Synchronisation of base stations in GSM and UMTS networks can Improve Quality of Service and reduce customer churn through reducing call drop off.

 Introduction

The mobile operator market is an aggressive and changing environment, customer retention is critical to the success and growth of a business.

Operators have worked hard to improve efficiency of customer support, service packages and network performance. However there are still problems customers will commonly quote the following three reasons for changing operator.

1. Call drop out
2. Poor or no network coverage in certain areas
3. Level of customer support

Now from an engineering aspect customer support is another issue, network coverage is and has improved greatly, we are nearing total coverage. However the biggest problem is call drop out and is often the most visible frustrating quality of service issue a customer will encounter

 How can poor Synchronisation cause a call drop out?

The basis of any robust and reliable GSM / UMTS network is good quality synchronisation references. This is particularly applicable to base stations an area often overlooked in terms of synchronisation.

The accuracy of the synchronisation at the base station is critical to the cell hand over; the typical accuracy requirement is +/- 50 parts per billion ppb. This level of accuracy is required to ensure hand over between base stations without drop out. When a hand over occurs between two base stations the potential for frequency difference can be as high as 100ppb. This in turn can be equated to a Doppler shift of 100kph in vehicle speed.

If the mobile cannot react quickly enough to this Doppler shift the call will be dropped.

The problem

The majority of GSM base stations employ a system of clock recovery or retiming. The synchronisation reference is normally recovered from a fixed leased line typically E1 or T1, in reality this works reasonably well until there is a problem.

The mobile network operator will have no direct control over the leased line reference therefore quality of the synchronisation reference cannot be guaranteed (remember we are dealing with an accuracy of -/+50 ppb!).

Also if the lease line synchronisation reference is compromised the fault resolution time can be slow, difficult and costly. While this is happening the base station will at best have low quality oscillator in holdover mode providing frequency accuracy of around 10 x -5 meaning regular call drop outs at the user end.

Due to the changes in backhaul requirement relying on a leased fixed line for the provision of synchronisation is not feasible.

The Solution

In order for the mobile operator to maintain, directly control the synchronisation in their network and guarantee synchronisation quality a simple and very effective GPS based retiming solution can be used.

The GPS re-timer device retimes leased lines removing any transient movement in the synchronisation reference before it can cause a call drop out at the subscriber device. The GPS based devices are traceable to ITU-T Recommendation G.811 quality and adhere to that of the 50ppb quality required at the base station.

Additionally the GPS unit have higher grade built in oscillators to provide holdover protection in the event of lossof GPS reception, thus giving the mobile operator direct control of the synchronisation source and time to fix, without compromising Q.O.S to the user.

In field trials is has been demonstrated that the use of GPS re-timing devices at the base station reduce call drop out by 25%.

As such it is vital that the synchronisation plan of the wireless network is revisited and assessed, with a changing and more demanding wireless arena on the horizon GPS based devices will be critical to customer satisfaction and control of churn.

E-mail: info@horsebridge.net

Application Overview.

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Oscilloquartz - OSA453x GPS Receiver Range
The existing and emerging standards in the world of mobile telecommunications and digital broadcasting demand the highest quality of synchronisation. A proper timing signal therefore becomes more and more important for media Centers as well as for cellular networks where location services such as E911 will have more importance.

The unique architecture of the OSA 453x Series is designed to maintain the strict CDMA holdover specifications. The OSA 453x Series are also a natural choice as a synchronisation source for UMTS, WCDMA and cdmaOne, as well as GPRS, CDMA and TDMA base station and mobile switches. Their 1 PPS and 10MHz outputs make them ideal for synchronising DAB/DVB equipment.

Common features
All models provide standard 1 PPS, 10 MHz and TOD (Time Of Day) outputs referenced to UTC1. All models include a highly accurate ovencontrolled oscillator, which filters the reference signal and provides a holdover quality comparable to that of an SSU in case of loss of external reference.

Oscilloquartz OSA 4530/1/2 GPS
The OSA 4530/1/2 GPS are GPS Receivers providing E1/DS1/2.048 MHz outputs according to ITU-T G.703-X. Models 4531/2 also accept an auxiliary E1/DS1/MHz reference as a robust alternative to GPS.

Oscilloquartz OSA 4533/4 GPS & OSA 4535/6 RTM
The OSA 4533/4 GPS and OSA 4535/6 RTM are Re-Timing Modules that re-time one E1 or DS1 traffic signal. Models 4533/4 take GPS as reference, while 4535/6 smooth out timing impairments from the traffic signal and therefore need no external reference.

Features:
• Fully compliant to ITU-T G.811 / Stratum 1 when locked to GPS
• Integrated high stability holdover functionality < 2x10^-10 /day
• Holdover compliant to ITU-T G.812 type I, V, VI
• Output 10MHz, 1PPS
• Various outputs types (E1 / T1 / 2.048MHz)
• Re-timing of one E1 / T1 traffic signal

Benefits:
• Low cost GPS receiver module
• Compact reference for Cellular networks application like UMTS, GPRS and CDMA
• Re-timing of traffic, e.g. for transit switches
• Optimum clock for Wireless and wireline telecom sync requirements
• Ideal source for specialised ATM and LAN/WAN applications
• External input filtering capabilities

Horsebridge Services:
As well as supplying the OSA453x range, we also provide a host of supporting services, including:
• Installation.
• Commissioning.
• Technical support.
• Spares support.
• Product training.

If you would like to know more about the OSA453x range, then why not contact us now.

Oscilloquartz OSA453x Datasheet