# D1101ET007DE 23 July IVC

## OFDM
OFDM is a multiplexing techniqure that divides bw into multiple orthogonal subcarrier
each subcarrier can be modulate independently

OFDM subcarrier will only experience flat fading to to narrow band
it resistant to fading

OFDM can uses various modulation type e.g QPSK,16QAM,64QAM
OFDM can apply adaptive coding e.g convolution coding,block conding

OFDM uses cylic prefix and preamble so itseasy to do syhncronization frequency and time
channel response in freq domain can be easily calc from the magnitude or reinformacemtn

### disadvantage of OFDM
1. sensitive to frequency shifting
2. shifting freq can cause ICI inter carrier intereference and MAI multiple access ainterference
3. frequency shift can caused doppler effect due to movement or use of non precision osilator

Doppler effect result in lost of orthogonaloty between subcarrier resukting in high BER

ifft will convert freq to time domain sgnal
fft converts time to freq domain

CP is used to maintain ortholity of ofdm to avoid ICI, the longer the cp can lower snr and signal efficiency

on OFDM, parameters should be considered:
1. Number of subcarrier
2. guard time
3. symbold uration
4. sub carrier spacing
5. modilation for each type sub carrier
*look at ofdm parameter table

higher complexty of midulation, the higher the bit rate but more suubceptible to noise

BPSK:
ignal represented as 2 values; 1 and 0

QPSK:
signal repsented as 4 values; 00,01,10,11

Demodulation

BPSK divided region into 2 in constelation. positive will be conclude as 1 otherwiser is 0

QPSK is the same as BPSK, only have more constelation. the value will depend on real (y)

16QAM and 64 QAM requires reading of coordinate constrlation to determine the values

# FDMA
uses different frequency for each user at the same time

# TDMA
uses time slots different for each user

# OFDMA
utilize diversity of time and frequency for users

# SC-FDMA
SC-FDMA is used to refuce PAPR problem