Page 195 - From GMS to LTE

Universal Mobile Telecommunications System (UMTS) and High-Speed Packet Access (HSPA) 181

transport Format and Resource indicator (channel coding information);
●
modulation format (QPSK, 16‐QAM, 64‐QAM, MIMO);
●
HARQ process number (see below);
●
whether the block contains new data or is used for retransmission and which redun-
●
dancy version (RV) is used (see below).
Each frame on the HS‐SCCH is split into three slots. The information in the control
frame is arranged in such a way that the mobile device has all information necessary to
receive the frame once it has received the first two of the three slots. Thus, the network
does not wait until the complete control frame is sent but starts sending the user data
on the HS‐PDSCH once the mobile device has received the first two slots of the control
frame. This means that the Shared Control Channel and the Downlink Shared Channels
are sent with a time shift of one slot. After reception of a user data frame, the mobile
device has exactly 5 milliseconds to decode the frame and to check if it was received
correctly. If the frame was sent correctly, the mobile device sends an Acknowledge
(ACK) message in the uplink direction on the High Speed Dedicated Physical Control
Channel (HS‐DPCCH). If the mobile device is not able to decode the packet correctly, a
Not Acknowledge (NACK) message is sent. To save additional time, the uplink control
channel is also slightly time‐shifted against the downlink shared channel. This allows
the network to quickly retransmit a frame.
As HARQ can only transmit a frame once the previous frame has been acknowledged,
the mobile device must be able to handle up to eight simultaneous HARQ processes.
Thus, it is ensured that the data flow is not interrupted by a problem with a single frame.
As higher layers of the protocol stack expect the data in the right order, the data stream
can only be forwarded once a frame has been received correctly. Therefore, the mobile
device must have a buffer to store frames of other HARQ processes that need to be
reassembled with other frames that have not yet been received correctly.
For the network, there are two options for retransmitting a frame. If the Incremental
Redundancy method is used, the network uses error correction information that was
punctured out after channel coding to make data fit into the MAC‐frame. Puncturing is
a method that is already used in UMTS Release 99, GPRS and EDGE, and further infor-
mation can be obtained in Section 2.3.3. If a frame needs to be retransmitted, the net-
work sends different redundancy bits and the frame is thus said to have a different RV
of the data. By combining the two frames, the overall redundancy is increased on the
receiving side and the chance that the frame can be decoded correctly increases. If the
frame still cannot be decoded, there is enough redundancy information left that has not
yet been sent to assemble a third version of the frame.
The second retransmission method is called Chase Combining and it involves retrans-
mission of a frame with the same RV as before. Instead of combining the two frames on
the MAC layer, this method combines the signal energy of the two frames on the physi-
cal layer before attempting to decode the frame again. The method that is used for
retransmission is controlled by the network. However, the mobile device can indicate to
the network during bearer establishment which of the two methods it supports.

3.10.3 Node‐B Scheduling
The HS‐DSCH channels have been designed in such a way that different channels can
be assigned to different users at the same time. The network then decides for each