this is a new episode of our series where G explained in this video we
discussed the basics of 5g on our waveforms frame structure and numerology
we will look at the you FDM waveforms this really important characteristic of
5gn are to accommodate different subcarrier spacings and its impact on
slots and subframes who will then explore the concept of carriers and
bandwidth part and see how resource blocks in v GN are differ from LTE v GN
r defines the waveform to be OFDM with cyclic prefix it also decreases the
guard band for the waveform which means that 5g waveforms are not limited to 90%
of a bandwidth as they are LTE as in LTE the standard does not specify any
particular technique to limit the bandwidth and spectral leakage several
well-known techniques such as filtered OFDM or windowed OFDM have received a
lot of attention and you can try them out easily with an example that ships
with MathWorks communication system toolbox key to many 5g features is the
fact that the subcarrier spacing in fact G&R can take not just one value 15
kilohertz in LTE but powers of 2 multiples of 15 kilohertz up to 240 kilo
hertz as shown on the first line as a consequence as the OFDM symbol duration
is the reciprocal of the subcarrier spacing the symbol duration shrinks by a
factor 2 for 8 or 16 for the highest subcarrier spacing at the same time the
same number of subcarriers when spaced further apart occupies a larger
bandwidth this is what can be seen on the third line the maximum supported
bandwidth is 50 megahertz at 15 kilohertz spacing and it doubles each
time to reach 100 200 and 400 megahertz at 120 Kidd heart spacing for the
highest subcarrier spacing of 240 kilo Hertz the number of supported sub
carriers is half which means that the bandwidth is still 400 megahertz so what
are the advantages of having different subcarrier spacings for one a higher
subcarrier spacing increases the bandwidth as we just discussed at the
same time it shortens the transmission time for
one OFDM symbol therefore enabling shorter latencies at the physical layer
v GN r retains the concept of ten millisecond frame divided into ten one
millisecond for sub frames there is also a concept of slot but its definition is
different from a slot in LTE one slot is defined as fourteen OFDM symbols which
corresponds to one subframe or one millisecond in LTE because of variable
subcarrier spacing the number of slots per subframe is also variable as the
subcarrier spacing doubles the slot length is half because the OFDM symbol
length is half for that reason the number of slots per subframe doubles
this is shown in the last column of this table also the picture at the bottom of
the slide illustrates the shortening of a slocked length as the subcarrier
spacing increases so far having nor the fact that there is an interdependency
between some parameters here I want to introduce the dependency on the carrier
frequency frequency range one ofr one is for carrier frequencies below six
gigahertz in that range only subcarrier spacing of fifteen thirty or sixty
kilohertz are available for fr2 which corresponds to minimal wave frequencies
data transmission can only use 60 or 120 kilohertz
subcarrier spacing finally note that 240 keloid spacing is reserved for non data
channels namely the synchronization signal block which
described in another episode of his series 5g explained getting a little
deeper into the detail of the slot structure 5g uses the same idea as LTE
where the cyclic prefix for the first OFDM symbol of a slot is longer than the
cyclic prefix of the next six this begs the question of how symbols align when
there is a mix of subcarrier spacing within the same bandwidth let's just
state here that five GNR defines the cyclic prefix length for all subcarrier
spacing in such a way that oh f DM symbols align regularly irrespective of
the subcarrier spacing has illustrated on this picture it is now helpful to
introduce two notions carriers and bandwidth parts for each numerology one
carrier is defined a carrier is characterized by a subcarrier spacing in
number of resource box and a starting resource to each carrier corresponds an
OFDM resource grid we can see here two examples of how to define carriers in
MathWorks while G 2 box one with 15 kilohertz subcarrier spacing and 216
resource blocks and one with 30 kilo Hertz subcarrier spacing in 106 resource
blocks the larger subcarrier spacing means that 106 resource blocks cover
almost the same bandwidth as 216 resource blocks at 15 kilohertz now let
us look at bandwidth parts 1 bandwidth part is also characterized by a
subcarrier spacing in number of resource blocks in a starting resource the
bandwidth part is associated with the carrier that has the same subcarrier
spacing but they're generally are several bandwidth parts with the same
carrier spacing bandwidth parts can be seen as a way to address the spectrum
available to a UE bandwidth path so address the following issues some
devices may not be able to receive the all bandwidth remember that the
bandwidth can be as large as 400 megahertz also even when a device is
capable of receiving a larger bandwidth it will save power if it can be
addressed with a smaller bandwidth for example because it doesn't need a high
data rate at that point in time here also you can see how you can easily
configure a bandwidth part in math works by G toolbox by defining the subcarrier
spacing bandwidth and location of each bandwidth part one Yui can be associated
with up to four bandwidth parts but 1ue can only have a single bandwidth part
active at a time those bandwidth parts are pre-configured and the UE can be
instructed to switch between those different bandwidth parts over time on
this diagram one can see the UE switching between bandwidth part 1 2 2
back to 1 and then to bond with part 3 which covers a larger bandwidth likely
when more data has to be exchanged with the gene would be the definition of a
resource element in v GN r is the same as for LTE it is the smallest time by
frequency unit which is one subcarrier in one OFDM symbol on the other hand a
resource block is defined as a group of 12 subcarriers with no associated time
duration you may remember that in LTE a resource block was defined as 12
subcarriers by 7 WebM symbols finally note that the DC subcarrier is modulated
in five G&R whereas it was kept and modulated in LTE the reason for the
difference is that in v GN our bandwidth parts and therefore transmissions need
not be centered in the carrier bandwidth consequently the zero subcarrier does
not play any specific role in the v GN our OFDM grid this concludes this
episode of the 5g explained video series on v GN r waveforms
preme structure and numerology