LTE-Advanced Release 10 is a major enhancement of the Long Term Evolution (LTE) standard developed by the 3rd Generation Partnership Project (3GPP). This new technology is targeting peak data rates up to 1Gbps and introduces new concepts with the ultimate goal of designing a system that is drastically enhanced in both cell capacity and coverage.
Today’s mobile backhaul network architectures will face additional challenges with LTE-Advanced on its way to being introduced by all major mobile network operators. Inter-cell interference coordination (ICIC) and coordinated multi-point transmission (CoMP) are two functions from the LTE-Advanced toolkit that target a better user-experience at the cell edge. ICIC limits 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 the X2 interface as defined in the LTE and LTE-Advanced standard. It facilitates direct communication between adjacent base stations. In order to meet stringent latency requirements of less than 1ms, the physical and logical path of the X2 interface needs to be as short as possible. Supporting this requirement is not trivial for most of the existing mobile backhaul deployments. Today’s underlying architecture is often designed according to a strict hub-and-spoke principle, where traffic distribution and re-direction is architected in the distribution layer of the backhaul network.
In addition to low-latency connections, base station clocks need to be in phase to enable proper operation of ICIC and CoMP. This leads to 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 until now.. The new LTE-Advanced functions, however, require base stations to be in phase with an accuracy of 500ns to efficiently operate ICIC and CoMP. This is nearly impossible to achieve without on-path support, i.e., the backhaul network needs to support the timing distribution architecture actively. The 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 start deploying LTE TDD radio interfaces operating in unpaired spectrum. Many operators have already 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. As ICIC and CoMP, TDD requires phase alignment of neighboring base stations in addition to the traditional frequency synchronization used in mobile networks today.
Heterogeneous radio access networks (HetNets) create further challenges. HetNets are quickly becoming a reality, 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 RAN technology, including 3G, 4G and WiFi.
No matter what type of base station a user is connected to, he should have a superior user experience and be able to roam seamlessly between base stations. The HetNet architecture drives capacities but also requires the backhaul network architecture to be 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 macro cells and small cells including metro cells, pico cells and femto cells 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.
And there is more to come. Since further increasing radio access network performance can only be achieved by very tight coordination between base stations, the base band architecture of the radio access network is foreseen to change as well. Controlling distributed antenna systems from a centralized baseband unit promises additional gains and system simplification. With such a centralized architecture, the requirements on backhaul connectivity will change significantly. Capacities in the range of multiple Gbps and ultra-low latency are key attributes when connecting radio heads to the baseband unit by the Common Public Radio Interface (CPRI).
Simple things are tedious. The challenge is more exciting.