Modern digital signal generators with internal arbitrary waveform generators can be used to create the multicarrier test signals needed to evaluate 3G basestation power amplifiers.
Testing multicarrier power amplifiers (MCPAs) for wideband-code-division-multiple-access (WCDMA)/third-generation partnership project (3GPP) wireless systems places great demands on the test-signal source. To obtain the optimum test setup for MCPAs, specific spectral and statistical characteristics of WCDMA multicarrier signals must be taken into account. Fortunately, modern vector signal generators such as the SMIQ03HD from Rohde & Schwarz (Munich, Germany) can produce signals with adequate dynamic range for testing these amplifiers.
The complementary-cumulative-distribution function (CCDF) defines the statistics of a test signal for WCDMA/3GPP, specifying the signal's probability of exceeding a specific power threshold. To minimize costs, the dynamic range of the signal should be limited. This can be achieved in different ways. Tests of the US CDMA System IS-95 (cdmaOne) have shown that the CCDF of a CDMA carrier can be influenced by selecting specific Walsh code combinations.1,2 The approach can also be applied analogously to 3GPP although because of its different spreading factors (in contrast to IS-95), the approach is somewhat more complex for 3GPP. Adjacent orthogonal codes in the code domain usually yield high crest factors (Fig. 1). Combinations with the code channels spread evenly across the code domain are better. Figure 1 shows the CCDF of a 3GPP base-station signal with the five obligatory control channels and eight traffic channels. The crest factor can be decreased by about 3 dB through code selection.
Moreover, 3GPP allows a timing offset of carrier traffic channels. Since the pilot data of the DPCH is not channel-coded, all DPCH signals have the same pilot symbols. This inevitably causes constructive interference and high power levels (if the pilot data of all channels is transmitted simultaneously). Timing offset circumvents this effect, reducing the crest factor by another 2 dB.3
A similar method is used with 3GPP multicarrier signals to avoid high peak envelope power values. Although the 3GPP standard defines different scrambling codes for different carriers, this is not sufficient in practice where, for this reason, the timing of the overall signals is offset on the individual carriers. The 3GPP standard advises a delay of 1/5 slot (133 µs) for signals on adjacent carriers,4 thus helping to effectively minimize the crest factor of a multicarrier signal (Fig. 2).
Clipping is another method of minimizing the crest factor. The sum signal of a base station is mathematically reduced by means of a saturation function directly before pulses are shaped in the baseband. The advantage of this method is that it works for all signal configurations, independent of code combinations or timing offset. However, correct transmission of constellations with high instantaneous power is no longer ensured, and the bit-error rate (BER) increases. Clipping therefore requires improved forward-error correction. For this reason, producers of base stations combine all of these methods to maximize dynamic range.
Test signals for 3GPP PAs must emulate these possible variations. Moreover, nearly all parameters of a 3GPP test signal should be accurately determined to achieve comparable results. For this reason, 3GPP has defined test models. The 3GPP TS 25.141 specification4 also defines unambiguous configurations for multicarrier signals (Fig. 3).
Every amplifier affects a signal in two ways: it generates additional noise, and its transmission function is linear only over a limited domain. Nonlinear components cause intermodulation. For 3GPP signals, this results in unwanted spectral components in the adjacent channels (Fig. 3). A 3GPP carrier has a width of 3.84 MHz and channel spacing of 5 MHz. The third-order intermodulation products (IM3) are in the 1.92-to-5.76-MHz range (relative to the carrier center frequency). The IM3 and wideband noise therefore contribute to the adjacent-channel leakage power. These power components may interfere with the transmission in the adjacent channel and must be minimized by achieving maximum amplifier dynamic range. Good results are usually obtained if IM3 and wideband noise make the same contributions. Wideband noise and fifth-order intermodulation (IM5) occur in the alternate channel. Since IM5 is one order less than IM3, the IM5 contribution is negligible compared to the wideband noise. The measurand is the adjacent-channel leakage ratio (ACLR), i.e. the ratio of the power in the useful channel to the power in the adjacent channel.
The situation is slightly different for multicarrier signals. A signal with four adjacent 3GPP carriers has a width of 18.84 MHz (Fig. 3). The IM3 now occurs in the 9.42-to-28.26-MHz range, both in adjacent and in alternate channels. In this case, the amplifier has to be driven to a lower level to achieve optimum ACLR.
Measuring instruments also generate intermodulation and noise and may contribute to the measured ACLR. Figure 4 shows a quantitative recording for a spectrum analyzer. If the inherent noise of the analyzer is 5 dB less than the measured value (consisting of the input signal and inherent noise), it is still necessary to deduct just under 2 dB from the measured value to obtain the correct value of the input signal. To ensure that the measuring instruments do not significantly influence the overall result of the ACLR, they must exhibit ACLR values that are at least 10 dB better than those of the device under test (DUT). The measurement uncertainty of the ACLR value of the DUT is significantly higher if the contributions of the measuring instruments are of the same order as those of the DUT, or even if they are dominant (Fig. 4).
The 3GPP base-station standards specify a minimum value of 45-dB ACLR in the adjacent channel. Most producers aim for an ACLR of 50 dB for the entire base station. As a result, ACLR values of minimum 60 dB are obtained for the associated PAs. For these reasons, a signal generator should exhibit an ACLR of 70 dB or better in the adjacent channel, which is a great challenge when designing signal generators (Fig. 5).
As noted, the maximum dynamic range of an amplifier also depends on the signal to be amplified. For ACLR improvements, this principle can also be applied to signal generators. The SMIQ03HD vector signal generator actually features two output modes: low distortion and low noise. In the first, some internal dynamic range is sacrificed to minimize intermodulation products. This mode is optimally suited for measurements in the adjacent channel and is preferable if a 3GPP multicarrier signal is generated by a single generator. In the second mode, the generator features a high internal dynamic range to maintain low wideband noise levels. This mode is for measurements in alternate channels with 3GPP single-carrier signals.
I/Q filters suppress the wideband noise of the baseband modules, thus improving the ACLR. Although these I/Q filters do influence the I/Q signals, it is possible to avoid higher vector errors by means of appropriate precorrection of the baseband signal. The SMIQ03HD provides this for every signal, whether it is generated internally or calculated externally and replayed by means of the internal arbitrary waveform generator. I/Q filters for 3GPP signals with one to four carriers are available for four different I/Q bandwidths (2.5/5/7.5/10 MHz). The I/Q filters can also be used for all RF output frequencies and levels.
To achieve maximum dynamic range in the ACLR, band-optimized solutions should be considered. The SMIQB57 option for the SMIQ03HD is based on a surface-acoustic-wave (SAW) filter operating at 380 MHz. The filter processes a generated single carrier without influencing the carrier edges and attenuates adjacent-channel interference with typically 31-dB suppression. A subsequent PA stage increases the RF signal to an output power of +30 dBm peak envelope power (PEP).
By using the SMIQB57 option, the SMIQ03HD achieves ACLR values of typically 77 dB in the adjacent channel and typically 82 dB in the alternate channel for a 3GPP single-carrier signal (3GPP test model 1, 64DPCH). These values satisfy the most-stringent demands and were previously never achieved without resorting to external bandpass filters (which are limited to a single frequency and increase insertion loss) or interference methods.
Another strategy for expanding the dynamic range is based on the interference method. In this approach, reference and test-signal paths are created by means of a power splitter. Unfortunately, this approach is not suitable for production due to the difficulty of matching the delay of the reference-signal path to that of the measurement-signal path.
Fortunately, the SMIQB57 option simplifies this approach, allowing the generation of reproducible RF level settings in particular for amplifiers using feedforward correction. By using an internal arbitrary waveform generator and personal-computer (PC) software, it is possible to generate 3GPP single-carrier and multicarrier signals as well as combinations of 3GPP carriers and signals of other standards.
Figure 6 shows a typical setup for testing WCDMA MCPAs. To generate a WCDMA/3GPP four-carrier signal with optimum ACLR, four R&S SMIQ03HD generators, fitted with the high ACLR R&S SMIQB57 option, are linked by means of a four-port power combiner. Timing delays between the individual carriers are implemented via suitable generator triggering. Power sensor 1 measures the power of the sum signal after the combiner, while power sensor 2 measures the reflected power. The output power can be exactly determined by means of power sensor 3. For spectral and ACLR measurements, a spectrum analyzer (such as a model FSQ) featuring wide dynamic range and high linearity is connected to the second port of the output coupler.
Since measurement of ACLR is a relative power measurement, the high linearity of the spectrum analyzer can be exploited. However, the use of a power meter, such as the R&S NRP with a model NRP-Z11 power sensor, is indispensable for a sufficiently accurate measurement of the absolute power of the amplified signal. It allows simultaneous operation of as many as four power meters with a dynamic range of 90 dB. In addition, it also measures wideband modulated signals with a precision associated with thermal power meters.
The dynamic range achieved by means of this test setup can be easily determined by measuring the ACLR without the DUT. By using four Vector Signal Generators R&S SMIQ03HD with a 3GPP four-carrier signal, an ACLR of typically 74 dB in the adjacent channel and typically 76 dB in the alternate channel (Fig. 7) can be achieved.
A one-generator approach offers the most cost-effective solution for testing MCPAs (Fig. 8). In this method, a four-carrier signal is generated by means of the SMIQ03HD's arbitrary waveform generator. The arbitrary-waveform-generator signal includes the required timing delay between carriers. The one-generator approach does not provide the same ACLR dynamic range as a four-generator approach since the bit resolution of the digital-to-analog converter (DAC) in the arbitrary waveform generator and the wideband noise of the I/Q modulator limit the available dynamic range. For each 3GPP four-carrier signal (always test model 1, 64DPCH), typically 62 dB in the adjacent channel and typically 64 dB in the alternate channel can still be obtained.
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