  In this paper, we propose two adaptive detection schemes based on
single-carrier frequency domain equalization (SC-FDE) for multiuser
direct-sequence ultra-wideband (DS-UWB) systems, which are termed structured
channel estimation (SCE) and direct adaptation (DA). Both schemes use the
minimum mean square error (MMSE) linear detection strategy and employ a cyclic
prefix. In the SCE scheme, we perform the adaptive channel estimation in the
frequency domain and implement the despreading in the time domain after the
FDE. In this scheme, the MMSE detection requires the knowledge of the number of
users and the noise variance. For this purpose, we propose simple algorithms
for estimating these parameters. In the DA scheme, the interference suppression
task is fulfilled with only one adaptive filter in the frequency domain and a
new signal expression is adopted to simplify the design of such a filter.
Least-mean squares (LMS), recursive least squares (RLS) and conjugate gradient
(CG) adaptive algorithms are then developed for both schemes. A complexity
analysis compares the computational complexity of the proposed algorithms and
schemes, and simulation results for the downlink illustrate their performance.
