Telecommunications – Transmitter and receiver at same station – With transmitter-receiver switching or interaction prevention
Reexamination Certificate
1999-09-10
2003-02-04
Le, Thanh Cong (Department: 2684)
Telecommunications
Transmitter and receiver at same station
With transmitter-receiver switching or interaction prevention
C455S088000, C455S296000, C375S346000
Reexamination Certificate
active
06516183
ABSTRACT:
BACKGROUND
The present invention generally relates to a full duplex transceiver in portable cellular phone systems, pager systems, etc., and more specifically to an error estimator which compensates for various disturbances caused to signals being received in a direct conversion receiver located within the transceiver.
Receivers in cellular systems and other fields noted above are preferably small, lightweight and inexpensive. To make a portable receiver such as a hand-held telephone smaller and less expensive, the integration of parts has become very important. Heterodyne receivers usually are of high cost to produce and have many parts such as bandpass filters that are unable to be integrated. To overcome such drawbacks, direct conversion receiver architecture has been developed in which the frequency of the local oscillator is the same as the frequency of the received radio carrier. Consequently, the received radio signal is down-converted directly to base band in one step. Since a direct-conversion receiver does not have any intermediate frequency (IF) stages, many filters can be omitted or simplified.
Direct conversion was introduced for single-sideband receivers in the 1950's, but the technique is not limited to such systems. Direct conversion can be used with many different modulation schemes and is especially well suited for the quadrature modulation schemes of today, such as minimum shift keying (MSK) and quadrature amplitude modulation (QAM). Various aspects of direct-conversion receivers are described in U.S. Pat. No. 5,530,929 entitled “Radio Receiver.”
The operation of a conventional direct-conversion receiver can be described as follows with reference to
FIG. 1. A
radio frequency (RF) signal having center frequency f
c
and bandwidth BW
rf
is received by an antenna
10
and then is filtered by a bandpass filter
20
. The filtered signal produced by the bandpass filter is amplified by an amplifier
30
, which preferably has low noise to improve the total noise figure of the receiver.
The amplified and filtered signal produced by the amplifier
30
is then down-converted to base band in an in-phase (I) channel and a quadrature phase (Q) channel by balanced mixers
40
,
50
. The mixers are driven by respective ones of sine (I) and cosine (Q) signals, produced from a sinusoidal signal generated by a local oscillator
60
, by a suitable divider and phase shifter
70
. According to the direct-conversion principle, the local oscillator signal also has the frequency f
c
.
The mixers
40
,
50
effectively multiply the signal from the amplifier
30
and the I and Q signals of the local oscillator. Each mixer produces a signal that has frequencies that are the sum and difference of the frequencies of the amplified filtered received signal and the local oscillator signal. The difference (down-converted) signals each have a spectrum that is folded over around zero frequency (DC) and that spans from DC to ½BW
rf
.
The I and Q signals produced by the mixers
40
,
50
are filtered by low-pass filters
80
,
90
that remove the sum (up-converted) signals, as well as components that might be due to nearby RF signals. The filters
80
,
90
set the noise bandwidth, and thus, the total noise power in the receiver. The I and Q signals are then amplified by amplifiers
100
,
110
, and provided to further processing components that produce the demodulated output signal. The further processing can include phase demodulation, amplitude demodulation, frequency demodulation, or hybrid demodulation schemes.
One major problem with the direct-conversion receiver is that baseband signal distortion can be caused by a pure DC signal generated by local oscillator leakage, in addition to second-order products of interferers (e.g., signals on the same and nearby RF communication channels) produced by active elements located close to the receiver. The distortions, located at the base band, interfere with the desired base band signal, thereby degrading performance of a direct conversion receiver. In some situations, this problem totally blocks communication in high-performance receivers for today's time division multiple access (TDMA) and wide band code division multiple access (WCDMA) digital cellular systems.
A second order non-linearity together with a strong constant envelope RF interferer will cause a DC component within a received signal. Such a DC component can be blocked, for example, with a DC blocking capacitor.
Amplitude Modulated (AM) interferers, however, present a larger problem since the disturbance at baseband will not be a pure DC signal and therefore cannot be easily removed. In non-linear devices such as an amplifier, an input signal, V
in
, will produce an output signal, V
out
. The characteristics of an amplifier located within a transceiver can be defined as V
out
=c
1
V
in
+c
2
V
in
2
+c
3
V
in
3
+ . . . , where an input signal can be V
in
(t)=v
1
(t)·Cos(&ohgr;
1
t). If the input signal is applied to the input of the amplifier, the output is described by the following equation:
v
out
(
t
)
=
1
2
c
2
v
1
2
(
t
)
+
(
c
1
v
1
(
t
)
+
3
4
c
3
v
1
3
(
t
)
)
cos
ω
1
t
+
1
2
c
2
v
1
2
(
t
)
cos
2
ω
1
t
+
1
4
c
3
v
1
3
cos
3
ω
1
t
+
…
(
1
)
From this equation it can be seen that the input signal will generate a baseband distortion component described by:
v
bb
(
t
)
=
1
2
c
2
v
1
2
(
t
)
,
(
2
)
where constant c
2
depends on a second order intercept point of the amplifier. The second order intercept point is determined by applying two signals at two different frequencies, f
1
and f
2
, to the amplifier. The output power, P
out
, is plotted at the first frequency, the sum of the first and second frequency against the input power P
in
of either f
1
or f
2
. Extrapolation of P
out
versus P
in
of either f
1
or f
2
yields the second order intercept point. If V
iIP2
is the voltage input of the amplifier at the second order intercept point, the baseband voltage can be written as:
v
bb
(
t
)
=
1
2
c
1
V
iIP2
v
1
2
(
t
)
.
(
3
)
In an application which consists of both I and Q modulators the interference problem can be described with reference to FIG.
2
. An interfering signal, IF, is an AM modulated signal that generates baseband interference in both the Q and I channels, where
V
bbI
(
t
)=
K
i
·v
1
2
(
t
)
V
bbQ
(
t
)=
K
q
·v
1
2
(
t
), (4)
and K
i
and K
q
are constants. The disturbance on the I and Q channels, however, is not necessarily equal, due to the differing levels of interfering signals on the channels, and the different components in the IQ demodulator. The resulting signal is an error signal that is combined with the desired signal. The error signal can be written as:
&egr;
i
(
t
)=
K
i
·v
1
2
(
t
)
&egr;
q
(
t
)=
K
q
·v
1
2
(
t
), (5)
The AM interferer together with second order non-linearities in the receiver generate a baseband error vector. The phase of the error vector is constant, or varies slightly, and the magnitude is proportional to the squared envelope of the interferer. Since the disturbance is not necessarily equal on I and Q channels, K
i
is not necessarily equal to K
q
in the general case. Accordingly, the error signal can be written as:
&egr;(
t
)=&egr;
i
(
t
)+
j&egr;
q
(
t
)=
r
&egr;
(
t
)
e
jy
, (6)
where
r
&egr;
(
t
)=
K
y
·v
1
2
(
t
)
K
y
=|K
i
+jK
q
|
and
&dgr;=
arg
(
K
i
+jK
q
) (7)
where &ggr; is an arbitrary phase shift that is constant or changes slightly with, for example, a temperature variation.
As discussed above, in the case where the input has a constant envelope, the baseband interference will be a pure DC component. The offset caused by a pure DC component can be compensated for, in the simplest case, by a DC blocking
Burns Doane Swecker & Mathis L.L.P.
Cong Le Thanh
Nguyen Thuan T.
Telefonaktiebolaget LM Ericsson (publ)
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