9.4 3G Services
9.7 Channels on the Air Interface
• controlling radio bearers, transport channels and physical channels
• controlling security functions (ciphering and integrity protection)
• RRC connection mobility functions, such as cell or UTRAN registration area (URA) updates and handovers
• supporting SRNS relocation (information to the target SRNS)
• performing downlink outer loop power control and open-loop power control
• higher-layer signalling with core network (direct transfer).
Third Generation Network (3G), UMTS 147 Table 9.7 UTRA logical channels.
Control logical channels Use Direction
BCCH (Broadcast Control Channel)
Cell broadcast of System Information message that contains cell identifier, code sequences, timers etc.
DL
PCCH (Paging Control Channel) Notify the users in location area of incoming calls or other messages.
DL DCCH (Dedicated Control
Channel)
Bi-directional signalling channel for every user with RRC connection to RNC. It transmits measurements report, RRC control messages.
DL/UL
CCCH (Common Control Channel)
Connection set up, channel allocation, cell reselection
UL and DL Traffic logical channels
DTCH (Dedicated Traffic Channel)
Transfers information for specific service dedicated to a single user. One user may have multiple services provided on different co-existing DTCHs simultaneously.
DL and UL
CTCH (Common Traffic Channel)
Point to multipoint channel carrying information for group of users, such as SMS cell broadcast message.
DL
9.7.2.1 Dedicated Transport Channel (DCH)
DCHs transport user data and control information coming from the upper layers for a single user. This information may include speech frames or packet data, as well as upper layer control data such as handover commands or measurement reports. There is no need for a UE identifying parameter.
One user may have multiple services carrier by multiple DCHs simultaneously. Even in the case of single service, two DCHs have to be allocated: one for transfer of a traf- fic logical channel (DTCH) and another, DCCH, for a signalling associated with traffic channel. The physical resource allocated to DCH is available for the whole duration of the call.
9.7.2.2 Common Transport Channels
Common transport channels are shared between users in the cell. When a message to a specific user is transmitted over the common transport channel, it should contain explicit UE identification. Table 9.8 lists common transport channels.
When asingle user is allocated multiple transport channels, then all channels are mul- tiplexed together over the same physical resources forming a Coded Composite Trans- port Channel (CCTrCH). One physical control channel and one or more physical data channels form a single Coded Composite Transport Channel. There can be more than one CCTrCH on a given connection but only one physical-layer control channel is trans- mitted in such a case.
Table 9.8 Common transport channels.
Common transport channels Use Direction
BCH (Broadcast Channel) Provides transport for the BCCH logical channel. DL PCH (Paging Channel) Provides transport for the PCCH logical channel DL RACH (Random Access
Channel)
Short signalling information during the initial access to the system, before traffic channel can be allocated
UL FACH (Forward Access
Channel)
Carries a logical channel for a specific UE. Response to RACH.
DL CPCH (Common Packet
Channel)
Extension to the RACH channel for the transmission of longer data packets.
UL DSCH (Downlink Shared
Channel)
Pool of physical resources allocated on a TTI basis to different users according to packet scheduling policy, similar to GPRS.
DL
The users that transmits through the DSCH channel must also have an associated bi-directional DCH channel through which control information is sent.
This control information indicates the TTI when the DSCH is allocated to the specific UE as well as power control commands for closed-loop power control for that UE.
9.7.3 Physical Channels and Physical Signals
Physical channels are defined by a combination of carrier frequency, scrambling code, channelization code, time start and stop (giving a duration) and, on the uplink, relative phase (0 orπ/2).
Time durations are defined by start and stop instants and measured in integer multi- ples of chips. The main time reference is the radio frame with a length of 38 400 chips and 10 ms in duration. The radio frame processing duration consists of 15 slots of 0.666 ms each, with 2560 chips/slot. In contrary to GSM or any other TDMA system, the time slot structure has nothing to do with separation of the users in the time domain. Instead it defines a closed-loop power control cycle for a transmission to a given user with rate 15 periods over the radio frame; that is, 15/10 ms=1500 Hz.
Physical signals are entities with the same basic on-air attributes as physical chan- nels but do not have transport channels or indicators mapped to them. Physical signals may be associated with physical channels in order to support the function of physical channels. As in the case of transport channels, physical channels are classified as ded- icated and common physical channels. Table 9.9 lists some of the physical channels in UTRA-FDD.
9.7.4 Parameters of the Transport Channel
Multiplexing of transport channels onto physical channel is performed at the physical layer. Appropriate configuration of the physical channel is defined by a set of channel parameters, TTI, TB, TF and TFC, listed in Table 9.10.
Third Generation Network (3G), UMTS 149 Table 9.9 Different types of physical channels in UTRA-FDD.
Name Use
Dedicated Physical Data Channel (DPDCH)
Transmission of user data and higher layer signalling (RRC, NAS) in the uplink direction coming from higher layers.
Dedicated Physical Control Channel (DPCCH)
Transmission of radio control information in uplink direction.
This channel exists only once per radio connection.
Dedicated Physical Channel (DPCH)
Transmission of user data and control information in downlink direction. Both types of information will be mapped onto the DPCH
Synchronization Channel (SCH)
Cell search and synchronization of the UE to the Node B signal.
Subdivided into Primary Synchronization Channel (P-SCH) and Secondary Synchronization Channel (S-SCH)
Common Control Physical Channel (CCPCH)
Transmission of common information and is divided into Primary Common Control Physical Channel (P-CCPCH) and Secondary Common Control Physical Channel (S-CCPCH).
P-CCPCH transmits the broadcast channel (BCH) and S-CCPCH transports the Forward Access Channel (FACH) and the Paging Channel (PCH). FACHs and PCH can be mapped to the same or to separate S-CCPCHs.
Common Pilot Channel (CPICH)
Supports channel estimation and allows estimations in terms of power control. It is subdivided into Primary Common Pilot Channel (P-CPICH) and Secondary Common Pilot Channel (S-CPICH), which differ in scrambling code and availability within a cell.
Physical Random Access Channel (PRACH)
Transmission of the Random Access Channel (RACH), which is used for the random access of UE and for transmission of a small amount of data in the uplink direction.
Physical Common Packet Channel (PCPCH)
Common data transmission using the collision detection CSMA/CD method.
Paging Indicator Channel (PICH)
Transmission of the Page Indicator (PI) to realize the paging in the downlink direction. One PICH is always related to an S-CCPCH, which transports the PCH.
Acquisition Indicator Channel (AICH)
Transmits the positive acknowledgment of a random access of a UE via PRACH or PCPCH.Phys signal
Physical Downlink Shared Channel (PDSCH)
Common transmission of data in downlink direction. Parallel UEs will have different codes assigned.
The TFC is fixed during TTI but can be changed at the next TTI. One of the possible changes may be ‘no transmission’ during a given TTI. The TFC change may involve alteration to physical layer parameters, such as spreading factor. As a consequence, each transport channel is accompanied by the Transport Format Indicator (TFI) at each TTI.
The physical layer combines the TFI information from different transport channels to the Transport Format Combination Indicator (TFCI). The details of the processing are given in Section 9.8.1.
Figure 9.21 shows a simplified example for grouping services/applications according to QoS and mapping the logical, transport and physical channels.
Table 9.10 Transport channel parameters.
Transmission Time Interval (TTI)
Time interval for transmission of transport channel that indicates how often data arrives from upper layers to the physical layer. The TTI value is a multiple of the frame time 10 ms, e.g. 10, 20, 40,…ms.
Transport Block (TB) Data part+MAC+RLC headers. Total number of bits is denoted as transport block size (TBS).
Transport Format (TF) Number of transport blocks that are transmitted in the
corresponding TTI, which consequently defines the instantaneous bit rate or equivalently the spreading factor that should be used in the physical layer. Additionally, TF defines channel coding type (e.g.
convolutional or turbo-code), code rate and the number of CRC bits.
Transport Format Combination (TFC)
Used exclusively when several transport channels are multiplexed onto a CCTrCH. TFC defines the number of transport blocks that are transmitted from each of the multiplexed transport channels in a given TTI.
Services/
Applications QoS2
QoS1
Logical Channels
AMR Voice e.g. AMR Voice Call
DTCH 8
RRC Control DCCH 1
CCTrCH CCTrCH CCTrCH CCTrCH
... n
DTCH 5
e.g. 12.2 kpbs e.g. 3.4 kpbs e.g. 12.2 kpbs
Depending on UE Features (Radio Access Capabilities) e.g. 3.4 kpbs
kpbs: Best Effort kpbs: Best Effort
... 7
DCH1DL ...
3 ..
m DCH1UL
DPDCH1 UL
..n DPDCH1
UL
...3 DCH32
DL DCH32
UL FACHDL
RACHUL PCPCH
UL S-CCPCH
DL
...n
Transport Channels Physical Channels
Figure 9.21 Example of mapping services to radio interface channels [10].