Changing the bit rate

Asked by Nikola on 2009-02-08


I have a question regarding the bit rate in WiFiMAC.

Is there a way to change the bit rate of the system? I mean whether it is e.g. 2 MBit/s, 11 MBit/s and so on? Which is the default value? Which standard is taken as default, e.g. 802.11a, b, g?


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Sebastian Max
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Best Sebastian Max (smx-comnets) said : #1

Hi Nikola,

I assume that by 'bitrate' you mean the selected Modulation- and Coding Scheme (MCS) of the transmitting STA.

By default, WiFiMAC is using the MCSs defined by the amendment IEEE 802.11a: For example, using BPSK as modulation, coding rate 1/2, 4us symbol length, 20MHz channel, etc., results in a PHY rate of 6MBit/s. Usually, MCSs are ordered by the resulting PHY rate.

Of course, there is no such thing as the MCS of the 'system': Each transmitter selects for the upcoming transmission a MCS according to its Rate Adaptation (RA) strategy, which can be based on many different factors, i.e., knowledge that the transmitter has gained from previous transmissions. Of course, each transmitter can have a different strategy.

In the current WiFiMAC, several different RA strategies are available (the implementations can be found in modules/dll/wifimac/src/lowerMAC/rateAdaptationStrategies):
* ConstantLow: The transmitter always selects the lowest (and most robust) MCS available (which is BPSK1/2 by default)
* Opportunistic: The transmitter tries to maximize the PHY rate by increasing the MCS until frame errors occur, which are identified by missing ACKs
* SINR: Each node measures the SINR of received frames and feedbacks the average to the transmitter. The latter selects the MCS based on this information. If no SINR measurements are available, the behavior of 'Opportunistic' is used.
* SINRwithMIMO: Same as SINR, but also uses multiple streams if multiple antennas are available at transmitter and receiver.
In the literature there are numerous other RA strategies with different purpose, required knowledge and efficiency in different scenarios...

The RA strategy is selected by the configuration parameter layer2.ra.raStrategy, see, e.g., the in the wifimac-test.


Hi Nikola,

did this solve your question? If yes, could you please indicate this by changing the status to "Solved"?


Nikola (nkz) said : #3

Thanks Sebastian Max, that solved my question.