Page 158 - From GMS to LTE

144 From GSM to LTE-Advanced Pro and 5G

other things, the subscriber for whom the MAC‐frame is intended. This part of the
MAC layer is called MAC c/sh (common/shared).
For DCHs, the MAC layer is also responsible for multiplexing several data streams on
●
a single transport channel. As can be seen in Figure 3.11, several logical user data
channels (DTCH) and the logical signaling channel (DCCH) of a user are mapped
onto a single transport channel. This permits the system to send user data and signal-
ing information of the MM, PMM, CC and SM subsystems in parallel. This part of the
MAC layer is called the MAC‐d (dedicated).

Before the frames are forwarded to the physical layer, the MAC layer includes addi-
tional information in the header to inform the physical layer of the transport format it
should select for transmission of the frames over the air interface. This so‐called
Transport Format Set (TFS) describes the combination of datarate, the TTI of the frame
and the channel coding and puncturing scheme to be used.
For most channels, all layers described above are implemented in the RNC. The only
exception is the physical layer, which is implemented in the Node‐B. The Node‐B,
therefore, is responsible for the following tasks.
In order not to send the required overhead for error detection and correction over the
Iub interface, channel coding is performed in the Node‐B. This is possible as the header
of each frame contains a TFS field that describes which channel encoder and punctur-
ing scheme is to be used. UMTS uses the half‐rate convolutional decoder already known
from GSM as well as a new 1/3 rate and Turbocode coder for very robust error correc-
tion. These coders double or even triple the number of bits. It should be noted that
puncturing is used to remove some of the redundancy again before transmission to
adapt the data to the fixed frame sizes of the air interface. Later, the physical layer per-
forms the spreading of the original data stream by converting the bits into chips, which
are then transferred over the air interface.
Finally, the modulator converts the digital information into an analog signal which is
sent over the air interface. QPSK modulation is used for the UMTS Release 99 air inter-
face, which transmits two chips per transmission step. This is done in the Node‐B in the
downlink direction by sending one chip over the complex I‐path and a second chip over
the complex Q‐path. As each path uses a fixed transmission rate of 3.84 MChips/s, the
total datarate of the transmission is 2 × 3.84 MChips/s. The DPDCH and the DPCCH,
which only use a small percentage of the frames, especially for low spreading factors, are
thus time multiplexed in the downlink direction as shown in Figure 3.16.
For the uplink direction, which is the direction from the mobile device to the network,
a slightly different approach was chosen. As in the downlink direction, QPSK modulation
is used. Instead of multiplexing user and signaling data over both the I‐path and Q‐path,
user data is only sent on the I‐path in the uplink. The Q‐path is used exclusively for trans-
mission of the DPCCH, which carries layer 1 messages for power control (see 3GPP
25.211, 5.2.1 [4]). Thus, only one path is used for the transmission of user data in the
uplink direction. This means that for an equal bandwidth in uplink and downlink direc-
tion, the spreading factor in uplink direction is only half that of the downlink direction.
Note: DPCCH is used only to transmit layer 1 signaling for power control. Control
and signaling information of the MM, PMM, CC and SM subsystems that are exchanged
between the mobile device and the MSC or SGSN are not transferred over the DPCCH
but use the logical DCCH. This channel is sent together with the logical DTCH (user
data) in the DPDCH transport channel (see Figures 3.11, 3.16 and 3.17).