080123 2 OFDM(a) Competence Development PartII Final

8/13/2019 080123 2 OFDM(a) Competence Development PartII Final

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

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

[email protected]

[email protected]


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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


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OFDMAOrthogonal Frequency Division MultipleAccess

OFDM can be used as a multiple access scheme allowing simultaneousfrequency-separated transmissions to/from multiple mobile terminals

The number of sub-carriers can be scaled to fit the bandwidth ScalableOFDMA

Contiguous (localized) mappingDistributed (diversity) mapping


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Subcarrier allocation techniques (I)

Contiguous or blockwisemapping

Adjacent sub-carriers

Frequency selective fadingcan erase a full block

For satisfactory performanceit must be combined withdynamic scheduling orfrequency 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 selectivefading

The scheduled user isalways allocated the

best time-frequencyblock

Channel variesdifferently for differentusers


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Synchronisation aspects

Impairments in time- and frequency synchronization reducesperformance: 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< NCsubcarriers are allocated to one user

An N-point Discrete Fourier Transform (DFT) is applied

New output symbols (Xk) are linear combinations of all Ninput 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

+C

P,D/A+RF

Channel

RF

+A/D,-CP

NC

-pointDFT

SC

de-mapping

NC-pointIDFT

NCNCN NN

-po

intDFT

N-po

intIDFT

OFDMADFT-spread

1

0

2N

n

knN

j

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 Ninput symbols are spread over all Nsubcarriers

All Nsubcarriers are modulated with a weighted sum of all Ninput symbols

The DFT/IDFT pair in the transmitter cancel each other outretaining 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 destroyedorthogonality

Out-of-band emission problem due to higher PAPR in thefrequency domain

4 6 8 10 12 14 16 18 20 22 2410

-2

10-1

100

av. SNR per subcarrier(dB)

PER

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 allowingsimultaneous frequency separated transmissions to andfrom multiple mobile terminals

Subcarriers can be allocated blockwise or distributed

Channel dependent scheduling can be used todynamically allocate frequency/time blocks to differentusers

Terminals must be sufficiently time and frequencysynchronised to avoid multiple access interference on theuplink

DFT spread OFDMA is beneficial in reducing the PAPRproblem employed by 3GPP E-UTRA on the uplink

080123 2 OFDM(a) Competence Development PartII Final

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