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3G Long-Term Evolution - Testing Times
Evan Gray of the Wireless Division, Aeroflex Test Solutions describes how testing LTE and the test solutions that are available will play a key part in the development of LTE by feeding back vital information into the development.
GSM/3GSM radio access technology dominates today’s global cellular landscape with over 2.5 billion users and this figure is steadily rising. To keep pace with this growth, and the demand for even faster, more bandwidth-intensive mobile telecommunications services, it is essential that the infrastructure technology continues to evolve to become more efficient. The key to sustaining this success is the development of the underpinning technology. The responsibility for defining this lies with the 3GPP standards forum. For past three years 3GPP has been specifying the next major evolution of the GSM/UMTS standards. This evolution has two threads: The first is the enhancement of the existing WCDMA Universal Terrestrial Radio Access Network, UTRAN, through the High Speed Packet Access Plus, HSPA+, specifications. The second is the Long Term Evolution, LTE, specifications for the Evolved Universal Terrestrial Radio Access Network, E-UTRAN. This also includes an evolution of the underlying core network in a development known as System Architecture Evolution, SAE.
Long Term Evolution Status
HSPA technology is now firmly established as a technology and as a commercial reality. There are over 120 networks delivering broadband mobile capability and more than 300 different device types. HSPA+ will access the remaining potential of the existing 5MHz WCDMA radio access networks, offering peak data rates up to three times that of HSPA as well as improved response times. Long Term Evolution, LTE, together with the associated System Architecture Evolution, SAE, will provide a step improvement to the capabilities of the system. It will deliver peak data rates of 300Mbps in the downlink and up to 75Mbps in the uplink, as well as providing a faster response through reduced latency. The LTE/SAE specifications define a new radio interface as well as a simplified, optimized, all-IP core network. These will provide a higher level of spectral efficiency and flexibility, higher numbers of users per cell and lower per-Mbyte cost. The LTE/SAE network architecture will also accommodate the co-existence and interoperation with other Radio Access Technologies including GERAN/UTRAN and even WiMAX. Much of the radio spectrum required for Long Term Evolution, LTE will come from 3G extension bands and GSM spectrum “re-farming.” This radio spectrum is fragmented being spread over a wide frequency range from 400MHz to over 3 GHz. 3G LTE specifications will address this issue of diverse allocations of radio spectrum resource through support of variable RF bandwidths (ranging from 1.25MHz up to 20MHz), paired and unpaired frequencies and multiple RF bands.
Other radio access technology changes employed in the long term evolution are the adoption of OFDMA (Orthogonal frequency-division multiple access (OFDMA), and increased spectral efficiency from multiple antenna (MIMO) technology and higher order modulations. The demanding pace of standards development has so far yielded the specification at around 90% of the fundamental layers of the 3G LTE Radio Access Network (RAN). Major milestone was achieved in January 2008 when these specifications received approval for inclusion into the 3GPP Release 8 standards. Progress has been no less impressive on the implementation side with several long term evolution experimental demonstrations and pre-commercial trials already under way and major new trials due to run in 2008. Aeroflex supports a number of these programs and general LTE test and testing through its TM500 LTE test mobile and PXI waveform generator solutions. This momentum bodes well for the roll out of the first 3GPP LTE commercial systems, forecast to start in early 2010. In the meantime, 3GPP HSPA+ is expected to enable operators to offer early access to evolved services as well as facilitating a smooth transition to 3G LTE. It ensures that there is no competitive gap between today’s HSPA and future 3G LTE networks.
Early Challenges
Significant progress has been made in establishing the first formal 3GPP LTE RAN specifications and early proof of concept trials. However there are still plenty of challenges ahead. The completion of the specifications, followed by implementation, test and initial roll out of commercial systems will continue to require huge efforts from the cellular industry. In these early stages, the fast pace of development, implementation of a new air interface and the architectural changes in the RAN are likely to be demanding.
Long Term Evolution Development
The fast pace of LTE development means that the implementation of early systems is, in a number of areas, running ahead of the formal 3GPP specifications. In particular, the early “proof of concept” systems have been developed in the period preceding the availability of the specifications. This has led to solutions incorporating substantial elements of proprietary assumption and customization. There also remain holes in the 3GPP specification. From the physical layer perspective, these undefined areas are mainly concentrated on the uplink and downlink control signalling. From a higher layers perspective the specifications are not expected to be ready for formal release until September of 2008. In the meantime, proprietary solutions continue to add more customization as required in order to enable the early higher layer operation. For these Long Term Evolution, LTE systems under development, it is essential that the test process and test equipment can support the latest core 3GPP specifications as well as the proprietary assumptions and customization. Thus the availability of 3G LTE test equipment is of paramount importance to assist in the swift location and cure of problems. LTE test equipment ranging from that required in the development laboratories to items such as an LTE test mobile for testing out in the field are all required.
Long Term Evolution Air Interface
The LTE/SAE architecture enables significant re-use of legacy infrastructure, especially in the core network. Within the air interface, the 3G LTE E-UTRAN borrows from earlier access technologies, but it is essentially new, thus requiring a significant development program. It will be important to gain early debug, test and validation of the key enabling features of the new specification. This includes MIMO, fast, low latency HARQ procedures, 64QAM and the broad set of RF band and BW combinations/ configurations that provide spectrum flexibility. Establishing the fundamental building blocks early on will enable LTE test and validation to proceed up to system level as quickly as possible
Changing Architecture
The LTE/SAE requirements to minimize the overall network architecture and protocol, and reduce latency, lead to significant differences between the E-UTRAN and the UTRAN architectures. The UTRAN employs relatively ‘dumb’ physical layer radio base stations (called NodeBs). These connect in a star topology into Radio Network Controllers (RNCs) which carry out the management of the radio resource and connect in turn to the core network. By contrast, in E-UTRAN, much of the radio resource management is devolved into the base stations (called eNodeBs or eNB). eNBs now connect directly into the core network gateway via a newly defined “S1 interface.” eNBs are also interconnected to adjacent eNBs in a mesh via the “X2 interface”. In addition to the new layer 1 and layer 2 functionality, the eNB will also handle radio resource control, admission control, load balancing and mobility. The high level of functionality and performance required from the eNB base station make it a complex and critical entity in the LTE architecture.
Solutions
To achieve fast time-to-market of E-UTRAN infrastructure with evolving specifications, new technology, and different architecture demands an especially rapid and efficient development and test program. Having the right LTE test equipment, when it is needed, is a critical factor in this process. Building on a successful track record of providing such test equipment to mobile wireless programs, including HSPA and HSPA+, Aeroflex is already supplying equipment to a number of LTE developments. Advanced test equipment like the Aeroflex TM500 LTE test mobile and PXI 3000 Series of LTE test equipment have been specifically designed to address the challenges outlined above. The Aeroflex solutions employ both a powerful software defined radio platform (SDR) an, a fast and efficient software development process. This enables the equipment to support not only the core 3G LTE specification and its evolution, but also proprietary customization. Conclusion Being based on the highly successful GSM/UMTS ecosystem, there must be few who would bet against the future success of its long-term evolution. The rate at which momentum is currently building on Long Term Evolution trials and commercial programs reflects this opportunity. There will certainly be testing times ahead, but is seems clear that the “Long” in Long Term Evolution, LTE means that it will serve the mobile industry for the long term-and without delay. LTE is happening now.
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Evan Gray has 20 years experience in the wireless communications industry. Currently based in Cambridge, UK, Evan is the TM500 General Manager for Aeroflex.