Telecommunications – Transmitter and receiver at separate stations – Distortion – noise – or other interference prevention,...
Reexamination Certificate
1998-09-30
2001-07-03
Le, Thanh Cong (Department: 2684)
Telecommunications
Transmitter and receiver at separate stations
Distortion, noise, or other interference prevention,...
C455S067700, C455S296000, C455S067700, C455S067150
Reexamination Certificate
active
06256477
ABSTRACT:
TECHNICAL FIELD
The invention relates to wireless communication and, more particularly, to avoiding interference from a potentially interfering transmitter in a wireless communication system.
BACKGROUND
Wireless communication systems allow remote and often portable transceiving devices, e.g., radio telephones (“mobile units”), to communicate bi-directionally with each other and with stationary transceiving devices, e.g., cellular stations (“fixed stations”) over great distances.
FIG. 1
shows a typical wireless network
10
, in which several mobile units
12
,
14
,
16
communicate with two fixed stations
18
,
20
. The fixed stations
18
,
20
communicate with each other, e.g., via radio frequency (RF) signals
22
or via a public switching telephone network
24
(PSTN).
Many wireless networks, including cellular networks such as the Personal Handyphone System (PHS) networks in Japan and the Digital European Cordless Telephone (DECT) networks in Europe, utilize dynamic channel assignment, through which fixed stations with overlapping coverage areas use and reuse preassigned carrier frequencies. In some systems, such as PHS, the mobile and fixed units employ time division multiple access (TDMA) and/or time division duplex (TDD) communication, which allows each fixed station to communicate with multiple mobile units in a given transmit/receive time period. Each transmit/receive period may include several discrete time slots, each dedicated to transmitting information between a fixed station and a particular mobile unit. A transceiver unit may be assigned to transmit at any given carrier frequency during each time slot, but tends to remain assigned to that slot for a relatively long period of time.
The fixed stations in many TDMA/TDD-based networks, including PHS networks, may be either public or private. In general, a public fixed station may be accessed by any mobile unit within its range, and a private fixed station may be accessed only by phones registered to it. While public fixed stations typically operate under the control of synchronized internal clocks, the clocks among private fixed stations are not synchronized in practice. Moreover, the clocks in private PHS fixed stations must be accurate only to 5 ppm, so over time the clocks in private fixed stations tend to drift with respect to one another. Because a virtually limitless number of PHS mobile units and fixed stations may exist within a given geographic area, and because PHS networks use Dynamic Channel Assignment, private fixed stations are subject to great variations in the interference they experience from other fixed stations. For example, a carrier frequency that presents relatively little interference when first selected by a fixed station may become too noisy for adequate communication when another fixed station suddenly switches to the carrier or as the transmit/receive periods of the fixed stations gradually drift onto each other.
SUMMARY
The invention, in one aspect, allows a wireless receiver to avoid interference from a potentially interfering transmitter. The receiver first determines whether a signal from the transmitter is likely to overlap an expected signal at an assigned carrier frequency or channel. If so, the receiver is allowed to switch to another channel to receive the expected signal.
In some embodiments, the receiver is allowed to switch to the other channel only if the interference is likely to exceed a predetermined threshold. The receiver can determine whether the interference is likely to exceed this threshold, e.g., by measuring the strength of the potentially interfering signal and comparing it to the strength of the expected signal. In a slotted system, e.g., one in which the receiver is scheduled to receive the expected signal within a predefined time or frequency slot, the receiver can determine the strength of the potentially interfering signal, if any, by measuring the received signal strength just outside of the reception slot. Alternatively, if the reception slot is preceded or followed immediately by a blind slot, the receiver can measure the received signal strength just before or just after the blind slot to determine the strength of the potentially interfering signal. The receiver can determine the strength of the expected signal by measuring the received signal strength within the reception slot. The receiver then can determine whether interference is likely to occur by determining the difference between the signal strength measured just outside of the reception slot and the signal strength measured within the reception slot.
In other embodiments, the receiver can use the signal strength measured within the reception slot to update a running average of signal strengths. The receiver then can determine which value is larger: the signal strength measured within the reception slot or the updated average of measured signal strengths. The receiver then can calculate the difference between the larger value and the strength of the potentially interfering signal to determine whether interference is likely to occur.
In another aspect, the invention allows a wireless receiver in a slotted system to avoid interference from a potentially interfering transmitter. The receiver includes a control element, such as an internal clock, that sets the boundaries of a slot within which the receiver is to receive an expected signal. In general, the control element allows the reception slot to drift with respect to similar slots defined by a controller in the potentially interfering transmitter. The receiver measures the received signal strength just outside of the reception slot to determine the strength of the potentially interfering signal, unless the measurement will be preceded or followed immediately by an adjacent slot that is blind with respect to the reception slot. If the adjacent slot is a blind slot, the receiver instead measures the received signal strength just outside of the adjacent slot. The receiver also measures the received signal strength during the reception slot to determine the strength of the expected signal.
The receiver then uses the signal strength measured during the reception slot to update a running average of signal strengths and determines which is the larger value: the signal strength measured during the reception slot or the updated running average. The receiver then determines the difference between the larger value and the signal strength of the potentially interfering signal. If the difference does not exceed a predetermined threshold value, the receiver is allowed to switch to another carrier frequency to receive the expected signal.
In some embodiments, the receiver can employ TDMA. Moreover, the reception slot can be a time slot of given duration.
Each embodiment of the invention may provide one or more of several advantages. A wireless unit need not wait until interference has occurred and has disrupted communication quality to act to avoid the interference. The unit may predict imminent interference from another unit and may switch carrier frequencies or channels before the interfering unit significantly degrades communication quality. Also, the wireless unit rapidly may detect the sudden appearance of threatening interference caused, e.g., when another unit switches to the same carrier frequency. The unit also may measure the power levels of the signal of interest and of the interfering signal independently, which results in a more sensitive mechanism to detect deleterious levels of interference. A wireless communication unit employing the invention is relatively immune to Rayleigh fading, shadowing effects, and false alarms and may be used even in a blind-slot environment.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description, the drawings, and the claims.
REFERENCES:
patent: 5148548 (1992-09-01), Meche et al.
patent: 5551064 (1996-08-01), Nobbe et al.
patent: 5649303 (1997-07-01),
Eidson Donald B.
Flores William R.
Akin Gump Strauss Hauer & Feld L.L.P.
Conexant Systems Inc.
Cong Le Thanh
Rourk Christopher J.
Zewdu Meless
LandOfFree
Avoiding interference from a potentially interfering... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Avoiding interference from a potentially interfering..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Avoiding interference from a potentially interfering... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2527888