1. OFDM

 

OFDM technology divides a given channel into many orthogonal sub-channels in the frequency domain. One subcarrier is used for modulation on each subchannel, and each subcarrier is transmitted in parallel. In this way, although the overall channel is non-flat and frequency selective. But each subchannel is relatively flat. Narrowband transmission is performed on each sub-channel, and the signal bandwidth is smaller than the corresponding bandwidth of the channel. Therefore, the interference between signal waveforms can be largely eliminated.

Since the carriers of each sub-channel are orthogonal to each other in the OFDM system. Their spectrums overlap each other. This not only reduces the mutual interference between sub-carriers, but also improves spectrum utilization.

 

2. COFDM

 

COFDM is Coded Orthogonal Frequency Division Multiplexing, which means

before OFDM modulation, the digital code stream is encoded.

What does this Coded do? It is channel coding (source coding is to solve the problem of efficiency, and channel coding is to ensure the reliability of transmission).

 

The specific method is:

 

2.1. Forward Error Correction(FEC)

 

For instance, 100 bits of data needs to be modulated for transmitting. First change it to 200 bits,. When the signal is received, even if there is a problem with the transmission of 100 bits, the correct data can still be demodulated. In short, it is to add redundancy before modulation to improve the reliability of transmission. This is called Internal Error Correction (FEC) in COFDM systems. And it is an important parameter of COFDM system.

 

 

2.2. Guard Interval

 

For purpose of solving the multi-path problem that is the transmitted signal reaches the receiving end through multiple transmission paths. A guard interval is inserted between the transmitted data bits.