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EE578 Assignment #4 Abdul-Aziz.M Al-Yami Nov 1 st 2010.

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Presentation on theme: "EE578 Assignment #4 Abdul-Aziz.M Al-Yami Nov 1 st 2010."— Presentation transcript:

1 EE578 Assignment #4 Abdul-Aziz.M Al-Yami Nov 1 st 2010


3 OFDM was invented more than 40 years ago. OFDM has been adopted for several technologies: Asymmetric Digital Subscriber Line (ADSL) services. IEEE a/g, IEEE a. Digital Audio Broadcast (DAB). Digital Terrestrial Television Broadcast: DVD in Europe, ISDB in Japan 4G, IEEE n, IEEE , and IEEE

4 High bit rate needs are clumped by the nature of communication channels. Multi-path Propagation effects forbid increasing of transmission rates.

5 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


7 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.

8 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.

9 Frequency Division Multiplexing OFDM frequency dividing EARN IN SPECTRAL EFFICIENCY

10 zThe baseband OFDM signals can be written as Where is the central frequency of the mth sub- channel and is the corresponding transmitted symbol. zThe signals are orthogonal over [0, T ] as illustrated below:

11 FEC IFFT DAC Linear PA add cyclic extension bits fcfc OFDM symbol Pulse shaper & view this as a time to frequency mapper Generic OFDM Transmitter Complexity (cost) is transferred back from the digital to the analog domain! Serial to Parallel

12 AGC fcfc VCO Sampler FFT Error gross offset Slot & fine offset Freq. Offset Estimatio n Timing Sync. (of all tones sent in one OFDM symbol) Generic OFDM Receiver Recovery P/S and Detection

13 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

14 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.

15 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

16 Digital Audio Broadcasting (DAB) Digital Video Broadcasting (DVB) Asymmetric Digital Subscriber Line (ADSL) Wireless LAN IEEE a Wireless networking, device connectivity Proposed for standard Connection between subscriber's transceiver station and a base transceiver station

17 Coded OFDM Transmitter COFDM Transmitter

18 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 kHz for double channel Data Rates of 31 – 72kbit/s

19 Proprietary OFDM Flavours Wideband-OFDM (W-OFDM) of Wi-LAN Flash OFDM from Flarion Vector OFDM (V-OFDM) of Cisco, Iospan,etc GHz band Mbps in 40MHz -- large tone-width (for mobility, overlay) -- Freq. Hopping for CCI reduction, reuse 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 Wireless MAN Cisco leads the Broadand Wireless Internet Forum (BWIF)

20 Program 4.2 (ofdm_fading)

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