OFDM/OFDMA for 4G Wireless Systems

1OFDM/OFDMAfor 4G wireless systems

IEEE LATINCOM 2011Belem do ParOctober 25th, 2011

Presenter: Renny Badra, Ph.D.Universidad Simn BolvarCaracas, [email protected]

2In this tutorial

SECTION 1: Back to Basics SECTION 2: TDMA SECTION 3: CDMA SECTION 4: OFDM/OFDMA SECTION 5: WiMAX and LTE SECTION 6: Conclusion

R. Badra: OFDM/OFDMA for Wireless Communications

3SECTION 1Back to Basics


4Shannon capacity of a channel

C = W log2 (1+S

N)

C is the maximum achievable information transfer rate [bps]W is the channels bandwidth [Hz]S/N is the channels signal-to-noise ratio

Shannon capacity is a theoretical limit It can only be achieved when both transmitted

signal and additive noise are Gaussianprocesses


5Central limit theorem Given a random variable Z obtained by

adding N independent random variables:

For large N, the distribution of Z approachesthe Gaussian distribution, no matter whatthe distributions of Xi are.!

Z = Xi

i=1

N

"


6The mobile wireless channel

01diffractionreflection

dispersionmotion

multipathdistance-dependent attenuation


7Flat Fading (viewed in time)Power

Time

Symbol duration Ts

Symbol duration Ts >> Channel delay spread

Channel delay spread is a measure of the channels energy dispersion

propagation paths


8Flat Fading (viewed in frequency)

Approximation corresponding to 50% correlation coherence bandwidth

Signal Bandwidth

FrequencyResponse ofthe Channel

frequency

Signal Bandwidth B

!

B > TS

Time-Domain Condition:

timeTc

one symbol

Coherence Time

Rs >> FDoppler

Frequency-Domain Condition:

Fdoppler = v /


12

Fast Fading Channel coherence time is comparable to

or less than one symbol time Symbols are severely distorted Special adaptive receiver techniques are

requiered Fast fading needs to be avoided in

terrestrial wireless communications


13

Typical limits for symbol rate

50 kbaud2.5 kbaudTypical Urban,2300 MHz

35 kbaud2.5 kbaudHighly dispersive,2300 MHz

50 kbaud1 kbaudTypical Urban,900 MHz

35 kbaud1 kbaudHighly dispersive,900 MHz

Upper limit (**) (toensure flat fading)

Lower limit (*) (toensure flat fading)

Conditions

(*) assuming a vehicle speed of 120 kph.(**) assuming a delay spread of 4 - 7 S.


14

Frequency reuse in cellularsystems

BCA

DE

FG

BCA

DE

FG

BC

DE

FG

A

Reuse Factor R=7FDMA, TDMA

AAA

AA

AA

AAA

AA

AA

AA

AA

AA

A

Universal Frequency Reuse R=1CDMA

cluster


15

Power in

Power outMax Output Power

1 dB1-dB compression point

Max Average PowerCrest Factor

PAPR = Crest Factor - 3 dB

Peak-to-Average Power Ratio(PAPR)

Lowest PAPR is 0 dB (unmodulated carriers, constant-envelopemodulated signals).

Higher PAPR values mean lower maximum average transmitpower for a given power amplifier. PAPR is actually a power LOSS in the uplink power budget.

Power Amplifier Output Characteristic


16

History of Cellular systems

cdmaOne

PDC

TDMA(IS-136)

2G

GSM

CDMA2000

3G

EDGE

UMTS

AMPS

NMT,Radiocom,TACS, etc.

NTT,JTACS

1G IMTAdvanced

LTE

WiMAX

UMB

GPRS

EVDO

HSPA

NAMPS

LTE-A

B3G

WiMANA

3.5 G 4G


17

SECTION 2TDMA


18

Time Division Multiple AccessTDMA (2G)

frequency

time

Users are assigned periodictime slots in a TDMA frame

N slots per frame mayserve up to N users

Slot allocation decided atbase station controller(centralized control)

Current systems:GSM/GPRS/EDGE


19

TDMA radio resourceaggregation

TDMA frame

1 2 N

1 2 N

1 2 N

::

Data users may be assigned more thanone time slot in order to increase data rate


20

TDMA symbol rate,MS average power

TDMA frame

1 2 N

one TDMA slot

one symbol

user symbol rateRu

over-the-airsymbol rateROTA=N Ru

MS Average PowerPave =Ppeak /N


21

Interference in cellular TDMA Number of interference sources (both up

and downlink) in cellular TDMA is small.

112Sectorized(60o)

223Sectorized(120o)

666Omni (360o)

Freq. ReuseFactor N=7

Freq. ReuseFactor R=4

Freq. ReuseFactor R=3

Antenna type

Number of dominant interference sources in a cellular FDMA/TDMAsystem assuming a regular hexagonal cell grid (up/downlink)


22

TDMA capacity Being an orthogonal multiple access scheme, capacity

is only limited by other-cell interference and thermalnoise.

However, due to small number of interferers,interference is far from being Gaussian and thereforeShannon capacity cannot be approached Central Limit Theorem does not apply.

Also, average power per user is reduced by a factor ofN (number of slots per frame), which impacts uplinkbudget. Ultrawideband TDMA (5-20 MHz) would require very high

values of N (100 or more) for fine granularity.R. Badra: OFDM/OFDMA for Wireless Communications

23

TDMA Summary (1/2) Radio resource (RR) granularity is poor and

allocation mechanism is slow RR aggregation possible but limited to N slots per

users (frequency channel aggregation not practical). Because of periodic slot assignment, RR allocation

scheme well suited for circuit-switched voice, butnot for packet-switched data.

High symbol rate induces frequency selective fading(equalizers typically needed in GSM/GPRS/EDGE,with symbol rate 270 ksps)


24

TDMA Summary (2/2) PAPR is 0 dB (GMSK/8PSK modulation), but

average power reduced by a factor of N Adaptive coding (GPRS/EDGE) and modulation

(EDGE only) used to compensate for varying SNRacross cells.

Theoretical capacity is high because oforthogonality

However, Shannon capacity cannot be trulyapproached because of reduced number ofinterferers.


25

SECTION 3CDMA


26

Code Division Multiple AccessCDMA (3G)

Users are assigned different codes Tight power control needed to manage self-interference Variable spreading factor allow for variable data rates per code RR assignment is centralized at base station controller

frequency

time

code

Current systems:WCDMA (UMTS)CDMA2000 1X


27

code aggregation

Data rate per code

1 2 3

CDMA radio resourcegranularity

Two dimensions for data rate granularity Code aggregation rarely used in 3G CDMA, but frquently used

in 3.5G CDMA. Modulation is fixed (BPSK or QPSK) (PAPR is 0 dB). Code aggregation in the uplink increases Peak-to-

Average Power Ratio (PAPR)

R2

R1

RU=R1+R2


28

CDMA symbol rate,MS average power

one symbol = S chipsS is the spreading factor

1

one symbol

user symbol rateRu

over-the-airsymbol rate

ROTA=Ru

2 S

one chip

over-the-airchip rateRch=S Ru

user symbol stream

MS Average PowerPave = (1-) Ppeak is the fraction of power allocated

to overhead code channels R. Badra: OFDM/OFDMA for Wireless Communications

29

Multipath Treatment in CDMA

Base Station

SEARCHER

DEMODULATOR

DEMODULATOR

DEMODULATOR

CO

MB

INER

RAKE RECEIVER

Multipath coherent combining improves SNR indispersive channels


30

CDMA multipath resistancepropagation paths

time

Received signalcomponents

time

Channelpower profile

Local despreading code at receiver

Resulting channelpower profile


31

CDMA Over-the-air symbol rateSystem Chip rate Minimum

SpreadingFactor

Maximum Over-the-airSymbol rate

WCDMA (UMTS) 3.84 Mcps 4 chips/symb 960 kbaudHSDPA 3.84 Mcps 16 chips/symb 240 kbaudHSUPA 3.84 Mcps 2 chips/symb 1920 kbaud

CDMA2000 1X downlink 1.2288 Mcps 4 chips/symb 307 kbaudCDMA2000 1X uplink 1.2288 Mcps 2 chips/symb 614 kbaud

CDMA2000 EVDO downlink 1.2288 Mcps 16 chips/symb 77 kbaudCDMA2000 EVDO uplink 1.2288 Mcps 4 chips/symb 307 kbaud

Multipath resistance allows for equalizer-free operation, even at highover-the-air symbol rates

However, higher spreading factors provide better multipath resistance.R. Badra: OFDM/OFDMA for Wireless Communications

32

CDMA capacity Orthogonal CDMA (downlink)

codes for different users are orthogonal and do not interferewith each other.

high number of interferers due to universal frequency reuse(six times as many as in TDMA with R=7).

however, multipath components and other-cell interference arenot orthogonal and therefore limit performance.

Non-orthogonal CDMA (uplink) codes from different users cannot be practically aligned and

therefore are not orthogonal. all same-cell and other-cell signals are sources of interference. due to very high number of interferers, interference

approaches Gaussian distribution and Shannon is approachedR. Badra: OFDM/OFDMA for Wireless Communications

33

CDMA compared to TDMA Voice capacity in CDMA is 5-8 times that of

TDMA Many independent sources of interference Tight power control Better exploitation of voice activity factor

However, TDMA (GSM) has prevailed More comprehensive standards have lead to more

products, better compatibility and economies of scale SIM card concept facilitates roaming and product

migrationR. Badra: OFDM/OFDMA for Wireless Communications

34

3G CDMA summary (1/2) Fine RR granularity due to two-dimensional bit

rate adjustment with code aggregation, PAPR becomes an issue

RR assignment still slow because of centralizedcontrol

Better tolerance to multipath than TDMA Over-the-air symbol rate equals user symbol rate per

code Interchip interference not an issue (all chips in one

symbol are modulated by same data)


35

3G CDMA summary (2/2) Better multipath resolution due to short chip time

(exploited through rake receivers) improves SNR. Better mobile power efficiency ( only 10-30%). Higher capacity than TDMA due to:

Downlink: orthogonal codes and higher number ofinterference sources

Uplink: tight power control and very high number ofinterferers.

Tight closed-loop power control used tocompensate for varying SNR across the cell.


36

Voice vs. Data

Data traffic has seen exponential growth Data revenues is still less than voice But voice will eventually become data (VoIP) Cellular data technologies will be key

time

Traffic

data

voice


37

3.5G CDMA/Statistical TDMA(downlink)

Current systems:Downlink HSPA (HSDPA)Downlink EVDO

HSDPA: Codes are assigned on demand

to potentially different users 1-15 codes Time resolution is 2 ms

Downlink EVDO: All codes are assigned on

demand to one user 16 codes Time resolution is 1.67 ms

1-15

code

s16

code

s

time

time


38

3.5G CDMA/Statistical TDMA(downlink) Distributed RR management: key MAC

decisions are made at the base station RR allocation is much faster

Adaptive modulation and coding based on fastchannel quality reporting

Proportional-fair packet scheduler at basestation optimizes capacity Multiuser diversity improves cell throughput


39

1. seal piloto deambos sectores

1. Mobile monitors pilot channel2. Mobile reports channel quality3. Base station adapts transmission

123

Channel Quality Estimation


40

Multiuser diversity

tiempo

tiempo

tiempo

tiempoAdaptive Packet Schedulling selects

best link


41

3.5G CDMA (uplink)

Distributed RR management: some of the RR allocation decisionsare made by mobile units based on status information broadcast bycell RR allocation is less deterministic, leaner and faster

Code aggregation use: In HSUPA, in order to increase data rate In uplink EVDO, in order to multiplex data and control channels

Remaining features of 3G CDMA still apply

Current systems:Uplink HSPA (HSUPA)Uplink EVDO


42

3.5G CDMA summary Statistical TDMA provides excellent RR granularity

Medium access is optimized for packet data RR assignment much faster because of distributed

medium control Some control decisions performed at cells and mobiles

Limited Scalability Multicarrier CDMA used in HSPA+ and EVDO RevB (2 carriers)

CDMA signal maintains good tolerance to multipath Different techniques used to compensate for varying

SNR across the cell: Downlink: rate control via adaptive modulation and coding Uplink: closed-loop fast power control


43

SECTION 4OFDM/OFDMA


44

Conventional vs. Orthogonalmulticarrier techniques

independent symbol streams overindependently generated carriers

synchronized symbol streams over harmonically related subcarriers

freq

freq

f1 f2 f3 f4 f5 f6

k f0(k+1)f0

(k+2)f0(k+3)f0


45

OFDM concept

rb

r1

r2

rN

Modulator@ F1

Bit d

istrib

utio

n Modulator@ F2

Modulator@ FN

+OFDM Signal

M-PSK or M-QAM

!

ri

i=1

N

" = rb


46

OFDM signal features All subcarriers operate at the same symbol rate,

but (possibly) at different bit rates due to differentmodulation indices M. Channel coding rates can also be different among

subcarriers. Adaptive techniques compensate varying SNR.

Each symbol time contains an integer number ofcycles of every subcarrier:

Ts = nK / FK , K=1,2,, N, nK+1 = nK +1, nK integer. These features ensure orthogonality among

subcarriers.R. Badra: OFDM/OFDMA for Wireless Communications

47

OFDM subcarriers

time

one OFDM symbol time

freq

no Inter-Carrier Interference (ICI)R. Badra: OFDM/OFDMA for Wireless Communications

48

OFDM realization using FFT(1/2)

IFFTMOD S/Pdata

P/S Tx

pilots

CHANNELCODING to

channel


49

OFDM realization using FFT(2/2)

FFTDEMODS/P

to channelestimation

P/SRx

pilots

from channel

CHANNELDECODING data


50

Scalability in OFDM By varying the length of the IFFT/FFT

operations, OFDM can be easily scaled Carrier separation is maintained

Scalability facilitates adaptation todifferent spectrum availability options

IFFT/FFT size always a power of 2 Computational complexity is greatly reduced

by using FFT algorithm


51

Interference immunity in OFDM

Narrowband interferers disable only asubset of subcarriers.

Disabled subcarriers may still be availableto other terminals

F1 F2 F3 FN... ... frequency

narrowband interferencedisabled subcarriers


52

Flat fading in OFDM

Narrowband OFDM subcarriers do not suffersignificant frequency selective fading

Guard time completely eliminates any residualdistortion (ISI)

F1 F2 F3 FN... ... frequencyrespuesta del canal


53

Guard time in OFDM

receiver integration time = 1 / carrier spacing Fguard time Tg

symbol time Ts

Ts = Tg + 1/F

cyclic extensions


54

Multipath immunity in OFDMwith guard time

1/FTgTs

first arriving path

reflectiondelay

Ts

reflection

Ts

55

OFDMA (downlink LTE/WiMAX)

User1 User2 User3 User4 Idle

SUC

CAR

RIE

RS

Symbol block T time

Smallest theoretical resource allocation (granularity) is T [ms] x F [kHz]

F


56

PAPR problem in OFDM

4 co-phased,unit-amplitude, OFDM subcarriers(symbol time = 64)

Instant power

average power

peak power = 16average power = 2crest factor = 16/2 = 8PAPR = 4 (6 dB)


57

DFT-spread OFDM or SingleCarrier OFDM (SC-FDM) (1/2)

IFFTP/S Tx

MOD S/Pdata CHANNELCODING to

channel

DFT

back and forth between time and frequency domainsoperations sort-of cancel each other (depending on IFFT mapping)as a result, symbols are sequentially embedded in IFFT outputcyclic prefix maintains ISI immunity


58

FFTS/PRxfrom

channel DEMODP/SCHANNEL

DECODING dataIDFT

DFT-spread OFDM or SingleCarrier OFDM (SC-FDM) (2/2)

FDE

one-tap frequency-domain equalization (FDE) neededSC-FDM exhibits lower PAPR than OFDM (about 3 dB)SC-FDM therefore preferred for uplink (LTE)


59

OFDMA (uplink LTE)

IFFTP/S Tx

DFT

IFFTP/S Tx

DFT

User 1

User 2

freq

OFDM band

freq

OFDM band

user1

user2


60

Fractional Frequency Reuse

Number of dominant independentinterferers is higher than in TDMA, but notas high as in CDMA

F1

F2

F3

EntireOFDM band

Example:R = 1/3


61

Downlink OFDMA Capacity Because of multipath immunity in OFDMA, capacity is

only limited by other-cell interference and thermal noise. Compared to orthogonal CDMA in highly dispersive

environments, OFDMA downlink capacity exhibitspotential gains due to multipath immunity.

27%17%f=1.051%28%f=0.5

237%55%f=0.0SNR=10 dBSNR=0 dBOther-cellinterference factor

(COFDMA - CCDMA) / CCDMA [%]


62

Uplink OFDMA Capacity Because of orthogonality, capacity is limited only by

thermal noise and other-cell interference. Potential gains in OFDM with respect to CDMA due to

mitigating non-orthogonal multiple access interference.

35%32%f=1.058%52%f=0.5

342%152%f=0.0SNR=10 dBSNR=0 dBOther-cellinterference factor

(COFDMA - CCDMA) / CCDMA [%]


63

OFDM/OFDMA Summary (1/2) Excellent RR granularity and fast allocation

mechanisms RR granularity based on time and frequency Channel quality estimation performed on pilot

subcarriers Excellent spectrum scalability Multipath immunity achieved through low

symbol rates (10-20 kbaud) and intersymboltime guard (cyclic prefix) Time guard slightly reduces capacity (around 10%)


64

OFDM/OFDMA Summary (2/2) PAPR becomes a significant issue at mobile

station PAPR proportional to number of subcarriers, and

therefore, it can be mitigated by reducing bandwidth PAPR also mitigated by use of DFT-spread OFDM

(SC-FDM) Uplink capacity significantly better than non-

orthogonal CDMA Downlink capacity only slightly better than

orthogonal CDMAR. Badra: OFDM/OFDMA for Wireless Communications

65

SECTION 5WiMAX and LTE


66

WiMAX WiMAX= Worldwide interoperability for Microwave Access Based on the IEEE standards 802.16 for wireless

metropolitan access 802.16 is a collection of standards supporting multiple operation

modes and parameters WiMAX is created to boost interoperability WiMAX Forum is in charge of defining equipment certification

rules WiMAX currently defines two subsets of specifications:

802.16-2004 or 802.16d (Fixed WiMAX) 802.16-2005 or 802.16e (Mobile WiMAX) A number of profiles are defined for these subsets (only 2 are active

for fixed and 6 are active for mobile)R. Badra: OFDM/OFDMA for Wireless Communications

67

Mobile WiMAX parameters(active profiles only)

Bands 2.3 GHz, 2.5 GHz,

3.5 GHz

Downlink

Channel coding

Convolutional +

Reed-Solomon

Multiple Access TDMA / OFDMA Uplink Channel

Coding

Convolutional

Duplex TDD Retransmission ARQ, Hybrid ARQ

(options)

Scalable YES Subcarrier

spacing

10.94 kHz

Bandwidth 5, 7, 8.75, 10 MHz Symbol Time 102.9 S

(91.4 S + 11.5 S)

FFT size 512, 1024 Guard Time

Overhead

12.5 %

Max Bit Rate 75 Mbps over 10

MHz (*)

Frame Time 5 mS

Aplication Fixed and Nomadic

Access

Bit Rate

TDD 1:1

32 Mbps (dwnlnk)

4 Mbps (uplnk) (*)

Downlink

Modulation

QPSK, 16QAM,

64QAM

Switching Mode End-to-end IP-based

packet switching

Uplink

Modulation

QPSK, 16QAM Uplink Mode OFDM/OFDMA

(*) Assuming 2x2 MIMOR. Badra: OFDM/OFDMA for Wireless Communications

68

RR granularity inMobile WiMAX Subchannels are defined, consisting of a subset of

subcarriers over a certain time. AMC (Adaptive Modulation and Coding) Subchannels comprised of

adjacent subcarriers. PUSC (Partial Usage of Subcarriers) Subchannels comprised of non-

adjacent, pseudorandomly selected subcarriers. RR granularity is based on subchannels.

Minimum block size is 48 symbols. AMC granularity is 8 subcarriers x 6 symbols, or 16 subcarriers

x 3 symbols, or 24 subcarriers x 2 symbols. Downlink PUSC granularity is 24 subcarriers x 2 symbols. Uplink PUSC granularity is 16 subcarriers x 3 symbols.


69

Long Term Evolution (LTE) 3GPP specification (release 9, 2009) for a highly

flexible, high-performance radio interface. It is a forerunner to LTE-Advanced Main features are 172 / 57 Mbps peak data rates

(down/uplink) and network delays of 5 mS for small IPpackets.

Fully flat, IP-based architecture. Interoperable with previous 3GPP technologies

(GSM/GPRS/EDGE, WCDMA, HSPA).


70

LTE parametersBands Multiband (25 FDD

bands, 11 TDD bands) Channel coding Turbo Coding

Multiple Access TDMA / OFDMA Aplication Fixed, nomadic and

fully mobile access

Duplex FDD, TDD Retransmission ARQ, Hybrid ARQ

Scalable YES Subcarrier

spacing

15 kHz

Bandwidth 1.4, 3, 5, 10, 20

MHz

Symbol Time (regular guard t )

71.4 S

(66.7 S + 4.7 S)

FFT size 128, 256, 512, 1024,

2048

Symbol Time (extended guard t )

83.4 S

(66.7 S + 16.7 S)

Max Bit Rate

(downlink)

172 Mbps over 20

MHz (*)

Guard Time

Overhead

6.6 % (regular)

25.0 % (extended)

Max Bit Rate

(uplink)

57 Mbps over 20

MHz

Frame Time 10 mS

Downlink

Modulation

QPSK, 16QAM,

64QAM

Switching Mode End-to-end IP-based

packet switching

Uplink

Modulation

QPSK, 16QAM Uplink mode DFT-spread

OFDMA

(*) Assuming 2x2 MIMOR. Badra: OFDM/OFDMA for Wireless Communications

71

RR granularity in LTE 10 ms-frames are subdivided into 1-ms

subframes Resource blocks are defined:

One subframe (1 ms) x 12 adjacentsubcarriers (180 kHz)

One resource block carries 168 OFDMsymbols (regular guard time) or 144 OFDMsymbols (extended guard time for highlydispersive environments)


72

SECTION 6CONCLUSION


73

Why OFDM for 4G? Multipath immunity Narrowband interference immunity Highly scalable Low signal overhead Computationally efficient (IFFT/FFT)


74

Why OFDMA for 4G? Fine RR granularity Highly flexible RR allocation Higher capacity than any previous

multiple access method Allows for fractional frequency reuse


75

OFDM limitations High PAPR reduces MS available power

and therefore impacts coverage Two practical approaches to PAPR

mitigation: DFT-spread OFDM Limit number of subcarriers


76

OFDM/OFDMAfor 4G wireless systems

IEEE LATINCOM 2011Belem do ParOctober 25th, 2011

Presenter: Renny Badra, Ph.D.Universidad Simn BolvarCaracas, [email protected]

OFDM/OFDMA for 4G Wireless Systems

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