Patent Publication Number: US-2011077044-A1

Title: Power control for wireless lan stations

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims benefit of U.S. Provisional Patent Application Serial No. 61/312,428, entitled “Power Control Mechanism for Uplink Multiuser MIMO”, filed Mar. 10, 2010 and is a continuation-in-part of U.S. patent application Ser. No. 12/352,733, entitled “Power Control for Wireless LAN Stations”, filed Jan. 13, 2009, which claims benefit of U.S. Provisional Patent Application Ser. No. 61/090,365, entitled “Power Control for SDMA Stations”, filed Aug. 20, 2008, all of which are herein incorporated by reference in their entirety. 
    
    
     FIELD  
     Certain embodiments of the present disclosure generally relate to wireless communication using multi-antenna transmission for spatial division multiple access (SDMA) in a multiple-input multiple-output (MIMO) communication system and, more specifically, to controlling the power of uplink (UL) signals from multiple SDMA stations in such a system. 
     BACKGROUND 
     In order to address the issue of increasing bandwidth requirements demanded for wireless communication systems, different schemes are being developed to allow multiple user terminals to communicate with a single base station by sharing the same channel (same time and frequency resources) while achieving high data throughputs. Spatial Division Multiple Access (SDMA) represents one such approach that has recently emerged as a popular technique for the next generation communication systems. SDMA techniques may be adopted in several emerging wireless communications standards such as IEEE 802.11 (IEEE is the acronym for the Institute of Electrical and Electronic Engineers, 3 Park Avenue, 17th floor, New York, N.Y.) and Long Term Evolution (LTE). 
     In SDMA systems, a base station may transmit or receive different signals to or from a plurality of mobile user terminals at the same time and using the same frequency. In order to achieve reliable data communication, user terminals may need to be located in sufficiently different directions. Independent signals may be simultaneously transmitted from each of multiple space-separated antennas at the base station. Consequently, the combined transmissions may be directional, i.e., the signal that is dedicated for each user terminal may be relatively strong in the direction of that particular user terminal and sufficiently weak in directions of other user terminals. Similarly, the base station may simultaneously receive on the same frequency the combined signals from multiple user terminals through each of multiple antennas separated in space, and the combined received signals from the multiple antennas may be split into independent signals transmitted from each user terminal by applying the appropriate signal processing technique. 
     A multiple-input multiple-output (MIMO) wireless system employs a number (N T ) of transmit antennas and a number (N R ) of receive antennas for data transmission. A MIMO channel formed by the N T  transmit and N R  receive antennas may be decomposed into N S  spatial channels, where, for all practical purposes, N S ≦min {N T ,N R }. The N S  spatial channels may be used to transmit N S  independent data streams to achieve greater overall throughput. 
     In a multiple-access MIMO system based on SDMA, an access point can communicate with one or more user terminals at any given moment. If the access point communicates with a single user terminal, then the N T  transmit antennas are associated with one transmitting entity (either the access point or the user terminal), and the N R  receive antennas are associated with one receiving entity (either the user terminal or the access point). The access point can also communicate with multiple user terminals simultaneously via SDMA. For SDMA, the access point utilizes multiple antennas for data transmission and reception, and each of the user terminals typically utilizes less than the number of access point antennas for data transmission and reception. When SDMA is transmitted from an access point, N S ≦min {N T , sum(N R )}, where sum(N R ) represents the summation of all user terminal receive antennas. When SDMA is transmitted to an access point, N S ≦min {sum(N T ), N R }, where sum(N T ) represents the summation of all user terminal transmit antennas. 
     Orthogonal Frequency Division Multiple Access (OFDMA) is another technique for allowing multiple user terminals to communicate with a single base station. In an OFDMA-based system, multiple user terminals may communicate on different OFDM subcarriers (i.e., different frequencies) to a base station. 
     SUMMARY 
     Certain embodiments of the present disclosure provide a method for wireless communication. The method generally includes receiving a plurality of uplink (UL) signals, determining power of the received plurality of UL signals, determining adjustment information for a UL signal in the plurality of UL signals based on at least the power of the received UL signals and a target carrier-to-interference (C/I) ratio, and transmitting the adjustment information. 
     Certain embodiments of the present disclosure provide a computer-program product for wireless communications. The computer-program product typically includes a computer-readable medium having instructions stored thereon, the instructions being executable by one or more processors. The instructions generally include instructions for receiving a plurality of UL signals, instructions for determining power of the received plurality of UL signals, instructions for determining adjustment information for a UL signal in the plurality of UL signals based on at least the power of the received UL signals and a target C/I ratio, and instructions for transmitting the adjustment information. 
     Certain embodiments of the present disclosure provide an apparatus for wireless communications. The apparatus generally includes means for receiving a plurality of UL signals, means for determining power of the received plurality of UL signals, means for determining adjustment information for a UL signal in the plurality of UL signals based on at least the power of the received UL signals and a target C/I ratio, and means for transmitting the adjustment information. 
     Certain embodiments of the present disclosure provide an access point (AP). The AP generally includes a receiver configured to a plurality of UL signals, logic for determining power of the received plurality of UL signals, logic for determining adjustment information for a UL signal in the plurality of UL signals based on at least the power of the received UL signals and a target C/I ratio, and a transmitter configured to transmit the adjustment information. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited features of the present disclosure can be understood in detail, a more particular description, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only certain typical embodiments of this disclosure and are therefore not to be considered limiting of its scope, for the description may admit to other equally effective embodiments. 
         FIG. 1  illustrates a spatial division multiple access (SDMA) multiple-input multiple-output (MIMO) wireless system, in accordance with certain embodiments of the present disclosure. 
         FIG. 2  illustrates a block diagram of an access point (AP) and two user terminals, in accordance with certain embodiments of the present disclosure. 
         FIG. 3  illustrates various components that may be utilized in a wireless device, in accordance with certain embodiments of the present disclosure. 
         FIG. 4  illustrates performance degradation in an interleaved OFDMA scheme for multiple users, in accordance with certain embodiments of the present disclosure. 
         FIG. 5  illustrates example operations for open loop (and optional closed loop) power control of an uplink (UL) signal from the perspective of a user terminal, in accordance with certain embodiments of the present disclosure. 
         FIG. 5A  is a block diagram of means corresponding to the example operations of  FIG. 5  for UL signal power control, in accordance with certain embodiments of the present disclosure. 
         FIGS. 6A and 6B  illustrate transmissions of power control information of an access station, in accordance with certain embodiments of the present disclosure. 
         FIG. 7  illustrates example operations for closed loop power control of a UL signal from the perspective of an AP, in accordance with certain embodiments of the present disclosure. 
         FIG. 7A  is a block diagram of means corresponding to the example operations of  FIG. 7  for controlling the power of a UL signal from the perspective of an AP, in accordance with certain embodiments of the present disclosure. 
         FIG. 8  illustrates example operations for power control of a UL signal based on the power of a plurality of received UL signals from the perspective of an AP, in accordance with certain embodiments of the present disclosure. 
         FIG. 8A  is a block diagram of means corresponding to the example operations of  FIG. 8  for controlling the power of a UL signal from the perspective of an AP, in accordance with certain embodiments of the present disclosure. 
         FIGS. 9-11  illustrate various examples of applying a power control mechanism for UL multiuser MIMO (UL-MU-MIMO), in accordance with certain embodiments of the present disclosure. 
         FIG. 12  illustrates an example UL-MU-MIMO protocol that may be used for power control of the UL signals, in accordance with certain embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Certain embodiments of the present disclosure provide techniques and apparatus for controlling the transmit power of an uplink (UL) signal from a user terminal in a wireless communications system in an effort to achieve some target characteristic, such as a target carrier-to-interference (C/I) ratio, at an access point (AP). In this manner, such a user terminal may help avoid or compensate for imbalances in received radio frequency (RF) power between UL signals received from multiple user terminals by the AP. For example, the transmit power at each user terminal may be controlled in an effort to achieve a target post-processing C/I ratio of 28 dB per spatial stream in an effort to reduce large power imbalances and optimize throughput per user terminal The user terminal and the AP may compose part of a multiple-input multiple-output (MIMO) communication system utilizing spatial-division multiple access (SDMA) techniques. 
     The word “exemplary” 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. Also as used herein, the term “legacy stations” generally refers to wireless network nodes that support 802.11n or earlier versions of the IEEE 802.11 standard. 
     The multi-antenna transmission techniques described herein may be used in combination with various wireless technologies such as Code Division Multiple Access (CDMA), Orthogonal Frequency Division Multiplexing (OFDM), Time Division Multiple Access (TDMA), and so on. Multiple user terminals can concurrently transmit/receive data via different (1) orthogonal code channels for CDMA, (2) time slots for TDMA, or (3) sub-bands for OFDM. A CDMA system may implement IS-2000, IS-95, IS-856, Wideband-CDMA (W-CDMA), or some other standards. An OFDM system may implement IEEE 802.11 or some other standards. A TDMA system may implement GSM or some other standards. These various standards are known in the art. 
     An Example MIMO System  
       FIG. 1  shows a multiple-access MIMO system  100  with access points and user terminals. For simplicity, only one access point  110  is shown in  FIG. 1 . An access point (AP) is generally a fixed station that communicates with the user terminals and may also be referred to as a base station or some other terminology. A user terminal (UT) may be fixed or mobile and may also be referred to as a mobile station (MS), a station (STA), or some other terminology. A user terminal may be a wireless device, such as a cellular phone, a personal digital assistant (PDA), a handheld device, a wireless modem, a laptop computer, a personal computer, etc. 
     Access point  110  may communicate with one or more user terminals  120  at any given moment on the downlink and uplink. The downlink (i.e., forward link) is the communication link from the access point to the user terminals, and the uplink (i.e., reverse link) is the communication link from the user terminals to the access point. A user terminal may also communicate peer-to-peer with another user terminal. A system controller  130  couples to and provides coordination and control for the access points. 
     While portions of the following disclosure will describe user terminals  120  capable of communicating via spatial division multiple access (SDMA), for certain embodiments, the user terminals  120  may also include some user terminals that do not support SDMA. Thus, for such embodiments, an AP  110  may be configured to communicate with both SDMA and non-SDMA user terminals. This approach may conveniently allow older versions of user terminals (“legacy” stations) to remain deployed in an enterprise, extending their useful lifetime, while allowing newer SDMA user terminals to be introduced as deemed appropriate. 
     System  100  employs multiple transmit and multiple receive antennas for data transmission on the downlink and uplink. Access point  110  is equipped with a number N ap  of antennas and represents the multiple-input (MI) for downlink transmissions and the multiple-output (MO) for uplink transmissions. A set N u  of selected user terminals  120  collectively represents the multiple-output for downlink transmissions and the multiple-input for uplink transmissions. For pure SDMA, it is desired to have N ap ≧N u ≧1 if the data symbol streams for the N u  user terminals are not multiplexed in code, frequency, or time by some means. N u  may be greater than N ap  if the data symbol streams can be multiplexed using different code channels with CDMA, disjoint sets of sub-bands with OFDM, and so on. Each selected user terminal transmits user-specific data to and/or receives user-specific data from the access point. In general, each selected user terminal may be equipped with one or multiple antennas (i.e., N ut ≧1). The N u  selected user terminals can have the same or different number of antennas. 
     MIMO system  100  may be a time division duplex (TDD) system or a frequency division duplex (FDD) system. For a TDD system, the downlink and uplink share the same frequency band. For an FDD system, the downlink and uplink use different frequency bands. MIMO system  100  may also utilize a single carrier or multiple carriers for transmission. Each user terminal may be equipped with a single antenna (e.g., in order to keep costs down) or multiple antennas (e.g., where the additional cost can be supported). 
       FIG. 2  shows a block diagram of access point  110  and two user terminals  120   m  and  120   x  in MIMO system  100 . Access point  110  is equipped with N ap  antennas  224   a  through  224   ap.  User terminal  120   m  is equipped with N ut,m  antennas  252   ma  through  252   mu,  and user terminal  120   x  is equipped with N ut,x  antennas  252   xa  through  252   xu.  Access point  110  is a transmitting entity for the downlink and a receiving entity for the uplink. Each user terminal  120  is a transmitting entity for the uplink and a receiving entity for the downlink. As used herein, a “transmitting entity” is an independently operated apparatus or device capable of transmitting data via a wireless channel, and a “receiving entity” is an independently operated apparatus or device capable of receiving data via a wireless channel. In the following description, the subscript “dn” denotes the downlink, the subscript “up” denotes the uplink, N up  user terminals are selected for simultaneous transmission on the uplink, N dn  user terminals are selected for simultaneous transmission on the downlink, N up  may or may not be equal to N dn , and N up  and N dn  may be static values or can change for each scheduling interval. The beam-steering or some other spatial processing technique may be used at the access point and user terminal. 
     On the uplink, at each user terminal  120  selected for uplink transmission, a TX data processor  288  receives traffic data from a data source  286  and control data from a controller  280 . TX data processor  288  processes (e.g., encodes, interleaves, and modulates) the traffic data {d up,m } for the user terminal based on the coding and modulation schemes associated with the rate selected for the user terminal and provides a data symbol stream {s up,m }. A TX spatial processor  290  performs spatial processing on the data symbol stream {s up,m } and provides N ut,m  transmit symbol streams for the N ut,m  antennas. Each transmitter unit (TMTR)  254  receives and processes (e.g., converts to analog, amplifies, filters, and frequency upconverts) a respective transmit symbol stream to generate an uplink signal. N ut,m  transmitter units  254  provide N ut,m  uplink signals for transmission from N ut,m  antennas  252  to the access point  110 . 
     A number N up  of user terminals may be scheduled for simultaneous transmission on the uplink. Each of these user terminals performs spatial processing on its data symbol stream and transmits its set of transmit symbol streams on the uplink to the access point. 
     At access point  110 , N ap  antennas  224   a  through  224   ap  receive the uplink signals from all N up  user terminals transmitting on the uplink. Each antenna  224  provides a received signal to a respective receiver unit (RCVR)  222 . Each receiver unit  222  performs processing complementary to that performed by transmitter unit  254  and provides a received symbol stream. An RX spatial processor  240  performs receiver spatial processing on the N ap  received symbol streams from N ap  receiver units  222  and provides N up  recovered uplink data symbol streams. The receiver spatial processing is performed in accordance with the channel correlation matrix inversion (CCMI), minimum mean square error (MMSE), successive interference cancellation (SIC), or some other technique. Each recovered uplink data symbol stream {s up,m } is an estimate of a data symbol stream {s up,m } transmitted by a respective user terminal An RX data processor  242  processes (e.g., demodulates, deinterleaves, and decodes) each recovered uplink data symbol stream {s up,m } in accordance with the rate used for that stream to obtain decoded data. The decoded data for each user terminal may be provided to a data sink  244  for storage and/or a controller  230  for further processing. 
     On the downlink, at access point  110 , a TX data processor  210  receives traffic data from a data source  208  for N dn  user terminals scheduled for downlink transmission, control data from a controller  230 , and possibly other data from a scheduler  234 . The various types of data may be sent on different transport channels. TX data processor  210  processes (e.g., encodes, interleaves, and modulates) the traffic data for each user terminal based on the rate selected for that user terminal TX data processor  210  provides N dn  downlink data symbol streams for the N dn  user terminals. A TX spatial processor  220  performs spatial processing on the N dn  downlink data symbol streams, and provides N ap  transmit symbol streams for the N ap  antennas. Each transmitter unit (TMTR)  222  receives and processes a respective transmit symbol stream to generate a downlink signal. N ap  transmitter units  222  provide N ap  downlink signals for transmission from N ap  antennas  224  to the user terminals. 
     At each user terminal  120 , N ut,m  antennas  252  receive the N ap  downlink signals from access point  110 . Each receiver unit (RCVR)  254  processes a received signal from an associated antenna  252  and provides a received symbol stream. An RX spatial processor  260  performs receiver spatial processing on N ut,m  received symbol streams from N ut,m  receiver units  254  and provides a recovered downlink data symbol stream {s dn,m } for the user terminal The receiver spatial processing is performed in accordance with the CCMI, MMSE, or some other technique. An RX data processor  270  processes (e.g., demodulates, deinterleaves, and decodes) the recovered downlink data symbol stream to obtain decoded data for the user terminal 
     At each user terminal  120 , N ut,m  antennas  252  receive the N ap  downlink signals from access point  110 . Each receiver unit (RCVR)  254  processes a received signal from an associated antenna  252  and provides a received symbol stream. An RX spatial processor  260  performs receiver spatial processing on N ut,m  received symbol streams from N ut,m  receiver units  254  and provides a recovered downlink data symbol stream {s dn,m } for the user terminal The receiver spatial processing is performed in accordance with the CCMI, MMSE, or some other technique. An RX data processor  270  processes (e.g., demodulates, deinterleaves, and decodes) the recovered downlink data symbol stream to obtain decoded data for the user terminal 
       FIG. 3  illustrates various components that may be utilized in a wireless device  302  that may be employed within the system  100 . The wireless device  302  is an example of a device that may be configured to implement the various methods described herein. The wireless device  302  may be an access point  110  or a user terminal  120 . 
     The wireless device  302  may include a processor  304  which controls operation of the wireless device  302 . The processor  304  may also be referred to as a central processing unit (CPU). Memory  206 A, which may include both read-only memory (ROM) and random access memory (RAM), provides instructions and data to the processor  304 . A portion of the memory  206 A may also include non-volatile random access memory (NVRAM). The processor  304  typically performs logical and arithmetic operations based on program instructions stored within the memory  206 A. The instructions in the memory  206 A may be executable to implement the methods described herein. 
     The wireless device  302  may also include a housing  308  that may include a transmitter  310  and a receiver  312  to allow transmission and reception of data between the wireless device  302  and a remote location. The transmitter  310  and receiver  312  may be combined into a transceiver  314 . A plurality of transmit antennas  316  may be attached to the housing  308  and electrically coupled to the transceiver  314 . The wireless device  302  may also include (not shown) multiple transmitters, multiple receivers, and multiple transceivers. 
     The wireless device  302  may also include a signal detector  318  that may be used in an effort to detect and quantify the level of signals received by the transceiver  314 . The signal detector  318  may detect such signals as total energy, energy per subcarrier per symbol, power spectral density and other signals. The wireless device  302  may also include a digital signal processor (DSP)  320  for use in processing signals. 
     The various components of the wireless device  302  may be coupled together by a bus system  322 , which may include a power bus, a control signal bus, and a status signal bus in addition to a data bus. 
     Power Control for 802.11 Stations for Multiple Access 
     The next generation of the IEEE 802.11 standard is moving towards SDMA and Orthogonal Frequency-Division Multiple Access (OFDMA). These technologies include provisions for multiple stations (STAs) to be simultaneously transmitting to an access point (AP). However, large power imbalances in the received power from multiple stations may result in performance degradation due to signal-dependent RF noise floors and frequency offset. For example, each AP-STA link may have different frequency offsets, which may lead to inter-channel interference (ICI) distortion. The signal-dependent RF noise floors may arise from I-Q imbalance and RF nonlinearities in each STA. 
     For example,  FIG. 4  illustrates performance degradation in an interleaved OFDMA scheme with uplink (UL) signals from four users. In  FIG. 4 , three of the four user UL signal tones are power-boosted OFDMA UL signal tones  401 ,  402 ,  403 , while the desired user UL signal  404  is not power-boosted in the same manner and, thus, has a lower power level. Such large uplink power differences across user terminals may lead to increased performance degradation for user terminals with lower received OFDMA signal power at the AP. To the first order, even larger performance degradation may be expected from SDMA power imbalances. 
     As shown, tones adjacent to power-boosted OFDMA (or SDMA) tones  401 ,  402 ,  403  may suffer from ICI distortion  406  (due to frequency offset error) and from phase noise distortion  408 . Furthermore, tones  412  that are mirrors of power-boosted tones may suffer from I-Q imbalance distortion  410  above the thermal noise floor  414 . 
     Accordingly, what is needed are techniques and apparatus for controlling the power of uplink signals from multiple user terminals in an effort to reduce power degradation at an access point, especially for user terminals with lower received signal power at the AP. 
     Example Open Loop Power Control  
       FIG. 5  illustrates example operations  500  for power control of a UL signal from the perspective of a user terminal, according to certain embodiments of the present disclosure. The operations  500  may begin, at  510 , by adjusting the power of a UL signal to meet a target carrier-to-interference (C/I) ratio of an access point (AP). This adjustment at  510  may be performed by the user terminal upon reception of DL packets and prior to UL multiple access transmission. The AP&#39;s target C/I ratio may be a post-processing target such that the target C/I ratio reflects a desired C/I ratio of a UL signal received from any capable user terminal after reception and signal processing by the AP. For some embodiments, the transmit power of a UL signal may be adjusted to meet the AP target C/I ratio and a client peak power constraint. 
     For some embodiments, the target C/I ratio may be 28 dB per spatial stream in an effort to maximize throughput per user terminal Although a target C/I ratio of 28 dB may yield a heightened spectral efficiency, the target C/I ratio may change depending on the code-rate used. Furthermore, for link-budget limited user terminals, a C/I ratio of 28 dB may not be achievable due to power amplifier (PA) limitations in the transmitter. 
     To meet the target C/I ratio at the AP, the user terminal&#39;s transmit power (P client ) for the UL signal may be calculated at  510  according to the following formula: 
     
       
      
       P 
       client 
       =SNR 
       Target 
       −G 
       OFDMA 
       −G 
       SDMA 
       −G 
       CDMA 
       +N 
       TH 
       +C+P 
       AP 
       −RSSI 
       client  
      
     
     where SNR Target  is the target C/I ratio at the AP, G OFDMA  is an optional orthogonal frequency-division multiple access (OFDMA) processing gain, G SDMA  is an optional spatial-division multiple access (SDMA) processing gain at the AP, G CDMA  is an optional code-division multiple access (CDMA) processing gain, N TH  is a thermal noise floor, C represents parameters calibrated during association or other representative packet exchange protocols, P AP  is an AP transmit power (e.g., advertised by the AP), and RSS client  is a received signal strength indication (RSSI) of a received downlink (DL) signal measured at the user terminal Some of these parameters may be calibrated out, and other parameters may be provided by the AP, for example. 
     The calculation for P client  may most likely include at least one of G OFDMA , G SDMA , and G CDMA . G OFDMA  may be equal to 10 log 10  (64/N tones ) where N tones  is the number of frequencies used for transmission, and G SDMA  may be equal to 10 log 10  (M T /N S ) where M T  is the number of transmit antennas and N S  is the number of SDMA spatial streams. The parameters calibrated during association or other representative packet exchange protocols may include, for example, a noise figure at the AP (NF AP ) and a radio frequency (RF)/antenna gain at the AP (G AP,RF ). Such parameters may be advertised by the AP. 
     At  520 , the user terminal may transmit the power-adjusted UL signal. The transmitted signal may meet the calculated transmit power P client , for example, unless the desired power exceeds the capabilities of the transmitter circuit components, such as the power amplifier. In this manner, the UL signal may be received by an AP and, after post-processing, may achieve the desired target C/I ratio. If multiple user terminals implement the operations at  510  and  520  described above in an effort to transmit multiple UL signals attempting to meet the desired AP target C/I ratio, there need not be large power differences between the UL signals received by the AP, and the performance degradation to UL signals with lower received signal power may most likely be reduced. In other words, according to certain embodiments of the present disclosure, by having the transmit power of UL signals from different user terminals adjusted to meet a target C/I ratio at the AP, significant differences in received UL signal power may be eliminated, and the effects of ICI distortion, phase distortion, and I-Q imbalance, for example, may be mitigated. 
     Example Closed Loop Power Control 
     The operations at  510  and  520  may be considered as open loop power control operations because these operations adjust the transmit power of UL signals without feedback from the AP. However, as illustrated by optional operations at  530  to  550 , closed loop operations (based on AP feedback) may also be performed. Closed loop power control may be employed in an effort to provide better power control and account for any imperfections in open loop power control due to, for example, an imperfect or outdated RSSI measurement and any changes to AP RF and processing gains. Further, closed loop power control may allow the AP to manage client transmit powers for optimum UL SDMA/OFDMA performance, police rogue clients (i.e., clients that are transmitting with excessive power, perhaps due to incorrect RSSI measurements or estimates), and limit interference generated to neighbor base station subsystems (BSSs), for example, in enterprise applications. 
     Therefore, for closed loop power control operations as illustrated in  FIG. 6A , the user terminal  120  may transmit a value  600  (e.g., P client ) indicative of the power of the power-adjusted UL signal at  530 . At  540 , the user terminal  120  may receive adjustment information  650  (represented as ΔP) from the AP  110  as shown in  FIG. 6B . For some embodiments, the adjustment information  650  may be based on the power value  600  and the target C/I ratio of the AP  110 . For example, the adjustment information  650  may be a new, adjusted target C/I ratio (e.g., SNR′ Target ) or an adjustment to the target C/I ratio (e.g., ΔSNR Target ) based on the difference between the target C/I ratio and the actual received C/I ratio after post-processing. At  550 , the user terminal  120  may adjust the transmit power of the UL signal based on the adjustment information  650  received from the AP  110 . For certain embodiments, the user terminal  120  may only adjust the transmit power of the UL signal if the difference between the target C/I ratio and the actual received C/I ratio is greater than a threshold. 
     For certain embodiments, the user terminal  120  may communicate the currently used transmit power value  600 , such as P client , in units of dBm, for example, to the AP  110  using an N-bit field in a Media Access Control (MAC) header of a UL packet such that bit values ranging from 0 to 2 N −1 indicate representative values. In such a scenario with N=6 as an example, the bit-representation may cover a range of [0:1:63] corresponding to a power value  600  ranging from [31 8.5:0.5:23.0] dBm, for example, with a resolution of 0.5 dBm, for some embodiments. 
     For other embodiments, the power value  600  communicated by the user terminal  120  may represent a back-off value from a peak transmit power. This feedback may allow the AP  110  to determine the amount of PA headroom available to the user terminal  120 . Such information may be needed for DL closed-loop power control signaling for multiple stations. Hence, a range of [0:1:63] with N=6, for example, may correspond to a peak transmit power that is [0.0:0.5:31.5] dB below the peak transmit power with a resolution of 0.5 dB. The user terminal  120  may communicate the peak transmit power to the AP  110  during association, the initial handshake when the user terminal  120  first enters the network. 
       FIG. 7  illustrates example operations  700  for closed loop power control of a UL signal from the perspective of an AP  110 . The operations  700  may begin, at  710 , by receiving a value  600  (e.g., P client  as shown in  FIG. 6A ) indicative of the power of a UL signal transmitted by a user terminal  120 . The AP  110  may be able to decode the power value in a MAC header of a received UL packet. 
     At  720 , the AP  110  may determine adjustment information  650  based on at least the received power value  600  and a target C/I ratio. For example, the adjustment information  650  may be a new, adjusted target C/I ratio (e.g., SNR′ target ) or an adjustment to the target C/I ratio (e.g., ΔSNR Target ) based on the difference between the target C/I ratio and the measured C/I ratio of the UL signal after reception at  710  and post-processing. For some embodiments, the AP target C/I ratio may be 28 dB per spatial stream as described above. 
     At  730 , the AP  110  may transmit the adjustment information  650  (e.g., ΔP as shown in  FIG. 6B ) to the user terminal  120 . To communicate the adjustment information  650 , the AP  110  may encode the adjustment information  650  in a MAC header of a DL packet. For certain embodiments, the AP  110  may use an M-bit field in the MAC header of the DL packet such that bit values ranging from 0 to 2 M −1 indicate representative values. In such a scenario with M=6 as an example, thereby covering a range of [0:1:63], the bit-representation may correspond to an adjustment range of [−16.0:0.5:15.5] dBm for some embodiments. 
     For other embodiments, the adjustment information  650  communicated to the user terminal  120  may represent a back-off value from a peak transmit power. As noted above, the adjustment information  650  may take into account the PA headroom available to the user terminal  120 . Hence, a range of [0:1:63] with M=6, for example, may correspond to a transmit power that is [0.0:0.5:31.5] dB below the peak transmit power with a resolution of 0.5 dB. 
     The operations  700  may be performed for multiple user terminals, each of which may provide a value indicative of transmit power used for UL transmissions. Thus, the AP may send different power adjustment information to different user terminals. 
     Example Power Control For UL-MU-MIMO 
     As described above, large power imbalances in the received power from multiple user terminals  120  (or multiple STAs) may result in performance degradation. In other words, the multiple user terminals may have different path losses, and strong user terminals may cause interference with weaker ones. In a multiuser multiple-input multiple-output (MU-MIMO) scheme, user terminals  120  close to the AP  110  may interfere with user terminals further from the AP. For certain aspects, the AP  110  may determine the power of the received signals and a power correction value for each of the user terminals, and each user terminal  120  may apply the individual power correction value received from the AP. 
       FIG. 8  illustrates example operations  800  for power control of an uplink (UL) signal based on the power of a plurality of received UL signals from the perspective of an AP, in accordance with certain embodiments of the present disclosure. The operations  800  may begin, at  802 , by receiving a plurality of UL signals. For example, the plurality of UL signals may be UL signals received from at least some, if not all, the user terminals  120  that are part of a UL-MU-MIMO scheme. 
     At  804 , the AP  110  may determine power of the received plurality of UL signals. For example, the AP may measure the received power (P i ) of each UL signal received from a user terminal  120  in a UL-MU-MIMO scheme. For certain aspects, these measurements may be taken from received RTX (Request to Transmit) messages  1202  and/or UL physical layer convergence protocol (PLCP) protocol data units (UL-PPDUs), both of which are illustrated in  FIG. 12  and described in greater detail below. 
     At  806 , the AP  110  may determine power adjustment information for a UL signal (e.g., a UL-MU-MIMO signal) in the plurality of UL signals based on at least the power of the received UL signals and a target C/I ratio. For certain aspects, the AP  110  may calculate a power back-off (B i ) each user terminal should apply to the UL signal the particular user terminal is transmitting. The power back-off may be computed according to the following equation: 
       B i   =P   i −max [max j ( P   j )−MAX_BO, Noise_floor+ SNR   Target ]
 
     where max j (P j ) is the maximum received power of all the UL signals, MAX_BO is the maximum power back-off a user terminal can apply, Noise_floor is a measure of the thermal noise floor in dB, and SNR Target  is a target received power-to-noise ratio (e.g., the target C/I ratio at the AP). For example, MAX_BO may be about 30 or 40 dB, and SNR Target  may be about 28 or 30 dB. For certain aspects, B i  may be bounded by a lower bound of 0 dB and an upper bound of MAX_BO in units of dB such that the power back-off can neither be negative, nor exceed the maximum power back-off. Also for certain aspects, B i  may be quantized in increments of power control steps (PC_step), where PC_step may be around 3 dB, for example. 
     At  808 , the AP  110  may transmit the power adjustment information. For example, the AP  110  may communicate B i  to all the user terminals in the UL-MU-MIMO scheme, only to user terminals where B i  is not zero, or only to user terminals where B i  is above the quantized power control step (PC_step). For certain aspects, the AP  110  may transmit CTX (Clear to Transmit) messages  1204  (illustrated in  FIG. 12  and described in greater detail below) with a field signaling the back-off value. 
       FIGS. 9-11  illustrate various examples of applying the power control mechanism described above to three stations (STA 1 , STA 2 , and STA 3 ) based on the equation for calculating B i  for each individual user terminal in a UL-MU-MIMO scheme. In all of these examples, MAX_BO is 30 dB, Noise_floor is negligent (0 dB), SNR Target  is 30 dB, and STA 1 , STA 2 , and STA 3  all transmit with a power of 106 dB. 
       FIG. 9  illustrates a scenario where the received power (P i ) from STA 1 , STA 2 , and STA 3  are all above the target power-to-noise ratio (SNR Target ). The power transmitted from STA 1  and STA 2  experiences a pathloss of 60 dB, such that the received power at the AP  110  is 46 dB for both UL signals. The power transmitted from STA 3  experiences a pathloss of only 40 dB, such that the received power at the AP is 66 dB for this UL signal. Calculating B 1  for STA 1  and B 2  for STA 2  according to the equation for B above leads to B 1 =B 2 =46−max [66−30, 0+30] dB=46−36=10 dB. Therefore, the AP may transmit B 1  to STA 1  and B 2  to STA 2  such that after STA 1  and STA 2  apply the back-off values, the received power of the UL signals at the AP may be 36 dB as illustrated. Calculating B 3  for STA 3  results in B 3 =66−max [66−30, 0+30] dB=66−36=30 dB. Therefore, the AP may transmit B 3  to STA 3  such that after STA 3  applies the back-off value, the received power of the UL signal at the AP may be 36 dB as illustrated. 
     In  FIG. 9 , note that the received power after power control for STA 1 , STA 2 , and STA 3  are all equal such that none of the UL signals is stronger than another as received at the AP. Therefore, performance degradation due to power imbalance may most likely be reduced. Furthermore, note that the received power of the UL signals did not reach the target power-to-noise ratio. This is due to the maximum power back-off value a user terminal can apply (30 dB in this example). 
       FIG. 10  illustrates a scenario where the received power (P i ) from STA 1 , STA 2 , and STA 3  are all below the target power-to-noise ratio (SNR Target ). The power transmitted from STA 1  and STA 2  experiences a pathloss of 80 dB, such that the received power at the AP  110  is 26 dB for both UL signals. The power transmitted from STA 3  experiences a pathloss of 95 dB, such that the received power at the AP is 11 dB for this UL signal. Calculating B 1  for STA 1  and B 2  for STA 2  according to the equation for B i  above leads to B 1 =B 2 =26−max [26−30, 0+30] dB=26−30=−4 dB. However, B i  is bounded with a lower bound of 0 dB, such that B 1 =B 2 =0 dB. Similarly, calculating B 3  for STA 3  results in B 3 =11−max [26−30, 0+30] dB=11−30=−19 dB, but B 3 =0 dB due to the lower bound. Therefore, for certain aspects, the AP may not transmit any adjustment information to STA 1 , STA 2 , or STA 3 . For other aspects, the AP may transmit B 1  to STA 1 , B 2  to STA 2 , and B 3  to STA 3  such that after STA 1 , STA 2 , and STA 3  apply the back-off values of 0 dB, no back-off occurs. In other words, the received power of the UL signals at the AP may remain at 26 dB and 11 dB as illustrated in  FIG. 10 . 
       FIG. 11  illustrates a scenario where the received power (P i ) from STA 1  and STA 2  are below the target power-to-noise ratio (SNR Target ), but the received power from STA 3  is above SNR Target . The power transmitted from STA 1  and STA 2  experiences a pathloss of 95 dB, such that the received power at the AP  110  is 11 dB for both UL signals. The power transmitted from STA 3  experiences a pathloss of 70 dB, such that the received power at the AP is 36 dB for this UL signal. Calculating B 1  for STA 1  and B 2  for STA 2  according to the equation for B i  above leads to B 1 =B 2 =11−max [36−30, 0+30] dB=11−30=−19 dB, but B 1 =B 2 =0 dB due to the lower bound as described above. Therefore, for certain aspects, the AP may not transmit any adjustment information to STA 1  or STA 2 . For other aspects, the AP may transmit B 1  to STA 1  and B 2  to STA 2  such that after STA 1  and STA 2  apply the back-off values of 0 dB, no back-off occurs. In other words, the received power of the UL signals at the AP may remain at 11 dB as illustrated in  FIG. 11 . 
     Calculating B 3  for STA 3  results in B 3 =36−max [36−30, 0+30] dB=36−30=6 dB. Therefore, the AP may transmit B 3  to STA 3  such that after STA 3  applies the back-off value, the received power of the UL signal at the AP may be 30 dB as illustrated in  FIG. 11 . Note that since the difference between the received power for the UL signal transmitted by STA 3  and the target power-to-noise ratio was within the maximum back-off value, STA 3  was able to adjust the UL signal transmission power to meet the target ratio for the received power at the AP  110 . 
       FIG. 12  illustrates an example UL-MU-MIMO protocol that may be used for power control of the UL signals, in accordance with certain embodiments of the present disclosure. The user terminals  120  may transmit RTX messages  1202  to request for UL-MU-MIMO transmission. For example, STA 1  may send RTX 1 , and STA 2  may send RTX 2 . The AP  110  may measure the power of a received RTX message to determine the power of a UL signal from a particular user terminal  120 . 
     The AP  110  may respond to an RTX message with a CTX message  1204 . The CTX message  1204  contains a list of user terminals  120  that can take part in the UL-MU-MIMO scheme, such that a particular user terminal knows to start a UL-MU-MIMO transmission. The AP may select user terminals with a simple round robin among a given set of user terminals. The CTX message  1204  may contain a field indicating the back-off value(s) (B i ) for the user terminal(s) to apply to UL-MU-MIMO signal transmissions. 
     Once a user terminal  120  receives a CTX message  1204  from the AP where this user terminal is listed, the user terminal may transmit UL-MU-MIMO data  1206 . In  FIG. 12 , STA 1  and STA 2  both transmit UL-MU-MIMO data  1206  containing physical layer convergence protocol (PLCP) protocol data units (PPDUs). Upon receiving the UL-MU-MIMO data  1206 , the AP  110  may transmit block acknowledgments (BAs)  1208  to the user terminals  120 . 
     With this power control scheme, user terminals operating in a UL-MU-MIMO scheme can each apply a back-off value determined by an AP up to a maximum back-off value when the power of UL signals received at the AP exceeds a target power-to-noise ratio. In this manner, performance degradation due to power imbalances may be reduced. 
     The various operations of methods described above may be performed by various hardware and/or software component(s) and/or module(s) corresponding to means-plus-function blocks illustrated in the figures. Generally, where there are methods illustrated in figures having corresponding counterpart means-plus-function figures, the operation blocks correspond to means-plus-function blocks with similar numbering. For example, blocks  510 - 550  illustrated in  FIG. 5  correspond to means-plus-function blocks  510 A- 550 A illustrated in  FIG. 5A . Similarly, blocks  710 - 730  of  FIG. 7  correspond to means-plus-function blocks  710 A- 730 A illustrated in  FIG. 7A . 
     As used herein, the term “determining” encompasses a wide variety of actions. For example, “determining” may include calculating, computing, processing, deriving, investigating, looking up (e.g., looking up in a table, a database or another data structure), ascertaining, and the like. Also, “determining” may include receiving (e.g., receiving information), accessing (e.g., accessing data in a memory), and the like. Also, “determining” may include resolving, selecting, choosing, establishing, and the like. 
     Information and signals may be represented using any of a variety of different technologies and techniques. For example, data, instructions, commands, information, signals, and the like that may be referenced throughout the above description may be represented by voltages, currents, electromagnetic waves, magnetic fields or particles, optical fields or particles, or any combination thereof 
     The various illustrative logical blocks, modules, and circuits described in connection with the present disclosure may be implemented or performed with a general purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array signal (FPGA) or other programmable logic device (PLD), discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general purpose processor may be a microprocessor, but in the alternative, the processor may be any commercially available processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. 
     The steps of a method or algorithm described in connection with the present disclosure may be embodied directly in hardware, in a software module executed by a processor, or in a combination of the two. A software module may reside in any form of storage medium that is known in the art. Some examples of storage media that may be used include random access memory (RAM), read only memory (ROM), flash memory, EPROM memory, EEPROM memory, registers, a hard disk, a removable disk, a CD-ROM and so forth. A software module may comprise a single instruction, or many instructions, and may be distributed over several different code segments, among different programs, and across multiple storage media. A storage medium may be coupled to a processor such that the processor can read information from, and write information to, the storage medium. In the alternative, the storage medium may be integral to the processor. 
     The methods disclosed herein comprise one or more steps or actions for achieving the described method. The method steps and/or actions may be interchanged with one another without departing from the scope of the claims. In other words, unless a specific order of steps or actions is specified, the order and/or use of specific steps and/or actions may be modified without departing from the scope of the claims. 
     The functions described may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored as one or more instructions on a computer-readable medium. A storage media may be any available media that can be accessed by a computer. By way of example, and not limitation, such computer-readable media can comprise RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium that can be used to carry or store desired program code in the form of instructions or data structures and that can be accessed by a computer. Disk and disc, as used herein, include compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk, and Blu-ray® disc where disks usually reproduce data magnetically, while discs reproduce data optically with lasers. 
     Software or instructions may also be transmitted over a transmission medium. For example, if the software is transmitted from a website, server, or other remote source using a coaxial cable, fiber optic cable, twisted pair, digital subscriber line (DSL), or wireless technologies such as infrared, radio, and microwave, then the coaxial cable, fiber optic cable, twisted pair, DSL, or wireless technologies such as infrared, radio, and microwave are included in the definition of transmission medium. 
     Further, it should be appreciated that modules and/or other appropriate means for performing the methods and techniques described herein can be downloaded and/or otherwise obtained by a user terminal and/or base station as applicable. For example, such a device can be coupled to a server to facilitate the transfer of means for performing the methods described herein. Alternatively, various methods described herein can be provided via storage means (e.g., RAM, ROM, a physical storage medium such as a compact disc (CD) or floppy disk, etc.), such that a user terminal and/or base station can obtain the various methods upon coupling or providing the storage means to the device. Moreover, any other suitable technique for providing the methods and techniques described herein to a device can be utilized. 
     It is to be understood that the claims are not limited to the precise configuration and components illustrated above. Various modifications, changes and variations may be made in the arrangement, operation and details of the methods and apparatus described above without departing from the scope of the claims.