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2009 Morrow Technical ServicesMWJ OFDMA

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ORTHOGONAL FREQUENCY

DIVISION MULTIPLEXING

Bob Morrow, Ph.D.Morrow Technical Services

6976 Kempton Rd., Centerville IN 47330 USA

+1-765-855-5109 [email protected]

MULTIPLE ACCESS

TECHNIQUES

Sponsored by Rohde & Schwarz


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OVERVIEW

Multiple access methods: TDMA and FDMA

OFDM basics and signal constructionOFDM challenges

OFDM application: IEEE 802.11g Wi-Fi

OFDM application: IEEE 802.16e-2005 WiMAX


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MEDIUM ACCESS CONTROL

(MAC): WHEN TO TRANSMIT?Used for multiple access and duplexing

Multiple access: multiple users

Duplexing: two-way communication

Scheduled access Structured communications

Collisions cannot usually occur

High overhead; requires transmitter coordination

Random access (contention-based) Unstructured communications

Collisions can occur

Low overhead; no transmitter coordination


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Frequency-division multiple access FDMA Each user transmits on separate frequency

Transmissions may overlap in time

MULTIPLE ACCESS: SCHEDULED

Time-division multiple access TDMA

Each user transmits during separatetime

Transmissions may overlap in frequency


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MULTIPLE ACCESS: RANDOM

ALOHA

Transmit without listening first

Often used in code division multiple access (CDMA)

Carrier-Sense Multiple-Access (CSMA) Listen before transmitting

Collision detection (CSMA/CD)

Node must be able to transmit and receive simultaneously

More suitable for wired networks such as 802.3 Ethernet

Collision avoidance (CSMA/CA) Doesnt require simultaneous transmit and receive

CSMA reserves channel for subsequent longer data packets

Used in many wireless networks


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ORTHOGONAL FREQUENCYDIVISION MULTIPLEXING (OFDM)

Several subcarriers are modulated, each with slow data

Subcarriers are orthogonal (no cross-carrier interference)

Modulation is via binary/quadrature phase shift keying(BPSK/QPSK), or quadrature amplitude modulation (QAM)

Fast aggregate data rate Advantages

Spectrally efficient

Each subcarrier experiences slow, flat fading

Error correction compensates for the loss of a few subcarriers

Disadvantages

Complex implementation

Intrasymbol interference between subcarriers

Frequency relationship must be precise

High peak-to-average power (PAP) at transmitter

Requires highly linear RF amplifiers


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OFDM SUBCARRIERS

Source: Communication Systems Design, February 2001

Time domain: Each subcarrier hasan integral number of cycleswithin the symbol duration

Frequency domain: Each subcarrierpeak is at a zero amplitude point for allother subcarriers. Subcarrier spacing is

the reciprocal of the symbol duration

symbol duration

subcarrierspacing


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OFDMSIGNALGENERATIONANDDETECTION


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OFDM SUBCARRIER MAPPING

(16-QAM SHOWN)

Data string is broken into four bits per subcarrier

Each subcarrier is assigned a point on its 16-QAMsignal constellation corresponding to its data

IFFT is performed on the composite I-Q vector toobtain one OFDM symbol in the time domain

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subcarriers

OFDM symbol

SAMPLE OFDM SYMBOL

(TIME DOMAIN)

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OFDM GUARD INTERVAL (GI)USING CYCLIC PREFIX (CP)

GI allows previous symbols multipath to die out

Prevents multipath-induced intersymbol interference (ISI)

GI must exceed the RMS delay spread

CP maintains subcarrier orthogonality

Linear convolution during RX FFT becomes circular convolution

Time domain signal is continuous within an OFDM symbol

Avoids TX turn-on transients at beginning of RX FFT period

GI reduces OFDM efficiency in two related ways

Increased bandwidth due to increased subcarrier spacing

Increased time overhead since GI contains no additional information


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OFDM USES

802.11 a/g/n

802.16 WiMAX

Long-term evolution (LTE)

Digital subscriber line (DSL)

Digital audio broadcasting (DAB)

Digital video broadcasting (DVB)

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802.11g OFDM SIGNAL

Signal contains 48 data subcarriers and 4 pilot subcarriers

Subcarriers are spaced 312.5 kHz apart

Symbol duration is 4 s, including 800 ns cyclic prefix

Subcarriers are modulated with BPSK, QPSK, 16-QAM, or 64-QAM

Pilot subcarriers provide channel condition info and phase reference

Pilot subcarrier spacing should be less than the coherence bandwidth


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IEEE 802.11g OFDM CODING

Data RateModulation

(each subcarrier)

CodingRate

Coded Bitsper OFDMSubcarrier

Coded Bitsper OFDM

Symbol

Data Bitsper OFDM

Symbol

6 Mb/s BPSK 1/2 1 48 24

9 Mb/s BPSK 3/4 1 48 36

12 Mb/s QPSK 1/2 2 96 48

18 Mb/s QPSK 3/4 2 96 72

24 Mb/s 16-QAM 1/2 4 192 96

36 Mb/s 16-QAM 3/4 4 192 144

48 Mb/s 64-QAM 2/3 6 288 19254 Mb/s 64-QAM 3/4 6 288 216

Maximum range decreasesas data rate increases


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802.11 CSMA: DEVICES A AND BCOMPETING FOR ACCESS


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BEYOND 802.11: TECHNICALCHALLENGES FOR BROADBAND

WIRELESS ACCESS (BWA)

Reliable signaling in a hostile outdoor/indoor environment

High spectral efficiency to support a large number of users

Multiplexing services with different quality-of-service (QoS)requirements

Supporting mobility through seamless handover and roaming

Low power consumption for battery use

Robust security

Adapts IP-based protocols for integration into existingnetworks

Low cost

Source: Andrews, J., Ghosh, A., and Muhamed, R., Fundamentals of WiMAX, Prentice Hall, 2007


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WiMAXUSAGE

Source: WiMAX Forum Plugfest, 24 Sep - 1 Oct 2006 white paper

Base Station

(BS)

Mobile Stations(MS)

Worldwide Interoperabilityfor Microwave Access


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WiMAX MULTIPLE-ACCESS

FEATURES (802.16e-2005) OFDM-based PHY

Anti-multipath and non-line-of-sight (NLOS) operation

Simultaneous multi-user transmissions

Different users are assigned different OFDM subcarriers

Scalable bandwidth/data rates with adaptive coding

Supports multiple access and user roaming in changingchannels

Hybrid automatic repeat request (ARQ)

Uses previous erroneous packets in decoding process

Time and frequency division duplexing (TDD/FDD) support System adjusts uplink (UL) and downlink (DL) usage ratios

QoS support

Variable bit rate, latency, and reliability



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SCALABLE OFDM PARAMETERS:

MOBILE WiMAXParameter DL Value

Channel bandwidth 1.25 MHz 5 MHz* 10 MHz* 20 MHz

Total number of subcarriers 128 512 1024 2048

Number of data subcarriers 72 360 720 1440

Number of pilot subcarriers 12 60 120 240

Number of null subcarriers 44 92 184 368

Subcarrier spacing 10.94 kHz

Total symbol duration 102.9 s

FFT interval 91.4 s

Guard interval (1/8 FFT) 11.4 s

OFDM symbols in 5 ms frame 48

* Initial mobile WiMAX system profiles


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TYPICAL DATA RATES:

MOBILE WiMAX Shown are aggregate PHY

data rates without overhead

Shared among all users in

the channel Assumptions:

5 MHz channel bandwidth

5 ms frame size

1/8 FFT for GI

No space-time coding

SubcarrierModulation

(512 subcarriers)

CodingRate

Data Rate(Mb/s)

DL UL

BPSK 1/2 not used

QPSK1/2 2.5 0.7

3/4 3.8 1.0

16-QAM1/2 5.0 1.3

3/4 7.6 2.0

64-QAM*

1/2 7.6 2.02/3 10.0 2.6

3/4 11.3 2.9

5/6 12.6 3.3

Source: Andrews, J., Ghosh, A., and Muhamed, R., Fundamentals of WiMAX, Prentice Hall, 2007 * Optional for uplink


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ORTHOGONAL FREQUENCY DIVISIONMULTIPLE ACCESS (OFDMA)

OFDMA (802.16e-2005) allows users to sharedifferent subcarriers and different time periods

Hybrid of FDMA and TDMA

Several mobile stations are supported by one base stationOFDMA subchannelization

The BS assigns to each MS a block (subchannel) ofOFDM subcarriers

OFDMA messages from BS to MS:

MS subcarrier map MS burst profile (modulation and coding method)


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OFDMA SUBCARRIER MAPPINGEXAMPLE (802.16e-2005)

Downlink full usage of subchannels (FUSC)

All data subcarriers are used

Eachsubchannelhas 48 data subcarriers

Subchannel carriers are not adjacent, but are distributedthroughout the OFDM subcarrier set

Pilot subcarriers are both fixed and variable

Variable pilot subcarriers change position in different OFDMsymbols for increased RX channel estimation accuracy


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OFDMA UPLINK RANGING

Process of equalizing BS received power levels andtiming among active MS

Prevents intercarrier and intersymbol interference

BS determines signal power and propagationdelay for each MS using special rangingsubchannels

Each MS receives its power and delay information in theburst profile sent by the BS

MS then adjusts its own:

TX power (1 dB max step size over 30 or 50 dB span)

TX start time


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MAC OFDMA/TDD EXAMPLE

DL frame preamble provides sync and other PHY functions FCH: system control information (subcarriers used, etc.)

MAP: specifies data regions in DL and UL for various MS

Ranging subchannels: BS uses these to find MS power and delay



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OFDMA HELPS SUPPORT

WiMAX QoS SCHEDULINGUnsolicited grant services (UGS)

Fixed size packets at a constant bit rate (e.g., some VoIP)

Real-time polling services (rtPS)

Variable size packets periodically (e.g., video streaming)

Non-real-time polling services (nrtPS)

Variable size packets that are delay-tolerant (e.g., FTP)

Best-effort (BE)

Packets without a minimum service guarantee (e.g., Web)

Extended real-time variable rate (ERT-VR)

Variable data rates requiring guaranteed delay (e.g.,some VoIP)


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CONCLUSIONS

OFDM has several advantages over single carriermodulation methods

Efficient use of bandwidth

Fast aggregate data rate Anti-multipath (Doppler spread and delay spread)

Multiple access using OFDM is highly flexible

TDMA: Scheduled access in time

FDMA: Scheduled access in frequency

CSMA: Random access using carrier sense

OFDMA: Assigning multiple users to different subcarriers


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