  In this paper, we study covert communications between {a pair of} legitimate
transmitter-receiver against a watchful warden over slow fading channels. There
coexist multiple friendly helper nodes who are willing to protect the covert
communication from being detected by the warden. We propose an uncoordinated
jammer selection scheme where those helpers whose instantaneous channel gains
to the legitimate receiver fall below a pre-established selection threshold
will be chosen as jammers radiating jamming signals to defeat the warden. By
doing so, the detection accuracy of the warden is expected to be severely
degraded while the desired covert communication is rarely affected. We then
jointly design the optimal selection threshold and message transmission rate
for maximizing covert throughput under the premise that the detection error of
the warden exceeds a certain level. Numerical results are presented to validate
our theoretical analyses. It is shown that the multi-jammer assisted covert
communication outperforms the conventional single-jammer method in terms of
covert throughput, and the maximal covert throughput improves significantly as
the total number of helpers increases, which demonstrates the validity and
superiority of our proposed scheme.
