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. 23 Jan 2008 OFDM Competence Development 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

3. 23 Jan 2008 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) mappingDistributed (diversity) mapping

4. 23 Jan 2008 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

5. 23 Jan 2008 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

6. 23 Jan 2008 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

7. 23 Jan 2008 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

8. 23 Jan 2008 OFDM Competence Development 8 DFT-spread OFDMA Linear precoding of OFDMA symbols N < N C subcarriers are allocated to one user –An N-point Discrete Fourier Transform (DFT) is applied –New output symbols (X k ) are linear combinations of all N input symbols (x n ) Conventional OFDMA has a PAPR problem in the time domain. Linear precoding with DFT moves the PAPR to the frequency domain SC mapping +CP, D/A+RF Channel RF+A/D, -CP N C -point DFT SC de-mapping N C -point IDFT NCNC NCNC NN N-point DFT N-point IDFT OFDMADFT-spread

9. 23 Jan 2008 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

10. 23 Jan 2008 OFDM Competence Development 10 ~2 dB Source: Myung et al. Peak-to-average power ratio of single carrier FDMA signals with pulse shaping. PIMRC 2006 N = 64, M = 256, 16-QAM Benefit of the SC-FDMA signal Reduces PAPR with 2-3 dB N = 64, M = 256, QPSK

11. 23 Jan 2008 OFDM Competence Development 11 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 3 dB loss IFDMA OFDMA Source: Alamouti. Mobile WiMAX: Vision & Evolution. Intel presentation. 2007

12. 23 Jan 2008 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