Conducting Physical Layer Measurements on LTE Base Station Transmitters
by Lynne Patterson, Anritsu Company

The advent of Long Term Evolution (LTE) introduces Orthogonal Frequency Division Multiplexing (OFDM) into the cellular world. LTE is also bringing with it bandwidths of up to 20 MHz, signific

antly wider than that of earlier technologies such as GSM and W-CDMA. Engineers preparing to measure LTE face many challenges when selecting test equipment. Understanding the tests defined in the specification, knowing the issues associated with particular tests, ensuring that the equipment is in place and training are only some of the considerations. Another is that many tests procedures are not currently defined in the specification. So, test equipment must be flexible enough to accommodate new tests as they are defined and test equipment vendors must show how their products will keep pace with the evolving LTE specification.

One solution to testing LTE is to upgrade existing instruments by adding expensive software and hardware options. While this solution may seem cost-effective, the existing equipment must have the processing power to handle LTE analysis. OFDM splits the channel bandwidth into multiple subcarriers that are transmitted simultaneously to the receiver. The FFT analysis required to analyze the incoming LTE signal is computationally intense. If an instrument is more than 2-3 years old, the upgrades may make the instrument LTE-capable, however, the new software may actually slow measurement time. If speed is critical, upgrading equipment designed for 2G and 3G may not deliver the desired performance.

While OFDM has distinct advantages – such as easily and efficiently handling multi-path, and robustness against narrow-band interference – it has disadvantages. Two potentially critical concerns are its sensitivity to frequency offset and phase noise, and a peak-to-average-power-ratio (PAPR) problem that reduces the power efficiency of the RF amplifier at the transmitter. All of this makes selecting the proper test instrument essential to ensure performance.

Are You Ready to Test LTE?
To ensure the proper performance of an LTE base station transmitter, physical layer measurements must be made. Among the most important are power, Error Vector Magnitude (EVM), Occupied Bandwidth (OBW), Adjacent Channel Leakage power Ratio (ACLR), and unwanted emissions. 3GPP has released two standards in reference to making LTE measurements. TS 36.104 V8.30 (2008-09) establishes general test guidelines with limits. TS 36.141 V8.00 (2008-09) defines both test procedures and test models.

In TS 36.141, test models are defined to measure the E-UTRA enhanced base station used for LTE. In order to satisfy the requirements of the model, it is imperative that a signal analyzer can conduct measurements on all portions of the test model waveforms. Likewise, any signal generator selected for LTE testing must produce the test model signals to ensure appropriate evaluation.

The LTE Physical Layer
An LTE frame has dimensions for frequency and time. In time, the LTE frame is 10 ms long and contains 10 subframes, each of which is divided into two slots. Typically, the first reference of the frequency dimension is defined as a Resource Block, which is exactly one slot by 12 subcarriers. The slot is divided into six or seven symbols in a Resource Block, depending on system parameters. Resource Blocks are composed of Resource Elements (RE). The REs are the smallest unit in an LTE frame with dimensions of one subcarrier by one symbol. Some measurements will be made on RE, some on Resource Blocks, and some on the entire frame. When evaluating test equipment, the resolution of the various measurements is an important consideration for these measurements.

Power Measurements
Because OFDM can reduce power efficiency, measurement of LTE’s signal power is critical. Engineers developing and testing LTE base stations will need to measure PowerOUT, RE Power Control Dynamic Range, and Total Power Dynamic Range.

All tests described in the specifications first define the item being measured. Power Control Dynamic Range is defined as the difference between the power of an RE and the average RE power for a base station at maximum output power for a specified reference condition. Total Power Dynamic Range is the difference between the maximum and minimum power of an OFDM symbol for a specified reference condition (Table 1).

These definitions require that the signal be measured at the RE level and at the OFDM symbol level. When conducting Total Power Dynamic Range measurements, the OFDM symbol shall carry PDSCH (Physical Downlink Shared Channel) and not contain Reference Signal (Reference Signal), PBCH (Physical Broadcast Channel) or synchronization signals. To perform this test, a signal analyzer must measure the signal without the presence of the Reference Signal or the synchronization signals.

Signal Quality – Frequency Error and EVM
Frequency Error is a critical metric for OFDM. OFDM is highly sensitive to frequency error as measurements of the subcarriers must be extremely precise and the specification requires that the modulated carrier frequency shall be accurate to within ±0.05 ppm observed a 1 ms subframe. Test equipment used to measure frequency error must have an excellent residual frequency error specification to ensure that this critical parameter is measured correctly.

EVM is always important when measuring digitally modulated signals. In LTE, where multiple modulation types are used, EVM becomes a critical figure of merit. When measuring EVM, the specification provides specific test models and defines the configuration of the test signal. Table 2 shows EVM requirements for the various modulation schemes used in LTE.

Unwanted Emissions
When performing base station transmitter testing, meeting unwanted emission specifications is critical to the success of the design. Unwanted emission testing includes measuring OBW (Occupied Bandwidth), Out-of-Band (OOB) emissions, and spurious emissions. OBW is defined as the frequency band that contains 99% of the signal energy. OOB emissions are frequency components that appear outside the channel bandwidth and are the result of non-linearities in the modulation chain. The LTE specifications fix the requirements for two types of OOB emissions: ACLR and Operating Band Unwanted Emissions. Spurious emissions are frequency components that appear far from the carrier frequency, particularly high-order harmonics.

ACLR and OBW are “one-button tests” on some signal analyzers. The parameters of these one-button tests must be flexible enough to accommodate the specification’s requirements. Any filters used by the instrument must conform to the specification’s requirements. Understand how the signal is measured and how parameter settings can affect the results. When measuring the adjacent channel frequencies on multi-carrier base stations, the adjacent channels apply to the lowest carrier frequency transmitted by the base station and the highest carrier frequency transmitted by the base station for each supported multi-carrier transmission configuration. Attention must be paid to band switch points in spectrum analyzers to ensure that lower band measurements and upper band measurements use the same receiver chain in the instrument.

The Operating Band Unwanted Emissions are defined as all unwanted emissions in the transmitter-operating band plus a frequency range 10 MHz above and 10 MHz below the operating band. Unwanted emissions outside of this frequency range are limited by the spurious emissions requirement. The specification states that the resolution bandwidth of test equipment should be equal to the measurement bandwidth when performing this testing. To improve measurement accuracy, sensitivity, and efficiency, the resolution bandwidth can be smaller than the measurement bandwidth. Using a smaller resolution bandwidth requires that the results be integrated over the measurement bandwidth in order to obtain the appropriate results.

The specification defines the spurious emissions frequency range from 9 kHz to 12.75 GHz, excluding the band of frequencies defined for the Operating Band Unwanted Emissions. Analyzers best suited for spurious emissions will allow the user to set a spurious emissions mask that divides the spurious band into segments. Each segment must be configurable independently of the other segments. When evaluating requirements for spurious emissions testing, be aware that additional spurious requirements may exist.

Intermodulation
The intermodulation requirement is a measure of the transmitter’s ability to prevent the generation of signals in its non-linear elements when the transmitter’s signal and an interfering signal reach the transmitter through the antenna. The specification defines the characteristics of the interfering signal and requires measurements be made at various offsets about the center carrier.

Conclusion
Designers and manufacturers of LTE base station transmitters have a number of considerations when conducting measurements. They must understand the specifications related to testing, as well as the potential dynamic range of each test. Equally important is understanding the caveats associated with various measurement procedures and the capabilities and strengths of the test equipment. Whether selecting new equipment or upgrading existing equipment, evaluate test equipment for both performance and suitability. Understanding the necessary tests and developing a strong working relationship with your test equipment vendor will ensure that you get the products you need, when you need them, with the specifications that you require. With the appropriate hardware and software on your lab bench, successful and accurate testing will be fast, accurate, and simple.

Anritsu Company
www.anritsu.com
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