  This paper studies a two-tier CDMA system in which the microcell base is
converted into a data access point (DAP), i.e., a limited-range base station
that provides high-speed access to one user at a time. The microcell (or DAP)
user operates on the same frequency as the macrocell users and has the same
chip rate. However, it adapts its spreading factor, and thus its data rate, in
accordance with interference conditions. By contrast, the macrocell serves
multiple simultaneous data users, each with the same fixed rate. The
achieveable throughput for individual microcell users is examined and a simple,
accurate approximation for its probability distribution is presented.
Computations for average throughputs, both per-user and total, are also
presented. The numerical results highlight the impact of a desensitivity
parameter used in the base-selection process.
