Page 258 - From GMS to LTE

244 From GSM to LTE-Advanced Pro and 5G

HARQ round trip time: 5 milliseconds
PDSCH

Earliest retransmission
1 millisecond later

PUSCH
or
PUCCH
NACK is sent 4 milliseconds later
Figure 4.14 Synchronous HARQ in the downlink direction.

The eNode‐B can set an upper limit for the number of retransmissions and can discard
the data block if the transmission is not successful even after several attempts. It is then
left to higher layers to detect the missing data and initiate a retransmission if necessary
or desired. This is not the case for all kinds of data streams. For VoIP transmissions, it
can be better to discard some data if it does not arrive in time as it is not needed any-
more anyway. As described in Chapter 1, voice codecs can mask missing or faulty data
to some degree.
As faulty data can only be repeated after 5 milliseconds at the earliest, several HARQ
processes must operate in parallel to fill the gap between the transmission of a data
block and the ACK. Up to eight HARQ processes can thus run in parallel to also cover
cases in which the eNode‐B does not immediately repeat the faulty data. Figure 4.14
shows five HARQ processes transmitting data. When downlink data is scheduled via
the PDCCH, the scheduling grant message has to describe the modulation and coding,
the HARQ process number to which the data belong, whether it is a new transmission
or a repetition of faulty data and which RV of the data stream is used.
At this point, it is interesting to note that the shortest HARQ delay in HSPA is 10
milliseconds, because of a block size of 2 milliseconds, and thus twice as long. In prac-
tice, LTE has hence even shorter jitter and round‐trip delay times compared to the
already good values in HSPA. Together with the shorter LTE HARQ delay in the uplink
direction as described below, overall round‐trip delay times in the complete LTE system
of less than 20 milliseconds can be achieved.
For the data stream in the uplink direction, synchronous HARQ is used where the
repetition of a faulty data block follows the initial transmission after a fixed amount of
time. If uplink data of a 1‐millisecond subframe has been received by the eNode‐B cor-
rectly, it acknowledges the proper receipt to the mobile device four subframes later. The
ACK is given via the PHICH, which is transmitted over a number of symbols in the first
symbol row of each subframe. As several mobile devices can get a scheduling grant for
different RBs during a subframe interval, a mathematical function is used to describe
which symbols of the PHICH contain the feedback for which mobile device. Once the
positive ACK has been received by the mobile device, the next data block of a HARQ
process can be sent in the uplink direction.
If the network did not receive the data of a subframe correctly, it has to request a
retransmission. This can be done in two ways. The first possibility for the network is to
send a NACK and order a retransmission in a new format and possibly different