Pulse or digital communications – Receivers – Angle modulation
Reexamination Certificate
1998-10-07
2001-11-06
Ghayour, Mohammad H. (Department: 2734)
Pulse or digital communications
Receivers
Angle modulation
C375S344000, C375S354000, C375S371000, C455S164100
Reexamination Certificate
active
06314145
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATION
This application claims priority of European Patent Application No. 97308743.0, filed Oct. 7, 1997.
FIELD OF THE INVENTION
The present invention relates to the tracking of carrier timing in a receiver in which a single frequency source is used to generate a reference clock and to generate a carrier signal. The invention particularly relates to transceivers in which part of the functionality takes place in discrete time.
BACKGROUND OF THE INVENTION
In most modern transceivers, for both technical and cost reasons a single frequency reference is used for the reference clock and the carrier oscillator in the transmitter, and a single frequency reference is used for the reference clock and the carrier reference in the receiver. The carrier oscillator and the carrier reference are respectively generated by multiplying the respective frequency reference by a predetermined value.
In the transmitter the reference clock is used as a sample clock in the modulator and in the digital-to-analogue converter, and in the receiver the reference clock is used in the analogue-to-digital converter and in digital signal processing circuitry (including the demodulator).
In such transceiver designs, in which part of the functionality takes place in discrete time, it is required to synchronise the sample timing of the receiver with the sample timing of the transmitter, i.e. synchronise the reference clock of the receiver with that of the transmitter. There are known techniques to derive sample tracking out of the received signal. However, such known techniques require complex circuitry in the receiver to ensure timing tracking of the carrier.
A method is usually provided in the receiver to synchronise the receiver with the carrier on which the received signal is modulated. This synchronisation involves determining, and removing, the error between the carrier frequency of the transmitter and the reference carrier frequency in the receiver. Especially in coherent detection receivers, the receiver must be very rigidly synchronised, or locked, to the received carrier.
SUMMARY OF THE INVENTION
It is an object of the present invention to simplify the circuitry required in the receiver for timing tracking of the receive reference clock to the transmit reference clock by utilising circuitry provided in the receiver for synchronising the receiver to the carrier.
According to the present invention there is provided circuitry for receiving a transmitted signal, the transmitted signal comprising a modulated signal generated under the control of a transmit timing signal of a first predetermined frequency and up-converted by a carrier frequency at a multiple n of the first predetermined frequency, the circuitry comprising: a signal source for generating a receiver timing signal of a second predetermined frequency; a frequency multiplier having a multiply ratio of n and connected to receive as an input the receiver timing signal and for generating a carrier reference signal having a reference frequency of a multiple n of the second predetermined frequency; a down-converter connected to receive as a first input the transmitted signal and as a second input the carrier reference signal, and for generating at an output an offset modulated signal corresponding to the modulated signal offset by a frequency equal to the difference between the carrier frequency and the reference frequency, a frequency compensator connected to receive as an input the offset modulated signal and for generating at an output a difference signal having a frequency corresponding to the difference between the carrier frequency and the reference frequency; a frequency divider, having a divide ratio of n, connected to receive the difference signal and to generate at an output thereof an error signal having a frequency equal to the difference between the first and second predetermined frequencies; timing control means, connected to receive the error signal and for generating at least one timing control signal in dependence on the error signal; and timing adjustment means coupled to the output of the down-converter, and for adjusting the time phase of the output of the down-converter responsive to said at least one timing control signal.
The present invention thus provides a technique for timing tracking in which error information normally generated for carrier frequency tracking is also used to track the reference timing of the receiver relative to the transmitter in dependence on a known ratio between the transmitter carrier frequency and the transmitter reference timing.
In a preferred embodiment there is provided offset removal circuitry connected between the down-converter and the timing adjustment means for removing the offset from the offset modulated signal and having a first input connected to the output of the down-converter and a second input connected to the output of the frequency compensator, and for generating at an output thereof the modulated signal. Thus in a preferred embodiment the invention advantageously utilises an efficient and economic circuit for carrier tracking.
In a further preferred embodiment the timing control means includes comparison means for comparing the error signal to a predetermined error value, wherein the at least one timing control signal is generated in dependence on the error signal being greater than the predetermined error value. Thus in a further preferred embodiment the invention advantageously enables the timing of the modulated signal to be adjusted in the receiver only if the reference timing in the receiver is in error by greater than a predetermined amount.
The present invention also provides a method of tracking the carrier timing of a transmitted signal in a receiver, the transmitted signal comprising a modulated signal generated under the control of a transmit timing signal of a first predetermined frequency and up-converted by a carrier frequency at a multiple n of the first predetermined frequency, the method comprising: receiving the transmitted signal; generating a receiver timing signal of a second predetermined frequency; generating a carrier reference signal having a reference frequency of a multiple n of the second predetermined frequency; down-converting the transmitted signal by mixing it with the carrier reference signal, thereby generating an offset modulated signal corresponding to the modulated signal offset by a frequency equal to the difference between the carrier frequency and the reference frequency; generating a difference signal having a frequency corresponding to the difference between the carrier frequency and the reference frequency; dividing the difference signal by n, thereby generating an error signal having a frequency equal to the difference between the first and second predetermined frequencies; generating at least one timing control signal in dependence on the error signal; and adjusting the time phase of the down-converted signal responsive to said at least one timing control signal.
In a further aspect, the present invention provides a communication system, including a transmitter comprising: a signal source for generating a transmitter timing signal of a first predetermined frequency; a frequency multiplier having a multiply ratio of n and connected to receive as an input the transmitter timing signal and for generating a carrier signal having a frequency of a multiple n of the first predetermined frequency; and an up-converter connected to receive as a first input a modulated signal to be transmitted and as a second input the carrier signal, and for generating at an output a transmit signal; the communication system further including a receiver comprising: a signal source for generating a receiver timing signal of a second predetermined frequency; a frequency multiplier having a multiply ratio of n and connected to receive as an input the receiver timing signal and for generating a carrier reference signal having a reference frequency of a multiple n of the second predetermined frequency; a down-converter connected to rece
Agere Systems Guardian Corp.
Garceran Julio A.
Ghayour Mohammad H.
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