  This paper studies a new decentralized resource allocation strategy, named
iterative spectrum shaping (ISS), for the multi-carrier-based multiuser
communication system, where two coexisting users independently and sequentially
update transmit power allocations over parallel subcarriers to maximize their
individual transmit rates. Unlike the conventional iterative water-filling
(IWF) algorithm that applies the single-user detection (SD) at each user's
receiver by treating the interference from the other user as additional noise,
the proposed ISS algorithm applies multiuser detection techniques to decode
both the desired user's and interference user's messages if it is feasible,
thus termed as opportunistic multiuser detection (OMD). Two encoding methods
are considered for ISS: One is carrier independent encoding where independent
codewords are modulated by different subcarriers for which different decoding
methods can be applied; the other is carrier joint encoding where a single
codeword is modulated by all the subcarriers for which a single decoder is
applied. For each encoding method, this paper presents the associated optimal
user power and rate allocation strategy at each iteration of transmit
adaptation. It is shown that under many circumstances the proposed ISS
algorithm employing OMD is able to achieve substantial throughput gains over
the conventional IWF algorithm employing SD for decentralized spectrum sharing.
Applications of ISS in cognitive radio communication systems are also
discussed.
