Method and system for rejecting interfering signals

Details Method and system for rejecting interfering signals Method and system for rejecting interfering signals

1997-07-01

2001-01-30

Nguyen, Lee (Department: 2746)

Telecommunications

Receiver or analog modulated signal frequency converter

Noise or interference elimination

C455S296000, C455S063300, C455S067700, C455S110000, C455S506000, C455S303000, C455S297000, C455S278100, C370S483000

Reexamination Certificate

active

06181924

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to wireless communication systems. More particularly, the present invention is directed to a method and system for rejecting interfering signals in a communication system employing antenna diversity.
BACKGROUND OF THE INVENTION
Mobile communication systems have experienced explosive growth, as an ever- increasing number of subscribers demand mobile communication services. To meet this demand, increased system capacity is necessary, and can be achieved in many ways. One way is to reduce the “cell” size (that is, the coverage area of an individual base station). However, this approach also results in frequencies being reused within a smaller area, thereby increasing interference and decreasing speech quality. While factors such as fading and noise can also reduce speech quality, interference is typically the dominating form of disturbance; therefore, methods to reduce the effects of interference are highly desirable to improve speech quality.
Interference typically manifests itself as discordant noise, and can be difficult to remove. There are generally two types of interference: co-channel interference and adjacent channel interference. Co-channel interference (interference between two or more channels which are assigned to the same frequency, as mentioned above) is typically the dominant type, and varies with frequency reuse distance. Adjacent channel interference is generally controlled more easily, as it can be reduced to an acceptable level through appropriate filtering of the received signal.
To improve speech quality, many conventional systems employ diversity schemes, in which communication signals are received at two or more input branches. The branches are combined according to one of a variety of diversity combining techniques. One known diversity algorithm uses equal-gain combining to reduce interference. Equal-gain combining is a co-phase combining technique in which a combined signal is generated as the sum of the instantaneous phase envelopes of the individual branches. While relatively simple, this technique does not adequately reduce the effects of co-channel interference.
It would be desirable to reduce the effects of co-channel interference in a communication system, particularly in a system employing antenna diversity.
SUMMARY OF THE INVENTION
The present invention overcomes the above-noted problems, and achieves other advantages, by providing for a mobile communication system and method which substantially maximizes the carrier-to-interference (C/I) ratio of a received signal. The present invention assumes that a source of an interfering communication signal is typically found to be a certain angle from the affected receiving antennas, and therefore the interfering angle is offset by some phase angle from the desired carrier signal. According to exemplary embodiments of the present invention, when a modulated communication signal which includes a desired carrier signal and one or more interfering signals is received at a receiving antenna, an offset value is determined which, when added to the received communication signal, will substantially maximize the carrier-to-interference (C/I) ratio. The offset value can be first estimated (based on, e.g., the angle to the source of the interfering signal), and adjusted incrementally by comparing the C/I ratio calculated using the estimated offset angle with the C/I ratio calculated using the incrementally-adjusted estimated offset angle, and selecting the offset angle corresponding to the better C/I ratio.


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P. Petrus et al., “Cochannel Interference Rejection for AMPS Signals Using Spectral Correlation Properties and an Adaptive Array”, Proceedings of the Vehicular Technology Conference, Chicago, Jul. 25-28, 1995, vol. 1, IEEE, pp. 30-34.

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