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Description:
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Data intensive applications , requiring reliability and strict delay constraints ,
have emerged recently and they necessitate a different approach to analyzing system
performance . In my work , I establish a framework that relates physical channel parameters
to the queueing performance for a single -user wireless system . I then seek to
assess the potential benefits of multirate techniques , such as hybrid -ARQ (Automatic
Repeat reQuest ) , in the context of delay -sensitive communications . Present methods
of analysis in an information theoretic paradigm define capacity assuming that
long codewords can be used to take advantage of the ergodic properties of the fading
wireless channel . This definition provides only a limited characterization of the channel
in the light of delay constraints . The assumption of independent and identically
distributed channel realizations tends to over -estimate the system performance by
not considering the inherent time correlation . A finite -state continuous time Markov
channel model that I formulate enables me to partition the instantaneous data -rate
received at the destination into a finite number of states , representing layers in a
hybrid -ARQ scheme . The correlation of channel has been incorporated through level
crossing rates as transition rates in the Markov model .
The large deviation principle governing the buffer overflow of the Markov model ,
is very sensitive to channel memory , is tractable , and gives a good estimate of the
system performance . Metrics such as effective capacity and probability of buffer
overflow , that are obtained through large deviations have been related to the wireless
physical layer parameters through the model . Using the above metrics under QoS constraints , I establish the quantitative performance advantage of using hybrid -ARQ
over traditional systems . I conduct this inquiry by restricting attention to the case
where the expected transmit power is fixed at the transmitter . The results show that
hybrid -ARQ helps us in obtaining higher effective capacity , but it is very difficult to
support delay sensitive communication over wireless channel in the absence of channel
knowledge and dynamic power allocation strategies . |