619 research outputs found
Communication under Strong Asynchronism
We consider asynchronous communication over point-to-point discrete
memoryless channels. The transmitter starts sending one block codeword at an
instant that is uniformly distributed within a certain time period, which
represents the level of asynchronism. The receiver, by means of a sequential
decoder, must isolate the message without knowing when the codeword
transmission starts but being cognizant of the asynchronism level A. We are
interested in how quickly can the receiver isolate the sent message,
particularly in the regime where A is exponentially larger than the codeword
length N, which we refer to as `strong asynchronism.'
This model of sparse communication may represent the situation of a sensor
that remains idle most of the time and, only occasionally, transmits
information to a remote base station which needs to quickly take action.
The first result shows that vanishing error probability can be guaranteed as
N tends to infinity while A grows as Exp(N*k) if and only if k does not exceed
the `synchronization threshold,' a constant that admits a simple closed form
expression, and is at least as large as the capacity of the synchronized
channel. The second result is the characterization of a set of achievable
strictly positive rates in the regime where A is exponential in N, and where
the rate is defined with respect to the expected delay between the time
information starts being emitted until the time the receiver makes a decision.
As an application of the first result we consider antipodal signaling over a
Gaussian channel and derive a simple necessary condition between A, N, and SNR
for achieving reliable communication.Comment: 26 page
Training-Based Schemes are Suboptimal for High Rate Asynchronous Communication
We consider asynchronous point-to-point communication. Building on a recently
developed model, we show that training based schemes, i.e., communication
strategies that separate synchronization from information transmission, perform
suboptimally at high rate.Comment: To appear in the proceedings of the 2009 IEEE Information Theory
Workshop (Taormina
Optimal Sequential Frame Synchronization
We consider the `one-shot frame synchronization problem' where a decoder
wants to locate a sync pattern at the output of a channel on the basis of
sequential observations. We assume that the sync pattern of length N starts
being emitted at a random time within some interval of size A, that
characterizes the asynchronism level between the transmitter and the receiver.
We show that a sequential decoder can optimally locate the sync pattern, i.e.,
exactly, without delay, and with probability approaching one as N tends to
infinity, if and only if the asynchronism level grows as O(exp(N*k)), with k
below the `synchronization threshold,' a constant that admits a simple
expression depending on the channel. This constant is the same as the one that
characterizes the limit for reliable asynchronous communication, as was
recently reported by the authors. If k exceeds the synchronization threshold,
any decoder, sequential or non-sequential, locates the sync pattern with an
error that tends to one as N tends to infinity. Hence, a sequential decoder can
locate a sync pattern as well as the (non-sequential) maximum likelihood
decoder that operates on the basis of output sequences of maximum length A+N-1,
but with much fewer observations.Comment: 6 page
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