在扩频码（伪随机数）下调制使带宽增加

Spread Spectrum

Frequency Hopping(FHSS)

• signal is broadcast over seemingly random series of frequencies
• receiver hops between frequencies in sync with transmitter
• Slow $T_{\text{shift}}\geq T_{\text{symbol}}$
• Fast $T_{\text{shift}}< T_{\text{symbol}}$
• 信号带宽远大于数据信号带宽
• 提高抗人为干扰能力

直接序列扩频(DSSS)

• each bit is represented by multiple bits using a spreading code

Code Division Multiple Access

• break each bit into k chips according to a fixed chipping code specific to each user
• 用户码
• 码片率：新信道码片数据率
• 接收器解码：用相同用户码做乘积，若为 $\pm N$ 则接受，否则拒绝