Abstract: With the rapid growth of wireless data services, the continuous introduction of new air interface technologies, and market competition from WiMAX, UMTS faces challenges in supporting real-time data services and large data volume services, and needs to evolve continuously. LTE has the characteristics of flexible spectrum usage, seamless interoperability with existing technologies, and low network deployment and management costs. Whether a new technology can survive and thrive in the market depends on the recognition of the technology by end-users, and the key is whether these technologies can meet their expectations. Therefore, operators will conduct extensive testing on network devices and mobile terminals.

With the rapid growth of wireless data services, the continuous introduction of new air interface technologies, and market competition from WiMAX, UMTS faces challenges in supporting real-time data services and large data volume services, and needs to evolve continuously. LTE has the characteristics of flexible spectrum usage, seamless interoperability with existing technologies, and low network deployment and management costs. Whether a new technology can survive and thrive in the market depends on the recognition of the technology by end-users, and the key is whether these technologies can meet their expectations. Therefore, operators will conduct extensive testing on network devices and mobile terminals.

LTE terminal RF testing is not only to examine the RF chip indicators of the terminal, but also to conduct overall testing of the terminal and evaluate its performance.

The overall requirements for LTE terminal RF indicators are:

For transmitters, on the one hand, it is required to accurately generate LTE useful signals that meet the standard requirements, and on the other hand, it is required to control the useless transmission and interference levels within a certain level.

For receivers, it is required to be able to reliably and accurately receive and demodulate useful signals under certain environmental conditions, while also resisting certain interference signals.

The LTE terminal RF testing project is divided into four major parts, namely transmitter indicators, receiver indicators, performance requirements, and channel status information reporting. Although the signal structure of LTE is different from UMTS, the RF testing requirements for LTE terminals mainly come from the defined RF requirements of UMTS, with only a few new testing items added. In terms of receiver and performance statistics, UMTS systems measure reception performance through BER and BLER, while LTE systems measure throughput. In the performance testing section, corresponding channel demodulation performance indicators have also been added for the channel structure of LTE. In addition, for LTE terminal RF testing, it is necessary to test the various bandwidths, RB configurations, and modulation methods supported by the terminal, and the amount of testing is also very large. Below is a brief description of these four major testing projects.

(1) In the transmitter indicators, the following types of test items are included:

● Projects related to transmission power

Such as UE maximum output power, maximum power fallback (MPR), UE configured output power, etc. These testing projects mainly examine whether the transmission power of the terminal meets the standard requirements. If the maximum transmission power of the terminal is too large, it will cause interference to other channels or systems, and if the maximum transmission power is too small, it will cause a decrease in system coverage. The maximum power fallback is a new test item, which will be analyzed in detail later.

● Dynamic range of output power

Such as minimum output power, transmission shutdown power, switching time template, etc. These testing projects mainly examine whether the output power range of the terminal meets the standard requirements. If the minimum output power and turn off power are too high, it will cause interference to other terminals and systems. The switch time template verifies whether the terminal can accurately turn on or off its transmitter, otherwise it may cause interference to other channels or increase the transmission error of the uplink channel.

Power control

Such as absolute power control tolerance, relative power control tolerance, etc. The purpose of power control is to limit the interference level of the terminal and compensate for channel fading. This part of the test is mainly to verify whether the terminal can correctly set its transmission power, and the transmission power is within a certain tolerance range.

● Transmission signal quality

Such as frequency error, error vector amplitude EVM, carrier leakage, non allocated RB in band transmission, EVM equalizer spectrum flatness, etc. The signal quality of terminal transmission is a very important indicator for evaluating the modulation performance of terminal transmitters. We know that OFDM systems are sensitive to frequency offset and phase noise, and the method of distinguishing each subcarrier in OFDM technology is to use the strict orthogonality between each subcarrier. Frequency offset and phase noise can deteriorate the orthogonal characteristics between subcarriers, leading to a decrease in the performance of LTE systems. So frequency error, EVM, carrier leakage (IQ imbalance), etc. are indicators that LTE terminals must consider.

● Output RF spectrum transmission

Such as bandwidth occupation, spectrum transmission template, adjacent channel leakage power ratio (ACLR), transmitter stray radiation, etc. The useful spectrum transmission of the terminal must strictly comply with the standard requirements, while out of band transmission and spurious transmission are useless transmissions that require strict limitations, otherwise it will cause serious interference to the systems of other users.

● Transmission intermodulation

When the power of two or more frequency RF signals simultaneously appears in passive RF devices, passive intermodulation products are generated, and generally, third-order intermodulation is the most severe. The testing principle of transmission intermodulation is to set the terminal at maximum transmission power, configure interference signals, and observe whether the intermodulation products exceed the limit within the frequency band. The requirement is that the ratio of useful signal to intermodulation product power (in dBc) is lower than the limit value. The main purpose of this test project is to verify the ability of the terminal to suppress its intermodulation products.

(2) In the receiver indicators, the following types of test items are included:

● Reference sensitivity level

Assess the terminal’s ability to receive small signals. If the terminal sensitivity is too poor, it will reduce the effective coverage range of eNodeB.

● Maximum input level

Assess the terminal’s ability to receive large signals. If the maximum input level of the terminal is not qualified, it will reduce the coverage range of the eNodeB near end.

● Neighborhood selectivity, blocking characteristics, spurious response, intermodulation characteristics

The above categories are to examine the reception performance of terminals in the presence of interference signals (single tone/dual tone/modulation interference). If the anti-interference ability of the terminal is too poor, it will reduce the performance of the terminal receiver.

● Stray radiation

Assess the ability of the receiver to suppress the power of spurious signals generated or amplified in the receiver.

(3) In the performance requirements section, the following types of testing items are included:

Demodulation of PDSCH channels.

Demodulation of PCFICH/PDCCH channels.

Demodulation of PHICH channels.

Demodulation of PBCH channels.

The performance testing section mainly examines the channel demodulation performance of LTE terminals. Performance testing is conducted by examining the signal-to-noise ratio (SNR) under certain throughput conditions.

The calculation formula for SNR is as follows:

 

The energy received on the symbol in the formula represents white noise, and the superscript in the formula represents the corresponding antenna port received.

The performance testing section can be divided into single antenna port and multi antenna (diversity, spatial interleaving, MU-MIMO) related UE performance testing. The performance of a single antenna port is measured by the SNR under multipath fading conditions when meeting certain throughput requirements. The performance of dual antenna ports is mainly evaluated for the MIMO performance of terminals (diversity, spatial multiplexing, MU-MIMO), and is also measured by SNR under multipath fading conditions when meeting certain throughput requirements.

(4) In the channel state information reporting section, there are several types of testing items as follows:

CQI reporting under additive Gaussian white noise (AWGN) environment.

CQI reporting in a fading environment.

● Pre coding matrix indication (PMI) reporting.

Rank Indicator reporting.

This part of the testing project mainly examines the MIMO feedback performance of the terminal. The performance testing of spatial reuse can be divided into open-loop and closed-loop. Open loop MIMO without prior channel information; A closed-loop MIMO system is a system where the receiving end feedbacks channel information to the transmitting end, and then performs pre coding, beamforming, or antenna selection operations on the transmitted data. The feedback methods of closed-loop MIMO can be divided into full feedback and partial feedback.

Below is an analysis and explanation of some key test items for LTE terminal RF testing.

Power fallback

The signal structure of LTE is different from R99 (WCDMA), with OFDM signal used for downlink and SC-FDMA signal used for uplink. Although the power peak to average ratio of SC-FDMA signal is lower than that of OFDM signal, when the power peak to average ratio of SC-FDMA signal is higher, it means that the RF power amplifier of the terminal must have a high degree of linearity to ensure that the terminal transmission signal is not distorted. However, using a linear RF power amplifier can significantly increase the cost of the transmitter, and even using a linear RF power amplifier can seriously reduce the efficiency of the entire system. However, actual systems are mostly systems with limited peak power. In order to ensure a certain level of efficiency, most actual systems usually use nonlinear power amplifiers to amplify signals under certain output power backoff conditions. Therefore, it is necessary to examine the power backoff test term. When the power peak to average ratio of the uplink signal is relatively high, it is necessary to perform power backoff to the linear region of the amplifier. When the number of resource blocks (RB) is higher and the modulation method is higher, a larger power backoff value is required.

Carrier leakage (IQ imbalance)

IQ imbalance has a significant impact on the results of signal quality EVM. IQ imbalance is manifested as IQ offset in the initial constellation diagram, which is caused by DC offset caused by DSP rounding and other factors. The two channels of IQ signals are amplified separately. Due to the inconsistency of the devices, it is inevitable that the gain of the I and Q channels will be unbalanced, resulting in different IQ amplitudes. As a result, the originally square constellation will become a rectangle, meaning that the amplitude and phase of the signal will change at the same frequency point. However, there are no relevant tests on the R99 specification, and the modulation quality of the signal is mainly measured by measuring EVM. Due to the sensitivity of OFDM technology to phase and frequency offset, the measurement of IQ imbalance can better measure the performance of transmitter modulation.

The IQ origin offset is measured by the relative carrier leakage power (IQ origin offset power). According to the different transmission power of UE, the relative carrier leakage power requirements vary, ranging from -10dBc to -25dBc.

Spectral Flatness

Spectrum flatness is a newly added test item in LTE terminal RF testing. The flatness of the spectrum corresponds to the size of the ripple within the frequency band, which directly affects the stability of the terminal RF. Therefore, it is necessary to test the flatness of the spectrum. For example, if the ripple changes greatly, it is equivalent to a significant change in the output power of the terminal, then the output efficiency of the power amplifier is constantly changing, which in turn causes changes in the supply voltage and has an adverse impact on the performance of the terminal transmitter. This test item is divided into two situations: the test signal is located in the middle and the edge of the entire frequency band

 

As shown in Table 1, Range 1 means that the test signal is located in the middle of the entire frequency band, and the fluctuation range of the spectrum is required to be 4dB. Range 2 means that the test signal is located at the edge of the entire frequency band, and the fluctuation range of the spectrum is required to be within 8dB. For the normal operation of the system, it is necessary to have better flatness in the middle of the frequency band than at the edges.