Telecommunications – Transmitter – With feedback of modulated output signal
Reexamination Certificate
1999-05-24
2003-07-08
Appiah, Charles N. (Department: 2682)
Telecommunications
Transmitter
With feedback of modulated output signal
C455S091000, C330S151000, C375S296000, C375S297000
Reexamination Certificate
active
06591090
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a transmitter and a method for transmitting a signal. In particularly, but not exclusively, the present invention relates to a transmitter and a transmitting method for 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
The signal which is modulated prior to transmission is generally modulated using a digital modulation method. If the transmitter is not linear, spectrum spreading to adjacent channels can occur. This leads to a reduction in the system capacity. If the transmitter is linear or substantially linear, the problem of spectrum spreading to adjacent channels can be reduced. The linearity of the transmitter is largely dependent on the operating characteristics of the power amplifier. Highly linear power amplifiers could be used to reduce the amount of spectrum spreading to adjacent channels. However, the power efficiency of linear amplifiers is poor. Less linear amplifiers are more efficient and in particular consume less power for the required amplification.
It has therefore been proposed to use nonlinear amplifiers in spread spectrum transmitters but with compensation for the non-linearity of the amplifier. One method of compensation is digital predistortion. With this method, before a signal is input to a power amplifier, it is predistorted in a nonlinear manner. This predistortion is the inverse of the distortion which is applied by the amplifier. Accordingly, the predistorted signal is input to the amplifier which provides a linear output. However, whilst this method provides improved power consumption if the signal is to be transmitted with a relatively high power level, the power efficiency is lower when the signal is transmitted with a lower power level. This is because the predistortion part of the transmitter consumes the same amount of power regardless of the power level of the signal to be transmitted. Since CDMA mobile stations will tend to use lower power levels, there may be little power saving as compared to simply using a linear power amplifier.
Another problem in predistortion is related to power control. An adaptive predistorter, which is not capable of real time compensation of the non-linearity of the amplifier, requires a certain amount of time to adapt if the non-linearity is changed. In known transmitters, the power control uses adjustable analogue gain and/or attenuation. In practice, this means that the power is changed after predistortion and prior to amplification. Although the change in power level does not necessarily change the characteristics of the amplifier, it does change the non-linearity which is seen by the predistorter. This is because every output point of the predistorter corresponds to a certain input point of the amplifier, and an unexpected gain or attenuation occurring between the predistortion and the amplification causes a shifting of the signal so that those points do not correspond to each other anymore.
FIG. 1
shows such an arrangement which is known from U.S. Pat. No. 5,049,832. The transmitter
50
has a power amplifier
52
which operates in a non linear manner. The signal which is to be transmitted is input to an address calculator
54
which calculates an address from the input signal. The calculated address corresponds to a location in a look up table (LUT)
56
. The values at the calculated address are output to a multiplier which predistorts the signal in accordance with these values. The transmitter also includes an adaption algorithm block which receives part of the signal to be transmitted. That signal is compared by the adaption algorithm block
60
with the signal which is input to the multiplier
62
. The same signal which is input to the multiplier
62
is input to a delay block
58
which outputs that signal to the adaption algorithm block
60
after a predetermined delay. This ensures that the part of signal to be transmitted is compared by the adaption algorithm block
60
with the corresponding input signal. If the transmitted signal is not linear with respect to the input signal, the adaption algorithm block
60
calculates corrected values which are output to the LUT
56
and stored in the respective address. This arrangement suffers from the problem discussed hereinbefore, and especially it does not consider the problems associated with power control.
SUMMARY OF THE INVENTION
It is an aim of certain embodiments of the present invention to provide a transmitter including predistortion which is power efficient and which takes into account the problem associated with power control.
According to a first aspect of the present invention, there is provided a transmitter comprising: amplifier means for amplifying a received signal; predistortion means for predistorting a received signal prior to the signal passing through said amplifier means; transmitting means for transmitting a signal; and control means for controlling said transmitter, whereby if the power level of the signal to be transmitted by the transmitter means is below a predetermined level, the signal is not predistorted by said predistortion means and if the power level of the signal to be transmitted by the transmitter means is above a predetermined level said received signal passes through said predistortion means and said amplifier means.
Preferably, said predistortion means is arranged substantially to compensate for the non-linearity of said amplifier means, when used.
Thus, the predistortion means is only used when the power level of the transmitted signal is relatively high and the non-linearity of the amplifier means is most likely to cause problems. The predistortion means thus compensates for the nonlinear characteristics of the amplifier means. If the amplifier means are nonlinear, more efficient use of power can be achieved. However, when the power level falls below a predetermined level, the predistortion means are not used, which may allow the power required to operate the predistortion means to be saved. As the power level is much lower, any distortion at the output of the amplifier means may not cause significant problems in terms of interference with adjacent channels.
Preferably, when the power level of the signal is below the predetermined level, the predistortion means is switched off. This allows power to be saved.
A bypass path is preferably provided for bypassing said amplifier, whereby if a power level of a signal to be transmitted by the transmitting means is below a given level (which may be the same as the predetermined level), a received signal passes through the bypass path. Thus, the amplifier means may also be turned off in this situation.
Preferably, bias control means are provided for controlling the biassing applied to the amplifier means, whereby when the power level of the signal to be transmitted by the transmitting means is above the predetermined level, then the amplifier means is controlled by the bias control means to operate in a more power efficient class. In practice this means that the amplifier means may be less linear. This gets the maximum power efficiency out of the transmitter even allowing for the extra power consumption required by the predistortion means.
If the power level of the signal to be transmitted by said transmitter is below the predetermined level, the amplifier means are controlled by the bias control means to operate substantially linearly. Thus, the signal may pass through the amplifier which as it is controlled to operate in a linear fashion, gives a linear output.
The amplifier means may comprise a plurality of amplifiers arranged in series. The bypass path may bypass at least one of said plurality of amplifiers and the received signal may be arranged to pass through at least one amplifier, and in another mode of operation, a different number of amplifiers may be bypassed.
In this arrangement, a plurality of amplifiers are connected in series at least some of which ar
Lilja Harri
Vuorio Juha
Appiah Charles N.
Nokia Mobile Phones Limited
Perman & Green LLP
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