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Digital Radio Test Set Addresses LTE Test Challenges

February 1, 2009
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Although standardization work is currently on-going, the 3GPP evolved UTRAN LTE specifications are close to providing a workable description for the implementation of complete devices. With the introduction of next-generation mobile network technology, new challenges will face device developers, including how to confirm that their new design functions correctly, meets the standards under all conditions and without over-heating or draining the battery too quickly. The Aeroflex 7100 Digital Radio Test Set is a test solution that has been developed to enable R&D engineers working on LTE terminal design to meet these challenges.


Why is it a challenge?

High performance, wide bandwidths, high data rates, fast response times (reducing latency), more complex antenna configurations—and that’s just the LTE part—all combine to present greater challenges to the development of next generation devices. In order to support roaming onto other network technologies, multiple radio standards will need to be supported, especially with the lack of voice support in early LTE networks.

For much of the world, LTE devices will need to be backed up with GSM/GPRS, WCDMA/HSPA and/or CDMA2000/1xEVDO support in a range of frequency bands, with downlink frequencies potentially ranging from 746 MHz to 2.69 GHz (Ref: 3GPP TS 36.101). Initial certification of LTE devices is expected to be in bands 1 (2100 MHz), 3 (1700 MHz), 7 (2.6 GHz) and 8 (900 MHz) for FDD mode and bands 38 (2.6 GHz) and 40 (2.4 GHz) for TDD mode. The WRC-07 conference allocated further spectrum for mobile use, meaning both lower (down to 450 MHz) and higher frequencies (up to 3.6 GHz) are likely to be seen as LTE rolls out over the next five years.

In addition, emphasis is being placed by major network operators on support for both FDD mode and TDD mode. There are two reasons for this: LTE TDD mode is the accepted upgrade path for Chinese 3G networks and is being championed by China Mobile, a major network operator. Secondly, many network operators already have unused spectrum that is allocated for non-paired operation, so this could be used for LTE deployment. Spectrum (or lack of it) is one of the major issues facing LTE operators, with the need for up to 20 MHz blocks to achieve the 100 Mbps headline data rate.

Another challenge for LTE devices is to maintain data throughput rates at the cell edges, where the SNR is usually worst, and also in crowded cell conditions. For these situations it is essential that the receiver performance is optimized, making the best use of the available signal in a noisy environment.

The typical form factors for LTE devices are likely to be USB sticks, dongles and PC cards in addition to the internal chipsets that will be integrated into lap-tops and high-end PDAs and smartphones. Thermal management will be important in these compact devices when so much functionality needs to be incorporated.

Meeting the Challenge

The 7100 integrates all the major functions needed in a bench-top instrument to enable comprehensive testing during the R&D stages of new devices. It simulates the radio and core networks and provides all the key measurements for characterizing the performance of LTE mobile devices, both at the radio interface and throughout the protocol stack, including the PCDP and IMS layers. End-to-end performance can be accurately assessed, along with correct idle mode and connected mode behavior.

Measurement Needs

There are a number of key performance measurements that need to be made. Some of these are familiar from previous technologies, including 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.

Figure 1 EVM per sub-carrier.

Other measurements are specific to LTE, with its OFDMA transmission scheme; for example, EVM per sub-carrier becomes an essential test of modulator performance. As the modulation bandwidth becomes a higher percentage of the centre frequency, this can pose a challenge with some modulator architectures. As a result, the EVM can be seen to rise at the band edges, as shown in Figure 1. With the availability of the 700 MHz analogue TV spectrum, the likelihood is that LTE will be deployed at lower frequencies than GSM or WCDMA, resulting in 20/700 MHz = 2.8% bandwidth compared to 5/2100 MHz = 0.24% for typical WCDMA devices.

Figure 2 Measuring occupied bandwidth.

There are six channel bandwidth allocations specified for LTE operation (1.4, 3, 5, 10, 15 and 20 MHz) and it is necessary to measure the occupied bandwidth to ensure that the transmitter output remains within the channel bandwidth for all channel allocations (see Figure 2). The same applies to measurement of 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, including power control, the measurement conditions need to be established using the signaling protocol, making it essential that the test equipment includes the protocol stack, simulating the evolved Node B (eNB) base station. The 7100 provides this, allowing standard and user-definable test conditions to be established. For the RF measurements, the signaling protocol operates automatically, using user-definable parameters such as channel number, 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, this needs to be tested to ensure correct operation. The 7100 incorporates a baseband fading option, enabling the impact of multipath fading on the end-to-end throughput to be assessed. This allows a real-world view of the behavior of the device in the field to be seen in the lab, before field trials are conducted.

Figure 3 LTE/SAE protocol partitioning.

The digital radio test set also incorporates a 3GPP Rel-8 compliant protocol stack and physical layer to emulate an eNB and the Evolved Packet Core (EPC) network (see Figure 3). A comprehensive range of RF tests is featured, including some based on the 3GPP TS 36.521 RF test specification, covering the key transmitter, receiver and transceiver measurements. An integrated IMS server allows complete functional testing to be performed, allowing end-to-end throughput and latency to be measured in a controlled environment.

Advanced 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 for the messaging sequence involved (see Figure 4). 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.

Figure 4 RRC connection establishment (from 3GPP TS 36.331).

Protocol testing with the 7100 is based on the Aeroflex Script Editor environment and uses a C++ API to construct signaling sequences. A message editor allows programmers to build customized messages for use in protocol test scripts, useful for both positive and negative testing. Although this can be accessed from the front panel, a VGA monitor and USB mouse and keyboard can also be connected for longer test script development and analysis sessions.

All radio interface protocol layers can be tested:

  • 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

In addition, the control plane signaling layers can also be tested:

  • RRC: handling of idle and connected state activity, system information, mobility management and connection control
  • AS: 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 include time-stamped message logging and decoding, providing the ability to trace through signaling message flows in detail, ensuring timing requirements are met.

Versatility

The range of features included in the 7100 make it suitable for a broad range of test duties in the R&D stages of LTE device development, with the integrated RF, baseband and protocol stack allowing the entire device to be tested. This combination of features makes the digital radio test set appeal to hardware engineers and software engineers as well as system test and integration engineers, right through to the pre-conformance test stage as illustrated in Figure 5.

Figure 5 R&D test stages.

As with all new technologies, there will be a period of time before the specifications are fully stable, so it is essential that the test equipment can be easily upgraded to track the changes. The 7100 is based on a software-defined radio architecture enabling it to be updated in the field as the standards evolve and as new tests emerge. Internally, the RF section is capable of 90 MHz bandwidth, well beyond the 20 MHz likely to be seen on first generation LTE devices. The modular nature of the 7100’s architecture allows future enhancements to the standards to be readily adopted.

Beyond the technical features of the 7100, its touch screen control and large (12.1 inch) LCD screen provides an elegant and intuitive user interface that enables engineers to easily navigate the extensive range of test functionality while providing clear results.

Conclusion

The next generation of mobile technology is rapidly emerging from the standardization process and is bringing with it a new generation of technical problems that need to be solved. Aeroflex is continuing to introduce new test solutions to help engineers working in this challenging new area, in both the RF and protocol domains. The 7100 Digital Radio Test Set is part of the company’s range of LTE test solutions, which also includes the TM500 LTE Test Mobile, the 3410 Signal Generator and the PXI modular system.

Aeroflex Test Solutions,
Stevenage, UK,
+44 (0) 1438 742200,
info-test@aeroflex.com.

RS No. 300

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