# IVC 5G O-RAN Summary

###### tags: `UNILA`

## Assigment Second Week

Nama : Dimas Danu Saputra
Student ID : 1915031076
Team : B-2

# 1. Study and modify the code to understand how to implement different FFT size as described in the table (FFT size = 128 and FFT size = 256). Explain the difference of the spectrum for each FFT size !


**EXPLANATION** :
- the spectrum of OFDM transmission with FFT size = 64

- the spectrum of OFDM transmission with FFT size = 128

- the spectrum of OFDM transmission with FFT size = 256

From the simulation above, the spectrum of OFDM transmission with FFT different size (64, 128 and 256). When the FFT size is 64, the power spectral density is lower than spectrum which the size is 128. And the same condition when we compare the spectrum of OFDM transmission between 128 and 256. So we can conclude that the more FFT size in the spectrum transmission of OFDM, the more power spectral density that will get. The bigger size of FFT, the more guard intervals are available, which are used between OFDM symbols. By making the guard interval larger than the expected multipath delay spread, ISI can be completely eliminated. Adding a guard interval, however, implies power wastage and a decrease in bandwidth efficiency. The amount of power wasted depends on how large a fraction of the OFDM symbol duration the guard time is. Therefore, the larger the symbol period & dash ; for a given data rate, this means more subcarriers & dash ; the smaller the loss of power and bandwidth efficiency.
The size of the FFT in an OFDM design should be as a balance between protection against multipath, Doppler shift, and design cost/complexity. For a given bandwidth, selecting a large FFT size would reduce the subcarrier spacing and increase the symbol time. This makes it easier to protect against multipath delay spread. A reduced subcarrier spacing, however, also makes the system more vulnerable to intercarrier interference owing to Doppler spread in mobile applications. The competing influences of delay and Doppler spread in an OFDM design require careful balancing. As we can see in each picture subcarrier spacing is getting smaller once the FFT size decreasing. So, we can conclude that, the more FFT size given, the less subcarrier spacing obtained.

# 2. Study and modify the code to understand how to implement different FFT size as described in the table (FFT size = 128 and FFT size = 256). Explain the difference of the BER performance for each FFT size!
**EXPLANATION **:
- the spectrum of OFDM transmission with FFT size = 64

- the spectrum of OFDM transmission with FFT size = 128

- the spectrum of OFDM transmission with FFT size = 256

From the simulation, there are three results from the spectrum of OFDM transmission that lied from 64, 128 and 256 FFT size. When FFT size show 64, the comparison between theoretical bit and Bit Error Rate is almost the same. It means that the ratio data loss because of the noise is more than the ratio data loss from 128 FFT size. When FFT size is 128, as we can see, Bit Error Rate is decreasing. So the probability of Bit Error Rate (BER) is also become lower. It contious to FFT which has 256 size. So, we can conlude that the bigger size FFT given, the less probability of Bit Error Rate (BER) obtained and and also the more data that can be transmitted