System for cancelling internal interference in a receiver

Details System for cancelling internal interference in a receiver System for cancelling internal interference in a receiver

2000-02-01

2001-07-10

Chin, Stephen (Department: 2734)

Pulse or digital communications

Receivers

Interference or noise reduction

C455S283000, C455S277100

Reexamination Certificate

active

06259752

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates generally to a system for canceling interference in a receiver, and more specifically, to a system for canceling internal interference in a time-division multiple access (TDMA) receiver, such as in a GSM receiver (Global System For Mobile Receiver).
GSM receivers typically derive many different clock frequencies from a single reference frequency, and are susceptible to “self-jamming” or interference at particular frequency channels. For example, it is common to use crystal oscillator as a system clock operating at a frequency of either 13 MHz or 19.5 MHz because use of such frequencies permits other clock frequencies to be easily derived. A problem arises when a harmonic of the system clock is the same frequency as one of the GSM communication channels. For example, the 72
nd
harmonic of a 13 MHz system clock has energy at 936 MHz (72*13=936). Channel 5 of the GSM system happens to be centered at 936 MHz. Thus, the harmonics produced by the system clock causes interference with GSM channel No. 5. Further, additional interference is caused by other harmonics, which may not necessarily be generated by the system clock, but rather, by other clocks in the system and by spurious mixes of local oscillators. In heterodyne or super-heterodyne receivers, local oscillators, mixers, and various digital clocks emit a wide range of harmonics that interfere with various GSM channels.
One approach to solve this problem in prior art systems is to add shielding to the “noisy” components of the system so that the harmonic energy does not substantially escape or enter the receiver circuitry. This involves using various metal enclosures and screens, and may include specialized printed circuit board design. This, however, adds significant weight and increases the size of the device. Because weight and size are extremely important considerations in the design of a GSM system, such as a cellular telephone, the disadvantages of this approach are apparent. Further, use of shielding increases the cost of the device, which is usually sold in a cost-competitive market.
SUMMARY
Reduction of internal interference in a radio-frequency (RF) receiver may be accomplished by providing a plurality of time slots within a frame where the receiver is configured to receive external RF signals during a predetermined receive time slot within the frame. External RF signals may be prevented from reaching a front-end portion of the receiver just prior to the receive time slot, the receiver is then activated, and a predetermined period of time is allowed to elapse, thus permitting the receiver to settle and become stable. An interference data collection is performed during a period of time prior to the predetermined receive time slot where the data collected represents internally generated interference signals. The data collected during the interference data collection is processed to determine a bias value corresponding to the interference signals, and the receiver is then permitted to receive the external RF signals during the predetermined receive time slot so that data is collected during the predetermined receive time slot. The data collected during the predetermined receive time slot is processed and the bias value is subtracted therefrom to provide output data corresponding to the external RF signals absent the interference signals.


REFERENCES:
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