An optimization algorithm based on equal margin criteria for selection of modulation index is proposed to allocate and optimize the limited transmit power of the downlink of deep space USB (Unified-S-Band) systems.Theoretical demonstration is given.Three different cases in the downlink are numerated in terms of operation modes:telemetry only channel,ranging only channel and simultaneous telemetry and ranging channels.The optimum modulation index can be determined based on equal link margin criteria in the first two cases.In the third case,however,constraint condition is needed to compensate the deficiency of equal link margin criteria.Finally,the feasibility of equal margin criteria is demonstrated with simulation results.%针对深空USB(统一S频段)系统下行链路的功率分配问题,提出了基于余量等均准则的调制指数优化算法,并给出了算法的理论证明.根据工作方式的不同,将下行链路分为仅有遥测信道、仅有测距信道和同时存在遥测与测距信道3种情况,并设定链路预算余量相等,其中在仅有遥测、仅有测距信道的情况下,根据余量等均准则可确定调制指数的最优值；在同时存在遥测与测距信道的情况下,需要增加约束条件,以进一步缩小并限制调制指数的取值范围.最后仿真结果验证了该准则的有效性.
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