Test and Measurement for LTE Products

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LTE offers tremendous opportunities for test equipment vendors. As it is still a rather new technology, market research indicates that service providers prefer to purchase backward- and forward-compatible testing tools for future use, LTE trials or deployments. Since the presence of LTE is only expected to increase with every passing day, T&M players are bracing their services for a variety of challenges and increased activity.

Ashwin Gopinath


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FSW43 signal and spectrum analyser by Rohde and Schwarz (Courtesy: www.rohde-schwarz.com)

Long-term evolution (LTE) is a standard for wireless communication of high-speed data for mobile phones and data terminals. Based on the GSM/EDGE and UMTS/HSPA network technologies, it increases the capacity and speed using a different radio interface along with core network improvements. This standard was developed by the 3rd Generation Partnership Project and is specified in its Release 8 document series, with further enhancements described in Release 9. The LTE eco-system is steadily building, with several LTE platforms already announced and demonstrated by leading device/chipset manufacturers.

According to market research studies, the global LTE handset sales will increase from 70 million units in 2012 to 150 million by 2013, and LTE subscribers will reach 380 million by 2015. By 2013, operators would have spent over $8.6 billion on LTE base stations infrastructure and LTE will grow faster than 3G.

LTE testing: What to focus on
With any new technology, one of the greatest challenges before production engineers is what to test and why. For LTE, the degree of complexity is unprecedented and testing it all would have devices sitting on the tester all day long.

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In production, the basic assumption has to be that the design handed off from engineering meets all the requirements of the customer and, when assembled correctly, will do so consistently. While supporting this assumption places a burden on the engineering team and its processes, without this assurance, the dimension of tests is simply too large to examine all the possibilities with today’s extremely complex devices.

Here are some of the many ways in which LTE testing is different from other testing domains:

1. Orthogonal frequency-division multiplexing (OFDM), a method of encoding digital data on multiple carrier frequencies, is used extensively in areas like DSL broadband Internet access and LTE mobile communications. OFDM is, in general, very sensitive to frequency and clock offsets.

2. Single-carrier FDMA (SC-FDMA) is a frequency-division multiple access scheme. Like other multiple access schemes, it deals with the assignment of multiple users to a shared communication resource. It has been adopted as the uplink multiple access scheme in 3GPP LTE. It is imperative to test and ensure that the peak-to-average power ratio is really low so as to minimise uplink interference.

3. Bandwidths up to 20 MHz are used by LTE, which requires power amplifier and IQ modulators to have a flat frequency response. Use of higher-order modulation schemes (16QAM and 64QAM) requires a high modulation accuracy even when noise, fading and interferences are present and as such, testing LTE for high performance is a no-brainer.

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4. There isn’t any transmit filter definition in LTE, as is the case for 3G technologies. Hencein-channel performance (error vector magnitude) and out-of-channel performance (adjacent channel leakage ratio, spectrum emission mask) requirements need to be met by the design, which necessitates the testing process.

5. Testing multiple use of antennae (MIMO) as part of performance requirements is a must for a system which essentially uses that feature as a differentiator. Transmit diversity and spatial multiplexing (closed-looped and open-loop) are among the most important parameters to be tested.

Madhukar Tripathi, regional manager, Anritsu India, says, “The major parameters to be measured in LTE domain depend on the device under test (DUT). Peak data rate, control plane latency, user throughput and spectrum efficiency are measured for radio interface. Since LTE has different modulation schemes, those are also tested. The supported downlink data-modulation schemes are QPSK, 16QAM and 64QAM. The possible uplink data modulation schemes are (pi/2-shift) BPSK, QPSK, 8PSK and 16QAM.”

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Stephen Hire, general manager, Aeroflex Asia-India, adds, “While each generation of mobile phone technology brings greater benefits to the subscriber, it also adds greater complexity—especially in RF design, protocols and signalling, and battery life. This drives the need for ever more complex test equipment to bring mobile handsets and network equipment to market. Handset and chipset vendors are using our 7100 LTE radio test set and network emulator to optimise battery life and maximise data transfer rates to improve the user experience when browsing the Web or watching video. The larger the network and the greater the number of subscribers, the more important it becomes to squeeze every last drop of performance out of each part of the network.”

Tackling challenges
Being a relatively new technology, LTE poses unique challenges to test engineers. RamaRao Anil, product support and application, Rohde and Schwarz, explains, “LTE is fast changing, as we move between the different 3GPP releases. There is demand for new features to be incorporated in our test solutions. Also, these feature sets are quite new from the implementation perspective both for the chipset/UE and test vendors. Hence it becomes very important to collaborate and align roadmaps with the UE/chipset vendors to be successful.”

Most LTE devices are expected to support both legacy 2G and 3G technologies. The testing has to take that into account. A host of valid scenarios for inter-technology mobility need to be validated to ensure seamless performance.

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LTE itself offers two variants, viz, time-division LTE and frequency-division duplexing LTE, and separate testing is required for device flavours catering to each variant. Additionally, device vendors need to customise and maintain a large number of firmware releases to address the varying needs of individual operators.

Sarma Yanamandra, wireless systems application engineer, Agilent Technologies, adds, “With the blistering speeds LTE offers, the major concern for the T&M vendors is with the HW to support it. MIMO is the way going forward for increased diversity and throughputs and there are several configurations that are defined. So, it becomes even more difficult to use the same HW to support such configuration due to some obvious reasons. In addition, as LTE has to co-exist with the older technologies like 2G and 3G along with 1x and EvDo the T&M vendors have to feature the support in their products. ”

An LTE network is composed of multiple types of eNodeB’s—femto, pico, metro and macro—each with a specific form factor and capacity. There is a clear need to focus not only on individual node types but also on how they stack up and work collectively, which is extremely essential as the collapse of the smallest layer can mess up the whole network. Also, functionality and performance of the ‘brains’ behind the success of heterogeneous networks (or HetNets), viz, self-organising networks and radio resource management, need to be carefully evaluated.

Ravichandran Raghavan, technical marketing engineer, National Instruments, India, summarises, “Traditional RF test and measurement vendors rely on existing tool-sets and methodology and try to extend it for newer communication standards, thereby not fulfilling the need for higher throughput and performance. With the ever increasing demand for testing LTE products, the number of LTE chip manufacturers is on the rise and there is a need for bringing down the cost and time to test these products and maintain a high quality.”

The future of the ‘future’ of mobile networks
In telecommunication circles, the evolution to 4G is a topic that has garnered significant interest from operators and network equipment manufacturers alike. With LTE being touted as the future of mobile broadband communication, communication test equipment companies will play a pivotal role in the development, support and optimisation of LTE. The LTE test equipment market is highly competitive with several companies vying for the position of the leading technology provider with solutions that incorporate the latest standards and technologies. As a result, an in-depth understanding of the trends impacting the LTE market is imperative to maintain a competitive edge in this market.

Anil explains, “As LTE as a technology is a fully packet-switched network, focused on high-speed data services, more emphasis on ensuring QoS and key performance indicators is a must while testing. Rohde & Schwarz has developed the PQA test system, exactly for this purpose. PQA ensures that all key performance indicators of the device are fully tested for optimal performance under field conditions.”

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With only about 9 per cent fixed-broadband penetration of households, mobile broadband is well-positioned to become the main means of Internet access for many Indians. This has already started with 3G, and LTE—both TDD and FDD—will further enhance access to the Internet over the coming years.

For equipment vendors and operators, growth in mobile broadband shifts the testing emphasis from simple compliance to 3GPP standards to more towards benchmarking and performance measurements. As network deployments accelerate, the numbers of handsets and devices ‘supported’ by LTE base station vendors is becoming a key marketing metric; however, some have struggled with real-world delivery of lab-measured metrics.

Hire adds, “In order to meet the capacity demands, there will be a shift away from centrally-managed networks based on macro base stations to HetNets comprising a mix of home-based femtocells, small cells (micro- and picocells) for hotspot and in-building coverage, and traditional macrocells. Creating a heterogeneous network is an attractive approach to meet traffic demands and performance expectations, particularly in situations where traffic is concentrated in hotspots, or areas that cannot be suitably covered by the macro layer. A heterogeneous network combines big, traditional base stations with small cells. It will reduce carriers’ reliance on large, macro base stations wherever they need coverage, and enable mobile operators and end customers alike to increase capacity where needed.”

Yanamandra says, “The most and immediate push is towards making the handsets as well as the operators stabilise the interworking between the existing technologies and LTE. Apps development on the most prominent mobile OSs’ like IOS, Android and Windows is at its peak and similarly the Network operators enhancing their value-added services utilising the data speeds in providing rich multimedia services like IPTV (which was never to be a reality earlier), LTE Direct, etc. This will be future of wireless.”

Changing test landscape
The next generation of mobile devices will need to provide a mobile broadband experience that matches the continually increasing expectations of the various 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. The most difficult challenge will be to ensure that the performance is maintained throughout the cell, especially as the number of users in the cell, and hence the signal noise level, increases.


The author is a tech correspondent at EFY Bengaluru

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