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Performance of cellular communication systems is typically enhanced by leveraging the three dimensions of system transmission bandwidth, area frequency reuse, i.e. system access point (AP) density, and spectral efficiency of employed radio air interface, referred to as the radio access technology (RAT). In particular, refined RATs are continuously introduced to compensate for limited spectrum availability and restrictions on system AP densification in cellular communication systems. While the performance of cellular communication systems is maximized by employing a single RAT providing maximal spectral efficiency, varying capabilities of user equipment compels the co-deployment of multiple RATs in cellular communication systems and entails fragmenting system radiofrequency resources between co-deployed RATs. Nevertheless, the inefficient structuring of multi-RAT systems as independently operated collocated single-RAT subsystems results in unbalanced system loading, suboptimal spectrum utilization, and the omission of multiuser diversity as a performance enhancement dimension in multi-RAT systems. The omitted dimension of multiuser diversity is exploited in this paper, through multiple means and techniques, to further enhance the performance of multi-RAT data cellular communication systems. Unifying the architectural structure of multi-RAT systems is proposed to eliminate the redundant duplication of radio access network functions and elements, reduce system deployment costs and operational complexity, improve system scalability, and enable the joint execution of non-radio transmission functions for all co-deployed RATs. By fully exploiting system multiuser diversity, the joint allocation of system radiofrequency resources under autonomous spectrum assignment is shown to substantially enhance the performance of all employed RATs, in addition to the overall performance of multi-RAT systems, without extending any of the three typical performance enhancement dimensions of cellular communication systems.

Multi-radio access technology (RAT) cellular communication systems limit connected users to utilizing a single RAT even when employing multi-mode user equipment (UE) capable of utilizing multi-RATs. Single-mode access, combined with static spectrum partitioning between co-deployed RATs and independent resource allocation for employed RATs, results in suboptimal spectrum utilization in multi-RAT systems. This paper models user access in multi-RAT systems and proposes enabling multi-mode UE to simultaneously utilize multiple RATs, using multi-RAT carrier aggregation, to improve the performance and spectrum utilization of multi-RAT systems. Several realizations of multi-mode access with varying implementation requirements are presented and discussed. Detailed system-level simulations, for a system co-deploying High Speed Packet Access (HSPA) and Long-Term Evolution (LTE), are performed to investigate the gains and limitations of different user access configurations in multi-RAT systems.