Encoding phase information on 1090 MHz extended squitter (1090ES) pulse position modulation ( PPM) signals of Automatic Dependent Surveillance-Broadcast( ADS-B) systems is a promising technical method to increase capacity without extra transmissions, additional frequency band and any influence on current ADS-B systems. Two approaches to increase 1090ES capacity are compared,which explains the reasons to choose phase modulation approach. Several key techniques are indicated in this paper,including phase modulation on PPM,receiving,demodulation and decoding,synchronization,design of error correction code,assessment of feasibility and safety. A carrier synchronization method is proposed for single-input-multiple-output ( MIMO) burst communication according to different deviation. Linear transition of phase between adjacent symbols is introduced to restrain spectrum expanding for phase mutation.%对广播式自动相关监视( ADS-B)系统的1090ES脉位调制信号做脉内调相，可在不增加发射次数、不占用新的频段、不影响现有ADS-B系统运行的情况下增加信号的数据传输能力。论证了两种可能采用的1090 ES数据链扩容方法，说明了选择相位调制实现扩容的原因。讨论了相位调制扩容1090 ES信号需解决的几项关键技术，包括扩容信号的调制，信号的接收、解调和解码，信号的同步，数据的校验码设计，可行性及安全性评估。针对多发单收的突发通信特点，提出了一种按偏差分区的载波粗同步方法；针对相位突变引起的频谱扩展问题，提出了一种相位线性过渡的解决方法。
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