# Baseband Technology
[TOC]
## OFDM (Orthogonal Frequency Division Multiplexing)
OFDM is a multiplexing technique that divides the available bandwidth into multiple orthogonal sub-carriers. Each sub-carrier can be modulated independently and carrier different information. OFDM uses many sub-carriers with narrow bandwidth (smaller than coherence bandwidth), then each sub-carrier will only experience at fading. This reduces This reduces the complexity of the equalization process required on the receiver to compensate for the flat fading. Characteristics of OFDM are:
- Adaptive modulation: OFDM systems can user various modulation types such as QPSK, 16-QAM and 64-QAM.
- Adaptive coding: OFDM system can use various types of coding, both convolution coding and block coding, such as Viterbi, Turbo Deco-ding, RSA, BCH and so on.
- MIMO: OFDM systems also use a variety of MIMO methods to improve communication quality and capacity, such as spatial multiplex MIMO, and time multiplex MIMO
- Duplex mode: Ordinary OFDM systems can be applied to both TDD and FDD systems.
## OFDMA (Orthogonal Frequency Division Multiple Access)
OFDMA is a multi-user version of OFDM enabling concurrent AP communication (uplink & downlink) with multiple clients by assigning subsets of subcarriers, called Resource Units (RUs) to the individual clients. OFDMA Increasing system flexibility by multiplexing multiple users onto the same subcarriers, efficient trunking of many low-rate users onto a shared channel, enable per-user frequency hopping to mitigate the effects of narrowband fading.
## Modulation and Demodulation
Modulation is the process of changing a periodic wave with a certain frequency in order to be able to carry information. With the modulation process, an information which usually has a low frequency, can be superimposed onto a carrier wave which is usually a sine wave with a high frequency. On the other hand, at the receiver, a demodulation process is carried out, namely the process of extracting data or information from the received carrier signal. On the other hand, at the receiver, a demodulation process is carried out, namely the process of extracting data or information from the received carrier signal. There are types of modulations, such as BPSK, QPSK, 16 QAM, 64 QAM, and 256 QAM.
Demodulation is the process of returning the bit value of the modulated signal y which consists of Real (I) and Imaginary (Q) components received. All data received by the receiver, before entering the demodulator, must first be multiplied by the Normalization Factor. This normalization factor is different for each modulation type. The multiplier values are obtained from the calculation of the average point amplitude on the constellation diagram for each modulation scheme.

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# Comment
Modulation on 5G is based on Orthogonal Frequency-Division Mutiplexing (OFDM) and also uses OFDMA.

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