  Reconfigurable intelligent surfaces (RISs) are expected to be a main
component of future 6G networks, due to their capability to create a
controllable wireless environment, and achieve extended coverage and improved
localization accuracy. In this paper, we present a novel cooperative
positioning use case of the RIS in mmWave frequencies, and show that in the
presence of RIS, together with sidelink communications, localization with zero
access points (APs) is possible. We show that multiple (at least three)
half-duplex single-antenna user equipments (UEs) can cooperatively estimate
their positions through device-to-device communications with a single RIS as an
anchor without the need for any APs. We start by formulating a
three-dimensional positioning problem with Cram\'er-Rao lower bound (CRLB)
derived for performance analysis. After that, we discuss the RIS profile design
and the power allocation strategy between the UEs. Then, we propose
low-complexity estimators for estimating the channel parameters and UEs'
positions. Finally, we evaluate the performance of the proposed estimators and
RIS profiles in the considered scenario via extensive simulations and show that
sub-meter level positioning accuracy can be achieved under multi-path
propagation.
