3GPP Long Term Evolution Status & Challanges
>> Testing Times For 3G Evolution
Evan Gray (Aeroflex Test Solutions)
Componenents In Electronics (CIE)
May 2008
cieonline.co.uk
Long Term Evolution is happening now but having the right test equipment, according to Evan Gray, will be vital to its success.
With over 2.5 billion users the GSM/3GSM radio access technology dominates the global cellular landscape. To keep pace with this growth, and the demand for ever-faster, more bandwidth-intensive services, it is essential that the technological infrastructure continues to evolve, while remaining efficient, competitive and successful.
The key to sustaining this success is the development of the underpinning technology. The responsibility for defining this lies with the 3GPP standards forum which, for past three years has been specifying the next major evolution of the GSM/UMTS standards. This evolution has two threads: the enhancement of the existing WCDMA Universal Terrestrial Radio Access Network (UTRAN) through the High Speed Packet Access Plus (HSPA+) specifications and the Long Term Evolution (LTE) specifications for the Evolved Universal Terrestrial Radio Access Network (E-UTRAN).
Current Status
HSPA technology is now an established commercial reality and 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 and improved response times.
LTE, together with the System Architecture Evolution (SAE), promises a more significant evolution to a system that will deliver peak data rates of 300Mbps in the downlink and up to 75Mbps in the uplink
The LTE/SAE specifications define a simplified, optimized, all-IP core network that will deliver operational improvements. This will include higher 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 spectrum required for LTE will come from 3G extension bands and GSM spectrum refarming, which is fragmented and spread over a wide range (from 400MHz to over 3 GHz). 3GPP LTE specifications will address this issue of diverse allocations of 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 LTE Radio Access Network (RAN). January of 2008 saw an important milestone 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 LTE experimental demonstrations and pre-commercial trials already under way.
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 LTE.
Early Challenges
While great progress has been made in establishing the first formal 3GPP LTE RAN specifications and early proof of concept trials, there are plenty of challenges ahead and in these early stages, the fast pace of development, the implementation of a new air interface and the architectural changes in the RAN are likely to be demanding.
The fast pace of LTE development means that the implementation of early systems is 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 formal 3GPP specifications. This has led to solutions incorporating substantial elements of proprietary assumption and customisation.
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 LTE systems under development, it is essential that the test process and equipment can support the latest core 3GPP specifications as well as the proprietary assumptions and customization.
New Air Interface
The LTE/SAE architecture enables significant re-use of legacy infrastructure, especially in the core network. Within the air interface, the 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 test and validation to proceed up as quickly as possible
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.
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 test equipment, when it is needed, is a critical factor in this process.
Being based on the ubiquitous and 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 trials and commercial programs reflects this opportunity.
There will certainly be testing times ahead, but is seems clear that the “Long” in LTE means that it will serve the mobile industry for the long term—and without delay. LTE is happening now. ###
- Eric - |
