Femtocell synchronization

- an overview of the basics the femtocell synchronization, why femtocell synchronisation is needed and how it is achieved.

Femtocell synchronization is an important aspect of their design. Many aspects of their operation requires reasonable levels of femtocell synchronisation.

3GPP specifies that base station frequencies need to be very accurate, and there needs to be close synchronisation with precise clock signals. Although Release 6 of the 3GPP standard relaxed the requirements for femtocell synchronization, and further relaxation my be possible in the future, methods of providing sufficient femtocell synchronization provide a challenge at the design stages.

Reasons for femtocell synchronization

There are a number of areas of femtocell operation that require synchronisation and frequency accuracy if they are to be able to operate satisfactorily.

  • Supply frequency information to handsets:   It is not possible for the handsets or user equipments to achieve the level of frequency accuracy that can be achieved by base stations. As a result, the handsets synchronize to base stations.
  • Ensure reliable handover:   If the femtocell is not synchronized to the network, and the adjacent base-station, then a variety of offsets may exist that would make any handovers fail or become disrupted.
  • Interference reduction:  By having all base stations synchronised the elvels of interference can be reduced. High levels of interference can reduce call quality and reduce the network capacity.
  • Ensures femtocell to be aware of adjacent cell sites:   If the femtocell is accurately synchronized to the rest of the network it can detect other cells more quickly and thereby improve the operation of the femtocell.

Levels of accuracy

Accuracy levels are defined in many standards for different types of cell. These tend to vary according to the type of cell which the base station is serving.

Base station type Frequency accuracy (ppb)
Wide area 50
Local area 100
Home - femtocell 250

It would be possible to utilise a very high stability standard to produce the required level of accuracy and hence femtocell synchronization. However cost can be an issue and therefore other methods of obtaining the required level of femtocell synchronisation and frequency accuracy may be more effective.

Crystal oscillators are generally at the heart of any clock system used to provide a timing and frequency reference. Different types of crystal oscillator may be used, but these have cost implications.

  TCXO OCXO Hybrid
Price Lower Higher Medium
Accuracy Lower Higher Medium

Cost performance for crystal oscillator types

NB: TCXO = Temperature compensated crystal oscillator, OCXO = Oven controlled crystal oscillator.

It can be seen that the accuracy is broadly reflected in the cost. As OCXOs can be particularly expensive, and often relatively large, other ways are normally sought of gaining he required level of stability.

As the cost of both the OCXO and hybrid solutions are normally too high for the maximum manufacturing costs that are viable for femtocell production, and therefore a TCXO combined with another form of external femtocell synchronization.

Time references for femtocell synchronisation

There are a number of ways in which femtocell synchronization can be achieved. There are several sources of time information that can be used. Some sources are better than others, or two types may be aggregated if required. Also the femtocell may choose the optimum for of synchronisation for the particular area in which it is located.

The major time synchronization sources that are used are:

  • Timing from the Internet:   It is possible to utilise the backhaul and Internet to gain timing information.
  • GPS timing:   Although most femtocells will be located within buildings, it is often still possible to obtain a GPS signal sufficient to gain the GPS timing.
  • Neighbouring cells:   It is often possible to obtain timing information from base stations which will are required to be more accurate than femtocells.
  • Broadcast sources:   There are a number of terrestrial radio broadcast sources that the femtocell can used to gain synchronization. Typically these would include television stations.

Femtocell synchronization and timing from the Internet

Using this method, the femtocell access point can use the backhaul connection to access the clock of the network operator - time servers. There are problems with this method because this form of femtocell synchronization can suffer delays resulting from the varying delays introduced by the Internet - different packets will take different amounts of time as a result of the nature of packet networks.

In order to be able to manage the nature of the Internet, protocols such as IEEE 1588 which specifies the use of PTP - Precision Time Protocol may be used. Essentially this operates in two stages:

  • Establishes a master slave hierarchy of suitable clocks
  • Synchronise the slave clock with the relevant master clock

While IEEE 1588 and PTP may be used, another protocol, called NTP - Network Time Protocol may be used. There are many NTP time servers that can be used

The use of the Internet connection to provide timing suffers from two main disadvantages:

  • Variable delays of packets and timing information which reduces the femtocell synchronization. This becomes more pronounced the further the femtocell is located from the clock.
  • Bandwidth required can be significant, especially if the precision is low because the synchronization process has to be performed on a more regular basis.

In view of these issues, network timing is best when a high bandwidth is available and the timing clock is located close to the femtocell in network terms.

GPS timing for femtocell synchronisation

GPS is now widely used, not only for navigation but also as a very accurate source of timing. GPS receivers for timing applications are widespread and cheap to include in femtocells. However GPS has the disadvantage that when used within buildings the signal suffers significant levels of attenuation and may often not operate.,/p>

To overcome this the femtocell could be located by a window, or it may be possible to install and external antenna, but this makes installation more complicated and expensive.

It may also be possible to utilise a form of GPS reception known as assisted GPS, or A-GPS. Here the femtocell would receive assistance data, possibly from a neighbouring macro cell to help in the de-correlation of the signal. A further advantage of the femtocell is that it would not be expected to move and therefore position data would remain the same and this too would assist in the decorrelation of the signals.

Using nearby macro cells for femtocell synchronization

As the femtocell will need to listen to nearby macro cells for self-organisation and handover, it is relatively simple to utilise the reception of these signals to provide the femtocell synchronisation information.

This can provide a particularly cost effective solution because little extra needs to be added to provide the required functionality.

The main drawback of this technique is that femtocells are used when macro coverage is poor. Although the signal may be sufficient to provide synchronisation in many cases, this method may not always prove to be a viable solution.

TV transmitters for femtocell synchronization

In many countries these days, most areas are able to receive terrestrial television transmissions at good strength. Also as they are broadcast at frequencies below 1GHz, they are able to penetrate buildings well. Additionally the stability of many transmissions is maintained to a high level - especially digital transmissions such as DVB, although analogue transmissions in less well developed countries may be held to less exacting tolerances. Accordingly many of these transmissions can be used to provide a reference source for the femtocell synchronization.

Television receivers suitable for providing femtocell synchronisation can be built into femtocells quite cheaply, and in fact cost less than a GPS receiver. As a result they can represent a very cost effective solution.

Summary of femtocell synchronization options

Each solution for providing femtocell synchronization has its own advantages and disadvantages. Some of the main points are outlined in the table below:

  Internet timing Neighbouring cells GPS Broadcast
Advantages Easy to implement Low cost Available in most locations (not in shielded buildings) Accurate
Disadvantages Low accuracy Femtocell must be within a coverage area of a macro cell May not be available in all buildings or may need external antenna Requires TV coverage

It can be seen that no single method is ideal for all situations and therefore the choice will depend upon a variety of factors including the operator preference, location. Security may also have an impact upon the choice of the method of femtocell synchronization.

By Ian Poole

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