Fractional-N Synthesizer State Shifting for LO Phase Noise Decorrelation
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Solution Overview
Problem
In 5G cellular systems using millimeter waves, implementing local oscillator (LO) generation circuitry for beamforming systems is challenging due to stringent phase noise requirements and the need to distribute LO signals efficiently without excessive power consumption, while maintaining low phase noise correlation across transceivers.
Innovation Solution
A fractional-N frequency synthesizer circuit with a modulator circuit that uses memory elements to store and restore its internal state, enabling a time shift of control word sequences for frequency dividers, thereby reducing phase noise correlation between LO signals generated by different transceivers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If LO signals are distributed to all transceivers in a beamforming system, then the system range and capacity are increased, but power consumption becomes excessive
Solution Approach 1:
The patent divides the LO generation function into two parts: a central reference oscillator that generates the base LO signal, and distributed frequency synthesizers at each transceiver that locally adjust the frequency. This segmentation allows the high-power central oscillator to serve all transceivers while each transceiver only needs low-power local adjustment circuitry, reducing total power consumption while maintaining system capacity and range.
2Measurement precision
If fractional-N frequency synthesizers are used to generate LO signals with high frequency resolution, then frequency precision is improved, but phase noise correlation between different transceivers increases
Solution Approach 1:
The patent introduces a randomization mechanism as an intermediary between the deterministic fractional-N frequency synthesizer and the LO signal output. By randomly varying the frequency divisor sequence among different transceivers, the system maintains the high frequency resolution provided by the fractional-N synthesizer while breaking the phase noise correlation between transceivers, thus resolving the contradiction between precision and reliability.
3Measurement precision
If the same modulated sequence of frequency divisors is used in different fractional-N frequency synthesizers, then frequency resolution is maintained, but phase noise correlation increases
Solution Approach 1:
The patent transforms the static, identical frequency divisor sequence used in all transceivers into a dynamic, randomized sequence. Each transceiver generates its own varied sequence of frequency divisors based on the same base modulation but with random offsets. This dynamic approach maintains the frequency resolution benefits of structured modulation while eliminating the harmful phase noise correlation that arises from using identical sequences across all transceivers.
Data Source
AI summary
A fractional-N frequency synthesizer circuit is disclosed. It comprises a frequency divider circuit configured to receive a first oscillation signal having a first frequency, to receive a control word indicating a divisor, and to frequency divide the first oscillation signal with the divisor to generate a second oscillation signal having a second frequency, lower than the first frequency. It also comprises a modulator circuit configured to generate a sequence of control words to the frequency divider circuit. The modulator circuit comprises a set of memory elements configured to store an internal state of the modulator circuit in response to a first control signal and to restore the internal state of the modulator circuit in response to a second control signal, thereby enabling a time shift of the sequence of control words. A communication circuit, a communication apparatus, and a method are also disclosed.