Meeting The Challenge

To match such demanding requirements, it’s necessary to break design down into sub-systems and build a test plan that allows each part of the design to be characterized. Thorough testing at each integration stage is essential before testing the complete device. Without it, the diagnosis of problems can occur so late in the program that it becomes difficult to manage the final release stages, including field trials and compliance testing.

How the tasks are broken down will depend to some extent on how the device has been developed. Device manufacturers currently buy in at least some of the hardware or software. Thus, the RF section, baseband processor, system software, or application software may have been procured externally, so it can be expected to have been tested to some extent. Alternatively, an existing platform may be extended to add support for LTE, so the embedded operating system and legacy radio access technologies will be reused. Finally, some designs will be completely new, leaving behind all the associated compromises, though all of aspects of the design will be unproven.

Measurement Needs

Regardless of how the design has come about, several key performance measurements need to be made. Some of these are familiar from previous technologies, like maximum output power, power control, and receiver sensitivity. But due to the transmission schemes used (OFDMA in the downlink, SC-FDMA in the uplink), new measurement equipment will be needed to support these tests.

Other measurements are specific to LTE. With its OFDMA transmission scheme, for example, error vector magnitude (EVM) per sub-carrier becomes an essential test of modulator performance. As the modulation bandwidth becomes a higher percentage of the center frequency, this can pose a challenge with some modulator architectures.

As a result, the EVM can be seen to rise at the band edges, so it needs special attention at the design stage. With the availability of the 700-MHz analog TV spectrum, LTE likely will be deployed at lower frequencies than GSM or WCDMA, resulting in 20 MHz/700 MHz = 2.8% bandwidth compared to 5 MHz/2100 MHz = 0.24% for typical WCDMA devices.

Six channel bandwidth allocations are specified for LTE operation: 1.4, 3, 5, 10, 15, and 20 MHz. It’s necessary to measure the occupied bandwidth to ensure that the transmitter output remains within the channel bandwidth for all channel allocations. The same applies to measurement of adjacent-channel leakage power ratio (ACLR) to ensure the interference between devices using adjacent frequency allocations is kept within specification.

Due to the dynamic nature of some of the tests, such as power control, the measurement conditions need to be established using the signaling protocol. This makes it essential for the test equipment to include the protocol stack, simulating the evolved Node B (eNB) basestation. Since RF engineers usually perform these measurements, the test equipment used must be easily configured, allowing the engineer to focus on the measurement being made.

Although the LTE physical layer uses a cyclic prefix to add resistance to multipath effects, it needs to be tested to ensure correct operation. Leaving this testing until the field trial stage adds risk to the development. Fortunately, test equipment suppliers provide facilities for simulating real-world signal conditions in the lab, with built-in fading simulators and noise generators.

These features enable the impact of multipath fading and noise on the end-to-end throughput to be assessed. This enables a real-world view of the behavior of the device in the field under worst-case conditions to be seen early in the design cycle, allowing time for any problems to be resolved.

An important factor determining the performance of an LTE device will be its ability to achieve and maintain synchronization with the downlink signal from the basestation . The LTE OFDMA scheme uses 15-kHz spaced subcarriers. To remain orthogonal, the receiver must stay precisely tuned to the subcarriers even under the effect of Doppler shift. Lack of synchronization results in inter-subcarrier interference, consequently reducing SNR. To characterize the device’s behavior, the ability to simulate Doppler shift in the lab again is essential.

Protocol Testing

One of the challenges for the protocol stack developer will be to ensure that the state change response requirements are met. Although the LTE specifications have reduced the number of states that a terminal can be in to RRC­_IDLE and RRC_CONNECTED, the time it takes to change from one to the other will be a major part of the delay budget when data needs to be sent.

In RRC_IDLE mode, as much of the device as possible will be in a low-power consumption state to ensure good battery life, with only the receiver activated periodically to check for paging messages. When data transmission is to be scheduled, the device must wake up and rapidly synchronize its uplink.

Protocol testing can often involve spending as much effort generating test cases as creating the protocol stack, so access to comprehensive and efficient test facilities is vital. To be able to break down the testing, it is important to be able to test each sub-layer in both the User Plane and Control Plane:

● MAC: scheduling and hybrid ARQ, logical channel multiplexing
● RLC: in-sequence delivery of SDUs, segmentation/concatenation of IP packets, retransmission of erroneous packets and duplication removal
● PDCP: IP header compression, ciphering, and integrity protection
● RRC: handling of idle and connected state activity, system information, mobility management, and connection control
● NAS: control of cell selection/re-selection, location registration, and de-registration
● IMS: SIP/SDP IP real-time service and connectivity handling

Protocol test diagnostic features are key to tracing faults. Typically, this would include time-stamped message logging and decoding. But it is important that this is available for each sub-layer, providing the ability to trace through signaling message flows in detail, from MAC PDUs up to RRC messages, ensuring timing requirements are met.

The ability to create test scenarios for each layer requires detailed control of the test equipment, but this needs to remain as easy to use as possible to avoid a painful learning curve. Graphical test description, which the Aeroflex 7100 Scenario Wizard can provide, offers the clearest method of defining new tests (Fig. 1).

Both positive test (does it work as intended?) and negative test (does it recover from fault conditions such as timeouts and dropped signals?) scenarios need to be created. Typically, many hundreds of test cases are created during the protocol stack development cycle, and it’s important to manage, maintain, and reuse them later in the regression test phase when bug fixes and new features are introduced.

Performance Testing

Once the device is integrated, the overall performance needs to be fully characterized. During this stage, it will be necessary to trace and eliminate bottlenecks to maximize data throughput. This needs to be done under normal and extreme conditions of temperature and power supply voltage. Power consumption, thermal characteristics, electromagnetic compatibility (EMC), emissions, and susceptibility also all need to be measured under full load conditions. Generally, this will involve using 2x2 downlink multiple-input multiple-output (MIMO).

The ability to seamlessly hand over between cells while minimizing the interruption to data throughput needs to be assessed, as does the ability to hand over between different radio access technologies while maintaining the data connection. Compact, flexible, and modular instruments are already available from multiple vendors. For example, Aeroflex’s LTE test products support all the features necessary to characterize the performance of LTE devices (Fig. 2).

Conclusion

For the next generation of mobile devices to provide a mobile broadband experience that matches the hopes and expectations of the network operators, it will be necessary to test new LTE devices using a layer-by-layer approach, building up to an end-to-end test scenario that uses real-world signal conditions. Ensuring performance is maintained throughout the cell will be the most difficult challenge, especially as the number of users in the cell grows—and with it the signal noise level.

The thorough and efficient testing of LTE devices requires comprehensive test coverage: RF, baseband, and protocol. Test equipment vendors are providing this capability with both new and upgraded instruments, test sets, and systems already available.

References

1. The Global Mobile Suppliers Association, www.gsacom.com
2. 3GPP, TS 36.101