Multicast multimedia transmission over wireless local area networks

Abstract

Multicast over IEEE 802.1 la/b/g/n Wireless Local Area Networks (WLANs) is an efficient means of transmission exploiting the broadcast nature of the wireless medium. Moreover the use of the unlicensed 2.4GHz or the 5GHz bands does not incur licensing costs. But IEEE 802.lla/b/g/n Access Points (APs) transmit multicast unreliably, because the receivers do not transmit feedback regarding packet reception. Hence, multicast over IEEE 802.lla/b/g/n WLANs suffers from a high Packet Error Rate (PER) which is inappropriate for multimedia transmission. In 2012, the IEEE 802.llaa standard added reliability to multicast using Directed Multicast Service, Groupcast with Retries (GCR) Unsolicited Retry and GCR Block Acknowledgement. However, packet repetition does not result in an optimal code rate. The work in this thesis first verified, using empirical and semi-analytical analyses, that two antennas of an IEEE 802.1 ln AP can be spatially distributed to mitigate the PER experienced. A main contribution was the proposal of a distributed antennas system (DAS), consisting of seven antennas, which was shown to multicast video over IEEE 802.1 ln WLANs with a Peak Signal-to-Noise Ratio (PSNR) of at least 36dB, with the same power but better code rate than other infrastructures. Even packets are multicast over one set of four transmit antennas with an antenna placed in the centre of the coverage area and three transmit antennas placed equidistantly at the periphery. The centre antenna and three other transmit antennas, also placed equidistantly at the periphery, are then used to multicast odd packets. The proposed DAS can also use antenna switching so that unicast transmission uses a centralized antennas system. The proposed DAS is shown to scale up well using a multi-cell approach. It was shown that packet repetition at the application layer, retransmitting each packet proactively once (code rate 0.5), does not result in the entire multicast group receiving good Quality of Service (QoS) with a legacy infrastructure or a DAS using only one set of four transmit antennas, due to burst erasures on the channel. The proposed seven-antenna DAS does guarantee the required QoS to the entire multicast group, on the other hand. Hence, schemes such as IEEE 802.11 aa GCR Unsolicited Retry should be deployed with the proposed DAS. The best code rate is achieved with Block Erasure Coding (BEC) on the proposed DAS. However, Network Coding, achieved by XORing packets to create parity packets, achieves a higher code rate than packet repetition and a lower delay than BEC, and results in good QoS with a PHY data rate of 58.5Mbps. Another original contribution is the proposal and study of two new MAC layer protocols. The first protocol uses a Binary Search guided by feedback from the receivers to determine the necessary number of R-S encoded parity packets. The second protocol combines Network Coding and packet repetition with feedback from the receivers, resulting in smaller channel occupancy than reactive packet repetition. Although this second protocol results in a larger maximum delay than reactive packet repetition, it is still appropriate for live-video streaming since the maximum delay is less than 8 ms. Finally, an infrastructure using distributed IEEE 802.1 ln APs is shown to perform better than the legacy infrastructure, at a code rate of 1/3. The advantage of using distributed APs is that IEEE 802.1 ln MCSs employing Spatial Division Multiplexing can be used for multicasting.