Patent Publication Number: US-9900066-B2

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

Description:
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  102   a . This OFDM processing includes sub-carrier allocation and OFDM modulation of each sub-carrier. The modulated sub-carriers are then mapped to multiple antennas  103   1  . . .  103   m  according to a MIMO algorithm in a MIMO Tx processing unit  102   b . Once mapped, the sub-carriers are transmitted to receiver  104  over multiple antennas  103   1  . . .  103   m  simultaneously. 
     At receiver  104 , the modulated sub-carriers are received on multiple antennas  105   1  . . .  105   n . A MIMO processing unit  104   a  prepares the sub-carriers for demodulation. The sub-carriers are then demodulated in OFDM Rx processing unit  104   b , 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 (SIR t ) is set (step  202 ). This target SIR t  is preferably obtained from a pre-defined storage within the receiver  201 , such as for example, a look-up table. Alternatively, the SIR t  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 SIR t  (step  202 ), an initial reference data rate (q r ) 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 SIR t  and q r  are set, (steps  202  and  204 , respectively), the receiver  201  measures the SIR of the j th  frame of a received i th  sub-carrier group (SIR m ) (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,  302   1 ,  302   2 , . . . ,  302   8 , and  304   1 ,  304   2 , . . .  304   8 , respectively. In  FIG. 3A , each sub-carrier,  302   1 - 302   8  and  304   1 - 304   8  is pre-defined as a sub-carrier group  306   a - 306   p  comprising a single sub-carrier. In  FIG. 3B , sub-carriers  302   1 - 302   8  from antenna  302  are grouped into two sub-carrier groupings,  308   a  and  308   b . Similarly, sub-carriers  304   1 - 304   8  from antenna  304  are grouped into two sub-carrier groupings,  308   c  and  308   d .  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 (SIR m ) (step  206 ) of the i th  sub-carrier group is then compared to the SIR t  to calculate their difference according to Formula 1 below:
 
ΔSIR mt(i,j) =SIR m(i,j) −SIR t(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 SIR m  and the SIR t  (ΔSIR mt(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 ΔSIR mt(i,j)  yields a negative variance that is greater than that allowed by the threshold, i.e., ΔSIR mt(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  210   a ). This “00” CQI indicates to the transmitter (not shown) to cease transmitting on the current, i th  sub-carrier group.
 
     Otherwise, if ΔSIR mt(i,j)  is not beyond the pre-defined threshold level, ΔSIR mt(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 ΔSIR mt(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 ΔSIR mt(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., ΔSIR mt(i,j) &gt;ΔSIRq (i,q) /2), ΔSIR mt(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  212   a ). 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  212   b ) and to adjust the target SIR (i,j)  (step  212   c ) in accordance with Formula 2 below:
 
SIR t(i,j) =SIR t(i,j−1) +ΔSIR q   (i,q) /2,  Formula (2)
 
where SIR t(i,j−1)  represents the previous target SIR. Alternatively, the SIR t(i,j)  can be adjusted (step  212   c ) in accordance with Formula 3 below:
 
SIR t(i,j) =SIR t(i,j−1) +[ΔSIR mt(i,j) −ΔSIR mt(i,j−1) ].  Formula (3)
 
     If, however, it is determined that ΔSIR mt(i,j)  is not greater than ΔSIRq (i,q) /2 (step  212 ), ΔSIR mt(i,j)  is compared to ΔSIRq (i,q)  (step  214 ) in order to determine whether ΔSIR mt(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 ΔSIR mt(i,j)  is less than one-half of the negative of ΔSIRq (i,q) , (i.e., ΔSIR mt(i,j) &lt;−(ΔSIRq (i,q) /2)), a 2-bit CQI, such as for example “01”, is generated and sent to the transmitting transmitter (not shown) (step  214   a ). 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  214   b ) and to adjust the SIR t(i,j)  (step  214   c ) in accordance with Formula 4 below:
 
SIR t(i,j) =SIR t(i,j−1) −ΔSIR q   (i,q) /2,  Formula (4)
 
where SIR t(i,j−1)  represents the target SIR of the previous data frame. Alternatively, the target SIR t(i,j)  can be adjusted (step  214   c ) in accordance with Formula 5 below:
 
SIR t(i,j) =SIR t(i,j−1) −[ΔSIR mt(i,j) −ΔSIR mt(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&#39;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 SIR m  and the SIR t  for a given frame (j) in a given sub-carrier group (i) (ΔSIR mt(i,j) ) to the threshold value in step  210  and to ΔSIRq (i,q)  in steps  212 - 214 , it is determined if ΔSIR mt(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 ΔSIR mt(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 SIR t  and reference data rate (q r ), respectively, for the next frame (j+1) in the i th  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  402   a . This OFDM processing includes sub-carrier allocation and OFDM modulation of each sub-carrier. The modulated sub-carriers are then mapped to multiple antennas  403   1  . . .  403   m  according to a MIMO algorithm in a MIMO Tx processing unit  402   b . Once mapped, the sub-carriers are transmitted to a receiver  404  over multiple antennas  403   1  . . .  403   m  simultaneously. 
     At the receiver  404 , the modulated sub-carriers are received on multiple antennas  405   1  . . .  405   n . The received sub-carriers are sent to a MIMO Rx processing unit  404   a  where an inverse MIMO algorithm prepares the sub-carriers for demodulation. The MIMO decoded sub-carriers are then sent to an OFDM Rx unit  404   b  where they are demodulated. Next, the demodulated data is sent to a Channel Quality Measurement unit  404   c , 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  404   d . Depending on the comparison, a Channel Quality Indicator (CQI) Signaling unit  404   e  generates a one or two bit CQI for each measured data frame and sends the CQIs to a MUX unit  404   f  for processing. These CQIs are then modulated in an OFDM Tx unit  404   g , and mapped to multiple antennas  405   1  . . .  405   n  via MIMO Tx unit  404   h  for transmission to transmitter  402 . 
     At the transmitter  402 , the encoded CQIs are received on multiple antennas  403   1  . . .  403   m , prepared for demodulation in a MIMO Rx unit  402   c , and demodulated in an OFDM Rx unit  402   d . Once demodulated, the extracted data is sent to a CQI recovery unit  402   e  where the one or two bit CQI is extracted and processed. The OFDM processing unit  402   a  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&#39;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.