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Controllable Load
minor load control
(packet scheduler)
procedures invoked
planned load
50




Non Controllable Load



time

Figure 6.14 Load control in UMTS
282 UNIVERSAL MOBILE TELECOMMUNICATIONS SYSTEM


6.7.3 Load control
Load control constantly monitors the air interface to maintain a clear picture of interference
levels in the cells. It interacts with the admission control and packet scheduler to try to
keep the loading below some target threshold level. Should the threshold be exceeded,
then procedures will be invoked to bring it back down below again. These are regarded
as emergency procedures and would include extreme actions such as suspension of NRT
bearers or dropping of real-time bearers.
There are a number of strategies for admission and load control to enable a new user
to enter a cell:

• The current mobile devices could increase their power but this causes more interference
for other users. The mobile device is limited in the amount of power it can transmit
due to the battery ampere-hour.
• The new mobile device could be granted a lower data rate than requested.
• Other voice call users could be shifted over to GSM (for example) if there is coverage.
• Soft handover could be performed for high data rate users. This would reduce the
average transmitted power and hence reduce interference.
• Users on the fringe of the cell could be asked to move to other cells.
• Calls could be dropped in some controlled way.
• Use outer loop power control to reduce uplink signal to interference ratio (SIR).
• Decrease bit rate of speech calls by use of adaptive multirate (AMR) CODECS.
• Users who have been allocated dedicated channels but who have a low utilization can
be moved to the shared or common channels.

By its nature, the air interface is prone to interference. The WCDMA system is designed
to work within these constraints by constantly monitoring system loading.


6.7.4 Handover control
The RNC is in charge of management of handovers. In UMTS, two categories of handover
exist: soft handover and hard handover. Soft handover is when a new radio link is initiated
before the old link is released. It is unique to CDMA technology and possible since
adjoining cells can operate at the same frequency. This provides for a better connection
quality. In the context of the UTRAN, the handovers summarized in Tables 6.4 and 6.5
are available.

Table 6.4 Soft handover types
Handover type Explanation
Softer handover Handover between cells under the control of the same BTS
Intra-RNC soft handover Handover between BTSs under the control of the same RNC
Inter-RNC soft handover Handover between BTSs under the control of different RNCs
6.7 RADIO RESOURCE MANAGEMENT (RRM) 283


Table 6.5 Hard handover types
Handover type Explanation
Intra-frequency hard handover Handover between BTSs under the control of different
RNCs where the Iur interface is unavailable
Inter-frequency hard handover Handover between cells or BTSs operating at different
frequencies
Inter-system hard handover Handover between UMTS and another cellular technology.
Initially, the focus is on support for UMTS/GSM
handover
SRNC relocation Moving of the SRNC, as detailed later


6.7.4.1 Soft handover
Soft handover is a mechanism which enables the mobile device to communicate with a
number of base stations at the same time. This is only possible with the CDMA system
since adjacent base stations will be working on the same carrier frequencies. When a
mobile device transmits, it transmits in an omnidirectional area, which means that much
of the signal will actually be in the wrong direction, away from the receiving antenna. It is
interesting to note that transmission power from mobile devices is considered to be greater
for non-voice services. This is since typically the voice user will place the device close to
the head when making a call, and thus a large percentage of the radiated signal is absorbed.
This is not generally the case for other services, where the device is held away from the
body. Calculations for loss generally factor in a 3 dB ˜head loss™ ¬gure for voice calls.
Soft handover allows the UE to communicate through another cell to maximize the
utilization of this signal. This scenario is not possible with GSM since adjacent cells
transmit and receive on different frequencies. In such a case, the mobile device would
have to send the data on two separate frequencies and this defeats the whole purpose of
soft handover. With reference to the soft handover, the cells that a UE can detect are
broken into three categories (Table 6.6).
Figure 6.15 shows a mobile device in soft handover. In this example the mobile device
is connected to three separate base stations and thus there are three separate connections
over the Iub interface. The lower base station is controlled by a different RNC to the
BTSs at the top of the diagram. This is referred to as a drift RNC (DRNC) and it simply
passes the information for this mobile device across to the serving RNC. There is only
one connection through to the core network and this is via the SRNC. All of the cells in
the ¬gure must be using the same frequency as each other for this system to function.

Table 6.6 Cell sets
Cell set Description
Active set Current set of cells in which the UE has an active connection and is
sending and receiving information (i.e. >1 for soft handover)
Monitored set A set of cells, not in the active set, that the UE has been instructed by the
RNC to monitor as possible handover candidates
Detected set All other cells that the UE has detected. Reporting of measurements only
occurs for intra-frequency measurements
284 UNIVERSAL MOBILE TELECOMMUNICATIONS SYSTEM




RAN
Con
nec
tion
to
UE
BTS
Iub
SRNC
Iu




Core Network
Iub
Connection
Connection to UE to UE




Connection to
UE




Iur
BTS




UE
Iub
Connection to UE

BTS DRNC


Figure 6.15 Soft handover


In the uplink the different base stations receive exactly the same information from
the mobile device and these are combined at the SRNC in a procedure referred to as
macrodiversity. Only one stream of information is passed across the Iu interface from
the RNC towards the CN. The RNC takes the information streams from the Iub and Iur
interfaces and simply ensures that the stream passed on conforms to the QoS requirements.
The soft handover mechanism can generate a lot of traf¬c over the RAN Iub and Iur
interfaces and this increased data transfer is weighed against ef¬ciencies that can be
made over the air interface. During the macrodiversity process at the SRNC, the different
data streams are checked on a per-transport block basis for block error rate (BLER) or
bit error rate (BER) against a reference target value. The best-quality block in each case
is forwarded to the core network. Since the RNC has multiple streams from which to
work, the overall data quality is increased. If this increase pushes the quality in excess
of the required target to meet the QoS, this indicates that it is possible to reduce the
transmit power of the mobile device, thus reducing the interference caused to other mobile
subscribers in the cells, and increasing cell capacity.
In the downlink, a number of base stations may transmit exactly the same information to
the mobile device. Again, this is used to reduce the downlink power over the air interface
and the mobile device will internally perform a diversity process. The base station that
is to be added to the active set needs to know the existence of the mobile device. Thus
the RNC needs to pass it the scrambling code for the mobile device, as well as other
connection parameters such as coding schemes, number of parallel code channels etc.
Although the same wideband frequency is used by adjacent cells, the cells are physically
identi¬ed by their different scrambling codes. The mobile device constantly monitors the
common pilot channel (CPICH) of cells in its surrounding area for power levels. This
information can then be used to assess whether or not the cell should be added to the
active set. The mobile device passes this information in the form of measurement reports
to the SRNC.
6.7 RADIO RESOURCE MANAGEMENT (RRM) 285


If a single base station is controlling a number of cells, typically referred to as sectors,
each of which is utilizing the same frequency, it is possible for soft handover to occur
within these cells. This form of soft handover is termed softer handover. In the downlink
direction, there is no difference; however, in the uplink, the diversity operation is per-
formed at the BTS, rather than the RNC. This diversity is an RF level procedure, where
the combination is performed by Maximal Ratio Combining (MRC) as was explained
in Section 2.8. For softer handover, however, the UE will need to take into account the
different scrambling codes used by the two branches. A mobile device can simultaneously
be in soft and softer handover.
Figure 6.16 shows a simpli¬ed graph of when a mobile device will perform a soft
handover. It is assumed in the example that the mobile device is currently connected to
cell-1 and is physically moving over timeout of this cell and into another cell, cell-2.
The mobile device constantly monitors the cells in the surrounding area and passes the
measurement reports of the CPICH signal strength to the SRNC. It can be seen that the
measurement reports for cell-1 will indicate that the signal is getting weaker and for cell-2
it is getting stronger. When the reports from cell-2 indicate to the SRNC that the signal
strength of the CPICH-2 is within a certain threshold of the stronger cell-1, a timer is set.
If the CPICH-2 continues to be within the threshold of cell-1 for a predetermined amount
of time then cell-2 will be added to the active set via an active set update message from
the RNC to the mobile device. The timer is required to reduce the amount of exchanges,
since if the subscriber decides to turn around and walk back towards cell-1 and CPICH-2
falls outside the threshold before the timer reaches its predetermined value then there
will not be an update. During the time the mobile device is connected to both cells, the
power it transmits at can be reduced, reducing interference for other users in both cells
and also saving the battery life of the mobile device. As the subscriber moves further



UE
Connected
CPICH-1
to both Cell-
1 and Cell-2
Measured Quantity




Hysteresis




UE Connected to Cell-1 UE Connected to Cell-2



T



CPICH-2



Time

Figure 6.16 Example soft handover procedure
286 UNIVERSAL MOBILE TELECOMMUNICATIONS SYSTEM


away from cell-1, eventually it will be dropped, and, again, there will be a threshold
value and a timer. A mobile device may be connected to more than two cells during
soft handover. The hysteresis in the diagram indicates that the two connections are being
used simultaneously, and that the decision to add or remove a link is made independently
based on the measurements recorded.
The advantages presented by soft handover are only really useful if the two (or more)
signals are fairly equal in power, typically less than 3 dB difference.



6.7.5 Power control
Power control in a WCDMA system is crucial to its successful operation. This is because
each handset transmits on the same frequency and at the same time as other handsets. Each
of the handsets therefore generates interference, raising the overall noise level in the cell,
and the base station has to be able to distinguish a particular user out of this interference.
If a single mobile device is transmitting with too much power, or is physically closer to the
BTS, this may drown out the other UEs. Conversely, if a UE is transmitting with too little
power, or is physically further away, the base station will never hear it. This is commonly
referred to as the near“far problem. There are two main concerns regarding power control:
distance from the base station and fast fading. Within the WCDMA system three types of
power regulation are used, open loop, inner loop and outer loop power control. Each of

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