BRIEF DESCRIPTION OF THE DRAWINGS
Referring now to the drawings,
FIG. 1 illustrates an exemplary prior art wireless LAN arrangement including a plurality of overlapping coverage access points (APs);
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;
FIG. 3 contains a diagram of an exemplary embodiment of the present invention, 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;
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;
FIG. 5 illustrates an alternative embodiment of the present invention where the pair of cables carrying the data signal are also used to carry the synchronization signal;
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 invention;
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
FIG. 8 illustrates an exemplary demodulator for use in an AP to recover the various synchronization pulses from the propagating synchronization signal.
DETAILED DESCRIPTION
The present invention 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.
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.
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 invention, 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.
With this understanding of the capability of distributing low voltage power signals over Ethernet cable, the subject matter of the present invention 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 invention, 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.
In accordance with the present invention, 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).
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.
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.
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 Idc) 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.
In an alternative embodiment of the present invention, 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.
FIG. 6 illustrates, in a simplified overview, the arrangement of the present invention 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.
FIG. 7 illustrates an exemplary synchronization generator module 40 formed in accordance with the present invention. 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's or all 1'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's and 1's. Alternatively, a longer PN sequence may be used for PN-2 so as to make the signal more noise-like.
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.
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.
While the present invention 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 invention is intended to be limited only by the scope of the claims appended hereto.
Patent Application
20070253382
