OFDM(A) Competence Development – part II

OFDM(A) Competence Development – part IIPer Hjalmar Lehne, Frode Bøhagen, Telenor R&IR&I seminar, 23 January 2008, Fornebu, Norway

[email protected]@telenor.com

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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

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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

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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

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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

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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

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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

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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

SC mapping

+CP, D/A+RF

Channel

RF+A/D, -CP

NC-point DFT

SC de-mapping

NC-point IDFT

NCNCN NN-point DFT

N-point IDFT

OFDMADFT-spread

1

0

2N

n

knNj

nk exX

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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

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~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

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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

4 6 8 10 12 14 16 18 20 22 2410

-2

10-1

100

av. SNR per subcarrier(dB)

PE

R

16 QAM 1/2, Red: OFDMA, Blue:IFDMA, FFT size:1024, M=128

3 dB loss

IFDMA

OFDMA

-2000 -1500 -1000 -500 0 500 1000 1500 2000-60

-50

-40

-30

-20

-10

0

10

subcarrier

Inst. PSD (4 symbols), N=1024, M=128

SC-FDMAOFDMA

Source: Alamouti. Mobile WiMAX: Vision & Evolution. Intel presentation. 2007

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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