ECE 6332, Spring, 2012

Wireless Communications

Zhu Han

Department of Electrical and Computer Engineering

Class 22

April 23th, 2012

                                                           

                                                           

OutlineOutline Chapter 13 CDMA, Spread Spectrum

– FHSS

– DSSS

– Multiuser case

                                                           

spread-spectrum transmission spread-spectrum transmission

Three advantages over fixed spectrum – Spread-spectrum signals are highly resistant to noise and

interference. The process of re-collecting a spread signal spreads out noise and interference, causing them to recede into the background.

– Spread-spectrum signals are difficult to intercept. A Frequency-Hop spread-spectrum signal sounds like a momentary noise burst or simply an increase in the background noise for short Frequency-Hop codes on any narrowband receiver except a Frequency-Hop spread-spectrum receiver using the exact same channel sequence as was used by the transmitter.

– Spread-spectrum transmissions can share a frequency band with many types of conventional transmissions with minimal interference. The spread-spectrum signals add minimal noise to the narrow-frequency communications, and vice versa. As a result, bandwidth can be utilized more efficiently.

                                                           

Pseudo Random Sequence GeneratorPseudo Random Sequence Generator Pseudorandom sequence

– Randomness and noise properties

– Walsh, M-sequence, Gold, Kasami, Z4

– Provide signal privacy

                                                           

Example: Linear Congruential Generators

                                                           

Frequency Hopping Spread SpectrumFrequency Hopping Spread Spectrum

Frequency-hopping spread spectrum (FHSS) is a spread-spectrum method of transmitting radio signals by rapidly switching a carrier among many frequency channels, using a pseudorandom sequence known to both transmitter and receiver.

Military

                                                           

Frequency Hopping ExampleFrequency Hopping Example

                                                           

Who is Bluetooth?Who is Bluetooth?

Harald Blaatand “Bluetooth” II– King of Denmark 940-981 AC

– Harald Bluetooth was first Christian king of Denmark

– He united Denmark under his rule in the mid-900's

– Similarly, Bluetooth seeks to unite personal computing devices wirelessly

In 1994 – need for low power consumption wireless devices to substitute for cable

Ericsson – driving force behind Bluetooth– Pre-Cell phone– 1998, Ericsson, Nokia, IBM, Toshiba, Intel formed the

Bluetooth Special Interest Group (SIG)– 1999 – Release of Bluetooth protocol– 2002 – IEEE adopted Bluetooth standard, 802.15

working group

                                                           

Adaptive Frequency-hopping spread Adaptive Frequency-hopping spread spectrum In Bluetoothspectrum In Bluetooth

Works like this …– During a connection, radio transceivers hop from one channel to another– One packet is sent on a channel, two devices then retune their frequencies (hop) to

send the next packet on a different channel. So, if one frequency channel is blocked, limited disturbance to the

Bluetooth communication– Allows several Bluetooth networks to run concurrently without interrupting one

other– Link rate: 1 Mbps, but with overhead, this reduces to 721 kbps– Range for Bluetooth:

10m, can reach up to 100m depending on the power class of the device

Bluetooth version 2.0 + EDR uses an enhanced technology called: Adaptive Frequency Hopping (AFH)

– AFH allows Bluetooth devices to measure quality of wireless signal– Determines if there are bad channels present on specific frequencies due to interference from

other wireless devices.– If bad channels present on a specific frequency, Bluetooth device will adjust its hopping

sequence to avoid them– As a result, the Bluetooth connection is stronger, faster, and more reliable

                                                           

S-72.3240 Wireless Personal, Local, Metropolitan, and Wide Area Networks 10

Bluetooth radio and baseband parameters

Topology Up to 7 simultaneous linksModulation Gaussian filtered FSK

RF bandwidth 220 kHz (-3 dB), 1 MHz (-20 dB)RF band 2.4 GHz ISM frequency band

RF carriers 79 (23 as reduced option)Carrier spacing 1 MHzAccess method FHSS-TDD-TDMAFreq. hop rate 1600 hops/s

                                                           

S-72.3240 Wireless Personal, Local, Metropolitan, and Wide Area Networks 11

Frequency hopping spread spectrum (1)

Bluetooth technology operates in the 2.4 GHz ISM band, using a spread spectrum, frequency hopping, full-duplex

signal at a nominal rate of 1600 hops/second.

2.4000 GHz 2.4835 GHz

Time1 MHz

The signal hops among 79 frequencies (spaced 1 MHz apart) in a pseudo-random

fashion.

83.5 MHz

                                                           

S-72.3240 Wireless Personal, Local, Metropolitan, and Wide Area Networks 12

The adaptive frequency hopping (AFH) feature (from Bluetooth version 1.2 onward) is designed to reduce

interference between wireless technologies sharing the 2.4 GHz spectrum.

2.4000 GHz 2.4835 GHz

Time Interference e.g. due to microwave oven => this frequency in the hopping sequence should be

avoided.

Frequency hopping spread spectrum (2)

                                                           

S-72.3240 Wireless Personal, Local, Metropolitan, and Wide Area Networks 13

In addition to avoiding microwave oven interference, the adaptive frequency hopping (AFH) feature can also avoid

interference from WLAN networks:

2.4 GHz 2.48 GHz

79 FHSS frequencies

2.4 GHz 2.48 GHz

WLAN channel

... ...

22 MHz (802.11b)16.5 MHz (802.11g)

Frequency hopping spread spectrum (3)

                                                           

Direct Sequence (DS)-CDMADirect Sequence (DS)-CDMA

It phase-modulates a sine wave pseudo-randomly with a continuous string of pseudo-noise code symbols called "chips", each of which has a much shorter duration than an information bit. That is, each information bit is modulated by a sequence of much faster chips. Therefore, the chip rate is much higher than the information signal bit rate.

It uses a signal structure in which the sequence of chips produced by the transmitter is known a priori by the receiver. The receiver can then use the same PN sequence to counteract the effect of the PN sequence on the received signal in order to reconstruct the information signal.

                                                           

System Block DiagramSystem Block Diagram

Jammer/Noise/Interferencej(t)

BPSKModulator

BPSKMatched

Filter

Channel

PseudorandomSequence Generator

PseudorandomSequenceGenerator

SourceData

OutputData

(to detector)

b(t)

c(t)

s(t) x(t) u(t)

c(t)

y(t) rn

y(t) = j(t) + x(t)u(t) = s(t) + j(t)c(t)

rn = bn + jammer projection

s(t) = b(t)cos(wot)x(t) = s(t)c(t)

Unique code to differentiate all users

Sequence used for spreading have low cross-correlations

Allow many users to occupy all the frequency/bandwidth allocations at that same time

Processing gain is the system capacity– How many users the system can support

                                                           

Spreading & DespreadingSpreading & Despreading

Spreading– Source signal is multiplied by a PN signal

Processing Gain:

Despreading– Spread signal is multiplied by the spreading code

Polar {±1} signal representation

DataRate

ChipRate

T

T

T

TG

b

c

c

bp

1

1

                                                           

Direct Sequence Spread Spectrum ExampleDirect Sequence Spread Spectrum Example

                                                           

CDMA Example – transmission from two sourcesCDMA Example – transmission from two sources

1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0

CodeData

0 1 0 0 1 1 0 1 0 0 1 10 1 0 0 1 1 0 1 0 0 1 1

1 0 1 1 0 0 0 1 0 0 1 1 1 0 1 1 0 0

0 0 1 0

1 0 1 0 1 0

1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 0 1 0 1 0 1

1 0 1 1

1 0 1 1 0 0

TransmittedA+B

Signal

A Data

A Codeword

B Data

B Codeword

CodeDataA Signal

B Signal

                                                           

CDMA Example – recovering signal A at the receiverCDMA Example – recovering signal A at the receiver

0 1 0 0

A+BSignal

received

A Codeword

atreceiver

CodeB)(A

IntegratorOutput

ComparatorOutput

Take the inverse of this to obtain A

                                                           

CDMA Example – recovering signal B at the receiverCDMA Example – recovering signal B at the receiver

1 1 0 1

A+BSignal

received

B Codeword

atreceiver

CodeB)(A

IntegratorOutput

ComparatorOutput

Take the inverse of this to obtain B

                                                           

CDMA Example – using wrong codeword at the receiverCDMA Example – using wrong codeword at the receiver

X 0 1 1 Noise

A+BSignal

received

Wrong Codeword

Used atreceiver

IntegratorOutput

ComparatorOutput

Wrong codeword will not be able to decode the original data!

                                                           

Wideband Interference SuppressionWideband Interference Suppression

Transmitter

Receiver

                                                           

Narrowband Interference SuppressionNarrowband Interference Suppression

Transmitter

Receiver

                                                           

CDMACDMA

Rake Receiver

                                                           

Road MapRoad Map

1XRTT/3XRTT

cdma2000CDMA

(IS 95 A) IS 95 B

GSM

TDMA EDGE UWC-136

GPRS W-CDMA

3X3X3X3X

No 3XNo 3XNo 3XNo 3X

cdmaOnecdmaOneIS-95AIS-95A

cdmaOnecdmaOneIS-95AIS-95A

1999 2000 2001 2002

1X1X1X1XIS-95BIS-95BIS-95BIS-95B

2G 2.5G 3G Phase 1 3G Phase 2

                                                           

2G: IS-95A (1995)2G: IS-95A (1995)

Known as CDMAOne

Chip rate at 1.25Mbps

Convolutional codes, Viterbi Decoding

Downlink (Base station to mobile):– Walsh code 64-bit for channel

separation

– M-sequence 215 for cell separation

Uplink (Mobile to base station):– M-sequence 241 for channel

and user separation

Standard IS-95, ANSI J-STD-008

Multiple Access CDMA

Uplink Frequency 869-894 MHz

Downlink Frequency

824-849 MHz

Channel Separation 1.25 MHz

Modulation Scheme BPSK/QPSK

Number of Channel 64

Channel Bit Rate 1.25 Mbps (chip rate)

Speech Rate 8~13 kbps

Data Rate Up to 14.4 kbps

Maximum Tx Power

600 mW

                                                           

2.5G: IS-95B (1998)2.5G: IS-95B (1998)

Increased data rate for internet applications– Up to 115 kbps (8 times that of 2G)

Support web browser format language– Wireless Application Protocol (WAP)

                                                           

3G Technology3G Technology

Ability to receive live music, interactive web sessions, voice and data with multimedia features

Global Standard IMT-2000– CDMA 2000, proposed by TIA– W-CDMA, proposed by ARIB/ETSI

Issued by ITU (International Telecommunication Union) Excellent voice quality Data rate

– 144 kbps in high mobility– 384 kbps in limited mobility– 2 Mbps in door

Frequency Band 1885-2025 MHz Convolutional Codes Turbo Codes for high data rates

                                                           

3G: CDMA2000 (2000)3G: CDMA2000 (2000)

CDMA 1xEV-DO– peak data rate 2.4 Mbps– supports mp3 transfer and video conferencing

CDMA 1xEV-DV– Integrated voice and high-speed data multimedia service up to 3.1

Mbps

Channel Bandwidth: – 1.25, 5, 10, 15 or 20 MHz

Chip rate at 3.6864 Mbps Modulation Scheme

– QPSK in downlink – BPSK in uplink

                                                           

3G: CDMA2000 Spreading Codes3G: CDMA2000 Spreading Codes

Downlink – Variable length orthogonal Walsh sequences for channel separation

– M-sequences 3x215 for cell separation (different phase shifts)

Uplink– Variable length orthogonal Walsh sequences for channel separation

– M-sequences 241 for user separation (different phase shifts)

                                                           

3G: W-CDMA (2000)3G: W-CDMA (2000)

Stands for “wideband” CDMA

Channel Bandwidth: – 5, 10 or 20 MHz

Chip rate at 4.096 Mbps

Modulation Scheme– QPSK in downlink

– BPSK in uplink

Downlink – Variable length orthogonal sequences for channel separation

– Gold sequences 218 for cell separation

Uplink– Variable length orthogonal sequences for channel separation

– Gold sequences 241 for user separation

                                                           

Near/Far ProblemNear/Far Problem

Performance estimates derived using assumption that all users have same power level

Reverse link (mobile to base) makes this unrealistic since mobiles are moving

Adjust power levels constantly to keep equal

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