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Plugging Loopholes: Handset and chipset testing key to a superior user experience

October 13, 2017
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The proliferation of 4G networks worldwide has resulted in the emergence of new mobile capabilities, which can support a plethora of advanced value-added services and applications. These networks come with advanced modulation features and leverage multiple antenna paths and diversity techniques to deliver a superior handset user experience.

While the wireless networks before 4G were all analog and were dedicated to voice communications, 4G networks are all digital, and use internet protocol (IP) technology with packet data and concatenated forward error correction to achieve high data rates with low bit-error rates. Even voice is transferred digitally in 4G networks by means of voice over internet protocol technology.

4G-enabled handsets require an end-to-end testing and monitoring solution to deliver high quality offerings and support a wide range of activities. Test instruments intended to evaluate a 4G phone must emulate not only the transmit and receive characteristics of a 4G base station but also environmental conditions such as signal fading and multipath.

Emerging trends in handset testing

Handset users are increasingly demanding a superior broadband experience given the availability of high speed networks. This is driving manufacturers to develop new technologies such as multiple input, multiple output (MIMO) antenna designs to improve the radio performance of mobile devices. Devices are consequently becoming more complex and hence require more extensive testing prior to market introduction. MIMO over-the-air (OTA) performance testing can be used to assess the end user’s experience of accessing data services on a mobile device by replicating real-world conditions.

OTA testing is conducted in the laboratory and involves the testing of a wireless device without any connected cables. It works on the principle that when a radio wave’s path interacts with an object, the radio wave is scattered, diffracted, reflected or absorbed. A radio channel emulator accurately simulates this behaviour and essentially replicates real-world radio channel conditions within a laboratory environment. These conditions include multipath propagation, such as per-path delay, the Doppler effect, angles of departure, angles of arrival or polarisation, which affect the base station antennas as well as noise and interference. MIMO OTA testing uses channel emulators to accurately emulate different environments such as urban, suburban, rural and indoor environments.

MIMO tests are typically undertaken in two stages. The first stage involves taking measurements in a cabled environment to establish a performance baseline for the receiver and baseband processing. The main purpose of this is to measure the radio frequency (RF) throughput rather than the IP throughput. After this, most test labs undertake external signal fading to get a sense of how the throughput varies under changing signal conditions. In the second stage, RF measurements are taken again, but in an anechoic non-echoing chamber, duplicating the environment of a cabled framework. The effects of the antenna are considered in this stage.

However, the major issue with regard to MIMO technology is how well the conditions captured in the field can be translated at the test bench. Recreating the environment in a test lab that is replete with multipath reflections and the Doppler effect is exceedingly difficult, but is critically important for network operators, infrastructure vendors and chipset makers.

Testing VoLTE-enabled devices

Instead of supporting data on a voice network, a large number of service providers globally are now delivering voice over LTE (VoLTE) networks. Transmitting voice over packet-based networks such as long term evolution (LTE) with acceptable quality requires the right mechanisms and architectures in both the radio and the core network.

Since LTE coverage is still limited, LTE services need to coexist with legacy networks. In many cases, LTE voice calls are being carried by the 3G network, particularly when a user accessing data receives an LTE call. However, the process of shifting LTE calls and data access to the 3G network, known as circuit-switched fallback, can compromise the quality of service (QoS) experienced by the user. Another consideration with regard to VoLTE services is the mobility of voice subscribers. A voice call initiated within LTE coverage must not be dropped when the subscriber moves out of LTE coverage to a 2G or 3G network coverage area.

IP multimedia subsystem (IMS) is a key technology for integrating voice services in an LTE network. IMS offers a framework that supports IP-based multimedia services. The session initiation protocol (SIP) is the base protocol of IMS. SIP uses an IP network to establish the connection between subscribers. To ensure a consistently high QoS, network operators have started to roll out VoLTE services for voice and video telephony based on IMS, providing a seamless transition to circuit-switched services, that is, to 2G and 3G networks.

While developing an LTE-or VoLTE-enabled device, operators need to conduct tests on several areas such as on the functional aspects of the device (LTE attach, IMS registration, etc.), how well it performs in a noisy or faded environment, IP traffic and how this affects battery life, and whether it can meet 3rd Generation Partnership Project and mobile network operator conformance requirements.

In the case of VoLTE, handset manufacturers seek to ensure that the underlying protocol features are present and functioning correctly in their devices. In addition to layering the IMS subsystem on top, they attempt to simulate the delivery of a voice call in an end-to-end scenario. In this process, they attempt to ensure that the underlying IMS and protocol is functioning and performing as intended in order to deliver the expected QoS.

VoLTE testing includes development testing, conformance testing and interoperability testing. Once VoLTE technology becomes mainstream, testing will have to be applied at all levels of the product value chain, from chipsets to the network. Conducting tests as lab-based simulations rather than relying solely on field trials can help save time and money. However, a key challenge with regard to VoLTE testing is that not all operators are deploying the technology in a consistent way, and there is a degree of ambiguity based on the different interpretations of the standards. In order to address this problem, testing solution providers can create their own IMS implementation mechanisms rather than using integrated third-party IMS servers. When the VoLTE market evolves, vendors will be able to fine-tune the services as per the required IMS implementation. This approach will allow vendors to be more flexible in supporting the different testing requirements of their customers.

Chipset testing

The testing of chipsets is critical to ensure that devices deliver exceptional performance under all use cases and do not impair the performance of other devices sharing these crowded networks. Testing should be conducted under many different radio conditions and active frequency bands.

Chipset and device manufacturers develop platforms and reference designs that provide wireless modem channels to multiple end products. For the development of these core platforms, protocol testers need to support leading-edge technologies to help in initial feature development such as a digital baseband interface that can be used prior to baseband integration with the RF front-end.

Developers need to ensure that the chipsets continue to work perfectly even after code changes are made. Further, regular builds and frequent regression tests are needed to detect any unwanted side effects of a new feature. Moreover, protocol testers for regression analysis need to support the automated testing of multiple technologies so that broad functional test coverage can be achieved with minimal human intervention. It is also important for smartphone and other device manufacturers to integrate the core wireless platform into their end products. They need to find a way to create a comprehensive set of reference tests that can be easily extended as new platform features are integrated into handsets.

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