Transcript Introducing multiple access: OFDMA, SC-FDMA

OFDM(A) Competence Development – part II
Per Hjalmar Lehne, Frode Bøhagen, Telenor R&I
R&I seminar, 23 January 2008, Fornebu, Norway
[email protected]
[email protected]
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
Distributed (localized)
(diversity) mapping
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 timefrequency 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
•
•
N 1
Linear precoding of OFDMA symbols
Xk 
N < NC subcarriers are allocated to one user
x
n
e
j
n 0
–
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
NC
NC
SC de-mapping
NC-point DFT
RF+A/D, -CP
+CP, D/A+RF
NC-point IDFT
SC mapping
N-point DFT
Channel
DFT-spread OFDMA
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N-point IDFT
N
2
kn
N
N
Single-Carrier (SC) FDMA
• Special case of DFT-spread OFDMA with contiguous subcarrier 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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Benefit of the SC-FDMA signal
•
Reduces PAPR with 2-3 dB
N = 64, M = 256, QPSK
N = 64, M = 256, 16-QAM
~2 dB
Source: Myung et al. Peak-to-average power ratio of single carrier
FDMA signals with pulse shaping. PIMRC 2006
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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
Inst. PSD (4 symbols), N=1024, M=128
16 QAM 1/2, Red: OFDMA, Blue:IFDMA, FFT size:1024, M=128
0
10
10
SC-FDMA
OFDMA
0
-10
IFDMA
PER
-20
-1
10
3 dB loss
-30
-40
OFDMA
-50
-2
10
4
6
8
10
12
14
16
18
av. SNR per subcarrier(dB)
20
22
24
-60
-2000
-1500
-1000
-500
0
subcarrier
Source: Alamouti. Mobile WiMAX: Vision & Evolution.
Intel presentation. 2007
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500
1000
1500
2000
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
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