请总结以下_phb exponent_词条内容，并特别指出phb exponent主要用于描述什么，在120字内完成总结。  
In chaos-based spread-spectrum communications, a p-unit for the PN sequence is employed as a basic sequence, denoted hereinafter as a pseudorandom binary (PRB) sequence. The periodicity of the sequence allows for several cyclic shifts, each shift relating to a certain phb exponent value, which then determines a unique spreading code. Thus, the phb exponent is the parameter that primarily defines the position of a specific cyclic shift within a spreading code, effectively acting as an identifier of the spreading code in the system.
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在基于混沌的扩频通信中，p-unit 作为伪随机二进制（PRB）序列的基础序列。该序列的周期性允许有多个循环移位，每个移位对应一个特定的 phb 指数值，用于确定唯一的扩频码。phb 指数主要用于标识系统中扩频码的位置。
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