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Wireless Communication

Session 5

Modulation & Multiple Access

M. Daneshvar Farzanegan

Soourosh.blogfa.com

smdanesh@yahoo.com

IIT Madras2

TDMA (with FDMA) Principle

Power

Time

Freq.

Time-slots

Carriers

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Direct Sequence CDMA Principle

(with FDMA)

Power

Time

Freq.

User Code

Waveforms

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OFDM (with TDMA & FDMA) Principle

Power

Time

Freq.

Time-slots

Carriers

Tones

IIT Madras5

Other Multiple Access Techniques

Multi-Carrier TDMA

DECT, PACS

Frequency Hopped Spread Spectrum

Bluetooth

CSMA/CA

IEEE 802.11 (1 or 2 Mbps standard)

DS-CDMA with Time Slotting

3GPP W-CDMA TDD (Time Division Duplex)

Packet Switched Air Interface is vital for high bit-rates

and high capacity (for data users) -- GPRS, DPRS, etc.

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What is an OFDM System ?

Data is transmitted in parallel on multiple carriers that

overlap in frequency

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FEC IFFT

DAC

Linear

PA

add cyclic extension

bits

fc

OFDM symbol

Pulse shaper

&

view this as a time to

frequency mapper

Generic OFDM Transmitter

Complexity (cost) is transferred back from the digital to the analog domain!

Serial to

Parallel

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Add

Cyclic

Prefix

Serial/

Parallel

]0,[ns

]1,[ns

],[ Nns

Parallel/

SerialIFFT

]0,[nd

]1,[nd

],[ Nnd

OFDM Transmitter -- contd.

S/P acts as Time/Frequency mapper

IFFT generates the required Time domain waveform

Cyclic Prefix acts like guard interval and makes equalization easy (FFT-cyclic

convolution vs. channel-linear convolution)

1

0

2

],[1

],[N

k

N

kij

eknsN

ind

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OFDM Receiver

Cyclic Prefix is discarded

1

0

2

],['1

],[N

i

N

ikj

eindN

knr

FFT

]0,[nr

]1,[nr

],[ Nnr

Parallel/

Serial

Serial/

Parallel

Remove

Cyclic

Prefix

]0,[' nd

]1,[' nd

],[' Nnd

FFT generates the required Frequency Domain signal

P/S acts like a Frequency/Time Mapper

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AGC

fc

VCO

Sampler FFTError

gross offset

Slot &

fine offset

Freq. Offset

Estimation

Timing

Sync.

(of all tones sent in one OFDM symbol)

Generic OFDM Receiver

RecoveryP/S and

Detection

IEEE Symp./ IISc -2001 IIT Madras11

OFDM Basics

To maintain orthogonality where

• = sub-carrier spacing

• = symbol duration

If N-point IDFT (or FFT) is used

Total bandwidth (in Hz) =

= symbol duration after CP addition

fTs

1

f

sT

fNW

CPS TT

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Condition for Orthogonality

Time

T

Base frequency = 1/T

T= symbol period

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OFDM Basics -- contd.

If the Cyclic Prefix > Max. Delay Spread, then

the received signal after FFT, at the nth

tone for the kth OFDM block can be

expressed as

where

is additive noise

is channel frequency response

],[],[],[],[ knwknsknHknr

],[ knw

],[ knH

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Tx Waveform over a OFDM Symbol

(magnitude values, for 802.11a)

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Sync Basis Functions(of equal height for single-ray channel)

Shape gets upset by

(a) Fine Frequency Offset

(b) Fading

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OFDM -- PHY layer tasks

Signals sent thro’ wireless channels encounter one or moreof the following distortions:

additive white noise

frequency and phase offset

timing offset, slip

delay spread

fading (with or without LoS component)

co-channel interference

non-linear distortion, impulse noise, etc

OFDM is well suited for high-bit rate applications

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Frequency Offset

Carrier recovery and tracking critical for OFDM

Offsets can be comparable to sub-carrier spacing in OFDM

Non-coherent detectors possible with differential coding

Residual freq. offset causes

constellation rotation in TDMA

loss of correlation strength over integration window in CDMA(thereby admitting more CCI or noise)

increased inter-channel interference (ICI) in OFDM

OFDM can easily compensate for gross freq. offsets (offsetswhich are an integral multiple of sub-carrier width)

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Timing Synchronisation

Timing recovery (at symbol level) is easily achieved in OFDMsystems

Can easily overcome distortions from delay spread

Can employ non-coherent timing recovery techniques by introducingself-similarity

• => very robust to uncompensated frequency offsets

If cyclic prefix is larger than the rms delay spread, range of (equallygood) timing phases become available

• => robust to estimation errors

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Slot and Timing Synchronization in OFDM

Example: 4 tones per slot (OFDM symbol)T

self-symmetry can be

exploited for non-

coherent timing recovery

zero tones

IFFT PA

T secs

t

IFFT PA

T secs

t

T/2 T

Traffic Slot

Preamble/Control Slot

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Effect of Delay Spread

Typical rms delay spread in macro-cells

• Urban : 1-4 msecs, Sub-urban : 3-6 msecs

• Rural (plain, open country) : 3-10 msecs

• Hilly terrain : 5-15 msecs

TDMA requires equalization (even if rms delay spread is only20-30% of symbol duration)

higher bit-rates would imply more Inter-Symbol Interference (ISI)

therefore, equalization complexity increases with bit rate

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Effect of Delay Spread -- contd. 1

Effect of delay spread on DS-CDMA is multi-fold

On the Uplink, the time diversity inherent in the delay spread canbe used to mitigate fading

On the Downlink, multipath delay spread upsets channelization(short) code orthogonality

Sectorisation vital in CDMA to reduce CCI on the Uplink

However, sectorisation reduces delay spread as well, therebyreducing the RAKE performance

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Effect of Delay Spread in OFDM

Delay spread easily compensated in OFDM using :

Cyclic Prefix (CP) which is longer than the delay spread

Thereby, converting linear convolution (with multipath channel) toeffectively a circular convolution• enables simple one-tap equalisation at the tone level

However, the frequency selectiveness could lead to certain tones

having very poor SNR=> poor gross error rate performance

Data Payload CP

3.2msecs 0.8msecs

Example: IEEE 802.11 A (and also in HiperLAN)

IEEE Symp./ IISc -2001 IIT Madras23

Delay Spread Compensation in OFDM

Two basic ideas to combat freq. selectivity in OFDM

Feed-forward only techniques

• Temporal FEC and interleaving

• Transmit diversity and space-time coding

Feed-back based techniques (similar to approaches used inMulti-Carrier Modulation in the ADSL modems)

• Water-pouring (bit-loading)

• Pre-equalisation or pre-distortion

Sectorisation in macro-cell OFDM can help reduce delayspread

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AGC

Sampler DFTError

-- Gross Freq. Offset

-- Channel Estimation

and Equalization

OFDM Receiver Algorithms -- Recap

RecoveryP/S and

Detection

Freq.

-- Fine Freq. Offset

-- Timing Estimation

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Conventional

OFDM

Frequency Domain Equalization

-- Conventional OFDM

DFTFrequency

Domain

Equaliser

Remove

CPRx

Algos.

Detection

& P/S

IDFTAdd

CPTx

Mod.

Symbol

Mapping

& S/P

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Tx -- low-complexity, TDMA

Rx -- implements SC-FDE;

Linear Equaliser or DFE

Frequency Domain Equalization

-- Single Carrier FDE (SC-FDE)

DFTFrequency

Domain

Equaliser

Remove

CPRx

Algos.DetectorIDFT

Add

CP

(of symbols)

Tx

Mod.

Symbol

Mapping

to permit FDE

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TDE + FDE

for OFDM

Time & Frequency Domain Equalization

-- for OFDM in large delay spread channels

DFTFrequency

Domain

Equaliser

Remove

CPRx

Algos.

Detection

& P/S

IDFTAdd

CPTx

Mod.

Symbol

Mapping

& S/P

Time-

Domain

Equaliser

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Fading and Antenna Diversity

Short-term fading exhibits spatial correlation

• Two antennas, spaced l/4 meters or greater apart, fade

independently

• Spatial diversity combining can mitigate fading

– Switch diversity (least complex, modest improvement)

– Selection diversity

– Equal gain combining

– Maximal ratio combining (most complex, optimal)

TDMA, CDMA, and OFDM systems will invariably require

antenna diversity to overcome fading

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Fading and Channel Estimation

Use of midamble in GSM and EDGE to avoid channel tracking

within the slot duration

Unlike in TDMA and OFDM, fading affects not only signal

quality, but also system capacity in DS-CDMA

• Fast closed-loop power control required which can track

short-term fading

• For RAKE combining, multipath delays and gains are

required to be estimated and tracked

– By using orthogonal signaling, IS-95 uplink does not need gain

estimation, but requires delay estimation

In OFDM systems, the long symbol duration makes channel

estimation and tracking very important

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Channel Estimation in OFDM -- Example

Traffic slots may contain a few equally spaced tones for phase correction(due to residual freq. offset, phase noise, fading)

Control slot may also contain MAC messages

Frame (say, 4 slots)

Control +

Training SlotTraffic Slot 1 Traffic Slot 3Traffic Slot 2

Phase

Correction

Tones

Training

Tones

(for channel

identification)

MAC message

(broadcast)

Control +

Training Slot

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Fading Compensation in OFDM

OFDM using a FDE, observes only “flat” fading at the sub-

carrier level

Fading manifests as ICI terms in the Frequency Domain

In OFDM Phy Layer, two basic ways to reduce ICI

Reduce OFDM symbol duration (increase sub-carrier width)– 802.16 has FFT sizes ranging from 256 to 4096

• Transmit pulse shaping can reduce ICI

– (by providing excess “time-width”)

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Other PHY Issues in OFDM

High peak-to-average ratio of the signal envelope

Linear Power Amp., with 5-8dB back-off required (costly)

To support mobility (fast fading) it will require

More training tones per symbol and also in every slot

Tx diversity and/or ST coding support

Exploit time, frequency, and space diversity / processing

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Phy Layer Issues in Macro-cell OFDM

Macrocells will require larger cyclic extensions / prefix

Microcells may not be economical during initial deployment

GPS locked base stations required

To control ACI from neighbor BS sites (at cell edge)

CCI can be estimated / controlled only if it is tone-aligned

Strict power control required may be required on uplink

To minimize cross-talk between tones of different users

sharing the same OFDM symbol (time slot)

To avoid uplink power control

• allocate only one user per uplink slot

• or, make uplink a pure TDMA (not OFDM)