Abstract: Mathematical models for fractionally spaced differentially detected π/4-DQPSK signals with bit majority decision rule，symbol majority decision rule and differential cumulative summary decision rule are established through analyzing the error probabilities of in-phase and quadrature channels.Symbol error rate expression for every decision rule is derived and verified by Monte-Carlo simulation.The comparison of symbol error rate for every decision rule shows that the differential cumulative decision rule has the best performance，which has about 8.3 dB improvement at error rate of 10－3 when signals are sampled every 1/8 symbol period.

Key words: π/4-Differential Quadrature Phase-Shift Keying（DQPSK）, differential detection, fractional space, symbol error rate

摘要： 通过分析同相与正交两路信号，建立了适用于分数间隔采样π/4-DQPSK差分解调的比特大数判决法、符号大数判决法及差分累加和判决法三种判决方式的数学模型，推导出了各种判决方式下符号误码率的数学表达式，并通过Monte-Carlo仿真实验对得出数学表达式进行了验证。误码率对比结果显示差分累加和判决法具有最佳的性能，在误码率为10－3，1/8符号间隔采样时，较传统差分解调有8.3 dB的性能提升。

关键词: π/4偏移差分正交相移键控（π/4-DQPSK）, 差分解调, 分数间隔, 符号误码率