  We introduce Noise Recycling, a method that substantially enhances decoding
performance of orthogonal channels subject to correlated noise without the need
for joint encoding or decoding. The method can be used with any combination of
codes, code-rates and decoding techniques. In the approach, a continuous
realization of noise is estimated from a lead channel by subtracting its
decoded output from its received signal. The estimate is recycled to reduce the
Signal to Noise Ratio (SNR) of an orthogonal channel that is experiencing
correlated noise and so improve the accuracy of its decoding. In this design,
channels only aid each other only through the provision of noise estimates
post-decoding.
  For a system with arbitrary noise correlation between orthogonal channels
experiencing potentially distinct conditions, we introduce an algorithm that
determines a static decoding order that maximizes total effective SNR. We prove
that this solution results in higher effective SNR than independent decoding,
which in turn leads to a larger rate region. We derive upper and lower bounds
on the capacity of any sequential decoding of orthogonal channels with
correlated noise where the encoders are independent and show that those bounds
are almost tight. We numerically compare the upper bound with the capacity of
jointly Gaussian noise channel with joint encoding and decoding, showing that
they match.
  Simulation results illustrate that Noise Recycling can be employed with any
combination of codes and decoders, and that it gives significant Block Error
Rate (BLER) benefits when applying the static predetermined order used to
enhance the rate region. We further establish that an additional BLER
improvement is possible through Dynamic Noise Recycling, where the lead channel
is not pre-determined but is chosen on-the-fly based on which decoder provides
the most confident decoding.
