Telecommunications – Receiver or analog modulated signal frequency converter – Noise or interference elimination
Reexamination Certificate
1999-04-28
2004-10-19
Appiah, Charles N. (Department: 2682)
Telecommunications
Receiver or analog modulated signal frequency converter
Noise or interference elimination
C455S278100, C455S306000, C370S318000, C378S148000
Reexamination Certificate
active
06807405
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to wireless communications, and more particularly to use of notch filters to minimize the adverse affect of narrow band interference upon the performance of code-division multiple access systems.
2. Background Description
The worldwide market for wireless phone service is experiencing growth at an accelerated rate. It is well known that service reliability and performance are key competitive criteria in the telecommunications industry. Existing and new 800 MHz Code-Division Multiple Access (CDMA) wireless sites are in need of a solution to address problems caused by narrow band interfering signals, such as those generated by existing analog sites which continue to provide cellular coverage in urban and non-urban areas.
When deploying a CDMA telecommunications system within a frequency spectrum traditionally allocated to narrow band systems, such as the Analog Mobile Phone System (AMPS) or Global System for Mobile communications (GSM), a frequency block slightly wider than the bandwidth of the CDMA spectrum is normally cleared, in and around the geographic region of the CDMA coverage, to protect the CDMA system from narrow band interference and hence maintain the performance quality of the system. However, it is not always practicable or economical, and in some cases not possible, to clear all the required spectrum. An example where it may be impossible is in country or state boundary regions. Deleterious effects of narrow band interference on CDMA telecommunication systems include: an increase in blocked call and dropped call rates, disruption of the RF power control system, increase in average mobile station-power consumption, reduction of cell capacity, and shrinkage of cell site coverage area. In the worst case, a high level interferer can jam the entire cell or sector, blocking all CDMA communications which would normally be routed through the location.
The use of adaptive notch filters in communications is not new. U.S. Pat. No. 3,911,366 to Bagdhady describes a frequency demodulation receiver for separating a stronger and weaker signal, and removing the undesired signal. Baghdady's invention employs a first mixer, a bandpass filter with a fixed tuned trap (notch), and a second mixer, to achieve a frequency variable notch filter. This filter bears some similarities to the frequency variable notch filter employed in this invention, however, Baghdady addresses only two FM signals and does not address scanning of a broadband signal for multiple narrowband interferors or tracking such interferors and notching them after acquisition.
U.S. Pat. No. 4,027,264 to Gutleber describes a system that scans over the spectral range of an intelligence bearing signal and locks onto and tracks the interfering signal. But the interfering signal is excised by generating a replica and subtracting that replica from the intelligence bearing signal.
U.S. Pat. No. 5,307,517 to Rich describes an improved adaptive notch filter for removing undesired cochannel FM interference, using an approach similar to that employed by Baghdady. The incoming signal is frequency translated to baseband signals and sent through a high pass filter.
U.S. Pat. No. 5,263,048 to Wade describes a method for excising narrowband interferors in a spread spectrum signal, wherein the incoming signal is digitized and transformed to the time domain and then the amplitude of the signal is discarded and replaced with a normalized value.
U.S. Pat. No. 5,703,874 to Schilling describes a spread spectrum CDMA communications system for operation within the same geographic region as occupied by a mobile-cellular system, the spectrum of which overlays the operating frequency of the mobile-cellular system. In this invention, the base station employs a comb filter for attenuating predetermined channels of the mobile-cellular system. It does not seek and attenuate narrowband transmissions at arbitrary frequencies and occurrences.
U.S. Pat. No. 5,640,385 to Long et al. describes a system for simultaneous wideband and narrowband communication, where the narrowband FM signal is embedded in the wideband signal in the base station transmitter circuitry. A notch filter is employed in the receiving circuitry and simultaneous narrowband and wideband wireless communication is supported, i.e. both the narrowband and wideband signals are recovered. Long et al. describes a system for employing both wideband and narrowband signals in an overlapping spectrum. In particular, the system embeds narrowband signals within the wideband frequency band to form a composite wideband signal, and it is important that narrowband and wideband carriers be transmitted from a common transmitter; the receiver in this system takes this composite signal, digitizes, transforms and frequency filters it so as to separate the individual narrowband and wideband signals contained in the composite. The invention provides means for optimizing the capacity of a system using such composite signals. However, Long et al. does not address the problem of conflicting systems, where a wideband system is faced with interference from narrowband signals from other systems which appear at random within the wideband spectrum.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to reduce the adverse effects of narrow band interference on CDMA communication systems, by employing an adaptive notch filter and, in wireless telecommunications applications, thereby restoring blocked call and dropped call rates, reducing or eliminating disruption of the RF power control system, avoiding increases in average mobile station power consumption, maintaining cell capacity, and maintaining cell site coverage area.
Another object of the invention is to prevent a high level interferer from jamming an entire cell or sector, blocking all CDMA communications which would normally be routed through a location.
It is a further object of the invention to provide a more convenient, less complex and less expensive method for overcoming narrow band interference.
The invention provides a device for suppressing narrow band interference in a wideband telecommunications system. Means are provided for rapidly analyzing the wide frequency band with respect to signal power levels in specified narrow frequency bands and detecting narrow band signal power levels received within the specified bands. There is shown how to derive an average composite wideband power level from signal power levels in the specified narrow bands, and then how to use these signal power levels to derive an adaptive threshold for identifying the narrow band interference. Finally, means are provided for setting one or more notch filters for suppressing the identified narrow band interference.
In accordance with the invention as applied to wideband CDMA systems, the spectrum used by the wideband CDMA signal (e.g. having a band of 1.23 MHz) is frequency scanned for narrowband analog signals (e.g. having a band of 30 KHz). An identified frequency band is then assigned to a notch filter and excised. A notable feature of the invention is that this process is fast—in contrast to more generic methods of filtering which take more time to be effective—and appears to a telecommunications user as no more than a brief “click” in reception, if at all, and excises the interferer quickly enough to prevent or greatly reduce the probability of the occurance of a blocked or dropped call due to the interference.
A further notable feature of the invention is that additional notch filters may be added to allow removal of multiple interferors. In a preferred embodiment of the invention, responsive to an environment where there are more interferors than notch filters, those interferors with the largest amplitude are assigned to the notch filters.
The present invention solves or mitigates the problem of narrow band interference in a manner which is less expensive and more convenient than other methods, and in some cases can complement other methods. The most
Jagger Charles E.
Tobia Micolino
Willetts Mark N.
Appiah Charles N.
ISCO International, Inc.
Marshall & Gerstein & Borun LLP
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