7 OFDM- IntroductionThe major requirement of modern wireless communication systems are high capacity and variable bit rate transmission with high BW efficiency.But, the wireless environment signal usually impaired by fading and multipath delay spreading .The traditional single carrier mobile communication systems do not perform well because it suffers extreme fading of the signal and Inter Symbol Interference(ISI).This leads to a high probability of errors and the system’s overall performance becomes very poor.Because of its high-speed data transmission and effectiveness in combating the frequency selective fading channel, OFDM technique is widely used in wireless communication nowadays.Orthogonal frequency division multiplexing (OFDM) is a multi-carrier transmission technique, which divides the available spectrum into many subcarriers, each one being modulated by a low data rate stream.
8 Introduction to ofdmOrthogonal Frequency Division Multiplexing (OFDM) is a frequency division multiplexing (FDM) scheme used as a digital multi carrier modulation (MCM) method.OFDM is a method of digital modulation in which a signal is split into several narrowband channels at different frequencies.The main idea behind the OFDM is that since low-rate modulations are less sensitive to multipath, the better way is to send a number of low rate streams in parallel than sending one high rate waveform.A large number of closely spaced orthogonal subcarriers are used to carry data.OFDM is a promising technique for achieving high data rate and combating multipath fading in Wireless Communications.
9 OFDM DEFINITION OFDM = Orthogonal FDM Carrier centers are put on orthogonal frequenciesORTHOGONALITY - The peak of each signal coincides with other signalsSubcarriers are spaced by 1/Ts
12 OFDM-Introduction Contd.. OFDM can be viewed as either a modulation technique or a multiplex technique.Modulation techniqueViewed by the relation between input and output signalsMultiplex techniqueViewed by the output signal which is the linear sum of the modulated signals
13 Why OFDM? Single Carrier Multicarrier Uses the entire bandwidth OFDM – Orthogonal Frequency Division MultiplexingSingle CarrierMulticarrierUses the entire bandwidthSplits bandwidth into subchannelsShort symbol timesSends information in parallelThis causes ISIOFDM: orthogonal subcarriersOFDM is a considerable option when the channel introduces ISIApplications: ADSL, DAB, DVB, Hiperlan/2, ...
14 Modulation techniques: monocarrier vs. multicarrier ChannelChannelizationN carriersBPulse length ~ N/BSimilar toFDM technique– Data are shared amongseveral carriersand simultaneously transmittedGuard bandsBPulse length ~1/B– Data are transmited overonly one carrier– Selective Fading– Very short pulses– ISI is compartively long– EQs are then very long– Poor spectral efficiencybecause of band guardsDrawbacks– It is easy to exploitFrequency diversity– Flat Fading per carrier– N long pulses– ISI is comparatively short– N short EQs needed– Poor spectral efficiencybecause of band guardsAdvantagesFurthermore– It allows to deploy2D coding techniques– Dynamic signallingTo improve the spectral efficiency:To use orthogonal carriers (allowing overlapping)Eliminate band guards between carriers
15 OFDM and Multicarrier Transmission 2/8Single and multicarrier transmission
16 Orthogonality Orthogonality in OFDM Time domain frequency domain Band pass signalWhere is the equivalent low pass signal ofif , n is an non-integer i.e Then
17 OFDM and Multicarrier Transmission 4/8OrthogonalOrthogonal, n=3Orthogonal, n=2(OFDM)Orthogonal, n=1Non-orthogonal
18 OFDM and Multicarrier Transmission 5/8Time domainFrequency domain
21 Orthogonal Frequency Division Modulation N carriersBSymbol: 2 periods of f0TransmitSymbol: 4 periods of f0+ffSymbol: 8 periods of f0Channel frequencyresponse.Data coded in frequency domainTransformation to time domain:each frequency is a sine wavein time, all added up.BDecode each frequencybin separatelyReceivetimefTime-domain signalFrequency-domain signal
22 Principles OFDMSome processing is done on the source data, such as coding for correcting errors, interleaving and mapping of bits onto symbols. An example of mapping used is QAM.The symbols are modulated onto orthogonal sub-carriers. This is done by using IFFT.Orthogonality is maintained during channel transmission. This is achieved byadding a cyclic prefix to the OFDM frame to be sent. The cyclic prefix consists ofthe L last samples of the frame, which are copied and placed in the beginning ofthe frame. It must be longer than the channel impulse response.Synchronization: the introduced cyclic prefix can be used to detect the start of each frame. This is done by using the fact that the L first and last samples are the same and therefore correlated. This works under the assumption that one OFDM frame can be considered to be stationary.Demodulation of the received signal by using FFTChannel equalization: the channel can be estimated either by using a trainingsequence or sending known so-called pilot symbols at predefined sub-carriers.Decoding and de-interleaving
23 Advantages of OFDM High spectral efficiency Simple implementation by FFTLow receiver complexityRobust ability for high-data rate transmission over multipath fading channel.High link in terms of link adaption.OFDM eliminates Inter Symbol Interference (ISI) through the use of a cyclic prefix.OFDM is less sensitive to sample timing offsets than the single carrier systems.OFDM eliminates the need for equalizers
24 Disadvantages of OFDM Synchronization Need FFT units at transmitter, receiverSensitive to carrier frequency offsetHigh peak to average power ratio
25 Application of OFDM Worldwide Interoperability for Microwave Access (WiMAX).Terrestrial Digital Audio Broadcasting (DVB-T).Wireless Metropolitan Area Network (WMAN).(IEEE d).Wireless Local Area Network (WLAN). ETSI HiperLAN.Digital Audio Broadcasting (DAB).Digital Video Broadcasting (DVB).High Definition Television (HDTV).Broadband Internet Access