Transmitter

Details Transmitter Transmitter

1999-05-27

2003-05-13

Chin, Stephen (Department: 2634)

Pulse or digital communications

Transmitters

C375S146000

Reexamination Certificate

active

06563883

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a transmitter and a method for gain control in a transmitter. In particularly, but not exclusively, the present invention relates to a transmitter and a method for gain control in a transmitter such as in a spread spectrum multiple access system using, for example code division multiple access (CDMA). The transmitter and method may be used in a cellular telecommunications network.
BACKGROUND TO THE INVENTION
FIG. 1
shows a known transmitter of a mobile station used in a cellular telecommunications network. The transmitter
1
comprises an antenna
2
which is used to receive and transmit signals. It should be appreciated that the transmit part only of the mobile station is shown in FIG.
1
. The signal to be transmitted can be regarded in the illustrated transmitter
1
as being two signals, one of which is the sine component and the other of which is the cosine component. These compounds are alteratively referred to as the I and Q components. The I and Q components are initially at a baseband frequency. The I and Q signals are in digital form initially and are converted to analogue signals by respective digital to analogue convertors (DAC)
3
a
and
3
b.
The output of each of the digital to analogue converters
3
a
and
3
b
is connected to a respective lowpass filter
4
a
and
4
b.
The lowpass filters
4
a
and
4
b
filter out undesired components which are introduced by the digital to analogue converters
3
a
and
3
b.
The output of each of the digital to analogue converters
4
a
and
4
b
are input to an IQ modulator
5
. The IQ modulator
5
includes two mixers
5
a
and
5
b
which mix each of the I and Q signals with a signal from a first local oscillator
7
to provide resulting bandpass signals at an intermediate frequency. It should be noted that the signal which is mixed with the Q component is 90° out of phase with the signal which is mixed with the I component of the signal. This 90° phase delay is introduced by delay element
5
c.
The resulting I and Q signals which are now at the intermediate frequency are then summed by a summer
5
d
of the modulator
5
to provide a single bandpass signal.
The output of the summer
5
d
is input to a first amplifier
9
which amplifies the output of the summer
5
d.
The output of the first amplifier
9
is input to a first bandpass filter
11
which filters out any undesired components of the signal which have been introduced by the first amplifier
9
. The output of the first bandpass filter
11
is input to a first gain control block
13
which applies a gain to the signal output by the first bandpass filter
11
. The first gain control block
13
receives a control signal
13
a
which determines the amount of gain to be applied by the first gain control block
13
.
The output of the first gain control block
13
is input into a mixer
6
which also receives an input from a second local oscillator
8
. The output from the second local oscillator
8
is mixed with the output from the first gain control block
13
to provide an output signal which is at the radio frequency, i.e. the frequency at which the signal is to be transmitted by the antenna
2
.
The output of the mixer
6
is input to a second bandpass filter
15
which filters out any undesired components introduced by the mixer
6
. The output of the second bandpass filter
15
is input to a second amplifier
17
which amplifies the signal. The output of the second amplifier
17
is input to a second gain control block
10
. The second gain control block
10
receives a control signal
12
which determines the gain to be applied to the signal. In particular, the second gain control block
10
varies the amount of gain applied to the input signal in dependence on the control signal
12
. The output of the second gain control block
10
is input to a high power amplifier
14
which amplifies the signal by a fixed amount. The output of the high power amplifier
14
is output to the antenna
2
via a duplex filter
42
.
However, it is often useful to be able to measure the power of the signal which is transmitted. Accordingly, a directional coupler
16
or similar device is provided. The coupler
16
allows a small proportion of the signal to be transmitted to be removed. The power level of that small proportion of the signal is measured using a radio frequency to DC rectifier
18
, consisting of a diode and passive component(s). By suitable scaling, a voltage value indicative of the power level of the signal which is to be transmitted can be obtained.
The duplex filter
42
has a transmit portion
42
b
which is tuned to the radio frequency. The transmit portion
42
b
removes undesired components introduced by the transmissive chain. The receive frequency is different from the transmit frequency. The duplex filter
42
also has a receive portion
42
a
which is tuned to the receive frequency.
The signal to be transmitted may be either a speech or data transmission and may be a combination of the two, depending on the use being made of the transmitter. For convenience, any references hereinafter to the type of signal being transmitted will be termed speech mode and data mode of the transmitter. In speech mode, the required power of the transmitted signal may be relatively low, because the gain of CDMA systems is relatively high for low bit rate services. However, in data mode the required power of the transmitted signal may be relatively high, because the gain lowers when the user data rate is increased.
The transmitter
1
shown in
FIG. 1
is not particularly suitable for a system which requires high power control accuracy and high power control dynamic range. One example of such kind of system is CDMA. This is because in a CDMA system, the mobile station transmitter will often operate at a relatively low power level. If the arrangement shown in
FIG. 1
is used, the entire transmission chain, particularly the power amplifier, will still consume power even if the power level required for the transmitted signal is relatively low. This means that the average power consumption is high and the life of the battery between chargings is reduced.
Typically, in WCDMA systems, the information bit rate of the transmitted signal may be in the approximate range of 12.2 kbps for speech signals 144 kbps (or even up to 384 kbps) for data transmission.. It is understood that about 10.7 dB (10 log 144+12.2) less transmission power is needed for transmission at 12.2 kbps and for 144 kbps. This 10.7 dB difference in the power requirement decreases the efficiency of power amplifier in speech mode since the power efficiency of a power amplifier decreases at lower output powers.
Reference is now made to the arrangement shown in FIG.
2
.
FIG. 2
shows a transmitter
19
which is used for TDMA mobile stations and which is disclosed in U.S. Pat. No. 5,152,004. In the arrangement shown in
FIG. 2
, the signal which is to be transmitted and which is at the radio frequency is input to a power divider
20
. The power divider
20
divides a signal into two parts. One part of the signal is input to an amplifier
22
whilst the other part of the signal is input to an attenuator
24
. When a high power transmitted signal is required, the signal is amplified by the power amplifier
22
and output to the antenna
2
. However, when the transmitted signal is to have low power, the power amplifier
22
is not used and the signal only passes through the attenuator
24
to provide a lower power signal. The lower power signal is output by the attenuator
24
to the antenna
2
. Whilst power consumption is reduced, the transmitter
19
shown in
FIG. 2
has the disadvantage that the output power level will not always have a smooth transition when a change is made from the path using the power amplifier
20
and the path using the attenuator
24
. This is because the arrangement of U.S. Pat. No. 5,152,004 does not have any circuitry which can provide accurate and hence smooth power control when changes between the power amplifier and th

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