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<h2>Required signal strength </h2>

<p>Signal-to-noise levels in digitally modulated systems are
commonly expressed in terms of Eb/No, Es/No or C/N.</p>


<ul>
<li><p>Eb/No is the available bit energy (received
power x bit duration) divided by noise spectral density (-174 dBm/Hz). </p>
</li>
<li><p>Es/No is the equivalent for the symbol
case (1 bit = 1 symbol in GMSK, 3 bits = 1 symbol in 8PSK).</p>
</li>
<li><p>C/N is the received power divided by
the total noise in the relevant RF bandwidth that is typically 180 kHz. </p>
</li>
</ul>


<p>C/N and Eb/No are linked by the spectral efficiency of
the modulation scheme. For schemes with 1bit/s/Hz, Eb/No is equal to C/N.
In GSM the spectral efficiency is 271kbit/s/200kHz =1.35 bit/s/Hz for GMSK
modulation, assuming the receiver noise bandwidth is matched to the channel
bandwidth. This represents on offset of 1.3dB in log terms, with C/N being
Eb/No + 1.3dB. For 8PSK, Es/No = 3*Eb/No which is 10*log(3) = 4.8 in log terms. </p>


<p>The above is summarised in the table below.</p>


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<p>Table: Signal-to-noise measurement equivalence</p>


<table border="1">
<thead>
<tr align="left" char="" charoff="50" valign="bottom">
<th colspan="1" rowspan="1" valign="top"><p>GSM modulation</p>
</th>
<th colspan="1" rowspan="1" valign="top"><p>Es/No</p>
</th>
<th colspan="1" rowspan="1" valign="top"><p>C/N</p>
</th>
</tr>
</thead>
<tbody>
<tr align="left" char="" charoff="50" valign="top">
<td colspan="1" rowspan="1"><p>GMSK</p>
</td>
<td colspan="1" rowspan="1"><p>Es/No=Eb/No</p>
</td>
<td colspan="1" rowspan="1"><p>C/N = Eb/No + 1.3 dB</p>
</td>
</tr>
<tr align="left" char="" charoff="50" valign="top">
<td colspan="1" rowspan="1"><p>8PSK</p>
</td>
<td colspan="1" rowspan="1"><p>Es/No = Eb/No + 4.77 dB</p>
</td>
<td colspan="1" rowspan="1"><p>C/N = Es/No + 1.3 dB = Eb/No + 6.07 dB</p>
</td>
</tr>
</tbody>
</table>
</div>


<p>The required Es/No is based on the required Eb/No (bit
energy divided by noise spectral density) from simulation results. Typically,
link budgets may consider a certain modulation and coding scheme at a certain
block error rate. However, it is also possible to calculate for a given data
rate. This latter case becomes more widely used as functionalities such as
link adaptation (LA) and incremental redundancy (IR) tend to mask, to some
extent, the actual underlying channel performance. </p>
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