Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
1999-07-30
2004-08-24
Tran, Khai (Department: 2631)
Pulse or digital communications
Spread spectrum
Direct sequence
Reexamination Certificate
active
06782038
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method and apparatus for performing radio communication, and in particular to a portable communications device incorporating such a method or apparatus.
BACKGROUND OF THE INVENTION
Within most common radio communication protocols (e.g. Groupe Speciale Mobile—GSM), portable communication devices must be able to receive and transmit radio signals at a plurality of different radio frequencies which correspond to different channels (or groups of channels). In order to receive radio signals at different radio frequencies, conventional radio receivers have employed a superheterodyne receiver in which the incoming radio signal is mixed with a first locally generated signal whose frequency may be varied as desired. In this way it is possible to generate an in termediate frequency (IF) signal whose frequency is given approximately by f
IF
=f
RF
−f
LO
where f
IF
is the frequency of the IF signal, f
RF
is the frequency of the wanted radio signal and f
LO
is the frequency of the locally generated signal; since f
LO
may vary, it is always possible to choose an f
LO
such that f
IF
occupies a single frequency range regardless of the value of f
RF
. Conventionally, within GSM portable communication devices, f
IF
is chosen to have a value such that the image (f
RF
−2f
IF
) is out of the GSM band so it can be filtered by suitable RF filters. The IF signal thus obtained is then filtered by a band-pass filter in order to permit the wanted signal to pass through while removing unwanted signals adjacent thereto. Thereafter, a second locally generated signal whose frequency corresponds to the frequency of the IF signal is mixed with the IF signal to generate the base-band signal. However, a significant drawback of such a superheterodyne receiver is that the band-pass filter required can not be easily incorporated onto an integrated circuit and has a significant cost associated with it.
In order to overcome the above-mentioned drawback with a superheterodyne receiver, a direct down-conversion receiver has been proposed in which f
LO
is set to equal f
RF
such that the IF signal corresponds directly to the base-band signal which is desired. In this case only a low pass filter is required which can be formed on an integrated circuit as desired. However, the locally generated signal may itself get received by the aerial of the receiver and interfere with the wanted rf signal thus generating noise at dc within the base-band signal which will not be filtered by the low-pass filter. In a similar manner, any non-linear distortion on the signal (or on high level interfers) caused by non-linear components within the receiver may also cause an unwanted dc noise or second order AM components within the base-band signal which cannot be easily filtered out without adversely affecting the wanted base-band signal (N.B. the wanted base-band signal will also have a dc component).
Thus there is a need for a method and apparatus for performing radio communication which overcomes the drawbacks associated with the prior art referred to above.
SUMMARY OF THE INVENTION
According to a second aspect of the present invention, there is provided a method of modulating a carrier signal with a modulating signal and transmitting the modulated carrier signal thus formed, the method comprising the steps of generating a low frequency local oscillator signal, which can be mathematically described as having a fixed frequency of zero and a phase which varies in time according to a predetermined phase function; mixing the modulating signal with the low frequency local oscillator signal to generate a mixed signal; converting the mixed signal from a digital signal to an analogue signal; generating a high frequency local oscillator signal, which can be mathematically described as having a fixed frequency corresponding to the frequency of the carrier signal to be modulated and a phase which varies in time according to the predetermined phase function when delayed by a predetermined delay corresponding to the time taken for the signal to propagate between the first mixing step and the second mixing step; mixing the mixed signal with the high frequency local oscillator signal to generate a generated signal including both the wanted modulated carrier signal whose phase is not dependent on the predetermined phase function and a noise signal whose phase is dependent upon the predetermined phase function; and transmitting the generated signal.
According to a third aspect of the present invention, there is provided a radio receiver for receiving a wanted modulated radio frequency signal and demodulating it to recover the wanted modulating signal therefrom, the receiver comprising receiving means for receiving a radio frequency signal; a high frequency local oscillator for generating a high frequency local oscillator signal, which can be mathematically described as having a fixed frequency corresponding to the frequency of the carrier wave of the wanted modulated radio frequency signal and a phase which varies in time according to a predetermined phase function; an analogue mixer for mixing the received radio frequency signal with the high frequency local oscillator signal to generate a mixed signal; an analogue to digital converter for converting the mixed signal from an analogue signal into a digital signal; a low frequency local oscillator for generating a low frequency local oscillator signal, which can be mathematically described as having a fixed frequency of zero and a phase which varies in time according to the predetermined phase function when delayed by a predetermined delay corresponding to the time taken for the signal to propagate between the analogue mixer and the digital mixer; and a digital mixer for mixing the mixed signal with the low frequency local oscillator signal to recover a recovered signal including both a wanted base-band signal whose phase is not dependent on the predetermined phase function and a noise signal whose phase is dependent upon the predetermined phase function.
According to a fourth aspect of the present invention, there is provided a radio transmitter for modulating a carrier signal with a modulating signal and transmitting the modulated carrier signal thus formed, the transmitter comprising a low frequency local oscillator for generating a low frequency local oscillator signal, which can be mathematically described as having a fixed frequency of zero and a phase which varies in time according to a predetermined phase function; a digital mixer for mixing the modulating signal with the low frequency local oscillator signal to generated a mixed signal; a digital to analogue converter for converting the mixed signal from a digital signal to an analogue signal; a high frequency local oscillator for generating a high frequency local oscillator signal, which can be mathematically described as having a fixed frequency corresponding to the frequency of the carrier signal to be modulated and a phase which varies in time according to the predetermined phase function when delayed by a predetermined delay corresponding to the time taken for the signal to propagate between the digital mixer and the analogue mixer; an analogue mixer for mixing the mixed signal with the high frequency local oscillator signal to generate a generated signal including both the wanted modulated carrier signal whose phase is not dependent on the predetermined phase function and a noise signal whose phase is dependent upon the predetermined phase function; and transmitting means for transmitting the generated signal.
The receiving and demodulating method preferably includes the step of filtering the recovered signal to remove unwanted components from the recovered signal including at least some of the noise whose phase is dependent upon the predetermined phase function. By this method, it is possible to remove more of the noise generated after the first mixing stage than would be the case if the phase of the noise signal was independent of the predetermined pha
Burd Kevin M
Motorola Inc.
Tran Khai
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