  In this paper, the throughput of relay networks with multiple
source-destination pairs under queueing constraints has been investigated for
both variable-rate and fixed-rate schemes. When channel side information (CSI)
is available at the transmitter side, transmitters can adapt their transmission
rates according to the channel conditions, and achieve the instantaneous
channel capacities. In this case, the departure rates at each node have been
characterized for different system parameters, which control the power
allocation, time allocation and decoding order. In the other case of no CSI at
the transmitters, a simple automatic repeat request (ARQ) protocol with fixed
rate transmission is used to provide reliable communication. Under this ARQ
assumption, the instantaneous departure rates at each node can be modeled as an
ON-OFF process, and the probabilities of ON and OFF states are identified. With
the characterization of the arrival and departure rates at each buffer,
stability conditions are identified and effective capacity analysis is
conducted for both cases to determine the system throughput under statistical
queueing constraints. In addition, for the variable-rate scheme, the concavity
of the sum rate is shown for certain parameters, helping to improve the
efficiency of parameter optimization. Finally, via numerical results, the
influence of system parameters and the behavior of the system throughput are
identified.
