Multiplex communications – Communication techniques for information carried in plural... – Combining or distributing information via time channels
Utility Patent
1998-02-26
2001-01-02
Patel, Ajit (Department: 2738)
Multiplex communications
Communication techniques for information carried in plural...
Combining or distributing information via time channels
C370S331000, C375S340000
Utility Patent
active
06169752
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of electronic communications, in particular a method and system for preventing information losses during alternative frequency searches.
2. Description of the Related Art
Cellular systems rely on “handoffs” to maintain call continuity. When a mobile station, for example a hand held cellular phone, moves away from the serving base station, the call is transferred, or “handed off,”to a new and closer base station. In this way a call is minimally disrupted by the movement of the mobile station. In general, digital cellular systems require the mobile station to aid the base station in making handoff decisions by reporting the strength at which the mobile receives signals from alternative base stations.
In direct sequence Code Division Multiple Access (CDMA) systems, many base stations and radio signals share the same band of the frequency spectrum; different users and base stations are distinguished by the use of uniquely identifying “spreading codes,” which allow a user to separate its desired signal from the undesired interference of other users. It is easy for a mobile station in this system to simultaneously receive data from the serving base station and to measure the power of an alternative base station without changing the radio frequency. However, sometimes it is required that the mobile search alternative frequencies, making continued data reception impossible. Since there are generally no dead times in the transmission of data in CDMA systems, this results in the loss of some information.
The conventional, direct approach to such alternative frequency searches is to perform the following process steps:
1. At the beginning of a data frame, stop demodulation.
2. Tune to the search frequency.
3. Perform a search.
4. Retune to the serving frequency.
5. Begin demodulation at the next frame.
This entails the loss (“nulling out”) of some integer number of frames of data. For a cellular phone being used for a voice conversation, this loss results in a degradation of audio quality. In particular, the user may hear clicks or nulled voice or other undesired noises.
Therefore, an object of this invention is to perform alternative frequency searches without losing any frames of data (information).
SUMMARY OF THE INVENTION
This object is achieved by the present invention, which comprises a method for preventing information losses during alternative frequency searches by a receiving unit in a communication system, wherein data is channel coded, interleaved, and segmented into a plurality of frames. The method comprises the steps of discontinuing demodulation at a predetermined time before a frame ends, inserting zero values into the frame, and performing a search for alternative frequencies while continuing to insert zero values into the frame and then a next frame. After the search is completed, insertion of zero values is discontinued and demodulation is resumed.
As a result of this novel procedure, the present invention allows searching of frequencies other than a serving frequency without necessarily losing any data. This applies to any systems where alternate frequencies must be searched but there are no free times provided to do so. As compared to conventional methods which require a “hard” degradation from throwing away complete frames of data (i.e. information will certainly be lost), the present invention only results in “soft” degradation (effective loss of signal power without a certain loss of information).
In one preferred embodiment of the present invention, the receiving unit comprises a mobile radio station operating at a serving frequency. The step of inserting zero values is followed by a step of programming the mobile radio station to a search frequency and waiting for the mobile radio station to settle. The step of discontinuing insertion of zero values is preceded by a step of programming the mobile radio station to the serving frequency and waiting for the mobile radio station to settle.
A preferred version of a system or integrated circuit embodying the present invention includes a demodulator which discontinues demodulation at a predetermined time before a frame ends, a deinterleaver which deinterleaves data received from the demodulator, and a channel decoder which decodes data received from the deinterleaver. There is also a control processor which directs zero values to be inserted into the frame after demodulation is discontinued, and a search unit which is directed to perform a search for alternative frequencies by the control processor, which continues to insert zero values into the frame and then a next frame. The control processor discontinues insertion of zero values after the search is completed, and then allows the demodulator to resume demodulation.
These and other aspects, features, and advantages of the present invention will be apparent to those persons having ordinary skill in the art to which the present invention relates from the foregoing description and the accompanying drawings.
REFERENCES:
patent: 4476581 (1984-10-01), Bragas
patent: 5625876 (1997-04-01), Gilhousen et al.
patent: 5764687 (1998-06-01), Easton
patent: 5883888 (1999-03-01), St-Pierre
patent: 6005855 (1999-12-01), Zehavi et al.
patent: 6038263 (2000-03-01), Kotzin et al.
LSI Logic Corporation
Mitchell Silberberg & Knupp LLP
Patel Ajit
Tsegaye Saba
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