080123 2 ofdm(a) competence development partii final
TRANSCRIPT
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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
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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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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