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<h1>Overview of BSS Synchronisation</h1>

<p>BSS Synchronisation means that all the BTS sites in the
synchronisation area are fixed to a single external clock source, and the
connections in different cells are aligned and synchronised at TDMA burst
level. In other words, the time slots in the air interface are constantly
aligned. This enhances or enables some key features in the BSS system, such
as Dynamic Frequency and Channel Allocation (DFCA).</p>


<p>Conventionally, the achievable synchronisation in GSM/EDGE systems has
been limited to the synchronisation of transmitted air interface signals of
the cells belonging to the same BTS site (one or more BCFs). This is called
site synchronisation. This is possible as the different logical cells within
the same site use one single master clock.</p>


<p>Nokia's solution of BSS synchronisation utilises a Global Positioning
System (GPS) time reference. Signals from the GPS satellites are received
by the Location Measurement Unit (LMU) at each BTS site. The LMU delivers
the clock signal to the BCF cabinet(s) of the site. When all the clocks of
the different sites along the network are synchronised to a global time reference,
BSS synchronisation is achieved.</p>


<p>The figure below illustrates how the timeslots of different cells cause
interference to each other.</p>


<a name="aug1241719141" shape="rect"></a>
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<img alt="urn:mars:dn04130491:1:en:global:cgm_fixed:data:data" border="0" src="NED?action=retrieve&amp;identifier=dn04130491&amp;edition=1&amp;language=en&amp;coverage=global&amp;encoding=gif&amp;component=data&amp;item=data" /><p class="figure-caption">Figure: Time slot coincidence</p>
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<p>The following figure presents the TDMA burst synchronisation (or TSL
synchronisation). The guard period within each burst is the existing margin
in the GSM air interface, which guarantees that a certain connection only
causes interference to the coincidently broadcast connections in other cells
that are using a traffic channel (TCH) with the same time slot (TSL) number.</p>


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<img alt="urn:mars:dn04130519:1:en:global:cgm_fixed:data:data" border="0" src="NED?action=retrieve&amp;identifier=dn04130519&amp;edition=1&amp;language=en&amp;coverage=global&amp;encoding=gif&amp;component=data&amp;item=data" /><p class="figure-caption">Figure:  TSL synchronisation</p>
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<p>Once BSS synchronisation has been achieved, further alignment of transmitted
downlink signals can be controlled by the settings of the TSL number and the
Frame Number (FN) at every site. The <span class="parameter-sem">TSL Offset</span>
and <span class="parameter-sem">Frame Number Offset</span> parameters allow shifting
of the TDMA frames as shown in the figure below.</p>


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<img alt="urn:mars:dn04130522:1:en:global:cgm_fixed:data:data" border="0" src="NED?action=retrieve&amp;identifier=dn04130522&amp;edition=1&amp;language=en&amp;coverage=global&amp;encoding=gif&amp;component=data&amp;item=data" /><p class="figure-caption">Figure: TDMA frame and multiframe alignment with TSL Offset and FN Offset</p>
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<p>FN Offset is necessary for dealigning the SCH bursts of different cell
sites. A mobile station needs to decode the BSIC of neighbouring cells to
be measured and reported to the network. Fast BSIC decoding benefits cell
reselection and handover processes. In synchronised BSS, all SCH transmissions
coincide making BSIC decoding and neighbour measurements very slow unless
frame numbers are de-aligned. By distributing FN Offset values evenly over
the BSS so that each BTS site uses different FN from surrounding sites, BSIC
decoding performance is secured.</p>


<p>Similarly, FR SACCH performance may decrease in a fully synchronised
network. To avoid degradation, the <span class="parameter-sem">TSL Offset</span>
parameter shifts BTS timing so that FR SACCH channels are not all sent simultaneously.
By distributing the TSL offset evenly over the network, SACCH performance
degradation is avoided.</p>


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<h2>Related topics</h2>

<ul>
<li><p><a href="NED?action=retrieve&amp;identifier=dn04122227&amp;edition=1&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=dec331522341#dec331522341" onclick="sync(this);return true" shape="rect">Functional description
of BSS Synchronisation</a></p>
</li>
<li><p><a href="NED?action=retrieve&amp;identifier=dn04122239&amp;edition=1&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=feb451332382#feb451332382" onclick="sync(this);return true" shape="rect">System impact of BSS
Synchronisation</a></p>
</li>
<li><p><a href="NED?action=retrieve&amp;identifier=dn04122242&amp;edition=1&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=jan1651149141#jan1651149141" onclick="sync(this);return true" shape="rect">Implementing BSS Synchronisation</a></p>
</li>
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<h2>Benefits for the operator</h2>

<p>Synchronising all BTS sites in the network decreases overlapping of
the TDMA bursts and thus minimises interference and the frame error rate (FER).
Higher tolerance to interference leads to better performance and makes frequency
reuse as well as the cell re-selection and handover processes more effective.
It also gives more accuracy for the MS locationing functionality.</p>


<p>When a burst is causing interference to a serving connection, the FER
of the serving connection increases as the overlapping between interfering
and serving bursts increases. When the overlapping is low, below 20%, the
degradation is small. When overlapping is over 40%, the training sequence
is affected, and the degradation is as high as with full overlapping. This
means that the worst performance case is the 50/50 case, which means that
two serving connection bursts are 50% interfered by the interfering burst
(see Figure <em>Time slot coincidence</em>). Simulation results have shown
that in a synchronised network the overall gain over the network is 0.5 dB.
This means that the network FER versus C/I performance will be enhanced by
0.5 dB.</p>
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<p><a href="NED?action=retrieve&amp;identifier=dn04122203&amp;edition=1&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=ned_2#ned_2" onclick="sync(this);return true" shape="rect">Benefits for applications</a></p>
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