Specifications
Chapter 4 - Operation and Administration Menus and Parameters
BreezeACCESS 4900 170 System Manual
Adaptive Modulation: The decisions made by the Adaptive Modulation
algorithm for the modulation level to be used are based on multiple
parameters, including information on received signal quality (SNR) that is
received periodically from the destination unit, the time that has passed since
the last transmission to the relevant unit, and the recent history of successful
and unsuccessful transmissions/retransmissions. In the AU, the decision
algorithm is performed separately for each SU.
The transmission/retransmission mechanism operates as follows:
1 Each new frame (first transmission attempt) will be transmitted at a
modulation level selected by the Adaptive Modulation algorithm.
2 If the first transmission trial fails, the frame will be retransmitted at the
same modulation level up to the maximum number of retransmission
attempts defined by the Number of HW Retries parameter.
Statistics-Based Rate Control: The Statistics-Based Rate Control decision
algorithm uses statistical analysis of the successfully/unsuccessfully sent
packets to determine when lowering the modulation would increase the actual
data rate. If the connection is stable, it will periodically check the unit’s
behavior on the next higher modulation (except on modulation 8) in an
attempt to increase the modulation.
In general, the Statistics-Based Rate Control algorithm provides an overall better
performance in case of interference and a decrease in the retransmissions’
percentage over the original Adaptive Modulation algorithm.
4.2.6.5.10.3 Statistics-Based Rate Control Parameters
This submenu configures parameters for the Statistics-Based Rate Control
algorithm. When enabled, the Statistics-Based Rate Control algorithm, constantly
evaluates the achievable throughput for a particular modulation by counting the
number of packets that are successfully transmitted and the packets that are not
received and that need to be retransmitted. Based on these statistics, and on each
modulation’s specific data rate, it will calculate the real throughput that the unit
can support in the current conditions for a particular modulation.
When choosing between modulations, up to a limit, a small number of
retransmissions on a higher modulation is compensated by the overall better
performance ensured by that modulation. When a critical retransmissions
percentage is reached however, it is necessary to decrease the modulation to
achieve better throughputs as shown in Table 4-18.