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In future X-ray Pulsar-based Navigation (XPNAV),the combination of navigation and X-ray communication (XCOM) will be a new and significant research issue.XCOM,as a new free space optical communication method,shows great advantage over traditional communication method,especially in the combination with XPNAV.In this paper,the idea of applying XCOM in XPNAV is proposed and the key techniques for realizing the application are introduced.Then,a key technique of XCOM is proposed that selects the MPPM constellation under the constellation-constrained average symbol error rate (SER).We first build the model of the MPPM constellation selection problem and then give the derivation of the average SER under constellation constraints mathematically.Then the constellation selection problem is converted to a constrained optimization problem.The direct random searching method and the ergodic searching method are proposed to solve the constrained optimization problem.Numerical experiments show that both methods are feasible for the constrained constellation problem and that the ergodic searching method shows better performance than the direct random searching method in computation efficiency.