Abstract:
A method and apparatus for determining the transmission power level between a plurality of stations located within the coverage area of a basic service set (BSS) in a wireless local area network (WLAN). The receiving station measures a received signal power from the transmitting station, then the path loss estimation is computed based on the difference between the received signal power and the transmit power level extracted from the incoming signal. The computed path loss is updated according predetermined criteria. Based on the updated path loss information, the transmit power level and/or the transmission rate of a receiving station is adjusted.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]    This application claims the benefit of U.S. Provisional Applications Serial No. 60/290,141 filed May 10, 2001, the teachings of which are incorporated herein by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to communication systems. More particularly, the present invention relates to a system and method for estimating path loss between wireless stations in an IEEE 802.11 wireless local area network (WLAN) and for using this value to more accurately adjust the transmit power level and/or transmission rate of each station.  
           [0004]    2. Description of the Invention  
           [0005]    In general, there are two variants of wireless local area networks (WLAN): infrastructure-based and ad hoc-type. In the former network, communication typically takes place only between the wireless nodes, called stations (STAY), and the access point (AP), whereas communication takes place between the wireless nodes in the latter network. The stations and the AP, which are within the same radio coverage, are known as a basic service set (BSS).  
           [0006]    The IEEE 802.11 standard specifies the medium access control (MAC) and physical characteristics for a wireless local area network (WLAN) to support physical layer units. The IEEE 802.11 standard is defined in International Standard ISO/IEC 8802-11, “Information Technology—Telecommunications and information exchange area networks”, 1999 Edition, which is hereby incorporated by reference in its entirety.  
           [0007]    Currently, the IEEE 802.11 does not provide any mechanism for providing dynamic transmit power control (TPC) between wireless stations within a BSS. Typically, each 802.11 STA uses a fixed transmission power level for all the frame transmissions throughout its lifetime. However, a new standard, IEEE 802.11h contemplates implementing the dynamic transmit power control (TPC). Accordingly, the present invention provides an improved TPC mechanism that can be implemented within the firmware of the proposed 802.11h MAC implementation without much complexity.  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention is directed to a system and method of estimating path loss by a communication receiver to determine accurate transmission power control (TPC) or to adjust transmission rate in a wireless local area network (WLAN).  
           [0009]    According to an aspect of the present invention, a method for determining the transmission power level and/or transmission rate between a plurality of stations located within the coverage area of a basic service set (BSS) in a wireless local area network (WLAN) is provided. The method includes the steps of: measuring total received signal power of an incoming frame; calculating a path loss based on the difference between the measured received signal power and the transmit power level extracted from the incoming frame and adjusting the transmit power level or the transmission rate of the receiving station according to the said calculated path loss.  
           [0010]    Another aspect of the present invention provides a method for determining the transmission power level and/or transmission rate between a plurality of stations located within the coverage area of a basic service set (BSS) in a wireless local area network (WLAN), each station having a means for transmitting a signal and a means for receiving a signal. The method includes the steps of: transmitting a first frame from a transmitting station to a receiving station; measuring the receive power level of said received first frame by the receiving station; extracting the transmit power level from the received first frame by the receiving station; calculating a path loss based on the difference between the measured received signal power and the extracted transmit power level from the first frame; and, adjusting the transmit power level and/or the transmission rate of a future frame transmitted by the receiving station based on the calculated path loss information received therein. The path loss information is determined based on the proportionate weight of the calculated path loss information and, one or more, previously calculated path loss information.  
           [0011]    Another aspect of the present invention provides an apparatus with a power measurement circuit for determining the transmission power level between a plurality of stations located within the coverage area of a basic service set (BSS) in a wireless local area network (WLAN). The apparatus includes a receiver circuit for demodulating an incoming signal; a power measurement circuit for measuring the received signal power of the incoming signal received therein; a processor, coupled to the power measurement circuit, for calculating path loss information based on the difference between the received signal power and the transmit power level extracted from the incoming frame; a memory, coupled to the processor, for storing the calculated path loss information for a predetermined time period for a subsequent retrieval; and, a transmitter circuit coupled to the processor  
           [0012]    The foregoing and other features and advantages of the invention will be apparent from the following, more detailed description of preferred embodiments as illustrated in the accompanying drawings in which reference characters refer to the same parts throughout the various views. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a simplified block diagram illustrating the architecture of a wireless communication system whereto embodiments of the present invention are to be applied;  
         [0014]    [0014]FIG. 2 illustrates a simplified block diagram of an access point and each station within a particular basic service set (BSS) according to the embodiment of the present invention;  
         [0015]    [0015]FIG. 3 illustrates the format of an 802.11 frame, including the modification of the SERVICE field, that can be used to transmit information between stations according to an embodiment of the present invention;  
         [0016]    [0016]FIG. 4 is a flow chart illustrating the operation steps of selectively adjusting the power level according to an embodiment of the present invention;  
         [0017]    [0017]FIG. 5 is a flow chart illustrating the operation steps of updating path loss per frame reception according to an embodiment of the present invention; and,  
         [0018]    [0018]FIG. 6 is a flow chart illustrating the operation steps of updating path loss prior to a frame transmission according to an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0019]    In the following description, for purposes of explanation rather than limitation, specific details are set forth such as the particular architecture, interfaces, techniques, etc., in order to provide a thorough understanding of the present invention. However, it will be apparent to those skilled in the art that the present invention may be practiced in other embodiments that depart from these specific details.  
         [0020]    [0020]FIG. 1 illustrates a representative network whereto embodiments of the present invention are to be applied. As shown in FIG. 1, an access point (AP)  2  is coupled to a plurality of mobile stations (STA i ), which, through a wireless link, are communicating with each other and to the AP via a plurality of wireless channels. A key principle of the present invention is to provide a mechanism to update and estimate the path loss between a transmitting station and a receiving station by the receiving station of a frame. The updated path loss is useful in, but not limited to, saving the battery power, avoiding interference to other systems, adjusting radio coverage and adjusting transmission rate, by transmitting frames at just the right power level and the right transmission rate. IEEE 802.11 Physical Layers (PHYs) define a plurality of transmission rates based different modulations and channel coding schemes so that the transmitter of a frame can choose one of the multiple rates based on the wireless channel condition between the receiver and itself at a particular time. Typically, the lower the transmission rate, the more reliable the transmission. It should be noted that the network shown in FIG. 1 is small for purposes of illustration. In practice most networks would include a much larger number of mobile stations.  
         [0021]    Referring to FIG. 2, the AP and each STA within the WLAN of FIG. 1 may include a system with an architecture that is illustrated in the block diagram of FIG. 1. Both the AP and STA may include a receiver  12 , a demodulator  14 , a power measurement circuit  16 , a memory  18 , a control processor  20 , a timer  22 , a modulator  24 , and a transmitter  26 . The exemplary system  10  of FIG. 2 is for descriptive purposes only. Although the description may refer to terms commonly used in describing particular mobile stations, the description and concepts equally apply to other processing systems, including systems having architectures dissimilar to that shown in FIG. 2.  
         [0022]    In operation, the receiver  12  and the transmitter  26  are coupled to an antenna (not shown) to convert received signals and transmit desired data into corresponding digital data via the demodulator  14  and the modulator  24 , respectively. The power measurement circuit  16  operates under the control of the processor  20  to determine the path loss by subtracting the received signal strength from the transmission power level (in dBm), which is conveyed in the frame received thereon. The path loss with respect to other stations is estimated and stored in the memory  18  that is coupled to the processor  20  for subsequent retrieval. The estimated path loss with respect to other stations within the same BSS is updated and later used to calculate the transmission power level. The timer  22  is used to eliminate the outdated path loss estimation, which is stored in the memory  18 . In the embodiment, the path loss is updated as it tends to change due to the time-varying nature of the wireless channel as well as the potential mobility of WLAN STAs.  
         [0023]    [0023]FIG. 3 represents the format of PHY Protocol Data Unit (PPDU) frame that is used to convey the transmission power level information between the stations. As shown in the lowest part of FIG. 3, the transmission power level (represented by TXPWR_LEVEL) is transmitted in the SERVICE field of the 802.11a/h PPDU frame. The SERVICE field of the 802.11a is slightly modified to include the four-bit TXPWR_LEVEL field. The original SERVICE field format of 802.11a is found in the middle part of FIG. 3. The TXPWR_LEVEL field is defined from  1  to  16 , where each value represents a particular transmission power level. The TXPWR_LEVEL is used to determine the path loss by subtracting the received signal strength via Received Signal Strength Indicator (RSSI) from the transmitted signal power via TXPWR_LEVEL (explained later). After obtaining the path loss by receiving frame(s), the receiving STA can determine both the PHY rates as well as the transmission power intelligently for its future transmission to other STA. Thus, the transmission power level and rate are determined solely up to the transmitting STA&#39;s discretion. It should be noted that the transmission power should not exceed the maximum transmission power specified by the AP through a beacon frame; an 802.11h-compliant AP shall broadcast such maximum transmission power via beacon frames periodically. Hence, receiving an erroneous TXPWR_LEVEL, which causes an adverse effect on the system performance, can be avoided. An Extended Hamming Code may be used for the error detection code operation.  
         [0024]    Now, the principle of operation steps according to the present invention of updating the path loss to determine the transmission power level is explained hereafter.  
         [0025]    Referring to FIG. 4, the inventive process includes the following steps: in step  100 , a station STA 2  receives a frame. In step  110 , the STA 2  measures the power level of the received frame. Measuring the power level is a well-known art that can be performed in a variety of ways. In step  120 , the STA 2  calculates the path loss, which is the difference between the transmitted power level and the received power level, and updates the path loss information. The path loss PL is updated by giving a different weight to the new and old path loss values, as follows: PL−al*PL_new+a 2 *PL (a 1 +a 2 −1, a 1 ≧0, and a 2 ≧0), wherein PL_new represents the estimated path loss from the new frame reception. The updated path loss is then used to determine the transmitter power required to obtain the desired carrier-to-noise ratio within the BSS. In step  140 , the STA 2  adjusts the transmission power level and/or the transmission rate based on the adjustment level that was determined in step  120 .  
         [0026]    Although a limited number of STAs is shown in FIG. 4 for illustrative purposes, it is to be understood that the WLAN can support communications between a much larger number of STAs. Thus, the number of STAs in the figure should not impose limitations on the scope of the invention. In such event, each STA keeps track of the path loss between other STAs within the BSS and to the AP, then each transmitting station may use the path loss estimation to adjust the transmit power level as it transmits a frame to another STA or to the AP. With non-802.11e WLAN, a STA needs to keep track of the path loss with the AP only as the STA must transmit frames to its AP. Here, each transmitting station may want to keep track of the path loss with a selected number of STAs to reduce the complexity. In addition, to prevent using outdated and stale path loss information, the present invention may adopt the path loss information lifetime. To this end, whenever STA  2  updates the path loss estimation with STA  1  by receiving a frame from STA  1 , STA  2  sets a timer for each updated path loss estimation using the timer  22  of FIG. 2. Hence, the STA  2  will compare the most recent updated time of the frame with the current time when it is to transmit a frame.  
         [0027]    [0027]FIG. 5 illustrates the principles of updating the path loss adjusting the transmission power level and/or the transmission rate of a frame. In step  200 , upon receiving a frame from another station in step  200 , the newly estimated path loss is calculated and updated in step  220 . Thereafter, the new path loss estimation is compared to determine whether it is different from the previously stored path loss estimation by more than the threshold in step  220 . If so, the receiving station will calculate the path loss according to a particular condition (PL=al*PL_new+a 2 *PL, wherein a 1 +a 2 =1, al≧0, and a 2 ≧0) in step  240  and will reset the flag PL_var=0. Otherwise, the receiving station will erase the old path loss information and store the new information by setting the flag PL_var=1, which indicates a change in the path loss information in step  230 . Thereafter, the STA  2  will use the stored path loss information received from the STA  1  only if the lifetime of the path loss does not pass the preset threshold and only when PL_var=0. This same method could be used to update path information between AP and a STA. Also, the same method could be used by AP to update path loss information to the STAs.  
         [0028]    [0028]FIG. 6 illustrates the operation steps of using the path loss information by the transmitting station. To transmit a frame to another station in step  300 , it is determined whether the path loss estimation that is received from the receiving station exists in step  310 . If so, it is determined whether the time of the path loss estimation and current time is less than the preset threshold in step  330 . Otherwise, a new estimation of path loss may be performed via a Request-to-Send (RTS) / Clear-to-Send (CTS) frame exchange. The CTS/RTS frame can measure the path loss without service interruption. The transmitting station could measure the path loss by sending an RTS frame to the supposed receiving STA and receiving the corresponding CTS frame from the said receiving STA. Note that the CTS frame shall include the transmission power level in its SERVICE field as well so that the sender of the RTS frame can estimate the path loss. Alternatively, the transmitting station may use the maximum power level announced by the AP within the BSS via a beacon frame for its frame transmission in step  320 . If the difference is less than the preset threshold and PL_var=0, the station uses the path loss estimation found in step  310 . If the difference is not less than the preset threshold in step  330 , a new estimation of path loss is performed via RTS/CTS, or the station may use the maximum power level announced by the AP within the BSS via the beacon frame in step  340 .  
         [0029]    While the preferred embodiments of the present invention have been illustrated and described, it will be understood by those skilled in the art that various changes and modifications may be made, and equivalents may be substituted for elements thereof without departing from the true scope of the present invention. In addition, many modifications may be made to adapt to a particular situation and the teaching of the present invention without departing from the central scope. Therefore, it is intended that the present invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out the present invention, but that the present invention include all embodiments falling within the scope of the appended claims.