25-09-2013, 12:59 PM
Orthogonal frequency-division multiplexing
Orthogonal frequency.ppt (Size: 1.08 MB / Downloads: 22)
Orthogonal Frequency-Division Multiplexing
Can easily adapt to severe channel
conditions without complex equalization
Robust against narrow-band co-channel interference
Robust against Intersymbol interference (ISI) and fading caused by multipath propagation
High spectral efficiency
Efficient implementation using FFT
Low sensitivity to time synchronization errors
Tuned sub-channel receiver filters are not required (unlike conventional FDM)
Facilitates Single Frequency Networks, i.e. transmitter macrodiversity.
continuation:
IEEE 802.20 or Mobile Broadband
Wireless Access (MBWA) systems.
Flash-OFDM cellular systems.
The WiMedia Alliance's Ultra wideband (UWB) implementation.
Power line communication (PLC).
MoCA home networking.
Optical fiber communications and Radio over Fiber systems (RoF).
CHARACTERISTICS OF OFDM:
Orthogonality
Guard interval for elimination of inter-symbol interference
Simplified equalization
Channel coding and interleaving
Adaptive transmission
OFDM extended with multiple access
Space diversity
Linear transmitter power amplifier
ORTHOGONALITY
The sub-carrier frequencies are chosen so that the sub-carriers are orthogonal to each other, meaning that cross-talk between the sub-channels is eliminated and inter-carrier guard bands are not required. This greatly simplifies the design of both the transmitter and the receiver; unlike conventional FDM, a separate filter for each sub-channel is not required.
Guard interval for elimination of inter-symbol interference
One key principle of OFDM is that
since low symbol rate modulation schemes (i.e. where the symbols are relatively long compared to the channel time characteristics) suffer less from intersymbol interference caused by multipath, it is advantageous to transmit a number of low-rate streams in parallel instead of a single high-rate stream. Since the duration of each symbol is long, it is feasible to insert a guard interval between the OFDM symbols, thus eliminating the intersymbol interference.
OFDM extended with multiple access
Orthogonal Frequency Division
Multiple Access (OFDMA),
frequency-division multiple access
is achieved by assigning different OFDM sub-channels to different users. OFDMA supports differentiated quality-of-service by assigning different number of sub-carriers to different users in a similar fashion as in CDMA, and thus complex packet scheduling or media access control schemes can be avoided. OFDMA is used in the uplink of the IEEE 802.16 Wireless MAN standard, commonly referred to as WiMAX.