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By Phil Windred of Aeroflex Test Solutions, Wireless Division
Introduction
Currently the GSM/EDGE/W-CDMA radio access technology dominates the global cellular market. Continued development, that keeps pace with customer demands for ever-richer services and also meets the expectations generated by connected lifestyles, is essential if 3GPP technology is to remain competitive going forward.
Beyond the increasing deployment of important technology enhancements such as High Speed Packet Access (HSPA), both on the uplink and the downlink, it is accepted that significant, further 3G evolution is required to satisfy the expected burgeoning future requirements for data-intensive applications. Global telecommunications industry analyst, Analysys, reports that full-featured mobile TV and video broadcast is already pretty much a must-have application. Also, if fixed-mobile substitution is to become a reality, then 3G must also compete head-on with DSL, which means more capacity, lower latency (the time it takes data to travel within the network) and lower cost per Mbyte transmission. Alternative technologies such as WiMAX for broadband wireless access and DVB-H for broadcast are developing rapidly and 3G will need to develop fast to keep pace.
The 3GPP is anything but complacent. Back in 2004, it set the wheels in motion to chart the long-term evolution of 3G and provide the necessary technology advances to ensure its continued competitiveness through the next decade. The result of the process will be the 3G Long-Term Evolution (LTE) standard, or Super 3G as it is sometimes called. With the conclusion of the Study Item phase in September 2006, the standard specification is expected to be delivered in 2007. Initial availability of 3G LTE products for network deployment is targeted for the 2009/2010 timeframe.
The progress of the standardisation work for 3G LTE is based on a set of high-level requirements, the principal aim of which is to further improve service provisioning and coverage but at a reduced cost per bit compared to 3G for both operators and users. All this is to be achieved within the context of an enhanced user experience, operational flexibility that covers both existing and new frequency bands, improved data rates and reduced latency. As such, 3G LTE is expected to provide the major leap forward to ensure 3G remains the dominant cellular architecture.
With the evolutionary focus on enhancing the packet-switched (PS) domain due to the convergence towards the use of internet protocols, 3G LTE holds out the prospect of providing a significant capacity increase of some three to four times that of HSPA in the same bandwidth. Peak data rates of 100Mbit/s over the downlink and 50Mbit/s over the uplink are being targeted. These higher data rates are also being targeted within the context of wide-area coverage. The throughput, efficiency and mobility targets should be met for 5km cells with only a slight degradation for 30km cells. However, 100km cells should not be precluded. Latency is also being targeted both in the control-plane to improve the performance of higher layer protocols such as TCP, for example, and also in the user-plane. Overall, a latency of 20ms is being targeted.
Of added appeal to operators using 3G-based networks is the fact that 3G LTE is truly being designed as an evolutionary upgrade that will utilise existing network resources where possible and not as a revolutionary technology that demands a completely new system to be built from scratch. Inter-working with 3G legacy systems is also a key target to facilitate service continuity.
3G LTE test challenges
Instead of W-CDMA, 3G LTE will utilise the OFDM (Orthogonal Frequency Division Multiplexing) modulation technique in order to obtain an all-important performance advantage for higher data rate mobile applications. Although OFDM is also being developed on WiMAX and EVDO Rev C, this does represent a significant change at the very lowest level of the radio communications. Achieving synchronisation will be a major challenge, as was also the case in the early days of W-CDMA development. As such it will be critical to have test equipment specifically oriented to 3G LTE that can provide observability of the very lowest levels of the radio modem as standard test equipment will not provide any diagnostic data, only a message stating that synchronisation has not succeeded. As such, Aeroflex is designing a pair of test products, the TM500 LTE and 6401 LTE, which will support the physical layer testing of networks and mobile devices respectively. With complete visibility into the very lowest layers of the radio modem, rather than just knowing a failure has occurred, the detailed data generated will allow users to diagnose the actual cause of a synchronisation problem. In the very early development stages of 3G LTE equipment development, synchronisation failure is more than likely to be the result of a misinterpretation of the 3G LTE specification, particularly if a very early version of the specification is being used.
The configuration of prototype test equipment is a major challenge for early 3G LTE development testing. Early testing of the 3G LTE physical layer will need to be done without the support of the higher layer protocol. Early prototypes need to be tested at the lowest possible layer to isolate problems associated with the implementation of the physical layer. This is a separate issue entirely from the problems of integrating the protocol onto the physical layer. Without the actual protocol, it is necessary to completely configure the physical layer using test scripts. As a consequence, many early test failures may not be the result of real problems but rather by a mismatch in the setup between the prototype and the test equipment. With 100 or more parameters that need to be selected, the risk of a mismatch is significant. One way to minimise the risk is, rather than requiring users to write software to execute the test, for test equipment to incorporate a GUI that facilitates parameter configuration through dialogue boxes which allows users to select their values using engineering terms and units.
A further implication of physical layer testing when the higher layer protocol is not available is that test automation is essential to ensure extensive and complete testing. 3G test tools must be designed from the start to be integrated into an automated test environment. The incorporation of test script configuration tools will allow the easy generation of all scripts needed to select the different configurations and tests. These various scripts can then be initiated by a test controller as required to synchronise control of the prototype and the test equipment. It should be possible to alter parameters in real time to enable test coverage to be extended across the extensive range of different configurations used in a live system both in relation to the 3G LTE network and early 3G LTE prototype mobile devices. This will allow the early detection of software bugs associated with particular parameter values that would not otherwise be found without such systematic automated testing.
The implementation of Multi-In Multi-Out (MIMO) antennas to improve the signal strength received by mobile devices from the network is a key new feature of 3G LTE. Test features will be needed that are designed specially for MIMO to ensure that both the network and mobile devices are able to get the signalling right and then also transmit and receive in full synchronisation with the signalling.
As 3G LTE is an evolution of existing UMTS systems based on W-CDMA and will also be fully integrated with existing GSM/GPRS/EDGE networks, seamless handovers will be critical to the gradual roll-out of the first 3G LTE networks and deployment of the first LTE mobile devices. Such handovers might simply be inter-cell between neighbouring 3G LTE cells or they could be handovers to W-CDMA or GSM/GPRS/EDGE as a user moves in or out of LTE coverage. 3G test systems should allow testing of these handovers both at an early physical layer and eventually of the complete system.
Figure 2 – Comprehensive measurements will get to the heart of development problems.
Conclusion
Historically, new technology roll-outs in mobile telecommunications industry have been subject to delays. This has often been due to problems experienced when the new technology mobile devices are tested against the new technology networks. Powerful test tools are therefore needed to support both network and mobile device testing at the very earliest development stages of 3G LTE products. Experience shows that the earlier problems are detected, the lower the cost of correcting them. The availability of appropriate test equipment providing diagnostic data geared specifically to the needs of 3G LTE will not only ensure the early detection of implementation problems but also help accelerate the rate at which they are resolved. This will ensure that the industry can achieve a smooth and rapid introduction of the 3G LTE technology, bringing the benefits of high data rates and low latency that will enable new applications to be supported by 3G LTE mobile devices. To date, Aeroflex's test tools have played a major role within the industry in terms of providing the deep diagnostic data critical to resolving the implementation problems that inevitably arise in the development of any new radio technology and 3G LTE will be no different.
Photo 1 – The Aeroflex TM500 LTE will provide 3G LTE infrastructure design and development testing.
Photo 2 – The Aeroflex 6401 will provide comprehensive protocol analysis and conformance testing for 3G LTE mobile devices.
Author Biography
Phil Windred graduated from Cambridge University in 1987 and joined British Aerospace working as a systems engineer on the design of communications satellites.
He joined UbiNetics in May 2000 working on business development of the WCDMA Test Mobile where he progressed to become Vice-President for the Test & Measurement division.
Phil was appointed General Manager of Aeroflex, Cambridge when Aeroflex acquired UbiNetics in May 2005. Phil has experience in all stages of product development, in engineering, business development product strategy and senior management roles
Aeroflex Test Solutions, Wireless Division 480 Bath Rd. Slough Berkshire United Kingdom SL1 6BE
Tel: +44 (0)1628 604455 Email: wireless@aeroflex.com
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