080123 1 OFDM(a) Competence Development PartI Final

OFDM(A) Competence Development – Part IPer Hjalmar Lehne, Frode Bøhagen, Telenor R&I

R&I seminar, 23 January 2008, Fornebu, Norway

Per-hjalmar.lehne@telenor.com

Frode.bohagen@telenor.com

23 Jan 2008

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 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 – 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