Use of Measurement Reports

The measurement report in IMT2000DS does the same job as the measurement report
in GSM. It provides the information needed for the Node B to decide on power control
or channel coding or for the RNC to decide on soft handover.
In IMT2000DS, the measurement report is based on received signal power. This is
the received power on one code after despreading defined on the pilot symbols. The
decoded pilot symbols provide the basis for the measurement report, which provides
the basis for power control or channel coding and soft handover. It also provides the
basis for admission control and load balancing. Effectively, it is providing information
on the noise floor as perceived by individual users on individual OVSF/long code
channels. It provides additional information over and above wideband noise measurements
(which can also be used to set admission control policy).
The measurement report includes Eb/No (the received signal code power divided by
the total received power), signal-to-interference ratio (which is determined partly by cell
orthogonality), and block error rate measurements (used for outer-loop power control).
Load estimation can be done either by measuring wideband received power, which
will be the sum of intercell, intracell interference, and background receiver noise, or by
measuring throughput, which can be measured in terms of bit rate or Eb/No. An additional
option would be to measure buffer occupancy. Initially, wideband power estimation
is probably an adequate way to decide on admission control and load balance
at the RNC.
Cell sizes can be increased or decreased physically by increasing or decreasing
downlink transmit power or by physically changing antenna patterns (see Chapter 13).
It may, for example, be the case that interference patterns change through the day as
traffic changes. The loading and interference from users traveling to work by car in the
morning may be different from the loading and interference generated from users traveling
home at night. The cell site configuration can be adapted to match the offered
traffic (and offered noise) as it changes through the day.
In our later chapter on Service Level Agreements, we review how radio and network
performance has to be integrated into provable performance platforms, providing
proof that a requested grade of service has been delivered.
At radio system level we need to comprehend a number of factors. One important
factor is soft handover gain. This is the effect of the handset serving two Node Bs. It can
be considered as a macro diversity factor, both on the uplink and the downlink.
Because the ultimate objective of network control is to maintain an acceptable BER, the
additional factor of soft handover gain enables the handset transmission power to be
decreased, which in turn reduces both the intercell and intracell interference and so
may be expressed as a capacity gain. As the effective path gain is increased—by virtue
of the aggregate signal up to two Node Bs—but the uplink transmit power is reduced,
the receive power to the network remains the same.
However, the receive sensitivity is no longer a constant design factor of the handset
but is also dependent on a number of dynamic factors.

Intercell and intracell interference adds to the noise power. Processing gain influences
sensitivity. The Eb/No needed will vary (with service, data rate, speed, and multipath
channel). In addition a fast fading margin needs to be added to account for the
deterioration in Eb/No caused by power limiting at the cell edge.
The link budget will change depending on mobility factors as a user moves within
the cell, as users move in other cells, and as users move into and out of cells. The
link budget will also change on the service factor as users change data rate (the service
factor).
We also need to take into account processing gain. Processing gain (see Table 11.13)
is based on the ratio of the user data rate to the chip rate (whereas spreading gain is the
ratio of the channel data rate to the chip rate—see Chapter 3). These figures need to
include the coding gain available from convolutional encoding and interleaving. This
will vary depending on what convolutional encoding or interleaving is used, which, in
turn, is dependent on the service being supported. This is allowed for by changing the
required Eb/No for each service.
Table 11.14 shows typical Eb/Nos needed for particular services. The Eb/No is lower
for the UDD service (unconstrained delay). The delay tolerance effectively makes the
data easier to send. Services at a higher mobile speed require a higher Eb/No because
of power control errors (the inability to follow the fast fading envelope at higher
speeds means the fade margin has to be increased). Services in rural areas need a
higher Eb/No because the delay spread (and nonorthogonality) is greater.
Fast fading allows all handsets operating in a cell to have equally received powers
at the Node B receiver. Fast fading power control effectively equalizes the fading character
of the radio channel so that the receiver sees a Gaussian-like radio channel. The
BER versus Eb/No will improve in a Gaussian channel. The improvement in Eb/No is
called the fast power control gain.

Coverage can actually improve for fast mobility users, since they become part of the
channel averaging process. Fast power control works well for slow mobility users and
provides useful gain particularly (as in the above example) where only minimal multipath
diversity is available. Table 11.15 shows how important it is to optimize the power
control algorithms in the handset and base station.
The process of power control is as follows:
Open-loop power control. This sets Tx power level based on the Rx power
received by the mobile and compensates for path loss and slow fading.
Closed-loop power control. This responds to medium and fast fading and
compensates for open-loop power control inaccuracies.
Outer-loop power control. This is implementation-specific, for example, the
outer loop adjusts the closed loop control threshold in the base station to maintain
the desired frame error rate. Closed-loop implementation at 1500 Hz uses
1/2 dB steps for urban and 1-dB steps for rural areas.

Power control needs to be optimized for certain operational conditions. Power
control inaccuracies will substantially reduce capacity and coverage (by adding to
noise rise). At higher speeds, fast power control is less effective in compensating channel
fading. When carrying out a link budget for 3G, we therefore usually use an Eb/No
figure for the service, channel type, and data rate assuming fast power control and then
subtract a fast fading margin. The fast fading margin is approximately equivalent to
the fast fading gain achieved by fast power control when the transmitter had no power
limits. We normally expect the fast fading margin to be a few dB.