This paper models the cooperative transmission via caching helpers (cache-based joint transmission), which cache one of the K most popular files, in wireless cellular networks and analyzes its performance in terms of the file transmission success probability in the stochastic geometry framework. For given caching placement, the caching helpers can either provide a file diversity gain by serving different files or a cooperative gain by jointly transmitting the same files. In order to account for the tradeoff between the file diversity gain and the cooperative gain, we derive the cache hit probability and the rate coverage probability. We also find the optimal caching placement balancing the tradeoff and investigate the effects of various system parameters, such as the cooperative region, the density of caching helpers, and the file popularity exponent, on the optimal caching placement.
For a user cooperation system with multiple antennas, we consider a trust
degree based cooperation techniques to explore the influence of the
trustworthiness between users on the communication systems. For the system with
two communication pairs, when one communication pair achieves its quality of
service (QoS) requirement, they can help the transmission of the other
communication pair according to the trust degree, which quantifies the
trustworthiness between users in the cooperation. For given trust degree, we
investigate the user cooperation strategies, which include the power allocation
and precoder design for various antenna configurations. For SISO and MISO
cases, we provide the optimal power allocation and beamformer design that
maximize the expected achievable rates while guaranteeing the QoS requirement.
For a SIMO case, we resort to semidefinite relaxation (SDR) technique and block
coordinate update (BCU) method to solve the corresponding problem, and
guarantee the rank-one solutions at each step. For a MIMO case, as MIMO is the
generalization of MISO and SIMO, the similarities among their problem
structures inspire us to combine the methods from MISO and SIMO together to
efficiently tackle MIMO case. Simulation results show that the trust degree
information has a great effect on the performance of the user cooperation in
terms of the expected achievable rate, and the proposed user cooperation
strategies achieve high achievable rates for given trust degree.Comment: 15 pages,9 figures, to appear in IEEE Transactions on Wireless
communication
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