Abstract:
A station for accessing the resources of an access point in a wireless network, the station including a transmitter that is arranged to formulate probe requests and a transmit chain that is arranged to transmit probe requests on multiple channels of the access point simultaneously, wherein the probe requests are configured to elicit responses from the access point. Also disclosed is a station for accessing the resources of an access point in a wireless network, wherein the network includes a plurality of channels at different frequencies and the station includes a receiver and a receive chain arranged to deliver to the receiver a signal that has been acquired wirelessly from the network and which spans a plurality of the channels, wherein the receiver is arranged to produce a first spectrogram of the signal and to make a determination, by comparing the first spectrogram with one or more earlier spectrograms of the signal, of whether there is communication starting in the network in a channel covered by the signal.

Description:
FIELD 
       [0001]    This application relates to wireless communications networks. 
       BACKGROUND 
       [0002]    It is known to organise wireless local area networks in accordance with the IEEE 802.11 standards. In such a network, an access point provides stations with access to a resource. Typically, the access point is a wireless router that provides access to a wired connection to the internet and the stations are WiFi enabled laptops or smart phones or the like. According to the IEEE 802.11 standards, an access point will transmit beacon frames and probe response frames that can be used by stations that want to access the resource (e.g. a broadband connection to the internet) that the access point controls. 
         [0003]    A station that wants to join a wireless local area network can perform scans in order to find wireless local area networks to associate with. These scans can be “passive scans” or “active scans”. 
         [0004]    In an active scan, a station will transmit a probe request frame on a selected frequency channel and will inspect the contents of any probe response frames that are received on that channel as a result. A probe request frame contains a service set ID (SSID) which may be a wild card SSID or a specific SSID (e.g. “BT Openzone”). 
         [0005]    In a passive scan, a station will listen for wireless local area network traffic, including beacon frames and unsolicited probe response frames and will inspect the contents of the received frames in order to determine if it is in range of a network that it should associate with. 
         [0006]    In portable devices, such as smart phones and tablet computers, scanning for wireless local area networks whilst roaming can consume a lot of energy. Often, portable or mobile devices spend a lot of time in transit, where no already-known networks are in range. Hence, it is beneficial to reduce the current that such devices consume in this scanning mode. 
       SUMMARY 
       [0007]    According to one aspect, certain embodiments of the invention provide a station for accessing the resources of an access point in a wireless network, the station comprising a transmitter that is arranged to formulate probe requests and a transmit chain that is arranged to transmit probe requests on multiple channels of the access point simultaneously, wherein the probe requests are configured to elicit responses from the access point. 
         [0008]    The transmit chain may include a plurality of upconverters that all operate on a probe request from the transmitter, with each upconverter raising the frequency of the probe request to a respective frequency channel of the network. 
         [0009]    Typically, the network has a plurality of channels which together form an operating band and each of the plurality of channels may have a respective upconverter in the plurality of upconverters. 
         [0010]    According to another aspect, certain embodiments of the invention provide a station for accessing the resources of an access point in a wireless network, wherein the network comprises a plurality of channels at different frequencies and the station comprises a receiver and a receive chain arranged to deliver to the receiver a signal that has been acquired wirelessly from the network and which spans a plurality of the channels, wherein the receiver is arranged to produce a first spectrogram of the signal and to make a determination, by comparing the first spectrogram with one or more earlier spectrograms of the signal, of whether there is communication starting in the network in a channel covered by the signal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    By way of example only, various embodiments of the invention will now be described with reference to the accompanying drawings, in which: 
           [0012]      FIG. 1  is a schematic block diagram of a station in a wireless local area network; 
           [0013]      FIG. 2  is a diagram illustrating communication activity in a wireless local area network; 
           [0014]      FIG. 3  is a schematic block diagram of an upconverter; 
           [0015]      FIG. 4  is a schematic block diagram of a down converter; 
           [0016]      FIG. 5  is a schematic block diagram of a transmitter; 
           [0017]      FIG. 6  is a schematic block diagram of a receiver; 
           [0018]      FIG. 7  is a schematic block diagram of a variant of the station shown in  FIG. 1 ; and 
           [0019]      FIG. 8  is a schematic block diagram of a transceiver. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    The drawings show various devices, but only in such detail as is sufficient to permit an efficient description of the invention. That is to say, it will be apparent to the skilled person that, in practice, the devices shown in the drawings will contain many more elements than are shown in the drawings or, indeed, than are described in the following text. It should also be borne in mind that elements carried over from one figure to another retain the same reference numerals. 
         [0021]      FIG. 1  shows a station  10  that can join an 802.11 wireless local area network that is established by an access point (not shown). The station  10  includes a transmitter  12 , a receiver  14 , a diplexer  20 , and an antenna  22 . In the transmit chain leading from the transmitter  12  to the antenna  22 , there is a bank of parallel up converters  16 - 1  to  16   m . In the receive chain leading from the antenna  22  to the receiver  14  is a downconverter  18 . 
         [0022]    The station  10  is designed to probe multiple channels of an 802.11 band simultaneously in order to search for networks that the station  10  can associate with. In order to achieve this, the transmitter  12  formulates a probe request frame and supplies it to the upconverters  16 - 1  to  16 - m . Each of the upconverters  16 - 1  to  16 - m  upconverts the probe request frame in frequency to occupy a different channel of the 802.11 band that is being probed. The differently upconverted versions of the probe request frame that are produced by the upconverters  16 - 1  to  16 - m  and which now occupy distinct channels of the 802.11 band in question are merged and are fed to an antenna  22  (via a diplexer  20 ) for wireless transmission. Thus, the station  10  transmits probe request frames into m 802.11 channels simultaneously. 
         [0023]    Signals that are received at the antenna  22  are routed by the diplexer  20  to a down converter  18 . The down converter  18  converts received signals down in frequency to baseband, or to an intermediate frequency, so that they can be processed by the receiver  14 . The down converter  18  admits to the receiver  14  only a span of the range of frequencies in the signal received by the down converter  18  from the antenna  22 . 
         [0024]    The receiver  14  can be operated in a channel reception mode or in a band reception mode. In the channel reception mode, the down converter  18  admits to the receiver  14  just that part of the signal from the antenna  22  that relates to a channel of interest within the 802.11 band that is in use. In the band reception mode, the down converter  18  admits to the receiver  14  the whole of the 802.11 band in question. The channel reception mode is used by the station  10  for receiving a dedicated communication in the selected channel or for studying, e.g. by way of passive scan, the communications, if any, are being conducted on that channel. The band reception mode is used by the station  10  to perform a Full Band Scan of all channels in the band, in a manner that will now be described. 
         [0025]    In order to perform the Full Band Scan, the down converter  18  is placed in the band reception mode and the receiver  14  is arranged to calculate by a known discrete Fourier transform (DFT) technique a spectrogram for the signal that the receiver then receives from the down converter  18 . The spectrogram is a list, plot or array of power values (typically in dB) for a series of frequency bins (each frequency bin is a relatively small frequency range). 
         [0026]    The receiver  14  calculates the discrete Fourier transform of discrete blocks of the signal that is received from the down converter  18 , these blocks extending over a predetermined time interval. These blocks are weighted by a window function before being applied to the discrete Fourier transform in order to reduce interference between non-adjacent frequency bins. Weighting signal blocks in this way is well known in the field of digital signal processing. A spectrogram can be considered as being composed of a series of values s(f,n), where f is an index specifying the frequency bin and n is an index specifying the time instant (i.e., n is an index denoting the block of the signal from the down converter  18  on which the spectrogram value was established). 
         [0027]    The receiver  14  processes the spectrogram values in order to determine whether there are any probe response frames present in the signal that is provided by the down converter  18 . To do this, the receiver  14  processes the spectrogram values according to the following equation: 
         [0000]    
       
         
           
             
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         [0028]    The binary values b(f,n) that are thus produced are dependent on the historical values of the frequency bins of the spectrogram. The value K is an integer, and is not necessarily 1. These binary values can then be used to collate a detection criterion d c (n) for each channel: 
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         [0029]    Where F c  is the set of frequency bins attributed to the channel c that is being tested and L is the number of decisions in time that are summed to produce d c (n). A good choice of L is L=K. It will be understood that the equation for a detection criterion d c (n) is in fact a two dimensional filter that filters over a range F c  of frequency and over a period L of time. 
         [0030]    Having determined the detection criteria value for a given channel, the transmitter  14  compares this with a predetermined threshold. If the d c (n) value for a given channel c exceeds the corresponding threshold then the receiver  14  assumes that a transmission has been received in that channel. The thresholds that are used with the d c (n) values are determined on an experimental basis having regard to the precise nature of the system and operating environment in which they are to be used. 
         [0031]    Once the receiver  14  determines that a transmission has been received in one of the monitored channels, the receiver  14  can then, operating in the channel reception mode, undertake a detailed scan of that channel, either a passive scan or an active scan, in order to determine if there is indeed a network on that channel with which the station  10  can associate. 
         [0032]      FIG. 2  outlines the stages in a process that is used by the station  10  to search for networks with which it can associate. In  FIG. 2 , the m channels are arranged vertically and activity in the channels is shown as shaded blocks. In time interval t a , the transmitter  10  sends a frequency multiplexed probe frame. That is to say, the transmitter  12  transmits via upconverters  16 - 1  to  16 - m , probe requests in the m channels simultaneously. The probe requests appear as blocks  24 - 1  to  24 - m . Then, in time interval t b , the receiver  14  performs a Full Band Scan in order to analyse the signal that it receives from down converter  18  in an attempt to detect any transmissions that are being received on the m channels. If no transmissions are detected, i.e., if none of the d c (n) exceed the threshold, then the station  10  can power down, by switching off at least some of its parts, for an interval since it can be assumed that there is presently no 802.11 access point for the station  10  to interact with. The duration of the power down interval can usefully be set so that the station  10  powers up once in every 802.11 frame to scan for access points. 
         [0033]    In this example however, transmissions are received on channels  2  and  3 , as indicated by shaded blocks  26  and  28 . It will be observed that, in this example, the transmission on channel  2  is received slightly before the transmission that is received on channel  3 . Assuming that the detection criterion values d c (n) for channels  2  and  3  exceed the threshold, the station  10  performs, in interval t c , sequential active scans of channels  2  and  3 , as indicated by shaded blocks  30  and  32 . Of course, one or both of the detected channels  2  and  3  could be subjected to a passive scan rather than an active scan. 
         [0034]      FIG. 3  shows upconverter  16 - 1  in more detail. It will be understood that the other upconverters  16 - 2  to  16 - m  have a similar construction. In essence, the upconverter  16 - 1  includes a mixer  34  and a local oscillator  36 . The local oscillator  36  produces a radio frequency carrier signal onto which the signal  38  from the transmitter is modulated by the mixer  34 . The resulting upconverted signal  40  is then combined with the outputs of the other upconverters  16 - 2  to  16 - m , and supplied to the diplexer  20 . 
         [0035]      FIG. 4  shows the construction of the down converter  18 . The down converter  18  comprises a mixer  42  and a local oscillator  44 . The local oscillator  44  produces a radio frequency signal which the mixer  42  mixes with the signal  46  that is received from the diplexer  20  in order to produce the down converted, baseband signal  48  that is supplied to the receiver  14 . The range of frequencies that the down converter  18  admits to the receiver  14  is controlled by a filter (not shown) that acts on signal  48 . It is the pass band of that filter that is adjusted to change the down converter  18  between the band reception and channel reception modes. 
         [0036]      FIG. 5  shows the construction of the transmitter  12  in a little more detail. As shown, the transmitter  12  comprises a processor  50 , a memory  52  and a digital to analogue converter (DAC)  54 . The processor  50 , by drawing on instructions and data stored in the memory  52 , produces a digital version of the probe request frame that is to be channelised by the upconverter  16 - 1  to  16 - m . This probe request is then converted to the analogue domain by DAC  54  and the resulting analogue signal  56  is then supplied to the upconverters  16 - 1  to  16 - m.    
         [0037]      FIG. 6  shows the construction of the receiver  14  in a little more detail. As shown, the receiver  14  comprises a processor  58 , a memory  60  and an analogue to digital converter (ADC)  62 . The ADC  62  receives the analogue baseband signal  64  from the down converter  18  and converts it into a digital baseband signal that can be processed by the processor  58 . The processor  58  could, for example, be a general purpose processor or a digital signal processor with its functionality set in hardware. The processor  58  performs discrete Fourier transformation on time-sliced blocks of the signal provided by the ADC  62  in order to produce the spectrograms that are used to detect the presence of transmissions received from access points. The processor also undertakes the calculations and algorithms necessary to calculate the various parameters b(f,n) and d c (n). The processor  58  performs these tasks by utilising instructions and data and storage space that is provided by the memory  60 . 
         [0038]      FIG. 7  shows a variant of the station  10  that is shown in  FIG. 1 . The station  66  is largely the same as station  10  except that the transmitter  12  and the receiver  14  have been replaced by a transceiver  68  which performs the functions of both the transmitter  12  and the receiver  14 . 
         [0039]      FIG. 8  shows the construction of the transceiver  68  in a little more detail. As shown, the transceiver comprises a processor  70 , a memory  72  a DAC  74  and an ADC  76 . The processor  70  is arranged to utilise instructions and data that are stored in the memory  72  in order to formulate the probe request frame that is converted by the DAC  74  and supplied to the bank of upconverters  16 - 1  to  16 - m . Likewise, the processor  70  utilises instructions and data in the memory  72  in order to process a version of the signal from the down converter  18  that has been digitised by ADC  76  in order to determine whether the received signal includes transmissions from a wireless access point. That is to say, the processor  70  is arranged to perform the calculations and algorithms necessary to calculate the spectrograms and the parameters b(n,f) and clan) and to make conclusions based on those parameters. 
         [0040]    In one variant, the b(f,n) values are modified before being used to calculate the detection criteria d c (n). In this case, b′ values are used in place of the b values and the equation for determining the b′ values is: 
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         [0041]    Here, the constants A m  are chosen in relation to the magnitudes of the discrete Fourier transform of the window that is applied to the blocks of the received signal prior to their discrete Fourier transformation. In other words, b′(f,n) is only 1 if the power in bin f at time n:
       exceeds the power in that bin at time n-K and   does not, for each value of m, fall more than an amount A m  below the larger of the two neighbouring channels at frequency f−m and f+m.       
 
         [0044]    Where b′ is used, the detection criteria equation is modified to: 
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         [0045]    That is to say, b′ is used in place of b. 
         [0046]    Various modifications are possible to the embodiments described above, and some of these will now be discussed. 
         [0047]    Although the embodiments described above use the Full Band Scan concept together with the multiplexed probe frames concept, this need not be the case and the one could be used without the other. For example, the Full Band Scan could be used to do a passive scan, i.e. a scan that is not driven by any probe request, multiplexed or otherwise. As another example, the receiver  14 , instead of being configured to do a Full Band Scan, which is based on spectrum analysis, could be configured to investigate multiple channels simultaneously by performing a clear channel assessment as per the 802.11 standards or having multiple receive chains each tuned to receive a respective one of the channels to be investigated.