Transcript OFDMA for wireless communications - Part I

OFDM(A) Competence Development – Part I
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
OFDM Competence Development
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OFDM Basic Concept
• Orthogonal Frequency Division Multiplexing (OFDM) is a
multi-carrier modulation scheme
– First break the data into small portions
– Then use a number of parallel orthogonal sub-carriers to transmit
the data
• Conventional transmission uses a single carrier, which is
modulated with all the data to be sent
Single Carrier Company
Multi Carrier Company
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OFDM Basic Concept
•
OFDM is a special case of
Frequency Division Multiplexing
(FDM)
•
For FDM
– No special relationship between
the carrier frequencies
– Guard bands have to be inserted
to avoid Adjacent Channel
Interference (ACI)
•
For OFDM
– Strict relation between carriers:
fk = k·Df where Df = 1/TU
(TU - symbol period)
– Carriers are orthogonal to each
other and can be packed tight
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OFDM Transmission model
Channel, h(t)
Modulator
and transmitter
Wireless channel
Receiver and demodulator
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Orthogonality – the essential property
• Example: Receiver branch k
– Ideal channel: No noise and no multipath
N c 1
aq

1  N c 1
j 2 qDft
 j 2 kDft
  aq  e
e
dt  



TU 0  q 0
q  0 TU

TU
TU
e
0
j 2  q  k 
1
t
TU
a k , k  q
dt  
 0, k  q
Received signal, r(t)
Tu = 1/Df gives subcarrier orthogonality over one Tu
=> possible to separate subcarriers in receiver
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OFDM – Signal properties
Frequency domain
Time domain
Power Spectrum for OFDM symbol
frequency
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OFDM – Signal properties
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Multipath channel
Diffracted and Scattered Paths
[ k , k ]
LOS Path
[ 0 , 0 ]
[1 , 1 ]
Reflected Path
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Multipath channel (cyclic prefix)
Multipath introduces inter-symbol-interference (ISI)
TU
Prefix is added to avoid ISI
Example multipath profile
TCP
TU
Amplitude
[]
0
1
2
Time
[]
The prefix is made cyclic to avoid inter-carrier-interference (ICI)
(maintain orthogonality)
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Multipath channel (cyclic prefix)
• Tcp should cover the maximum length of the time
dispersion
• Increasing Tcp implies increased overhead in power and
bandwidth (Tcp/ TS)
• For large transmission distances there is a trade-off
between power loss and time dispersion
TS
CP
Useful symbol
CP
Tcp
Useful symbol
CP
Useful symbol
TU
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Multipath channel (frequency diversity)
•
The OFDM symbol can be exposed to a frequency selective
channel
•
The attenuation for each subcarrier can be viewed as “flat”
– Due to the cyclic prefix there is no need for a complex equalizer
•
Possible transmission techniques
– Forward error correction (FEC) over the frequency band
– Adaptive coding and modulation per carrier
=
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Multipath channel (frequency diversity)
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Multipath channel (pilot symbols)
•
The channel parameters can be estimated based on known
symbols (pilot symbols)
•
The pilot symbols should have sufficient density to provide
estimates with good quality (tradeoff with efficiency)
•
Different estimation methods exist
– Averaging combined with interpolation
– Minimum-mean square error (MMSE)
Pilot carriers /reference signals
Data carriers
Time
Frequency/subcarrier
Pilot symbol
Frequency
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The Peak to Average Power Problem
•
A OFDM signal consists of a number of independently modulated
symbols
•
The sum of independently modulated subcarriers can have large
amplitude variations
x(t ) 
N c 1
a
k 0
•
k
 e j2 kDf t
Results in a large peak-to-average-power ratio (PAPR)
PA
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The Peak to Average Power Problem
•
Example with 8 carriers and
BPSK modulation
– x(t) plotted
•
It can be shown that the PAPR
becomes equal to Nc
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The Peak to Average Power Problem
•
High efficiency power amplifiers
are desirable
AM/AM characteristic
– For the handset, long battery life
– For the base station, reduced
operating costs
•
A large PAPR is negative for the
power amplifier efficiency
•
Non-linearity results in intermodulation
– Degrades BER performance
POUT
OBO
IBO
– Out-of-band radiation
PA
Average
Peak
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PIN
The Peak to Average Power Problem
• Different tools to deal with large PAPR
– Signal distortion techniques
Clipping and windowing introduces distortion and out-of-band
radiation, tradeoff with respect to reduced backoff
– Coding techniques
FEC codes excludes OFDM symbols with a large PAPR
(decreasing the PAPR decreases code space). Tone reservation,
and pre-coding are other examples of coding techniques.
– Scrambling techniques
Different scrambling sequences are applied, and the one
resulting in the smallest PAPR is chosen
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OFDM Synchronization
• Timing recovery
– No problem if offset is within D
max
D
CP
Useful symbol
Integration period, TU
• Frequency synchronization
– A carrier synchronization error will introduce phase
rotation, amplitude reduction and ICI
– Frequency offsets of up to 2 % of Df is negligible
– Even offsets of 5 – 10 % can be tolerated in many
situations
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Choosing the OFDM parameters
• Symbol time (TU) and subcarrier
spacing (Df) are inverse
– TU = 1/Df
• Consequences of increasing the
subcarrier spacing
Increase CP
overhead
Increasing
subcarrier spacing
– Increase cyclic prefix overhead
• Consequences of decreasing the
subcarrier spacing
TU
Decreasing
subcarrier spacing
– Increase sensitivity to frequency
inaccuracy
– Increasing number of subcarriers
increases Tx and Rx complexity
Increase sensitivity to
frequency accuracy
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Summary
•
Advantages
– Splitting the channel into narrowband channels enables significant
simplification of equalizer design
– Effective implementation possible by applying FFT
– Flexible bandwidths enabled through scalable number of subchannels
– Possible to exploit both time and frequency domain variations (time
domain adaptation/coding + freq. domain adaptation/coding)
•
Challenges
– Large peak to average power ratio
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Summary
Pilot carriers /reference signals
Data carriers
Frequency/subcarrier
PA
CP
Channel, h(t)
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