Pulse or digital communications – Transceivers
Reexamination Certificate
1999-12-14
2004-04-06
Tse, Young T. (Department: 2634)
Pulse or digital communications
Transceivers
C455S078000, C455S083000, C455S069000
Reexamination Certificate
active
06717981
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of communications, and in particular to time-division-duplex (TDD) transceivers with a common transmit and receive frequency.
2. Description of Related Art
Time-division-duplex (TDD) transceivers are commonly used to provide two-way communications using a single carrier signal frequency.
FIG. 1
illustrates an example block diagram of a conventional time-division-duplex transceiver
100
that utilizes quadrature modulation. The transceiver
100
includes a transmitter
130
that transforms an input data signal into quadrature signals TI
131
and TQ
132
. A local oscillator
120
provides an in-phase oscillation signal
121
, and a phase shifter
125
provides a quadrature-phase oscillation signal
122
that is 90 degrees out of phase with the in-phase oscillation signal
121
. The quadrature signal TI
131
is modulated, at
142
, by the in-phase oscillation signal
121
, and the quadrature signal TQ
132
is modulated, at
144
, by the quadrature-phase oscillation signal
122
. The adder
150
combines these modulated signals to produce a composite signal
151
.
The transmit/receive switch
160
alternately selects the composite signal
151
for transmission, via an antenna
165
. On the alternate cycle, the transmit/receive switch
160
provides an input signal
161
from the antenna
165
. Although an antenna
165
is illustrated in
FIG. 1
(and FIG.
3
), the use of other communications media, such as a wire, or cable, is also common in the art.
The input signal
161
is a composite signal that is segregated into corresponding quadrature signals RI
173
and RQ
175
by demodulators
172
and
174
, respectively. Common in the art, the local oscillator
120
that is used to modulate the transmit quadrature signals TI and TQ is used to demodulate the received input signal
161
into receive quadrature signals RI and RQ. A number of advantages are achieved by using a common local oscillator
120
. In particular, the local oscillator
120
is typically a phase-locked oscillator, and using the same oscillator
120
during both phases of the transmit/receive switch
160
eliminates the need to re-phase or re-synchronize the oscillator
120
with each transition. Additionally, the use of the same local oscillator
120
provides a material cost savings compared to the use of a separate oscillator for each of the transmit and receive operations. The receiver
110
processes the quadrature signals RI
173
and RQ
175
to provide an output signal
102
.
As is common in the art, the transmitter
130
provides the transmit quadrature signals TI
131
and TQ
132
at a predetermined intermediate frequency (IF). In like manner, the quadrature signals RI
173
and RQ
175
, being produced by a distant transmitter that is similar to the transmitter
130
, are also produced at the predetermined intermediate frequency. The modulation
142
,
144
of the quadrature signals TI
131
, TQ
132
at the intermediate frequency IF with the local oscillation signals
121
,
122
at a carrier frequency Fc results in two sidebands of modulation, one at Fc+IF, and the other at Fc−IF. Ideally, the quadrature signals TI
131
and TQ
132
are structured such that one of the sidebands, the intended sideband, contains maximum power, while the other sideband, the “image” sideband contains minimum power.
Due to component variations and other factors, however, a difference in phase or amplitude from the ideal relationship between the quadrature signals TI
131
and TQ
132
can result in an image sideband having a considerable power content.
FIG. 2
illustrates an example spectral power density plot of a convention transmitter
130
having a less-than-ideal relationship of amplitude or phase between the quadrature signals TI and TQ. As illustrated, a majority of power is located at the intended sideband at Fc+IF, at
220
, but a considerable amount of power is illustrated at the carrier frequency FC, at
210
, and at the image sideband at Fc−IF, at
230
. To minimize the distortion of the demodulated intended signal, the transmitter or a distant receiver must filter this unintended and undesirable carrier and image signal power.
As is known in the art, the cost and complexity of a filter process is highly dependent upon the degree of “roll-off” required of the filter. The selective filtering of two signals that are close in frequency requires a very steep roll-off, and therefore is more costly and complex than the selective filter of two signals that are widely separated in frequency. By implication, then, the preferred intermediate frequency IF should be large, because the separation between the intended
220
and unwanted
230
signals is twice the intermediate frequency. However, a high intermediate frequency introduces additional costs and complexities to the components utilized within the transmitter
130
and receiver
110
compared to a lower intermediate frequency. Preferably, the transmitter
130
should be designed to conform as close to the ideal as possible, so that the degree of required filtering at the transmitter or distant receiver can be minimized, and so that a lower intermediate frequency can be utilized. The use of precision components and robust design techniques that provide for this idealized transmitter performance, however, is also a costly and complex approach.
BRIEF SUMMARY OF THE INVENTION
It is an object of this invention to provide a method and apparatus that minimizes the transmission of unwanted image frequency signals from a transceiver. It is a further object of this invention to provide a method and apparatus that minimizes the transmission of unwanted image frequency signals from a transceiver that does not require the use of precision components in the transceiver. It is a further object of this invention to provide a method and apparatus that minimizes the transmission of unwanted image frequency signals from a transceiver that allows for a dynamic adjustment of the transceiver performance to compensate for component variations and environmental changes.
These objects and others are achieved by providing a method and apparatus for determining the characteristics of the image signal energy being transmitted from a transceiver and thereafter providing feedback to the transmitter to reduce this image signal energy. The image signal energy is measured by the receiver component of the transceiver and fed back to the transmitter component of the transceiver. The transmitter component uses the fed back information to adjust the gain and or phase relationship between the quadrature signals that are subsequently quadrature-phase modulated and transmitted. A variety of techniques can be employed to allow the image signal energy to be measured directly by the receiver component. The phase modulation signals at the transmitter can be interchanged, so that the image signal energy is transmitted in the sideband of the intended signal. Alternatively, the phase modulation signals at the receiver can be interchanged, so that the receiver's operating frequency is shifted from the frequency of the transmitter's intended signal sideband to the frequency of the transmitter's image signal sideband.
REFERENCES:
patent: 5423076 (1995-06-01), Westergren et al.
patent: 5446422 (1995-08-01), Mattila et al.
patent: 5896562 (1999-04-01), Heinonen
patent: 6009124 (1999-12-01), Smith et al.
patent: 6163708 (2000-12-01), Groe
patent: 6278864 (2001-08-01), Cummins et al.
patent: 6553018 (2002-04-01), Ichihara
patent: 6404293 (2002-06-01), Darabi et al.
patent: 0624004 (1994-11-01), None
Patent Abstract of Japan Publication No.: 10242765A, Date of Publication Sep. 11, 1998.
Halajian Dicran
Koninklijke Philips Electronics , N.V.
Tse Young T.
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