  Communication systems are traditionally designed to have tight
transmitter-receiver synchronization. This requirement has negligible overhead
in the high-SNR regime. However, in many applications, such as wireless sensor
networks, communication needs to happen primarily in the energy-efficient
regime of low SNR, where requiring tight synchronization can be highly
suboptimal.
  In this paper, we model the noisy channel with synchronization errors as an
insertion/deletion/substitution channel. For this channel, we propose a new
communication scheme that requires only loose transmitter-receiver
synchronization. We show that the proposed scheme is asymptotically optimal for
the Gaussian channel with synchronization errors in terms of energy efficiency
as measured by the rate per unit energy. In the process, we also establish that
the lack of synchronization causes negligible loss in energy efficiency. We
further show that, for a general discrete memoryless channel with
synchronization errors and a general input cost function admitting a zero-cost
symbol, the rate per unit cost achieved by the proposed scheme is within a
factor two of the information-theoretic optimum.
