  In this paper, we formulate a new problem to cope with the transmission of
extra bits over an existing coded transmission link (referred to as coded
payload link) without any cost of extra transmission energy or extra bandwidth.
This is possible since a gap to the channel capacity typically exists for a
practical code. A new concept, termed as accessible capacity, is introduced to
specify the maximum rate at which the superposition transmission of extra bits
is reliable and has a negligible effect on the performance of the coded payload
link. For a binary-input output-symmetric (BIOS) memoryless channel, the
accessible capacity can be characterized as the difference between the channel
capacity and the mutual information rate of the coded payload link, which can
be numerically evaluated for very short payload codes. For a general payload
code, we present a simple lower bound on the accessible capacity, given by the
channel capacity minus the coding rate of the payload code. We then focus on
the scenarios where low-density parity-check (LDPC) codes are implemented for
the payload link. We propose to transmit extra bits by random superposition for
encoding, and exhaustive search (with the aid of statistical learning) for
decoding. We further propose, by establishing an auxiliary channel (called
syndrome channel) induced from "zero-forcing" over the binary field, to
transmit extra bits with structured codes such as repetition codes and
first-order Reed-Muller (RM) codes. Numerical results show that up to 60 extra
bits can be reliably transmitted along with a rate-1/2 LDPC code of length
8064.
