ECE 6332, Spring, 2014

Wireless Communications

Zhu Han

Department of Electrical and Computer Engineering

Class 23April 16th, 2014

                                                           

                                                           

OFDM Basic IdeaOFDM Basic Idea

Orthogonal frequency-division multiplexing

Divide a high bit- rate stream into several low bit- rate streams ( serial to parallel)

Robust against frequency selective fading due to multipath propagation

                                                           

Orthogonal frequency-division multiplexing Orthogonal frequency-division multiplexing

Special form of Multi-Carrier Transmission.

Multi-Carrier Modulation.– Divide a high bit-rate digital stream into several low bit-rate

schemes and transmit in parallel (using Sub-Carriers)

-6 -4 -2 0 2 4 6

-0.2

0

0.2

0.4

0.6

0.8

Normalized Frequency (fT) --->

Norm

alized A

mplitu

de -

-->

                                                           

OFDMOFDM

                                                           

Transmitted SymbolTransmitted Symbol

                                                           

Guard Time and Cyclic Extension...Guard Time and Cyclic Extension...

A Guard time is introduced at the end of each OFDM symbol for protection against multipath.

The Guard time is “cyclically extended” to avoid Inter-Carrier Interference (ICI) - integer number of cycles in the symbol interval.

Guard Time > Multipath Delay Spread, to guarantee zero ISI & ICI.

M ultipath component that does not cause IS I

guard Symbol guard

guard Symbol guard

guard Symbol guard

M ultipath component that causes IS I

                                                           

Mathematical description

                                                           

Mathematical description

                                                           

OFDM Timing ChallengeOFDM Timing Challenge

                                                           

OFDM bit loadingOFDM bit loading Map the rate with the sub-channel condition

Water-filling

                                                           

OFDM Time and Frequency GridOFDM Time and Frequency Grid

Put different users data to different time-frequency slots

                                                           

OFDM Transmitter and ReceiverOFDM Transmitter and Receiver

                                                           

OFDMOFDM

                                                           

Multiband OFDM Multiband OFDM

- Simple to implement

- Captures 95% of the multipath channel energy in the Cyclic Prefix

- Complexity of OFDM system varies Logarithmically with FFT size i.e.

- N point FFT (N/2) Log2 (N) complex multiplies for every OFDM symbol

                                                           

Pro and ConPro and Con Advantages

– Can easily be adopted to severe channel conditions without complex equalization

– Robust to narrow-band co-channel interference – Robust to inter-symbol interference and fading caused by multipath propagation – High spectral efficiency – Efficient implementation by FFTs – Low sensitivity to time synchronization errors – Tuned sub-channel receiver filters are not required (unlike in conventional

FDM) – Facilitates Single Frequency Networks, i.e. transmitter macro-diversity.

Disadvantages– Sensitive to Doppler shift. – Sensitive to frequency synchronization problems – Inefficient transmitter power consumption, since linear power amplifier is

required.

                                                           

OFDM ApplicationsOFDM Applications ADSL and VDSL broadband access via telephone network copper wires.

IEEE 802.11a and 802.11g Wireless LANs.

The Digital audio broadcasting systems EUREKA 147, Digital Radio Mondiale, HD Radio, T-DMB and ISDB-TSB.

The terrestrial digital TV systems DVB-T, DVB-H, T-DMB and ISDB-T.

The IEEE 802.16 or WiMax Wireless MAN standard.

The IEEE 802.20 or Mobile Broadband Wireless Access (MBWA) standard.

The Flash-OFDM cellular system.

Some Ultra wideband (UWB) systems.

Power line communication (PLC).

Point-to-point (PtP) and point-to-multipoint (PtMP) wireless applications.

                                                           

ApplicationsApplications

WiMax

Digital Audio Broadcast (DAB)

Wireless LAN

                                                           

ApplicationsApplications

High Definition TV (HDTV)

4G Cellular Communication systems

Flash -OFDM

                                                           

Proprietary OFDM FlavoursProprietary OFDM Flavours

Wideband-OFDM(W-OFDM) of Wi-LAN

www.wi-lan.com

Flash OFDMfrom Flarion

www.flarion.com

Vector OFDM(V-OFDM) of Cisco, Iospan,etc.

www.iospan.com

Wireless Access (Macro-cellular)

-- 2.4 GHz band-- 30-45Mbps in 40MHz

-- large tone-width (for mobility, overlay)

-- Freq. Hopping for CCI reduction, reuse

-- 1.25 to 5.0MHz BW -- mobility support

-- MIMO Technology-- non-LoS coverage,

mainly for fixed access-- upto 20 Mbps in MMDS

Wi-LAN leads the OFDM Forum -- many proposals submitted to IEEE 802.16 Wireless MAN

Cisco leads the Broadand Wireless Internet Forum (BWIF)

                                                           

20

OFDM based StandardsOFDM based Standards

Wireless LAN standards using OFDM are– HiperLAN-2 in Europe

– IEEE 802.11a, .11g

OFDM based Broadband Access Standards are getting defined for MAN and WAN applications

802.16 Working Group of IEEE– 802.16 -- single carrier, 10-66GHz band

– 802.16a, b -- 2-11GHz, MAN standard

                                                           

Key Parameters of 802.16a Wireless MANKey Parameters of 802.16a Wireless MAN

• Operates in 2-11 GHz• SC-mode, OFDM, OFDMA, and Mesh support

• Bandwidth can be either 1.25/ 2.5/ 5/ 10/ 20 MHz• FFT size is 256 = (192 data carriers+ 8 pilots +56 Nulls)

• RS+Convolutional coding • Block Turbo coding (optional)

• Convolutional Turbo coding(optional)• QPSK, 16QAM, 64QAM

• Two different preambles for UL and DL

                                                           

Calculations for 802.16a -- Example: 5MHzCalculations for 802.16a -- Example: 5MHz

Carrier frequency 2-11 GHz Channel Bandwidth 5 MHz Number of inputs to IFFT/FFT 256 Number of data subcarriers 192 Number of pilots 8 Subcarrier frequency spacing f 19.53125 KHz (5 MHz/256) Period of IFFT/FFT Tb 51.2 s (1 / f) Length of guard interval 12.8 s (Tb / 4) Length of the preamble for Downlink 128 s (640 sub-carriers) Length of the preamble for Uplink 76.8s (384/5 MHz) Guard interval for Uplink preamble 25.6 s (128/5 MHz) OFDM symbol duration 64 s (320/5 MHZ)

                                                           

IEEE 802.16(10 to 66 GHz)

Broadband Access Standards -- contd. Broadband Access Standards -- contd.

IEEE LAN and MAN standards

IEEE 802.11a or.11b, or .11g

IEEE 802.16a,b(2 to 11 GHz)

2-5 miles, LoS(> 11GHz)

1-3 miles, non-LoS

                                                           

The IEEE 802.11a/g StandardThe IEEE 802.11a/g Standard

Belongs to the IEEE 802.11 system of specifications for wireless LANs.

802.11 covers both MAC and PHY layers.

802.11a/g belongs to the High Speed WLAN category with peak data rate of 54Mbps

FFT 64, Carrier 2.4G or 5G. Total bandwidth 20 MHz x 10 =200MHz

                                                           

The IEEE 802.11 StandardThe IEEE 802.11 Standard

                                                           

Evolution of Radio Access TechnologiesEvolution of Radio Access Technologies

LTE (3.9G) : 3GPP release 8~9

LTE-Advanced :3GPP release 10+

802.16d/e

802.16m

In Nov. 2004, 3GPP began a project to define the long-term evolution (LTE) of Universal Mobile Telecommunications System

(UMTS) cellular technology

                                                           

27

LTE vs. LTE-AdvancedLTE vs. LTE-Advanced

                                                           

DS-CDMA versus OFDMDS-CDMA versus OFDM

channelInput (Tx signal)

Output(Rx signal)

ImpulseResponse h(t)

time

3

0

freq.

Frequency Response H(f)

DS-CDMA can exploit

time-diversity

OFDM can exploitfreq. diversity

                                                           

Comparing Comparing ComplexityComplexity of TDMA, DS- of TDMA, DS-CDMA, & OFDM TransceiversCDMA, & OFDM Transceivers

Timing Sync.

Freq. Sync.

Timing Tracking

Freq. Tracking

ChannelEqualisation

Analog Front-end(AGC, PA, VCO, etc)

TDMA OFDM

Very elegant, requiringno extra overhead

CDMA

Easy, but requiresoverhead (sync.) bits

Difficult, and requiressync. channel (code)

Easy, but requiresoverhead (sync.) bits

More difficult than TDMAGross Sync. Easy

Fine Sync. is Difficult

Modest Complexity Usually not requiredwithin a burst/packet

Requires CPE Tones(additional overhead)

RAKE Combining in CDMA usually more complex than

equalisation in TDMA

Modest Complexity(using dedicated correlator)

Easy, decision-directedtechniques can be used

Frequency DomainEqualisation is very easy

Complexity or cost is very high (PA back-off

is necessary)

Very simple(especially for CPM signals)

Complexity is high inAsynchronous W-CDMA

Modest to High Complexity(depending on bit-rate and

extent of delay-spread)

Fairly Complex(power control loop)

                                                           

Comparing Comparing PerformancePerformance of TDMA, DS- of TDMA, DS-CDMA, & OFDM TransceiversCDMA, & OFDM Transceivers

Fade Margin(for mobile apps.)

Range

Re-use & Capacity

FEC Requirements

Variable Bit-rateSupport

Spectral Efficiency

TDMA OFDM

Required for mobileapplications

CDMA

Required for mobileapplications

Modest requirement(RAKE gain vs power-

control problems)

Range increase by reducing allowed noise rise (capacity)

Difficult to support large cells (PA , AGC limitations)

Modest (in TDMA) andHigh in MC-TDMA

Re-use planning iscrucial here

FEC is vital even forfixed wireless access

FEC is usually inherent (to increase code decorrelation)

FEC optional for voice

Powerful methodsto support VBR

(for fixed access)

Very High(& Higher Peak Bit-rates)

Modest

Modest

Low to modest support

Poor to Low

Very elegant methodsto support VBR & VAD

Very easy to increasecell sizes

                                                           

31

LTE vs. LTE-AdvancedLTE vs. LTE-Advanced

                                                           

32

LTE vs. LTE-AdvancedLTE vs. LTE-Advanced