Urban small cells, especially LTE ones, need tight timing synchronisation to ensure they work well together with the macrocell layer. GPS is a popular method but may not be entirely reliable in urban canyons. We look at alternative methods around this problem.
Why is sync important for Urban small cells?
Outdoor, urban small cells installed in the street canyons are almost entirely installed to add extra capacity, squeezing the most out of available spectrum in dense metropolitan areas. This means they’ll be using some of the more advanced LTE features to make best use of spectrum, co-operating closely with existing macrocells using features such as eICIC (enhanced Inter-Cell Interference Co-ordination) or TD-LTE mode. These require not just frequency synchronisation but also phase timing alignment throughout the network. Macrocells can share the spectrum and go quiet for short periods (using Almost Blank Subframes) to allow the small cells a chance to communicate with their local users.
This is even more important for some of the network virtualisation architectures, where CoMP and similar features mandate very tight co-ordination and synchronisation.
A clear requirement for Sync – but which method?
This has led to clear operator requirements for timing and sync sources, preferably two different methods to allow for resilience and redundancy. GPS is often used as a primary source because it’s a straightforward low cost point solution which gives very high accuracy (as good as 30ns phase timing).
However, there are many practical difficulties ensuring GPS works in all cases. Although only one satellite signal is required to derive timing compared with three for location, satellites are constantly moving around with resulting signal variation.
A good view of the sky is not always a given. Narrow street canyons, footbridges and walkways above the small cell – even the backhaul transmitter installed directly above it on the same street pole – can all degrade or block adequate GPS reception.
Backhaul based synchronisation a popular second choice
SyncE and IEEE 1588v2 are a popular secondary source because they are delivered through the backhaul connection and are unaffected by interference, jamming and don’t need a view of the sky. A downside is that the SyncE and 1588 signals must be engineered end-to-end throughout the network, on an all-or-nothing basis.
Pretty much every wireless small cell backhaul vendor today complies with the ability to pass through these SyncE and 1588 signals. SyncE is embedded within the Layer 1 framing while 1588 uses IP packets with timestamps. There are some clever workarounds to cope with packet delay variation.
- 1588 TC (Transparent Clock) calculates and adjusts the timestamps immediately prior to restransmission from each node.
- Fitting a small edge-master dongle which cleans up and regenerates a new master clock at several points along the transmission chain.
- Adding router boxes which incorporate their own local GPS receiver and act as local master clock to the adjacent small cell.
Some backhaul designs inherently require synchronisation
Timing synchronisation is also inherent within some types of wireless backhaul, especially those using TDD across multiple links. Cambridge based CCS deal with this by equipping each of their backhaul nodes with a full GPS receiver but also distribute this internal synchronisation throughout their network using a dedicated signalling channel.
This means that each individual backhaul node doesn’t have to receive GPS for it to remain synchronised – worst case, only a single node somewhere needs to at any time. It’s an integral feature of the system and doesn’t require any additional hardware.
Adding backhaul based synchronisation option for small cells
CCS have gone one step further and output their own GPS derived timing signal as a master clock for the small cells. The format is presented as standard SyncE and IEEE 1588, so requires no different or custom adaption of the cellsite. Often this will be done locally within the backhaul node on the same street pole, but it can operate across as many as five or six hops.
Extensive testing by Chronos, an independent expert group of consultants focussed on network timing solutions, has shown that clock accuracy is maintained during transient changes where the master clock source or routing through the network is reconfigured.
You don’t have to use this feature if you don’t want to - CCS also support the standard 1588 TC and SyncE methods used by the rest of the industry – but it’s hard to see why you might choose not to.
This architecture provides three levels of resilience for timing and phase sync in Urban canyons:
- GPS sourced locally by the backhaul node or small cell
- GPS derived synchronisation from the wireless network
- External wireline based clocking using SyncE and 1588v2
Timing and phase synchronisation will become an increasingly important aspect of urban small cell deployment, and even more relevant for network virtualisation.
Bringing the benefits of synchronised wireless backhaul can only improve the resilience of the system and the efficient functioning of the full HetNet system.
Our thanks to David Turner, Head of Technical Sales at CCS, for his insights when preparing this article. CCS are a sponsor of ThinkSmallCell