空时格码技术是近年来无线通信领域的一个研究热点，空时格码的设计准则问题一直是该技术的一个难点。在平坦快衰落条件下且天线数目较多时，基于最小平方欧式距离最大化的空时格码设计准则虽然优于经典的距离－积准则，但仍不完善。从平方距离分布与平方距离和方－方和比分布的角度对此进行了改进，得到改进的准则以及性能为目前最优的新码。还对快慢2种衰落条件下码性能差异的原因给出了新的解释，并将改进的码设计准则应用于智贪码的设计。

In recent years space-time trellis codes (STTCs) have attracted a great amount of research interest in wireless communication. However, the design criterion for STTCs is still a difficult problem. In flat fast fading channels, when the number of antennas is large, the space-time trellis codes design criterion, which is based on maximizing the minimum squared Euclidean distance, is better than the traditional distance and product criteria. Nevertheless, the former criterion is still not perfect. In this paper, from the aspects of squared distance distribution and squared distance sum-square square-sum ratio distribution, an improved criterion is presented, and some new codes are also obtained, which are the best codes known. In addition, a new reason is given, which can explain the different performance of a code between in slow fading channels and in fast fading channels. Finally the improved criterion is introduced into the design of smart-greedy codes.