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LTE: Testing the physical layer of next-generation mobile communications

LTE (LONG-TERM EVOLUTION) IS THE NEXT STEP IN MOBILE COMMUNICATIONS AND SEEKS TO MAKE WIRELESS ACCESS TECHNOLOGY UBIQUITOUS. THERE ARE A NUMBER OF NEW CHALLENGES THAT ENGINEERS ARE FACING WHEN TESTING THE PHYSICAL LAYER, BUT APPROPRIATE TESTING FEATURES ARE ALREADY AVAILABLE.

Manufacturers of mobile phones and mobile infrastructure are working on a number of improvements of the current UMTS technology to enhance user experience, but also to drive down cost by reducing capital as well as operating expenses. They achieve these enhancements by measures that include high data rates—in excess of 100 Mbps; shorter user-plane latency—less than 5 msec; shorter control-plane latency— less than 50 and 100 msec, respectively, to change from a camped or dormant state to the active state; and higher spectral efficiency—three to four times that of HSPA (high-speed packet access). With the first set of specifications now available for the new 3GPP air interface, the challenges of testing LTE have become clearer. Since providers plan commercial roll-out for the year 2010, the first test systems are already available.

TEST CHALLENGES OF LTE PHY

The air interface of LTE is based, in the DL (downlink), on OFDMA (orthogonal frequency domain multiple access) and in the UL (uplink), on SC-FDMA (single-carrierfrequency- domain-multiple-access) technology. Both use the concept of densely-stacked low-data-rate subcarriers to convey the information. Since every subcarrier has a very low bandwidth—15 kHz— a flat-fading channel is a representation of what the signal experiences. In addition to this, a cyclic prefix combats intersymbol interference. This technology, however, makes the physical layer inherently frequency-selective; consequently, all procedures that the specifications define are also frequency-selective. Engineers have to keep this in mind when performing tests on wireless devices.

To boost the data rate of the physical layer and to make it more robust, LTE uses MIMO (multiple-input-multipleoutput) schemes in conjunction with the OFDMA scheme. This adds another level of complexity to the testing challenge. As 3GPP defines various modes of operation from the MIMO —such as single-user MIMO, multi-user MIMO, open- and closed-loop transmit diversity—the number of different set-ups increases. Thus, automated testing becomes a necessity if an engineer wants to be able to verify the correct operation of the complete system.

PHYSICAL-LAYER TESTING

When testing the physical layer, engineers typically increase the coverage in accordance with the integration status of the system. The process starts with low-level block testing that verifies the receiver and transmitter of the system. The strategy adds further processing step by step until the integreation of all required functional blocks.

During the testing campaign, it is important that engineers be able to observe the internal signals, such as payload transmitted and received or measurements of the timing offset between the UL and DL frames. Not having the relevant data available could hinder the debugging process. Furthermore, the system should be easy to use and not require substantial configurations outside of the layer under test. For example, users should not have to configure the PDCP (packet-data-convergence-protocol) layer when testing the physical layer. Similarly, when testing the RLC (radio-link-control) layer, configuring the physical layer should be as easy as possible.

Since most of the testing occurs concurrently with the development and debugging process, it is crucial that engineers be able to automate the testing and embed it into a regression environment with as little manual intervention as possible. The test environment should include all components from the testbed, such as fading-channel simulators, and external noise and interference sources.

DATA-PATH TESTING

Engineers have to go through several steps in the testing campaign. The first step within the testing campaign is to make sure that the individual channels operate in an open-loop fashion. This action validates the correct implementation of 3GPP specifications 36.211 and 36.212, which define the DL and UL transmission with the forwarderror correction. In order to verify the correct reception and transmission to and from the UE (user equipment), the tester provides a DL signal to the UE and/or receives an UL signal. During this phase, intermediate points in the encoding and decoding chains of the test equipment must be visible to assist in the debugging process. The process also requires special test features such as the ability to corrupt DL transmissions. During this step, test teams frequently use off-the-shelf signal generation and analysis equipment, with capabilities and software options appropriate to the relevant standards.

FUNCTIONAL TESTING

Once engineers have established the operation of the individual data paths in a noise-free environment, the functional testing phase begins. The engineers test the feedback procedures within the UE in a controlled and static environment, which makes the behaviour of these procedures easy to predict. The procedures under test in this phase include CQI (channel quality indication), HARQ (hybrid ARQ) and timing control, which the 3GPP specification 36.213 define. During this phase, the tester has to respond in real time to the control information: For testing the DL-HARQ operation, the tester has to adjust the DL transmission in accordance to the received ACK/NACK information. To test these features efficiently, the tester has to provide special test features such as built-in interference or channel models.

Figure 1 shows the block diagram of a test system for functional testing of an LTE UE. The grey blocks mark the special test behaviour—functionality that the 3GPP set of specifications does not specify. Most notable are the channel models that apply static flat channels on a per-subcarrier basis to enable the test engineer to test channel estimation and CQI reporting in a deterministic and repeatable environment.

Other special test features are the DL scheduling, which can be used to test HARQ processes; the UL scheduling to stimulate UL transmission; the timing control to verify that the UE obeys the timing commands sent within the PDSCH (physical-downlink-shared-channel) payload; and the UL power control to verify the correct behaviour of the power-control algorithms in the UE. In addition, the system requires extensive logging and measurement capability to validate the UE implementation. Measured parameters are, for example, UL power per subframe, UL-transmission quality, and UL/DL timing offset and throughput. The system requires OCNG (OFDMA channel-noise generation) to simulate transmissions to other UEs that are typically also present on the air interface.

PERFORMANCE TESTING

After satisfactory functional testing, engineers have to check the performance of the UE in a further step in order to validate the requirements of 3GPP specification 36.101, which defines performance requirements for the user equipment. This step is divided into two parts: First is the performance of individual blocks such as receiver and transmitter performance; second is the system performance— the performance including closed-loop operation and UE procedures.

Figure 2 shows the block diagram of a test system for performance tests of an LTE UE. The engineers have to substitute the static-channel models with a fading-channel simulator. Additionally, they have to add neighbouring channels, blockers and interferers to the DL signal in order to simulate an environment that the UE would experience in the field. While doing the block-level performance test for the PDSCH, the engineer needs to see how BLER (block-error rate) changes in the face of varying signal power, type and level of interference, the chosen transport format, as well as the fading-channel profile. During block-level tests on the PDCCH (physical downlink-control channel), the engineer is interested in false-detection ratios so he or she can characterise the performance of the detection algorithms—algorithms that carry out their function of detecting the wanted signal, without any prior knowledge of the characteristics of that signal— in various interference scenarios. In this case, the UE must carry out about 40 blind trials within a defined search space.

Once the block-level testing is complete, testing moves on to system-level testing in order to validate the interaction of various system components. Mandatory tests are throughput measurements with enabled HARQ operation— similar to the test that Section 9.2 of 3GPP specification 25.101 describes for HSDPA; CQI-validation tests—similar to the tests that Section 9.3 of 3GPP specification 25.101 describes; or PDCCH detection tests— similar to Section 9.4 of 3GPP specification 25.101.

In parallel with this step, engineers can verify the compliance of the UL transmission with 3GPP specification 36.101, checking whether in parameters such as EVM and spectral flatness it fulfils the required spectral-emission masks and the minimum-quality requirements of the UL transmission.

PRODUCTION TESTING

After the UE has completed all the R&D testing phases that this article has described thus far, it enters production. At this point, the design team ports some of the tests that it has deployed to the production environment. Therefore, it is a desirable feature that the same test platform and environment be usable for development as well as production testing. This guarantees high re-use, plus teams can transfer knowledge among each other and thus assist in solving problems. The requirements for production tests differ from those for development test, however: for instance, while it is less important to observe the internal workings, clear pass/fail criteria are crucial, and—as is always the case with production test—test times must be as short as possible.

An LTE-protocol-testing solution based on available test equipment is already in place—the scenario that this article outlines employs the R&S CMW 500, for example— to meet the challenges described in this article. Today’s test products are also capable of addressing a number of future test challenges that this article does not mention: These challenges include physical-layer testing for MIMO and other advanced physical-layer technologies, which the upcoming 3GPP releases will define; and testing of higher layers such as MAC (medium access control), RLC (radio link control), PDCP (packet data convergence protocol) and RRC (radio resource control). Inter-RAT (radio-access technology) handover to other radioaccess technologies, such as GSM and W-CDMA, are on the roadmap as well.

Author Information

Moritz Harteneck has been working at Rohde & Schwarz’ headquarters in Munich since 2007. After completing his MSc and his PhD at the University of Strathclyde (UK) in 1994 and 1998, respectively, he has been working in the area of wireless communication systems, focusing mainly on the development and testing of physical-layer implementations.