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Class Report

Class Report. 林宏穎 : OFDM Introduction. OFDM History. 1957: Kineplex multi-carrier HF modem 1966 : Chang, Bell Labs: OFDM paper & patent 1971 : Weinstein & Ebert propose use of FFT and guard interval 1985: Cimini describes use of OFDM for mobile communications

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Class Report

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  1. Class Report 林宏穎: OFDM Introduction

  2. OFDM History 1957: Kineplex multi-carrier HF modem 1966: Chang, Bell Labs: OFDM paper & patent 1971: Weinstein & Ebert propose use of FFT and guard interval 1985: Cimini describes use of OFDM for mobile communications 1987 Alard & Lasalle: OFDM for digital broadcasting 1995: ETSI DAB standard: first OFDM-based standard 1997: DVB-T standard 1998: Magic WAND project demonstrates OFDM modems for wireless LAN 1999: IEEE 802.11a and HIPERLAND/2 standards for wireless LAN 2000: V-OFDM for fixed wireless access 2001: OFDM considered for new IEEE 802.11 and 802.16 standards

  3. Introduction to OFDM • Basic idea • Using a large number of parallel narrow-band sub-carrier instead of a single wide-band carrier to transport information • Advantages • Very easy and efficient in dealing with multi-path • Robust against narrow-band interference • Disadvantages • Sensitive to frequency offset and phase noise • Peak-to-average problem reduces the power efficiency of RF amplifier at the transmitter • Adopted by various standards • DSL, 802.11a, DAB, DVB, etc.

  4. frequency frequency OFDM Definition FDM • The technique of OFDM is based on the well-known technique of FDM • FDM technique: • Different streams of information are mapped onto separate parallel frequency channels • Guard bands are inserted to reduce interference between adjacent channels • OFDM technique • Multiple carriers carry the information stream • Carrier spectrum are are overlapped and orthogonal to each other • A guard time is added to each symbol to combat the channel delay spread OFDM

  5. Concept of OFDM • A type of multi-carrier modulation • Single high-rate bit stream is converted to low-rate N parallel bit stream • Each parallel bit stream is modulated on one of N sub-carriers • Each sub-carrier can be modulated by QFSK or QAM • Add a guard time to each OFDM symbol to avoid inter-symbol interference of fading channel • To achieve high bandwidth efficiency, the sub-carriers are closely spaced and overlapped • Sub-carriers are orthogonal over the symbol time • Use coding to correct errors for sub-carriers in deep fading environment

  6. Advantages of OFDM • Robust in multi-path propagation environment • Successful Examples: • DAB, DVB-T, Wireless LAN • More tolerant of delay spread • Due to the use of many sub-carriers, the symbol duration is increased, relative to delay spread • Inter-symbol interference is avoided through the use of guard interval • Simplified or eliminate equalization needs, as compared to single carrier modulation • More resistant to fading • Low symbol rate per carrier provides the robustness against frequency selective fading or narrowband interference • FEC is used to correct for sub-carriers that suffer from deep fade • Multi-carrier with single frequency network (SFN)

  7. OFDM Good for Broadband Systems • Most broadband systems are subjects to multipath transmission • Conventional solution to multipath is an equalizer in the receiver • Equalizers are too complicated at high data rates • With OFDM there is a simple way of dealing with multipath • Relatively simple DSP algorithms

  8. Modulation System Single carrier modulation  Multi carrier modulation N subchannels N complex samples S/P quadrature amplitude modulation (QAM) encoder N-IFFT add cyclic prefix P/S D/A + transmit filter TRANSMITTER multipath channel RECEIVER N subchannels N complex samples Receive filter + A/D P/S QAM decoder N-FFT S/P remove cyclic prefix channel estima- tion & equalizer

  9. Multicarrier Rate R Mapping Filter f0 Rate R Mapping Filter f1 Rate NR Rate R Mapping Filter fN-1 Bandlimited signals f0 f1 f2 fN-1  The transmission bandwidth is divided into sub-bands which are transmitted in parallel  Ideally, each sub-band is narrow enough so that the fading it experiences is flat (no ISI)  Disadvantages -- Requires filter bank at receiver -- Spectrally inefficiency

  10. OFDM Source of Impairment Power Amplifier Non-Linear Insert Guard Interval FEC Coding QAM Mapping Pilot Insertion IQ Modulator DAC IFFT (TX) FFT (RX) HPA Fixed-Point Computation Error Multi-path Fading Channel Frequency Corrected Signal Phase noise ADC noise FEC Decoding Remove Guard Interval AGC Response Time QAM De- Mapping Channel Correction ADC Symbol timing AGC Amp LNA Timing Frequency Synchronization Phase noise Frequency offset

  11. Performance Loss • Detection Loss of synchronized Detection • SNR (dB) required to achieve the performance of perfect channel knowledge . (Infinite Precision arithmetic assumed) • Algorithms for channel model description • Implementation Loss • SNR (dB) resulting from finite precision arithmetic • Computation complexity, architecture selection, cost

  12. Problems of OFDM Modulation • ICI (Inter-channel interference): interference between symbol in adjacent frequencies • ISI (inter-symbol interference): interference of successive OFDM frames • Highly vulnerable to synchronization errors and frequency offsets • Highly vulnerable to the non-linearity of the Pas (in the RF analog front end)

  13. Challenges for OFDM • Synchronization challenges • Transmitter frequency  Receiver frequency • Mesochronous: same frequency, different phase • Pleisochrnous: slightly different frequencies • Asynchronous: totally different frequencies • Transmitter sampling time  Receiver sampling time • Symbol timing is unknown to receiver • Peak-to Average Power Ratio (PAPR) • Dynamic range at output of IFFT is much larger than at input • it is about 2 dB higher than that of the ATSC 8-VSB system. A larger Tx (more dynamic range) might be required or using pre-distortion and better filtering to reduce the first adjacent channel interference • Channel estimation for time varying environment

  14. Impact of Symbol Duration • The symbol duration of OFDM is much larger than that of single carrier system under the similar overall transmission bandwidth • A larger symbol duration will enhance the effective bit rate and power utilization if the delay spread is about fixed • The larger OFDM duration when compared with the channel coherence time can reduce the ability to combat the fast temporal fading • The channel coherence time is inversely proportional to the maximum Doppler shift

  15. Impact of Sub-Carrier Spacing • Because of the time-frequency duality, some of the time-domain arguments can be translated to the frequency domain • The large number of OFDM sub-carriers makes the bandwidth of the individual sub-carriers small relative to the overall signal bandwidth and the channel coherence bandwidth • The fading on each sub-carrier is frequency flat and can be better modeled as a constant complex channel gain. • The narrower sub-carrier spacing will be easier to cause inter-carrier interference

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