Comprehensive testing of LTE systems using channel emulators

As users, handset manufacturers, application developers and service providers focus on the next big application that will drive market growth, the current mobile phone industry has shifted its focus from "fewest dropped calls" to "fastest networks." This demand growth mainly comes from data transmission.

In just a few years, mobile data services have gone from being too slow to be useful to being as good as Wi-Fi today. As consumers and business application developers rush to find ways to take advantage of improved mobile data services, technology experts are working to deliver newer, faster, more powerful broadband wireless networks to support this demand.

Existing wireless network capacity is limited, so to maintain this growth, the network needs to have the following characteristics: smooth, all-IP (Internet Protocol) architecture, greater capacity, lower cost per bit, faster connections, lower latency, and improved video capabilities. As data services continue to explode, how can manufacturers and service providers ensure that new devices and new applications can provide the quality, throughput and performance that users expect?

Several leading industry groups are developing technology standards designed to drive the development of 4G broadband wireless networks, enable these increasingly complex devices to work properly, and enable these demanding applications to be implemented, including technologies called Long Term Evolution (LTE) and 802.16e-2005 or Mobile WiMAX. LTE and Mobile WiMAX have one major common feature in that they both use Orthogonal Frequency Division Multiple Access (OFDMA) and Multiple Input Multiple Output (MIMO) technology. Essentially, OFDMA and MIMO provide higher levels of data transmission capabilities with less bandwidth.

In terms of the impact on the air interface (the physical path of the RF signal between the transmitter and receiver), OFDMA and MIMO systems can improve the overall performance of the RF transmitter and receiver. The radio channel is affected by many factors, such as signal delay, attenuation, and obstacles. These factors can combine to improve signal transmission conditions or damage the transmitted signal, that is, they can have both positive and negative effects on the overall channel throughput/data rate.

OFDMA is a robust digital modulation scheme that improves overall spectral efficiency. It uses a lower symbol rate per carrier to reduce multipath interference, but it also uses multiple carriers to increase the data rate. OFDMA transmits multiple symbols simultaneously over several carriers, rather than one symbol at a time. The subcarriers are spread over multiple frequencies and are "orthogonal" to avoid any interference caused by adjacent subcarriers.

MIMO systems utilize multiple transmit and receive data paths to significantly increase throughput and extend link coverage without requiring additional bandwidth or increased transmit power. MIMO can be implemented using several different techniques, including spatial multiplexing, adaptive antenna systems (AAS), space-time coding (STC), and maximum ratio combining (MRC).

Spatial multiplexing generally improves performance by increasing system capacity. AAS expands network coverage by directing signal power to users or nullifying the impact of interference sources. STC (transmit diversity) and MRC (receive diversity) send and receive multiple copies of the same user data, respectively, to avoid impairments such as attenuation.

Traditional testing methods and limitations

As new complex and specialized hardware and software become available, the challenges of testing are increasing. One area that has changed radically is test tools, especially those used to validate next-generation RF transmitters and receivers. There are three approaches to testing transmitter and receiver performance: direct cable connection, over-the-air testing, and connection with a channel emulator, as shown in Figure 1.

Figure 1: Three methods for testing transmitter and receiver performance.

RF testing has traditionally been performed in the lab, then in space, and then in the field. Typically, the first step in testing an RF circuit is to directly connect the RF transmitter to the receiver using RF cables and connectors. While this test configuration allows the performance of the transmitter and receiver to be tested in a controlled and repeatable manner, it is an idealized, non-realistic scenario.

Cable-based testing cannot meet the more complex testing requirements needed to simulate real-world dynamic conditions when signals are transmitted through space and wireless devices are in motion. Because MIMO technology relies on real-world behavior to improve operational performance, laboratory/cable-based testing is ineffective in some test situations and inappropriate in many others.

Over-the-air (OTA) testing, which refers to the testing of the process of propagating RF signals through space to a physically remote receiver, can provide a certain degree of performance measurement, but it cannot provide consistent accuracy and repeatability. OTA tests are generally conducted in large rooms or in the field, and their conditions are not completely controllable or repeatable, resulting in different results from different tests. The effort to build hundreds of complex test environments under multiple field conditions of noise and different interference is a time-consuming and expensive task.

Advanced 4G Testing: Channel Emulation

The introduction of LTE and mobile WiMAX technologies, especially their OFDMA and MIMO features, requires advanced testing capabilities and tools to ensure that product designs meet or exceed the performance expectations of 3GPP/802.16e. The complex physical layer and open access requirements of 4G technologies based on MIMO and OFDMA also place higher demands on wireless performance/interface quality, making 4G testing more complex than ever before.

To bridge the gap between lab and OTA testing, channel emulators are often used to accurately characterize the impact of multi-channel RF interactions on conformance, performance, and interoperability of MIMO and OFDMA-based systems in a lab environment. [page]

Using complex channel models and multiple programmable parameters, channel emulators replicate real-world channel propagation conditions in a controllable and repeatable manner, allowing manufacturers and service providers to test devices under real-world conditions, thereby minimizing the time and cost of testing in the "real" world.

By reproducing real-world channel conditions in the lab, the channel emulation test process for base stations and mobile devices can be greatly simplified and efficiency can be significantly improved. Channel emulation is the key to effectively testing LTE, mobile WiMAX or any MIMO-based system. Channel emulation is required from initial research and design to system integration, quality assurance and even certification and competitive benchmarking. Using channel emulators, RF design and performance can be verified, test coverage can be expanded, test times can be reduced, and higher quality products can be brought to market in a relatively short time.

Suitable channel emulator

To meet the testing needs of the latest and emerging mobile wireless technologies, it is critical to evaluate whether the channel emulators selected for these tasks meet the diverse requirements of conformance, functionality, performance, and interoperability testing. As mentioned above, the latest wireless broadband technologies use OFDMA, which transmits on high-order modulation up to 64-state quadrature amplitude modulation (64QAM), and also use MIMO technology that utilizes multiple antennas.

Therefore, the channel emulator needs to have RF fidelity and scalability to handle and test these functions. Listed below are some of the key technical indicators and capabilities that engineers should focus on when selecting a channel emulator for test equipment and networks. Figure 2 shows an example of testing using a channel emulator. With a full bidirectional MIMO channel, the eNodeB can connect to multiple user equipment (UE). The channel emulation environment provides a different path for each user and provides a real-world scenario with attenuation in both the downlink and uplink. The characteristics of this channel emulation environment are given below.

Figure 2: Example of testing using a channel emulator.

Scalability/Multi-channel: Up to 4×4 structure to support spatial multiplexing, space-time coding/maximum ratio combining and beamforming. Flexible configuration: Includes the ability to configure point-to-point and point-to-multipoint, one-way and two-way for throughput and handover type testing. Two-way: Time division duplex (TDD), frequency division duplex (FDD), beamforming, etc., all require real-world uplink and downlink directions. High RF fidelity: Sufficiently wide error vector magnitude (EVM), noise floor and dynamic power range that will not hinder testing. Channel models: Standard and user-defined channel models are required to test a variety of scenarios. Real-time dynamic channel modeling: Long model repetition and playback time. Control and automation: Graphical user interface (GUI) and script-based simulation control to build test automation. Ease of use: Simple setup, configuration and test execution and collection of test results to support standalone or integrated testing.

Conclusion

Complex broadband wireless technologies such as LTE and mobile WiMAX that use MIMO and OFDMA technologies, coupled with high user expectations for new mobile broadband services, are driving the need for more in-depth and extensive testing to ensure network interoperability and performance. Using a channel emulator to test devices using recreated real-world channel conditions greatly simplifies the implementation of more comprehensive testing.

By providing a broad and cost-effective solution to test whether RF and MIMO algorithms are working properly and predict the performance of MIMO-based products in real-world environments, lab-controlled channel emulation can accurately characterize the impact of wireless interactions between devices and networks on conformance, performance, and interoperability. Only through such comprehensive testing can manufacturers ensure that new, data-intensive applications and devices can successfully function on 4G mobile networks.