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<h1>FOC Algorithm for 2nd Gen BTS</h1>

<p>The MS does not have an accurate frequency reference. Instead,
MS uses the signal received from BTS to derive the necessary information to
adjust its clock. When MS is moving, it continues to adjust its own clock
according to received signal, which is distorted by the Doppler shift. MS
uses this signal as a reference when it generates transmission towards BTS,
that is, the frequency of the transmission is already shifted by the amount
of Doppler.</p>


<p>Because of the movement of the mobile, BTS sees a signal
that has twice the Doppler shift for a given speed. In practice this means
that the BTS receiver has to cope with double Doppler shifts while MS receiver
can compensate totally for the Doppler shift it sees. The ETSI specifications
do not take this Doppler-doubling effect into account. The performance requirements
for both uplink and downlink are equal in this respect despite of the fact
that a BTS needs to cope with worse conditions.</p>


<p>Frequency Offset Correction (FOC) algorithm can correct
most of the above phenomena providing that the Doppler shift is either positive
or negative only. The frequency offset (FO) is calculated for each burst and
averaged over one SACCH-multi-frame. This averaged FO is used in the FOC to
correct the bursts of the subsequent SACCH-multi-frame. During the correction,
a new average is calculated and so on. In the Line-Of-Sight case, the correction
should be relatively easy since the signal is Doppler shifted to only one
direction.</p>


<p>The above described FOC algorithm will be based on TalkFamily
and PrimeSite solution originally implemented in DF2.0 and further improved
in DF4.0.</p>
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