- Published on Wednesday, 03 October 2007 13:01
- Written by David Chambers
Long term accurate frequency reference is essential for a Femtocell
High accuracy of the transmission frequency in a mobile phone system has always been extremely important, with very limited tolerance of frequency deviation permitted. This ensures the correct operation of the network, and also limits the interference with other users and other networks. A frequency reference with long term accuracy to 0.1 parts per million is required.
Without a long term frequency reference to correct the base frequency used within an ordinary mobile phone basestations, a very high quality crystal osciallator, costing several hundred dollars, would be required but which would not drift in frequency over a long lifetime. Alternatively, site visits to retune and correct the base frequency would be needed on a regular basis - a costly and commercially impractical option. Therefore, basestations have compromised by fitting a cheaper crystal oscillator and correcting the signal by deriving a long term timing base from the connected transmission lines, usually a 2 Mbit E1 (or in the US a 1.5Mbit T1).
Network Synchronisation saves money
Vendors are actively investigating techniques to extract accurate long term clock signals from the network, using methods such as IEEE 1588. This standard (version 2) has been used for MetroEthernet, and is also practical to use for femtocells. This is a recent standard and further work is ongoing to improve it.
Alternatively, a GPS receiver can be used to derive a very accurate clock signal and give additional benefit of location information (for emergency, information search and disabling use where the operator has no spectrum licence). However, the femtocell must be positioned with a view to the sky (eg by a window) in order to receive the satellite signal.
Crystals are the highest individual cost item in a Femtocell
The crystals to achieve this high accuracy can be very expensive - sometimes over $100 - which has previously made the economics of a low cost home base station unviable. The standards bodies have relaxed the frequency tolerances in some situations, and more ingenious methods have been invented to reduce costs. These items remain the highest individual cost element of a femtocell.
Crystal vendors are actively seeking lower cost alternatives
Rakon, a New Zealand based company manufacturing crystals, has come up with several options to reduce cost each with their own risks attached. A TCXO (temperature controlled oscillator) is cheaper than an OCXO (oven temperature controlled oscillator) and hybrid devices are also available. Typical prices for OCXO of $100 need to be reduced to $20-25 range in order for the total bill of materials for a femtocell to be comparable with WiFi hubs. However, there is higher risk in OCXO alternatives which can drift from their original reference frequency over a period of time.
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