Abstract:
An apparatus and method is provided for efficiently allocating a transmission period in a WLAN system. An access point (AP) transmits a PSMP message providing a downlink period and an uplink period provided to each station (STA), and at least one sub PSMP frame indicating a period of at least one of a downlink and an uplink for an STA requiring additional resource allocation. After exchanging data with the AP in the downlink and uplink periods provided by the PSMP frame, if there is a need for additional resource allocation, the STA receives the at least one sub PSMP frame and exchanges data with the AP in the period provided by the each sub PSMP frame.

Description:
PRIORITY  
       [0001]     This application claims the benefit under 35 U.S.C. § 119(a) of an application entitled “Phase Resource Allocation Method” filed in the United States Patent and Trademark Office on Oct. 18, 2005 and assigned Ser. No. 60/727,915, an application entitled “Multi-Phase Resource Allocation Method” filed in the United States Patent and Trademark Office on Oct. 27, 2005 and assigned Ser. No. 60/730,924, and application entitled “Method and Apparatus for Allocating Transmission Period in a Wireless Communication System, and System Therefor” filed in the Korea Intellectual Property Office on Jun. 8, 2006 and assigned Serial No. 2006-51604, the entire contents of all of which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates generally to a wireless communication system, and in particular, to an apparatus for efficiently allocating a transmission period in a transmission frame period of a wireless network system.  
         [0004]     2. Description of the Related Art  
         [0005]     Along with the recent development of wireless communication technologies and the resulting spread of wireless equipment, there is an increasing demand for high-speed, highly-reliable data transmission via a wireless link. A Wireless Local Area Network (WLAN) developed to meet the demand is basically composed of stations (STAs), which are mobile data communication equipment, and an access point (AP) capable of exchanging data with the STAs. The AP and the STAs, located in the same wireless service coverage area, are known as a Base Service Set (BSS).  
         [0006]     In a WLAN system, STAs located in one wireless service coverage area transmit or receive data using wireless resources allocated from an AP. In this case, the AP allocates the wireless resources in the form of phase resources, and the term “phase resources” refers to a period in which the STAs or AP can transmit data.  
         [0007]      FIG. 1  illustrates a structure of a transmission frame period in a conventional WLAN system.  
         [0008]     Referring to  FIG. 1 , in a transmission frame period  100  having a fixed length determined in the system, an AP transmits a MAP (mapping) frame  110  indicating phase resource allocation for the full transmission frame period  100 . The MAP frame  110  is composed of a downlink MAP  120  indicating a period in which the AP can transmit data, and an uplink MAP  122  indicating a period in which the STAs can transmit data. The downlink MAP  120  is composed of a Number-of-STAs field  130 , and at least one STA Information field  132  determined based on the Number-of-STAs field  130 . Similarly, the uplink MAP  122  is composed of a Number-of-STAs field  134 , and at least one STA Information field  136  determined depending on the Number-of-STAs field  134 . The STA Information fields  132  and  136  each include STA ID fields  140  and  144  indicating downlink or uplink periods allocated to the STAs, and Time Offset fields  142  and  146  indicating time offsets allocated to the STAs.  
         [0009]     An STA, that has been allocated a period in the MAP frame  110 , receives data in a period indicated by corresponding STA information, of a downlink period  112 , and transmits data in a period indicated by corresponding STA information, of an uplink period  114 . The STA holds a sleep mode in the other period except for the period in which the MAP frame  110  is transmitted and the period indicated by the MAP frame  110 . A contention period  116  following the uplink period  114  can be accessed by at least one STA on a contention basis.  
         [0010]     In this way, the AP estimates the amount of resources required for each STA in one transmission frame period  100 , and allocates downlink and uplink periods according to the estimation. However, when the AP overestimates the amount of resources required for the STA, the wireless resources are wasted, decreasing data throughput performance. In this case, the STA may not use the allocated wireless resources. Moreover that resource can not be reused by other STAs since that resource is already allocated to the particular STA. When the AP underestimates the amount of resources required for the STA, the STA cannot be allocated its required resources until at least the next transmission frame period, suffering transmission delay and jitter of the uplink service. The transmission delay and jitter affects a Quality of Service (QoS) required by the STA. In addition, if the STA accesses the contention period  116  due to a lack of its allocated resources, the STA cannot operate in the sleep mode for the contention period, wasting its power.  
         [0011]     Therefore, in the wireless communication system in which the AP determines the downlink and uplink periods required for the STA through scheduling, there is a need for technology for preventing the reduction in data throughput and the waste of STA power, and efficiently allocating downlink and uplink periods.  
       SUMMARY OF THE INVENTION  
       [0012]     To substantially solve at least the above problems and/or disadvantages and to provide at least the advantages below, the present invention provides a transmission period allocation apparatus for minimizing a waste of STA power without reducing data throughput in a wireless communication system.  
         [0013]     The present invention provides an apparatus for flexibly allocating a transmission period required for an STA using more than two MAP frames in a wireless communication system.  
         [0014]     According to one aspect of the present invention, there is provided an access point (AP) apparatus for allocating a transmission period in a wireless network system. The AP apparatus includes a processor for generating a power save multi-poll (PSMP) frame providing a downlink period and an uplink period to at least one station (STA), and after expiration of the downlink and uplink periods provided by the PSMP frame, generating at least one sub-PSMP frame providing at least one of an additional downlink period and an additional uplink period to at least one STA requiring additional resource allocation, the additional downlink period being provided in the frame period if there is additional data to be received by the STA and the additional uplink period being provided in the frame period if there is additional data to be transmitted by the STA; a transceiver for transmitting data to the at least one STA in the provided downlink period provided by the PSMP frame and receiving data from the at least one STA in the provided uplink period provided by the PSMP frame, and transmitting data to the at least one STA requiring additional resource allocation in the additional downlink period provided by the sub-PSMP frame if there is additional data to be transmitted by the STA and receiving data by the at least one STA requiring additional resource allocation in the additional uplink period provided by the sub-PSMP frame if there is additional data to be received by the STA. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which:  
         [0016]      FIG. 1  is a diagram illustrating a structure of a transmission frame period in a conventional WLAN system;  
         [0017]      FIG. 2A  is a diagram illustrating a configuration of a WLAN system according to the present invention;  
         [0018]      FIG. 2B  is a block diagram illustrating an exemplary structure of an AP and each STA in the WLAN system shown in  FIG. 2A ;  
         [0019]      FIG. 3  is a diagram illustrating a structure of a transmission frame period according to the present invention; and  
         [0020]      FIG. 4  is a timing diagram illustrating a power reduction effect of the phase resource allocation according to the present invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0021]     Preferred embodiments of the present invention will now be described in detail with reference to the annexed drawings. In the following description, a detailed description of known functions and configurations incorporated herein has been omitted for clarity and conciseness.  
         [0022]     A main feature of the present invention, which is related to allocation of transmission periods in a wireless communication system, is to provide more than one MAP (mapping) frames to indicate a period allocated by an access point (AP) to each station (STA) in a transmission frame period.  
         [0023]     The allocated period starts immediately after the MAP frame is transmitted, and in the transmission frame period, the MAP frames other than the first MAP frame will be referred to as “subsequent MAPs (sub MAPs).” 
         [0024]     Although a detailed description of the present invention will be made with reference to a Wireless Local Area Network (WLAN) system based on IEEE 802.11 standards, it will be understood by those skilled in the art that allocation of phase resources, a basic object of the present invention, can also be applied to other wireless communication systems having similar technical requirements and channel formats without departing from the spirit and scope of the invention.  
         [0025]      FIG. 2A  illustrates a configuration of a WLAN system according to the present invention.  
         [0026]     Referring to  FIG. 2A , each of APs  202  and  210  is connected to a wire network  200 , and a plurality of STAs  204 ,  206 ,  208 ,  212  and  214  access their associated APs  202  and  210  via an IEEE 802.11 Physical (PHY) layer and a wireless link based on a Media Access Control (MAC) protocol, and transmit/receive data over a plurality of wireless channels. The STAs  204  to  208  and the AP  202 , located in the same wireless service coverage area  220 , constitute one Base Service Set (BSS). In the same way, the STAs.  212  and  214  and the AP  210 , located in the wireless service coverage area  222 , form another BSS. STAs located in each BSS can exchange data with each other via a corresponding AP. The key functions of the APs  202  and  210  include delivery of data traffic, access to another network (e.g. wire network  200 ), roaming support, synchronization in a BSS, power management support, and control of media access for supporting time-bound service in a BSS.  
         [0027]      FIG. 2B  is a block diagram illustrating an exemplary structure of an AP and each STA in the WLAN system shown in  FIG. 2A . Both of the AP and the STA can include a display  232 , a processor  234 , a transceiver  236 , an input unit  238 , a storage  240 , a Random Access Memory (RAM)  242 , a Read Only Memory (ROM)  244 , and a common bus  230 . The illustrated exemplary structure is merely provided for convenience. Although specific elements and their operations of a computer system serving as an AP or an STA will be described herein with reference to  FIG. 2B , the exemplary description should not limit the present invention.  
         [0028]     Referring to  FIG. 2B , the transceiver  236 , connected to an antenna (not shown), receives desired data and converts the received signal into corresponding digital data. The processor  234  is a controller operating under the control of an operating system (OS) and other programs, included in the ROM  244 , and the data and information generated by an operation of the processor  234  are stored in the RAM  242 .  
         [0029]     The key operations of the processor  234  included in the AP include generation/analysis of data, allocation of periods for the STAs located in the same wireless service coverage area and connected to the AP, generation of at least one MAP frame indicating the allocated period, and operation mode control for the transceiver  236  based on the allocated period. Specifically, the processor  234  of the AP allocates initial resources R init  to the STAs through a MAP frame located in the first part of a transmission frame, and if there is a need for additional resource allocation, the processor  234  additionally allocates, for uplink/downlink transmission, the remaining period in the transmission frame period to at least one STA requiring the additional resource allocation, through the sub MAP frame transmitted immediately after a sequence duration associated with the first MAP frame of the transmission frame. It would be obvious to those skilled in the art that the term “immediately after” substantially refers to a lapse of a predetermined time.  
         [0030]     After the expiration of the downlink/uplink period indicated by the first MAP frame, if necessary, more than one sub MAP frames indicating the downlink and/or uplink periods allocated in the same transmission frame period can be additionally transmitted. The sub MAP frame is followed by the downlink and/or uplink periods indicated by the sub MAP frame.  
         [0031]     The key operations of the processor  234  included in the STA include generation/analysis of data, generation of a transmission frame, and operation mode control for the transceiver  236  based on a MAP frame received from the start point of the transmission frame. That is, the processor  234  controls the transceiver  236  such that it receives the MAP frame from the AP at the start point of every transmission frame, and analyzes the MAP frame to determine whether its own STA ID (identification) is included therein. If its own STA ID is included in the MAP frame, the processor  234  stores in the storage  240  information related to the allocated downlink and uplink periods indicated by the STA information associated with the STA ID, and then wakes up both a receiver and a transmitter of the transceiver  236  in the downlink and uplink periods so as to receive/transmit the data. The receiver and the transmitter of the transceiver  236  enter the sleep mode in the other periods except for the allocated periods. Further, the processor  234  of the STA monitors whether a sub MAP frame indicating additionally allocated resources is received from the AP after the expiration of the full period indicated by the MAP frame, and upon receipt of the sub MAP frame, analyzes the received sub MAP frame.  
         [0032]      FIG. 3  illustrates a structure of a transmission frame period according to the present invention. Although it is illustrated herein that frames and uplink/downlink periods are adjacent, intervals for transmission/reception switching and processing may exist in the actual system.  
         [0033]     Referring to  FIG. 3 , in a transmission frame period  300  having a fixed length, an AP first transmits a MAP frame  310   a  with a basic rate set (including modulation scheme, coding rate, and data rate), which is less than a normal rate set. This is to enable all STAs in the service coverage area to receive the MAP frame  310   a.  Initially, the MAP frame  310   a  is periodically transmitted according to a fixed period (for example, about 20 ms and 100 ms for Voice over Internet Protocol (VoIP) and Moving Picture Experts Group 4 (MPEG4), respectively) determined through a negotiation between the AP and the STA. The transmission frame period  300  is initialized by the transmission of the MAP frame  310   a.    
         [0034]     For example, the MAP frame  310   a,  in order to indicate phase resource allocation of its succeeding downlink period  312  and first uplink period  314   a,  is composed of a downlink MAP  320  indicating a period in which the AP can transmit data in the downlink period  312 , and a first uplink MAP  322   a  indicating a period in which the STA can transmit data in the first uplink period  314   a.  The downlink MAP  320  is composed of a Number-of-STAs field  330  and at least one STA Information field  332  determined based on the Number-of-STAs field  330 . In the same way, the first uplink MAP  322   a  is composed of a Number-of-STAs field  334   a  and at least one STA Information field  336   a  determined based on the Number-of-STAs field  334   a.    
         [0035]     The STA Information fields  332  and  336   a  each include STA ID fields  340  and  344   a  that indicate STAs that have been allocated the periods in the downlink period  312  and the first uplink period  314   a,  Time Offset fields  342  and  346   a  that indicate start times of the periods allocated to the STAs, and Duration fields  343  and  348   a  that indicate the lengths of the allocated periods. The STA ID fields  340  and  344   a  each include at least a part of an Association Identity (AID) of each STA, or at least a part of a hardware address, i.e. a MAC address, given to each STA. An STA ID that indicates a period for broadcast/multicast data is set to a particular value, for example, ‘0’. The Time Offset fields  342  and  346   a  each indicate an interval from the MAP frame  310   a  to a start time of a corresponding period as a multiple of a predetermined unit of time (for example, 4 μs). The Duration fields  343  and  348   a  each indicate an interval from a start time to an end time of the period, i.e. indicate a length of the period, as a multiple of a predetermined unit of time (for example, 16 μs).  
         [0036]     The first uplink MAP  322   a  of the MAP frame  310   a  indicates an uplink period first allocated for the STA, and can be determined according to a required data rate for individual service joined by each STA, or the amount of data estimated by the AP. The STA, allocated a period by the MAP frame  310   a,  receives data in the period indicated by the corresponding STA information  332 , in the downlink period  312 , and transmits data in the period indicated by the corresponding STA information  336   a,  in the first uplink period  314   a.  The downlink and uplink data can include MPDUs including one or more Aggregate MAC Protocol Data Unit (PDU) (A-MPDU) and/or payload, and an Acknowledgement. The STA maintains the sleep mode in the period where the MAP frame  310   a  is transmitted, and the other periods except for the periods indicated by the MAP frame  310   a.    
         [0037]     After transmitting the data in the allocated period of the first uplink period  314   a,  each STA determines whether there is a need for additional resource allocation. The need for additional resource allocation is determined based on whether there is additional transmission data, or whether there is information to receive from the AP. A detailed description thereof will be omitted in order not to obscure the clear understanding of the present invention. An STA requiring additional resource allocation, monitors a sub MAP frame  310   b  after expiration of the first uplink period  314   a.  The sub MAP frame  310   b  can indicate the downlink and/or uplink resources additionally allocated by the AP, and it is shown in  FIG. 3  that the sub MAP frame  310   b  indicates allocation of additional uplink resources, by way of example.  
         [0038]     That is, the sub MAP frame  310   b  includes a second uplink MAP  322   b  indicating phase resource allocation for its succeeding second uplink period  314   b.  Similarly, the sub MAP frame  310   b  can be transmitted with the basic rate set. The second uplink MAP  322   b  is composed of a Number-of-STAs field  334   b  and at least one STA Information field  336   b  determined based on the Number-of-STAs field  334   b.  The STA Information field  336   b  includes an STA ID field  344   b  that indicates an STA allocated a period in the second uplink period  314   b,  a Time Offset field  346   b  that indicates a start time of the period allocated to the STA, and a Duration field  348   b  that indicates a length of the period. The elements of the STA Information field  336   b  have been described above. An STA, allocated phase resources by the sub MAP frame  310   b,  receives data in the period indicated by the corresponding STA information  336   b,  of the second uplink period  314   b.    
         [0039]     Although not illustrated, if there is another need for additional resource allocation, more than one sub MAP frames and an uplink period or a downlink period based on the sub MAP frames can be additionally included in the transmission frame period  300 . A transmission scheme using a MAP frame capable of accompanying one or more sub frames is referred to as a Multi-Phase “Power Save Multi-Poll (PSMP),” and the MAP frame  310   a  and the sub MAP frame  310   b  are referred to as a PSMP frame and a sub PSMP frame, respectively. Each of the PSMP frame and the sub PSMP frame is followed by at least one downlink or uplink period indicated by the corresponding (sub) PSMP frame, and one (sub) PSMP frame and a corresponding period are referred to as a (sub) PSMP sequence. In other words, one PSMP sequence is initialized by transmitting the PSMP frame, and the STA wakes up only in the period indicated by the PSMP frame in the PSMP sequence starting with the PSMP frame, thereby minimizing power consumption.  
         [0040]     A contention period  316  following the last sequence duration can be accessed by at least one STA on a contention basis.  
         [0041]     It should be noted that elements of the MAP frames  310   a  and  310   b  shown in  FIG. 3  and their arrangements are subject to change without departing from the spirit and scope of the invention. For example, each of the MAP frames  310   a  and  310   b  includes one STA ID field, Time Offset and Duration fields for downlink transmission, and Time Offset and Duration fields for uplink transmission: In this case, if no period is allocated to the uplink or downlink, a corresponding Duration field is set to Null (0). In addition, each of the MAP frames  310   a  and  310   b  can include a field for indicating whether it will be followed by another sub MAP frame. The STA analyzes the field in the-sub MAP frame to determine whether the sub MAP frame is the last sub MAP frame in the current transmission frame period.  
         [0042]      FIG. 4  is a timing diagram illustrating a power reduction effect of the phase resource allocation according to the present invention.  
         [0043]     Referring to diagram (a) of  FIG. 4 , a transmission frame period  400  includes a MAP frame  410   a,  a downlink period  412 , a first uplink period  414   a,  a sub MAP frame  410   b,  and a second uplink period. The MAP frame  410   a  indicates phase resource allocation of the downlink period  412  and the first uplink period  414   a.    
         [0044]     Diagram (b) of  FIG. 4  shows a scenario in which the resources of the first uplink period  414   a  allocated by the MAP frame  410   a  are enough to transmit data and control messages desired by the STA. That is, after switching to the sleep mode after receipt of the MAP frame  410   a  in a period  420 , the STA wakes up in an allocated period  422  in the downlink period  412  indicated by the MAP frame  410   a  and receives data from the AP. After switching back to the sleep mode upon expiration of the period  422 , the STA maintains the sleep mode until an allocated period  424  in the first uplink period  414   a  indicated by the MAP frame  410   a.  In the period  424 , the STA transmits data and/or control messages to the AP. Here, the STA, as it has no need for additional resource allocation, holds the sleep mode until it receives a MAP frame of the next transmission frame period, without monitoring receipt of the sub MAP frame  410   b.    
         [0045]     Diagram (c) of  FIG. 4  shows a scenario in which after expiration of the first uplink period  414   a  allocated by the MAP frame  410   a,  there is a need for additional resource allocation for the STA. That is, after switching to the sleep mode after receipt of the MAP frame  410   a  in a period  430 , the STA wakes up in an allocated period  432  in the downlink period  412  indicated by the MAP frame  410   a,  and receives data from the AP. After switching back to the sleep mode upon expiration of the period  432 , the STA maintains the sleep mode until an allocated period  434  in the first uplink period  414   a  indicated by the MAP frame  410   a.  In the period  434 , the STA transmits data to the AP. Here, the STA, as it requires additional resource allocation, wakes up again in a period  436  and monitors receipt of the sub MAP frame  410   b.  Upon receipt of the sub MAP frame  410 , the STA switches back to the sleep mode and waits until period  438  in a second uplink period  414   b  indicated by the sub MAP frame  410   b.  In the period  438 , the STA transmits data to the AP. Although not illustrated, there is a possible allocated period mapped to additional one or more MAP frames before expiration of the transmission frame period  400 .  
         [0046]     As can be understood from the foregoing description, even though the amount of resources estimated by the AP is not accurate, the present invention minimizes a waste of resources and a reduction in the data throughput, and maximizes the time in which the STA maintains the sleep mode without unnecessarily monitoring the downlink or uplink, reducing power consumption of the STA. In addition, the present invention can perform additional resource allocation required for the STA in the current frame period, without reserving the additional resource allocation until the next transmission frame period.  
         [0047]     While the invention has been shown and described with reference to a certain preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.