Capacity and utilization enhancement

Considerations in Enhancing Cell Utilization
Levels in WCDMA

Revision History
Revision Date Status Revised By Revision Details
0.1 12 Nov 2010 Dr. Asoka Korale Estimating cell load for increasing utilization
through cell broadcast and similar methods
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1.0 Introduction
Strategies for enhancing utilization levels in under utilized cells through targeted
advertising using means such as SMS/ cell broadcast (SABP) require estimating the load
and usage levels in those UMTS cells.
This note lists the considerations involved in making this estimate and briefly describes
the kinds of algorithms employed to maximize utilization levels by maximizing the
efficient use of radio and network resources.
The load / utlization estimation in 2G systems is a relatively simple task based on traffic
channel utilization and Erlang traffic measurements. Unlike in 2G systems estimating cell
load levels in WCDMA systems is fairly complex as it depends on several variables and
optimizing the utilization level also depends on the interaction of many competing radio
resource management (RRM) algorithms.
2.0 Estimating Cell Load Levels
Techniques for measuring cell load levels in CDMA systems typically depends on the
direction of transmission. The measurements for this purpose are available from the RNC
and the messages passed over the Iub via NBAP protocol (3GPP 25.433).
2.1 Down link
In the down link (DL) the typical cell load measurements are transmitted carrier power,
and channelization code usage. Additionally aggregate transmitted bit rates are also some
times used to determine a throughput based estimate of cell load.
In the case of power measurements the load is thus expressed as a proportion of the total
DL transmitted carrier power capability of the cell. (Typically a proportion of the
transmit power is also reserved for HSDPA users.)
The proportion of the code tree utilized also provides a measure of the load in the cell and
is related to the total transmitted data rates relative to the total data rate capacity of the
cell.
In throughput based estimation of load, the ratio between the sums of the total data rates
to all users to the maximum allowed throughput of the cell forms such a measure.
2.2 Up link
In the up link the cell load measure is one of noise rise, calculated from measurement of
received total wide band power (RTWP) in the cell. Noise rise is typically computed as
the ratio between the total received wider band power to the noise floor (nose floor may
be a parameter or dynamically estimated when the cell is not supporting any users).
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Throughput based load estimation can also be derived here as well, utilizing a construct
that defines it to be the ratio between the received power at the cell from any given user
to the total interference power received at the cell (RTWP).
2.3 Cell load measure
The admission control algorithm operating in the RNC determines whether a call is
admitted in to a cell by checking the available resources of transmitted carrier power and
code on the down link and expected rise in the interference levels on the uplink. If both
conditions are satisfied the call is admitted in to the cell.
The admission control algorithm determines the load contribution of each potential call
by calculating a load factor for each service which is typically a function of EbNo target,
bit rate, activity factor.
Thus one way of estimating cell load and potential capacity of a cell is through this
algorithm. Access to statistics and logs relating to this algorithm will be implementation
dependent.
• Other factors like backhaul capacity on Iub and channel element usage that are
not directly related to cell load in the classical sense also determine whether a call
is allowed in to a cell or not.
3.0 Considerations in maximizing utilization level
The total data rate transmitted in the down link in a cell compared to the total data rate
capacity of the cell alone does not provide an accurate picture of the utilization level as it
is principally the degree of utilization of the code tree that represents the availability of
spare resources.
The distribution of the transmitted carrier power in the cell is a measure that indicates the
degree of utilization of transmitted carrier power, and so indicates the incidence of
unutilized capacity.
The optimization of load in a cell then would ideally benefit from the availability of the
cell load measurements from the admission control algorithm. However some
implementations do not necessarily use cell measurements described above and utilizes
“load factors” determined through simulations
4.0 Utilization enhancing strategies via RRM Algorithms
UMTS employs many proprietary algorithms to ensure efficient utilization of radio
resources and increase utilization levels. Principal among them are
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Transport channel switching: this algorithm changes the rate of the radio bearer allocated
to a particular user based on traffic volume and quality measurements (such as error rate).
Thus a user who is initially assigned a high rate radio bearer may be subsequently
downgraded to a lower rate bearer if for example the volume of traffic utilized by that
user is low freeing up resources for allocation to other users.
Radio access bearer (RAB) down sizing: This algorithm allocates a lower rate bearer at
the initiation of the call than requested depending on the available resources as reported
by the admission control algorithm. Thus if the admission control algorithm indicates that
there is code/power resource available for a 64kbit/s bearer when a 384kbit/s service has
been requested the lower rate may be allocated instead of rejecting the request altogether
due to the inability to meet the original request. (this is particularly true in the case of
interactive/background class services)
Preemption: The allocation/ retention priority of a radio link in a cell can be set to
preempt users of lower priority and admit users of higher priority in to cell. These
attributes are typically set based on the QOS profile of a user.
Power reservation: When allocating power for HSDPA users in a cell certain schemes
accomplish this objective by modifying the data rates of interactive/ background class
services.
Maximum power per code: Limits the maximum power transmitted to a user on a per
service basis with the objective of ensuring a subscriber in bad radio conditions does not
end up consuming a large amount transmit carrier power, leaving capacity to serve other
users.
5.0 Note
In UMTS the GSM cell broadcast function is enabled via Service Area Broadcast
Protocol , and defined in 3GPP 23.041/25.419.
As a cell broadcast server is typically required, certain vendors (Huawei) however have
implementations that do not require the server and is offered as an optional feature to the
RNC.
6.0 Appendix
In the estimation of cell load and utilization levels the following direct measurements are
available through the configuration of measurement reports to the RNC from Node B via
the NBAP protocol 25.433 and defined in 25.215.
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Received total wide band power
Definition The received wide band power, including noise generated in the receiver, within the bandwidth
defined by the receiver pulse shaping filter. The reference point for the measurement shall be
the Rx antenna connector. In case of receiver diversity the reported value shall be linear average
of the power in the diversity branches. When cell portions are defined in the cell, the total
received wideband power shall be measured for each cell portion.
Transmitted carrier power
Definition Transmitted carrier power, is the ratio between the total transmitted power on one DL carrier
from one UTRAN access point, and the maximum transmission power possible to use on that
DL carrier at this moment of time. Total transmission power is the mean power [W] on one
carrier from one UTRAN access point. Maximum transmission power is the mean power [W] on
one carrier from one UTRAN access point when transmitting at the configured maximum power
for the cell. Measurement shall be possible on any carrier transmitted from the UTRAN access
point. The reference point for the transmitted carrier power measurement shall be the Tx
antenna connector. In case of Tx diversity the transmitted carrier power is the ratio between the
sum of the total transmitted powers of all branches and the maximum transmission power. When
cell portions are defined in the cell, the transmitted carrier power for each cell portion shall be
measured and reported to higher layers.
Transmitted code power
Definition Transmitted code power, is the transmitted power on one channelisation code on one given
scrambling code on one given carrier. For DPCH, measurement shall be possible on the
DPCCH-field of any dedicated radio link transmitted from the UTRAN access point and shall
reflect the power on the pilot bits of the DPCCH-field. For F-DPCH, measurement shall be
possible on the TPC-field and shall reflect the power on the TPC bits. When measuring the
transmitted code power in compressed mode all slots shall be included in the measurement, e.g.
also the slots in the transmission gap shall be included in the measurement. The reference point
for the transmitted code power measurement shall be the Tx antenna connector. In case of Tx
diversity the transmitted code power for each branch shall be measured and summed together in
[W].
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