Pulse or digital communications – Spread spectrum – Frequency hopping
Reexamination Certificate
1998-09-30
2002-04-23
Pham, Chi (Department: 2631)
Pulse or digital communications
Spread spectrum
Frequency hopping
Reexamination Certificate
active
06377608
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates in general to wireless local area networks, and is particularly directed to an access point-associated, pulsed beacon-based mechanism for reducing the potential for or avoiding interference between wireless communication devices of users of a communication network, such as one employing the unlicensed industrial, scientific and medical (ISM) band, and wireless communication devices of non-users of the network, who are located within a basic service area of the network, and are capable of transmitting within the same bandwidth portion of the wireless communication spectrum as, and thereby potentially interfering with, the users of the network.
BACKGROUND OF THE INVENTION
Because the (2.400-2.4835 GHz) industrial, scientific and medical (ISM) band is unlicensed and reasonably wide, it is anticipated to soon undergo substantial crowding, as wireless devices capable of transmitting at data rates in excess of 10 Mbps are expected to become increasingly common in the enterprise environment in the next few years. For example, as illustrated diagrammatically in
FIGS. 1 and 2
, it can be expected that one or more ad hoc networks (or ‘piconets’)
10
, such as those comprised of a wireless phone
11
and laptop computer
13
wirelessly linked to each other (e.g., via associated Bluetooth radios), will be carried into a building having an infrastructure high speed wireless local area network (WLAN). Note that ad hoc network
10
is one which is temporary in nature, and typically contains two or more mobile nodes which communicate with each other and do not make use of the infrastructure WLAN.
Although not directly communicating with the access point (AP)
21
of the WLAN infrastructure's basic service area (BSA)
20
, by transmitting in the ISM band, wireless communications between the cell phone
11
and laptop computer
13
, such as downloading of e-mail to the laptop, for example, may interfere with, or suffer interference from, the infrastructure WLAN. Namely, as there will be multiple devices operating in the same unlicensed frequency band (i.e., 2.400-2.4835 GHz) as Bluetooth, HomerRF, and IEEE 802.11 specification-based frequency hopped spread spectrum (FHSS) radios, it is essential that the various radio systems utilizing the ISM band be capable of at least some degree of coexistence. This interference issue is of concern to all users of the band and, because most wireless communication devices are mobile, it is currently substantially impossible to predict the severity of the interference problem. Indeed, at present, none of the above-referenced systems are capable of operating in the presence of any of the others without causing potentially serious levels of interference.
SUMMARY OF THE INVENTION
In accordance with the present invention, this problem is effectively obviated by taking advantage of the availability of a prescribed unused region of portion of the ISM band (at either or both ends of the ISM band) to generate a pulsed warning beacon, that serves to alert ad hoc network radios (such as frequency hopped spread spectrum (FHSS) radios), that they are spatially close to a WLAN infrastructure access point, and thereby in the range of potential interference with the WLAN. To combat potential fading of the beacon signal, two beacons alternatively may be generated at lower and upper unused band regions of the ISM band. For example, by placing a first warning beacon at 2.401 GHz and a second warning beacon at 2.482 GHz, the probability of loss of beacon signal due to simultaneous fading on two non-correlated channels is significantly reduced.
The warning beacon is sourced from a beacon generator that is spatially located in close proximity to and linked with infrastructure access point. By close proximity is meant that the beacon generator and access point are spaced sufficiently far apart to provide isolation that minimizes adjacent channel interference; yet they are still close enough to ensure the AP and beacon coverage areas are approximately the same. The beacon generator is operative to periodically generate a pulsed beacon signal which, because of its physical proximity to access point, serves to indicate the presence of infrastructure WLAN and its associated BSA. The beacon generator may be serially interfaced with the access point in order to convey information about AP spectrum utilization to the mobile nodes. Alternatively, the beacon generator may communicate with the access point via a wired LAN (Ethernet). Communication from the access point to the beacon generator need not occur frequently, since it is only necessary to communicate when the access point is turned on or off, or when the beacon frequency channel is changed.
By periodically tuning the receive frequency synthesizer of its transceiver to this frequency, a node in an ad hoc network radio, such as an FHSS radio, is able to monitor whether it is in close proximity to an infrastructure network. Because FCC regulations permit a low power transmitter to periodically increase its transmit power by as much as 20+dBm, as long as average power remains below 0 dbm, the beacon signal emitted by beacon generator preferably has a pulsed beacon profile. The beacon may also be modulated with information relating the operation of the infrastructure network, such as the center frequency of the occupied channel, where the BSA is that of a DSSS infrastructure WLAN. In the case of an infrastructure FHSS network, the hop sequence and system clock data are embedded in the beacon signal.
In order to monitor and respond to the pulsed warning beacon that is sourced from the vicinity of the access point of the infrastructure of a WLAN, it is only necessary to modify the control software employed by the microcontroller of an ad hoc participant's radio, to incorporate a synthesizer tuning mechanism that controls a programmable synthesizer, through which operation of the radio's transceiver is controlled, so that operation of each of the ad hoc radio and the WLAN may proceed on a non-interfering basis. No modification to infrastructure mode operation is required. Non-limiting examples of such non-interfering operation include deferral mode, altered hop pattern mode, fixed frequency mode, deactivation mode, and a reduced transmit power mode.
In deferral mode, the tuning control mechanism employed by the ad hoc radio network is operative to defer transmission, in response to detecting that ad hoc radio and the infrastructure networks occupy the same frequency. This technique may be employed regardless of which type of infrastructure network is encountered. The ad hoc radio uses information contained in the monitored beacon which informs FHSS radios operating in the ad hoc mode of the operating parameters of the infrastructure network. For an IEEE 802.11 standards DSSS network, which occupies fixed channels which are about 25 MHz apart, to avoid interference, the FHSS ad hoc radio defers on 25 out of 79 1 MHz channels, or about 30% of the FHSS channels. For an FHSS WLAN infrastructure, in which each channel has an instantaneous bandwidth of only 1 MHz, a lower reduction in throughput is obtained, as the ad hoc network radio defers on only 3 out of 79 channels. As a result, throughput reduction is less than 4%.
Altered hop pattern mode may be used for FHSS systems, such as Bluetooth, that have nominal RF output of 0 dBm, and are able to operate in accordance with FCC low power rules, where there is no requirement to employ spread spectrum modulation, so that changing hop sequences to avoid interference is permissible. The advantage to this approach is that the band occupied by the infrastructure network may be avoided completely. Also, there is no reduction in throughput in either the ad hoc radio or the infrastructure network.
Fixed frequency mode is employed for avoiding interfering with DSSS WLANS, which use center frequencies shifted toward the lower end of the ISM band to avoid excessive out-of-band emissions in the lower restricted band. This frequency s
Bayard Emmanuel
Intersil America's Inc.
Pham Chi
Stanford Gary R.
LandOfFree
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