Patent Application
20100233977
The present invention relates to multi-mode radio transmitters and to a method of operating such transmitters. The invention has particular, but not exclusive, application to hybrid polar radio transmitters.
One of the key drivers in determining what constitutes the best architecture for a radio transmitter, especially for battery-powered applications, is the achievement of high power efficiency.
The Global System for Mobile Communications (GSM) is a second-generation (2G) cellular radio standard that uses constant-envelope modulation. It is therefore comparatively easy to achieve a high power efficiency in the transmitter of handsets targeted at this standard, as the power amplifier (PA) can be operated in saturation. Operating the PA in saturation has established a benchmark for power efficiency that the market expects to be maintained in products targeted at more advanced cellular radio standards than GSM.
Third-generation (3G) cellular radio standards, such as Code Division Multiple Access 2000 (CDMA2000) and the Universal Mobile Telecommunication System (UMTS), as well as transitional (2.5G) standards, such as Enhanced Data Rates for GSM Evolution (EDGE), all use non-constant-envelope modulation schemes. Standard architectures for the transmitter of handsets targeted at these applications necessarily operate the PA linearly, and this makes it difficult to achieve a power efficiency that looks attractive.
Three different techniques can be considered in the quest to improve the power efficiency of transmitters for handsets targeted at cellular radio standards that use non-constant-envelope modulation. Efficiency control involves adjusting the power supply voltage to the PA in accordance with the average RF output power that is being demanded. This improves the power efficiency at lower than the maximum RF output power by eliminating headroom when it is not needed. Since the rate at which the average RF output power can be changed is usually limited, efficiency control can almost always be applied. Envelope tracking extends this principle by adjusting the power supply voltage to the PA in accordance with the instantaneous RF output power that is being demanded by the modulation. This further improves the power efficiency, even at the maximum RF output power, but puts greater demands on the system, especially in those parts associated with the PA power supply, as the rate at which the instantaneous RF output power changes depends on the bandwidth of the amplitude component of the modulation. This bandwidth can be up to a factor of 10 greater than that of the baseband representation of the modulation. However, as the degree to which the supply track is made to follow the envelope of the modulation is a matter of choice in the case of envelope tracking, the approach is feasible for most of the current cellular radio standards and only becomes impracticable for standards that use an extremely wide bandwidth. Polar modulation looks superficially similar to envelope tracking but pushes the principle one stage further by operating the PA in saturation. The PA is driven by a constant-envelope RF input signal that contains only the phase component of the modulation, and adjusting the power supply voltage to the PA in accordance with the instantaneous RF output power being demanded then re-instates the amplitude component of the modulation, at high level, in the PA. This achieves the highest power efficiency, but puts even greater demands on the transmitter system, as timing issues become more severe, and the PA no longer offers any rejection of noise on its power supply. Polar modulation is therefore generally more difficult to implement than envelope tracking, especially for the wide-bandwidth standards.
Referring to
A modulator control means 58 controls a quadrature generation means 110 and a power supply control means 120. A primary function of the modulator control means 58 is to set the desired average output power level of the PA 40.
The quadrature generation means 110 generates, at baseband, quadrature related signal components, i.e. an in-phase component (I) and a quadrature phase component (Q), from the input data. The quadrature generation means 110 comprises a quadrature generator 12, 14, for generating, in the digital domain, the baseband I and Q components from the input data. The baseband signal path of the quadrature generation means 110 comprises digital-to-analogue (DAC) converters 22, 24 for converting the I and Q signal components from the digital to analogue domain, filters 26, 28 for filtering the analogue I and Q signal components and amplifiers 30, 32. The DACs 22, 24 may be coupled to the modulator control means 58 as DAC parameters, such as offset voltage, and scaling the maximum output of the DACs 22, 24 may depend on the control exerted by the modulator control means 58.
Optionally, predistortion means 18, 20 for predistorting the I and Q components may be included to compensate for distortion introduced by modulator 100 elements or by other elements of the transmitter. Such predistortion means 18, 20 may be coupled to the modulator control means 58 as the predistortion required may depend on the control exerted by the modulator control means 58, particularly the average output power of the transmitter.
The modulator 100 further comprises a quadrature modulation means 34 which modulates the carrier by mixing the I and Q signal components from the amplifiers 30, 32 with respective quadrature related components of the carrier signal and delivers the combined components on the first output 106. In particular, the quadrature generation means 110 is adapted to generate I and Q signal components for a modulation scheme that, on transmission, has a non-constant envelope carrier signal, such as required for example for UMTS.
In its basic form as described so far, the modulator 100 is used with a PA 40 that does not saturate but remains linear throughout its operating range. In order to control the average output power of such a non-saturating PA 40, for example as required for closed loop power control in a cellular mobile communication system, the modulator control means 58 controls the amplitude of the I and Q signal components delivered by the quadrature generation means 110.
The modulator 100 further comprises a power supply control means 120 for generating the power supply control signal at the second output 108. The power supply control means 120 has an input coupled to receive signals from the quadrature generation means 110. In particular, the power supply control means 120 is adapted to generate a power supply control signal that tracks the envelope of the modulated carrier signal appearing at the first output 106. Envelope tracking in this way enables power efficiency to be improved by ensuring that, as the carrier signal envelope fluctuates, the DC supply voltage is maintained at the minimum level required for the PA 40 to accurately amplify the fluctuations.
The power supply control means 120 comprises generation means 46 for generating the power supply control signal from the baseband I and Q signal components. To facilitate this, it may be convenient to extract the baseband I and Q signal components from the quadrature generator 12, 14 before completion of all the required baseband processing on the I and Q components, as denoted by separate processing elements 12 and 14.
The power supply control signal is scaled by a scaling means 48 which is coupled to the modulator control means 58 for control of the extent of scaling and control of DC offset.
Optionally, a predistortion means 50 for predistorting the power supply control signal may be included to compensate for distortion introduced by modulator 100 elements or by other elements of the transmitter. Such predistortion means 50 may be coupled to the modulator control means 58 as the predistortion required may depend on the control exerted by the modulator control means 58.
The power supply control signal path of the power supply control means 120 comprises a DAC 52 for converting the power supply control signal from the digital to analogue domain, and a filter 54 for filtering the analogue power supply control signal. The DAC 52 may be coupled to the modulator control means 58 as DAC parameters, such as offset voltage, and scaling of the maximum output of the DAC 52 may depend on the control exerted by the modulator control means 58. The power supply control signal path further includes a buffer amplifier 56 for driving the DC power supply 44.
It is also possible for the transmitter shown in
A key feature of the transmitter shown in
It is an object of the present invention to improve the power efficiency of radio transmitters capable of operating in constant envelope and non-constant envelope modes.
According to a first aspect of the present invention there is provided a method of operating a multi-mode transmitter in which an input signal is modulated independently of controlling the drive of a power amplifying means.
In implementing the method in accordance with the first aspect of the present invention the amplitude information is separated from phase information in the input signal to be transmitted. The phase information is used to produce a modulated constant envelope real signal at the frequency of the transmitter and the amplitude information is used to amplitude modulate the constant envelope signal. More particularly the amplitude modulation is applied in a selected one of two modes, in a first of the two modes the amplitude modulation is applied as a low level signal prior to power amplification in the power amplifying means operating in a linear envelope tracking mode in which an envelope tracking signal derived from the amplitude information is applied to the power amplifying means at high level and in a second of the two modes the constant envelope real signal is multiplied by a fixed voltage signal prior to being applied to the power amplifying means operating in a saturation mode and the amplitude modulation is applied to the power amplifying means at high level, the selection of the first or second of the two modes being dependent on the characteristics and required output power of the signal being transmitted.
According to a second aspect of the present invention there is provided a multi-mode transmitter comprising an input for an input signal, modulating means for producing a modulated signal, power amplifying (PA) means coupled to the modulating means, the PA means having a control voltage input, and means for providing a PA control voltage to be applied to the control voltage input independently of the modulating means.
In an embodiment of the multi-mode transmitter made in accordance with the second aspect of the present invention there is provided means for deriving separately phase (8) and amplitude (R) components present in an input signal, means for producing from the phase component information a modulated constant envelope real signal at the operating frequency of the transmitter, first means for producing from the amplitude (R) component information a first amplitude signal comprising a substantially faithful representation of the amplitude component of the input signal, multiplying means having a first input for the real signal and a second input coupled in a first optional condition to the first means for applying the first amplitude signal to effect amplitude modulation of the real signal for envelope tracking or in a second optional condition to means for setting the second input to a fixed voltage for polar modulation, the multiplying means having an output coupled to the power amplifying means, second means for producing a second amplitude signal from the amplitude (R) component information, a power control voltage generating means having a control input coupled to receive the second amplitude signal, the power control voltage generating means having an output coupled to the control input of the power amplifying means, said second means in the first optional condition providing a power control signal enabling envelope tracking to be applied to the power amplifying means working in a linear mode or in the second optional condition providing an optionally pre-distorted power control voltage for polar modulation when the power amplifying means is operating in saturation, and control means for controlling the first and second means to operate in the first optional condition for envelope tracking and a second optional condition for polar modulation dependent on the characteristics and required output power of the signal being transmitted.
In an embodiment of the present invention the low-level signal generation is based on a polar (R and Θ) approach, rather than on a Cartesian arrangement as shown in
A phase locked loop may be used to produce the modulated constant envelope real signal. This has the advantage over the transmitter shown in
A dual-point modulation arrangement may be used within the phased locked loop.
The present invention will now be described, by way of example only, with reference to the accompanying drawings, wherein:
In the drawings the same reference numbers have been used to indicate corresponding features.
As
Referring to
The modulator comprises a real signal generator 110 having a data input 102 and an output 106 for a real signal at the operating frequency of the transmitter. The data input 102 is coupled to a base band generator 12 which produces quadrature related I and Q signals. The I and Q signals are applied to an envelope extract block 60 which produces a constant envelope output containing just the phase component I′, Q′ of the modulation (for a constant radius) at all RF output power levels. This complex constant envelope output is converted into a real output signal, at the required carrier frequency, using a fractional-N phase locked loop (PLL) 62. A differentiating stage 64 determines the rate of change of phase and this is used by a Sigma-Delta modulator 66 to determine the division ratios (N/N+1). The output from the Sigma-Delta modulator 66 is coupled to the divider 68 in the PLL 62. In the interests of brevity the remainder of the PLL 62 will not be described in detail as its structure and operation are known in the art.
If desired a dual-point modulation arrangement may be used within the phase locked loop 62.
The output 106 of the real signal generator 110 is connected to a first input 70 of a multiplier 72, a second input 74 of which receives an output from a first amplitude component circuit 78. An attenuator 80 controlled by a modulator control circuit 58 is coupled to an output 76 of the multiplier 72. An output of the attenuator is coupled to a power amplifier (PA) module 40.
The first amplitude control circuit 78 has as an input, a digitised amplitude or radius component R extracted by the envelope extract block 60. The component R is converted into an analogue voltage by a digital-to-analogue converter (DAC) 82. The analogue voltage is filtered by a low pass filter 84 and the resulting signal is applied to a buffer amplifier 86. The output from the buffer amplifier 86 is the amplitude component of modulation and is applied to one pole 87 of a two-way switch 88 which is controlled by an output from the modulator control circuit 58. A fixed bias voltage Vg1 is applied to another pole 89 of the two-way switch 88.
In operation, for envelope tracking with the PA operating linearly, the switch 88 is connected to pole 87 of the switch 88 so that a faithful representation of the amplitude component of the modulation R is supplied to the input 74 of the multiplier 72 where it is re-instated into the real signal on the first input 70 by the operation of the multiplier 72. Although this multiplier could also provide a degree of power control, it is envisaged that it will be better to keep this function separate from that of amplitude modulation, hence the reason for providing the attenuator 80 that follows. For polar modulation, with the PA 40 operating in saturation, the output of the PLL 62 is already the RF input signal required, so the second input to the multiplier is then set by the two-way switch 88 to the fixed voltage Vg1. In this mode of operation no attenuation is necessarily required.
A second amplitude control circuit 120 is provided for controlling a second amplitude component of the modulation R path which is provided for controlling the power supply voltage to the control input 41 of the PA module 40. As the second amplitude control circuit 120 is essentially the same as the circuit 120 described with reference to
The hybrid supply modulator 44 comprises an input connected to the output 108 of the second amplitude control circuit 120. A summing amplifier 124 has a non-inverting input 126 coupled to the input 122 and an inverting input 128. An output 130 of the amplifier 124 is coupled by way of a switch 132 to a low pass filter 136, an output of which is coupled to a DC/DC converter 134. The filter 136 is responsible for the frequency response of the DC/DC converter 134, as shown in
The switch 132 is a change-over switch which in one position P1 connects the DC/DC converter 134 in parallel with the linear regulator 140 and in a second position P2 connects a power control offset signal derived from the second amplitude control circuit 120 to the DC/DC converter 134.
For both polar modulation and envelope tracking, the switch 132 is coupled to position P1 so that the DC/DC converter 134 and linear regulator 140 operate essentially in parallel. In a variant, for polar modulation, the DC/DC converter may be operated independently by connecting the switch 132 to the position P2 so that the DC/DC converter 134 derives its input from a separate voltage reference source in the second amplitude control circuit 120 delivering only the DC component of the modulation supplied to the PA module 40.
The hybrid supply modulator 44 is capable of supplying the required control voltage to the PA module 40 over a bandwidth of 0 to 50 MHz. By the action of the feedback loop between the junction 142 and the inverting input 128 of the amplifier 124, the DC/DC converter supplies most of the control voltage to the control input 41 of the PA module 40 at frequencies below 200 kHz and the linear regulator supplies most of the control voltage at frequencies above 200 kHz. This is illustrated in the simple frequency plot shown in
Whilst the first amplitude control circuit 78 only ever has to provide a faithful representation of the amplitude component of the modulation, this is not true for the second amplitude control circuit 120. For envelope tracking, an appropriate degree of scaling and offset has to be applied, to keep the PA working at a constant degree of (minimal) gain compression, and prevent its RF output from collapsing at very low power supply voltages. For polar modulation, pre-distortion may need to be applied, to compensate for any lack of linearity in the amplitude modulation characteristics of the PA. It is therefore not possible to combine the functions of the first and second amplitude control circuits into one.
It is not currently possible to use the DC/DC converter to adjust the power supply voltage to the PA module 40, when this is operating in saturation, as even a state-of-the-art device is too noisy to allow typical type approval specifications to be met. Only the linear modulator 140 can be used in these circumstances. As a result, polar modulation only provides the high power efficiency that one would expect from it at RF output power levels that are close to the maximum. At lower levels, the comparatively poor power efficiency of the linear regulator, when operating alone, rapidly degrades the overall performance. By contrast, at lower RF output power levels, envelope tracking provides a higher level of performance, despite the intrinsically poorer power efficiency of the PA module 40, when operating linearly, as this is more than offset by the improvement in power efficiency that is made possible by the DC/DC converter 134 being brought into play. One of the great strengths of this architecture is its ability to move from using one technique for improving power efficiency to another, at different RF output power levels. The presence of the attenuator 80 after the multiplier 72 helps in achieving such transitions in as seamless a manner as possible.
Reference will now be made to
In order to facilitate an understanding of the various operating modes reference will be made to the truth table shown below. The footnotes beneath the table explain the abbreviations used. Whilst it might ideally be desirable to use polar modulation, in all circumstances, the table describes what operating modes will provide the highest efficiency, given the current state of the art.
Referring to
A second amplitude control circuit 120 is coupled to the hybrid supply modulator 44 which in turn is connected to the control input 41 of the PA module 40. As this arrangement is the same as shown in
The invention has been conceived in the context of the radio transmitter in cellular radio handsets needing to provide the best possible power efficiency when working on the existing 2G and the latest 2.5G and 3G standards. It is of potential application to any other radio transmitter scenarios in which the need to operate on any one of a number of different standards or at different RF output power levels has traditionally led to irreconcilable constraints on choosing what constitutes the best architecture for the task.
In the present specification and claims the word “a” or “an” preceding an element does not exclude the presence of a plurality of such elements. Further, the word “comprising” does not exclude the presence of other elements or steps than those listed.
The use of any reference signs placed between parentheses in the claims shall not be construed as limiting the scope of the claims.
From reading the present disclosure, other modifications will be apparent to persons skilled in the art. Such modifications may involve other features which are already known in the design, manufacture and use of radio transmitters and component parts therefor and which may be used instead of or in addition to features already described herein.
| Number | Date | Country | Kind |
|---|---|---|---|
| 06112024.2 | Mar 2006 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IB2007/051044 | 3/26/2007 | WO | 00 | 9/20/2008 |
