  We investigate the physical layer security of uplink single-carrier
frequency-division multiple-access (SC-FDMA) systems. Multiple users, Alices,
send confidential messages to a common legitimate base-station, Bob, in the
presence of an eavesdropper, Eve. To secure the legitimate transmissions, each
user superimposes an artificial noise (AN) signal on the time-domain SC-FDMA
data block. We reduce the computational and storage requirements at Bob's
receiver by assuming simple per-subchannel detectors. We assume that Eve has
global channel knowledge of all links in addition to high computational
capabilities, where she adopts high-complexity detectors such as single-user
maximum likelihood (ML), multiuser minimum-mean-square-error (MMSE), and
multiuser ML. We analyze the correlation properties of the time-domain AN
signal and illustrate how Eve can exploit them to reduce the AN effects. We
prove that the number of useful AN streams that can degrade Eve's
signal-to-noise ratio (SNR) is dependent on the channel memories of Alices-Bob
and Alices-Eve links. Furthermore, we enhance the system security for the case
of partial Alices-Bob channel knowledge at Eve, where Eve only knows the
precoding matrices of the data and AN signals instead of knowing the entire
Alices-Bob channel matrices, and propose a hybrid scheme that integrates
temporal AN with channel-based secret-key extraction.
