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Base Station RNC
SGSN GGSN
Packet Core

Figure 6.10 End-to-end QoS


6.6 UTRAN CHANNELS
UMTS channels at the UE are divided into a three-layer hierarchy: logical layer, transport
layer and physical layer. A logical channel is a stream of information that is dedicated
to the transfer of a particular type of information over the radio interface. A transport
channel is how logical channels are transported between the UE and RNC. A physical
channel is the actual channel across the air interface, de¬ned by a WCDMA code and
frequency. Figure 6.11 shows the layered model
As an analogy, consider that the post of¬ce offers the logical channel of conveyance
of letters and parcels from one location to another. As transport, air mail, surface mail,




UE Base Station RNC


logical channels


transport channels


physical channels


Figure 6.11 UMTS channel structure
6.6 UTRAN CHANNELS 277


registered etc. are offered, and the physical channel could be the particular vehicle in
which the item is carried. The logical channel does not need to know that an Airbus
A340 cargo plane is used in this case; it only needs to know that it will be physically
transported and that the transport will meet the required QoS (e.g. a ¬‚ight is used to
minimize delay).
Figures 6.12 and 6.13, show the various channels available for logical, transport and
physical in the downlink and uplink directions, and how the channels are mapped. The
mapping of logical channels to transport channels is performed by the media access control
(MAC) layer, discussed later. For clarity there are a number of channels which have only
physical layer signi¬cance that are not included in this diagram. These channels, such
as the access indication channel (AICH), will be discussed in reference to the channels
below, as and when required.


6.6.1 Logical channels

• Broadcast control channel (BCCH): this is a downlink channel which carries all the
general system information that a UE needs to communicate with the network.
• Paging control channel (PCCH): this is a downlink channel which carries paging
information from the network to inform the user that there is a communications request.

UTRAN UE
Logical Channels Transport Channels Physical Channels


BCCH BCH P-CCPCH



PCCH PCH S-CCPCH



CTCH
FACH DPDCH


CCCH

DCH DPCCH

DTCH


DSCH PDSCH
DCCH




Figure 6.12 Downlink mapping of logical, transport and physical channels
278 UNIVERSAL MOBILE TELECOMMUNICATIONS SYSTEM


UE UTRAN
Logical Channels Transport Channels Physical Channels


CCCH RACH PRACH



DPDCH

DTCH DCH

DPCCH



DCCH CPCH PCPCH




Figure 6.13 Uplink mapping of logical, transport and physical channels


• Common traf¬c channel (CTCH): this is a downlink channel which carries dedicated
user information to a group of speci¬ed UEs.
• Common control channel (CCCH): this is a channel, both uplink and downlink, which
carries control information from the network to UEs that do not have any dedi-
cated channels.
• Dedicated traf¬c channel (DTCH): this is a bidirectional, point-to-point channel, ded-
icated to one UE, for the transfer of user information.
• Dedicated control channel (DCCH): this is a bidirectional, point-to-point channel that
transmits dedicated control information between the network and a UE.



6.6.2 Downlink transport and physical channels
The following transport channels offer information transfer of logical channels as shown
in Figure 6.12. There are often several options of which transport channel may be used
to perform this transfer.
The broadcast channel (BCH) transports the BCCH. This is then carried in the primary
common control physical channel (PCCPCH). The physical layer coding, i.e. the WCDMA
channelization code used, for this is a system constant as all UEs need to be able to decode
the information on the BCH. Once decoded, the BCCH will contain system information
indicating the coding of all other channels present. The paging channel (PCH) transports
the PCCH, and is carried in the secondary common control physical channel (SCCPCH).
Also carried in the SCCPCH is the forward access channel (FACH). The FACH can
transport a number of logical channels carrying common (CCCH, CTCH) and dedicated
(DCCH, DTCH) control and traf¬c. Dedicated traf¬c and control can also be transported
on a dedicated channel (DCH) or on a downlink shared channel (DSCH), where multiple
6.7 RADIO RESOURCE MANAGEMENT (RRM) 279


users can be statistically multiplexed on the one transport channel. As will be seen, it is
the RNC that decides, based on the user requirements, whether the user data is transported
on a DCH or on a common or shared channel (FACH, DSCH). For example, a user may
be allocated a low data rate DCH, and then simultaneously allocated larger, variable-rate
transport on the DSCH. A more detailed example of this is presented in Section 6.15.
At the physical layer, the DCH is carried in two physical channels (which are combined
into one channel, time multiplexed together), separated into dedicated physical data chan-
nel (DPDCH) and dedicated physical control channel (DPCCH). The DPCCH contains
physical layer control, such as the format of the data and power control information. The
DSCH is carried in the physical downlink shared channel (PDSCH).


6.6.3 Uplink transport and physical channels
In the uplink, as shown in Figure 6.13, the random access channel (RACH) is present
for the transport of dedicated traf¬c as well as common and dedicated control informa-
tion. It is on the RACH that the UE makes its initial access to the network. The RACH
is carried in the physical RACH (PRACH). The dedicated channel (DCH) is the same
as the downlink; however at the physical layer, it is actually carried in two separate
physical channels rather than multiplexed into a single channel as is the case in the
downlink. The common packet channel (CPCH) is another channel for data traf¬c, but
designed speci¬cally for transport of packet data. It is carried in the physical common
packet channel (PCPCH). The CPCH transport is associated with downlink transport on
the FACH, where a user would receive data on the FACH, but be permitted to transmit
on the CPCH. Since it is a common channel it requires a sophisticated control mech-
anism to ensure that two UEs do not try to access the channel at the same time. This
mechanism introduces a number of other channels which only have relevance at the
physical layer.



6.7 RADIO RESOURCE MANAGEMENT (RRM)
RRM is responsible for making sure that the resources of the UTRAN, and particularly of
the air interface, are used ef¬ciently. Its target is to maximize performance to provide all
users with coverage and quality, regardless of which service they wish to use. RRM can
be broken into two categories, those that are network based, and therefore performed for a
whole cell, and those that are connection based, which are performed per connection. The
RRM functions are summarized in Table 6.3 and a brief explanation of each is presented.
The actual operation of the algorithm is implementation dependent. RRM is implemented
in software and handled at the RNC.


6.7.1 Admission control
Admission control is used to provide resources for guaranteed, real-time services, such as
voice calls. When a user requests resources, the admission control algorithm will verify
280 UNIVERSAL MOBILE TELECOMMUNICATIONS SYSTEM


Table 6.3 Radio resource management
Category RRM function
Network based Admission control
Packet scheduler
Load control
Connection based Handover control
Power control


that it can meet the request by checking the loading of the cell and the available WCDMA
and transmission resources. Based on this, if resources are available, the RNC will allocate
and reserve them, and then update the load control and admit the user. It is at this point
that the RNC will decide what type of channel to allocate a user. Should the resources not
be available, the RNC will either reject the request or make a counter offer of resources
that are available. The admission control is designed to meet a planned load level, placed a
reasonable safety margin below the maximum resources available. A typical ¬gure would
be 50“70% planned load.
The decision process for admission control is referred to as call admission control
(CAC), and in the UTRAN it is performed by the RNC. There are also admission control
procedures performed in the core network. This role is executed in the CS domain by the
MSC, and in the PS domain by both the SGSN and GGSN. Before a call is established,
the network decides whether there are enough resources available for this connection. If
not, either the call is queued, offered a lower quality of service or rejected. The RNC has
to consider the effect on other users within the cell and possibly within adjacent cells if
this call proceeds.
This is one of the most technically demanding aspects of implementing a soft capacity
system. With GSM, CAC over the air interface is rather simple: a quick check to see if
there is a time slot available; if so, admit the call. There are further checks for authen-
tication and authorization, but these are separate issues to CAC. There are a number
of performance measurements over the air interface which may be taken into consider-
ation when considering the admission of another subscriber or modifying the data rate
of an existing subscriber. These measurements can include, but are not limited to, the
following:

• received signal strength of the current and surrounding cells;
• estimated bit error rate of the current and surrounding cells;
• received interference level;
• total downlink transmission power per cell.

Within the WCDMA system, each user shares the same 5 MHz spectrum at the same
time as other users, and users are separated by a code. As more and more users are per-
mitted connections in a cell, they cause interference for the other users in the cell and also
for other users in adjacent cells since the frequency reuse is 1, meaning that adjacent cells
6.7 RADIO RESOURCE MANAGEMENT (RRM) 281


share the same frequency. A user sending at a high data rate introduces a higher quantity
of wideband noise than a user who is simply making a voice call. The network therefore
has to know beforehand what data rate a user requires and estimate how much noise this
will generate in the cell. If this noise value is too high it will cause too much interference
for other users in the cell and their call quality will be reduced. WCDMA works on a
10 ms frame and it is therefore possible for users to modify their current connection data
rates and QoS frequently, placing enormous strain on the system. It is also important for
the CAC algorithm to take into consideration the load on surrounding cells. This is to
ensure that users who are on the move from cell to cell do not lose their connection.


6.7.2 Packet scheduler
The packet scheduler uses the resources not currently allocated to schedule non-real-time
(NRT) data. Since NRT data does not place any demands on the network in terms of delay
requirements, this traf¬c only needs to be scheduled in terms of relative priority. Should
real-time traf¬c need more resources, the NRT data will be rescheduled. Figure 6.14,
illustrates the concept. Real-time traf¬c is considered as non-controllable load since the
network must guarantee the resources and thus has no control over these. NRT traf-
¬c, however, offers such a compromise; it can expand and contract to ¬t the available
resources and hence is termed controllable load.

maximum load
percentage loading




100


major load control
procedures invoked

threshold

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