As wireless networks become more and more popular the increase of the users that demand high quality services by such networks is tremendous. Thus, the bandwidth limitation in the unlicensed bands in combination with the dense of wireless networks in rural areas increase the importance of problems as interference and efficient bandwidth allocation and demands new solutions for the manipulation of such issues. This project suggests one way to address these problems by using the notion of cooperation between wireless nodes.
The novel approach that is studied in this project, provides a unified framework for the development of cooperation communication in wireless networks. Conventional wireless systems are designed and optimized in such a way that any unicast message involves two parties only. The new framework constitutes a breakthrough, as it changes the conventional methods of wireless medium access by introducing additional collaboration from the nodes that otherwise do not directly participate in the transmission. The new approach may enable tremendous improvement in robustness, throughput, delay, significant reduction of interference and extension of coverage range.
In this project we design, study and implement a new MAC protocol called CoopMAC, which is based upon the IEEE 802.11 distributed coordination function (DCF) mode. The basic operation of CoopMAC is outlined and depicted below.
When a source station has a new MAC protocol data unit (MPDU) to send, it can either transmit directly to the destination, or use an intermediate helper for relaying, whichever consumes less total air time. The air time is compared using cached information on the feasible data rates between the three nodes.
Beyond its normal function, a request to send (RTS) message is also used by CoopMAC to notify the station that has been selected for cooperation. Moreover, CoopMAC introduces a new message called helper-ready to send (HTS), which is used by the helper to indicate its availability after it receives the RTS message from the source. If the destination hears the HTS message, it issues a clear to send (CTS) message to reserve the channel time for a two-hop transmission. Otherwise, it still sends out the CTS, but only reserves the channel time for a direct transmission.
If both HTS and CTS are received at the source, the data packet should be transmitted to the helper first, and then forwarded to the destination by the helper. If the source does not receive HTS, it should then initiate a direct transmission to the destination.
A normal ACK is used to acknowledge a correct reception, regardless of whether the packet is forwarded by the helper, or is directly transmitted from the source. If necessary, retransmission is attempted, again in a cooperative fashion.
It is crucial that each station obtains and constantly updates its information about the availability of potential helpers. The CoopMAC protocol deals with this issue mainly hrough maintaining a table called the CoopTable in its management plane. Each entry in the CoopTable corresponds to a potential helper, and contains such information as the ID (e.g, 48-bit MAC address) of the potential helper, and the the latest time at which a packet from that potential helper is overheard, the data rate used for direct transmission between the potential helper and destination, and between the current station and the potential helper. A set of protocols have been defined in CoopMAC to properly establish, manage and update the table in a timely manner.
* The material in this site is based upon work supported by the National Science Foundation under Grant No. 0520054.