Page 95 - ITUJournal Future and evolving technologies Volume 2 (2021), Issue 1

ITU Journal on Future and Evolving Technologies, Volume 2 (2021), Issue 1

mation provided to the receiver to reconstruct the data tion block to correct the effects of the channel), the
from its origin. In our communication model, we suppose guard bands, the cyclic pre ix and a raised cosine il‑
that the ECC will be in charge of correcting all the errors ter (which serves as windowing to focus our study
created by the OFDM communication model. on the used frequencies). Then, it includes also an
Inverse Fast Fourier transform (IFFT) module which
• ECC encoder: This block is the transmitter side of the
ECC. The communication channel alters the signal, allows the information to be transmitted in orthogo‑
which introduces errors. We can use error correct‑ nal frequencies through the communication channel.
ing codes to add redundancy in the transmitted data. • OFDM Demodulator: This block does the reverse op‑
Many encoding methods have been studied in the lit‑ eration of the previous one. An FFT module trans‑
erature, such as the Hamming and the Reed‑Solomon forms the information back into the time domain,
codes, which are detailed in Section 3. the pilot signals are used for the channel estimation,
then they are removed to restore the original mes‑
• ECC decoder: This block is on the reception side of sage (the guard bands and cyclic pre ix are removed
the model, receiving the information that has passed as well).
through the channel and that has already been de‑
modulated. The decoder has to decode the message
and translate it back into its’ original form by exploit‑ 2.5 The communication channel
ing the redundant bits that were added previously by Here, the communication channel is modelized by an Ad‑
the encoder and that allows the decoder to detect an ditive White Gaussian Noise (AWGN) block. We induce
error and correct it. In general, the decoder is more noise by using the which represents a normalized Sig‑

complex than the encoder, especially concerning its naltoNoiseRatio(SNR) bybit, andiswelladaptedtocom‑
implementation (for example Viterbi decoder [7]). pare the Bit Error Rate (BER) of different ECCs. The rela‑
tion between these two parameters is given by:
2.3 Mapping/De‑Mapping
= ∗ (1)
This block takes the coded messages and builds a con‑ ℎ ℎ
stellation in an I/Q plane, in order to transmit our digital The deviation (which is equal to 10 in our case) repre‑
stream of bits through the analog channel, under the form sents the thermal noise in our real Power Line Communi‑
of frequencies. cation (PLC) channel [9, 10]. We have estimated this devi‑

• Mapping: This block takes a binary stream and out‑ ation on LT Spice tools by adding the quanti ication noise
puts a point in the I/Q plane. There are many types and converting the sine wave by the ADC/DAC block. In
of modulation: such as shifting the phase of the sig‑ order to make the channel block more realistic, we ixed
nal ’Phase‑Shift Keying (PSK)’, modulate only the am‑ these parameters of the AWGN block the closest to reality.
plitude of the signal ’Amplitude Modulation (AM)’, In the following Section, we will focus only on the ECC
or both ’Quadrature amplitude modulation (QAM)’. blocks which are described in Section 2.2 and adapted to
In our communication model, we use a Quadrature our short‑frame OFDM communication system.
phase‑shift keying (QPSK) modulation. There are
= 4 possible points in the constellation and it can 3. TRADE‑OFF BETWEEN SIMPLE HAM‑
encode = log ( ) = 2 bits per symbol. MING CODE AND REED‑SOLOMON CODE

2
• De‑Mapping: This block does the inverse of the pre‑
vious mapping block. It takes the signal in the time InthisSection, wewillfocusontwoerrorcorrectingcodes
domain after the Fast Fourier transform (FFT) block (ECCs) that meet a trade‑off between the low implemen‑
(which is inside the OFDM modulator block), and tation complexity and the high error correction capacity.
with the constellation reference point it restores the Since we have considered a short‑frame OFDM commu‑
original binary message corresponding to a given nication model, we will study Hamming code and Reed‑
point. Solomon code which are the most adapted in our case.
2.4 Orthogonal Frequency Division Multiplex‑ 3.1 The Hamming code
ing (OFDM) MoDem The Hamming code is one of the error correcting codes
that can be used to detect and correct bit errors that can
OFDM is a type of digital transmission and a method occur when data is moved or stored. Like other error cor‑
of encoding digital data on multiple carrier frequencies. recting codes, the Hamming code makes use of the con‑
In OFDM, multiple closely spaced orthogonal sub‑carrier cept of parity bits which are bits that are added to data so
signals with overlapping spectra are transmitted to carry that the validity of the data can be checked when it is read
data in parallel.
or after it has been received in a data transmission.
• OFDM Modulator: This block introduces the pilot The Hamming code method is based on two methods
signals (which are used later in the channel estima‑ (even parity, odd parity) for generating redundancy bits.

© International Telecommunication Union, 2021 79