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Introduction

The evolution mobile phones as Personal Communication Devices and the explosion of devices

such as tablets have created a challenge for the mobile networks, starting from the Base Station and

the Mobile Backhaul. New initiatives such as Small Cells reduce the pressure of capacity crunch

and coverage limitations, but the importance of Macro Base Stations remains very high. With the

transition to the packet based backhaul mechanisms, the synchronization implementations havebecome extremely challenging.

Base Station Synchronization Requirements

Traditionally Base Stations were connected to the core network through circuit switched network

connections delivering data and the frequency synchronization. When Phase information was

required because of the air interface requirements, a GPS based system was used and a combination

of GPS and line extracted frequency were used for synchronizing base stations. We can generalize

the requirements to network side requirements and radio side requirements.

The network recovered clock had to meet the limit of 16ppb to the network and the air interface had

a frequency accuracy requirement of 50ppb and phase accuracy requirement of few S whenever

required. The 16ppb requirement is derived from the Stratum 2 frequency accuracy levels of and ina scenario where the network or GPS synchronization were lost, the base station be able to maintain

its accuracy within the air interface limits.

The following table shows the frequency accuracy requirement of most commonly used air

interface requirements.

Radio Technology BTS TypeFrequencyAccuracy

Phase/Time Accuracy

GSM Macro BTS 50ppb

CDMA2000 Macro BTS 50ppb3s (norm)10s (max)

WCDMA-FDD Wide Area BTS 50ppb

WCDMA-TDDTD-SCDMA

Wide Area BTS 50ppb 2.5s

LTEFDD & TDD

Wide Area BTS 50ppb

LTE-TDD

Wide area BTS, >3kmradius

10s

Wide area BTS, 3kmradius

3s

Home BTS, >500m rad. 1.33 +Tprop s

Home BTS, 500m rad. 3s

NetworkInterface

Modem

Timing &Synchronization

GPS

RadioInterface

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Base Station Synchronization Challenges

The challenges in Macro Base Station Synchronization are multi fold. GPS or other GNSS methods

are no longer preferred as a primary source of network timing because of the fear of signal loss or

jamming in various situations. Secondly, the circuit switch network is transitioning to packet

switched network forcing the Base Stations to use Synchronous Ethernet and Precision Time

Protocol based synchronization techniques. ITU-T has finalized the standards for frequency onlyPacket Equipment Slave Clocks and are working to finalize the network and equipment

requirements for phase and frequency. Many implementations are relying on a combination of

GNSS and packet clock methods to achieve the synchronization requirements.

Rakon Oscillators for Base Station Timing applications

Two main aspects of the oscillators are considered for Base Station synchronization designs. The

holdover requirement is one of the primary objectives. The service impact on the service provider is

the turnaround time to attend to and repair the systems, if anything were to disturb synchronizationmechanisms. Holdover time is required by the operator so that they system remains operations

within the time limit in which it can be attended to. Depending on the air interface requirements,

constraints may be placed on frequency only or frequency and phase to achieve certain holdover

time periods. In a synchronizer design, the holdover time depends on the oscillator, specifically on

the frequency versus temperature characteristics and the slope performances of frequency with

temperature. Secondly, Packet based networks impose extremely low bandwidths on clock recovery

PLLs, due to the varying nature of PDVs. Depending on the loop bandwidth, very stable oscillators

(MTIE and TDEV on such bandwidths ) are required for implementation of such PLLs. Oscillator

selection is carefully done identifying the holdover and loop bandwidth requirements, along with

other considerations such as phase noise.

At Rakon we have the expertise to help determine the best oscillator solution for our customers

systems requirements. Oscillator solutions require knowledge of the differential in frequency versus

the differential in temperature. Full characterization is determined by defining the temperature ramp

and measurement rate and a detailed analysis over the characterization time period. Rakon has

characterized various types of oscillators under changing environmental conditions for the loop time

periods relevant to IEEE 1588v2. Rakon has also performed extended tests on the holdover phase

movement performance of the OCXOs and features devices that has holdover performance of 1uS

over the temperature range over 24 hours. Rakon has wide range of OCXOs that economically fit a

wide range of holdover requirements of 1us to 10us across 8hours to 24 hours.

[ 3 OCXO families -2 ROX families with 1uS over 24 hours, 1 us over 8 hours with holdover performance plots. MTIE

and TDEV plots with 0.05mHz with G.8263 masks. Phase noise curves]

1 STP family. Performance maps and plots

Other products available Mercury and XO VCXOs for the solution]