In this blog, we will describe OFDM (Orthogonal Frequency Division Multiplexing), which is an important technology in LTE systems.
OFDM (Orthogonal Frequency Division Multiplexing) is a type of multi-carrier transmission. By dividing the information to be transmitted into multiple carrier waves (subcarriers) with different frequencies and sending them together, a large amount of information can be sent at once.
1. Multiple carrier waves can be packed into a frequency band for transmission, resulting in high-frequency utilization efficiency.
2. Resistant to interference caused by multipath (radio waves not only reach straight ahead but also propagate through multiple routes by reflecting off mountains, buildings, etc.)
A signal whose horizontal axis is represented by the time axis is broken down by frequency, and the horizontal axis is represented by the frequency axis.
The vertical axis represents the signal intensity.
To represent a signal whose horizontal axis is the time axis by a signal whose horizontal axis is the frequency axis, the Fast Fourier Transform (FFT) is performed.
Conversely, to represent a signal whose horizontal axis is the frequency axis by a signal whose horizontal axis is the time axis, the Inverse Fast Fourier Transform (IFFT) is performed.
Figure 4: Subcarrier arrangement in OFDM
It is a guard interval set in OFDM symbols to suppress the effects of multipath.
When CP is not set
If the CP is not set, the signal of the reflected wave symbol will be mixed with the direct wave, and interference will occur.
When CP is set
If the signal of the reflected wave symbol is within the CP set for the direct wave, the effect of interference can be suppressed.
Figure 9 shows a block diagram of the OFDM processing flow. OFDM processing follows steps (1) through (7) below on the transmitting side. (On the receiving side, the processing is the reverse of the transmitting side.)
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