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Chapter 4 of Hiroshi Harada Book (OFDM Transmission)

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1 Chapter 4 of Hiroshi Harada Book (OFDM Transmission)
EE578 Assignment #4 Chapter 4 of Hiroshi Harada Book  (OFDM Transmission) Abdul-Aziz .M Al-Yami Nov 1st 2010


3 OVERVIEW ABOUT OFDM 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 WHY OFDM ? High bit rate needs are clumped by the nature of communication channels. Multi-path Propagation effects forbid increasing of transmission rates.

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

6 Modulation

7 PRINCIPLES 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 PRINCIPLES 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 FDM  OFDM Frequency Division Multiplexing OFDM frequency dividing

10 OFDM THEORY 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:

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

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

13 OFDM ADVANTAGES 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 ADVANTAGES 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: for enhanced coexistence with the other devices.

15 OFDM DRAWBACKS 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 Standards 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 COFDM Transmitter Digital Radio Coded OFDM Transmitter

18 DRM 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

19 (V-OFDM) of Cisco, Iospan,etc.
Proprietary OFDM Flavours Wideband-OFDM (W-OFDM) of Wi-LAN Flash OFDM from Flarion Vector OFDM (V-OFDM) of Cisco, Iospan,etc. -- Freq. Hopping for CCI reduction, reuse to 5.0MHz BW -- mobility support GHz band Mbps in 40MHz -- large tone-width (for mobility, overlay) -- 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 MATLAB Results Program 4.2 (ofdm_fading)

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