  We introduce a framework to study slotted Aloha with cooperative base
stations. Assuming a geographic-proximity communication model, we propose
several decoding algorithmswith different degrees of base stations' cooperation
(non-cooperative, spatial, temporal, and spatio-temporal). With spatial
cooperation, neighboring base stations inform each other whenever they collect
a user within their coverage overlap; temporal cooperation corresponds to
(temporal) successive interference cancellation done locally at each station.
We analyze the four decoding algorithms and establish several fundamental
results. With all algorithms, the peak throughput (average number of decoded
users per slot, across all base stations) increases linearly with the number of
base stations. Further, temporal and spatio-temporal cooperations exhibit a
threshold behavior with respect to the normalized load (number of users per
station, per slot). There exists a positive load $G^\star$, such that, below
$G^\star$, the decoding probability is asymptotically maximal possible, equal
the probability that a user is heard by at least one base station; with
non-cooperative decoding and spatial cooperation, we show that $G^\star$ is
zero. Finally, with spatio-temporal cooperation, we optimize the degree
distribution according to which users transmit their packet replicas; the
optimum is in general very different from the corresponding optimal
distribution of the single-base station system.
