f f f f f 1 2 3 M 1 M In M -FSK, only one carrier is - - PowerPoint PPT Presentation

Spectrum of M-FSK

• 1

f

M

f

2

f

3

f

1 M

f

−

⋯

f ∆

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)

• Bandwidth

N

N W N f T ≈ ⋅ ∆ =

1

f

N

f

2

f

3

f

1

N

T

f ∆ =

1 N

f

−

⋯

In OFDM the spectrum (frequency) is divided into overlapping but

• rthogonal 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 IEEE 802.11a, g, n (WiFi) Wireless LANs ADSL and VDSL broadband access via POTS copper wiring IEEE 802.15.3a Ultra Wideband (UWB) Wireless PAN MoCA (Multi-media over Coax 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 bits/sec

b

r [0] X [1] X [ 1] X N − [0] x [1] x [ 1] x N −

( )

Q

x t

( )

cos 2

c

f t π

( )

cos 2

c

f t π

[0] y [1] y [ 1] y N −

[0] Y [1] Y [ 1] Y N −

• ✁

Y [ ] y n

( )

I

x t

( )

sin 2

c

f t π ( ) s t

( ) t r

( )

sin 2

c

f t π

EE456 – Digital Communications Ha H. Nguyen

Multipath Problem in High-Speed Wireless Transmission

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

[0], , [ 1] y y N − ⋯

• µ

N [0], , [ 1] y y N − ⋯

• µ

N [0], , [ 1] y y N − ⋯

• µ

N CP CP CP Data block Data block Data block

⋯ ⋯

[0], , [ 1] x x N − ⋯

• [0],

, [ 1] x x N − ⋯

• [0],

, [ 1] x x N − ⋯

• [

], [ 1], , [ 1] x N x N x N µ µ − − + − ⋯ [0], [1], [2], , [ 1] x x x x N µ − − ⋯⋯⋯ [ ], [ 1], , [ 1] x N x N x N µ µ − − + − ⋯ Append last symbols to the front µ Cyclic prefix (CP) of length µ Original signal sequence of length 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