ECE 4371, Fall, 2013 Introduction to Telecommunication Engineering/Telecommunication Laboratory

ECE 4371, Fall, 2015

Introduction to Telecommunication Engineering/Telecommunication Laboratory

Zhu Han

Department of Electrical and Computer Engineering

Class 20

Nov. 3rd, 2015

OutlineOutline CDMA(code division multiple access)

– Introduction– FHSS(frequency hopping spread spectrum)– DSSS(direct sequence spread spectrum)– Application– Road map

Spread Spectrum Modulation Techniques Spread Spectrum Modulation Techniques Definition:

– The bandwidth of the transmitted signal is much greater than the bandwidth of the original message.

– The bandwidth of the transmitted signal is determined by the message to be transmitted and by an additional signal known as the Spreading Code.

Two main Spread Spectrum modulation techniques – Frequency Hopping Spread Spectrum (FHSS)– Direct Sequence Spread Spectrum (DSSS)

Two major advantages: – Low power density– Redundancy

Spread Spectrum Transmission Spread Spectrum Transmission A spread-spectrum transmission offers three main advantages

over a fixed-frequency transmission:– 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. – Spread-spectrum transmissions can share a frequency band with

many types of conventional transmissions with minimal interference.

Spread Spectrum Transmission Spread Spectrum Transmission – Spread-spectrum signals are difficult to intercept.

A spread-spectrum signal may simply appear as an increase in the background noise to a narrowband receiver.

An eavesdropper may have difficulty intercepting a transmission in real time if the pseudorandom sequence is not known.

– 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.

Therefore, bandwidth can be utilized more efficiently.

OutlineOutline CDMA(code division multiple access)

– Introduction– FHSS(frequency hopping spread spectrum)– DSSS(direct sequence spread spectrum)– Application– Road map

Frequency Hopping Spread SpectrumFrequency Hopping Spread Spectrum Definition

– A 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 use– Highly resistant to deliberate jamming– Limited protection

The JTIDS/MIDS family, HAVE QUICK and SINCGARS Civilian use

– In the unregulated 2.4 GHz band

Frequency Hopping PatternFrequency Hopping Pattern A transmitter "hops" between available frequencies according to

a specified algorithm. The transmitter operates in synchronization with a receiver,

which remains tuned to the same center frequency as the transmitter.

System StructureSystem Structure

Pseudo Random Sequence GeneratorPseudo Random Sequence Generator Random sequence

Randomness and noise properties Provide signal privacy

Two properties– Randomness and unpredictability.

Pure randomness is hard to achieve. – Pseudorandomness, the sequences produced are long and there is

no way of predicting the next number from the sequence.

Pseudo Random Sequence GeneratorPseudo Random Sequence Generator Linear Feedback Shift Registers(LFSR).

– LFSR are implemented as a circuit consisting of XOR gates and shift register.

– The register is a string of 1-bit storage devices.

The m+1th bit is output as the result of an operation on the previous m bits in the register.

By using a long enough sequence of bits, the sequence may appear to be random, even though it is actually a long cycle.

ECE 4371 Fall 2008

FHSS Resistance of JammingFHSS Resistance of Jamming Narrow band jamming FHSS under broadband jamming FHSS under partial band jamming

– Jamming on one frequency affects only a few bits

Multiple User AccessMultiple User Access Only one user with a large bandwidth is too wasteful. Allows multiple user to be admitted over the same frequency.

– This creates the possibility of system data rates that are higher than the Shannon limit for a single channel.

Each transmitter is assigned a unique code which allows multiple users to be multiplexed over the same physical channel.

Multiple User AccessMultiple User Access

Well designed PN sequence can prevent user collision. In practice, collision cannot be avoided

– Lack of a common synchronization clock– More than L active users access

OutlineOutline CDMA(code division multiple access)

– Introduction– FHSS(frequency hopping spread spectrum)– DSSS(direct sequence spread spectrum)– Application– Road map

Compare with FHSSCompare with FHSS FHSS adopts noncoherent detection (FSK) DSSS adopts coherent detection (QAM, PSK, PAM)

Direct Sequence Direct Sequence Spread SpectrumSpread Spectrum Modulates with a continuous string of pseudo-noise code

symbols called "chips", each of which has a much shorter duration than an information bit.

Spreading code spreads signal across wider frequency band– In proportion to number of bits used– 10 bit spreading code spreads signal across 10 times bandwidth of

1 bit code

Spreads the bandwidth of the data uniformly for the same transmitted power.

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

DataRateChipRate

T

TTTG

b

c

c

bp

1

1

Direct Sequence Direct Sequence Spread SpectrumSpread Spectrum One method:

– Combine input with spreading code using XOR– Input bit 1 inverts spreading code bit– Input zero bit doesn’t alter spreading code bit– Data rate equal to original spreading code

Multiple User AccessMultiple User Access

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 projections(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

Narrowband Interference SuppressionNarrowband Interference Suppression

Transmitter

Receiver

Wideband Interference SuppressionWideband Interference Suppression

Transmitter

Receiver

OutlineOutline CDMA(code division multiple access)

– Introduction– FHSS(frequency hopping spread spectrum)– DSSS(direct sequence spread spectrum)– Application– Road map

ApplicationApplication FHSS

– Bluetooth– frequency-hopping code division multiple access (FH-CDMA)

DSSS– CDMA– GPS– WLAN

BluetoothBluetooth Operates in the globally unlicensed (but not unregulated)

Industrial, Scientific and Medical (ISM) 2.4 GHz short-range radio frequency band.

CDMACDMA

Rake Receiver

ComparisonComparison

GPSGPS Why spread spectrum in GPS ?

– Signal from satellite can be kept from unauthorized use.– Inherent processing gain of spread spectrum allows reasonable

power levels to be user.– Each satellite can use the same frequency band, yet no mutual

interference.

WLANWLAN Why spread spectrum in WLAN ?

– Operates in the range of 2.4 GHz short-range radio frequency band.

– Interference resistance from other wireless device

OutlineOutline CDMA(code division multiple access)

– Introduction– FHSS(frequency hopping spread spectrum)– DSSS(direct sequence spread spectrum)– Application– Road map

Cellular Telephony EvolutionCellular Telephony Evolution

ECE 4371 Fall 2008

1G

Analog voice

telephony

No data connectivity

Example: AMPS

2G

Digital voice telephony (9.6

kbps-14.4 kbps)

CDMA, TDMA

Example: GSM, IS-54,

IS-95A, cdmaOne

2.5G

Digital voice telephony

data connectivity

Example: GPRS, IS-

958, IS-136, (E)-GPRS

3G

Digital voice telephony

broadband and data

connectivity

Example: WCDMA,

CDMA2000 TD-SCDMA

3.9G

IP based protocol for voice and

data

Mobile broadband and data

connectivity

Example: LTE

4G

Built-in self organizing

mechanisms

Interference mitigation

and co-existence

Example: LTE-

Advanced

20011981 1992 1999 2007 2011

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 64Channel 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 indoor

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

Road MapRoad Map1XRTT/3XRTT

cdma2000CDMA

(IS 95 A) IS 95 B

GSM

TDMA EDGE UWC-136

GPRS W-CDMA

3X3X

No 3XNo 3X

cdmaOnecdmaOneIS-95AIS-95A

1999 2000 2001 2002

1X1XIS-95BIS-95B

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