multiple antennas. In addition, a MIMO system improves signal quality, such as for example signal-to-noise ratio (SNR), and increases data throughput.
Multi-path, once considered a considerable burden to wireless communications, can actually be utilized to improve the overall performance of a wireless communication system. Since each multi-path component carries information about a transmitted signal, if properly resolved and collected, these multi-path components reveal more information about the transmitted signal, thus improving the communication.
Orthogonal Frequency Division Multiplexing (OFDM) systems that are used with Multiple-Input Multiple-Output (MIMO) are used to properly process multi-path for improving the overall system performance. In fact, MIMO-OFDM systems are considered the technology solution for the IEEE 802.11n standard. An example of a MIMO-OFDM system is shown in FIG. 1. A transmitter 102 processes a data stream Tx in an OFDM Tx processing unit 102a. This OFDM processing includes sub-carrier allocation and OFDM modulation of each sub-carrier. The modulated sub-carriers are then mapped to multiple antennas 1031 . . . 103m according to a MIMO algorithm in a MIMO Tx processing unit 102b. Once mapped, the sub-carriers are transmitted to receiver 104 over multiple antennas 1031 . . . 103m simultaneously.
At receiver 104, the modulated sub-carriers are received on multiple antennas 1051 . . . 105n. A MIMO processing unit 104a prepares the sub-carriers for demodulation. The sub-carriers are then demodulated in OFDM Rx processing unit 104b, yielding the Rx data.
One of the challenges of the MIMO-OFDM system design of 802.11n, however, is system capacity. Presently, an efficient method for optimizing the system capacity of a MIMO-OFDM system does not exist, particularly when the system utilizes a large number of sub-carriers. The “water-pouring” solution, for example, is a technique for increasing system capacity by selectively performing power or bit allocation to each sub-carrier. This technique requires, however, that the transmitter be aware of channel state information. The transmitter estimates this channel state information using feedback from a receiver in the system. The signaling overhead of this feedback, however, is significant and can limit the increase in system performance, particularly in systems transmitting large amounts of data and/or utilizing a large number of sub-carriers.
Accordingly, it is desirable to have alternate schemes for optimizing the system capacity of an MIMO-OFDM.
SUMMARY
The present relates to a method and apparatus for optimizing the system capacity of an Orthogonal Frequency Division Multiplexing (OFDM) system that uses Multiple-Input Multiple-Output (MIMO) antennas. In a receiver, a target quality of service (QoS) metric and reference data rate are set. The target QoS metric may be set to a predetermined value and/or may be adjusted dynamically with respect to packet error rate (PER) by a slow outer-loop control processor. The QoS of received signals are measured and compared to the target QoS. Depending on the comparison, the receiver generates a channel quality indicator (CQI) which is sent back to the transmitting transmitter. The CQI is a one or two bit indicator which indicates to the transmitter to disable, adjust or maintain data transmission rates of particular sub-carriers, groups of sub-carriers per transmit antenna, or groups of sub-carriers across all transmit antennas. At the transmitter, the transmitted data rate is turned-off, increased, decreased, or maintained. At the receiver, the target QoS metric and reference data rate are adjusted accordingly. This process is repeated for each data frame of each sub-carrier group.



BRIEF DESCRIPTION OF THE DRAWINGS
A more detailed understanding of the invention may be had from the following description, given by way of example and to be understood in conjunction with the accompanying drawings wherein:
FIG. 1 illustrates a Multiple-In Multiple-Out (MIMO) Orthogonal Frequency Division Multiplexing (OFDM) system;
FIG. 2 is a flow diagram of a method for optimizing the system capacity of a MIMO-OFDM system;
FIGS. 3A, 3B, and 3C illustrate various sub-carrier groupings; and
FIG. 4 illustrates a MIMO-OFDM system with means for optimizing its system capacity utilizing quality measurement bits.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention may be implemented in a wireless transmit/receive unit (WTRU) or in a base station. The terminology “WTRU” includes but is not limited to user equipment, a mobile station, a fixed or mobile subscriber unit, a pager, or any other type of device capable of operating in a wireless environment. The terminology “base station” includes but is not limited to a Node-B, a site controller, an access point or any other type of interfacing device in a wireless environment.
Elements of the embodiments may be incorporated into an integrated circuit (IC), multiple ICs, a multitude of interconnecting components, or a combination of interconnecting components and IC(s).
In a preferred embodiment, the system capacity for Orthogonal Frequency Division Multiplexing (OFDM) systems that are used with Multiple-Input Multiple-Output (MIMO) antennas is optimized using quality measurements. These quality measurements may be taken continuously, periodically or preferably, over a sliding window of quality measurement observations. In a MIMO-OFDM receiver, an initial or target quality of service (QoS) metric and corresponding initial reference data rate are set. The QoS of received signals are measured and compared to the target QoS. Depending on the comparison, the receiver generates one of a plurality of channel quality indicators (CQI) which is sent back to the origin transmitter of the signals. The CQI is a one or two bit indicator which informs the transmitting transmitter to disable, adjust or maintain the data transmission rates, (i.e., the modulation order of Quadrature Amplitude Modulation (QAM) and channel code rate), of particular sub-carriers or groups of sub-carriers per transmit antenna. Once the CQI is sent back to the transmitter, the transmitted data rate is disabled, adjusted or maintained in accordance with the CQI, and at the receiver, the target QoS metric and the reference data rate are adjusted accordingly. This process is then repeated for each received signal on each sub-carrier group, gradually reaching optimal system capacity. This concept is further illustrated with reference to FIG. 2.
FIG. 2 illustrates a flow diagram representative of the system-optimizing algorithm of the present embodiment. For the purpose of this illustration, signal-to-interference-ratio (SIR) represents the QoS metric of a sample MIMO-OFDM system. It should be understood, however, that any QoS metric, such as for example, signal-to-noise ratio (SNR), bit error rate (BER), and the like, may be utilized in accordance with the present embodiment to accommodate the needs of a particular user.
Within receiver 201, an initial target SIR (SIRt) is set (step 202). This target SIRt is preferably obtained from a pre-defined storage within the receiver 201, such as for example, a look-up table. Alternatively, the SIRt may be obtained and adjusted dynamically with respect to packet error rate (PER) by a slow outer-loop control processor.
In conjunction with setting the SIRt (step 202), an initial reference data rate (qr) is set (step 204) to a predetermined value. Although the present embodiment describes optimizing the data transmission rate of a MIMO-OFDM system, it should be understood that a MIMO-OFDM system may alternatively be optimized in terms of transmission power.
Once the SIRt and qr are set, (steps 202 and 204, respectively), the receiver 201 measures the SIR of the jth frame of a received ith sub-carrier group (SIRm) (step 206). A sub-carrier group is pre-defined as a single sub-carrier, as a group of sub-carriers from a given transmit antenna, or as a group of sub-carriers from multiple transmit antennas. FIGS. 3A-3C illustrates these various sub-carrier groupings. Transmit antennas 302 and 304, for example, each transmit data over eight sub-carriers, 3021, 3022, . . . , 3028, and 3041, 3042, . . . 3048, respectively. In FIG. 3A, each sub-carrier, 3021-3028 and 3041-3048 is pre-defined as a sub-carrier group 306a-306p comprising a single sub-carrier. In FIG. 3B, sub-carriers 3021-3028 from antenna 302 are grouped into two sub-carrier groupings, 308a and 308b. Similarly, sub-carriers 3041-3048 from antenna 304 are grouped into two sub-carrier groupings, 308c and 308d. FIG. 3C illustrates sub-carrier groupings from FIGS. 3A and 3B each comprising a combination of sub-carriers from both antennas 302 and 304.
The measured SIR (SIRm) (step 206) of the ith sub-carrier group is then compared to the SIRt to calculate their difference according to Formula 1 below:

ΔSIRmt(i,j)=SIRm(i,j)−SIRt(i,j),  Formula (1)

where i is the respective sub-carrier group number and j is the respective frame number (step 208). The calculated difference between the SIRm and the SIRt (ΔSIRmt(i,j)) is then compared to a threshold value (step 210). The threshold value is a pre-defined value stored in the receiver 201 which represents an acceptable negative variance from the target SIR. If ΔSIRmt(i,j) yields a negative variance that is greater than that allowed by the threshold, i.e., ΔSIRmt(i,j) is less than (−)threshold value, a 2-bit CQI, such as for example “00”, is generated and sent to the transmitting transmitter (not shown) (step 210a). This “00” CQI indicates to the transmitter (not shown) to cease transmitting on the current, ith sub-carrier group.

Otherwise, if ΔSIRmt(i,j) is not beyond the pre-defined threshold level, ΔSIRmt(i,j) is compared to the difference between a SIR value associated with the transmitted data rate (q) and a SIR value associated with the next highest data rate (q+1) (ΔSIRq(i,q)) (step 212) in order to determine whether ΔSIRmt(i,j) is large enough to increase the current data rate. To make this determination, receiver 201 utilizes a look-up table which represents Data Rate (q) vs. ΔSIRq. This look-up table is produced from a series of measurements or from simulation(s) and is stored in the receiver 201. In this table, ΔSIRq represents the difference in SIR between a transmitted data rate q and the next highest data rate q+1 in the look-up table. Thus, if ΔSIRmt(i,j) is larger than one-half of ΔSIRq(i,q) for a given frame (j) in a given sub-carrier group (i), (i.e., ΔSIRmt(i,j)>ΔSIRq(i,q)/2), ΔSIRmt(i,j) is large enough to increase the data rate (q) to the next highest data rate (q+1) in the Data Rate look-up table.
Accordingly, a 2-bit CQI, such as for example “10”, is generated and sent to the transmitting transmitter (not shown) (step 212a). This “10” CQI indicates to the transmitter (not shown) to increase the current data rate (q) to the next highest data rate (q+1) in the Data Rate vs. ΔSIRq look-up table (step 212b) and to adjust the target SIR(i,j) (step 212c) in accordance with Formula 2 below:

SIRt(i,j)=SIRt(i,j−1)+ΔSIRq(i,q)/2,  Formula (2)

where SIRt(i,j−1) represents the previous target SIR. Alternatively, the SIRt(i,j) can be adjusted (step 212c) in accordance with Formula 3 below:

SIRt(i,j)=SIRt(i,j−1)+[ΔSIRmt(i,j)−ΔSIRmt(i,j−1)].  Formula (3)

If, however, it is determined that ΔSIRmt(i,j) is not greater than ΔSIRq(i,q)/2 (step 212), ΔSIRmt(i,j) is compared to ΔSIRq(i,q) (step 214) in order to determine whether ΔSIRmt(i,j) is small enough to decrease the data rate (q) to the next lowest data rate (q−1) in the look-up table. To make this determination, receiver 201 utilizes the same Data Rate vs. ΔSIRq look-up table described above with regards to step 212. In this comparison, however, if ΔSIRmt(i,j) is less than one-half of the negative of ΔSIRq(i,q), (i.e., ΔSIRmt(i,j)<−(ΔSIRq(i,q)/2)), a 2-bit CQI, such as for example “01”, is generated and sent to the transmitting transmitter (not shown) (step 214a). This “01” CQI indicates to the transmitter (not shown) to decrease the data rate (q) to the next lowest data rate (q−1) in the Data Rate vs. ΔSIRq look-up table (step 214b) and to adjust the SIRt(i,j) (step 214c) in accordance with Formula 4 below:

SIRt(i,j)=SIRt(i,j−1)−ΔSIRq(i,q)/2,  Formula (4)

where SIRt(i,j−1) represents the target SIR of the previous data frame. Alternatively, the target SIRt(i,j) can be adjusted (step 214c) in accordance with Formula 5 below:

SIRt(i,j)=SIRt(i,j−1)−[ΔSIRmt(i,j)−ΔSIRmt(i,j−1)].  Formula (5)

It should be understood that the difference between successive data rates, (i.e., step size), in the data rate table of steps 212 and 214 does not necessarily have to be uniform. In fact, it may be varied according to a user's needs. For example, the step size in the data rate table may be four (4) for the first x-number of frames (in transient state), while the step size for all frames after the xth frame can be one (1) (steady state).
After comparing the difference between the SIRm and the SIRt for a given frame (j) in a given sub-carrier group (i) (ΔSIRmt(i,j)) to the threshold value in step 210 and to ΔSIRq(i,q) in steps 212-214, it is determined if ΔSIRmt(i,j) is within the threshold value (step 210) and neither large enough to increase the current data rate (step 212) nor small enough to decrease the current data rate (step 214). If ΔSIRmt(i,j) does not meet that criteria, a 2-bit CQI, such as for example “11”, is generated and sent to the transmitting transmitter (not shown) (step 216). This “11” CQI indicates to the transmitter (not shown) to continue transmitting at the current data rate.
It should be noted that steps 206 through 216 of this process, illustrated by FIG. 2, comprise a looping algorithm which is repeated for all sub-carrier groups (i) and for all frames (j). In addition, the target SIR(i,j) and reference data rate (i,j) of a given sub-carrier group (i) and frame (j) act as the reference SIRt and reference data rate (qr), respectively, for the next frame (j+1) in the ith sub-carrier group. It is this continual updating of the transmitted data rate that causes the MIMO-OFDM system to gradually reach its optimal performance level.
A MIMO-OFDM system with means for optimizing its system capacity utilizing quality measurement bits in a manner described herein is shown in FIG. 4. A transmitter 402 processes a data stream Tx in an OFDM processing unit 402a. This OFDM processing includes sub-carrier allocation and OFDM modulation of each sub-carrier. The modulated sub-carriers are then mapped to multiple antennas 4031 . . . 403m according to a MIMO algorithm in a MIMO Tx processing unit 402b. Once mapped, the sub-carriers are transmitted to a receiver 404 over multiple antennas 4031 . . . 403m simultaneously.
At the receiver 404, the modulated sub-carriers are received on multiple antennas 4051 . . . 405n. The received sub-carriers are sent to a MIMO Rx processing unit 404a where an inverse MIMO algorithm prepares the sub-carriers for demodulation. The MIMO decoded sub-carriers are then sent to an OFDM Rx unit 404b where they are demodulated. Next, the demodulated data is sent to a Channel Quality Measurement unit 404c, frame by frame, wherein a quality measurement is taken for each data frame. Each of these quality measurements are then sequentially compared to a target quality metric in a Channel Quality Comparison unit 404d. Depending on the comparison, a Channel Quality Indicator (CQI) Signaling unit 404e generates a one or two bit CQI for each measured data frame and sends the CQIs to a MUX unit 404f for processing. These CQIs are then modulated in an OFDM Tx unit 404g, and mapped to multiple antennas 4051 . . . 405n via MIMO Tx unit 404h for transmission to transmitter 402.
At the transmitter 402, the encoded CQIs are received on multiple antennas 4031 . . . 403m, prepared for demodulation in a MIMO Rx unit 402c, and demodulated in an OFDM Rx unit 402d. Once demodulated, the extracted data is sent to a CQI recovery unit 402e where the one or two bit CQI is extracted and processed. The OFDM processing unit 402a then allocates and modulates the sub-carriers with the next Tx data stream according to the processed CQI information. This entire process is then repeated so as to iteratively increase (or decrease) the data transmission rate of a given sub-carrier thereby maximizing the system's capacity.
In an alternate embodiment, the CQI can be sent as a 1-bit indicator, where one state of the binary bit would indicate to the transmitter to increase the data rate to a higher level and the other state of the binary bit is sent to indicate to the transmitter to decrease the transmitted data rate.
Although the features and elements of the present invention are described in the preferred embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the preferred embodiments or in various combinations with or without other features and elements of the present invention. Further, the features and elements of the present invention may be implemented on a single IC, such as an application specific integrated circuit (ASIC), multiple ICs, discrete components, or a combination of discrete components and ICs. Moreover, the present invention may be implemented in any type of wireless communication system. In some deployments, the IC(s)/discrete components may have some of these features and elements, which are totally or partially disabled or deactivated.
While the present invention has been described in terms of the preferred embodiment, other variations which are within the scope of the invention as outlined in the claims below will be apparent to those skilled in the art.

## Claims

1. A wireless transmit/receive unit (WTRU) comprising:
a receiver configured to receive an orthogonal frequency division multiplexing (OFDM) signal that includes a plurality of subcarriers; and
a transmitter configured to transmit a channel quality indicator that includes an n-bit indication of a difference between a first value in a channel quality indicator table and a second value in the channel quality indicator table; wherein the second value is associated with a code rate and a modulation scheme for a group of subcarriers in the plurality of subcarriers;
wherein the receiver is further configured to receive a data stream using a modulation and coding scheme (MCS) and sub-carriers that are allocated subsequent to the transmission of the transmitted channel quality indicator.

2. The WTRU of claim 1 wherein the n-bit indication is a 2-bit indication.

3. The WTRU of claim 1 wherein the second value is associated with at least one particular transmit antenna of a base station.

4. The WTRU of claim 1 wherein the OFDM signal is a multiple-input multiple-output (MIMO) OFDM signal;
wherein the MIMO OFDM signal includes a plurality of MIMO signals; and
wherein the n-bit indication is associated with one of the plurality of MIMO signals and one of a group of subcarriers in the plurality of subcarriers.

5. A method comprising:
receiving, by a wireless transmit/receive unit (WTRU), an orthogonal frequency division multiplexing (OFDM) signal; wherein the OFDM signal includes a plurality of subcarriers;
transmitting a channel quality indicator that includes an n-bit indication of a difference between a first value in a channel quality indicator table and a second value in the channel quality indicator table; wherein the second value is associated with a code rate and a modulation scheme for a group of subcarriers in the plurality of subcarriers; and
receiving a data stream using a modulation and coding scheme (MCS) and sub-carriers that are allocated subsequent to the transmission of the transmitted channel quality indicator.

6. The method of claim 5 wherein the n-bit indication is a 2-bit indication.

7. The method of claim 5 wherein the second value is associated with at least one particular transmit antenna of a base station.

8. The method of claim 5 wherein the OFDM signal is a multiple-input multiple-output (MIMO) OFDM signal;
wherein the MIMO OFDM signal includes a plurality of MIMO signals; and
wherein the n-bit indication is associated with one of the plurality of MIMO signals and one of a group of subcarriers in the plurality of subcarriers.

9. A base station comprising:
a transmitter configured to transmit an orthogonal frequency division multiplexing (OFDM) signal that includes a plurality of subcarriers; and
a receiver configured to receive a channel quality indicator that includes an n-bit indication of a difference between a first value in a channel quality indicator table and a second value in the channel quality indicator table; wherein the second value associated with a code rate and a modulation scheme for a group of subcarriers in the plurality of subcarriers;
wherein the transmitter further configured to transmit a date stream using a modulation and coding scheme (MCS) and sub-carriers that are allocated subsequent to reception of the received channel quality indicator.

10. The base station of claim 9 wherein the n-bit indication is a 2-bit indication.

11. The base station of claim 9 wherein the second value is associated with at least one particular transmit antenna of the base station.

12. The base station of claim 9 wherein the OFDM signal is a multiple-input multiple-output (MIMO) OFDM signal;
wherein the MIMO OFDM signal includes a plurality of MIMO signals; and
wherein the n-bit indication is associated with one of the plurality of MIMO signals and one of a group of subcarriers in the plurality of subcarriers.

13. The base station of claim 9 wherein the channel quality indicator is based on signal to interference ratios.