080123 1 ofdm(a) competence development parti final
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OFDM(A) Competence Development – Part IPer Hjalmar Lehne, Frode Bøhagen, Telenor R&I
R&I seminar, 23 January 2008, Fornebu, Norway
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OFDM Competence Development
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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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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·f where f = 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
Tu = 1/f gives subcarrier orthogonality over one Tu
=> possible to separate subcarriers in receiver
qk,0
qk,adte
T
adteea
T
1 k1N
0q
T
0
tT
1kq2j
U
qT
0
ftk2j1N
0q
ftq2jq
U
c U
U
U c
Received signal, r(t)
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OFDM Competence Development
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OFDM – Signal properties
Time domain
Frequency domain
Power Spectrum for OFDM symbol
frequency
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OFDM – Signal properties
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Multipath channel
],[ 00
],[ 11
Diffracted and Scattered Paths
Reflected Path
LOS Path
],[ kk
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Multipath channel (cyclic prefix)
Time[]
Amplitude []
Example multipath profile
0 1 The prefix is made cyclic to avoid inter-carrier-interference
(ICI) (maintain orthogonality)
Multipath introduces inter-symbol-interference (ISI)
TU
Prefix is added to avoid ISITUTCP
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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
CP Useful symbol CP Useful symbolCP Useful symbol
TUTcp
TS
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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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Frequency/subcarrier
Pilot carriers /reference signals
Data carriers
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 symbol
Time
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
• Results in a large peak-to-average-power ratio (PAPR)
1N
0k
tfk2jk
c
ea)t(x
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
– 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 inter-modulation
– Degrades BER performance
– Out-of-band radiation
PA
PIN
POUT
IBO
AM/AM characteristic
OBO
Average Peak
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The Peak to Average Power Problem
• Different tools to deal with large PAPR
– Signal distortion techniquesClipping and windowing introduces distortion and out-of-band radiation, tradeoff with respect to reduced backoff
– Coding techniquesFEC 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 techniquesDifferent 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
• Frequency synchronization
– A carrier synchronization error will introduce phase rotation, amplitude reduction and ICI
– Frequency offsets of up to 2 % of f is negligible
– Even offsets of 5 – 10 % can be tolerated in many situations
max
CP Useful symbol
Integration period, TU
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Choosing the OFDM parameters
• Symbol time (TU) and subcarrier spacing (f) are inverse
– TU = 1/f
• Consequences of increasing the subcarrier spacing
– Increase cyclic prefix overhead
• Consequences of decreasing the subcarrier spacing
– Increase sensitivity to frequency inaccuracy
– Increasing number of subcarriers increases Tx and Rx complexity
Increasing subcarrier spacing
Decreasing subcarrier spacing
Increase sensitivity to frequency accuracy
TU
Increase CP overhead
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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 sub-channels
– 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
Channel, h(t)
PACP
Frequency/subcarrier
Pilot carriers /reference signals
Data carriers