OFDM & QAM

OFDM & QAMSubmitted byChhandarani meher14mtesd01,ESD

1OutlineBackground, historyReview of digital modulationFDMA vs. Multi-carrier modulationTheory of OFDM an QAMAdvantages and disadvantagesConclusion

2The first QAM-related so-called orthogonal frequency division multiplexing (OFDM) schem was proposed by Chang in 1966 for dispersive fading channels.OFDM was standardised as the European digital audio broadcast (DAB) as well as digital video broadcast (DVB) scheme.It constituted also a credible proposal for the recent third-generation mobile radio standard competition in Europe. The systems operational principle is that the original bandwidth is divided in a highthe original bandwidth is divided in a high number of narrow sub-bands, in which the mobile channel can be considered non-dispersive. HistoryQuadrature Amplitude Modulation, QAM is a signal in which two carriers shifted in phase by 90 degrees are modulated and the resultant output consists of both amplitude and phase variations.

In view of the fact that both amplitude and phase variations are present it may also be considered as a mixture of amplitude and phase modulation.

QAMMulti-carrier Amplitude modulationschemeEach Carriers amplitude is modulatedWell-suited for Wireless Communication

What Is Orthogonal Frequency Division Multiplexing (OFDM)?Multi-carrier modulationUse multiple channel (carrier frequency) for one data transmission4/17/2015System Arch 2007 (Fire Tom Wada)6data

LPFLPFLPFdataDEMULTIPLEXMULTIPLEXDigital modulation basicsDigital modulation modulates three parameters of sinusoidal signal. A, k fc,

Three type digital modulation: ASK : Amplitude Shift Keying PSK : Phase Shift Keying FSK : Frequency Shift Keying4/17/20157

OFDM uses combination of ASK and PSK such as QAM, PSKSymbol Waveform 4/17/20158

10100Digital InformationcarrierASKPSKFSK

Symbol length Ofdm /qam system

9The main idea behind OFDM is to partition the frequency selective fading channel into a large number of parallel sub channels.which are flat fading and there fore transform a very high data rate transmission in to a set of parallel transmission with very low data rate.The problem of high data rate transmission over frequency selective channel has been transformed into a set of simple problem, which don't require complicated time domain equalization. OFDM/QAM & Cyclic prefix OFDM Transmitter Block DiagramChannelSerialTo ParallelEncoding BankIFFTParallel To SerialCyclicPrefixSerial Data Source Advantages Very easy and efficient in dealing with multi-pathOfdm/qam provide high data rate. communication and effectively remove ISI.Disadvantages Sensitive to frequency offset and phase noise Peak-to-average problem reduces the power efficiency of RF amplifier at the transmitter.Advantages & Disadvantages Feature of OFDM/QAM

High Frequency utilization by the square spectrum shapeMulti-path problem is solved by GIMultiple services in one OFDM by sharing sub-carriersImplementation was complicated but NOW possible because of LSI technology progress4/17/201513CONCLUSION Performs better than a single modulated carrier in multipath fading

With a properly implemented guard interval: Time waveform appears periodic orthogonality of subcarriers is ensured ISI and ICI are eliminated

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