  This paper studies achievable information rates of backscatter communication
systems where the tag performs load modulation with a freely adaptable passive
termination. We find that the complex phasor of the tag current is constrained
to a disk and that the capacity problem can therefore be described with
existing results on peak-power-limited quadrature channels. This allows us to
state the channel capacity and the capacity-achieving distribution of the load
impedance, which is described by non-concentric circles in the right
half-plane. For the low-SNR case (SNR < 4.8 dB) we find that channel capacity
is achieved by a purely reactive load with Cauchy-distributed reactance. The
exposition is based on a system model that abstracts all relevant classes of
backscatter communication systems, including RFID. To address practicality, we
construct a symbol alphabet that allows for a near-capacity information rate of
more than 6 bit per load-switching period at reasonably high SNR. We also find
that the rate hardly decreases when typical value-range constraints are imposed
on the load impedance.
