Technical challenges of LTE terminal application design and testing

“As the first batch of LTE networks have been put into use, operators have shifted their business focus to providing mobile broadband services on a large scale. However, there are still some problems that need to be overcome before users can get the full benefits of next-generation cellular technology? In addition to technical issues, there are still some issues to be resolved in some broader aspects of LTE services. This article discusses these challenges from the perspective of a test engineer. But before discussing the technical challenges of user equipment (UE) design and testing, let’s take a broader view of some potential problems.

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As the first batch of LTE networks have been put into use, operators have shifted their business focus to providing mobile broadband services on a large scale. However, there are still some problems that need to be overcome before users can get the full benefits of next-generation cellular technology? In addition to technical issues, there are still some issues to be resolved in some broader aspects of LTE services. This article discusses these challenges from the perspective of a test engineer. But before discussing the technical challenges of user equipment (UE) design and testing, let’s take a broader view of some potential problems.

At the time of writing, the formal certification of LTE terminals has not yet begun. Major certification bodies (GCF, PTCRB) plan to launch a conformance test program for protocol, RF and wireless resource management in December 2010. But because some markets have begun to sell LTE terminals, we have to worry about whether these terminals can pass these conformance tests in the future.

For LTE Category 3 terminals that support high data rates (100Mbps on the downlink and 50Mbps on the uplink), can the backhaul capacity be able to handle it? In the long run, as the number of LTE users increases, the bandwidth sharing of all users in the cell on the wireless network will become an important factor. If there are too many users in the cell, performance may be affected. In addition, as the number of active users increases, cell edge performance will be affected by a higher signal-to-noise ratio (SNR).

For potential global mobile data networks, it is also necessary to meet the expectations of data users for global roaming. Although this is technically feasible, the problem of excessively high charges for roaming data services still needs to be resolved. On the other hand, for network operators, the flat rate data solution has become a problem, because it provides a constantly changing and definitely increasing amount of data, but obtains a fixed income.

Since LTE is the next-generation technology choice of CDMA2000 network operators (such as Verizon Wireless), interworking with 3GPP2 CDMA2000 high-rate packet data services is a clear requirement. The merging of 3GPP and 3GPP2 network topologies at the LTE wireless network interface is a meaningful development, which requires careful testing to ensure that its operation meets the expected goals.

For network operators, maintaining voice services that use IP networks and traditional Circuit-switched networks at the same time will be a big challenge. The agreement on VoLTE (Voice LTE) standardization announced by mainstream network operators at the Mobile World Congress 2010 (Mobile World Congress 2010) will solve this problem to a large extent. However, the use of 3GPP’s IMS (IP Multimedia Subsystem) technology still needs to be deployed on a large scale.

Test challenges and requirements

The key to meeting the stringent requirements of LTE terminal equipment is to divide the design into sub-systems and develop a test plan that allows for thorough verification of each part of the design before testing the complete terminal. If this modular approach is not adopted, problem diagnosis may be delayed to the final stage of the project, making it difficult to manage the final release stage, including field trials and conformance testing.

Whether the terminal design is from scratch, based on an earlier design, or using third-party integrated components, multiple key performance measurements are required. Some of these measurements (such as maximum output power, power control, and receiver sensitivity) are familiar to earlier technologies, but because of the different transmission methods used (OFDMA in the downlink and SC-FDMA in the uplink), so New measurement equipment is needed to support these tests.

Some other measurements are specific to LTE. For example, for OFDMA transmission, sub-carrier error vector magnitude (EVM) becomes an important test of modulator performance. With the availability of 700MHz analog TV spectrum, LTE will be deployed at a lower frequency than GSM or WCDMA, resulting in a much larger bandwidth: 20MHz/700MHz=2.8%. In contrast, for a typical WCDMA terminal: 5MHz/2100MHz=0.24%. This will bring challenges to some modulator architectures because it will result in higher EVM values ​​at the edge of the cell, so special attention should be paid to this in the design stage.

Due to the dynamic nature of some tests (such as power control), a signaling protocol is needed to establish measurement conditions. This requires that the test equipment must include a protocol stack and a simulated evolved Node B (eNB) base station. Since these measurements are usually performed by RF engineers rather than protocol experts, the test equipment used must be easy to configure so that engineers can focus on the measurement itself.

Protocol test

One of the main challenges facing protocol stack developers is to ensure that the state change response requirements are met. Although the LTE specification has reduced the number of possible states of a terminal device to RRC_IDLE and RRC_CONNECTED, when data needs to be sent, the time required to change from one state to another will constitute a large part of the delay budget.

In the RRC_IDLE mode, the terminal device puts itself in a low power consumption state as much as possible to ensure excellent battery life, and the receiver is periodically activated to check paging messages. When the data transmission time is scheduled, the terminal must be in an awake state and quickly synchronize its uplink.

In the protocol testing process, the workload used to generate test cases is often as much as the workload of creating the protocol stack, so it is important to have comprehensive and efficient testing facilities. In order to be able to subdivide the test, it is important to be able to measure each sub-layer in the user plane and the control plane. The protocol test and diagnosis function is very important when finding faults. Typically, this includes time-stamped message logging and decoding. But what is important is that this can also be used in each sub-layer to provide the function of tracking the details of the signaling message flow (from MAC PDU to RRC message) to ensure that the timing requirements are met.

To be able to create test scenarios for each layer, you must have detailed control of the test equipment, but this needs to be as easy to use as possible to avoid painful learning. Graphical test descriptions (such as those provided by the scenario wizard) provide the clearest way to define a new test (Figure 1).

Figure 1: Graphical test description (such as provided by the scenario wizard) provides the clearest way to define a new test

Performance Testing

Once the RF, baseband, protocol stack, and application layer are integrated, then the overall terminal performance needs to be fully tested. During this phase, data transmission bottlenecks must be detected and eliminated to maximize data throughput-including under normal and extreme temperature and power supply voltage conditions. In addition, it is necessary to measure power consumption, thermal characteristics, electromagnetic compatibility (EMC), emission and susceptibility under full load conditions. Under normal circumstances, this requires the use of 2×2 downlink multiple input multiple output (MIMO) technology.

Test equipment must be able to seamlessly transfer data between cells while minimizing the impact on data throughput, just as it must be able to transfer data between different wireless access technologies while maintaining data connections. Many suppliers have introduced compact, flexible and modular test instruments. For example, Aeroflex’s LTE test products support all functions used to test the performance of LTE terminals.

Although the LTE physical layer uses a cyclic prefix to increase resistance to multipath effects, it needs to be tested to ensure correct operation. Leaving this test in the field trial phase will increase development risk. Fortunately, test equipment vendors provide tools with built-in fading simulators and noise generators to simulate actual signal conditions in the laboratory.

An important performance parameter of the LTE terminal is its ability to achieve and maintain synchronization with the downlink signal. The LTE OFDMA transmission method uses sub-carriers with a frequency interval of 15 kHz. The receiver must also be precisely tuned to the sub-carriers, even if there is a Doppler shift effect. Asynchrony will cause mutual interference between sub-carriers, thereby reducing the signal-to-noise ratio. To test the behavior of the terminal, it is also necessary to be able to simulate the Doppler shift effect in the laboratory.

Summary of this article

Next-generation mobile terminals need to provide a mobile broadband experience that can meet the wishes and expectations of network operators. Therefore, a layer-by-layer solution must be used to test new LTE terminals and establish end-to-end test scenarios based on actual signal conditions. Ensuring that terminal performance is maintained in the entire cell will be the most difficult challenge, especially when the number of users in the cell increases and the resulting signal noise level rises.

LTE terminal testing must not only be accurate and efficient, but also cover a comprehensive range of testing-including RF, protocol, and system-level testing. Test equipment manufacturers are providing this function, and new and upgraded instruments, testers and systems have been listed successively. Achieving high data throughput and low latency (in terms of power consumption and RF spectrum usage) as efficiently as possible is the main purpose of introducing LTE technology, but this can only be achieved by careful testing during the development and deployment phases .