WCDMA Air Interface Protocol Architecture

We have already mentioned some of the types of physical channels defined
in WCDMA. In fact, many different channel types exist, and the various
types of channels are defined in a logical hierarchy.
Figure 6-8 shows the overall logical structure of the WCDMA air interface.
At the lowest level, we have the physical layer. The functions of the
physical layer include RF processing, spreading, scrambling and modulation,
coding and decoding for support of forward error correction, power control,
timing advance, and soft handover execution. Physical channels, such
as those already mentioned, exist at the physical layer and are used for transmission across the RF interface.A given physical channel is defined by
a combination of frequency, scrambling code, channelization code, and, in
the uplink, phase. Some physical channels exist solely for the correct operation
of the physical layer. Other physical channels are used to carry information
provided to or from higher layers.
Higher layers that want to transmit information across the RF interface
pass information to the physical layer through the Medium Access Control
(MAC) layer using a number of logical channels. MAC maps these logical
transport channels to channels.
The physical layer maps transport channels to physical channels.
Above the MAC layer, we find the Radio Link Control (RLC) layer.
Among the services provided by RLC are the following:
■ RLC connection establishment and release A given upper layer
may request the use of a certain radio bearer. For each radio bearer, an
RLC connection is established between the MS and the network.
■ Error detection RLC includes a sequence number check function
that enables the detection of errors in received protocol data units
(PDUs).
■ Ensuring error-free delivery through acknowledgements (if the
upper layer protocol has requested an acknowledged service)
RLC can request that the peer entity retransmit in the event that a
PDU is received incorrectly, lost, or received out of sequence. Note that
this type of error correction is different to the error correction that is
achieved through coding schemes on the air interface.
■ In-sequence delivery This ensures that PDUs are passed to the
upper layer in the correct order.
■ Unique delivery This ensures that a given PDU is passed to an
upper layer only once, even if erroneously received twice at RLC.
■ Quality of service (QoS) management Upper layers can request a
certain QoS. It is RLC that ensures that the QoS is controlled.
RLC supports both acknowledged and transparent services.With transparent
service, any errors in received PDUs will cause the PDU to be discarded,
in which case it is up to the upper layer to recover from the loss
according to its own capabilities.With acknowledged service, RLC recovers
from errors in received data by requesting a retransmission by the peer
entity (the UE or the network).
One of the protocols above the RLC layer is the Packet Data Convergence
Protocol (PDCP). The main objective of PDCP is to enable the lower layers (RLC, MAC, and the physical layer) to be common regardless of the type or
structure of the user data. For example, packet data transfer from a UE
could use either IPv4 or IPv6. One does not want the RLC and lower layers
to be different depending on which of those two protocols a subscriber uses.
Moreover, if new protocols are introduced, one would want them to be supported
by the same radio interface. PDCP meets these objectives by maintaining
a standard interface to RLC regardless of the type of user data.
PDCP is similar to the Subnetwork Dependent Convergence Protocol
(SNDCP) of GPRS.
In Figure 6-8, we also find Broadcast/Multicast Control (BMC). This is a
function that handles the broadcast of user messages across the cell. In
other words, BMC supports the cell broadcast function, similar to cell
broadcast, as defined in GSM. This enables users in a cell to receive broadcast
messages, such as traffic warnings and weather information. In GSM,
cell broadcast has also been used as a means of informing users of the geographical
zone that they are in as part of zone-based tariffing.
One of the most important components depicted in Figure 6-8 is the
Radio Resource Control (RRC). RRC can be considered the overall manager
of the air interface and, as such, is responsible for the management of radio
resources, including the determination of which radio resources shall be
allocated to a given user. As can be seen, all control signaling to or from
users passes through RRC. This is necessary so that requests from a user or
from the network can be analyzed and radio resources can be allocated as
appropriate. Also, a control interface exists between RRC and each of the
other layers. Among the functions performed or controlled by RRC are
■ The broadcast of system information.
■ The establishment of initial signaling connections between the UE and
the network. When the user and network want to communicate, an
RRC connection is first established. It is this RRC connection that is
used for the transfer of signaling information between the UE and the
network for the purpose of allocation and management of the radio
resources to be used.
■ The allocation of radio bearers to a UE. A given UE may be allocated
multiple radio bearers for the transfer of user data.
■ Measurement reporting. RRC determines what needs to be measured,
when it should be measured, and how it should be reported.
■ Mobility management. It is the RRC that determines when, for
example, a call should be handed over. RRC also executes cell
reselection and location area or routing area updates.

Quality of Service (QoS) control. The allocation of radio resources has a
direct consequence for the QoS perceived by the user. Since RRC
controls the allocation of radio resources, it has a direct influence on
QoS. The resources allocated by RRC must be aligned with the QoS
offered to the subscriber. 240