A communication system comprising at least two mobile structures (113,114), each mobile structure comprising: - a base station (11,12) adapted to establish cellular communication links with mobile communication terminals of a respective mobile structure via a determined radio interface, - a subscriber subscription server (41,42) adapted to manage security information for authenticationof the mobile communication terminals of the respective mobile structure; - a security management entity (31,32) adapted to interact with the subscriber subscription server in order to obtain and store security information associated with the mobile communication terminals served by the base station; - at least one link entity (21,22) adapted to establish a direct mode link with another link entity (22,21) of the other mobile structure (114,113), said link entity having an identification module (211,221) adapted to store a unique identifier (IMSI#1,IMSI#2) associated with the link entity; - an emulator (51,52) of the security management entity of the mobile structure, accessible by the link entity and adapted to emulate the interface for accessing the subscriber subscription server (41,42) of said mobile structure; and, - a proxy module (213,223) adapted to transport an authentication request from the link entity (21,22) of the other mobile structure to the emulator (51,52) of the security management entity (31,32) of the mobile structure (113,114), for the authentication of said link entity (21,22) of the other mobile structure (114,113) on the subscriber subscription server (41,42) of the mobile structure (113,114), based on the unique identifier (IMSI#1,IMSI#2) of said link entity (21,22) of the other mobile structure (114,113). - a base station (11,12) adapted to establish cellular communication links with mobile communication terminals of a respective mobile structure via a determined radio interface, - a subscriber subscription server (41,42) adapted to manage security information for authenticationof the mobile communication terminals of the respective mobile structure; - a security management entity (31,32) adapted to interact with the subscriber subscription server in order to obtain and store security information associated with the mobile communication terminals served by the base station; - at least one link entity (21,22) adapted to establish a direct mode link with another link entity (22,21) of the other mobile structure (114,113), said link entity having an identification module (211,221) adapted to store a unique identifier (IMSI#1,IMSI#2) associated with the link entity; - an emulator (51,52) of the security management entity of the mobile structure, accessible by the link entity and adapted to emulate the interface for accessing the subscriber subscription server (41,42) of said mobile structure; and, - a proxy module (213,223) adapted to transport an authentication request from the link entity (21,22) of the other mobile structure to the emulator (51,52) of the security management entity (31,32) of the mobile structure (113,114), for the authentication of said link entity (21,22) of the other mobile structure (114,113) on the subscriber subscription server (41,42) of the mobile structure (113,114), based on the unique identifier (IMSI#1,IMSI#2) of said link entity (21,22) of the other mobile structure (114,113).