1 EQ2430 Project Course in Signal Processing and Digital Communications - Spring 2011 On phase noise and it effect in OFDM communication system School of Electrical Engineering KTH

2 EQ2430 Project Course in Signal Processing and Digital Communications - Spring 2011 To provide an overview of oscillor phase noise and its influence on OFDM systems. Objective

3 EQ2430 Project Course in Signal Processing and Digital Communications - Spring 2011 Over view of the phase noise phenomenon  Ideal oscillator delivers a pure sinusoid Fig. 1: Spectrum of a pure sinusoid by an ideal oscillator.  Practically,oscillators output a sinusoid with time varaying phase Fig. 2: Spectrum of a sinusoid from a practical oscillator.

4 EQ2430 Project Course in Signal Processing and Digital Communications - Spring 2011 Cond.... An Oscillator is characterized by a plot of the phase noise power versus frequency offset from carrier frequency. What is measured?: Total side band power in a range of 1Hz band width at a given frequency offset from the carrier and with respect to carrier power. Unit is in dBc/Hz. Mainly poses a problem for OFDM systems, but others also - Limits precision of GPS - Destroys quality of data transmission by making clock s synchronization difficult OFDM is currently employed extensively in many standards; a solution to phase noise mitigation is therefore highly sought for.

5 EQ2430 Project Course in Signal Processing and Digital Communications - Spring 2011 OFDM: Distributes data over a large number of orthogonal carriers. - High spectral efficiency - Robust in multi-path distortions (ISI) - High data rate Implemented in DAB, DVB, WLAN,4G, IEEE standards. Phase noise in OFDM

6 EQ2430 Project Course in Signal Processing and Digital Communications - Spring 2011 Current Researchs focus toward utilization of the ultra high frequency bandwidth (60 Ghz ).  More bandwidth available  Suitable for high data rate short range transmission Why the interest in phase noise?  Effect of Phase noise increases quadratically with carrier frequency.  Destroys orthogonality of sub-carriers in OFDM and thus cause ISI  Oscillator’s phase noise major parameter in design of OFDM systems. - Sets limit on Subcarrier spacing - Cheap oscillator => poor phase noise performance Contd...

7 EQ2430 Project Course in Signal Processing and Digital Communications - Spring 2011 Reciever phase noise in OFDM system Channel Parallel to Serial/ Demodulation FFT Serial to Parallel Remove Cyclic Prefix DAC Recieved Data Modulation/ Serial To Parallel IFFT Parallel to serial Cyclic Prefix ADC/ LPF Serial Data Source low frequency phase noise which modulates the oscillator output Fig. 3: Block diagram of an OFDM Communication system

8 EQ2430 Project Course in Signal Processing and Digital Communications - Spring 2011 The recieved signal at the reviever would be Where is the convolution operation on the transmitted signal and the channel impulse response and an additive gaussian noise. The sampled and baseband form of the recieved signal disturbed by phase noise where is the gaussian noise, rotated by the phase noise Interfering components of phase noise

9 EQ2430 Project Course in Signal Processing and Digital Communications - Spring 2011 The k th subcarrier component after the FFT block Where and is FFT of the Gaussian noise Common phase error (CPE) Inter carrier interference (ICI)

10 EQ2430 Project Course in Signal Processing and Digital Communications - Spring 2011 fcfc f1f1 f2f2 f3f3 f4f4 f c +f 1 f c +f 2 f c +f 3 f c +f 4 1) Ideal local oscillator fcfc f1f1 f4f4 f3f3 f2f2 f c +f 1 f c +f 2 f c +f 3 f c +f 4 2) Practical local oscillator ICI Fig. 4: illustration of phase noise effect in an OFDM View of Inter carrier interference

11 EQ2430 Project Course in Signal Processing and Digital Communications - Spring 2011 Try to mitigate the CPE and ICI components of phase noise individualy.  CPE Estimation: considers the ICI term as an additive gaussian noise term. Where Least squares estimater of using known pilot sequences : Where is pre-estimate (sometimes assumed known) of the channel at the k th sub-carrier. Algorithms for Phase noise mitigation

12 EQ2430 Project Course in Signal Processing and Digital Communications - Spring 2011  ICI estimation: Estimating the N terms for large number of sub-carriers in computationaly demanding  Estimate the components around the center frequency, i.e. circular indexing  Remaining ICI terms considered as additive gaussian noise. Where, Unestimated ICI terms plus channel noise  For subset of reliable sub-carriers and, the above term is written in matrix form Where

13 EQ2430 Project Course in Signal Processing and Digital Communications - Spring 2011  Estimation of the vector with out requiring knowledge of the statistical properties of the phase noise by Pseudo inverse of the matrix Improvment on performance of the algorithms by iterative method. i.e: Improve estimations by using partially corrected symbols Deconvolve effect of phase noise from the recieved signal

14 EQ2430 Project Course in Signal Processing and Digital Communications - Spring 2011 Extensive going research focuse on develping numerically efficient DSP algorithms for estimation and mitigation of phase noise in OFDM systems along with other RF artifacts. In conclusion...

15 EQ2430 Project Course in Signal Processing and Digital Communications - Spring 2011 References T. H. Lee, and A. Hajimiri, Oscillator Phase Noise: A Tutorial, IEEE Journal of solid-state circuits, vol. 35, No. 3, March A. G. Armada, Understanding the Effects of Phase Noise in Orthogonal Frequency Division Multiplexing (OFDM), IEEE Transactions On Broadcasting, vol. 47, No. 2, June D. Petrovic, W. Rave, and G. Fettweis, Limits of Phase Noise Suppression in OFDM,” IEEE Trans. on Inform. Theory,, no. 10, October V. Syrjälä, M Valkama, N. N. Tchamov, and Jukka Rinne, Phase Noise Modelling and Mitigation Techniques in OFDM Communications Systems, on Wireless Communication Symposium, (WTS) 2009.