1 Introduction
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We study the joint user association and content placement problem of cache-enabled HetNets which consists multiple RANs. To achieve joint resource management and performance enhancement of various access networks, we propose a joint resource management architecture, based on which a joint user association and content placement algorithm is designed.
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The problem of user association or content placement in HetNets has been studied separately in previous works [2‐7] and [11‐16]. In this paper, we jointly consider the user association and content placement problem in cache-enabled HetNets and design jointly optimal strategies so that the overall performance of the networks can be maximized.
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Stressing the tradeoff between user download delay and caching cost, we characterize the joint network performance as the weighted sum of user download delay and the caching cost via applying the concept of utility function and formulate the joint user association and content placement problem as a network utility minimization problem.
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Since the formulated optimization problem is a nonlinear integer optimization problem which cannot be solved conveniently using traditional optimization tools, we apply Lagrange partial relaxation and McCormick envelopes method and transform the original optimization problem equivalently into three convex subproblems, which can then be solved by a proposed iterative algorithm. Within each iteration, for a given set of Lagrange multipliers, the three subproblems are solved respectively by means of the modified Kuhn-Munkres (K-M) algorithm and the locally optimal solutions can be obtained, based on which the Lagrange multipliers can be updated through applying subgradient method.
2 Related works
2.1 User association schemes of HetNets
2.2 Content placement schemes of HetNets
3 System model and proposed joint resource management architecture
3.1 System model
3.2 Proposed joint resource management architecture
4 Utility function optimization formulation
4.1 Objective function
4.1.1 Wireless transmission delay
4.1.2 Content fetching delay
4.1.3 Backhaul transmission delay
4.2 Optimization constraints
4.2.1 Transmission rate constraint
4.2.2 Maximum cache capacity constraint
4.2.3 User association constraints
4.3 Optimization problem
5 Solution of the optimization problem
5.1 Equivalent transformation of original optimization problem
5.1.1 Introduction of new variable
5.1.2 Lagrangian dual problem formulation
5.1.3 Subproblem formulation
5.2 Proposed iterative method
5.2.1 K-M algorithm-based optimal solutions to the subproblems
5.2.2 Updating Lagrange multipliers
6 Simulation results
Parameters | Value |
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Maximum cache capacity of BS | 20 Mbits |
Maximum cache capacity of AP | 10 Mbits |
File sizes | 9, 8, 10, 9, 8, and 10 Mbits |
Content fetching delay at BS | 0.2 s |
Content fetching delay at AP | 0.3 s |
Maximum numbers of active users of BS and AP | 3 |
Minimum data rate requirements of users | 1, 2, 0.5, 0.5, 1, and 2 Mbps |
Small scale fading distribution | Rayleigh fading with zero mean and unit variance |
Channel path loss model | 128.1+27 log(d) dB, d denotes the distance |