For OFDM synchronisation, we are using two training symbols according to the Schmidl and Cox method of frequency synchronization and timing synchronization. The paper says:
The first OFDM training symbol has only even numbered subcarriers applying a PN sequence.The result is two identical half symbols in time domain each consisting of Nc/2 samples each.
The second training symbol consists of even numbered subcarriers that are differentially modulated with even numbered subcarriers of first training symbol using a PN sequence. The odd numbered subcarriers of second training symbol can be used for data, pilot or reference symbols.
I have two questions on this.
In OFDM, every subcarrier corresponds to one symbol which is a complex value. So what does it mean by saying even numbered subcarriers are modulated in a symbol using a PN sequence? How does that ensure that it results in two identical halves of a time domain symbol.
what is differential modulation using a PN sequence. How is it achieved?