OFDM(A) Competence Development – part II

1. OFDM(A) Competence Development – part II 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. OFDMA– Orthogonal Frequency Division Multiple Access • OFDM can be used as a multiple access scheme allowing simultaneous frequency-separated transmissions to/from multiple mobile terminals • The number of sub-carriers can be scaled to fit the bandwidth – Scalable OFDMA Contiguous (localized) mapping Distributed (diversity) mapping OFDM Competence Development

4. Subcarrier allocation techniques (I) • Contiguous or blockwise mapping • Adjacent sub-carriers • Frequency selective fading can erase a full block • For satisfactory performance it must be combined with dynamic scheduling or frequency hopping • Examples: • E-UTRA • Mobile WiMAX – Band AMC OFDM Competence Development

5. Subcarrier allocation techniques (II) • Distributed or diversity mapping • Carriers allocated to one user are spread across the total OFDM bandwidth • Permutation changes from time-slot to time-slot • Examples: • Mobile WiMAX – UL/DL PUSC, DL FUSC • Robust against frequency selective fading OFDM Competence Development

6. Channel dependent scheduling • Exploits time-frequency selective fading • The scheduled user is always allocated the best time-frequency block • Channel varies differently for different users OFDM Competence Development

7. Synchronisation aspects • Impairments in time- and frequency synchronization reduces performance: ISI and ICI • Downlink • Time- and frequency synchronization • Uplink • Control is distributed between terminals • Frequency synchronization • Impact on orthogonality between SCs belonging to different users • Timing synchronization • Impact on inter-symbol interference (ISI) • Different received power at the base station • Base station receiver dynamic range exceeded. Power control necessary OFDM Competence Development

8. NC NC N N NC-point DFT SC de-mapping SC mapping NC-point IDFT +CP, D/A+RF RF+A/D, -CP Channel N-point IDFT N-point DFT DFT-spread OFDMA • Linear precoding of OFDMA symbols • N < NC subcarriers are allocated to one user • An N-point Discrete Fourier Transform (DFT) is applied • New output symbols (Xk) are linear combinations of all N input symbols (xn) • Conventional OFDMA has a PAPR problem in the time domain. • Linear precoding with DFT moves the PAPR to the frequency domain DFT-spread OFDMA OFDM Competence Development

9. Single-Carrier (SC) FDMA • Special case of DFT-spread OFDMA with contiguous sub-carrier mapping • Used in Evolved UTRA uplink • Resulting spectrum becomes continuous – Single-Carrier • All N input symbols are spread over all N subcarriers • All N subcarriers are modulated with a weighted sum of all N input symbols • The DFT/IDFT pair in the transmitter cancel each other out retaining the time domain symbols with a shorter symbol (chip) rate OFDM Competence Development

10. Benefit of the SC-FDMA signal • Reduces PAPR with 2-3 dB N = 64, M = 256, QPSK N = 64, M = 256, 16-QAM ~2 dB Source: Myung et al. Peak-to-average power ratio of single carrier FDMA signals with pulse shaping. PIMRC 2006 OFDM Competence Development

11. IFDMA 3 dB loss OFDMA Drawbacks of the SC-FDMA signal • Performance loss in fading channels due to destroyed orthogonality • Out-of-band emission problem due to higher PAPR in the frequency domain Source: Alamouti. Mobile WiMAX: Vision & Evolution. Intel presentation. 2007 OFDM Competence Development

12. Summary - OFDMA • OFDM can be used a multiple access scheme allowing simultaneous frequency separated transmissions to and from multiple mobile terminals • Subcarriers can be allocated blockwise or distributed • Channel dependent scheduling can be used to dynamically allocate frequency/time blocks to different users • Terminals must be sufficiently time and frequency synchronised to avoid multiple access interference on the uplink • DFT spread OFDMA is beneficial in reducing the PAPR problem – employed by 3GPP E-UTRA on the uplink OFDM Competence Development