  In this paper, optimal power allocation and capacity regions are derived for
GSIC (groupwise successive interference cancellation) systems operating in
multipath fading channels, under imperfect channel estimation conditions. It is
shown that the impact of channel estimation errors on the system capacity is
two-fold: it affects the receivers' performance within a group of users, as
well as the cancellation performance (through cancellation errors). An
iterative power allocation algorithm is derived, based on which it can be shown
that the total required received power is minimized when the groups are ordered
according to their cancellation errors, and the first detected group has the
smallest cancellation error.
  Performace/complexity tradeoff issues are also discussed by directly
comparing the system capacity for different implementations: GSIC with linear
minimum-mean-square error (LMMSE) receivers within the detection groups, GSIC
with matched filter receivers, multicode LMMSE systems, and simple all matched
filter receivers systems.
