Patent Publication Number: US-2004057507-A1

Title: Link estimation in a communication system

Description:
FIELD OF THE INVENTION  
       [0001] The present invention relates generally to improved link estimation in a wireless or wired communication system, for example, a wireless local area network system.  
       BACKGROUND OF THE INVENTION  
       [0002] Referring to FIG. 1, in a typical wireless local area network (“WLAN”) system  100 , an access point (“AP”; i.e., infrastructure device)  102  transmits messages to a plurality of mobile stations (“MS”; i.e., subscriber station)  104 . A typical MS  104  receives a message over a communication link  106 , uses the information contained in the message to identify the AP  102 , and processes the message in a conventional manner as known in the art. There are, however, a few problems with the current method.  
       [0003] First, if transmit power control is used, a power control message needs to be transmitted at maximal power. Second, the power control message does not provide a method of fully evaluating the communication link; assuming that there are multiple APs with different peak power outputs, and that the MS has yet a different power output from the different APs, there is no way for the MS to evaluate which link is better and how much power to use for the uplink power. Third, due to the time division multiplexing nature of the channel, there is no immediate link quality indication, such as the quality indicator channel that exists in the various data oriented cellular standards. Fourth, lacking immediate channel knowledge, the MS transmit power control and adaptive modulation and coding are sub-optimal, especially in outdoor application where the channel changes with time (e.g., reflections from a moving car may change the channel even if both AP and MS are stationary).  
       [0004] Thus, there exists a need for improved link estimation in a communication system. 
     
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
     [0005] A preferred embodiment of the invention is now described, by way of example only, with reference to the accompanying figures in which:  
     [0006]FIG. 1 illustrates a typical wireless local area network (“WLAN”) system diagram;  
     [0007]FIG. 2 illustrates the typical WLAN system of FIG. 1 with added power and interference levels in accordance with the present invention;  
     [0008]FIG. 3 illustrates a message format in accordance with the present invention;  
     [0009]FIG. 4 illustrates a sequence diagram of communications between an access point and a mobile station in accordance with the present invention;  
     [0010]FIG. 5 illustrates a block diagram of a receiver in accordance with the present invention; and  
     [0011]FIG. 6 illustrates a plot of power versus time for the input data to the receiver of FIG. 5 in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
     [0012] It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements are exaggerated relative to each other. Further, where considered appropriate, reference numerals have been repeated among the figures to indicate corresponding elements.  
     [0013] The present invention uses measurements at both ends of the communication link  106  to allow the transmitting device to more accurately predict the channel conditions, and use the measurements, in whole or in part, for link estimation, transmit power control and adaptive modulation and coding.  
     [0014]FIG. 2 illustrates a typical WLAN system  100  as previously described with respect to FIG. 1. As noted in the preceding discussion of FIG. 1, preferably both the AP  102  and the MS  104  have the capabilities to act as a transmitting device and a receiving device. For ease of explanation, the following description assumes that the AP  102  will first act as the transmitting device and the MS  104  will first act as the receiving device.  
     [0015] In the preferred embodiment, after participating devices have registered with the system, preferably all the participating devices enter into a quiet period where each device estimates/measures its local interference level. The interference level  204  is perceived at the AP  102 , and the interference level  208  is perceived at the MS  104 . Typically, the devices monitor the communication link  106  to identify the quiet period, however, the quiet period can, in addition to or alternatively, be scheduled at predetermined times or randomly by a device.  
     [0016] After at least the AP  102  has estimated its perceived local interference level  204 , the AP  102  generates and/or prepares a message by defining a transmit power level  202  at which the message will be sent to the MS  104 . Preferably, as illustrated in FIG. 3, the AP  102  inserts an indication of the transmit power level  202  used by the AP  102  and an indication of the interference level  204  as perceived locally by the AP  102  into the message  300 , and transmits the message  300  to the MS  104 . It should be noted that the transmit power level  202  and the interference level  204  can be communicated to the MS  104  in a single message (such as message  300 ) or a plurality of messages depending the system design parameters.  
     [0017] The MS  104  receives the message  300  at a given power  206 , which may be different than the transmit power level  202  used by the AP  102  to transmit the message  300  over the communication link  106 . Upon receipt of the message  300 , the MS  104  identifies, from the message  300 , the transmit power level  202  used by the AP  102  to transmit the message  300  and the interference level  204  perceived locally by the AP  102 . Based on the transmit power level  202  used by the AP  102 , the receive power level  206 , the interference level perceived by the AP  102 , and the interference level  208  perceived locally by the MS  104 , the MS  104  deduces the “link path loss” and calculates at least one optimal transmission parameter (e.g., a transmit power level, a data rate, a modulation format, a modulation mode, error correction, spreading, coding, etc.) by which a response message will be transmitted to the AP  102 . It is important to note that the transmit power level  202  used by the AP to transmit the message  300  is not necessarily the same as the receive power level  206  as received by the MS  104 ; moreover, the interference level  204  perceived locally by the AP  102  is not necessarily the same as the interference level  208  perceived locally by the MS  104 .  
     [0018] As stated above, both the AP  102  and the MS  104  have the capabilities of acting as a transmitting device and a receiving device. After the MS  104  receives the message(s) from the AP  102  identifying the transmit power level  202  used to the transmit the message and the interference level  204  perceived locally by the AP  102 , the two devices may switch roles and the MS  104  becomes the transmitting device an the AP  102  becomes the receiving device. Once the MS  104  generates and/or prepares the response message, the MS  104  transmits the response message to the AP  102  using at least one optimal transmission parameter (preferably, both the optimal transmit power level and optimal data rate).  
     [0019] The MS  104  informs the AP  102  of the transmit power level  212  it used to transmit the response message and the interference level  208  as perceived locally by the MS  104 . As noted above, the transmit power level  212  and the interference level  208  can be communicated to the AP  102  in a single message (such as the response message) or a plurality of messages depending the system design parameters.  
     [0020] The AP  102  receives the response message at a given power  210 , which may be different than the transmit power level  212  used by the MS  104  to transmit the response message over the communication link  106 . Upon receipt of the response message, the AP  102  identifies the transmit power level  212  used by the MS  104  to transmit the response message, and the interference level  208  perceived locally by the MS  104  from the response message. Based on the transmit power level  212  used by the MS  104 , the receive power level  210 , the interference level  208  perceived locally by the MS  104 , and the interference level  204  perceived locally by the AP  102 , the AP  102  deduces the “link path loss” and calculates at least one optimal transmission parameter (e.g., a transmit power level, a data rate, a modulation format, a modulation mode, error correction, spreading, coding, etc.) by which a subsequent message will be transmitted to the MS  104 . It is important to note that the transmit power level  212  used by the MS  104  to transmit the response message is not necessarily the same as the receive power level  210  as received by the AP  102 ; moreover, the interference level  208  perceived locally by the MS  104  is not necessarily the same as the interference level  204  perceived locally by the AP  102 .  
     [0021] The process described above is an iterative process with the devices  102 ,  104  switching roles as the transmitting device and the receiving device; more importantly, messaging information extracted from a message(s) received when acting as a receiving device is used to facilitate the transmission of a message(s) transmitted when acting as a transmitting device. For ease of understanding, FIG. 4 pictorially illustrates the above process in a sequence diagram.  
     [0022] To elaborate further as to how the receiving device processes messages, let us now refer to FIG. 5. FIG. 5 illustrates a block diagram of a receiver  500  in accordance with the present invention. The receiver  500  resides on both the AP  102  and the MS  104  since both devices are capable of acting as a receiving device; as above, the following assumes that the MS  104  first acts as the receiving device. In operation, input  502  is received by an analog-to-digital converter (“ADC”)  504  and converted into digital signals  506 . The digital signals  506  are then fed into a modem  508  that demodulates the digital signals  506  and transfers them to a host computer (not shown). In the preferred embodiment, the modem  508  further extracts messaging information (e.g., transmit power level  202  and local interference level  208 ) inserted into the message by the AP (currently acting as a transmitting device)  102  from the digital signals  506  and stores the transmit power level  202  as defined by the AP  102  into a first storage medium  510  and stores the interference level  208  as perceived locally by the AP  102  into a second storage medium  512 .  
     [0023] Typically, the digital signals  506  are further used by an automatic gain control (“AGC”) circuit  514  to maintain constant signal energy at the output of the ADC  504  with the help of an analog multiplier  516  located in the radio frequency path. A power meter  518  measures the energy on the digital signals  506  and receives an AGC adjustment from the AGC  514  to calculate the power of the input  502  as illustrated in FIG. 6 in accordance with the present invention. Unless otherwise noted, all operations described herein use linear calculations rather than logarithmic calculation.  
     [0024] Once the power meter  518  has estimated the receive power  206  as perceived by the MS  104 , the estimated receive power  206  is stored into a third storage medium  520  if the MS  104  receives a message (such as message  300 ) that contains the transmit power  202  used by the AP  102  to transmit message(s) and the interference level  204  as perceived locally by the AP  102 . The content of the third storage medium  520  is illustrated in FIG. 6 as power level  602 . If the MS  104  does not receive a message(s) that contains the transmit power  202  used to transmit message(s) by the AP  102  and the interference level  204  perceived locally by the AP  102 , a processor  522  searches for a minima of the received power  206 , and stores the minima in the fourth storage medium  522 ; the content of the fourth storage medium  522  is illustrated in FIG. 6 as power level  604 . The content of the fourth storage medium  522  tends to be noisy, and is thus fed into a low pass filer (“LPF”)  524  to reduce the noise component.  
     [0025] Subtractor  526  subtracts the output of the LPF  524  (which is the total background noise) from the content from the third storage medium  520  (which is the estimated receive power  206  as perceived by the MS  104 ). A first divider  528  divides the output of subtractor  526  by the contents of the first storage medium  510 . The output of the first divider  528  is the total channel attenuation of the communication link  106 .  
     [0026] Processor  530  computes a desired rate and modulation mode that needs to be applied to the response message based on the total channel attenuation of the communication link  106 ; in the preferred embodiment, as noted above, the response message is the message that the MS  104  will eventually transmit to the AP  102 . The calculated desired rate and modulation mode is fed into processor  532  that calculates the signal-to-noise ratio (“SNR”) required by the AP  102  to successfully receive/decode the response message that will be eventually transmitted by the MS  104 . Multiplier  534  multiplies the output of the processor  532  with the content of the second storage medium  512  (which is the interference level  204  perceived locally at the AP  102 ). The output of the multiplier  534  produces the desired receive power level in which the AP  102  should receive the response message. A divider  536  divides the desired receive power level in which the AP  102  should receive the response message  210  by the total channel attenuation to produce a minimum transmit power level  212  in which the MS  104  should apply when transmitting the response message in order to compensate for “link path loss” and local interference  204  as perceived by the AP  102 .  
     [0027] The receiver performs in the same manner on the AP  102  when the AP  102  is acting as the receiving device.  
     [0028] The receiver process described in FIG. 5 is further detailed by the following mathematical analysis.  
     [0029] When the power meter  518  receives the digital signals  506 , the power meter  518  estimates the total received power  206  at the MS  104 . This estimated received power  206  could be expressed as: 
     
       P 
       RX 
       =P 
       Tx 
       ×L 
       P 
       +I 
       MP 
       +I 
       OC 
       +N 
       0 
     
     [0030] where:  
     [0031] P Rx  is the total estimated receive power as perceived by the MS;  
     [0032] P Tx  is the transmit power as defined by the AP;  
     [0033] L P  is the path loss;  
     [0034] I MP  is the multipath induced interference;  
     [0035] I OC  is the interference induced by adjacent transmitting devices; and  
     [0036] N 0  is the thermal noise.  
     [0037] Taking into account that the multipath interference is proportional to the transmit power  202  as defined by the AP  102 ; the total received power  206  becomes a function of the transmit power  202  and environmental interference  208 : 
       P   Rx   =P   Tx ×( L   P   +L   MP )+( I   OC   +N   0 ) or 
     
       P 
       Rx 
       =P 
       Tx 
       ×L 
       T 
       +I 
     
     [0038] where:  
     [0039] P Rx  is the total estimated receive power as perceived by the MS;  
     [0040] P Tx  is the transmit power as defined by the AP;  
     [0041] L P  is the path loss;  
     [0042] L MP  is the multipath path loss;  
     [0043] I OC  is interference induced by adjacent transmitting devices;  
     [0044] N 0  is the thermal noise;  
     [0045] L T  is the total path loss; and  
     [0046] I is the total interference as perceived by the MS.  
     [0047] Given that the link is time division multiplexed between multiple users, a continuous scan of the communication link  106  yields the total interference, I, by monitoring the lowest absolute receive power  206  as perceived by the MS  104  over time. 
       I=min   time     —     interval ( P   RX ) 
     [0048] The total interference, I, is the power received by the MS  104  when the AP  102  and all other MSs in communication with the MS  104  are silent (i.e., the quiet period). In the preferred embodiment, the AP may optionally schedule at least one quiet period where all the devices are required to be silent and measure their local interference level.  
     [0049] Once the total interference, I, is known, the MS  104  can deduce the communication link  106 , L T , by the following equation:  
         L   T     =         P   Rx     -   I       P   Tx                     
 
     [0050] Knowing the communication link  106 , L T , is not sufficient for the MS  104  to define the correct transmit power  212  for the response message because the interference  204  perceived by the AP  102  may be different than the interference  208  perceived by the MS  104 . To overcome the discrepancy in perceived interference levels, the AP  102  transmits its perceived local interference level  204  to the MS  104 .  
     [0051] Knowing the communication link, L T , the MS  104  estimates the optimal data rate by any appropriate method, and using the optimal data rate, deduces a target SNR at the AP  102  for the response message. Once the SNR is known, the MS  104  can calculate the required transmit power  212  for the response message using the following equation:  
         SnR   Target     =           P   MTx     ×     L   T         I   AP                     or               P   MTx     =         SnR   Target     ×     I   AP         L   T                     
 
     [0052] where:  
     [0053] P MTx  is the total transmitted power at the MS;  
     [0054] L T  is the total path loss;  
     [0055] I AP  is the interference at the AP; and  
     [0056] SnR Target  is the required signal to noise ratio at the AP for a given transmission rate.  
     [0057] When moved to dB notations, the equation reads: 
       P   MTx ( dB )= SnR   Target ( dB )+ I   AP ( dB )− L   T ( dB ) 
     [0058] While the invention has been described in conjunction with specific embodiments thereof, additional advantages and modifications will readily occur to those skilled in the art. The invention, in its broader aspects, is therefore not limited to the specific details, representative apparatus, and illustrative examples shown and described. Various alterations, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. Thus, it should be understood that the invention is not limited by the foregoing description, but embraces all such alterations, modifications and variations in accordance with the spirit and scope of the appended claims.