This application relates to wireless communications networks.
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.
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”.
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”).
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.
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.
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.
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.
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.
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.
By way of example only, various embodiments of the invention will now be described with reference to the accompanying drawings, in which:
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.
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.
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.
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.
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).
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).
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:
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 dc(n) for each channel:
Where Fc 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 dc(n). A good choice of L is L=K. It will be understood that the equation for a detection criterion dc(n) is in fact a two dimensional filter that filters over a range Fc of frequency and over a period L of time.
Having determined the detection criteria value for a given channel, the transmitter 14 compares this with a predetermined threshold. If the dc(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 dc(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.
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.
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 dc(n) for channels 2 and 3 exceed the threshold, the station 10 performs, in interval tc, 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.
In one variant, the b(f,n) values are modified before being used to calculate the detection criteria dc(n). In this case, b′ values are used in place of the b values and the equation for determining the b′ values is:
Here, the constants Am 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:
Where b′ is used, the detection criteria equation is modified to:
That is to say, b′ is used in place of b.
Various modifications are possible to the embodiments described above, and some of these will now be discussed.
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.