Voice channels (FTTH, DOCSIS, VoLTE)

Jean-Francois Mezei jfmezei_nanog at vaxination.ca
Tue Nov 22 07:01:25 UTC 2016

On 2016-11-21 21:56, joel jaeggli wrote:

> Not really the air interface uses OFDMA coding scheme, so it is both
> divided into sub-carriers from 1.4 to 20mhz wide which are then also
> scheduled accordingly.

I have read in a number of places that 1 * 20mhz yields much more
capacity than 2 * 10mhz for LTE. but...

On the other hand, just read something on
> https://www.nxp.com/files/wireless_comm/doc/white_paper/3GPPEVOLUTIONWP.pdf

and it states:
Unlike single carrier systems described above, OFDM communication
systems do not rely on increased symbol rates in order to achieve higher
data rates. This makes the task of managing ISI much simpler.
***OFDM systems break the available bandwidth into many narrower
sub-carriers and transmit the data in parallel streams.***
 Each subcarrier is modulated using varying levels of QAM modulation,
e.g. QPSK, QAM, 64QAM or possibly higher orders depending on signal
quality. Each OFDM symbol is therefore a linear combination of the
instantaneous signals on each of the sub-carriers in the channel.
Because data is transmitted in parallel rather than serially, OFDM
symbols are generally MUCH longer than symbols on single carrier systems
of equivalent data rate.

At page 8:
In OFDMA, users are allocated a specific number of subcarriers for a
predetermined amount of time. These are referred to as physical resource
blocks (PRBs) in the LTE specifications. PRBs thus have both a time and
frequency dimension.

At page 9, a table shows that a PRB is 180KHz, and that if you have
20mhz of spectrum, you have 100 PRBs.

And more importantly:
A PRB is the smallest element of resource allocation assigned by the
base station scheduler.

Intertingly, the data I have read in that document points to performance
that is linear with more spectrum, no mention that 1 block of 20mhz
yields more capacity than 2 blocks of 10mhz.

So, if I read this right, (and please confirm if I understand
correctly), an LTE system of 20mhz breaks itself into 100 180KHz chunks
(a PRB) and the base station then schedules which user gets to use which

So instead of giving each use a time slot, OFDMA gives it one or more
PRB, a frequency slot  180KHz wide ?

I assume that this is how VoLTE gets priority, with VoLTE bandwidth
causing the base station to give the handset enough PRBs to handle the
VoLTE connection, at the expense of normal users who will see a reduced
number pf PRBs given to them for default data ?

Is that how it works ?

Would it be correct to assume there is some baseband signalling so the
base station tells each user which PRBs it should be listening to (and
sending on for the uplink) ?

(One piece of text which helped me understand was stating that LTE
doesn't transmit packets).

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