The integration of a high altitude platform station (HAPS) with existing terrestrial communication systems would be beneficial as several kinds of wireless services can be easily extended into remote areas. In this letter, we first examine the uplink capacity of a sharing-band overlaid HAPS-terrestrial code division multiple access (CDMA) system and an improved version using a doughnut-like cell model. Numerical results indicate that the uplink capacity is poor under both the proposals due to serious inter-system interference. To mitigate this problem, a ring structure is proposed for HAPS when integrated with a terrestrial cellular system.
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