Packet delay for CSMA and multi-channel ALOHA multicast schemes in WLANs with fading and co-channel interference

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Abstract
Multicast links can be utilized in WLANs to decrease the average transmission time of a message in situations when several receivers require the same information by simultaneously addressing all the receivers instead of addressing the receivers one-by one as for unicast links. The performance of CSMA and multichannel ALOHA access schemes adapted for multicast transmissions in wireless indoor packet radio networks have been investigated with respect to the average packet delay. In the study, both fading and co-channel interference have been considered. Considerable gains may be achieved by utilizing point-to-multipoint links instead of point-to-point links. Firstly, the average delay before all addressed units have received a packet may be significantly reduced. Secondly, since the packet delay is decreased for multicast links, higher packet rates may be allowed and thirdly, with multicast links, co-channel interference may be reduced, since a packet is in general transmitted fewer times before it is received by all addressed units. Thereby the scarce radio spectrum can be more efficiently utilized, supporting a higher user density. However, our results indicate that when the number of receivers in a multicast group grows, there are situations when the fade margin, i.e. the transmitted power, must be increased to maintain a specified level of performance.

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