  The novel concept of simultaneously transmitting and reflecting
reconfigurable intelligent surface (STAR-RIS) is investigated, where incident
signals can be transmitted and reflected to users located at different sides of
the surface. In particular, the fundamental coverage range of STAR-RIS aided
two-user communication networks is studied. A sum coverage range maximization
problem is formulated for both non-orthogonal multiple access (NOMA) and
orthogonal multiple access (OMA), where the resource allocation at the access
point and the transmission and reflection coefficients at the STAR-RIS are
jointly optimized to satisfy the communication requirements of users. For NOMA,
we transform the non-convex decoding order constraint into a linear constraint
and the resulting problem is convex, which can be optimally solved. For OMA, we
first show that the optimization problem for given time/frequency resource
allocation is convex. Then, we employ the one dimensional search-based
algorithm to obtain the optimal solution. Numerical results reveal that the
coverage can be significantly extended by the STAR-RIS compared with
conventional RISs.
