Abstract:
A method includes receiving at a wireless communication device via a wired connection, a signal including a synchronization signal, the synchronization signal including a baseband data signal and a plurality of pseudo noise sequences. The method includes decoding the synchronization signal to determining timing information. The method further includes synchronizing operation at the wireless communication device with a second wireless communication device based on the timing information.

Description:
CLAIM OF PRIORITY 
       [0001]    This application is a continuation of and claims priority from U.S. patent application Ser. No. 14/273,028, filed on May 8, 2014, which is a continuation of U.S. patent application Ser. No. 14/031,098, filed on Sep. 19, 2013, now issued as U.S. Pat. No. 8,767,779, which is a continuation of U.S. patent application Ser. No. 12/636,866, filed on Dec. 14, 2009, now issued as U.S. Pat. No. 8,571,072, which is a continuation of U.S. patent application Ser. No. 11/505,553 filed on Aug. 16, 2006, now issued as U.S. Pat. No. 7,656,855, which claims the benefit of Provisional Application No. 60/796,177, filed Apr. 29, 2006, the contents of each of which are expressly incorporated herein by reference in their entirety. 
     
    
     FIELD OF DISCLOSURE 
       [0002]    The present disclosure relates to an arrangement for synchronizing transmission between a plurality of access points (APs) in a wireless local area network (WLAN) and, more particularly, to an arrangement that utilizes direct-sequence spread spectrum (DS-SS) signaling. 
       BACKGROUND 
       [0003]    A wireless local area network (WLAN) is generally implemented to provide local connectivity between a wired network and one or more mobile computing devices. In a typical wireless network, all of the computing devices within the network broadcast their information to one another using radio frequency (RF) communications. WLANs are based on the IEEE 802.11 standard, which designates a wireless-Ethernet specification using a variety of modulation techniques at frequencies in the 2.4 GHz and 5 GHz license-free frequency bands, although other bands might be used with the same basic framework. 
         [0004]    The IEEE 802.11 standard (generally referred to as “Wireless Fidelity”, or Wi-Fi) enables wireless communications with throughput rates up to 54 Mbps. Wi-Fi is essentially a certification that a manufacturer&#39;s product is interoperable with the IEEE 802.11 standard. For example, equipment carrying the “Wi-Fi” logo is certified to be interoperable with other cards that operate in peer-to-peer mode, but Wi-Fi usually incorporates at least one access point, or edge device. Most access points have an integrated Ethernet controller to connect to an existing wired-Ethernet network. A Wi-Fi wireless transceiver connects users via the access point to the rest of the LAN. The majority of Wi-Fi wireless transceivers available are in Personal Computer Memory Card International Association (PCMCIA) card form, particularly for laptop, palmtop, and other portable computers. However, Wi-Fi transceivers can also be implemented through an Industry Standard Architecture (ISA) slot or Peripheral Component Interconnect (PCI) slot in a desktop computer, mini-PCI cards embedded in a laptop computer, a Universal Serial Bus (USB), or can be fully integrated within a handheld device. 
         [0005]    Modern WLANs composed of more than one access point with overlapped cells require a frequency re-use pattern to minimize interference. However, available frequencies may be too few to allow for a sufficient amount of co-channel interference reduction. Recently, new quality-of-service (QoS) protocols allow for APs to share “time” as well as “frequency”. However, synchronization between APs is required for such time division multiplexing schemes. One prior art arrangement for providing synchronization between access points is discussed in U.S. Pat. No. 6,917,804, issued to T. Takayama et al. on Jul. 12, 2005. In this case, one access point is defined as the “master” access point and periodically transmits a “master beacon” to all remaining access points in the WLAN. Such an arrangement, however requires each access point to be equipped with additional receiving and synchronization components. 
         [0006]    A need remains, therefore, for an arrangement that allows for synchronization of access points with little additional overhead, while maintaining the reliability of a wired connection. 
         [0007]    Other and further aspects of the present disclosure will become apparent during the course of the following discussion and by reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  illustrates an exemplary prior art wireless LAN arrangement including a plurality of overlapping coverage access points (APs); 
           [0009]      FIG. 2  illustrates an exemplary prior art Power over Ethernet (PoE) arrangement for providing both data and low power signals from a power sourcing equipment (PSE) module to a powered device (PD) end station; 
           [0010]      FIG. 3  contains a diagram of an exemplary embodiment of the present disclosure, where the PoE Ethernet cable connection is used to provide direct-sequence spread spectrum (DS-SS) synchronization signals from a hub connection to an exemplary access point over a “spare” cable pair; 
           [0011]      FIG. 4  illustrates an exemplary embodiment of a power separation filter utilized to combine a power signal (I) and DS-SS sync signal (S) onto the same cable pair within a CAT5 Ethernet cable; 
           [0012]      FIG. 5  illustrates an alternative embodiment of the present disclosure where the pair of cables carrying the data signal are also used to carry the synchronization signal; 
           [0013]      FIG. 6  illustrates, in simplified block diagram form, a plurality of access points coupled to a single hub location via an Ethernet cable that supports the transmission of the same synchronization signal to each access point, thus allowing for frequency re-use in accordance with the present disclosure; 
           [0014]      FIG. 7  illustrates an exemplary synchronization generator module for use in generating an exemplary DS-SS synchronization signal to be transmitted over the Ethernet cable; and 
           [0015]      FIG. 8  illustrates an exemplary demodulator for use in an AP to recover the various synchronization pulses from the propagating synchronization signal. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    The need remaining in the prior art is addressed by the present disclosure, which relates to an arrangement for synchronizing transmission between a plurality of access points (APs) in a wireless local area network (WLAN) and, more particularly, to an arrangement that utilizes direct-sequence spread spectrum (DS-SS) signaling. The present disclosure is based on leveraging the communication capability provided by the emerging Power over Ethernet (PoE) technology to “piggyback” synchronization signals onto the power signals, thereby distributing the synchronization signals to each of the APs within the wireless LAN without requiring additional overhead, and allowing for greater frequency re-use between adjacent, overlapping cells. 
         [0017]      FIG. 1  is a diagram of a conventional prior art wireless LAN system  10  comprising a plurality of access points (APs)  12  coupled to a communication switch (or hub)  14  via a conventional Ethernet CAT5 cable  16 . The radio frequency coverage for each AP  12  is illustrated as a shaded cell region  18 , where the overlap between adjacent cells  18  is illustrated as cross-hatched areas  20 . As mentioned above, an overlapping coverage condition may produce too much interference for some wireless devices in overlap regions  20 , illustrated as wireless devices  22  in  FIG. 1 . That is, unless there is a frequency re-use plan, or a radio resource time-sharing arrangement within wireless LAN  10 , devices  22  will experience transmission errors. 
         [0018]      FIG. 2  illustrates a conventional prior art PoE arrangement that is utilized to provide power signal input over Ethernet cable  16  between a power sourcing equipment (PSE) module  30  and a powered device (PD)  32 . While the intricacies of the power distribution are not germane to the subject matter of the present disclosure, an overall understanding of the PoE system is helpful. Referring to  FIG. 2 , Ethernet cable  16  comprises in this embodiment a CAT5 cable which includes four twisted pairs of copper signal paths. A first pair (designated  16 - 1 ) is used to provide data and power signals from PSE  30  to PD  32 . A second pair (designated  16 - 2 ) is used to provide the return data signals from PD  32  to PSE  30 . The remaining pairs ( 16 - 3  and  16 - 4 ) have previously been reserved as “spare” communication lines in the event of a failure of either primary pair. Alternatively, and particularly for high speed applications, cable pairs  16 - 3  and  16 - 4  may be used as a second data line. When implementing PoE, the power signal is transmitted over data pair  16 - 1 ,  16 - 2  (or “spare” pair  16 - 3 ,  16 - 4 ) in a “common mode” powering arrangement. If the spare pairs are used for powering, the individual wires in a pair are shorted together at both ends, allowing both conductors to share the current being supplied. 
         [0019]    With this understanding of the capability of distributing low voltage power signals over Ethernet cable, the subject matter of the present disclosure may be explained as follows. Referring to  FIG. 3 , an exemplary wireless LAN synchronization arrangement for overlapping APs is illustrated as comprising a hub  14  and an exemplary AP  12  connected via Ethernet cable  16 . For the sake of clarity, only a single AP  12  is shown in this view. In accordance with the present disclosure, hub  14  comprises a PSE  30 , similar to the arrangement of  FIG. 2 . Hub  14  further comprises an Ethernet line driver  31  for supplying an input data signal (E) to pair  16 - 1  of cable  16 , and an Ethernet line receiver  33  for recovering received data from cable pair  16 - 2  of CAT5 cable  16 . 
         [0020]    In accordance with the present disclosure, hub  14  further comprises a synchronization generator module  40  that is utilized to generate a direct-sequence spread spectrum (DS-SS) synchronization signal S that is coupled through a first power separation filter  42  and transmitted to AP  12  over cable  16 . It is to be understood that sync signal S may be generated and transmitted to AP  12  whether or not power I is also being transmitted to AP  12  via PoE over cable pair  16 - 3 . Advantageously, the creation of a DS-SS signal for sync signal S allows for a relatively low power, broadband signal to be formed that will not interfere with the data also being transmitted over cable pair  16 - 1  (or with any power signal than may be present along cable pair  16 - 3 ). 
         [0021]    At the receive side within AP  12 , a second power separation filter  44  is used to separate sync signal S from any power signal I that may be present along pair  16 - 3 . Synchronization signal S is thereafter applied as an input to a DS-SS synchronization demodulator  46  to recover the specific sync signal S, as identified below, that is used to maintain overall synchronization between the plurality of separate access points. The power signal I (if present) is then applied as an input to a power sinking device (PD)  32 , as shown in  FIG. 3 . 
         [0022]    In a preferred embodiment of the synchronizing method discussed herein and as particularly illustrated in  FIG. 3 , spare pairs  16 - 3 ,  16 - 4  of CAT5 cable  16  are employed to transmit the synchronization signals (and power signals) between hub  14  and a plurality of APs  12 . In this case, first power-separation filter  42  is interposed between PSE  30 , synchronization generator module  40 , and CAT5 cable  16  at hub location  14 . It is to be understood that if a plurality of CAT5 cables are associated with hub  14 , a single power supply, power separation filter, and synchronizing generator can be shared among the plurality of cables to provide the desired synchronization signal. 
         [0023]    The function of first power separation filter  42  is to prevent the high-frequency DS-SS sync signal S generated by module  40  from being attenuated by the low resistance of the power supply/powering sourcing device  30 , thus more efficiently coupling the sync signal S onto spare cable pair  16 - 3 .  FIG. 4  illustrates an exemplary arrangement of first power separation filter  42 , sync generator module  40  and cable pairs  16 - 3  and  16 - 4 . An LC circuit arrangement  43  is used to maintain separation between the DC power signals Lc) and the relatively high frequency sync signal S. In particular, as a result of its high-frequency content, sync signal S propagates through the cable, which behaves as a balanced transmission line, in a relatively unimpeded fashion. This minimizes coupling into the data pairs and eases compliance with FCC emissions rules. Second power separation filter  44  within each AP  12  is similar in form to first power separation filter  42  and functions to isolate sync signal S at the output thereof. 
         [0024]    In an alternative embodiment of the present disclosure, as illustrated in  FIG. 5 , sync signal S is directly applied to cables pairs  16 - 1  and  16 - 2  in combination with data signal E. Since DS-SS sync signal S is noise-like and spread so that its power is distributed over a wide frequency range wide, there will be a minimal amount of interference with Ethernet data transmission (particularly the carrier-sense function). However, this alternative embodiment requires that the synchronization and data signals be combined before being fed to a line-coupling transformer at the hub. Additionally, a power signal I may also be transmitted over cable pair  16 - 1 . As shown, power signal I is coupled through a center-tapped transformer  34  onto data pair  16 - 1  along with the DS-SS signal and the data signal. Upon reaching AP  12 , the low voltage DC current I is returned via another center-tapped transformer  36 , with the current passing through PD  32  as shown. The DS-SS sync signal S is removed from the Ethernet data signal E by DS-SS synchronization demodulator  46 , in a manner similar to that discussed above in association with the embodiment of  FIG. 3 . Aside from small degradation of the Ethernet signal-to-noise ratio, this alternative architecture also complicates mid-span powering since the pair of cables must be broken to impress the synchronization signal via metallic (rather than common mode) current while retaining line balance. This is best accomplished by turning the mid-span powering tap into an Ethernet repeater. 
         [0025]      FIG. 6  illustrates, in a simplified overview, the arrangement of the present disclosure where sync signal S as created within sync generator module  40  of hub  14  is thereafter transmitted along Ethernet cable  16  and received by each AP  12  connected to cable  16 . As long as each AP  12  includes the same demodulator  46 , each AP  12  will recover the same sync signal and be able to maintain synchronous communication within the wireless LAN formed by the plurality of APs  12 . In particular, sync signal S is used to align the superframes (beacons) for each AP  12  in the network, allowing them to time-share the same frequency channel without causing interference. The synchronization also facilitates power saving within the wireless client devices and provides for improved hand-off of client devices between APs. 
         [0026]      FIG. 7  illustrates an exemplary synchronization generator module  40  formed in accordance with the present disclosure. In a preferred embodiment, a pair of independent pseudo noise (PN) sequences are utilized to form the DS-SS sync signal S. A first PN sequence, denoted PN- 1 , is used to generate a precision phase signal that is utilized as the “pilot” tone for the other sync signals. PN- 1  may comprise either all 0&#39;s or all 1&#39;s for fast acquisition. A time mark signal TM that facilitates the ability to provide the superframe (beacon) timing among the APs (as mentioned above) and “time of day” (ToD) coding signal are combined within a multiplexer  50  and thereafter spread by a second PN sequence (PN- 2 ), for example, as a real-time clock facility for AP  12 , providing ranging, delay measurements and the like. Sequence PN- 2  is of the same length as PN- 1 , but comprises an equal number of 0 3  s and 1&#39;s. Alternatively, a longer PN sequence may be used for PN- 2  so as to make the signal more noise-like. 
         [0027]    As shown in  FIG. 7 , each of these PN sequences is applied as a separate input to a summer  54 , where Ethernet data signal E is applied as a third input to summer  54 . Summer  54  adds the synchronization information (pilot, ToD, TM) to the already-existing Ethernet data signal E (similar to adding white noise to a signal) and then transmits the signal via CAT5 cable  16  to the remote APs. It is to be understood that in various other embodiments, perhaps only PN- 1  is utilized, or a combination of PN- 1  and PN- 2 , or any other appropriate combination of one or more PN sequences that supply sync pulses to the plurality of separate APs. The resultant DS-SS sync signal S is generally in the form of a noise-like spectra, low enough to provide negligible crosstalk with the data and/or power signals propagating along cable  16 . 
         [0028]      FIG. 8  illustrates an exemplary demodulator  46  used within each AP  12  to extract the various PN sequences from the received signal traveling along cable  16 . As shown, the received signal is demodulated using the same baseband signal B and a relatively straightforward filtering arrangement is then used to separate the various PN sequences from the signal. Thus, as long as each AP  12  recovers the same sync pulses (pilot, TM, ToD), the various units will be able to provide time sharing of the frequency spectrum without the interference problems prevalent in the prior art. 
         [0029]    While the present disclosure has been described in relation to a particular preferred embodiment, many variations, equivalents, modifications and other uses will become apparent to those skilled in the art. Thus, the present disclosure is intended to be limited only by the scope of the claims appended hereto.