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Worldwide mobile data traffic doubled over the past 24 months and is expected to continue growing at a similar rate, due to the continuous rise in smartphone sales and increasing video traffic. To support the expected data growth and maintain superior user experience in the future, radio access network technology needs to continue evolving.
The 3rd Generation Partnership Project (3GPP), a collaboration among groups of telecommunications associations, is defining globally applicable mobile phone system specifications including radio access technologies. In order to support the predicted growth, the 3GPP has developed LTE-Advanced Release 10, a major enhancement of the LTE standard deployed in many mobile networks today. The new technology is targeting peak data rates up to 1 gigabit per second and introduces new concepts with the ultimate goal of designing a system that is drastically enhanced in both cell capacity and coverage.
As LTE-Advanced is introduced by major mobile network operators, however, new challenges are likely to present themselves across today’s mobile backhaul network architectures.
Inter-cell interference coordination and coordinated multi-point transmission are two functions from the LTE-Advanced toolkit that target a better user experience at the cell edge. ICIC is limiting cross-talk by coordinating spectrum allocation across multiple cells. CoMP allows multiple base stations to simultaneously serve a user device and increase the receive power level and, therefore, capacity. Both functions require very short latencies across the backhaul network to achieve real-time coordination between base stations. This implies implementing a high-performance X2 interface as defined in the LTE and LTE-Advanced standard. The X2 interface facilitates direct communication between adjacent base stations. In order to meet stringent latency requirements of less than 1 millisecond, the physical and logical path of the X2 interface needs to be as short as possible.
But supporting this requirement is not trivial with most of the existing mobile backhaul deployments. Today’s underlying architecture is often following a strict hub-and-spoke design introduced with 3G radio access technology, where traffic distribution and re-direction is performed at the security gateway in the distribution layer of the backhaul network.
In addition to providing low-latency connectivity, base station clocks need to be in phase to enable proper operation of ICIC and CoMP. This leads to the requirement for highly accurate phase or time-of-day synchronization. Most 3GPP base station clocks are currently synchronized on frequency only, since accurate phase synchronization was not a requirement up to now. The new LTE-Advanced functions, however, require base stations to be in phase with an accuracy of 500 nanoseconds to efficiently operate ICIC and CoMP. This is nearly impossible to achieve without on-path support (i.e., the backhaul network needs to contribute to timing distribution actively). The International Telecommunication Union -Telecommunication Standardization Sector (ITU-T) Study Group 15Q13 is currently defining telecom profiles facilitating end-to-end frequency and phase synchronization across packet-based backhaul networks with on-path support.
Mobile operators will also introduce LTE TDD radio interfaces operating in unpaired spectrum. Many operators already have acquired unpaired spectrum, since TDD provides more flexible scaling of the up- and down-link capacity and has additional benefits to the overall architecture of the radio access network. Like ICIC and CoMP, LTE TDD also requires phase alignment of neighboring base stations in addition to the traditional frequency synchronization used in mobile networks today.
Heterogeneous radio access networks create further challenges. Hetnets are becoming reality fast, with radio access networks being composed of different types of base stations for maximizing access capacities, optimizing user experience and reducing cost. Base stations can differ in terms of capacity, reach, transmission power and radio access network technology, including 3G, “4G” and Wi-Fi.
A user should have a superior user experience and the ability to roam seamlessly between base stations, no matter the type of base station being connected to. The hetnet architecture drives capacities but also requires the backhaul network architecture to become more flexible and scalable. And it will evolve into a more heterogeneous architecture connecting all base stations in a cost-effective manner. Backhaul service providers and mobile network operators are looking for complete solutions to connect macrocells and small cells including metro cells, picocells and femtocells in private locations.
LTE-Advanced and the tighter coordination between base stations will, therefore, challenge existing backhaul networks with respect to capacity, latency and synchronization performance. Current architectures need to evolve, enabling mobile network operators to seamlessly migrate to LTE-Advanced and enhance cell capacity, coverage and ultimately mobile user experience.
The distribution of timing information for frequency and phase synchronization of the radio access network constitutes a particular challenge. Backhaul networks now need to actively contribute to timing distribution and provide on-path support. This is a new requirement for most backhaul service providers. And it is more than just providing on-path support. Highly accurate synchronization of base stations is elementary for stable operation of the radio access network. Synchronization performance, therefore, needs to be monitored and assured, just in the same way as performance-assured carrier Ethernet services evolved from basic best-effort connectivity. Consequently, mobile operators also want to understand the actual performance of the timing network providing this information. Delivering synchronization is not enough for enabling stable operation of the radio access network. Assured delivery with guaranteed quality of service metrics is a must – no matter whether operating the backhaul network by oneself or leasing backhaul capacity and timing services from independent service providers.
Network timing behavior is not a stationary process. It is subject to dynamic conditions and changes over short and longer term. Appropriate tools for cost-effective and time-efficient end-to-end management of the synchronization network are, therefore, required during all phases of its lifecycle – installation, turn-up testing, monitoring and troubleshooting. Mobile network operators want to identify potential problems before they cause outages and degrade user experience.
