OFDM(A) Competence Development – part II
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Transcript of OFDM(A) Competence Development – part II
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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