OFDM(A) Competence Development – part IIPer Hjalmar Lehne, Frode Bøhagen, Telenor R&I

R&I seminar, 23 January 2008, Fornebu, Norway

Per-hjalmar.lehne@telenor.com

Frode.bohagen@telenor.com

23 Jan 2008

OFDM Competence Development

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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

map

pin

g

+C

P, D

/A+

RF

Channel

RF+

A/D

, -CP

NC-p

oin

t DFT

SC

de-m

ap

pin

g

NC-p

oin

t IDFT

NCNCN NN-p

oin

t DFT

N-p

oin

t 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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OFDM Competence Development

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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-FDMA

OFDMA

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