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ECE PhD Defense: "Random Linear Packet Coding over Fading Channels with Long Delays," Rameez Ahmed Rasheed Ahmed

05
Aug

210 Behrakis

August 5, 2016 11:00 am
August 5, 2016 11:00 am

Abstract:

We consider random linear packet coding over fading channels with long propagation delays, such as underwater acoustic channels, satellite communication and deep-space communication. We adapt random linear packet for such channels, particularly focusing on underwater acoustic channel. 

Underwater communication has garnered much interest in recent years with emerging applications in underwater sensor networks, warning systems, off-shore oil and gas platform operations, marine life monitoring, etc. 

Since electromagnetic waves do not propagate over long distances underwater, acoustic waves remain the preferred choice for a number of applications. The slow speed of sound in water, however, leads to long propagation delays which challenge the efficiency of traditional automatic repeat request (ARQ) techniques such as the stop-and-wait and its selective versions in acoustic communication systems

We first propose a packet coding scheme where the number of coded packets to transmit is determined so as to achieve a pre-specified outage/reliability criterion, and investigate joint power and rate control with constrained resources. Using the channel state information which is obtained via feedback from the receiver, the transmitter adjusts its power and the number of coded packets such that the average energy per successfully transmitted bit of information is minimized. Two optimization constraints are imposed: (a) the transmit power should not exceed a maximum level, and (b) the number of coded packets should not exceed a maximum value dictated by the desired throughput and delay.

We further extend the results to take into account the effect of inevitable channel estimation errors, and consider the case where the transmitter has only an estimate of the channel gain. We design adaptation policies for such a case based on minimum mean square error (MMSE) channel estimation, taking into account the effect of channel estimation errors in an optimal manner so as to satisfy the required outage/reliability criterion. Finally, we compare the proposed technique to standard automatic repeat request

(ARQ) protocols for underwater communications in terms of the throughput efficiency. Analytical results show that substantial energy savings and improvements in throughput efficiency are available from adaptive power/rate control.

We finally investigate reliable data packet delivery employing random linear packet coding for half-duplex communication links with long delays. 

In order to achieve full reliability, we regard a group of coded packets as one super-packet, on which we apply an ARQ technique. Specifically, we group several super-packets and apply a selective stop-and-wait acknowledgment procedure to the so-obtained unit (a super-group). We explore adaptive power and rate control to improve the performance of the proposed grouped packet coding technique on fading channels. We compare the performance of the proposed technique to that of conventional stop-and-wait, as well as an ideal full-duplex benchmark. Results are presented for a point-to-point link, as well as for a broadcast network. 

For the broadcast network, we investigate two adaptation rules. According to the first rule, the transmitter adjusts its parameters in accordance with the average of the channel gains on each link (average link rule). 

Under the second rule, the transmitter adjusts its parameters in accordance with the lowest channel gain among the links (worst link rule). Using numerical analysis we show that the proposed grouped packet coding, applied to a half-duplex link, can achieve a throughput efficiency that is very close to that of a full-duplex link.

Advisor: Professor Milica Stojanovic 

Committee:
Professor Milica Stojanovic
Professor Kaushik Chowdhury
Professor Mandar Chitre