Abstract:
Provided is an association control method for maximizing a message in message (MIM) function in a WLAN environment. According to an exemplary embodiment of the present invention, when all SINR values of transmission links to two or more first clients associated with the first access point are equal to or more than a predetermined capture threshold, SINR value of a transmission link to a second client associated with the second access point is less than the capture threshold, and at least one of the first clients, referred to as a third client group, can be associated with the second access point, associations are modified so that the third client group is associated with the second access point to enable concurrent transmission.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119 to Korean Patent Application No. 10-2010-0056171, filed on Jun. 14, 2010, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     The present invention relates to an opportunistic concurrent transmission method of a wireless network and a wireless network system using the same, and more particularly, to an opportunistic concurrent transmission method of a wireless network and a wireless network system using the same that can fully utilize the wireless capacity in terms of spatial reuse and maximize the system throughput. 
     BACKGROUND 
     Recent proliferation of IEEE 802.11 WLANs (Wireless local area networks) stems from its attractive features such as low chipset cost, ease of deployment, and sufficient bandwidth. As IEEE 802.11 WLANs becomes a dominant wireless access technology, it requires more efficient use of scarce wireless resources. 
     Distributed Coordination Function (DCF), the most popular MAC protocol for IEEE 802.11 WLANs, is very simple and its distributed operations show good performance in most environment. DCF which is based on CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) prohibits concurrent transmissions in order to avoid packet collisions and harmful interferences. 
     However, this basic collision protection scheme (CSMA/CA) may not fully utilize the wireless resources in terms of spatial reuse due to its conservative medium access control. If we adjust the transmission order and relative signal strength, we can successfully transmit multiple packets without the collision and channel error. We call this Capture Effect. 
     Previous wireless NICs (Network Interface Card) enables the PHY capture when an intended signal arrives until the middle of the preamble time of an interference signal. Of course, the SINR (Signal to Interference plus Noise Ratio) value of the intended signal must satisfy the required capture threshold. Recent MIM (Message in Message)-capable NICs such as Athelos increases the PHY capture probability by using enhanced preamble detection functionality. MIM-capable NICs can capture the intended signal with higher SINR (≈10 dB) even if the intended signal arrives after the preamble time of an interference signal. 
     This is shown in  FIG. 1 .  FIG. 1A  shows PHY capture, and  FIG. 1B  shows MIM, respectively. 
     As shown in  FIG. 1A , when an intended signal having high SINR of approximately 10 dB arrives within the preamble time of an interference signal, the intended signal can be captured. 
     With MIM function, an intended signal can be captured even though it arrives after the preamble time of an interference signal, as shown in  FIG. 1B . 
     U.S. Pat. No. 5,987,033 is the related art for maximizing the PHY capture using MIM function. In U.S. Pat. No. 5,987,033, there are disclosed a receiver and a method for operating the receiver, for a station in a wireless local area network using a common wireless communication channel and employing a CSMA/CA protocol includes various modes. In normal mode, the receiver follows typical states in order to detect a message and demodulate data from the message properly. Meanwhile, a process implements a message-in-message (MIM) mode when an energy increase above a specified level is detected. While in the MIM mode, if a carrier is detected, the energy increase is caused by a new message; otherwise, the energy increase is caused by an interfering station. If the carrier is detected, the receiver begins retraining so that it can start receiving the new message as soon as the first message ends. 
     SUMMARY 
     An exemplary embodiment of the present invention provides an association control method for controlling associations in a wireless network system including a first access point and a second access point, comprising: verifying signal to interference plus noise ratio (SINR) values of transmission links to two or more first clients associated with the first access point and SINR value of a transmission links to a second client associated with the second access point; and when all of the SINR values of the transmission links to the first clients from the first access point are equal to or more than a predetermined capture threshold, the SINR value of the transmission link to the second client from the second access point is less than the capture threshold, and at least one of the first clients, referred to as a third client group, can be associated with the second access point, modifying associations in the wireless network system so as to associate the third client group with the second access point. 
     Herein, the method may further comprises: when one of the first and the second access points has a packet to transmit to the first client associated with the one of the first and the second access points, the one of the first and the second access points concurrently transmitting the packet while the other of the first and the second access points is transmitting another packet to the first client associated with the other of the first and the second access point, after the modifying of the associations. 
     The SINR value may be verified by referring to an interference map. The wireless network system may further include a central controller associated with the first and second access points, and the interference map may be provided by the central controller. 
     Another exemplary embodiment of the present invention provides a wireless network system including a first access point and a second access point, in which, when all of signal to interference plus noise ratio (SINR) values of transmission links to two or more first clients associated with the first access point from the first access point are equal to or more than a predetermined capture threshold, a SINR value of a transmission link to a second client associated with the second access point from the second access point is less than the capture threshold, and at least one of the first clients, referred to as a third client group, can be associated with the second access point, associations of the wireless network system is modified so as to associate the third client group with the second access point, and when one of the first and the second access points has a packet to transmit to the first client associated with the one of the first and the second access points, the one of the first and the second access points concurrently transmits the packet while the other of the first and the second access points is transmitting another packet to the first client associated with the other of the first and the second access point. 
     Other features and aspects will be apparent from the following detailed description, the drawings, and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are diagrams showing transmission schedules of PHY capture and MIM capture, respectively; 
         FIG. 2  is a diagram showing an operation of a WLAN system according to an exemplary embodiment of the present invention; 
         FIG. 3  is a flowchart showing an opportunistic concurrent transmission method from a viewpoint of one AP according to an exemplary embodiment of the present invention; 
         FIGS. 4A and 4B  are diagrams showing frame schedules in a case of concurrent transmission and in an opposite case of non-concurrent transmission method according to the exemplary embodiment of the present invention, respectively; 
         FIG. 5  is a graph showing expected throughputs of DCF and an exemplary embodiment of the present invention; 
         FIG. 6  shows an example of association in which the opportunistic concurrent transmission method according to an exemplary embodiment of the present invention cannot be applied; and 
         FIG. 7  shows an example of association in which the opportunistic concurrent transmission method according to an exemplary embodiment of the present invention is made to be applied by changing association shown in  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Hereinafter, exemplary embodiments will be described in detail with reference to the accompanying drawings. Throughout the drawings and the detailed description, unless otherwise described, the same drawing reference numerals will be understood to refer to the same elements, features, and structures. The relative size and depiction of these elements may be exaggerated for clarity, illustration, and convenience. The following detailed description is provided to assist the reader in gaining a comprehensive understanding of the methods, apparatuses, and/or systems described herein. Accordingly, various changes, modifications, and equivalents of the methods, apparatuses, and/or systems described herein will be suggested to those of ordinary skill in the art. Also, descriptions of well-known functions and constructions may be omitted for increased clarity and conciseness. 
       FIG. 2  is a diagram showing a WLAN system to which an opportunistic concurrent transmission method is applied according to an exemplary embodiment of the present invention. 
     As shown in  FIG. 2 , the WLAN system according to the exemplary embodiment of the present invention includes a central controller  210 , two access points (APs) AP 1 ;  221  and AP 2 ;  222 , and client devices R 1 ;  231 , R 2 ;  232 , and R 3 ;  233  connected to each AP, respectively. Both APs are located within the transmission range of each other. Though two APs and three client devices are shown in  FIG. 2  for better comprehension and ease of description, the numbers of APs and the client devices are not necessarily limited thereto. 
     In the figure, solid arrows mean a transmission link between an AP and a client device, and dashed lines denote interferences among concurrent transmissions. The value in a box indicates received SINR when packets are transmitted concurrently. That is, the clients R 1  and R 2  are associated with AP 1  and a signal transmitted from AP 2  becomes an interference signal for R 1  and R 2 . On the contrary, the client R 3  is associated with AP 2  and, as a result, a signal transmitted from AP 1  becomes the interference signal for R 3 . When concurrent transmission is made from AP 1  and AP 2 , R 1 , R 2 , and R 3  receive signals having SINRs of 1 dB, 5 dB, and 13 dB, respectively. 
     AP 1  and AP 2  may transmit concurrently by referring to an interference map. The interference map is a table of relative signal strength of each transmission depending on the transmission orders. In the exemplary embodiment shown in  FIG. 1 , the central controller  210  makes interference map from the individual report of each AP and distributes it to all APs. However, there are lots of schemes that make an interference map without a central controller. 
     Hereinafter, an opportunistic concurrent transmission method according to the exemplary embodiment of the present invention will be described referring to  FIG. 2 . It is assumed that each of AP 1  and AP 2  has packets to transmit to its associated clients R 1  and R 3 , respectively. 
     Let AP 1  transmit a packet to R 1  first, and AP 2  transmit a packet to R 3  after the preamble time of the AP 1 &#39;s packet. AP 1 &#39;s transmission may result in a collision and cannot be decoded successfully by R 1  since the SINR value (1 dB) of the received signal does not satisfy the capture threshold (4 dB). Of course, AP 2 &#39;s transmission may succeed due to a higher SINR value of 13 dB. 
     Now, let us change the transmission link. If AP 1  transmits a packet to R 2  not to R 1 , then a following concurrent transmission of AP 2  may not corrupt the AP 1 &#39;s packet. The reason is that SINR value of R 2  (5 dB) is higher than the capture threshold (4 dB). 
     Consequently, AP 2  has an opportunity to transmit a packet concurrently with AP 1  when AP 1  send a packet to R 2 . AP 2  can overhear the transmission of AP 1  and knows which link is used in this transmission by sniffing the MAC header of the ongoing packet. Referring the interference map, AP 2  knows that its concurrent transmission will not destroy the ongoing transmission of AP 1 . That is, AP 2  assures its concurrent transmission satisfy the required SINR thresholds for capturing both packets. 
     When it is determined that the concurrent transmission will cause a problem, that is, when it is determined that the transmission of another AP will fail by the concurrent transmission, the AP defers its own transmission as a standard DCF operation. 
       FIG. 3  is a flowchart showing an opportunistic concurrent transmission method from a viewpoint of one AP according to an exemplary embodiment of the present invention. 
     First, an AP determines whether there are packets to be transmitted (S 310 ). If so, the AP overhears transmission from another AP to acquire information on a transmission link (S 320 ). Next, AP finds out the SINR value for the transmission link by referring to the interference map (S 330 ). If the SINR value is equal to or higher than the capture threshold (S 340 ), the AP transmits its packets concurrently (S 350 ). If the SINR value is lower than the capture threshold (S 340 ), the AP enters the back off period (S 360 ) and waits for the transmission to be completed. When the transmission in completed (S 370 ), the AP transmits its own packets (S 380 ). 
       FIGS. 4A and 4B  shows timings of the opportunistic concurrent transmission and non-concurrent transmission, respectively. 
       FIG. 4A  shows the case of concurrent transmission. While AP 1  is transmitting a frame, AP 2  determines whether concurrent transmission can be made through a MAC header of the frame being transmitted by AP 1  and the interference map. If AP 2  determines to transmit concurrently, AP 2  transmits its own frame right away. 
     On the contrary,  FIG. 4B  shows the case in which it is determined that concurrent transmission is not made. When AP 2  overhears the transmission of AP 1  and determines that concurrent transmission is not made, AP 2  waits until the transmission of AP 1  is completed and transmits its own frame later. 
     Meanwhile, the opportunistic concurrent transmission method according to the exemplary embodiment of the present invention operates well in broadcast scenario, because broadcasts do not use ACK packets. However, the opportunistic concurrent transmission method requires a more sophisticated ACK processing mechanism in unicast scenario. Here is one possible solution. We can avoid ACK collisions by scheduling ACK packets to be serialized by referring to the MAC header. That is, since a transmission time of the ACK frame can be found by referring a MAC header of a packet which another AP is transmitting, its own frame schedule may be planned not to be overlapped with the ACK frame of the packet which another AP is transmitting. For example, AP 2  knows an ACK transmission time of AP 1  by the MAC header information of AP 1 &#39;s packet in  FIG. 4A . 
     A simulation is performed in order to compare the performances of the opportunistic concurrent transmission method according to the exemplary embodiment of the present invention and DCF. The expected throughput means the number of transmitted data bits divided by the total transmission time. We compare the expected through put of both DCF and the opportunistic concurrent transmission method according to the exemplary embodiment of the present invention with the broadcast operation. To simplify the analysis, we assumed that there is no collision. Therefore, the expected throughput of DCF is expressed as follows. 
     
       
         
           
             ET_DCF 
             = 
             
               
                 data 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 size 
               
               
                 DIFS 
                 + 
                 BackOff 
                 + 
                 TXdur 
               
             
           
         
       
     
     where 
     
       
         
           
             TXdur 
             = 
             
               
                 
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                   M 
                   ⁢ 
                   
                       
                   
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                   A 
                   ⁢ 
                   
                       
                   
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                   C 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   header 
                 
                 + 
                 preamble 
               
               TXrate 
             
           
         
       
     
     In the opportunistic concurrent transmission method according to the exemplary embodiment of the present invention, it requires additional one preamble time plus one MAC header time to send two packets simultaneously. Thus, we obtain the expected throughput of the opportunistic concurrent transmission method according to the exemplary embodiment of the present invention as followings. Herein, OMCT is an abbreviation of Opportunistic MIM-aware Concurrent Transmission which represents the opportunistic concurrent transmission according to the exemplary embodiment of the present invention. 
     
       
         
           
             ET_OMCT 
             = 
             
               
                 2 
                 * 
                 data 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 size 
               
               
                 DIFS 
                 + 
                 BackOff 
                 + 
                 TXdur 
               
             
           
         
       
     
     where 
     
       
         
           
             TXdur 
             = 
             
               
                 
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                   ⁢ 
                   
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                         M 
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                         A 
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                         C 
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                         header 
                       
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                       preamble 
                     
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               TXrate 
             
           
         
       
     
     We insert the typical values of IEEE 802.11b parameters in equations above and get the results.  FIG. 5  shows the expected throughput of both DCF and the exemplary embodiment of the present invention as a function of data size. We set the data rate 11 Mbps and vary the data size from 10 bytes to 1500 bytes. The result shows that the exemplary embodiment of the present invention outperforms DCF up to 200% in terms of the expected throughput. 
     In the exemplary embodiment described above, the opportunistic concurrent transmission method is used in a WLAN system, but the method is not limited thereto and may be applied to another wireless network system such as a wireless ad hoc network, or the like. 
     It is preferable that the opportunistic concurrent transmission method according to the exemplary embodiment of the present invention is applied only to a downlink transmission, the transmission from an AP to a client device, while an uplink transmission, the transmission from the client device to the AP, is made as a standard DCF operation. Despite of that, transmission efficiency can be remarkably improved. The reason is that in a general WLAN system, most transmissions are made as the downlink. 
     Hereinafter, an association control method between the AP and the client for utilizing the opportunistic concurrent transmission method according to the exemplary embodiment of the present invention to the maximum will be described. 
       FIG. 6  shows an example in which the opportunistic concurrent transmission method according to the exemplary embodiment of the present invention cannot be utilized. 
     As shown in  FIG. 6 , R 1  and R 2  are associated with AP 1 , and R 3  is associated with AP 2 , respectively. Since a SINR value of R 3 , which is the client of AP 2 , is 1 dB, which cannot reach the capture threshold of 4 DB, AP 1  and AP 2  cannot utilize concurrent transmission using MIM. That is, AP 1  should stand by while AP 2  performs transmission, and AP 1  may perform transmission to R 1  or R 2  after the transmission from AP 2  to R 3  is completed. 
     However, if R 2  needs not to be associated with AP 1  and may be associated with AP 2 , association of R 2  can be changed to AP 2  to enable concurrent transmission. 
       FIG. 7  shows an example of the case in which concurrent transmission is available by changing the association. 
     As shown in  FIG. 7 , R 1  is associated with AP 1 , and R 2  and R 3  are associated with AP 2 , respectively. That is, association of R 2  is changed to AP 2  from AP 1 . Since the SNR value of R 2 , which is now the client of AP 2 , is 5 dB, which is higher than the capture threshold, capture becomes available despite of concurrent transmission. Therefore, as shown in  FIG. 7 , the transmission from AP 1  to R 1  may start during the transmission from AP 2  to R 2 . Accordingly, the performance of the entire WLAN system can be improved. 
     A number of exemplary embodiments have been described above. Nevertheless, it will be understood that various modifications may be made. For example, suitable results may be achieved if the described techniques are performed in a different order and/or if components in a described system, architecture, device, or circuit are combined in a different manner and/or replaced or supplemented by other components or their equivalents. Accordingly, other implementations are within the scope of the following claims.