Patent Publication Number: US-2009227263-A1

Title: Method and apparatus for using load indication for intereference mitigation in a wireless communication system

Description:
The present application claims priority to provisional U.S. Application Ser. No. 60/971,219, entitled “SUPERFRAME PREAMBLE WITH LOAD INDICATION,” filed Sep. 10, 2007, and U.S. Application Ser. No. 61/014,668, entitled “SUPERFRAME PREAMBLE WITH LOAD INDICATION,” filed Dec. 18, 2007, both assigned to the assignee hereof and incorporated herein by reference. 
    
    
     BACKGROUND 
     I. Field 
     The present disclosure relates generally to communication, and more specifically to techniques for mitigating interference in a wireless communication system. 
     II. Background 
     Wireless communication systems are widely deployed to provide various communication content such as voice, video, packet data, messaging, broadcast, etc. These wireless systems may be multiple-access systems capable of supporting multiple users by sharing the available system resources. Examples of such multiple-access systems include Code Division Multiple Access (CDMA) systems, Time Division Multiple Access (TDMA) systems, Frequency Division Multiple Access (FDMA) systems, Orthogonal FDMA (OFDMA) systems, and Single-Carrier FDMA (SC-FDMA) systems. 
     A wireless communication system may include a number of base stations that can support communication for a number of terminals. A terminal may communicate with a base station via the downlink and uplink. The downlink (or forward link) refers to the communication link from the base station to the terminal, and the uplink (or reverse link) refers to the communication link from the terminal to the base station. 
     Each base station may transmit data to zero or more terminals on the downlink and may receive data from zero or more terminals on the uplink at any given moment. On the downlink, a transmission from a base station to a terminal may observe interference due to transmissions from neighbor base stations. On the uplink, a transmission from a terminal to a base station may observe interference due to transmissions from other terminals communicating with neighbor base stations. For both the downlink and uplink, the interference due to interfering base stations and interfering terminals may degrade performance. 
     Various interference mitigation schemes or protocols may be used to mitigate strong interference from other transmissions in the same geographical or radio frequency vicinity. These interference mitigation schemes may attempt to orthogonalize transmissions from interfering stations in time, frequency, and/or code. Each transmission may then observe less or no interference from other transmissions and may thus achieve better performance. However, these interference mitigation schemes may have high overhead for signaling messages exchanged between base stations and terminals in order to implement interference mitigation. 
     There is therefore a need in the art for techniques to mitigate interference with less overhead. 
     SUMMARY 
     Techniques for mitigating interference in a wireless communication system with less overhead are described herein. In an aspect, a base station may periodically broadcast a load indication to convey information such as whether or not to use interference mitigation, which interference mitigation scheme to use among multiple possible interference mitigation schemes, time and/or frequency resources to apply interference mitigation, duration of interference mitigation, performance metrics for the base station, and/or other information pertinent for interference mitigation. Terminals within communication range of the base station may receive the load indication and may perform interference mitigation as indicated by the load indication. 
     In one design, a terminal may receive a load indication from a base station that the terminal desires to access. The terminal may determine from the load indication whether to obtain reserved resources having reduced interference from interfering stations. In another design, a terminal may receive a load indication from a neighbor base station. The terminal may determine whether to reduce its transmit power, to request for resources prior to transmission, or to perform some other action based on the load indication. 
     Various aspects and features of the disclosure are described in further detail below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a wireless communication system. 
         FIG. 2  shows a transmission scheme for a load indication. 
         FIG. 3  shows a design for obtaining reserved resources for downlink broadcast and uplink access. 
         FIG. 4  shows a design for obtaining reserved resources for uplink data. 
         FIG. 5  shows a process performed by a terminal for interference mitigation. 
         FIG. 6  shows an apparatus for interference mitigation at the terminal. 
         FIG. 7  shows a process performed by a base station for interference mitigation. 
         FIG. 8  shows an apparatus for interference mitigation at the base station. 
         FIG. 9  shows a block diagram of the terminal and the base station. 
     
    
    
     DETAILED DESCRIPTION 
     The techniques described herein may be used for various wireless communication systems such as CDMA, TDMA, FDMA, OFDMA, SC-FDMA and other systems. The terms “system” and “network” are often used interchangeably. A CDMA system may implement a radio technology such as Universal Terrestrial Radio Access (UTRA), cdma2000, etc. UTRA includes Wideband CDMA (WCDMA) and other variants of CDMA. cdma2000 covers IS-2000, IS-95 and IS-856 standards. A TDMA system may implement a radio technology such as Global System for Mobile Communications (GSM). An OFDMA system may implement a radio technology such as Evolved UTRA (E-UTRA), Ultra Mobile Broadband (UMB), IEEE 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, Flash-OFDM®, etc. UTRA and E-UTRA are part of Universal Mobile Telecommunication System (UMTS). 3GPP Long Term Evolution (LTE) is an upcoming release of UMTS that uses E-UTRA, which employs OFDMA on the downlink and SC-FDMA on the uplink. UTRA, E-UTRA, UMTS, LTE and GSM are described in documents from an organization named “3rd Generation Partnership Project” (3GPP). cdma2000 and UMB are described in documents from an organization named “3rd Generation Partnership Project 2” (3GPP2). 
       FIG. 1  shows a wireless communication system  100 , which may include a number of base stations and other network entities. For simplicity,  FIG. 1  shows only two base stations  110  and  112 , which are also referred to as base stations A and B, respectively, and one system controller  130 . A base station may be a fixed station that communicates with the terminals and may also be referred to as an access point, a Node B, an evolved Node B (eNB), etc. A base station may provide communication coverage for a particular geographic area. The overall coverage area of a base station may be partitioned into smaller areas, and each smaller area may be served by a respective base station subsystem. The term “cell” can refer to a coverage area of a base station and/or a base station subsystem serving this coverage area, depending on the context in which the term is used. 
     A base station may provide communication coverage for a macro cell, a pico cell, a femto cell, and/or other types of cell. A macro cell may cover a relatively large geographic area (e.g., several kilometers in radius) and may support communication for all terminals with service subscription in the system. A pico cell may cover a relatively small geographic area and may support communication for all terminals with service subscription. A femto cell may cover a relatively small geographic area (e.g., a home) and may support communication for a set of terminals having association with the femto cell (e.g., terminals belonging to residents of the home). The terminals supported by a femto cell may belong in a closed subscriber group (CSG). A base station for a macro cell may be referred to as a macro base station, a base station for a pico cell may be referred to as a pico base station, and a base station for a femto cell may be referred to as a home base station. The techniques described herein may be used for all types of base station and all types of cell. 
     System controller  130  may couple to a set of base stations and provide coordination and control for these base stations. System controller  130  may be a single network entity or a collection of network entities. System controller  130  may communicate with base stations  110  and  112  via a backhaul, as shown in  FIG. 1 . Base stations  110  and  112  may also communicate with one another, e.g., via a direct wireless or wireline interface or via a data network such as the Internet. 
     System  100  may support communication for a number of terminals. For simplicity,  FIG. 1  shows only two terminals  120  and  122 , which are also referred to as terminals X and Y, respectively. A terminal may be stationary or mobile and may also be referred to as an access terminal (AT), a mobile station (MS), a user equipment (UE), a subscriber unit, a station, etc. A terminal may be a cellular phone, a personal digital assistant (PDA), a wireless modem, a wireless communication device, a handheld device, a laptop computer, a cordless phone, etc. The terms “terminal” and “user” are used interchangeably herein. A terminal may communicate with a serving base station and may cause interference to and/or observe interference from other stations. A serving base station is a base station designated to serve a terminal on the downlink and/or uplink. An interfering base station is a base station causing interference to a terminal on the downlink. An interfering terminal is a terminal causing interference to another terminal on the uplink. An interfering station may be an interfering base station or an interfering terminal. 
     Terminal  120  may desire to communicate with base station  110  but may observe strong interference from base station  112  on the downlink and/or from terminal  122  on the uplink. For example, base station  110  may be a home base station covering a femto cell with restricted association and may transmit at a much lower power level than base station  112 , which may be a macro base station. Terminal  120  may then receive much higher power from interfering base station  112  compared to home base station  110  on the downlink. Terminal  122  may communicate with base station  112  and may transmit at much higher power level than terminal  120 . Base station  110  may then receive much higher power from interfering terminal  122  compared to terminal  120  on the uplink. 
     An interference mitigation scheme may be used to orthogonalize the downlink transmissions from base stations  110  and  112  so that terminal  120  can observe less interference from interfering base station  112 . An interference mitigation scheme may also be used to orthogonalize the uplink transmissions from terminals  120  and  122  so that base station  110  can observe less interference from interfering terminal  122 . Various signaling messages may be sent on the downlink and uplink to support interference mitigation on each link. These signaling messages represent overhead for implementing interference mitigation. The overhead may be particularly severe in deployments where many base stations are close to one another. For example, a femto cell deployment may have tens of home base stations in a single apartment building. The overhead may be prohibitive when many of the base stations do not have active sessions. 
     In an aspect, a load indication may be used to support interference mitigation with less overhead. The load indication may also be referred to as load information, loading information, etc. The load indication may convey information used for interference mitigation, information used for system access and communication with a base station, etc. The load indication may be broadcast periodically to all terminals within communication range of the base station. Communication range is the range in which a signal from a base station can be received by a terminal, or vice versa. 
       FIG. 2  shows a design of a transmission scheme  200  for the load indication. The transmission timeline for the downlink may be partitioned into units of radio frames. Each radio frame may cover a predetermined time duration, e.g., 10 milliseconds (ms), and may be partitioned into 20 slots with indices of 0 through 19. Each slot may cover a fixed or configurable number of symbol periods, e.g., six or seven symbol periods. 
     In the design shown in  FIG. 2 , the load indication may be sent in a broadcast message, which may include other information. The broadcast message may be processed and sent on a broadcast channel, which may be mapped to designated time and frequency resources. In the example shown in  FIG. 2 , the broadcast message may be sent on a set of subcarriers (e.g., 72 subcarriers) in four symbol periods of slot  1  in each radio frame. 
     In general, the load indication may be sent on a broadcast channel, a control channel, a traffic/data channel, a pilot channel, a preamble of a superframe covering a predetermined time duration, etc. The load indication may be sent in a transmission (e.g., a broadcast channel or a preamble) used by the terminals for system acquisition. The load indication may be sent periodically (i) whenever the channel or preamble carrying the load indication is transmitted or (ii) at a different rate. 
     The load indication may carry various types of information that may be used by terminals for interference mitigation and system operation. In one design, the load indication may convey one or more of the following:
         Whether or not to use interference mitigation,   Which interference mitigation scheme to use from among multiple interference mitigation schemes,   Time and/or frequency resources to apply interference mitigation,   Duration of interference mitigation,   Performance metrics for a base station,   Backhaul capability, and   Other information related to interference mitigation or cell performance.       

     The load indication may indicate whether or not to use interference mitigation for downlink and/or uplink transmissions within communication range of a base station transmitting the load indication. The load indication may be set to a first value to indicate no need for interference mitigation, to a second value to indicate use of interference mitigation, etc. For example, if a base station is not serving any active users, then the load indication may indicate that neighbor base stations and terminals can operate as if this base station is not present. If the base station is serving active users, then the load indication may indicate that interference mitigation is to be used for terminals communicating with the base station and/or for terminals communicating with neighbor base stations. 
     The load indication may indicate a particular interference mitigation scheme to use for downlink and/or uplink transmissions within communication range of a base station transmitting the load indication. Different interference mitigation schemes may be used to achieve different levels of interference mitigation. For the downlink, a terminal may first receive broadcast information, then receive control information, then receive data. For the uplink, the terminal may first transmit an access request, then transmit control information, and then transmit data. Interference mitigation for downlink broadcast, downlink control, downlink data, uplink access, uplink control, and uplink data may be considered as different levels of interference mitigation and may be achieved with different interference mitigation schemes, as described below. The load indication may indicate whether to use interference mitigation for downlink broadcast, downlink control, downlink data, uplink access, uplink control, and/or uplink data. 
     A terminal may receive a load indication from a base station and may perform interference mitigation as indicated by the load indication. The terminal may perform interference mitigation in different manners (e.g., use different interference mitigation schemes) depending on whether the base station is serving the terminal or is a neighbor base station. For example, a load indication from a serving base station may indicate whether the terminal should obtain reserved resources having reduced interference for communication with the serving base station. A load indication from a neighbor base station may indicate whether the terminal should reduce transmit power on resources used by the neighbor base station for communication with its terminals. 
     The load indication may indicate frequency and time resources on which interference mitigation should be used. The available frequency and time resources may be partitioned into resource blocks or tiles. Each resource block may cover a predetermined frequency and time dimension, e.g., 12 subcarriers in one slot. The available resource blocks may be assigned indices. The load indication may provide indices of resource blocks on which interference mitigation should be used. 
     In one design, the load indication may indicate the frequency and time resources on which to use interference mitigation as well as the type of information (e.g., control, data, etc.) to send on the resources and/or the particular link (e.g., downlink or uplink) for which the resources are used. In another design, the load indication may indicate the frequency and time resources on which to use interference mitigation, and the type of information to send on the resources may be implicit or known a priori by the terminals. In yet another design, the load indication may indicate whether or not to use interference mitigation on predefined frequency and time resources. For example, the load indication may be a 1-bit value to indicate whether or not certain predefined resources should be shared via a predetermined interference mitigation scheme. 
     The load indication may indicate the time duration over which to apply interference mitigation. In one design, the load indication may comprise a 1-bit value to indicate whether or not to apply interference mitigation for a predetermined time duration, e.g., a predetermined number of radio frames, one superframe duration, etc. In another design, the load indication may comprise a multi-bit value to indicate a specific time duration for interference mitigation. The load indication may comprise (i) only the interference mitigation duration or (ii) any of the information described above along with the interference mitigation duration. 
     The load indication may convey performance metrics for a base station (or a cell). The performance metrics may comprise statistics such as mean and variance of throughputs and/or delays for terminals communicating with the base station, the throughput achieved by a predetermined percentage of terminals, the percentage of terminals achieving a predetermined throughput, etc. The performance metrics may also convey other information such as the number of terminals being served by the base station, the resources (e.g., in terms of time, bandwidth, power, etc.) available to newly arriving terminals, typically performance of terminals currently being served, etc. The performance metrics may be used by terminals to determine whether or not to access the base station. For example, if the performance metrics indicate heavy loading or a low average throughput for terminals communicating with the base station, then a terminal may choose to access another base station with lighter loading. The performance metrics may also be used by the terminals for interference mitigation. For example, a terminal may invoke interference mitigation if the average throughput for its serving base station is below a low threshold and may skip interference mitigation otherwise. As another example, a terminal may receive a message from a neighbor base station requesting reduction in interference from interfering terminals. The terminal may determine whether or not to respond to the message or the amount of reduction in transmit power based on the performance metrics for the neighbor base station. The terminal may also use the performance metrics to set one or more thresholds for interference mitigation, to determine how to respond to interference management messages, etc. 
     The load indication may also convey other information useful for interference mitigation and system operation. For example, the load indication may convey a transmission protocol used at a base station (e.g., whether the base station is a relay station), the type of interference mitigation to apply (e.g., transmit power reduction, interference nulling for multi-antenna base stations or terminals, etc.), capability of joint transmission/reception with a set of neighbor cells, etc. 
     A terminal may receive a load indication from a base station that the terminal desires to access. The terminal may determine whether or not to access the base station based on the load indication. For example, if the load indication conveys the average throughput, then the terminal may decide to access the base station if the average throughput is above a throughput threshold. This throughput threshold may be dependent on the data requirements of the terminal and/or other factors. The terminal may also determine whether or not to access the base station based on the interference mitigation scheme and/or other information conveyed in the load indication. 
     A terminal may receive a load indication from a serving base station and may operate in accordance with this load indication. For example, the terminal may utilize the interference mitigation scheme and/or the frequency and time resources conveyed in the load indication for communication with the serving base station. 
     A terminal may receive a load indication from a neighbor base station and may operate in accordance with this load indication. The terminal may determine which interference mitigation scheme, if any, to invoke for uplink and/or downlink transmission based on the load indication from the neighbor base station. The load indication may inform the terminal to skip interference mitigation, e.g., due to light or no loading at the neighbor base station. The terminal may then transmit at any power level, including high power levels that may de-sense the neighbor base station, over all frequency and time resources except for those allocated for uplink access. The load indication may inform the terminal to apply interference mitigation, e.g., due to heavy loading at the neighbor base station. For example, the terminal may request for uplink resources and may send transmission on granted resources. As another example, the terminal may transmit at a specified power level or lower without requesting for resources and may need to request for resources to transmit at higher power levels. The load indication may also inform the terminal to use an interference mitigation scheme with a shorter request and transmission delay, e.g., when the neighbor base station has light or no loading. The load indication may also inform the terminal to use an interference mitigation scheme with a longer request and transmission delay, e.g., when the neighbor base station has heavy loading. For both cases, the terminal may request for time frequency resources prior to transmission and may send transmission on granted resources. 
     A terminal may receive a load indication from one base station and may send all or part of the information from the load indication to another base station. The terminal may forward load indication information from a neighbor base station to a serving base station, or vice versa. A base station may also receive load indication information from another base station via over-the-air transmission or the backhaul. 
     A base station may use load indication information from other base stations in various manners. The base station may configure its control and traffic channel structure based on the load indication information from other base stations. For example, the base station may decide that interference mitigation is not needed for its control and traffic channels if the load indications from the neighbor base stations indicate light or no loading. Conversely, the base station may use interference mitigation for its control and traffic channels if the load indications from the neighbor base stations indicate heavy loading. The base station may also use performance metrics from the neighbor base stations for frequency planning and selecting interference mitigation schemes. 
     A base station may periodically transmit broadcast information on designated downlink resources for use by terminals to access the base station. Some uplink resources may be reserved for terminals to send access requests to the base station. Some downlink and uplink resources may also be reserved for downlink and uplink control channels to send control information/signaling messages for various procedures for system access, resource assignment, interference mitigation, etc. After successfully accessing the base station, a terminal may be assigned dedicated downlink and uplink resources for sending data on the downlink and uplink. 
     A base station may have few or no active terminals communicating with the base station. Furthermore, terminals may infrequently access the base station. This may be the case, for example, if the base station is a home base station that serves a femto cell and has restricted association. The base station may also be located near other home base stations and/or may be within the vicinity of macro base stations. In this case, the downlink and uplink transmissions for the base station may observe high interference unless these transmissions are sent on resources not used by other base stations and are orthogonalized with other transmissions for these other base stations. The base station may have some reserved downlink resources to periodically transmit broadcast information, some reserved uplink resources to receive access requests, some reserved downlink and uplink resources for control information, and/or some reserved downlink and uplink resources for data. The reserved downlink and uplink resources may be allocated exclusively to the base station, and neighbor base stations may avoid using these reserved resources. However, if the base station has few or no active terminals, then reserving downlink and uplink resources specifically for the base station may represent inefficient use of the available resources. The inefficiency may be more severe when there are other nearby base stations, each with few or no active terminals but having reserved resources that are used infrequently by that base station but not usable by other base stations. 
     In one design, a load indication from a base station may indicate whether a terminal should perform bootstrapping for communication with the base station. Bootstrapping is a process in which one or more base stations and one or more terminals coordinate to reserve resources for a recipient station, which may be a base station for downlink or a terminal for uplink. The reserved resources may have less or no interference from other stations and may be used by the recipient station to achieve good performance. Bootstrapping may be performed for downlink broadcast, downlink control, downlink data, uplink access, uplink control, and/or uplink data 
     A terminal may go through a series of steps in order to communicate with a base station. These steps may include receiving the broadcast information from the base station, sending an access request to the base station, exchanging control information with the base station for system access and resource assignment, and exchanging data with the base station on assigned resources. Bootstrapping may be performed for any of these steps to reserve downlink and/or uplink resources. 
     Bootstrapping of downlink broadcast may be performed if no downlink resources are reserved for sending broadcast information. A base station may forego sending broadcast information in order to avoid consuming downlink resources and causing interference to neighbor base stations. Whenever a terminal desires to receive broadcast information, a mechanism may be used as a bootstrap to reserve downlink resources for the base station to periodically send broadcast information. 
     Bootstrapping of uplink access may be performed if no uplink resources are reserved for sending access requests to a base station. The base station may periodically transmit broadcast information on reserved downlink resources. A terminal may receive the broadcast information and may desire to access the base station. A mechanism may be used as a bootstrap to reserve uplink resources for the terminal to send an access request to the base station. 
     Bootstrapping of downlink and uplink control may be performed if no resources are reserved for sending control information on the downlink and uplink, respectively. A base station may periodically transmit broadcast information on reserved downlink resources, and a terminal may send an access request on reserved uplink resources. A mechanism may be used as a bootstrap to reserve downlink and uplink resources for sending control information on the downlink and uplink. Bootstrapping for downlink and uplink control may be performed together or separately. 
     Bootstrapping of downlink and uplink data may be performed if no resources are reserved for sending data on the downlink and uplink, respectively. A base station may periodically transmit broadcast information on reserved downlink resources, a terminal may send an access request on reserved uplink resources, and the base station and the terminal may exchange control information on reserved downlink and uplink resources. A mechanism may be used as a bootstrap to reserve downlink and uplink resources for sending data on the downlink and uplink. Bootstrapping for downlink and uplink data may be performed together or separately. 
       FIG. 3  shows a design of bootstrapping for downlink broadcast and uplink access. Terminal  120  may receive a load indication from base station  110  (step  1 ). Terminal  120  may determine from the load indication that broadcast information is not being transmitted by base station  110  and that bootstrap is needed for downlink broadcast (step  2 ). Terminal  120  may decide to associate with base station  110  (step  3 ). Since base station  110  may not have any reserved resources for uplink control, terminal  120  may send an association request to neighbor base station  112  to request association with base station  110  (step  4 ). Neighbor base station  112  may have reserved uplink resources for uplink control, which may be determined by terminal  120  from the broadcast information sent by base station  112 . 
     Base station  112  may receive the association request from terminal  120 . Base station  112  may send a message via the backhaul to inform base station  110  that terminal  120  desires to associate with base station  110  (step  5 ). Base station  112  may reduce its transmit power (e.g., to zero or a low level) on downlink (DL) resource R 1 , which may be reserved for downlink broadcast for base station  110  (step  6 ). Base station  112  may also instruct terminals (e.g., terminal  122 ) to reduce transmit power (e.g., to zero or a low level) on uplink (UL) resource R 2 , which may be reserved for uplink access for base station  110  (step  7 ). Resources R 1  and R 2  may be known a priori by both base stations  110  and  112  or may be signaled by base station  112  to base station  110  in step  5 . 
     Base station  110  may receive the message from base station  112  and may send broadcast information on downlink resource R 1  (step  8 ). Terminal  120  may receive the broadcast information and obtain applicable system parameters (step  9 ). Terminal  120  may then send an access request on uplink resource R 2  to base station  110  (step  10 ). Resources R 1  and R 2  may be known a priori by terminal  120  or may be signaled to terminal  120  in the broadcast information. 
       FIG. 3  shows a specific design of bootstrapping for downlink broadcast and uplink access. The bootstrapping may also be performed in other manners. For example, the association request in step  4  may simply request interference mitigation on the downlink and/or uplink. The downlink and uplink interference mitigation triggers may or may not occur at the same time. The message in step  5  may or may not include specific information for terminal  120 . The design in  FIG. 3  shows use of power reduction to mitigate interference. Interference mitigation may also be achieved via other means, e.g., spatial coordination between base stations  110  and  112  and/or terminals  120  and  122 . 
     In the design shown in  FIG. 3 , base station  110  and terminal  120  communicate via neighbor base station  112  to start transmission of broadcast information and to reserve resources for downlink broadcast and uplink access. Bootstrapping for downlink broadcast and uplink access may also be performed in other manners. In another design, terminal  120  may send the association request directly to base station  110  on predefined uplink resource or on different uplink resources. An access scheme such as Carrier Sense Multiple Access With Collision Avoidance (CSMA/CA) may be used to send the association request on uplink resources that may be used by other terminals. 
       FIG. 4  shows a design of bootstrapping for uplink data. Terminal  120  may have data to send on the uplink and may send a resource request to base station  110  (step  1 ). Base station  110  may receive the resource request and, in response, may send a transmit capability request to terminal  120  (step  2 ). Base station  110  may also send a reduce interference request to interfering terminals in neighbor cells to request these terminals to reduce their transmit power (e.g., to zero or a low level) on uplink resource R 3  (step  3 ). Each interfering terminal may reduce its transmit power on uplink resource R 3  in response to the reduce interference request from base station  110 . 
     Terminal  120  may receive the transmit capability request from base station  110  (step  2 ) and may also receive a reduce interference request from neighbor base station  112  (step  4 ). Terminal  120  may determine the maximum transmit power level that it can use on uplink resource R 3  in order to comply with the reduce interference request (if any) from neighbor base station  112  (step  5 ). Terminal  120  may then send a power decision pilot to base station  110  to convey its transmit capability (step  6 ). The power decision pilot may be a pilot sent at the maximum transmit power level that terminal  120  can use on uplink resource R 3 . Base station  110  may schedule terminal  120  for uplink data transmission and may assign all or part of uplink resource R 3  to terminal  120 , e.g., based on the transmit capability of terminal  120  ascertained from the power decision pilot. Base station  110  may then send a resource grant comprising the assigned uplink resource to terminal  120  (step  7 ). Terminal  120  may send data on the assigned uplink resource to base station  110  (step  8 ). 
       FIGS. 3 and 4  show two designs of bootstrapping for downlink broadcast/uplink access and uplink data. Bootstrapping for downlink control, uplink control, and downlink data may also be performed by exchanging messages between terminal  120 , base station  110 , and possibly neighbor base stations and/or interfering terminals. 
     In the design shown in  FIG. 4 , the reduce interference requests from base stations  110  and  112  may be triggered by resource requests from terminals  120  and  122 , respectively. Each reduce interference request may convey specific uplink resource for which reduced interference is requested, the priority of the request, the duration of the request, etc. Each terminal may reduce its transmit power as indicated by the reduce interference requests received from neighbor base stations. The interference mitigation in  FIG. 4  may thus be short-term and may be achieved by sending reduce interference requests to potentially interfering terminals in neighbor cells. 
     In another design of interference mitigation, a load indication from a base station may indicate whether terminals in neighbor cells (or neighbor terminals) should reduce their transmit power. For example, if the load indication indicates light or no loading, then the neighbor terminals may operate without regards to the base station. Conversely, if the load indication indicates heavy loading, then the neighbor terminals may reduce their transmit power. The amount of reduction in transmit power may be dependent on various factors such as the base station loading, the path loss to the base station, etc. 
     A terminal may receive load indications from one or more neighbor base stations and may adjust its transmit power accordingly. The terminal may use high transmit power when the load indications from the neighbor base stations indicate light or no loading. The terminal may use lower transmit power when the load indication from any neighbor base station indicates heavy loading. 
       FIG. 5  shows a design of a process  500  performed by a terminal for interference mitigation. The terminal may receive a load indication from a base station (block  512 ). The terminal may determine whether to perform interference mitigation based on the load indication from the base station (block  514 ). The terminal may perform interference mitigation in accordance with the load indication if directed by the load indication (block  516 ). 
     In one design of blocks  514  and  516 , the terminal may determine whether to obtain reserved resources having reduced interference based on the load indication. The reserved resources may comprise downlink resources for sending broadcast information, uplink resources for sending access request, downlink resources for sending control information, uplink resources for sending control information, downlink resources for sending data, and/or uplink resources for sending data. The terminal may send a message to a neighbor base station to obtain the reserved resources, e.g., as shown in  FIG. 3 . Reduced interference on the reserved resources may be achieved via the neighbor base station, which may reduce its transmit power and/or ask terminals to reduce their transmit power on the reserved resources. 
     In another design of blocks  514  and  516 , the terminal may receive the load indication from a neighbor base station and may determine whether to reduce its transmit power, to request for resources prior to transmission, or to perform some other action based on the load indication from the neighbor base station. The terminal may determine whether to reduce its transmit power to a predetermined level or lower based on the load indication from the neighbor base station. The terminal may (i) reduce its transmit power if the load indication indicates heavy loading at the neighbor base station or (ii) not reduce its transmit power if the load indication indicates light or no loading. 
     The terminal may determine an interference mitigation scheme to use from among multiple interference mitigation schemes based on the load indication. The terminal may also determine the duration of interference mitigation or the resources selected for interference mitigation based on the load indication. The terminal may also determine whether to access the base station based on the load indication. The terminal may also obtain at least one performance metric for the base station from the load indication and may send the at least one performance metric to a neighbor base station. The terminal may also obtain other information and/or perform other actions based on the load indication. 
       FIG. 6  shows a design of an apparatus  600  for interference mitigation. Apparatus  600  includes a module  612  to receive a load indication from a base station, a module  614  to determine whether to perform interference mitigation based on the load indication from the base station, and a module  616  to perform interference mitigation in accordance with the load indication if directed by the load indication. 
       FIG. 7  shows a design of a process  700  performed by a base station for interference mitigation. The base station may determine whether interference mitigation is applicable for transmissions within communication range of the base station (block  712 ). The base station may make this determination based on loading at the base station, the number of terminals communicating with the base station, at least one performance metric for the base station, and/or other factors, as described above. The base station may send a load indication indicating whether interference mitigation is applicable for the base station (block  714 ). The base station may communicate with terminals with interference mitigation, if applicable and as indicated by the load indication (block  716 ). 
     In one design, the base station may determine whether to seek reduction in interference from terminals communicating with neighbor base stations. If a decision is made to seek reduction in interference, then the base station may send the load indication to request the terminals communicating with the neighbor base stations to reduce their transmit power, to request for resources prior to transmission, and/or to perform some other action. 
     In another design, the base station may send the load indication to inform terminals communicating with the base station to request for reserved resources having reduced interference. The reserved resources may comprise downlink resources for sending broadcast information, uplink resources for sending access request, downlink resources for sending control information, uplink resources for sending control information, downlink resources for sending data, and/or uplink resources for sending data. The base station may exchange at least one message with a neighbor base station to obtain reserved resources having reduced interference and may use the reserved resources for communication with a terminal. 
     The base station may select an interference mitigation scheme from among multiple interference mitigation schemes. The base station may then generate the load indication to convey the selected interference mitigation scheme to be used by terminals within communication range of the base station. The base station may also generate the load indication to convey the duration over which interference mitigation is applicable and/or resources for which interference mitigation is applicable. The base station may also determine at least one performance metric for the base station and may send the at least one performance metric in the load indication. The base station may also send other information and/or direct terminals to perform other actions via the load indication. 
       FIG. 8  shows a design of an apparatus  800  for interference mitigation. Apparatus  800  includes a module  812  to determine whether interference mitigation is applicable for transmissions within communication range of a base station, a module  814  to send a load indication indicating whether interference mitigation is applicable, and a module  816  to communicate with terminals with interference mitigation, if applicable and as indicated by the load indication. 
     The modules in  FIGS. 6 and 8  may comprise processors, electronics devices, hardware devices, electronics components, logical circuits, memories, etc., or any combination thereof. 
       FIG. 9  shows a block diagram of a design of base station  110  and terminal  120 . In this design, base station  110  is equipped with T antennas  934   a  through  934   t , and terminal  120  is equipped with R antennas  952   a  through  952   r , where in general T≧1 and R≧1. 
     At base station  110 , a transmit processor  920  may receive data for one or more terminals from a data source  912 , process (e.g., encode and modulate) the data for each terminal based on one or more modulation and coding schemes, and provide data symbols for all terminals. Transmit processor  920  may also receive broadcast and control information (e.g., load indication, resource grant, reduce interference request, transmit capability request, etc.) from a controller/processor  940 , process the information, and provide overhead symbols. A transmit (TX) multiple-input multiple-output (MIMO) processor  930  may multiplex the data symbols, the overhead symbols, and pilot symbols, process (e.g., precode) the multiplexed symbols, and provide T output symbol streams to T modulators (MOD)  932   a  through  932   t . Each modulator  932  may process a respective output symbol stream (e.g., for OFDM) to obtain an output sample stream. Each modulator  932  may further process (e.g., convert to analog, amplify, filter, and upconvert) the output sample stream to obtain a downlink signal. T downlink signals from modulators  932   a  through  932   t  may be transmitted via T antennas  934   a  through  934   t , respectively. 
     At terminal  120 , R antennas  952   a  through  952   r  may receive the downlink signals from base station  110  and provide received signals to demodulators (DEMOD)  954   a  through  954   r , respectively. Each demodulator  954  may condition (e.g., filter, amplify, downconvert, and digitize) a respective received signal to obtain received samples and may further process the received samples (e.g., for OFDM) to obtain received symbols. A MIMO detector  960  may perform MIMO detection on the received symbols from all R demodulators  954   a  through  954   r  and provide detected symbols. A receive processor  970  may process the detected symbols, provide decoded data for terminal  120  to a data sink  972 , and provide decoded broadcast and control information to a controller/processor  990 . 
     On the uplink, at terminal  120 , data from a data source  978  and control information (e.g., access request, association request, resource request, etc.) from controller/processor  990  may be processed by a transmit processor  980 , precoded by a TX MIMO processor  982  (if applicable), conditioned by modulators  954   a  through  954   r , and transmitted via antennas  952   a  through  952   r . At base station  110 , the uplink signals from terminal  120  may be received by antennas  934 , conditioned by demodulators  932 , detected by a MIMO detector  936 , and processed by a receive processor  938  to obtain the data and control information transmitted by terminal  120 . 
     Controllers/processors  940  and  990  may direct the operation at base station  110  and terminal  120 , respectively. Controller/processor  940  at base station  110  may implement or direct process  700  in  FIG. 7  and/or other processes for the techniques described herein. Controller/processor  990  at terminal  120  may implement or direct process  500  in  FIG. 5  and/or other processes for the techniques described herein. Memories  942  and  992  may store data and program codes for base station  110  and terminal  120 , respectively. A scheduler  944  may schedule terminals for transmissions on the downlink and/or uplink and may assign resources to the scheduled terminals. A communication (Comm) unit  946  may support communication with other base stations and system controller  130  via the backhaul. 
     Those of skill in the art would understand that information and signals may be represented using any of a variety of different technologies and techniques. For example, data, instructions, commands, information, signals, bits, symbols, and chips 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. 
     Those of skill would further appreciate that the various illustrative logical blocks, modules, circuits, and algorithm steps described in connection with the disclosure herein may be implemented as electronic hardware, computer software, or combinations of both. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, circuits, and steps have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the present disclosure. 
     The various illustrative logical blocks, modules, and circuits described in connection with the disclosure herein 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 (FPGA) or other programmable logic device, 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 conventional 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 disclosure herein 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 RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, hard disk, a removable disk, a CD-ROM, or any other form of storage medium known in the art. An exemplary storage medium is coupled to the 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 processor and the storage medium may reside in an ASIC. The ASIC may reside in a user terminal. In the alternative, the processor and the storage medium may reside as discrete components in a user terminal. 
     In one or more exemplary designs, the functions described may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored on or transmitted over as one or more instructions or code on a computer-readable medium. Computer-readable media includes both computer storage media and communication media including any medium that facilitates transfer of a computer program from one place to another. A storage media may be any available media that can be accessed by a general purpose or special purpose 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 means in the form of instructions or data structures and that can be accessed by a general-purpose or special-purpose computer, or a general-purpose or special-purpose processor. Also, any connection is properly termed a computer-readable 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 medium. Disk and disc, as used herein, includes 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. Combinations of the above should also be included within the scope of computer-readable media. 
     The previous description of the disclosure is provided to enable any person skilled in the art to make or use the disclosure. Various modifications to the disclosure will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other variations without departing from the spirit or scope of the disclosure. Thus, the disclosure is not intended to be limited to the examples and designs described herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.