Synchronization Algorithms for OFDM Systems Denis Yusim Evgeniy Priziment Presented by: Supervised by: Dr. Shlomo Greenberg Mr. Ron Bercovich

Synchronization problems in OFDM Synchronization algorithms Outline IEEE 802.16 background OFDM basics Synchronization problems in OFDM Synchronization algorithms

Escalating business demand for: IEEE 802.16 Background Escalating business demand for: rapid Internet connection integrated data, voice and video services BWA is an affordable alternative to wired solutions which can be easily and quickly deployed BWA was limited due to lack of a universal standards IEEE standard 802.16 was developed to make BWA more widely available The standard specifies the PHY and the MAC layers for wireless metropolitan area networks

Global Wireless Standards IEEE 802.15 Bluetooth WAN MAN LAN PAN ETSI HiperPAN IEEE 802.11 WirelessLAN ETSI HiperLAN IEEE 802.16 WirelessMAN ETSI HiperMAN & HIPERACCESS IEEE 802.20 (proposed) 3GPP, EDGE (GSM)

T1+ LEVEL SERVICE ENTERPRISE FRACTIONAL T1 FOR SMALL BUSINESS IEEE 802.16 Applications RESIDENTIAL & SOHO DSL ALWAYS BEST CONNECTED 802.11 802.11 802.11 802.16 T1+ LEVEL SERVICE ENTERPRISE BACKHAUL FOR HOTSPOTS INTERNET BACKBONE FRACTIONAL T1 FOR SMALL BUSINESS

Synchronization problems in OFDM Synchronization algorithms Outline IEEE 802.16 background OFDM basics Synchronization problems in OFDM Synchronization algorithms

OFDM Orthogonal Frequency Division Multiplexing OFDM - What is it? Multi - Carrier Transmission technique Subcarriers are spaced by 1/Ts ORTHOGONALITY - The peak of each signal coincides with trough of other signals

OFDM – Principles FFT is the basic technique to generate overlapped orthogonal OFDM tones dN-1 SN-1 Data In Modulation (QPSK,QAM, BPSK, etc) S/P IFFT D/A d0 S0 Baseband OFDM signal Data Out De-Modulation (QPSK,QAM, BPSK, etc) P/S FFT A/D

Adaptive Modulation Modulation is chosen based on the channel conditions Closer the range – higher order modulations (QAM16, QAM64) Further the range – lower modulations (BPSK)

OFDM Advantages High Spectrum Efficiency Guard bands f FDM OFDM High Spectrum Efficiency The information is spread over frequencies, without any frequency guard bands Equalization is very simple compared to Single-Carrier systems

OFDM Advantages (cont.) OFDM has excellent robustness in multi-path environments Cyclic prefix preserves orthogonality between subcarriers

Synchronization problems in OFDM Synchronization algorithms Outline IEEE 802.16 background OFDM basics Synchronization problems in OFDM Synchronization algorithms

The goal is to determine where to start the FFT Time Offset Symbol # n-1 Symbol # n Symbol # n+1 Optimal FFT window Late FFT window Early FFT window The goal is to determine where to start the FFT FFT window can start from any sample within the Guard Period due to FFT cyclic properties When FFT window spans samples from two consecutive OFDM symbols, inter-OFDM-symbol interference (ISI) occurs

Frequency Offset Frequency offset is a multiple of a sub carrier spacing Sub carriers are still orthogonal Received data symbols are in the wrong position in the demodulated spectrum, resulting in a BER degradation Frequency offset is a fraction of sub carrier spacing Loss of the orthogonality BER of the system deteriorates

Synchronization problems in OFDM Synchronization algorithms Outline IEEE 802.16 background OFDM basics Synchronization problems in OFDM Synchronization algorithms

Uplink Synchronization The synchronization is based on the uplink preamble symbol Preamble Data From Subscriber # k From Subscriber # k+1 The preamble consists of one OFDM symbol utilizing only even subcarriers The time domain waveform consists of 2 times 128 samples preceded by a CP

Symbol Timing Estimation by M. Schmidl and C. Cox Time θ X X = M(θ) R(θ)2 2|P(θ)|2

Symbol Timing Estimation by M. Schmidl and C. Cox The estimator is defined as: where is the proposed metric is the correlation between the samples in two halves is the energy of the entire symbol

Symbol Timing Estimation by M. Schmidl and C. Cox

ML Algorithm by M. Sandell, et al. The estimator is defined as: where is the proposed metric is the energy of the signal sample is the energy of the noise sample

ML Algorithm by M. Sandell, et al.

Modified ML Algorithm for IEEE 802.16 uplink preamble CP … The algorithm is based on calculation of six correlations between three windows , and and correlations of two windows and . In addition the energy within these five windows is calculated. These windows slide through an observation window.

Modified ML Algorithm for IEEE 802.16 uplink preamble

Frequency Offset Estimation is shifted in frequency by is a sampled signal at intervals

Frequency Offset Estimation (Cont’) Within the preamble symbol and are identical Averaging over all pairs and calculating an argument

Frequency Offset Estimation (Cont’)

Simulation Results For Cyclic Prefix 1/16

Simulation Results (Cont’) For Cyclic Prefix 1/32

Questions? Thank you!