Ofdm for wireless

OFDM FOR WIRELESS
COMMUNICATION
Y SUNIL RAJ KUMAR
M.Tech 2nd year
Reg. NO: 14304029
Dept. of electronics engineering
Pondicherry university

CONTENT LAYOUT
1. INTRODUCTION TO MULTI-CARRIER SYSTEM
2. OFDM SYSTEM MODEL & GENERATION OF SUB CARRIERS
3. FADING
4. GUARD TIME & CYCLIC EXTENSIONS
5. WINDOWING
6. CHOICE OF OFDM PARAMETERS
9/9/2015 Course Code :EENG 564 Directed Study 2

INTRODUCTION TO MULTI-CARRIER
SYSTEM
Frequency Division
Multiplexing
Multicarrier Modulation
Multicarrier
Modulation
OFDM Discrete Multi-tone
(DMT)
MC-CDMA
MC-DS-CDMA
MT-CDMA

APPLICATIONS
• Wideband communication over mobile radio: Mobile radio
FM, Dig. Cellular telephony, WLAN, WMAN, UWB, . . . . .
• Digital subscriber lines: ADSL, HDSL, VHDSL, . . . . . .
• Digital audio broadcasting
• Digital video broadcasting
• HDTV broadcasting
• Optical communication – HFC
• Underwater communications

Single Carrier QAM Modulator
HT(f)
HT(f)
M-ary symbol
to quadrature
Signal
encoding
×
×
+
cos(w0t)
- sin(w0t)
{d0, d1, d2, d3, . . . . }
{a0 , a1 , a2 , a3 , . . . }
{b0 , b1 , b2 , b3 , . . . }
b0
b1 b2
O/P
a0
a1 a2

Multi-Carrier Comm. System
Modulator
Modulator
Modulator
De-
MUX
+
. . ,d1
i, d1
i+1, d2
i+2, . .
r bps
. ,d3
i, d3
i+1, . .
. ,dM
i, dM
i+1, . .
f0 f0+B f0+2B f0+(M-!)B
Signal spectrum
p(t)
p(t)
p(t)
f0
f0+B
f0+(M-1)B
Subcarrier
MC signal
{. . ,d1
i, d2
i, . . dM
i, d1
i+1, d2
i+1, . . dM
i+1, . . }
Mr bps9/9/2015 Course Code :EENG 564 Directed Study 6

OFDM SYSTEM MODEL & GENERATION OF
SUB CARRIERS

Frequency Division Multiplexing (FDM)
• Frequency Division Multiplexing (FDM) has been used for a
long time to carry more than one signal over a telephone
line.
• FDM divides the channel bandwidth into sub channels and
transmits multiple relatively low rate signals by carrying
each signal on a separate carrier frequency.
• To ensure that the signal of one sub channel did not
overlap with the signal from an adjacent one, some guard-
band was left between the different sub channels.

Orthogonal Frequency Division
Multiplexing (OFDM)
• In order to solve the bandwidth efficiency problem, orthogonal
frequency division multiplexing was proposed, where the
different carriers are orthogonal to each other. With OFDM, it is
possible to have overlapping sub channels in the frequency
domain, thus increasing the transmission rate.
• This carrier spacing provides optimal spectral efficiency. Today,
OFDM has grown to be the most popular communication
system in high-speed communications.
• OFDM is becoming the chosen modulation technique for
wireless communications. OFDM can provide large data rates
with sufficient robustness to radio channel impairments.

OFDM is a special case of FDM

An example of OFDM using 4 sub-carriers
• few bits are 1, 1, -1, -1, 1, 1, 1, -1, 1, -1, -1, -1, -1, 1, -1, -1, -1, 1,…

An example of OFDM using 4 sub-carriers

OFDM signal in time and frequency
domain

GENERATED OFDM SIGNAL

FADING
• The attraction of OFDM is mainly because of its way of handling the
multipath interference at the receiver.
• Multipath phenomenon generates two effects (a) Frequency
selective fading and (b) Inter symbol interference (ISI).
• The "flatness" perceived by a narrow-band channel overcomes the
frequency selective fading. On the other hand, modulating symbols
at a very low rate makes the symbols much longer than channel
impulse response and hence reduces the ISI.
• Use of suitable error correcting codes provides more robustness
against frequency selective fading.
• The insertion of an extra guard interval between consecutive OFDM
symbols can reduce the effects of ISI even more

FADING:
• DEFINING FADING: the path from the
transmitter to the receiver either has
reflections or obstructions, we can get
fading effects. In this case, the signal
reaches the receiver from many
different routes, each a copy of the
original. Each of these rays has a
slightly different delay and slightly
different gain. The time delays result
in phase shifts which added to main
signal component (assuming there is
one.) causes the signal to be
degraded

Fading….
• a)the signal we want to send
and the channel frequency
response are well matched.
• (b) A fading channel has
frequencies that do not allow
anything to pass. Data is lost
sporadically.
• (c) With OFDM, where we have
many little sub-carriers, only a
small sub-set of the data is lost
due to fading
a b
c

Fading…..
• OFDM signal offers an advantage in a channel that has a frequency
selective fading response .
• Instead Of the whole symbol being knocked out, we lose just a small
subset of the (l/N) bits. With proper coding, this can be recovered.
• The BER performance of an OFDM signal in a fading channel is much
better than the performance of QPSK/FDM which is a single carrier
wideband signal .

GUARD TIME & CYCLIC EXTENSIONS

GUARD TIME & CYCLIC
EXTENSIONS…..
• The PSK symbol and its delayed version
• Move the symbol back so the arriving
delayed signal peters out in the gray
region. No interference to the next
symbol
• we just extend the symbol, then the
front of the symbol which is important to
us since it allows figuring out what the
phase of this symbol is, is now corrupted
by the “splash”.
• we move the symbol back and just put in
convenient filler in this area, then not
only we have a continuous signal but one
that can get corrupted and we don’t care
since we will just cut it out anyway
before demodulating

• Cyclic prefix is copy of back of the symbol we add to the front of the
symbol.
• Slide the symbol to start at the edge of the delay spread time and then fill
the guard space with a copy of what turns out to be tail end of the
symbol .
• We will be extending the symbol so it is 1.25 times as long, to do this,
copy the back of the symbol and glue it in the front .
• We can add cyclic prefix just once to the composite OFDM signal. The
prefix is any where from 10% to 25% of the symbol time.
• the addition of cyclic prefix which mitigates the effects of link fading and
inter symbol interference, increases the bandwidth.

WINDOWING
• OFDM signals consists of a number of unfiltered QAM sub
carriers, as a result the out of band spectrum decreases
rather slowly, according to sinc function.
• Spectra of 16,64,and 256 subcarriers are plotted for larger
number of subcarriers, the spectrum goes down more
rapidly in the beginning which is caused by the fact that
the side lobes are closer together.
• To make the spectrum go down rapidly, windowing can be
applied to the individual OFDM symbols.
• Windowing an OFDM symbol makes the amplitude go
smoothly to zero at the symbol boundaries.

WINDOWING……….
• A commonly used window type is the raised cosine window
• 𝒘 𝒕 = {𝟎. 𝟓 + 𝟎. 𝟓 𝐜𝐨𝐬(𝝅 + 𝒕𝝅/(𝜷𝑻𝒔)) 0 ≤ 𝒕 ≤ 𝜷𝑻𝒔}
{1.0 𝜷𝑻𝒔 < 𝒕 ≤ 𝑻𝒔}
{𝟎. 𝟓 + 𝟎. 𝟓𝐜𝐨𝐬(
𝒕−𝑻𝒔 𝝅
𝜷𝑻𝒔
)} 𝑻𝒔 ≤ 𝒕 ≤ 𝟏 + 𝜷 𝑻𝒔
• Here Ts is the symbol interval, which is shorter than the
total symbol duration because we allow adjacent symbol
to partially overlap in roll-off region.

WINDOWING……….
• The practical OFDM signal is generated as fallows first Nc
input QAM values are padded with zero to get N input
samples that are used to calculate an IFFT.
• The last Tprefix samples of the IFFT output are inserted at
the start of the OFDM symbol, and the first Tpostfix
samples are appended at the end.
• The OFDM symbol is then added to the output of the
previous OFDM symbol with delay Ts, such that there is no
overlap region 0f 𝜷𝑻𝒔
• Where 𝜷 is the roll-off factor of the raised cosine window.

WINDOWING………
• Instead of windowing it is also possible to use conventional
filters techniques to reduce the out of band spectrum
• Windowing and filtering are duel techniques, multiplying an
OFDM signal by an window means the spectrum is going to be a
convolution of the spectrum of the window function with asset
impulse functions at the subcarriers frequencies.
• When filtering is applied ,a convolution is done in the time
domain and the OFDM spectrum is multiplied by the frequency
response of the filter
• Digital filtering techniques are more complex to implement than
windowing

CHOICE OF OFDM PARAMETERS
• OFDM system design, a number of parameters are up for
consideration
• Such as the
1. Number of subcarriers
2. Guard time
3. Symbol duration
4. Sub carrier spacing
5. Modulation type per sub carrier

CHOICE OF OFDM PARAMETERS…….
• The choice of parameters is influenced by system
requirements
• Usually there are three main requirements to start with
1. Available bandwidth
2. Required bit rate
3. Tolerable delay spread

CHOICE OF OFDM PARAMETERS……
• The delay spread is directly dictates the guard time.as a rule the
guard should be about two to four times the root-mean-squared
delay spread
• This value depends on the type coding and QAM modulation.
• Higher order QAM is more sensitive to ICI and ISI than QPSK, while
heavier coding obviously reduces the sensitivity to such
interference.
• Now that the guard time has been set, the symbol duration can be
fixed.
• To minimize the signal-to-noise ratio (SNR) loss caused by guard
time, it is desirable to have the symbol duration much larger than
the guard time

• Large symbol duration means more subcarriers with
smaller subcarrier spacing, a larger implementation
complexity, and more sensitivity to phase noise
• Practical design choice is to make the symbol duration at
least five times the guard time.
• The number of subcarriers follows directly as the required
-3-dB bandwidth divided by the subcarrier spacing, which
is the inverse of the symbol duration less the guard time
• Another way to determine the subcarrier is Required bit
rate divided by the bit rate per subcarrier
• The bit rate per subcarrier is defined by the modulation
type ,coding rate, and symbol rate

Ofdm for wireless

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Ofdm for wireless