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Technical Description
Buffering
Adaptive modulation might influence the design of the buffer dimensioning. In
case packet aging is not used, the maximum delay variation time will increase
due to that the buffer is configured in bytes and that data will travel at a slower
speed during lower modulation steps. When packet aging is enabled, the
maximum delay variation time will be kept regardless of modulation level.
This will also ensure that there is no old data in lower priority queues when
modulation is increased after a fading situation.
Adaptive modulation can influence the position of the narrowest congestion
point in the network, with too small buffers this might have a strong negative
impact on utilization and end user TCP performance. To ensure high link
utilization and high TCP performance, buffers for TCP traffic should be
dimensioned in the area above average Round Trip Time (RTT), which is
typically in the area of 100–200 ms.
RSEC
Reference Spectrum Efficiency Class (RSEC) defines the spectral mask, that
is to say, it is which SEC is used as a reference. The maximum output power
is dependent on the RSEC used. There is a relation between RSEC and the
maximum chosen modulation, which is illustrated in Table 3.
Table 3
4 QAM
16 QAM
32 QAM
64 QAM / 128 QAM
256 QAM / 512 QAM
When upgrading an existing path to Adaptive Modulation, it is important to
consider if licenses will have an impact on the choice of mask, or spectral class,
for a given country.
5.2
Transmit Power Control
The radio transmit power can be controlled in Remote Transmit Power Control
(RTPC) mode, selectable from the management system including setting of
associated parameters. In Automatic Transmit Power Control (ATPC) mode the
transmit power can be increased rapidly during fading conditions and allows
the transmitter to operate at less than the maximum power during normal path
conditions. The normally low transmit power allows more efficient use of the
available spectrum while the high transmit power can be used as input to path
reliability calculations, such as fading margin and carrier-to-interference ratio.
10
RSEC
Modulation
Reference Spectrum Efficiency
Class
RSEC=2
RSEC=4L
RSEC=4H
RSEC=5B
RSEC=6B
1/221 02-CSH 109 172/1-V1 Uen E | 2012-11-07
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