High-quality power ramping in a communications transmitter

BACKGROUND OF THE INVENTION

1. Field of the Invention

0001The present invention relates to power ramping in a communications transmitter.

2. State of the Art

0002High quality RF (radio frequency) signals must ramp quickly from a condition of minimal output power to a condition of information-bearing modulation at a specified output power and back down to the condition of minimal output power. Such power ramping capability, illustrated in <figref idref="f0001">Figure 1</figref>, is required for transmitters in many time division multiple access (TDMA) communication systems. Example systems include those specified by the GSM and ANSI-136 standards, and combinations of the same (so-called multi-mode systems).

0003A fundamental requirement of these transmitters is that the acts of ramping up and ramping down must not violate specified limits on peak power in spectral bands away from the assigned RF channel (e.g., bands that would be allocated to other transmitters); the associated measurement is called the transient spectrum in some systems or the transient adjacent channel power (transient ACP) in others.

0004Present power ramping techniques must be tailored for each modulation type, and typically require unit-by-unit calibration (at least in the same of typical GMSK transmitters and conventional multi-mode transmitters). Even so, transient ACP performance is usually very sub-optimal.

0005<patcit id="pcit0001" dnum="EP0895363A"><text>Patent application EP 0895 363</text></patcit> discloses a digital modulator in which modulation data or modulated signals are prevented from generating interference signals with adjoining circuit through control when power supply is turn on or system is changed. At turning on the transmission data is modulated with fixed data for a time required for completing a required system constitution set in.

0006<patcit id="pcit0002" dnum="EP0800267A"><text>Patent application EP 0800 267</text></patcit> discloses a transmission apparatus, with a simplified ramp circuit for performing smooth transition when turned on and off. The apparatus uses a mapping circuit for mapping the data to be transmitted in orthogonal coordinates. The data are converted to analog, to form channels of the analog data corresponding to the digital orthogonal data. Each channel is provided with filters means for band restricting outputs signals.

0007The present invention is applicable to both conventional (I/Q) and polar modulation architectures. Polar modulation architectures, and similar architectures in which separate amplitude and phase paths are provided, are described, for example, in <patcit id="pcit0003" dnum="US6191653B"><text>U.S. Patents 6,191,653</text></patcit>, <patcit id="pcit0004" dnum="US6194963B"><text>6,194,963</text></patcit>, <patcit id="pcit0005" dnum="US6078628A"><text>6,078,628</text></patcit>, <patcit id="pcit0006" dnum="US5705959A"><text>5,705,959</text></patcit>, <patcit id="pcit0007" dnum="US6101224A"><text>6,101,224</text></patcit>, <patcit id="pcit0008" dnum="US5847602A"><text>5,847,602</text></patcit>, <patcit id="pcit0009" dnum="US6043707A"><text>6,043,707</text></patcit>, and <patcit id="pcit0010" dnum="US3900823A"><text>3,900,823</text></patcit>, as well as <patcit id="pcit0011" dnum="FR2768574"><text>French patent publication FR 2768574</text></patcit>.

0008<patcit id="pcit0012" dnum="EP0535669A"><text>EP 0 535 669</text></patcit> discloses a burst signal transmitter with a ramp generator.

0009The present invention is defined in claim 1. Embodiments of the invention are defined by the features of claim 2 and 3.

BRIEF DESCRIPTION OF THE DRAWING

0010The present invention may be further understood from the following description in conjunction with the appended drawing. In the drawing: <ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001">Figure 1</figref> is a diagram illustrating power ramping in a communication system;</li><li><figref idref="f0001">Figure 2</figref> is a diagram illustration operation of a conventional QAM modulator using a pulse shaping filter having an impulse response given by p(t);</li><li><figref idref="f0001">Figure 3</figref> is a diagram of one example of p(t);</li><li><figref idref="f0001">Figure 4</figref> is a diagram illustrating operation of a QAM modulator using prepended and appended zero-valued symbols to control ramping;</li><li><figref idref="f0002">Figure 5</figref> is a timing diagram of timing signal used in with the circuitry of <figref idref="f0003">Figure 6</figref>;</li><li><figref idref="f0003">Figure 6</figref> is a diagram of a portion of a transmitter including ramp control circuitry in accordance with an exemplary embodiment of the invention;</li><li><figref idref="f0004">Figure 7</figref> is a signal plot of results obtained using the ramp control circuit of <figref idref="f0003">Figure 6</figref>;</li><li><figref idref="f0004">Figure 8</figref> is a diagram of a pulse shaping filter function p(t) used in the example of <figref idref="f0004">Figure 7</figref>;</li><li><figref idref="f0005">Figure 9</figref> is an exploded view of the rising edge of the ramp of a signal plot like that of <figref idref="f0004">Figure 7</figref>;</li><li><figref idref="f0006">Figure 10</figref> shows the rising edge of the ramp of <figref idref="f0005">Figure 9</figref> when viewed on a logarithmic (dB) scale;</li><li><figref idref="f0007">Figure 11</figref> is similar to <figref idref="f0005">Figure 9</figref> but shows the falling edge of the ramp;</li><li><figref idref="f0008">Figure 12</figref> is a block diagram illustrating application of the present ramping technique in a polar modulation architecture;</li><li><figref idref="f0005">Figure 13</figref> is a diagram of a pulse shaping filter function n(t) used for D-AMPS; and</li><li><figref idref="f0006">Figure 14</figref> is a diagram of a portion of a communications transmitter implementing ramping for D-AMPS.</li></ul>

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

0011For nearly all systems of interest, the complex envelope x(t) of an information-bearing modulation can be expressed by the well-known equation<maths id="math0001"><math display="block"><mi>x</mi><mfenced><mi>t</mi></mfenced><mo>=</mo><mstyle displaystyle="false"><mstyle displaystyle="true"><munder><mo>∑</mo><mi>n</mi></munder></mstyle><msub><mi>a</mi><mi>n</mi></msub><mo>⁢</mo><mi>p</mi><mo>⁢</mo><mfenced><mi>t</mi><mo>-</mo><mi mathvariant="italic">nT</mi></mfenced></mstyle></math><img file="EP1517452B1_D0001.tif" /></maths> which is equivalent to<maths id="math0002"><math display="block"><mi>x</mi><mo>⁢</mo><mfenced><mi mathvariant="italic">nT</mi><mo>+</mo><mi>τ</mi></mfenced><mo>=</mo><munderover><mo>∑</mo><mrow><mi>k</mi><mo>-</mo><mn>0</mn></mrow><mi>L</mi></munderover><msub><mi>a</mi><mrow><mi>n</mi><mo>-</mo><mi>k</mi></mrow></msub><mo>⁢</mo><mi>P</mi><mfenced><mi mathvariant="italic">Kt</mi><mo>=</mo><mi>τ</mi></mfenced></math><img file="EP1517452B1_D0002.tif" /></maths> where a_n is the n-th complex-valued symbol (typically drawn from a discrete constellation), p(t) is the impulse response at time t of a pulse-shaping filter, and T is the symbol period. Time t can be either continuous or discrete. Operation of a conventional QAM modulator using a pulse shaping filter having an impulse response given by p(t) is illustrated in <figref idref="f0001">Figure 2</figref>. Due to the desire to maintain spectral efficiency, p(t) is typically a smooth pulse-like function as shown for example in <figref idref="f0001">Figure 3</figref>.

0012An imported observation, previously unknown either with respect to polar modulators or conventional modulators, is exploited in accordance with the present invention to achieve ramping having the advantageous characteristics previously mentioned. It is that by prepending and appending a few zero-valued symbols to the finite-length sequence of information symbols belonging to a burst, the resulting complex envelope x(t) naturally ramps us and down precisely as required. Furthermore, it can be shown mathematically that the transient spectral properties of x(t) during these ramps are no worse than during the information-bearing modulation. A diagram illustrating operation of a QAM modulator using prepended and appended zero-valued symbols to control ramping is shown in <figref idref="f0001">Figure 4</figref>.

0013<figref idref="f0003">Figure 6</figref> shows a portion of a transmitter including ramp control circuitry in accordance with an exemplary embodiment of the invention. Prior to describing the circuitry of <figref idref="f0003">Figure 6</figref>, it will be useful to understand the relationship of certain timing signals used in the circuitry of <figref idref="f0003">Figure 6</figref>. These timing signal are shown in <figref idref="f0002">Figure 5</figref>. A sample clock signal is divided by some number T to obtain a symbol clock. A τ counter counts the sample clock pulse within one period of the symbol clock. In the example of <figref idref="f0002">Figure 5</figref>, T=4.

0014Referring now to <figref idref="f0003">Figure 6</figref>, a pulse shaping filter 601 having impulse response coefficients p (0) , p (I) , ..., p((L+1)T-1) receives from a tapped delay line or shift register a group of symbols a_n, a_n-1, a_n-2, ..., a_n-L. (For purpose of the present description, a shift-register implementation will be assumed.) As τ cycles through 0, 1, 2, ..., T - 1, the indices τ, τ + T, τ + 2T, ..., τ + LT select a subset of the impulse response coefficients for application within the circuit at a particular time. The subsets of impulse response coefficients applied at a particular time may be described as follows: at τ = 0, the subset is {p(0), p(T), ..., p(LT)}; at τ = 1, the subset is {p(1), p(T+1),..., p(LT+1)}; at τ = 2, the subset is {p(2) p(T+2),..., p(LT+2}, and so forth, until at τ = T-1, the subset is {p(T-1), p(T-1+T), ...; p(T-1+LT)}. Hence, as τ cycles through 0, 1, 2, ..., T - 1, the entire range of impulse response coefficients p (0) , p(1) , ..., p((L + 1)T - 1) will have been applied.

0015The pulse filter 601 forms an output signal 603 given by x(nT + τ), which is modulated using an I/Q modulator or polar modulator 605 to form an RF signal 607. Prepending and appending of zero-valued symbols for ramp control is accomplished by inputting values to a shift register 608 through an input selector or switch 609, connected to either a source of information symbols 611 or to a source of zero values 613. A sample clock 615 is input directly to the pulse-shaping filter 601, and is input also to a τ counter 617 and a divide-by-T counter 619. The τ counter produces a count 621 that is input to the pulse-shaping filter 601. The divide-by-T counter 619 produces from the sample clock a symbol clock 623 that is input to the shift register 608 and applied to clock the individual stages of the shift register.

0016In operation, upon receipt of the first information-bearing symbol a₀, the initial state (n=0) of the shift register 608 is a_n-1 = a_n-2 ... =a_n-L = 0. As additional symbols are received, they are shifted into the shift register. With each tick of the sample clock, the counter or index τ is updated, modulo T; τ therefore cycles through the sequence 0, 1, ..., T-1, 0, 1, ..., T-1, .... After the last information symbol enters the shift register 608, the input selector 609 switches to accept zeros during the next L ticks of the symbol clock, until the shift register 608 state is an = ... = a_n-L+1 = 0 and a_n-L = a_N-1, where N is the number of symbols in a burst. In this state, the ramp-down is complete once τ = T-1 is reached.

0017Results of this technique for one complete burst with N = 148 symbols are shown in the plot of <figref idref="f0004">Figure 7</figref>. In this example, an EDGE pulse, illustrated in <figref idref="f0004">Figure 8</figref>, was used, with T = 4 (i.e., four samples per symbol) and L + 1 = 5 (i.e., a shift register of length five).

0018<figref idref="f0005">Figure 9</figref> shows an exploded view of the rising edge of the ramp of a signal plot like that of <figref idref="f0004">Figure 7</figref>, annotated to show the value of the τ counter 617 at each output sample, along with the contents of the shift register 608, updated as each new symbol is input. Note that the ramp up is basically complete within three symbols periods of the first information symbol entering the shift register.

0019<figref idref="f0006">Figure 10</figref> shows the rising edge of the ramp when viewed on a logarithmic (dB) scale. Here it can be seen that the signal amplitude during the first symbol period is over 40dB down from the peak. In most systems (including those complying with the EDGE specification), such small signal components can be significantly distorted (e.g., clamped at zero) without causing measurable degradation of system performance (e.g., transient ACP). Various power amplifier control signals can therefore be abruptly switched during such low-amplitude times without performance degradation, described in greater detail hereinafter.

0020<figref idref="f0007">Figure 11</figref> is similar to <figref idref="f0005">Figure 9</figref> but shows the falling edge of the ramp, with zero-valued symbols entering the shift register after the last information symbol. This appending of zero-valued symbols is accomplished in the example embodiment of <figref idref="f0003">Figure 6</figref> when the input selector switches to the zero source, after the symbol clock at index n = 147 but before the next symbol clock at n = 148.

0021<figref idref="f0008">Figure 12</figref> is a block diagram illustrating application of the present ramping technique in a polar modulator architecture, i.e., one having separate amplitude and phase paths. A symbol source 1201 inputs data symbols to a pulse modulator 1203, such as an EDGE QAM modulator, in accordance with a symbol clock 1205. The modulator produces an envelope signal 1207, for example an envelope signal like that of <figref idref="f0004">Figure 7</figref>, given by x(nT + τ). The envelope signal is processed by a rectangular-to-polar converter 1109 (such as a Cordic converter), producing magnitude and phase signals p and θ.

0022In an exemplary embodiment, the latter signals are corrected for non-linearities and are time aligned to account for path delay differences. Hence, the magnitude signal is applied to an AM/AM look-up table 1211, an output p' of which is delayed a controlled amount by a magnitude delay element 1213 to produce an output ρ". Similarly, the phase signal is applied to an AM/PM look-up table 1215, an output θ' of which is delayed a controlled amount by a phase delay element 1217 to produce an output θ". The delays of the magnitude delay element and the phase delay element are controlled to achieve proper magnitude and phase alignment at an amplification chain 1220.

0023The amplification chain 1220, in an exemplary embodiment, includes three cascaded stages, realized for example using FET devices. The stages are drain modulated and driven in switch mode or, for low-power operation, in "multiplicative" mode An RF input port 1221 of the amplification chain may be regarded as the phase port, and the drains (or power supply inputs) of the stages may be regarded together as the amplitude port 1223.

0024The amplitude port is driven by a driver circuit 1225, responsive to the signal ρ" and to a power level input signal 1227.

0025The phase port is driven by a digital phase modulator 1230, preferably a digital phase modulator having a phase-stable frequency locked loop as described in <patcit id="pcit0013" dnum="US6094101A"><text>U.S. Patent 6,094,101</text></patcit> of the present assignee, in combination with a VCO 1231. The digital phase modulator 1230 is isolated from the amplification chain 1220 using a variable gain amplifier (VGA) or a variable attenuator that is responsive to another power level input signal. Alternatively, the digital phase modulator may be isolated from the power amplifier using a buffer amplifier. These alternative are represented in <figref idref="f0008">Figure 12</figref> by a variable gain amplifier 1233 that may have a gain that is zero (in the case of the buffer amplifier), negative (in the case of an attenuator) or positive.

0026A timing control block 1240 provides timing signals to the symbol source and to the driver circuit, as well as to the buffer amplifier, if present.

0027The transmitter of <figref idref="f0008">Figure 12</figref> is mainly digital, the digital and analog (right-portions) being separated by a dashed line.

0028The same principles described thus far, particularly with respect to ramping of the EDGE modulator, may be readily extended to embrace other modulation types, such as IS-136, also known as North American Digital Cellular or D-AMPS. The particulars of D-AMPS, however, require certain modifications to the foregoing approach.

0029In particular, the pulse shape used in D-AMPS, shown in <figref idref="f0005">Figure 13</figref>, is theoretically of infinite duration (unlike the EDGE pulse, which has a duration of 5 symbol periods). Of course, in practice, this infinite-duration pulse is truncated, the choice of the truncation interval (i.e., interval outside of which the pulse is truncated) determining the spectral characteristics (including ACP and transient ACP) of the output signal. Using the foregoing method of ramping, to obtain low side lobes, a truncation interval in the range of 8-16 symbols periods would be required, corresponding to a ramp-up time in the range of 4-8 symbol periods and a ramp-down time in the range of 4-8 symbols periods. Unfortunately, such prolonged ramp times exceed the 3 symbol period duration specified in the D-AMPS standard. Therefore, in order to use the foregoing method for D-AMPS, or for multiple QAM modulations including EDGE, D-AMPS, etc., a mechanism of ramp acceleration is required whereby the prolonged ramp times of D-AMPS may be shortened to satisfy the specified ramp mask.

0030One way of achieving such ramp acceleration is illustrated in <figref idref="f0006">Figure 14</figref>. Here, a D-AMPS QAM modulator 1401 is provided, zero-valued symbols being prepended and appended to the information symbols belonging to a burst, as previously described in relation to EDGE. The modulator produces a digital output signal 1403 having a prescribed symbol rate. This digital output signal is applied to a discard unit 1405 controlled by a control signal 1407 from a timing generator (not shown). During ramp-up and ramp-down, a control signal is applied to the discard unit to cause it to discard selected samples (which has the equivalent effect of accelerating the time base). For example, every other sample may be discarded, resulting in 2X acceleration. During the information burst, the discard unit passes the sample stream from the modulator unchanged.

0031In an exemplary embodiment, the ramp-up and ramp-down times using ramp acceleration are three symbols times in duration, satisfying the specified ramp mask.

0032Since the signal at the original sampling rate is oversampled and is naturally bandlimited, discarding every other symbol does not create spectral side lobes or aliasing, and does not destroy signal information.

0033Various other means of accomplishing ramp acceleration will be apparent to those skilled in the art. For example, instead of the discard unit, an arbitrarily variable sample rate converter (sometimes referred to as an asynchronous sample rate converter) of a type known in the art may be used. Using such a sample rate converter, the desired acceleration, instead of being limited to discrete values, may be arbitrarily chosen.

0034Thus there has been described a polar modulator architecture, amendable to a high level of integration, that enables ramping of both QAM (e.g., EDGE, D-AMPS) and non-QAM (e.g., GMSK) signals, and enabling glitch-free on-the-fly switching between different modulations (e.g., EDGE and GMSK). No unit-by-unit calibration is required, allowing ramp shapes to be fixed at design time. Timing control signals can also be fixed at design time, since they relate mainly to digital events or conditions. The particular ramping methods described produce narrow rising and falling edge ramps and very low transients (i.e., very good transient spectrum characteristics).

0035It will be appreciated by those of ordinary skill in the art that the invention can be embodied in other specific forms. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restrictive.