This paper presents an analytical approach to calculate the capacity that a UMTS terrestrial radio access-TDD (UTRA-TDD) pico cell can exploit from underused uplink radio resources of a UTRA-FDD macro cellular overlay. Also, the impact on the UTRA-FDD uplink capacity is evaluated. Underused FDD spectrum is a consequence of a growing demand for an asymmetrical use of communication channels. This study is carried out under the assumption of a spatially uniform user distribution. It was found that under the assumption made, the TDD underlay can be used to increase the flexibility of the UMTS air interface. However, the advantages of the underlay are dependent on the BS separation and the severity of lognormal shadowing. The optimal BS separation was found to be between 200 m and 300 m. Methods are proposed to overcome the revealed constraints