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Synchronization plays an essential role in all mobile networks. The cellular architecture has stringent sync requirements in order to affect clean hand-offs from cell to cell. As carriers respond to the pressures of their business — demand for more bandwidth, limited spectrum, competitive price pressure—they are evolving new ways to make networks more efficient, scalable and economical. To deliver high bandwidth, small cell sites are part of this direction and consequently a different approach to providing sync to these sites is required. This new approach must deliver the two things most important to the carriers: reliable, precise sync signals to maintain IP-based services and a deployment model that fits the economic parameters of small cell deployment. To better understand the benefits of the approach recommended here, we will begin with an understanding of synchronization in today’s mobile networks.
Traditional cellular networks and sync
Synchronization has always been a key element of digital communication networks. In mobile networks, lack of proper sync destabilizes the cellular base stations and could result in dropped calls when a user moves from one cell area to another.
Different methods were used to address the sync requirement for the different networks types. GSM/UMTS networks need only a frequency source while CDMA and TD-SCDMA networks need a source for both frequency and phase.
As CDMA and TD-SCDMA also required phase synchronization, GPS elements were deployed as the sync source to synchronize the network. In the early days GPS reception was considered unreliable, and vulnerabilities remain today. Consequently, backup technology was engineered into base stations to enable them to stay in sync through periods of GPS satellite invisibility, outage or equipment malfunction to holdover the signal.
Mobile backhaul and synchronization
The rising demand for bandwidth to support LTE requires the mobile backhaul to scale economically. Carriers are increasingly adopting Ethernet for their backhaul infrastructure and transitioning from TDM backhaul.
As TDM lines are replaced at cell sites, alternative modes of delivering synchronization are required. The focus has shifted to packet based sync distribution technologies such as precision timing protocol (PTP) and network timing protocol (NTP). Synchronous Ethernet (SyncE) is a rapidly emerging technology that can frequency lock an Ethernet network just like a SONET/SDH network.
PTP and NTP are both in-band client/server technologies. The PTP standard (IEEE 1588-2008 or PTP 1588v2) specifies hardware-based time stamping and therefore it is more precise. NTP, on the other hand, is standardized as a software implementation and is used for applications such as residential femtocell, IPTV, IMS, billing, CDR etc. In its standard form NTP is less precise, but some implementations use hardware time stamping and work very well for network infrastructure. SyncE is an end-to-end technology that requires new or upgraded equipment to support it from source to end.
As mobile networks increasingly deploy PTP as the network transitions to Ethernet, PTP is rapidly becoming the industry’s technology of choice for synchronization transfer supporting both frequency and phase required for LTE deployments.
Synchronization for LTE
LTE is designed to support IP-based traffic with no provision for TDM backhaul in LTE eNode Bs. The IP-oriented approach enables better integration with other multimedia services.
The core network called the evolved packet core (EPC) supports RAN (evolved UMTS RAN) through a reduced number of network elements and improved redundancy. It also enables carriers to deliver a seamless mobility experience with smooth connections and hand-over to fixed line and wireless access technologies.
Two different LTE variants have evolved: LTE-FDD (LTE-frequency division duplex using paired spectrum, and better suited for symmetric traffic) and LTE-TDD (LTE-time division duplex used with unpaired spectrum, and better suited for asymmetric traffic). LTE-TDD has very precise phase requirements that pose a challenge to any eNode B system design and to network sync distribution technologies.
To meet very strict frequency and phase requirements for LTE, service providers are increasingly adopting a combined approach to back-up their sync network with a combination of two of the following: GNSS, PTP 1588v2 or Rubidium to ensure holdover.
Small cells and a sync architecture for LTE
LTE architecture divides the area into cells of varying size (in-home cells, in-office cells, small cells and macro cells). This approach allows reuse of spectrum to the point where inter-cell interference becomes the limiting factor. This approach, coupled with highly efficient OFDM radio technology pushes the boundaries of data throughput.
A software based multi-sync approach is becoming the optimal way to deliver sync in an LTE network with small cells. This architecture will handle the exacting sync requirements of LTE and does so in an economical, scalable and robust manner.
This new approach recommends deploying multiple standard sync technologies and using the small cell clusters as “synchronization” sub-networks. Multi-standard sync means eNode Bs have the ability to be synchronized by any or a combination of the three primary sync technologies – SyncE, PTP, or GNSS. This enables each individual node to supplement the source of sync available and reinforce when any of the sources fades out (GNSS signal) or becomes degraded.
Multi-standard sync will deliver the robustness required as small cell eNode Bs will be deployed in many form factors and in many physically demanding environments. “Sync clusters” for small cells clusters will reduce technology investment costs and also decrease overall cost of deployment and operations. It is a powerful sync architecture for LTE as the number of small cells grows toward ubiquitous deployment.
To support LTE deployments, networks must ensure that precise sync signals are maintained to ensure QoS to support IP-based services (and quality of experience their customers require) and a sync deployment model that fits the economic parameters of the small cell concept.
A software based multi-sync approach is the most effective method to deliver a guaranteed sync signal taking the best sync input from either PTP, GNSS or SyncE to guarantee the best clock output that support precise IP-based applications. This software based architecture will meet the exacting sync requirements of LTE economically, and ensure scalability.