Page 194 - From GMS to LTE

180 From GSM to LTE-Advanced Pro and 5G

blocks, no packet loss is experienced on higher layers. However, every retransmission
increases the overall delay of the connection. Higher‐layer protocols like TCP, for exam-
ple, react very sensitively to changing delay times and interpret them as congestion. To
minimize this effect, HSDPA adds an error detection and correction mechanism on the
MAC layer in addition to the mechanisms which already exist on the RLC layer. This
mechanism is directly implemented in the Node‐B and is called Hybrid Automatic
Retransmission Request (HARQ). In combination with a block size of 2 milliseconds
instead of at least 10 milliseconds for DCHs, an incorrect or missing block can be
retransmitted by the Node‐B in less than 10 milliseconds. This is a significant enhance-
ment over Release 99 dedicated channels as they only use a retransmission scheme on
the RLC layer, which need at least 80–100 milliseconds for the detection and retrans-
mission of a faulty RLC frame.
Compared to other error detection and correction schemes that are used, for example,
on the TCP layer, HARQ does not use an acknowledgement scheme based on a sliding
window mechanism but sends an acknowledgement or error indication for every single
frame. This mechanism is called Stop and Wait (SAW). Figure 3.40 shows an example of
a frame which the receiver cannot decode correctly transmitted in the downlink direc-
tion. The receiver therefore sends an error indication to the Node‐B, which in turn
retransmits the frame. The details of how the process works are given below.
Before the transmission of a frame, the Node‐B informs the mobile device of the
pending transmission on the HS‐SCCH. Each HS‐SCCH frame contains the following
information:
ID of the mobile device for which a frame is sent in one or more HS‐PDSCH channels
●
in the next frame;
channelization codes of the HS‐PDSCH channels that are assigned to a mobile device
●
in the next frame;

2 ms = 1 HSDPA Frame = 3 slots
HS-SCCH

1. Assignment 3. New
of two HS-DSCH assignment of
after two slots two HS-DSCH

HS-DSCH
10 ms
HS-DSCH

2. NACK is sent after 7.5 slots
= 5 ms

DPCCH

Figure 3.40 Detection and report of a missing frame with immediate retransmission within
10 milliseconds.