Sin ttulo de diapositiva

1
Multicarrier vs. Monocarrier Modulation
Techniques An Introduction to OFDM
Jose M. Paez-Borrallo
BWRC Retreat 2000
Department Señales, Sistemas y
Radiocomunicaciones Institution ETSIT -
Universidad Politécnica de Madrid
2
Summary of the presentation

• Multicarrier vs. monocarrier modulation scheme
• Orthogonal Frequency Division Multiplex OFDM
• Modulation/Demodulation using FFTs
• Limitations of the OFDM modulation
• Sensitivity to synchronization
• Increase of peak-to-average ratio
• Combating channel time dispersion
• Pulse shaping for OFDM symbols
• An example of OFDM transceiver

3
Modulation techniques monocarrier vs.
multicarrier
Channelization
Channel
Guard bands
B
Pulse length 1/B
Data are transmitted over
only one carrier
4
Introduction to OFDM modulation
Data
Time-frequency grid
Frequency
B
Carrier
f
0
symbol OFDM
T1/f
0
Time
Intercarrier Separation Any integer Multiple
of 1/(symbol duration)
Modulation technique
One user utilizes all carriers simultaneously to
transmit its data (may be different modulations)
Access techniques (FDMA)
Several users share dynamically the carriers
(traffic or service dependent) to access to the
system
5
OFDM Modulation using FFTs
Orthogonality principle
1
-
N
å
å
k

y
-
x
(
t
)
c
(
t
lT
)
l
k
0

l
k
and fkk/T
Let us assume
(
p
e
x
p
j
2
f
t
)
p
(
t
)
k
Modulator bank of type
c
k
kth carrier modulator
Sampling at TsT/N
where
It can be expressed as
6
Loss of orthogonality (by time shift)
2 consecutive symbols
Let us assume a misadjustment??
Then
if mk-l
m is intercarrier separation
Zone of interest
7
Loss of orthogonality (by frequency offset)
Transmission pulses
Reception pulse with offset ??
Interference between channels k and km
Summing up ???m
1
-
23
N
1
å
å
(
)
(
)
2
2
)
I
2
(
d
)

T
d

T
d


N
gtgt
1
(
N
gt
5

Is enough
for
m
m
2
14

m
m
1
Independent of N
Asymmetric
8
Combating the channel time dispersion (avoiding
EQs)
Furthermore, it converts Lineal conv. Cyclic
conv.
(Method overlap-save)
Including the Cyclic Prefix
Without the Cyclic Prefix
Symbol 8 periods of fi
Symbol 8 periods of fi
CP
Y
(
t
)
i
Y
(
t
)
i
Channel
Passing the channel h(n)

n
Passing the channel h(n)
h
(
n
)

(
1
)
/
n
n

0
,

,
2
3
¹
Y
(
t
)
i
Final transient remains within the CP
Initial transient remains within the CP
The inclusion of a CP maintains the orthogonality
Initial transient
Decaying transient
Loss of orthogonality
Y
(
t
)
Y
j
(
t
)
j
Symbol 4 periods of fi
Symbol 4 periods of fi

• CP functions
• It accommodates the decaying transient of the
  previous symbol (ISI)
• It avoids the initial transient reaches the
  current symbol (ICI)

9
Peak-to-Average Ratio (PAR) of OFDM modulation
If x(t) is the OFDM signal and
Limiting case for a PSK signal
It requires very linear High Power Amplifiers
10
Simplified scheme of an OFDM transceiver
.
.
Transmitter
L
Cyclic
prefix (CP)
PC
S
S
CODER
BITS
N-IFFT
DAC
RF
P
P
1
N
.
Power
Control
Pre-equalization?
Receiver
L
N
N
BITS
S
P
DECOD.
ADC
NFFT
PC
P
S
1
PLL
Frame
PC Marker
To MAC
SYNCH
Synchronizes the frequency axis
Synchronizes the time axis
11
Need of shaping the OFDM symbol (e.g. DECT system)
Maintaining a fixed bandwidth, if N increases
ACI level decreases
BUT

• It is interesting to have few carriers within the
  DECT band (TDMA-TDD)
• To have shorter symbols --gt the synchronization
  is then short
• To introduce short delay in data gathering and
  signal processing (FFTs)
• To have a bigger intercarrier separation --gt It
  reduces the relative frequency offset

12
Robustness against the channel and ACI improvement

• With guards (Cyclic prefixes), the channels
  time dispersion is avoided

LNCP

PC
PC
PC
PC
PC
OFDM symbols with time guards (CPs)

• With smooth transitions between symbols, the
  Adjacent Channel Interference is minimized

PC
PC
PC
PC
OFDM symbols with time guards and shaping (extra
time guard)
Drawback it is implemented by using the TIME
resource
13
Simplified scheme for OFDM transmission
AWGN
Data
u(n)
x(n)
RX
X(k)
r(n)
Channel
IDFT
PC
DFT
H(?)

?
j2?n?/N
TX
e

?
Normalized Offset
Reference
Synchro. Symbol
OFDM Symbol PC
S
Lineal Convolution Circular Convolution
Slot DECT
Good correlation properties
IDFT
S
Random phases
Frequency components of the channel
14
Conclusions

• Multicarrier techniques are better in frequency
  selective channels, while maintaining spectral
  efficiency.
• Can be implemented digitally using FFTs.
• Very sensitive to time-frequency
  missynchronization
• As N increases the peak-to-average ratio
  decreases
• Addition of cyclic prefix combats time
  dispersion.
• ACI can be reduced with pulse shaping.