E-UTRAN Radio Interface Protocols

 
E-UTRAN Radio Interface Protocols
This section is to describe the radio-interface protocol evolution for Evolved UTRA and Evolved
UTRAN [821]. This activity involves the TSG RAN working group of the 3GPP studies for evolution
and has impacts both on the UE and the Access Network of the 3GPP systems. It should be noted
that the information provided in this section should not be considered as the final standard, but rather
only the results from the discussions made in various 3GPP TSG WG meetings held previously up
to the time when this book was written.
Before introducing the E-UTRAN Radio Interface Protocols, we would like to define various
acronyms used in the discussions followed, as shown in Table 10.1.
10.6.1 E-UTRAN Protocol Architecture
The E-UTRAN protocol architecture bears a similar form as the one defined for the UTRAN. Two
layered protocol stacks have been defined for the E-UTRAN, including the user-plane protocol stack
and the control-plane protocol stack, as shown in Figures 10.7 and 10.8, respectively.
It is to be noted that in the E-UTRAN user-plane protocol stack, a MAC sublayer exists right
above the physical layer (or Layer-1). The dashed line in Figure 10.7 means that the existence of a
separate RLC layer is still open. The Packet Data Convergence Protocol (PDCP) will exist in the
E-UTRAN protocol stack with its exact functionalities to be revisited in the future.

Table 10.1 Various acronyms used in the discussions on E-UTRAN Radio Interface
Protocols
ARQ Automatic Repeat Request
AS Access Stratum
CN Core Network
DL Downlink
E-UTRAN Evolved UMTS Terrestrial Radio Access Network
HARQ Hybrid Automatic Repeat Request
HO Handover
L1 Layer 1 (physical layer)
L2 Layer 2 (data link layer)
L3 Layer 3 (network layer)
MAC Medium Access Control
NAS Nonaccess Stratum
NW Network
PDCP Packet Data Convergence Protocol
PDU Protocol Data Unit
RAN Radio access network
RLC Radio Link Control
RRC Radio Resource Control
SDU Service Data Unit
TCH Traffic Channel
UE User Equipment
UL Uplink
UMTS Universal Mobile Telecommunications System
UTRA UMTS Terrestrial Radio Access
UTRAN UMTS Terrestrial Radio Access Network
 On the other hand, the E-UTRAN control-plane protocol stack also has a MAC sublayer. Similar
to the one in the user-plane, the existence of a separate RLC entity is still undetermined. A simplified
RRC layer will be used in the E-UTRAN standard.
10.6.2 E-UTRAN Layer 1
Layer 1 in E-UTRAN is defined as exactly the same as what we often refer to in the Physical Layer.
In this subsection, we will introduce the service, functions, and transport channels of Layer 1 (or the
physical layer) in the E-UTRAN architecture.
The physical layer offers information transfer services to MAC and all other higher sublayers/
layers, as shown in Figures 10.7 and 10.8. The physical layer transport services are described by
how and with what characteristics data are transferred over the radio interface. An adequate term for
this is “Transport Channel.” It should be noted that, on the other hand, the classification of what is
transported is what relates to the concept of logical channels at the MAC layer.
Downlink transport channels
There are three types of DL transport channels in total, which are explained as follows:
• Broadcast Channel (BCH): characterized by (1) low fixed bit rate; (2) requirement to be broadcast
in the entire coverage area of the cell. • Downlink Shared Channel (DL-SCH): characterized by: (1) the possibility of using HARQ;
(2) the possibility of applying link adaptation by varying the modulation, coding, and transmit
power; (3) the possibility to be broadcast in the entire cell; (4) the possibility to use beamforming;
(5) dynamic or semistatic resource allocation; (6) the possibility of channel-quality
indication (CQI) reporting;2 and (7) the support of UE power saving.3
• Paging Channel and Notification Channel (PCH and NCH): characterized by: (1) the support
of UE power saving; (2) the requirement to be broadcast in the entire coverage area of the cell.
Uplink transport channels
There are two types of UL transport channels, which are explained as follows:
• Uplink Shared channel (UL-SCH):4 characterized by: (1) the possibility to use beam-forming;
(2) the possibility of applying link adaptation by varying the transmit power and potentially
modulation and coding; (3) the possibility to use HARQ; (4) dynamic or semistatic resource
allocation;5 (5) the possibility of CQI reporting.6
• Random Access Channel(s) (RACH):7 characterized by: (1) limited data field; (2) collision
risk; and (3) the possibility of using HARQ.
10.6.3 E-UTRAN Layer 2
Layer 2 in the E-UTRAN protocol architecture consists of three sublayers, including the MAC sublayer,
the RLC sublayer and the PDCP sublayer.
The MAC sublayer provides data transfer services on logical channels. A set of logical channel
types is defined for different kinds of data transfer services as offered by MAC. Each logical channel
type is defined by what type of information is transferred.
Some RR control (scheduling of user data, common channel transmissions, resource allocations,
etc.) is also performed in MAC. MAC should: (1) be QoS aware; (2) assign resource blocks based on
QoS attributes, buffer occupancy, and radio measurements; (3) include support of HARQ mechanism;
(4) include segmentation/reassembly, if taken out of RLC and considered needed in L2. The possibility
to cipher all flows in MAC exists.
A general classification of logical channels is divided into two groups: Control Channels (for the
transfer of control-plane information) and Traffic Channels (for the transfer of user-plane information).
MAC Control Channels
Control channels are used for the transfer of control-plane information only. There are five different
control channels offered by MAC.
• Broadcast Control Channel (BCCH): A DL channel for broadcasting system control information. • Paging Control Channel (PCCH) and Notification Control Channel (NCCH): A DL channel
that transfers paging information (and notifications for MBMS). This channel is used when the
network does not know the location cell of the UE.
• Common Control Channel (CCCH): This channel is used by the UEs having no RRC connection
with the network.8
• Multicast Control Channel (MCCH):9 a point-to-multipoint DL channel used for transmitting
MBMS scheduling and control information from the network to the UE, for one, or several
MTCHs. After establishing an RRC connection, this channel is only used by UEs that receive
MBMS.10
• Dedicated Control Channel (DCCH): A point-to-point bidirectional channel that transmits dedicated
control information between a UE and the network. Used by UEs having an RRC
connection.
MAC Traffic Channels
Traffic channels are used for transferring user-plane information only. The traffic channels offered by
MAC include:
• Dedicated Traffic Channel (DTCH): A DTCH is a point-to-point channel, dedicated to one UE,
for the transfer of user information. A DTCH can exist in both UL and DL.
• Multicast Traffic Channel (MTCH): A point-to-multipoint DL channel for the transmission of
traffic data from the network to the UE.
Mapping between logical channels and transport channels
Another important function in the E-UTRAN MAC sublayer is to perform the mapping between the
logical channels and the transport channels.
The mapping in UL concerns the connections between logical channels and transport channels,
to be explained as follows:
• CCCH can be mapped to RACH;11
• CCCH can be mapped to UL SCH;12
• DCCH can be mapped to RACH;
• DCCH can be mapped to UL SCH;
• DTCH can be mapped to UL SCH. On the other hand, the mapping in DL concerns the connections between logical channels and
transport channels, as explained below.
• BCCH can be mapped to BCH;
• PCCH can be mapped to PCH;13
• PCCH can be mapped to DL SCH;14
• CCCH can be mapped to DL SCH;
• DCCH can be mapped to DL SCH;
• DTCH can be mapped to DL SCH;
• MTCH can be mapped to DL SCH;15
• MTCH can be mapped to MCH;16
• MCCH can be mapped to DL SCH;17
• MCCH can be mapped to MCH.18
RLC sublayer and PDCP sublayer
The exact functionalities of the other two sublayers in the E-UTRAN Layer 2, RLC sublayer and
PDCP sublayer, had not been determined at the time of writing this book.
Also, the proposals for the functions of the Layer 3 and many other detailed elements in the
E-UTRAN protocol architecture will be collected and discussed in the subsequent 3GPP TSG RAN
meetings scheduled in 2006, as shown in Figure 10.6.