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The efficiency of slotted-ALOHA scheme is upper limited by 37%. However, it is very easy to implement. To increase system efficiency, code division multiple accessing (CDMA) technique can be adopted in ALOHA scheme. This paper studies a medium access scheme, utilizing two dimensional reservations: time dimension and code dimension. The reservation rate and collision rate are analyzed theoretically. Simulation results are given under ideal channel condition and noisy single-path Rayleigh channel condition. Theoretical analysis and simulation results demonstrate that the reservation rate is increased and collision rate is reduced due to the anti-collision advantage of CDMA code.