Patent Document

FIELD OF THE INVENTION 
     This invention relates to wireless local area networks (WLANs), and more particularly to a method of improving throughput in a wireless network in a contention based environment. 
     BACKGROUND OF THE INVENTION 
     Many wireless systems  100 , most notably 802.11 wireless local area network (“WLAN”) systems, can operate in both an infrastructure mode and a peer-to-peer mode; in other words, as illustrated in  FIG. 1 , in an infrastructure mode, the mobile stations  102 ,  104  communicate with each other through the infrastructure device  106  (e.g., access point, base station, or the like), or the information can be communicated directly from mobile station-to-mobile station. The infrastructure device  106 , also provides the connection to the wired LAN, depicted by circle  108 , via the Ethernet bridge  110 . It should be noted that the coverage area of the infrastructure device  106  is depicted by circle  112 . It should be noted that WLANs protocols currently in use, such as variations on 802.11, evolved from the Ethernet wired protocols, where each participating mobile station contends equally for bandwidth resources. 
     As illustrated in  FIG. 2 , the partial OSI model  200  comprises the physical (PHY) layer  202  and the medium access control (MAC) layer  204 . The PHY layer  202  and MAC layer  204  make up the bottom portion of the OSI model  200 . The PHY layer  202  is the interface between the MAC layer  204  and the wireless media, which transmits and receives messages (typical referred to as “data frames” or data packets) over a shared wireless link. In 802.11 WLAN systems, the MAC protocol is the mechanism used to deliver reliable messages (e.g., Media Access Data Units, “MSDU&#39;s”) over the wireless link. In order to ensure that the messages in 802.11 are reliably sent over the wireless link, the protocol requires that the MSDU be converted to MAC protocol data unit (“MPDU”), by adding a header and a trailer. Once the messages have been appropriately formatted, the message is then forwarded to the appropriate PHY layer to be sent over the wireless link. Since there are multiple PHY layers capable of communicating with the MAC layer, each PHY layer  202  is structured uniquely based on modulation type, to allow a mobile station to transmit and receive messages at a designated data rate. It should be noted that the MPDU is referred to as a PLCP protocol data unit (“PPDU”) depending on the PHY layer type, however the format and the functions are essentially the same. It should also be noted that the PHY layer  202  is composed of physical layer convergence sublayer (PLCP)  206  and a physical medium dependent sublayer (PMD)  208 . 
     As shown in  FIG. 3 , the basic format of the PHY layer  300  includes a preamble  314 , a PHY header  306 , and message depicted by fragment 1    308 , fragment 2    310 , and fragment 3    312 . The preamble consists of a SYNC field  302  and a start of frame delimiter (SFD) field  304 . The mobile station uses the SYNC field to capture the incoming signal and to synchronize its receiver and the SFD field to indicate the start of the message. The SYNC field  302  also allows for ramping up and down the transmit power, establishing bit edge determination at the receiver of the mobile station, and an energy measurement interval for antenna diversity. Each preamble  300  is followed by a PHY header  306 , the next fragment 2    310 . The cycle is repeated with Preamble  314 , PHY header  306  and, fragment 3    312  until the all fragments in the set are received. 
     In actual practice, many WLANs implementations utilize the infrastructure mode, with all mobiles stations communicating to a wired LAN  108  through the infrastructure device  106 . Therefore, the majority of the traffic is “downlink” (e.g., messages flow over the wireless link from the fixed infrastructure device  106  to the mobile stations  102 ,  104 ). 
     A major disadvantage resulting from the manner in which the mobile stations  102 ,  104  communicate with each other when operating in the infrastructure mode is that the throughput on the air interface in increased, consuming twice the bandwidth. 
     Another disadvantage is that in applications, such as multimedia voice, individual messages are small such that functions performed by the SYNC field  302  (e.g., establishing bitsync headers and ramp up times, etc.) add significantly to the overhead. This is particularly impactful as the bitsync headers are sent at the slowest, least common denominator data rates available in order to support backward compatibility and derated performance. 
     Nevertheless, in the infrastructure mode, a mobile station  102 ,  104  is required to transmit to the infrastructure device  106  within the bounds of the existing protocols. Thus, there exists a need for a method to achieve greater throughput when operating in the infrastructure mode in a WLAN environment. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       A preferred embodiment of the invention is now described, by way of example only, with reference to the accompanying figures in which: 
         FIG. 1  (prior art) illustrates a plurality of mobile stations operating within range of an infrastructure device in a WLAN system; 
         FIG. 2  (prior art) illustrates an interface between the PHY and MAC layer in a WLAN system; 
         FIG. 3  (prior art) illustrates a timing diagram standard physical layer format in a WLAN system; 
         FIG. 4  illustrates a timing diagram of a concatenated physical layer format; 
         FIG. 5  illustrates a circuit diagram of a infrastructure device transmitting the concatenated messages to a mobile station utilizing diversity receiver for receiving an RF signal on a first receiver and demodulation path in a WLAN system; and 
         FIG. 6  illustrates a circuit diagram of a infrastructure device transmitting to a mobile station utilizing a diversity receiver for receiving an RF signal on a first receiver and demodulation path and a second receiver and demodulation path in a WLAN system. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention provides a method for improving system messaging throughput in a WLAN, particularly wireless infrastructure-based networks. The present invention discloses a method in which the throughput performance of the network is improved by fragmenting a single message into a plurality of fragments and transmitting each fragment separately, wherein only one fragment of the message comprises the synchronization for the entire message. In other words, unlike the prior art depicted in  FIG. 3 , the SYNC field typically attached to and transmitted with each fragment of a message is eliminated from each fragment of the message except one. Optionally, other redundant signaling, such as the SFD field, may also be removed from the plurality of fragments in accordance with the present invention. The present invention further discloses a diversity receiver that eliminates the increased interference associated with concatenating long transmission streams at the receiver end when there is only a single SYNC field for entire message (e.g., multiple fragments). The diversity receiver utilizes separate receiver and demodulation paths to allow the diversity receiver to decode each fragment on both paths and choose the best path (i.e., the path with the least amount of interference) to recover the fragment. Let us now refer to  FIGS. 4-6  to describe the present invention in greater detail. 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 identical elements. 
       FIG. 4  depicts the relative performance difference between the standard PHY layer format  300  in  FIG. 3  (shadowed for comparison only) and a concatenated PHY layer format  400  in a WLAN system. As previously stated, the PHY layer  202  is the interface between the MAC layer  204  and the wireless media, which transmits and receives messages (e.g., “data frames”) over a shared wireless link. Unlike wired LANs, WLAN systems must deal with significant interference problems. To deal with the interference issues, the MAC layer fragments its messages in order to increase the probability that a message is correctly received over the wireless media. A message is typically divided into one or more fragments (e.g., MPDUs or PPDUs) which is limited to a maximum value or threshold as defined by the IEEE 802.11 MAC specification. The fragments are transmitted in order by sequence with the lower number fragment (e.g. fragment 1    408 ) being transmitted first. Subsequent fragments (e.g., fragment 2    410 , fragment 3    412 ) are transmitted immediately after receiving verification (e.g., acknowledgements) that the previous fragment has been received. The SYNC field  402  contains a string of Is that are scrambled prior to transmission. As previously mentioned, the SYNC field  402  is utilized by the receiver portion  550 ,  650  of the mobile station  102 ,  104  to acquire the incoming signal and synchronize each fragment prior to receiving the SFD, as depicted by the SFD field  404 . 
     As noted above in the prior art depicted in  FIG. 3 , the SYNC field  302  precedes each fragment in the message (e.g. fragment 1    308 , fragment 2    310 , and fragment 3    312 ). In the preferred embodiment of the present invention, however, the SYNC field  402  precedes only the first fragment (e.g., fragment 1    408 ) and is not transmitted with or precedes the remaining fragments in the message. Thus, in operation, the transmitter of the present invention transmits the first fragment (i.e., fragment 1 )  408  of the message along with a SYNC field  402 , a SFD field  404 , and a PHY header field  406 . Since the preferred embodiment of the present invention transmits only a single SYNC field  402  to provide synchronization for the entire message, the remaining fragments of the message (i.e., fragment 2  and fragment 3 )  410 ,  412  are transmitted along with only the SFD field  404  and PHY header field  406 ; no additional synchronization signaling is required. 
     Thus, fragment 1    408 , fragment 2    410 , and fragment 3    412  are each concatenated at the receiver based on the target address (e.g., address of the mobile station  102 , 104 ) in accordance with the present invention. The target address can be unicast to a single address per message, broadcast to multiple addresses per message, multicast to multiple addresses per multiple messages or a combination of unicast, broadcast, and multicast, thus allowing multiple PHY headers  406  to be concatenated in one message. 
     It will be appreciated by those skilled in the art that this process continues until all fragments in the set  501  are received. It will also be appreciated by those skilled in the art that in an alternate embodiment, the throughput of the WLAN system can be further improved by eliminating the SFD field  404 . In both embodiments, however, the shorter overall transmission time enabled by this concatenation technique significantly improves system throughput and decreases the amount of bandwidth consumed. As previously noted in application such as multimedia voice the functions performed by the SYNC field  402  (e.g. establishing the bitsync headers and ramp up times) add significantly to the overhead, therefore removal of the SYNC field  402  from subsequent fragments is particularly impactful as the bitsync headers are sent at the slowest, least common denominator data rates available in order to support backward compatibility and derated performance. 
       FIG. 5  illustrates a circuit diagram  500  of an infrastructure device  590  transmitting the concatenated messages to the mobile station  102 ,  104  utilizing a diversity receiver for receiving an RF signal on a first receiver and demodulation path in a WLAN system. 
     When data packets become available for transmission in the transmit message queue (e.g., message set  501 ), the timing control unit  510  is signaled  502  starting the transmission burst event. The Timing Control unit  510  signals the RF transmitting circuitry  530  via output signal  512  to ramp up RF power. Then the Timing Control unit  510  combines the sequence signals  511  in the Formatter  520 . The formatter  520  presents the information in the SYNC field  402 , SFD field  404 , PHY Header field  406 , and fragement 1    408  to the RF transmit circuitry  530  via output signal  521 . The modulated RF waveform from the RF transmit circuitry  530  is transmitted from transmit antenna  540 , over radio link  545 . As previously stated, in this invention, once a data packet is transmitted, if there are remaining fragments in queue, the Timing Control unit  510  will recycle and transmit the next set of information in the SFD field  404 , PHY Header field  406 , and the next fragment 2    410 . As shown in this embodiment of the invention, the recycling of the information in the SFD field  404  and PHY Header  406  is repeated and the next fragment 3    412  is sent. The Timing control unit  510  will continue to do this until the final fragment n  in the message set  501  has been sent. When the queue is empty or some other event occurs forcing the transmitter to stop, the Timing Control unit  510  will signal the RF transmitting circuitry  530  to shut down. 
     The resulting RF waveform is received at one or more receiving antennas  551 , 552  and associated RF receiver amplification, filtering, and demodulation circuitry, and presented to energy detector  570  via RF signal  554 . This energy detector  570  determines a quantitative measure of received signal energy, and presents this measure to receive Timing Control unit  560 . As the RF waveform first begins to appear at the receiving antennas  551 ,  552 , the energy measure causes the Timing Control unit  560  to apply signal  561  to switch  553 , in order to maximize the received RF energy by toggling between receiving antennas  551  and  552 . This measurement is nominally made during the first few instants of the received waveform, normally during the SYNC interval. 
     The receiver demodulation circuitry of the receiver portion  550 ,  650  of the mobile station  102 ,  104  will begin to recover bit synchronization, and make received bit estimations. These estimations are passed to the SFD  574 , the PHY PHY Parser  578  can then extract its information fields, one of which indicates the length of the following data packet. The Timing Control unit  560  uses this length to extract the next fragment, and store it in the receive buffer  580  via control  562 . In this invention, once a data packet has been buffered (e.g. fragmented), the Timing Control unit  560  will recycle and again begin observing the output of the SFD  574  in anticipation of the next fragment. Should a subsequent SFD  574  not be detected, the Timing Control unit  560  will return to observing the Energy detector  570 . 
       FIG. 6  illustrates a circuit diagram  600  of an infrastructure device  690  transmitting to a mobile station  102 ,  104  utilizing a diversity receiver  550 ,  650  for receiving an RF signal on a first receiver and demodulation path and a second receiver and demodulation path in a WLAN system. 
     A potential difficulty with concatenating long transmission streams by eliminating SYNC intervals from separate messages is that in typical operation a receiver portion of the mobile station  650  will use the re-occurring SYNC intervals to check the antenna diversity decision. With the typically high carrier frequencies used in WLAN data systems, (2 to 5 GHz), signal conditions due to multipath and fading may change appreciably between the SYNC occurrence (in  FIG. 5 ) and some subsequent fragment n . 
     The improved embodiment of  FIG. 6  nullifies problems associated with increased interference due to multipath and fading by employing a separate receiver and demodulation path  655  for the diversity antenna  651 . This separate receiver and demodulation path comprises its own Energy Detector  671 , SFD Detector  675 , and PLCP parser  679 . Each path  654  and  655  continuously attempts to decode the incoming RF waveform, with one or the other paths receiving the better diversity signal at any given instant. The Timing and Control unit  660  via  662  continuously chooses the best path to recover the received fragments based on the continuous energy detection of both energy detectors  670 ,  671 , success of the SFD CRC check, or other criteria. 
     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.

Technology Category: 5