  The joint uplink/downlink (JUD) design of simultaneously transmitting and
reflecting reconfigurable intelligent surfaces (STAR-RIS) is conceived in
support of both uplink (UL) and downlink (DL) users. Furthermore, the dual
STAR-RISs (D-STAR) concept is conceived as a promising architecture for
360-degree full-plane service coverage, including UL/DL users located between
the base station (BS) and the D-STAR as well as beyond. The corresponding
regions are termed as primary (P) and secondary (S) regions. Both BS/users
exist in the P-region, but only users are located in the S-region. The primary
STAR-RIS (STAR-P) plays an important role in terms of tackling the P-region
inter-user interference, the self-interference (SI) from the BS and from the
reflective as well as refractive UL users imposed on the DL receiver. By
contrast, the secondary STAR-RIS (STAR-S) aims for mitigating the S-region
interferences. The non-linear and non-convex rate-maximization problem
formulated is solved by alternating optimization amongst the decomposed convex
sub-problems of the BS beamformer, and the D-STAR amplitude as well as phase
shift configurations. We also propose a D-STAR based active beamforming and
passive STAR-RIS amplitude/phase (DBAP) optimization scheme to solve the
respective sub-problems by Lagrange dual with Dinkelbach's transformation,
alternating direction method of multipliers (ADMM) with successive convex
approximation (SCA), and penalty convex-concave procedure (PCCP). Our
simulation results reveal that the proposed D-STAR architecture outperforms the
conventional single RIS, single STAR-RIS, and half-duplex networks. The
proposed DBAP of D-STAR outperforms the state-of-the-art solutions found in the
open literature for different numbers of quantization levels, geographic
deployment, transmit power and for diverse numbers of transmit antennas, patch
partitions as well as D-STAR elements.
