Congestion control meets medium access: throughput, delay, and complexity

This paper looks at the problem of designing medium access algorithm for wireless networks with the objective of providing high throughput and low delay performance to the users, while requiring only a modest computational effort at the transmitters and receivers. Additive inter-user interference at the receivers is an important physical layer characteristic of wireless networks. Today's Wi-Fi networks are based upon the abstraction of physical layer where inter-user interference is considered as noise leading to the 'collision' model in which users are required to co-ordinate their transmissions through Carrier Sensing Multiple Access (CSMA)-based schemes to avoid interference. This, in turn, leads to an inherent performance trade-off [1]: it is impossible to obtain high throughput and low delay by means of low complexity medium access algorithm (unless P=NP). As the main result, we establish that this trade-off is primarily due to treating interference as noise in the current wireless architecture. Concretely, we develop a simple medium access algorithm that allows for simultaneous transmissions of users to the same receiver by performing joint decoding at receivers, over time. For a receiver to be able to decode multiple transmissions quickly enough, we develop appropriate congestion control where each transmitter maintains a "window" of undecoded transmitted data that is adjusted based upon the "feedback" from the receiver. In summary, this provides an efficient, low complexity "online" code operating at varying rate, and the system as a whole experiences only small amount of delay (including decoding time) while operating at high throughput.