  We show how the iterative decoding threshold of tailbiting spatially coupled
(SC) low-density parity-check (LDPC) code ensembles can be improved over the
binary input additive white Gaussian noise channel by allowing the use of
different transmission energies for the codeword bits. We refer to the proposed
approach as energy shaping. We focus on the special case where the transmission
energy of a bit is selected among two values, and where a contiguous portion of
the codeword is transmitted with the largest one. Given these constraints, an
optimal energy boosting policy is derived by means of protograph extrinsic
information transfer analysis. We show that the threshold of tailbiting SC-LDPC
code ensembles can be made close to that of terminated code ensembles while
avoiding the rate loss (due to termination). The analysis is complemented by
Monte Carlo simulations, which confirm the viability of the approach.
