Networking Reference
In-Depth Information
twice per second. This type of power control is required because the open loop system
can only give a rough estimate and is not accurate enough to deal with problems such as
fast fading. To control the power level on the uplink, the base station performs frequent
signal to interference ratio (SIR) measurements on the received signals from each of the
mobiles. This value is compared to a target SIR value and if the power from the mobile
station is deemed to be too high or low it will be told to decrease or increase the power
accordingly. Since this task is executed 1500 times per second, it is much faster than
power control problems, such as fast fading, that may occur, and hence can compensate
for these. This fast power control is very effective for slow to moderate movement speeds
of the mobile device. However, benefits decrease as the speed of the mobile increases.
This also deals with the near-far problem, where signals from mobile devices which are
far from the base station will suffer greater attenuation. The object of the fast power
control is that the signal from each mobile should arrive at the BTS at its target SIR
value. The same type of power control is used on the downlink. When communicating
with mobile devices that are on the edge of the cell the base station may marginally
increase the power it sends. This is required since these particular mobiles may suffer
from increased other-cell interference. Outer loop
As noted, inner loop power control measures the power from the mobile device and
compares it to a set SIR target. This target value is set and adjusted by the outer loop
power control within the RNC. This value will change over time but does not need to be
adjusted at the same high frequency. The target value is actually derived from a target
BER or BLER that the service is expected to meet. Some errors with the data received
from the mobile device are expected. If there are no errors, then the UE is assumed to
be transmitting at too high a power, with the consequences of causing interference and
reducing the battery life of the device. To implement this method of power control the
mobile device will compute a checksum before sending any data. Once received, a new
checksum is computed on the data and this is compared to the one sent by the mobile
device. The BTS will also measure the quality of the received data in terms of BER. If
too many frames are being received with errors, or frames have too high a BER, then the
power can be increased. The target set-point is not static: it does change over time. This
is required so that the cell can be more efficiently utilized.
The physical layer is structured into radio frames, each of 10 ms duration, and a radio
frame is divided up into 15 slots, as shown in Figure 6.18. Blocks of data are transferred
across the air interface in each radio frame, and the data rate at which information is sent
may change with radio frame granularity. Within a slot, control functions such as power
control take place.
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