  The golden code is a full-rate full-diversity space-time code for two
transmit antennas that has a maximal coding gain. Because each codeword conveys
four information symbols from an M-ary quadrature-amplitude modulation
alphabet, the complexity of an exhaustive search decoder is proportional to
M^2. In this paper we present a new fast algorithm for maximum-likelihood
decoding of the golden code that has a worst-case complexity of only O(2M^2.5).
We also present an efficient implementation of the fast decoder that exhibits a
low average complexity. Finally, in contrast to the overlaid Alamouti codes,
which lose their fast decodability property on time-varying channels, we show
that the golden code is fast decodable on both quasistatic and rapid
time-varying channels.
