Proportionally fair

From Wikipedia, the free encyclopedia

In a proportionally fair scheduling algorithm, the data flows are assigned a data rate that is inversively proportional to its "cost" in terms of resource consumption. For example, a cell phone that is at far distnace from the base station may achive lower data rate than one that is close to the base station.

Proportionally fair scheduling can be achieved by means of weighted fair queuing (WFQ), by setting the scheduling weights for data flow $i$ to $w i = 1 / c i$ , where the cost $c i$ is the amount of consumed resources per data bit. For instance:

In CDMA spread spectrum cellular networks, the cost may be the required energy per bit in the transmit power control (the increased interference level).
In wireless communication with link adaptation, the cost may be the required time to transmit a certain number of bits using the modulation and error coding scheme that this required.
In wireless networks with fast Dynamic Channel Allocation, the cost may be the number of nearby base station sites that can not use the same frequency channel simultaneously, in view to avoid co-channel interference.

[edit] Alternative definition

The introduction to this article provides insufficient context for those unfamiliar with the subject matter.
Please help Wikipedia by improving the introduction according to the guidelines laid out at Wikipedia:Guide to layout. You can discuss the issue on the talk page.

A proportionally fair scheduling algorithm schedules the channel for the station that has the maximum of the priority function $P=\frac{T^\alpha}{R^\beta}$ , where $T$ denotes the data rate potentially achievable for the station at the present moment (in the present time slot), $R$ is the average data rate of this station. The time and station indexes are omitted for convenience. Parameters $α$ and $β$ tune the fairness of the scheduler, that is, is it fair to all stations giving them equal bandwidth or is the scheduler maximizing the throughput of the channel. If $α = 0$ and $β = 1$ , then the scheduler becomes a round-robin scheduler that is very fair to all stations. If $α = 1$ and $β = 0$ then the scheduler is called Maximum C/I scheduler, that maximizes the throughput of the channel while stations with low $T$ (that are far away, probably) are disadvantaged.