3G Base Station Transmitter Performance Test and Analysis-EEWORLD

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The current fast-developing third-generation wireless communication system has greatly improved system performance and service quality compared to the previous second-generation wireless network. In order to achieve a higher transmission rate, 3G uses a highly complex modulation system. The rapid development of various 3G base station systems has put forward higher requirements for the stable operation of the WCDMA radio frequency part and the code domain part. The maintenance measurement of the UMTS base station system increasingly requires measurement personnel to conduct a full range of performance evaluations in the frequency domain, time domain, amplitude domain, modulation domain, etc. This has brought new challenges to engineers.

The characteristic description of WCDMA UMTS Node B transmitter includes two parts: RF spectrum description and code domain demodulation description. The measurement based on this description can be performed using a handheld spectrum analyzer with demodulation function. For example, the new handheld base station tester MT8220A from Anritsu Company not only includes the traditional analysis capabilities based on signal spectrum, but also includes carrier frequency, frequency error, occupied bandwidth, channel power, peak-to-average ratio, noise level and adjacent channel interference ratio (ACLR), and also includes the modulation domain analysis capabilities that must be measured for WCDMA transmitters, including spurious characteristic measurement, vector error magnitude (EVM) measurement, carrier feedthrough measurement, primary/secondary common control physical channel (P-CCPCH/S-CCPCH), primary/secondary synchronization channel (P-SCH/S-SCH), paging indicator channel (PICH), common pilot channel (CPICH), absolute power level of primary pilot channel and other modulation domain indicators.

Figure 1: Code domain demodulation measurement;

Code Domain Demodulation Measurements

By utilizing advanced semiconductor technology, Anritsu has successfully developed a series of small RF/microwave devices. This allows the instrument to achieve indicators and performance comparable to ordinary benchtop instruments at a compact weight of 2.9kg. Based on the new high-performance 7.1GHz spectrum analyzer platform, coupled with powerful WCDMA demodulation function, it truly realizes measurement and analysis at any time and any place. The low average noise floor level of -153dBm is also far ahead in handheld instruments. The instrument also strengthens the performance and indicators of measuring Spectral Emission Mask and ACLR (Figure 1).

UMTS WCDMA transmitter testing

The built-in spectrum analysis mode of the MT8220A is optimized for WCDMA base station RF measurements. Its RF measurements include carrier frequency, frequency error, occupied bandwidth, channel power, peak-to-average ratio, noise level and ACLR.

ACLR measurement - usually due to the nonlinearity of the linear power amplifier, the system produces a higher ACLR, and the adjacent channel power leakage contributes to the channel background noise. It directly reduces the system redundancy/capacity, and the ACLR characteristics will greatly affect the working status and communication status of other sites. Too high a value will bring the so-called near-far effect to mobile phone users. Handheld spectrum analyzers usually have built-in signal filters. This filter strictly defines the bandwidth and shape factor of ACLR measurement to ensure that the 3dB bandwidth is strictly equal to the chip rate. Through the built-in ACLR measurement mode of MT8220A, users can accurately obtain the precise leakage values of 5MHz and 10MHz away from the carrier.

Signal spurious measurement --- MT8220A also provides WCDMA spurious test mode, with built-in standard 3GPP spurious template. As the signal size changes, the template measurement will indicate whether the spurious index is PASS or FAIL. The concise result display is easy for users to identify.

Code Domain Demodulation Measurements --- The code domain demodulation test mode provided by the MT8220A provides a test of the WCDMA signal quality. This test mode can measure the absolute power of the P-CCPCH, S-CCPCH, P-SCH, S-SCH, PICH, CPICH, P-CPICH channels and measure parameters such as code channel power, EVM, and carrier feedthrough. In this mode, the MT8220A demodulates the received signal and provides three display modes: code domain power display, code spectrum display, and modulation list display. Code Domain Power Display Appropriate power levels on all selected OVSF codes, P-CPICH, P-SCH, and S-SCH channels are critical to optimizing network capacity and quality. Only test equipment that can correctly demodulate the transmitted signal and analyze the power level in the code domain can make these power measurements. These signals determine the cell coverage and must be set within appropriate limits.

Figure 2: Modulation item list display.

Code Spectrum Test - Displays code domain power with a color scale, building a display of different times from bottom to top on the vertical axis, and including each subsequent trace of power and code channel. Code spectrum diagrams show the characteristics of base stations with different data rates and service requirements. By evaluating code spectrum diagrams, important diagnostic information related to W-CDMA systems can be obtained. This provides an effective way to verify and analyze the characteristics of service channels and signaling channels in base stations. Signals with high data rates are usually transmitted quickly and are difficult to observe by other means. Code spectrum analysis can also track service loads over time, showing call volume and call power.

Air measurement mode: Users can use the antenna or connect directly to the NodeB to perform these three modes. All three modes can directly display the carrier power, frequency and channel power, but not all base stations can support all three modes. For example, measurements directly connected to the NodeB can often only complete the measurement of occupied bandwidth, ACLR, spurious characteristics, and peak-to-average ratio. Through the air interface, the MT8220A can help users automatically capture the 6 groups of scrambling codes with the strongest signals, and users can also manually specify the scrambling code number. Scrambling code analysis detects the pilot power level of each received WCDMA downlink signal, which is displayed here as (Ec). Scrambling code analysis also detects the surrounding noise and calculates Ec/Io using the surrounding noise (Io). This is equivalent to the signal-to-noise ratio of each sector in WCDMA, so that the coverage quality of the location can be easily determined. In the air measurement mode, the measurement information provided by the MT8220A includes total power, pilot power (CPICH), Ec/Io, Ec and pilot tolerance parameters.

Modulation item list display

WCDMA networks provide an important evolutionary step in the 3G field. Having efficient diagnostic and test tools is very important for system operation and maintenance. This shortens system downtime and maintenance tasks, and ensures customer satisfaction. Traditional large-scale dedicated testers are too expensive, too bulky, and too complicated to use in field applications. MT8220A is designed specifically for base station technicians and field RF engineers. It takes the lead in providing excellent performance, usability and value for network operation managers. The test suite provided by MT8220A helps users to conduct detailed analysis of measurement results on the PC. Users can easily download the measured data to the computer in .JPG and .DAT formats to write base station maintenance reports.

The lightweight design provides engineers with a portable tool, which can reduce the burden on engineers when facing 3G test and measurement challenges.

Mastering key technical indicators to meet the challenges of 3G testing

Tan Hock Leong Anritsu Asia Pacific Marketing Manager

After entering 3G, Chinese engineers and overseas engineers face the same challenges. Mobile phones now pursue higher transmission rates. On GSM, it is more common to measure standing wave ratio and return loss, and there is no actual need to test demodulation parameters. Now QPSK, 16QAM or 8PSK modulation are used, which are all complex modulation methods. Therefore, it is necessary to test some items that did not need to be tested in the past 2G era, such as EVM (Error Vector Magnitude), adding parameter tests that were not previously focused on.

Of course, there are many parameters that can be measured or not measured, but if not measured, some problems may not be discovered, which may affect the quality of the entire service in the future. Our company launched a series of Site Master instruments 10 years ago. Handheld testing is mainly for on-site or field testing. More than 80% of customers use Site Master.

Currently, South Korea mainly uses CDMA or CDMA2000, or EVDO systems. They also have WCDMA systems. The items tested are more than what I have seen in other countries, including channel power (Channel Power), EVM, ACLR, OBW and spectrum emission mask (SEM, Spectrum Emission Mask), etc., because only through comprehensive testing can some interference signals be discovered and the entire system can be corrected.

At present, the problem in China is that engineers themselves may not know what parameters need to be tested. If 3G tests are conducted in the same way as 2G tests, the service quality may be affected. In the past six months, many engineers in the Asia-Pacific region have conducted road tests after testing the standing wave ratio and return loss, and have not tested the modulation parameters at all. Such problems may exist because engineers do not understand the importance of modulation parameter testing. For example, in EVM testing, if the modulation index is not met, it may cause problems such as dropped calls.

Specifically, EVM means error vector magnitude, which uses symbols to represent the data transmission rate. For example, simple BPSK only has two simple symbols, which are 180 degrees apart; however, in QPSK, there are four symbols. You can imagine them as four points, each 90 degrees apart, just like a square. The four corners are the positions of the symbols. EVM is to test whether the symbol is at the corner position to see how far it deviates. For example, in WCDMA, BPSK and QPSK modulation are mainly used. In HSDPA, 8PSK will be used, and 16QAM will be used further, represented by 16 symbols, thereby further improving the transmission rate. In 16QAM, there are a total of 16 symbols. EVM measurement represents the deviation of the symbol from its position as a percentage. It is a very important indicator in base station testing.

In general, as the transmission rate increases, some parameters that did not need to be tested before now need to be tested. On the one hand, engineers need to have a deeper understanding of the new test parameters. On the other hand, there may not be relevant test equipment on site, and the test time will increase by 50%. This is one of the challenges they face.

In the field of 3G test equipment, the MT8220A spectrum analyzer can reach a maximum frequency of 7.1GHz. With a body weight of 2.9Kg, the indicators of this spectrum analyzer have reached the level of desktop computers in many aspects, and the bottom noise level is -153dBm. MT8220A is the fifth generation of our handheld spectrum analyzer. About 5 years ago, we launched a handheld spectrum analyzer. At present, although many competitors have launched similar functions, we still occupy a leading position in the field of field testing.

Although 3G standards continue to change, our test equipment has a minimum resolution bandwidth of 10Hz, a noise floor of only -153dBm, and a bandwidth of 7.1GHz, so we can avoid new requirements for hardware upgrades in the short term. In the future, if 3G adds new demodulation functions, all we need to do is upgrade the software.

By comparing with competitors, we found that many competitors do not have the spectrum radiation mask test item. ACLR is a test that can only be done on benchtop instruments. Our third and fourth generation spectrum analyzers cannot achieve this performance. If the dynamic range or phase noise indicators of the instrument are not good enough, the measurement of EVM, SEM and ACLR will be affected.

For example, the measurement of ACLR requires the instrument to have high sensitivity, dynamic range and phase noise indicators. It tests the adjacent channel power leakage ratio. If the dynamic range of the instrument is not enough, the noise may appear higher, mainly because the bottom noise is not low enough. This will affect the effect of the entire test. Therefore, some test equipment manufacturers' instruments cannot even test SEM. It mainly tests whether the base station transmitter has some spurious or interfering signals. During the test, a MASK will be automatically set as a threshold to determine whether it is qualified.

With the help of handheld instruments, engineers will no longer need to carry large and heavy instruments to the site for testing, and the price is also lower than that of desktop instruments. The future development is to integrate more instrument functions. In order to shorten the test time, we have a storage function. One document can collect all test parameters, so there is no need to worry about missing field data storage. Our spectrum analyzer is equipped with Ethernet and USB ports, which can be remotely controlled through the network.

80% of mobile communication customers use our Site Master test equipment, mainly for maintenance and engineering testing. In the future, we will add new functions according to customer requirements, and mainly add new functions to this high-performance hardware platform and DSP, which will provide a good solution for measurement engineers to cope with measurement challenges. The focus is on HSDPA testing and other demodulation methods.

Reference address：3G Base Station Transmitter Performance Test and Analysis

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