Detailed explanation of complex measurement techniques for LTE transmitter design

Modern wireless service providers are working to expand bandwidth and provide Internet Protocol (IP) services to more users. Long Term Evolution (LTE) is a new generation of cellular technology that can enhance currently deployed 3GPP networks and create important new business opportunities to meet these needs. LTE's complex and evolving architecture brings new challenges to the design and testing of networks and user equipment. One of the key challenges on the air interface is how to manage power during signal transmission.

In digital communication systems such as LTE, the power of the transmitted signal leaking into adjacent channels may interfere with the signal transmission in the adjacent channels, thereby affecting the system performance. The adjacent channel leakage power ratio (ACLR) test verifies that the system transmitter is operating within the specified limits. Given the complexity of LTE technology, it can be challenging for testers to perform this critical test quickly and accurately (see Table 1). A signal generator with LTE-specific signal generation software, a modern signal analyzer with LTE-specific measurement software, and methods optimized for the analyzer can help testers overcome this challenge.

Understanding ACLR Testing Requirements

ACLR is an important transmitter characteristic in LTE RF transmitter conformance testing. The purpose of these tests is to verify that the device under test meets the minimum requirements in the base station (eNB) and user equipment (UE). Most LTE conformance tests for out-of-band emissions are similar in scope and purpose to those for WCDMA. However, WCDMA specifies the use of root cosine (RRC) filters for transmitter measurements, and the standard does not define an equivalent filter for LTE. Therefore, LTE transmitter testing can use different filters to optimize channel in-band performance and improve error vector magnitude; or optimize channel out-of-band performance to obtain better adjacent channel power characteristics.

Given the many complex transmitter configurations that can be used in testing transmitter performance, LTE specifies a series of downlink signal configurations to test eNBs. These configurations are called E-UTRA Test Models (E-TM). They can be divided into three categories: E-TM1, E-TM2, and E-TM3. The first and third categories can be further divided into E-TM1.1, E-TM1.2, E-TM3.1, E-TM3.2, and E-TM3.3. Note: The "E" in E-UTRA comes from "enhanced", which refers to LTE UMTS terrestrial radio access; while UTRA alone refers to WCDMA.

ACLR testing requirements vary depending on whether the transmitter testing is performed on the UE or the eNB. ACLR testing performed on the UE is not as demanding as on the eNB. Transmitter testing is performed using the Reference Measurement Channel (RMC) specified for eNB receiver testing.

The 3GPP LTE specification defines ACLR as the ratio of the filtered average power centered on the specified channel frequency to the filtered average power centered on the adjacent channel frequency. The minimum ACLR consistency requirement for eNBs is specified in two scenarios: adjacent E-UTRA channel carriers of the same bandwidth (E-UTRAACLR1); and UTRA adjacent and alternate channel carriers (UTRAACLR1 and UTRAACLR2, respectively).

Different limits and measurement filters are defined for E-UTRA and UTRA adjacent channels for paired spectrum (FDD) and unpaired spectrum (TDD) operation. E-UTRA channels are measured using a square measurement filter, while UTRA channels are measured using an RRC filter with a roll-off factor of 0.22 and a bandwidth equal to the chip rate.

Overcoming ACLR Measurement Challenges

Given the complexity of LTE technology and the complexity of transmitter configurations used to test transmitter performance, standard-compliant spectrum measurements such as ACLR can be tedious. Fortunately, the advent of advanced signal evaluation tools enables engineers to make these LTE measurements quickly and accurately. Power measurements, including ACLR, are typically made using a spectrum analyzer or signal analyzer, while the required test signals are generated using a signal generator.

To better illustrate how to use these instruments, consider the following scenario: According to the specification, the carrier frequency must be set in the frequency band supported by the base station under test, as specified in paired spectrum FDD operation or unpaired spectrum TDD operation, by measuring the ACLR of the frequency offset on both sides of the channel frequency. First test with an E-TM1.1 transmit signal, where all PDSCH resource blocks have the same power. Then test with an E-TM1.2 signal (increasing and decreasing power). The E-TM1.2 configuration is very useful because it can simulate multiple users (whose devices operate at different powers). The result of this scenario is a higher crest factor, which makes it more difficult to amplify the signal without generating additional invalid spectral content (such as ACLR).

In this example, Agilent Signal Studio for LTE is connected to an Agilent MXG signal generator to generate a standard-compliant E-TM1.2 test signal with a frequency set to 2.11 GHz. The output signal amplitude—an important consideration in determining ACLR performance—is set to -10 dBm. A 5 MHz channel bandwidth is selected over a bandwidth range that extends from 1.4 to 20 MHz.

Figure 1 shows the eNB setup with selected transport channels. The bottom shows the resource allocation block diagram for the test signal. Channels 1 and 2 are the channels to be measured, and they share the downlink.

Figure 1: The resource allocation block for the E-TM1.2 test signal is shown here (bottom). The Y-axis represents frequency or resource blocks, the X-axis represents time slots or time, the white area represents channel 1, the pink area represents channel 2, and the other colors represent synchronization channels, reference signals, etc.

The output power level of channel 1 is -4.3dB. Therefore, its channel power has been reduced. The output power of channel 2 has been increased and set to 3dB. Complex power increase and decrease options can be set for different resource blocks in the resource block allocation map. The resulting composite signal has a higher peak-to-average ratio than a single channel with all resource blocks at the same power level. Amplifying such a power-increased signal can be very difficult. Insufficient power back-off in the power amplifier can result in limiting.

The test signal can then be generated using Signal Studio software running on an Agilent X-Series signal analyzer. Once the signal is generated, the waveform is downloaded to the signal generator via LAN or GPIB. The RF output of the signal generator is connected to the RF input of the signal analyzer and the ACLR performance is measured using swept spectrum analysis. In this example, the signal analyzer is in LTE mode with a center frequency of 2.11 GHz and the ACP measurement selected. A quick one-button ACLR measurement is then made in accordance with the LTE standard by calling up the appropriate parameters and test limits from a range of options available in the LTE application, such as paired or unpaired spectrum, carrier types in adjacent channels and alternate channels. [page]

For FDD measurements, LTE defines two ACLR measurement methods: one is to use E-UTRA (LTE) at the center frequency and offset frequency; the other is to use LTE at the center frequency and UTRA (WCDMA) at adjacent and alternate offset frequencies. Figure 2 shows the ACLR measurement results of E-UTRA adjacent and alternate frequency offset channels. For this measurement, a 5MHz carrier is selected. Since the downlink has 301 subcarriers, the measurement noise bandwidth is 4.515MHz.

Figure 2: Shown here are ACLR measurements taken using an Agilent X-Series analyzer. The first frequency offset (A) is at 5 MHz with an integration bandwidth of 4.515 MHz. The other frequency offset (B) is at 10 MHz with the same integration bandwidth.

Optimizing Analyzer Settings

While the one-button measurements above provide very fast, easy-to-use, LTE standard-compliant ACLR measurements, engineers can still optimize the signal analyzer settings to achieve even better performance. There are four ways to optimize the analyzer to further improve measurement results:

*Optimizing signal levels at the mixer - Optimizing the signal level at the input mixer requires adjusting the attenuator to achieve the least amount of limiting. Some analyzers are able to automatically select the attenuation value based on the current measured signal value. This lays a good foundation for achieving the best measurement range. Other analyzers, such as the X-Series signal analyzers, have both electronic and mechanical attenuators that can be used in combination to optimize performance. In these cases, the mechanical attenuator only needs to be adjusted slightly to achieve better results, in steps of about 1 or 2 dB.

*Change the resolution bandwidth filter - Press the analyzer's bandwidth filter key to reduce the resolution bandwidth. Note: As the resolution bandwidth is reduced, the sweep time will increase. The reduction in sweep speed can reduce the variation in measurement results and measurement speed.

*Enable Noise Correction - Once the noise correction function is enabled, the analyzer will perform a sweep to measure the internal noise floor at the current center frequency and will subtract the internal noise floor from the measurement results in subsequent sweeps. This method can significantly improve ACLR, sometimes by as much as 5dB.

* Use an alternative measurement method. In addition to using the default measurement method (integrated bandwidth, or IBW), you can use the filtered IBW method. This method uses a steeper cutoff filter. Although this method reduces the absolute accuracy of the power measurement, it has no adverse effect on the ACLR result.

By combining these methods, the signal analyzer can automatically optimize the ACLR measurement using its embedded LTE application to achieve the best combination of performance and speed. For a typical ACLR measurement, the measurement results may be improved by up to 10dB or more (Figure 3). If the measurement requires the highest performance, the analyzer settings can be further adjusted.

Conclusion

Standards-compliant spectrum measurements, such as ACLR, are extremely important to RF engineers developing next-generation wireless systems. However, these measurements are very complex when performed using LTE applications due to a variety of factors: variations in adjacent channel bandwidths, the choice of transmit filters, and the interaction of RF variables between channels of different bandwidths and different interference sensitivities. A practical solution to this challenge is to use a spectrum analyzer or signal analyzer with a standards-specific measurement application installed. This combination reduces errors in complex measurements, automatically configures limit tables and specified test setups, and ensures excellent repeatability of measurements. Measurement results can be further improved using analyzer optimization techniques.

Figure 3: Shown here are ACLR measurements obtained using an Agilent X-Series signal analyzer with optimized settings. Compared to the results in Figure 2 using the embedded N9080A LTE measurement application, the ACLR in Figure 3 is an 11dB improvement.