際際滷

際際滷Share a Scribd company logo

Cfo in ofdm

Download as PPT, PDF
K
Khalid Hussain

This document discusses carrier frequency offset (CFO) in OFDM systems. It begins with an introduction to OFDM and the issues caused by timing offset and CFO. It then describes CFO in more detail, distinguishing between integer and fractional CFO. Methods for CFO estimation are presented, including pilot-based and non-pilot based approaches. Fast acquisition methods for initial CFO estimation are covered, along with references.

1 of 20
Downloaded 157 times
1 Carrier Frequency Offset in OFDM System Syed Khalid Hussain Department of Electrical and Computer Engineering
2 Outline Introduction Timing Offset (TO) Carrier Frequency Offset (CFO) Frequency Offset Estimation With Fast Acquisition References
3 Outline Introduction Timing Offset (TO) Carrier Frequency Offset (CFO) Frequency Offset Estimation With Fast Acquisition References
4 Introduction Multicarrier Amplitude modulation scheme. Each carriers amplitude is modulated. Carriers are spaced evenly and orthogonally(can be computed efficiently with IDFT and DFT. Idea is to split the signal into multiple smaller sub-signals that are transmitted simultaneously. OFDM signal in Time and Frequency domain
5 Thus , OFDM can provide large data rates with sufficient robustness to radio channel impairment ,therefor a popular choice for wireless broadband communication(Adopted in WiFi , WiMax ,DAB , DVB, LTE & LTA-A ) ADVANTAGES Immunity to Frequency selective fading Resilience to interference Robust to narrow-band interference Spectrum efficiency DISADVANTAGES Sensitivity to synchronization errors High Peak-to-average power ratio Introduction Block Diagram of OFDM Symbol
6 Introduction Impairment in OFDM Symbol Timing Offset (TO) Due to the unknown transmission time or propagation delay of OFDM symbol at receiver Carrier Frequency Offset (CFO) Frequency differences between the transmitter and receiver oscillator Oscillator instabilities Doppler shift of mobile channels Phase noise Sampling Clock Offset These errors cause Inter symbol Interference(ISI) and Inter carrier Interference(ICI)
7 Introduction Timing Offset Carrier Frequency Offset Frequency Offset Estimation With Fast Acquisition References
8 Timing Offset Window overlapping of the two successive OFDM symbols raises to ISI Due to incorrect position of the FFT window at receiver for Start Symbol. Introduces Inter-symbol Interference(ISI) and phase shift to the desired symbol components. (TO) can be avoided by keeping the window start point within the ISI free region of cyclic prefix (CP) for each symbol. Larger the CP, the more TO , a system can tolerate.
9 Timing Offset Estimator GOAL : To determine the start symbol position at receiver to avoid the ISI and ICI. Pilot Based Method : (useful for systems with low SNR) Pilots (pseudo-random sequences or null symbols) can be used to determine the start of an OFDM symbol. Pilot-symbols can be OFDM based or Non-OFDM based. In case of continuous data , a null signal is appropriate , for bursty data this is not true. Non-Pilot based Method : Most method exploits the redundancy of cyclic-prefix(CF) to find the start symbol position. Some algorithms use the periodicity of the correlation function of the time domain OFDM symbol.
10 Introduction Timing Offset Carrier Frequency Offset Frequency Offset Estimation With Fast Acquisition References
11 Carrier Frequency Offset
12 Carrier Frequency Offset Integer CFO : Do not introduces ICI between sub-carrier spacing but introduces a cyclic shift of data sub-carriers and a phase change proportional to OFDM symbol number. No effect on orthogonality. In Acquisition phase, we estimate the Integer CFO (also called Coarse Frequency Offset Estimation) Acquisition scheme generally have a wide range , but low accuracy.(Usually fast acquisition is required) Fractional CFO : Causes rotation and introduces ICI between sub-carriers, (destroy the orthogonality of sub-carrier ,results in BER degradation). In Tracking phase, we usually estimate Fractional CFO (also called Fine Frequency Offset Estimation). Tracking algorithm have a narrower range but fine accuracy.
13 Carrier Frequency Offset The OFDM system model with CFO : x ( t ) Channel h ( t) S ( k ) S/P X ( k ) x(n) Adding Pilots C(n)& IFFT Adding Cyclic Prefix & P/S DAC Signal Mapper z (t ) r (t ) ( )S k P/S ( )R k FFT Remove Cyclic Prefix & S/P ADC Signal Demapper ( )r n AWGN w ( t) 02 /fj n N e 隆 /f f隆 竜=
CFO Estimation Methods : Data Aided (Use of Pilots or training symbols , cause reduction in throughput as known data pattern also needs to send) Non-Data Aided (Use of the Intrinsic structure of OFDM symbols e.g. exploits the redundancy in Cyclic-Prefix) Blind Approaches (No data overhead but very high computational complexity and relies on the signal statistics) Frequency synchronization Steps in OFDM : I. Finding the correct Integer Offset II. Finding a coarse estimate of the fractional frequency offset. III. Refining the obtained fractional frequency offset. 14 Carrier Frequency Offset Estimator
15 Carrier Frequency Offset Estimator GOAL : To refine the initial analog estimate of the carrier frequency Pilot Based Method : can be used for both Integer and Fractional carrier frequency synchronization Pilot can be an extra sequence outside an OFDM symbol or known data interspersed with the OFDM symbol. Both OFDM based and non-OFDM based pilots can be used Non-Pilot based Method : Generally used for Fractional frequency synchronization. Redundancy in the cyclic-prefix (CP) is used for frequency offset estimation. Very similar to the pilot based method except that this method rely on the correlation within the OFDM symbol rather than adding known pilot symbols.
16 Frequency Synchronization can be categorized into : Pre-FFT Synchronization i.e. estimation is performed before computation of DFT at receiver so require less computing power and provides fast synchronization Post-FFT Synchronization i.e. estimation is performed after computing the DFT. Carrier Frequency Offset Estimator Without CFO With CFO
17 Introduction Timing Offset Carrier Frequency Offset Frequency Offset Estimation With Fast Acquisition References
CFO With Fast Acquisition 18
CFO With Fast Acquisition Training phase need to correct the remaining (Fraction CFO) after Acquisition. MLE of fraction CFO is evaluated by using the correlation properties of sample in received vector R 19 ADVANTAGE: Fast acquisition Acquisition range is as large as one-half of overall signal bandwidth High performance tracking Timing synchronization can be achieved with same (new training symbol)
20 References [1] Zhang, Zhongshan, et al. "Frequency offset estimation with fast acquisition in OFDM system." Communications Letters, IEEE 8.3 (2004): 171-173. [2] Mohseni, Saeed, and Mohammad A. Matin. "Study of the estimation techniques for the Carrier Frequency Offset (CFO) in OFDM systems." IJCSNS International Journal of Computer Science and Network Security 12.6 (2012): 73-80. [3] J. R. Foerster, Ed., Channel Modeling Sub-committee Report Final, IEEE P802.15 SG3a contribution. [4] Li, Ye Geoffrey, and Gordon L. Stuber. Orthogonal frequency division multiplexing for wireless communications. Springer Science & Business Media, 2006. [5] Chang, Dah-Chung. "Effect and compensation of symbol timing offset in OFDM systems with channel interpolation." Broadcasting, IEEE Transactions on 54.4 (2008): 761-770.

More Related Content

Cfo in ofdm

  • 1. 1 Carrier Frequency Offset in OFDM System Syed Khalid Hussain Department of Electrical and Computer Engineering
  • 2. 2 Outline Introduction Timing Offset (TO) Carrier Frequency Offset (CFO) Frequency Offset Estimation With Fast Acquisition References
  • 3. 3 Outline Introduction Timing Offset (TO) Carrier Frequency Offset (CFO) Frequency Offset Estimation With Fast Acquisition References
  • 4. 4 Introduction Multicarrier Amplitude modulation scheme. Each carriers amplitude is modulated. Carriers are spaced evenly and orthogonally(can be computed efficiently with IDFT and DFT. Idea is to split the signal into multiple smaller sub-signals that are transmitted simultaneously. OFDM signal in Time and Frequency domain
  • 5. 5 Thus , OFDM can provide large data rates with sufficient robustness to radio channel impairment ,therefor a popular choice for wireless broadband communication(Adopted in WiFi , WiMax ,DAB , DVB, LTE & LTA-A ) ADVANTAGES Immunity to Frequency selective fading Resilience to interference Robust to narrow-band interference Spectrum efficiency DISADVANTAGES Sensitivity to synchronization errors High Peak-to-average power ratio Introduction Block Diagram of OFDM Symbol
  • 6. 6 Introduction Impairment in OFDM Symbol Timing Offset (TO) Due to the unknown transmission time or propagation delay of OFDM symbol at receiver Carrier Frequency Offset (CFO) Frequency differences between the transmitter and receiver oscillator Oscillator instabilities Doppler shift of mobile channels Phase noise Sampling Clock Offset These errors cause Inter symbol Interference(ISI) and Inter carrier Interference(ICI)
  • 7. 7 Introduction Timing Offset Carrier Frequency Offset Frequency Offset Estimation With Fast Acquisition References
  • 8. 8 Timing Offset Window overlapping of the two successive OFDM symbols raises to ISI Due to incorrect position of the FFT window at receiver for Start Symbol. Introduces Inter-symbol Interference(ISI) and phase shift to the desired symbol components. (TO) can be avoided by keeping the window start point within the ISI free region of cyclic prefix (CP) for each symbol. Larger the CP, the more TO , a system can tolerate.
  • 9. 9 Timing Offset Estimator GOAL : To determine the start symbol position at receiver to avoid the ISI and ICI. Pilot Based Method : (useful for systems with low SNR) Pilots (pseudo-random sequences or null symbols) can be used to determine the start of an OFDM symbol. Pilot-symbols can be OFDM based or Non-OFDM based. In case of continuous data , a null signal is appropriate , for bursty data this is not true. Non-Pilot based Method : Most method exploits the redundancy of cyclic-prefix(CF) to find the start symbol position. Some algorithms use the periodicity of the correlation function of the time domain OFDM symbol.
  • 10. 10 Introduction Timing Offset Carrier Frequency Offset Frequency Offset Estimation With Fast Acquisition References
  • 12. 12 Carrier Frequency Offset Integer CFO : Do not introduces ICI between sub-carrier spacing but introduces a cyclic shift of data sub-carriers and a phase change proportional to OFDM symbol number. No effect on orthogonality. In Acquisition phase, we estimate the Integer CFO (also called Coarse Frequency Offset Estimation) Acquisition scheme generally have a wide range , but low accuracy.(Usually fast acquisition is required) Fractional CFO : Causes rotation and introduces ICI between sub-carriers, (destroy the orthogonality of sub-carrier ,results in BER degradation). In Tracking phase, we usually estimate Fractional CFO (also called Fine Frequency Offset Estimation). Tracking algorithm have a narrower range but fine accuracy.
  • 13. 13 Carrier Frequency Offset The OFDM system model with CFO : x ( t ) Channel h ( t) S ( k ) S/P X ( k ) x(n) Adding Pilots C(n)& IFFT Adding Cyclic Prefix & P/S DAC Signal Mapper z (t ) r (t ) ( )S k P/S ( )R k FFT Remove Cyclic Prefix & S/P ADC Signal Demapper ( )r n AWGN w ( t) 02 /fj n N e 隆 /f f隆 竜=
  • 14. CFO Estimation Methods : Data Aided (Use of Pilots or training symbols , cause reduction in throughput as known data pattern also needs to send) Non-Data Aided (Use of the Intrinsic structure of OFDM symbols e.g. exploits the redundancy in Cyclic-Prefix) Blind Approaches (No data overhead but very high computational complexity and relies on the signal statistics) Frequency synchronization Steps in OFDM : I. Finding the correct Integer Offset II. Finding a coarse estimate of the fractional frequency offset. III. Refining the obtained fractional frequency offset. 14 Carrier Frequency Offset Estimator
  • 15. 15 Carrier Frequency Offset Estimator GOAL : To refine the initial analog estimate of the carrier frequency Pilot Based Method : can be used for both Integer and Fractional carrier frequency synchronization Pilot can be an extra sequence outside an OFDM symbol or known data interspersed with the OFDM symbol. Both OFDM based and non-OFDM based pilots can be used Non-Pilot based Method : Generally used for Fractional frequency synchronization. Redundancy in the cyclic-prefix (CP) is used for frequency offset estimation. Very similar to the pilot based method except that this method rely on the correlation within the OFDM symbol rather than adding known pilot symbols.
  • 16. 16 Frequency Synchronization can be categorized into : Pre-FFT Synchronization i.e. estimation is performed before computation of DFT at receiver so require less computing power and provides fast synchronization Post-FFT Synchronization i.e. estimation is performed after computing the DFT. Carrier Frequency Offset Estimator Without CFO With CFO
  • 17. 17 Introduction Timing Offset Carrier Frequency Offset Frequency Offset Estimation With Fast Acquisition References
  • 18. CFO With Fast Acquisition 18
  • 19. CFO With Fast Acquisition Training phase need to correct the remaining (Fraction CFO) after Acquisition. MLE of fraction CFO is evaluated by using the correlation properties of sample in received vector R 19 ADVANTAGE: Fast acquisition Acquisition range is as large as one-half of overall signal bandwidth High performance tracking Timing synchronization can be achieved with same (new training symbol)
  • 20. 20 References [1] Zhang, Zhongshan, et al. "Frequency offset estimation with fast acquisition in OFDM system." Communications Letters, IEEE 8.3 (2004): 171-173. [2] Mohseni, Saeed, and Mohammad A. Matin. "Study of the estimation techniques for the Carrier Frequency Offset (CFO) in OFDM systems." IJCSNS International Journal of Computer Science and Network Security 12.6 (2012): 73-80. [3] J. R. Foerster, Ed., Channel Modeling Sub-committee Report Final, IEEE P802.15 SG3a contribution. [4] Li, Ye Geoffrey, and Gordon L. Stuber. Orthogonal frequency division multiplexing for wireless communications. Springer Science & Business Media, 2006. [5] Chang, Dah-Chung. "Effect and compensation of symbol timing offset in OFDM systems with channel interpolation." Broadcasting, IEEE Transactions on 54.4 (2008): 761-770.