Abstract:
A method of operating a wireless device having a first radio interface and a second radio interface. The method includes: using the first radio interface to transfer data between i) the wireless device and ii) a first access point; and using the second radio interface to transfer data between i) the wireless device and the first access point. While continuing to use the first radio interface to transfer data between i) the wireless device and ii) the first access point: suspending the transfer of data between i) the wireless device and ii) the first access point through the second radio interface; for a predetermined time period, using the second radio interface to search for a second access point; and in response to expiration of the predetermined time period, resuming the transfer of data between i) the wireless device and ii) the first access point through the second radio interface.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 11/894,179, filed on Aug. 20, 2007, which claims priority under 35 U.S.C. §119(e) to U.S. Provisional Application No. 60/823,197, filed Aug. 22, 2006. The disclosures of the applications referenced above are incorporated herein by reference. 
    
    
     BACKGROUND 
     In a wireless network, a wireless access point connects wireless communication devices together to form the wireless network. The access point usually connects to a wired network, and can relay data between wireless devices and wired devices. Several access points can link together to form a larger network that allows a user of a wireless device to roam between access points without the connection being dropped. 
     802.11 is an IEEE (Institute of Electrical and Electronics Engineers) standard for wireless area networks. 802.11 wireless devices typically have a mode for transmitting and receiving data traffic, and a mode for scanning for available access points during a process called background scanning. Up until 2004, 802.11 wireless devices had a single antenna (some devices had two antennas, but there was only one set of components to process the signal, or RF chain). 
     Because a 802.11 wireless device has a single antenna, in order to perform a background scan, the device has to stop transferring data over a data channel, switch to a scan channel, and then perform the background scan by transferring scan data over the scan channel for short periods time (e.g., a couple of milliseconds) to detect available access points. After the background scan is completed, the device has to switch back to the data channel, and restart the transmitter to transfer data traffic. The stopping and starting of data traffic to perforin a background scan negatively influences the data throughput of the wireless device, since the wireless device is not able to send or receive during this scanning period. 
     802.11 task group N (TGn) has recently proposed an 802.11n standard that has the goal of increasing the peak data throughput transmitted by a wireless multiple-input/multiple-output (MIMO) device to 100 Mbps. The basis of MIMO operation is to provide 11n devices with multiple radio interfaces to allow the devices to send data on different channels at the same time in order to achieve greater transmit/receive data rates than the pre-11n devices. In its present form, the 802.11n standard is silent as to how background scanning should be implemented. Using the traditional background scan process in which all data traffic is temporarily suspended would be counterproductive to the goal of the proposed 802.11n standard of increasing throughput. Accordingly, it would be desirable to provide an improved background scan process for use in multi-radio equipped wireless devices. 
     SUMMARY 
     The present invention provides a method and system for implementing a background scan in a wireless device having at least two independent radio interfaces. Aspects of the exemplary embodiment include using a first one of the radio interfaces for transferring data with an access point; and simultaneously using a second one of the radio interfaces for receiving scan data to search for a new access point. 
     According to the method and system disclosed herein, wireless devices are no longer required to entirely stop data traffic in order to perform a background scan, thereby minimizing the negative effects on data throughput caused by the background scan to just a decrease in the data rate, rather than a complete cessation of data traffic. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating an exemplary wireless communication system. 
         FIG. 2  is a flow diagram illustrating a process performed by a driver for implementing a background scan in a wireless device having at least two independent radio interfaces  16  in accordance with the exemplary embodiment. 
         FIG. 3  is a diagram illustrating operation of the background scan process between the wireless device and the access point. 
         FIG. 4  is a flow diagram illustrating the process performed by the driver for implementing the background scan according to one embodiment. 
         FIG. 5  is a flow diagram illustrating the process performed by the driver for implementing the background scan according to another embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention relates to an improved background scan process for wireless devices. The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the preferred embodiments and the generic principles and features described herein will be readily apparent to those skilled in the art. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features described herein. 
     The preferred embodiment provides an improved background scan process for use in wireless communication devices having at least two independent radio interfaces. The exemplary embodiment takes advantage of the presence of the two independent radio interfaces to transfer data traffic and background scan traffic in parallel. 
     The exemplary embodiments will be described in terms of an 802.11n standard multiple-input/multiple-output (MIMO) device that has multiple radio interfaces for sending data on different channels at the same time. However, one with ordinary skill in the art will readily recognize that the exemplary embodiments may be used with any type of wireless communication device that has at least two independent radio interfaces. The exemplary embodiments will also be described in the context of particular methods having certain steps. However, the method and system operate effectively for other methods having different and/or additional steps not inconsistent with the exemplary embodiments. 
       FIG. 1  is a block diagram illustrating an exemplary wireless communication system. The wireless communication system  10  includes a wireless device  12  that wirelessly communicates with an access point (AP)  14 . In one exemplary embodiment, the wireless device  12  comprises a MIMO device. In addition, the wireless device  12  may be a network device or client station (STA) used in a desktop/portable computer for communication. The access point  14  is the device that connects wireless devices  12  together to form a wireless network and permits wireless devices  12  to communicate-over the network or to each other. An example access point  14  is a router that has a broadband network connection. Several access points  14  can link together to form a larger network that allows roaming. The wireless device  12  searches for available access points  14  within range during a background scan process of the exemplary embodiments, as described below. 
     The wireless device  12  includes at least two independent radio interfaces  16   a  and  16   n  (commonly referred to as radio interfaces  16 ) for processing at least two data streams, a controller  18  coupled to the radio interfaces  16 , a memory  20  coupled to the controller  18 , and a bus interface unit  22  coupled to the controller  18  and to the memory  20  for transmitting data to a host  24  over a host system bus. 
     The radio interfaces  16  are independent from each other because each radio interface  16  has its own antenna and RF chain. Each RF chain and its corresponding antenna are responsible for transmitting and processing a data stream. A single frame of data can be broken up and multiplexed across multiple data streams and reassembled at the receiver, which may have the benefits of resolving multipath interference and improving the quality of the received signal. 
     In one embodiment, the devices in the wireless communication system  10  may have a different number of receive antennas than transmit antennas. An “Y×Z” antenna configuration may be employed, where Y and Z refer to the number of transmitter antennas on a transmitting device and the number of receiver antennas on a receiving device, respectively. At a minimum, the system  10  requires a 2×2 configuration that has two transmit chains and two receive chains, which allows for two data streams multiplexed across a radio link. A common hardware configuration may include two antennas and RF chains on the wireless device  12  to save cost and battery power, while at least three antennas and RF chains are used on the access point  14 . This configuration would use 2×3 MIMO for its uplink, and 3×2 MIMO on the downlink. 
     Each of the radio interfaces  16  may utilize 1 to N radio channels. Each of the channels may be used simultaneously for data transmission. One channel may be designated as the primary channel, and another channel may be designated as a background scan channel. The wireless device  12  may also include multiple operating modes including one or more data modes corresponding to the number of available radio interfaces  16 , and a powersave mode. 
     The driver  26  is software or firmware that controls the radio interfaces  16  and can process the data if needed. The driver  26  is executed by the controller  18 . The controller  18  may comprise an ASIC, a DSP or other type of processor. The memory  20  stores the incoming and outgoing data packets and any other data needed by the driver  26 . The bus interface unit  22  transfers data between the host system  24 , and the controller  18  and the memory  20 . 
       FIG. 2  is a flow diagram illustrating a process performed by the driver  26  for implementing a background scan in a wireless device  12  having at least two independent radio interfaces  16  in accordance with the exemplary embodiment. The process begins with the driver  26  using one of the radio interfaces  16   a  for transferring data with a current access point  14  (block  200 ). Simultaneously, the driver  26  uses another one of the radio interfaces  16   n  for receiving scan data to search for new access points (block  202 ). The driver  26  may then deliver the received scan data to the host system  24  while still using the first radio interface  16   a  for data transfer (block  204 ). 
       FIG. 3  is a diagram illustrating operation of the background scan process between the wireless device  12  and the access point  14 , where like components from  FIG. 1  have like reference numerals. The wireless device  12  is shown having two radio interfaces  16   a  and  16   b  that transmit and receive data on different channels. Likewise, the access point  14  is shown having two radio interfaces  16   c  and  16   d . The wireless device  12  is shown using first radio interface  16   a  to transfer data traffic  26  between the radio interface  16   c  of the access point  14 , while the second radio interface  16   b  is used to receive (and optionally send) scan traffic  28  from/to the radio interface  16   d  of the wireless access point  14  simultaneously or in parallel with the transfer of the data traffic  26 . 
     As can be seen, the exemplary embodiments eliminate the requirement for the wireless device  12  to entirely stop data traffic in order to perform a background scan. The advantage is that the negative effects on data throughput caused by the background scan to minimized to only a decrease in the data rate, rather than a complete cessation of data traffic. 
     There are several embodiments for performing the background scan. In one embodiment, when a wireless device  12 , which only has two radio interfaces  16  is to perform a background scan, the wireless device  12  signals the access point  14  that the wireless device  12  is entering a mode in which one of the radio interfaces  16  is unavailable for data traffic. In one embodiment, the powersave mode may be used for this purpose. In response, the access point  14  refrains from sending frames of data traffic  26  to the wireless device  12  with data rates that necessitate the wireless device  12  using more than one radio interface  16 . The wireless device  12  will then use one of the free radio interfaces  16  to perform the background scan. During the background scan, the wireless device  12  will be able to send data, but on a lower data rate that necessitate the use of only one radio interface  16 . When the background scan is complete, the wireless device  12  may signal the access point  14  that it is no longer in powersave mode meaning that all of the wireless device&#39;s radio interfaces  14  are available. 
     In another embodiment, if the wireless device  12  includes multiple radios (e.g. more than two), then the wireless device  12  can signal the access point  14  that only one of those radio interfaces  16  is in powersave mode and that the wireless device  12  will be able to send and receive data using the remaining radio interfaces  16 . 
     In another embodiment, the wireless device  12  does not signal to the access point  14  that one radio interface is unavailable for data traffic, and simply starts using one of the radio interfaces  16  for the background scan. In this case, the wireless device  12  will have to make sure it will not use that radio interface  16  for data traffic while the access point  14  is still sending its data to the device  12  if the access point features auto rate adaptation, since the device  12  will not acknowledge a frame until it is able to receive it properly using the available radios for data traffic. 
       FIG. 4  is a flow diagram illustrating the process performed by the driver  26  for implementing the background scan according to one embodiment. The process assumes that one of the radio interfaces  16  has been designated a scan radio interface, or that otherwise the driver  26  designates one of the radio interfaces  16  as the scan radio interface (block  400 ). Then for every channel to be scanned (block  401 ), the driver  26  signals the access point  14  that the scan radio interface has entered power save mode in which the scan radio interface is unavailable for data traffic (block  402 ). The driver  26  then switches the scan radio interface to a scan channel (block  404 ). 
     If the wireless device  12  is configured for non-passive scan mode, then the driver  26  sends through the scan radio interface  16  one or more probe requests (block  406 ). The driver  26  then listens for a predefined amount of tune for probe responses and beacons (block  408 ). Thereafter, the driver  26  switches the scan radio interface back to a data channel (block  410 ). The driver  26  then signals the access point  14  that the scan radio interface is again available for data traffic (block  412 ). The driver  26  then allows the scan radio interface to be used for data traffic for a specified amount of time (block  414 ). 
       FIG. 5  is a flow diagram illustrating the process performed by the driver  26  for implementing the background scan according to another embodiment. The process assumes that one of the radio interfaces  16  has been designated a scan radio interface, or that otherwise the driver  26  designates one of the radio interfaces  16  as the scan radio interface (block  500 ). The driver  26  signals the access point  14  that the scan radio interface has entered power save mode in which scan radio interface is unavailable for data traffic (block  502 ). 
     Then, for every channel to be scanned (block  503 ), the driver  26  switches the scan radio interface to a scan channel (block  504 ). If the wireless device  12  is configured for non-passive scan mode, then the driver  26  sends through the scan radio interface one more probe requests (block  506 ). The driver  26  then listens for a defined amount of time for probe responses and beacons (block  508 ). 
     Thereafter, the driver  26  switches the scan radio interface back to the data channel (block  510 ). The driver  26  then sends to the access point  14  that the scan radio interface is again available for data traffic (block  512 ). The driver  26  then allows the scan radio interface to be used for data traffic for a specified amount of time (block  514 ). 
     A method and system for implementing a background scan in a wireless device having at least two independent radio interfaces has been disclosed. The present invention has been described in accordance with the embodiments shown, and one of ordinary skill in the art will readily recognize that there could be variations to the embodiments, and any variations would be within the spirit and scope of the present invention. For example, the present invention can be implemented using hardware, software, a computer readable medium containing program instructions, or a combination thereof. Software written according to the present invention is to be either stored in some form of computer-readable medium such as memory or CD-ROM, or is to be transmitted over a network, and is to be executed by a processor. Consequently, a computer-readable medium is intended to include a computer readable signal, which may be, for example, transmitted over a network. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims.