OFDM(A) Competence Development – Part I

1. OFDM(A) Competence Development – Part I Per Hjalmar Lehne, Frode Bøhagen, Telenor R&I R&I seminar, 23 January 2008, Fornebu, Norway Per-hjalmar.lehne@telenor.com Frode.bohagen@telenor.com

2. Outline • Part I: What is OFDM? • Part II: Introducing multiple access: OFDMA, SC-FDMA • Part III: Wireless standards based on OFDMA • Part IV: Radio planning of OFDMA OFDM Competence Development

3. Single Carrier Company Multi Carrier Company OFDM Basic Concept • Orthogonal Frequency Division Multiplexing (OFDM) is a multi-carrier modulation scheme • First break the data into small portions • Then use a number of parallel orthogonal sub-carriers to transmit the data • Conventional transmission uses a single carrier, which is modulated with all the data to be sent OFDM Competence Development

4. OFDM Basic Concept • OFDM is a special case of Frequency Division Multiplexing (FDM) • For FDM • No special relationship between the carrier frequencies • Guard bands have to be inserted to avoid Adjacent Channel Interference (ACI) • For OFDM • Strict relation between carriers: fk = k·Df where Df = 1/TU(TU - symbol period) • Carriers are orthogonal to each other and can be packed tight OFDM Competence Development

5. Channel, h(t) OFDM Transmission model Wireless channel Modulator and transmitter Receiver and demodulator OFDM Competence Development

6. Received signal, r(t) Orthogonality – the essential property • Example: Receiver branch k • Ideal channel: No noise and no multipath Tu = 1/Df gives subcarrier orthogonality over one Tu => possible to separate subcarriers in receiver OFDM Competence Development

7. Frequency domain Time domain Power Spectrum for OFDM symbol frequency OFDM – Signal properties OFDM Competence Development

8. OFDM – Signal properties OFDM Competence Development

9. Multipath channel Diffracted and Scattered Paths LOS Path Reflected Path OFDM Competence Development

10. Multipath introduces inter-symbol-interference (ISI) TU Prefix is added to avoid ISI Example multipath profile TCP TU Amplitude [a] t0 t1 t2 The prefix is made cyclic to avoid inter-carrier-interference (ICI) (maintain orthogonality) Time [t] Multipath channel (cyclic prefix) OFDM Competence Development

11. TS CP CP CP Useful symbol Useful symbol Useful symbol Tcp TU Multipath channel (cyclic prefix) • Tcp should cover the maximum length of the time dispersion • Increasing Tcp implies increased overhead in power and bandwidth (Tcp/ TS) • For large transmission distances there is a trade-off between power loss and time dispersion OFDM Competence Development

12. = Multipath channel (frequency diversity) • The OFDM symbol can be exposed to a frequency selective channel • The attenuation for each subcarrier can be viewed as “flat” • Due to the cyclic prefix there is no need for a complex equalizer • Possible transmission techniques • Forward error correction (FEC) over the frequency band • Adaptive coding and modulation per carrier OFDM Competence Development

13. Multipath channel (frequency diversity) OFDM Competence Development

14. Time Pilot symbol Frequency Multipath channel (pilot symbols) • The channel parameters can be estimated based on known symbols (pilot symbols) • The pilot symbols should have sufficient density to provide estimates with good quality (tradeoff with efficiency) • Different estimation methods exist • Averaging combined with interpolation • Minimum-mean square error (MMSE) OFDM Competence Development

15. PA The Peak to Average Power Problem • A OFDM signal consists of a number of independently modulated symbols • The sum of independently modulated subcarriers can have large amplitude variations • Results in a large peak-to-average-power ratio (PAPR) OFDM Competence Development

16. The Peak to Average Power Problem • Example with 8 carriers and BPSK modulation • x(t) plotted • It can be shown that the PAPR becomes equal to Nc OFDM Competence Development

17. PA AM/AM characteristic POUT OBO IBO Average Peak PIN The Peak to Average Power Problem • High efficiency power amplifiers are desirable • For the handset, long battery life • For the base station, reduced operating costs • A large PAPR is negative for the power amplifier efficiency • Non-linearity results in inter-modulation • Degrades BER performance • Out-of-band radiation OFDM Competence Development

18. The Peak to Average Power Problem • Different tools to deal with large PAPR • Signal distortion techniquesClipping and windowing introduces distortion and out-of-band radiation, tradeoff with respect to reduced backoff • Coding techniquesFEC codes excludes OFDM symbols with a large PAPR (decreasing the PAPR decreases code space). Tone reservation, and pre-coding are other examples of coding techniques. • Scrambling techniquesDifferent scrambling sequences are applied, and the one resulting in the smallest PAPR is chosen OFDM Competence Development

19. tmax Dt CP Useful symbol Integration period, TU OFDM Synchronization • Timing recovery • No problem if offset is within Dt • Frequency synchronization • A carrier synchronization error will introduce phase rotation, amplitude reduction and ICI • Frequency offsets of up to 2 % of Df is negligible • Even offsets of 5 – 10 % can be tolerated in many situations OFDM Competence Development

20. Choosing the OFDM parameters • Symbol time (TU) and subcarrier spacing (Df) are inverse • TU = 1/Df • Consequences of increasing the subcarrier spacing • Increase cyclic prefix overhead • Consequences of decreasing the subcarrier spacing • Increase sensitivity to frequency inaccuracy • Increasing number of subcarriers increases Tx and Rx complexity Increase CP overhead Increasing subcarrier spacing TU Decreasing subcarrier spacing Increase sensitivity to frequency accuracy OFDM Competence Development

21. Summary • Advantages • Splitting the channel into narrowband channels enables significant simplification of equalizer design • Effective implementation possible by applying FFT • Flexible bandwidths enabled through scalable number of sub-channels • Possible to exploit both time and frequency domain variations (time domain adaptation/coding + freq. domain adaptation/coding) • Challenges • Large peak to average power ratio OFDM Competence Development

22. PA CP Channel, h(t) Summary OFDM Competence Development