this is a new episode in our series 5g explained in this video we discussed
downlink data transmission in 5g new radio we will look at the downlink share
channel chain which includes LDPC coding the physical downlink shared channel
chain our resource elements are allocated for PDSA transmission the
different types of PD ACH mapping and conclude with a quick word on transport
block sizes the downlink share channel or dl SCH is the channel that carries
user data it also carries other pieces of information such as the different
types of system information blocks or s ib the according chain includes the
usual steps such as CRC code block segmentation rate matching and
concatenation all steps were familiar with from LTE the main difference with
LTE is the use of LDPC coding the output of a coding chain is a code word 5g
supports transmission of up to 8 layers to a single user on the downlink this
means that they can be up to 8 streams transmitted in parallel those streams
are coming from one or two code words one code word if there are fewer than 4
layers and two code words if they are more the coded data is then mapped to
the physical downlink share channel or PD SCH here you can see how every stage
of the downlink share channel processing is mapped to functions in MathWorks 5g
toolbox you can recognize CRC encoding code block segmentation LDPC coding and
raid matching the physical downlink shell channel is highly configurable
much more so than in LTE and we will see some of the detail on the next slide it
is configured both by downlink control information which can change from slot
to slot and radio resource control which can set
up some parameters as well there isn't much of a surprise here of virtuality we
find scrambling modulation layer mapping precoding for my mo processing and
resource mapping while those are all known blocks there are few differences
worth pointing out chiefly the precoding step is not specified explicitly in the
standard although it is fully expected to be present the detail of precoding
will be addressed in another episode of this video series here we can see code
for complete downlink data processing in MathWorks 22 box
the PD SDH processing stage is highlighted but you can also see the DL
SDH stage as well as the insertion of the modulation reference signals or DM
RS 5 GN r uses the exact same list of modulations on the downlink as LTE from
QPSK through 256 QAM the NR symbol modulate function implements modulation
for 5g on or in one simple call layer mapping is the operation that map's one
or two code words to up to 8 layers this operation is somewhat simplified
compared to LTE where you could see one or two code words for a given number of
layers in 5g NR as mentioned earlier anything up to four layers uses a single
code word anything beyond four layers uses a second code word the mapping is
pretty straightforward direct for one layer alternatively for tube
similarly for three and four layers each group of three or four input bits is
mapped to a set of three or four layers
for five through eight layers the two code words are split as shown here
between the different layers
after one or two codewords got my up to between one and eight layers the layers
undergo precoding which interestingly is not specified in the standard for
downlink recording is the operation that map's the layers to as many or more
antenna ports using a matrix multiplication with the pre corner the
special case of pre coding is mapping one layer to multiple antennas which
enables beam forming for line-of-sight transmission this would likely mean
targeting a particular direction another case of pre coding is mapping several
layers to multiple antennas this more general case is sometimes referred to as
spatial multiplexing one key aspect of precoding in 5g is that the Associated
demodulation reference signals or dmrs must undergo the same precoding as a
result the ue doesn't need to be made aware of the pre coder as the effect of
a pre coder is included in channel estimation this is why the exact pre
coder the G note B is to use is not specified in the standard the pre-code
our output is then mapped to physical resource blocks either directly or
indirectly as we will see on the next two slides downlink channels and signals
including the PD SCH and associated dmrs share the OFDM grid PD SH symbols are
first mapped to virtual resource blocks one map to the grid PD SH symbols avoid
locations reserved for other purposes this includes all physical signals dmrs
channel State Information reference signals or si si RS and face tracking
reference signals or P TRS this also includes any resource block that is
fully or even partially used by synchronization signal blocks or SSB SS
beads are explained in detail in another episode
would of this video series mapping of virtual to physical resource blocks can
be interleaved on non interleaved non-internet mapping consists in
directly mapping each virtual block to the same position in the physical
resource grid interleave mapping provides frequency diversity by
distributing the virtual blocks over the whole bandwidth part the inter leaver
granularity is 2 or 4 resource blocks this scheme let's assign consecutive
virtual resource blocks to a PDS CH a pattern that is easy to signal simply
with a stunning resource block and number of resource blocks while still
getting frequency diversity here we see two examples of PD SCH resource
allocation in time the PD SCH may spend the whole slot as shown at the bottom of
the grid it may also use part of a slot this is sometimes referred to as partial
slot allocation and it is a new capability in 5g new radio compared to
LTE as you may remember LT always allocates a full subframe of one
millisecond for PDS ing let us explore some of those allocation options
interactively with a user interface that uses math works well G toolbox here we
are looking at tens of frames with 30 kilo Hertz subcarrier spacing which
means a total of 20 slots the PDA CH is shown in teal online blue and this is
what I want you to look at here we will focus on the rest of a resource grid in
detail in another episode of this series 5g explained resource block allocation
does not have to be contiguous although it is easier to signal when it is let's
make it contiguous from 0 to 20
we can see PDS transmission in the first 10 slots followed by five empty slots
this is because we are located slot zero through nine with a periodicity of 15
slots let's change the allocation to something different now slot 6 7 & 9 do
not have PTSD a transmission finally notice that within each slot the PD SCH
only uses symbols 2 through 10 this is called partial slot allocation you can
choose to allocate a 4 slot in which case there is no break between PDS the a
transmission reference signals for PD SCH are shown in yellow those locations
are not available for PD SCH mapping reference signals will be addressed in
detail in another episode of this series 5g explained here we can see some of the
parameters available in MathWorks 5g toolbox to specify one or multiple PDS
th we have just seen how these parameters impact the PDS CH chain in
the live example as we have just seen a PDS CH slot allocation may start at the
beginning of a slot or midway through a slot this corresponds to two different
mapping types mappings type A and B strictly speaking the PDS CH mapping
type only affects the location of the demodulation reference signals for
mapping type a DM RSI in symbol 2 or 3 of a slot while for mapping type b TM RS
are in the first symbol of the PDS CH allocation both mapping types a and B
support full and partial slot allocation in practice however mapping type B is
the preferred option for partial slaughter allocation and especially for
transmissions that do not start at the beginning of a slot having gmrs at the
beginning of the allocation enables to reduce processing Leighton
see which is critical for low latency communications we will cover dmrs
allocation in more detail in another episode of this 5g explained series to
conclude this section we want to turn to the receiver and explore how transport
block sizes are communicated to the receiver the receiver needs to be able
to figure out the transport block size in order to perform inverse read
matching an LDPC decoding the concept is similar to LTE in another episode of
this 5g explained video series we will explain how turning control information
is transmitted but it is important to know that the transport block size
itself is not signaled instead a few different pieces of information are
signal they include the modulation encoding scheme MCS as well as the
resource allocation which is which resource blocks are assigned to the PD l
CH and the duration of the assignment in OFDM symbols 5 DNR uses a formula based
approach to compute the transport block size where LTE uses a number of tables
because of the way the formula is defined including quantization there are
several slightly different configurations of all those parameters
the result in the same transport block size far from being an issue
this gives the operator more flexibility in selecting different parameters for
the retransmission of a packet that didn't go through the first time this
concludes this episode of the 5g explained video series on downlink data
transmission