Post on 26-Mar-2015
EE578 Assignment #4
Abdul-Aziz .M Al-Yami
Nov 1st 2010
OVERVIEW ABOUT OFDMDEFINITION AND PRINCIPLESOFDM ADVANTAGES & DRAWBACKSAPPLICATIONSCONCLUSIONS AND PERSPECTIVESRESULTS
OFDM was invented more than 40 years ago. OFDM has been adopted for several technologies:
Asymmetric Digital Subscriber Line (ADSL) services.
IEEE 802.11a/g, IEEE 802.16a. Digital Audio Broadcast (DAB). Digital Terrestrial Television Broadcast: DVD in Europe, ISDB in Japan 4G, IEEE 802.11n, IEEE 802.16, and IEEE
802.20.
High bit rate needs are clumped by the nature of communication channels.
Multi-path Propagation effects forbid increasing of transmission rates.
OFDM = Orthogonal FDM Carrier centers are put on orthogonal
frequencies ORTHOGONALITY - The peak of each signal
coincides with trough of other signals Subcarriers are spaced by 1/Ts
BASIC IDEA : Channel bandwidth is divided into multiple subchannels to reduce ISI and frequency-selective fading.
Multicarrier transmission : Subcarriers are orthogonal each other in frequency domain.
Time-domain spreading: Spreading is achieved in the time-domain by
repeating the same information in an OFDM symbol on two different sub-bands => Frequency Diversity.
Frequency-domain spreading: Spreading is achieved by choosing conjugate
symmetric inputs for the input to the IFFT (real output)
Exploits frequency diversity and helps reduce the transmitter complexity/power consumption.
Frequency Division Multiplexing
OFDM frequency dividing
EARN IN SPECTRAL EFFICIENCY
The baseband OFDM signals can be written as
Where is the central frequency of the mth sub-channel and is the corresponding transmitted symbol.
The signals are orthogonal over [0, T ] as illustrated below:
1
0
0 ,2exp)(N
mm Ttt
T
mjXtx
T
mfm
mX
tT
mj 2exp
ml
T
dttT
ljt
T
mjT )2exp().2exp(1
0
FEC IFFT
DAC
LinearPA
add cyclic extension
bits
fc
OFDM symbol
Pulse shaper &
view this as a time tofrequency mapper
Generic OFDM Transmitter
Complexity (cost) is transferred back from the digital to the analog domain!
Serial toParallel
AGC
fc
VCO
Sampler FFTError
gross offset
Slot &
fine offset
Freq. OffsetEstimation
TimingSync.
(of all tones sent in one OFDM symbol)
Generic OFDM Receiver
RecoveryP/S and
Detection
OFDM is spectrally efficient IFFT/FFT operation ensures that sub-carriers do not
interfere with each other.
OFDM has an inherent robustness against narrowband interference. Narrowband interference will affect at most a couple of subchannels. Information from the affected subchannels can be erased and recovered via the forward error correction (FEC) codes.
Equalization is very simple compared to Single-Carrier systems
OFDM has excellent robustness in multi-path environments. Cyclic prefix preserves orthogonality between sub- carriers. Cyclic prefix allows the receiver to capture multi- path energy more efficiently.
Ability to comply with world-wide regulations: Bands and tones can be dynamically turned on/off to comply with changing regulations.
Coexistence with current and future systems: Bands and tones can be dynamically turned on/off for enhanced coexistence with the other devices.
High sensitivity inter-channel interference, ICI
OFDM is sensitive to frequency, clock and phase offset
The OFDM time-domain signal has a relatively large peak-to-average ratio tends to reduce the power efficiency of the RF
amplifier non-linear amplification destroys the
orthogonality of the OFDM signal and introduced out-of-band radiation
Digital Audio Broadcasting (DAB) Digital Video Broadcasting (DVB) Asymmetric Digital Subscriber Line
(ADSL) Wireless LAN IEEE 802.11a
Wireless networking, device connectivity Proposed for 802.16 standard
Connection between subscriber's transceiver station and a base transceiver station
Coded OFDM Transmitter
COFDM Transmitter
For HF Frequencies 4.5 – 5 kHz for half channel
▪ Data Rates of 4 – 14.5 kbit/s 9 – 10 kHz for full channel
▪ Data Rates of 8 – 35kbit/s 18-20 kHz for double channel
▪ Data Rates of 31 – 72kbit/s
Proprietary OFDM Flavours
Wideband-OFDM(W-OFDM) of Wi-LAN
www.wi-lan.com
Flash OFDMfrom Flarion
www.flarion.com
Vector OFDM(V-OFDM) of Cisco, Iospan,etc.
www.iospan.com
-- 2.4 GHz band-- 30-45Mbps in 40MHz-- large tone-width (for mobility, overlay)
-- Freq. Hopping for CCI reduction, reuse-- 1.25 to 5.0MHz BW -- mobility support
-- MIMO Technology-- non-LoS coverage, mainly for fixed access-- upto 20 Mbps in MMDS
Wi-LAN leads the OFDM Forum -- many proposals submitted to IEEE 802.16 Wireless MANCisco leads the Broadand Wireless Internet Forum (BWIF)
Program 4.2 (ofdm_fading)
0 5 10 15 20 25 30 35 4010
-4
10-3
10-2
10-1
100
Eb/N
0 [dB]
BE
R
BPSK performance
QPSK AWGN theory
QPSK 1 path Rayleigh theory
QPSK AWGN (no compensation)QPSK 1 path Rayleigh (perfect compensation)
QPSK 1 path Rayleigh (no compensation)