Spectrum of M-FSK
∆f
f
f1 f2 f3 f M −1 fM
In M -FSK, only one carrier is active in any given symbol duration
to carry log2 M bits. This is why it is not a spectral-efficient
modulation scheme.
Why not using all the carriers to carry information at the same time
since they are orthogonal? This leads to OFDM (orthogonal
frequency-division multiplexing) technique.
EE456 – Digital Communications Ha H. Nguyen
OFDM (Orthogonal Frequency-Division Multiplexing)
∆f = 1
TN
f
f1 f2 f3 f N −1 fN
N
Bandwidth W ≈ N ⋅ ∆f =
TN
In OFDM the spectrum (frequency) is divided into overlapping but
orthogonal subcarriers. Each sub-carrier is independently modulated
by M -QAM. The minimum subcarrier separation is 1/TN , where
TN is the OFDM symbol length.
OFDM can be simply looked upon as a combination of amplitude,
phase and frequency modulation techniques.
EE456 – Digital Communications Ha H. Nguyen
Communication Services using OFDM
Wireless Wireline
ADSL and VDSL broadband access
IEEE 802.11a, g, n (WiFi) Wireless LANs
via POTS copper wiring
MoCA (Multi-media over Coax
IEEE 802.15.3a Ultra Wideband (UWB) Wireless PAN
Alliance) home networking
IEEE 802.16d, e (WiMAX), WiBro,
PLC (Power Line Communication)
and HiperMAN Wireless MANs
IEEE 802.20 Mobile Broadband
Wireless Access (MBWA)
DVB (Digital Video Broadcast) terrestrial TV
systems: DVB -T, DVB -H, T-DMB, and ISDB-T
DAB (Digital Audio Broadcast) systems:
EUREKA 147, Digital Radio Mondiale,
HD Radio, T-DMB, and ISDB-TSB
Flash-OFDM cellular systems
3GPP UMTS & 3GPP@ LTE (Long-Term Evolution),
and 4G
OFDM is spectrally efficient, tolerates environments with
high RF interference, works well in harsh multi-path
environments and can be elegantly implemented with
IFFT/FFT modules.
EE456 – Digital Communications Ha H. Nguyen
Implementation of OFDM
X [0] x[0] sin ( 2π fc t )
"" 1
X [1] x[1]
# ' x (t )
+
I
rb bits/sec
X
/ ! ,
$#% -
s (t )
" &'( '
)
*
+
xQ (t )
X [ N − 1] x[ N − 1]
cos ( 2π f ct )
sin ( 2π fc t ) y[0] Y [0]
.& ' 0
)
y[1]
Y [1]
+ 2
$#%
34 " ('& / ! , "
"
*
y[ n] Y
r (t )
)
*
"
.& ' "
+ y[ N − 1] Y [ N − 1]
cos ( 2π fc t )
EE456 – Digital Communications Ha H. Nguyen
Multipath Problem in High-Speed Wireless Transmission
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Example: Consider the symbol rate of 106 symbols/sec ⇒ The receiver expects a
specific symbol within a window of 1 µs. If multi-path delays the signal by more than
1 µs (easily happen in real propagation environment), then the receiver will also
receive the symbol in the next symbol period, causing inter-symbol-interference (ISI),
hence severe performance degradation.
EE456 – Digital Communications Ha H. Nguyen
How Does Cyclic Prefix Work in OFDM
Append last µ symbols to the front
x[ N − µ ], x[ N − µ + 1],⋯ , x[ N − 1] x[0], x[1], x[2],⋯⋯⋯ , x[ N − µ − 1] x[ N − µ ], x[ N − µ + 1],⋯ , x[ N − 1]
Cyclic prefix (CP) of length µ Original signal sequence of length N
CP x[0],⋯, x[ N − 1] CP x[0],⋯, x[ N − 1] CP x[0],⋯, x[ N − 1] CP
CP Data block CP Data block CP Data block
⋯ ISI y[0],⋯ , y[ N − 1] ISI y[0],⋯ , y[ N − 1] ISI y[0],⋯ , y[ N − 1] ⋯
µ N µ N µ N
An OFDM symbol is basically a super-symbol obtained by multiplexing many
M -QAM symbols in a complicated manner. The length of a super-symbol (TN )
becomes longer and hence more resistent to multipath effect.
One can also use zero padding to create a guard interval between consecutive
OFDM symbols, hence avoiding ISI.
EE456 – Digital Communications Ha H. Nguyen
