ECE 4371, Fall, 2013
Introduction to Telecommunication
Engineering/Telecommunication Laboratory
Zhu Han
Department of Electrical and Computer Engineering
Class 18
Nov. 3rd, 2014
Outline

CDMA(code division multiple access)
– Introduction
– FHSS(frequency hopping spread spectrum)
– DSSS(direct sequence spread spectrum)
– Application
– Road map
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

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 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.
Outline

CDMA(code division multiple access)
– Introduction
– FHSS(frequency hopping spread spectrum)
– DSSS(direct sequence spread spectrum)
– Application
– Road map
Frequency 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 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 Structure
Pseudo 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 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 Jamming

Narrow band jamming

FHSS under broadband jamming

FHSS under partial band jamming
– Jamming on one frequency affects only a few bits
Multiple 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 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
Outline

CDMA(code division multiple access)
– Introduction
– FHSS(frequency hopping spread spectrum)
– DSSS(direct sequence spread spectrum)
– Application
– Road map
Compare with FHSS

FHSS adopts noncoherent detection (FSK)

DSSS adopts coherent detection (QAM, PSK, PAM)
Direct Sequence Spread 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 & Despreading

Spreading
– Source signal is multiplied by a PN signal
1
Tb
Tc ChipRate
Gp  

1
Tc
DataRate
Tb

Processing Gain:

Despreading
– Spread signal is multiplied by the spreading code

Polar {±1} signal representation
Direct Sequence Spread 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 Access

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
Jammer/Noise/Interference
j(t)
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)
Channel
b(t)
Source
Data
BPSK
Modulator
s(t)
x(t)
c(t)
Pseudorandom
Sequence
Generator
y(t)
u(t)
c(t)
Pseudorandom
Sequence
Generator
BPSK
Matched
Filter
rn
Output
Data
(to detector)
Narrowband Interference Suppression
Transmitt
er
Receiver
Wideband Interference Suppression
Transmitt
er
Receiver
Outline

CDMA(code division multiple access)
– Introduction
– FHSS(frequency hopping spread spectrum)
– DSSS(direct sequence spread spectrum)
– Application
– Road map
Application

FHSS
– Bluetooth
– frequency-hopping code division multiple access (FH-CDMA)

DSSS
– CDMA
– GPS
– WLAN
Bluetooth

Operates in the globally unlicensed (but not unregulated)
Industrial, Scientific and Medical (ISM) 2.4 GHz short-range
radio frequency band.
CDMA
Rake Receiver
Comparison
GPS

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.
WLAN

Why spread spectrum in WLAN ?
– Operates in the range of 2.4 GHz short-range radio frequency
band.
– Interference resistance from other wireless device
Outline

CDMA(code division multiple access)
– Introduction
– FHSS(frequency hopping spread spectrum)
– DSSS(direct sequence spread spectrum)
– Application
– Road map
Cellular Telephony Evolution
1G
2G
2.5G
3G
3.9G
4G
Analog
voice
telephony
Digital voice
telephony (9.6
kbps-14.4
kbps)
Digital voice
telephony
Digital voice
telephony
Built-in self
organizing
mechanisms
data
connectivity
broadband and
data
connectivity
IP based
protocol for
voice and
data
No data
connectivity
Example:
AMPS
1981
CDMA,
TDMA
Example:
GSM, IS-54,
IS-95A,
cdmaOne
1992
ECE 4371 Fall 2008
Example:
GPRS, IS958, IS-136,
(E)-GPRS
1999
Example:
WCDMA,
CDMA2000
TD-SCDMA
2001
Mobile
broadband
and data
connectivity
Example:
LTE
2007
Interference
mitigation
and coexistence
Example:
LTEAdvanced
2011
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)

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 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)

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 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)

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
Outline

CDMA(code division multiple access)
– Introduction
– FHSS(frequency hopping spread spectrum)
– DSSS(direct sequence spread spectrum)
– Application
– Road map
Thanks
Road Map
1XRTT/3XRTT
CDMA
(IS 95 A)
IS 95 B
cdma2000
GSM
GPRS
W-CDMA
TDMA
EDGE
UWC-136
1999
2000
2001
2002
3X
cdmaOne
IS-95A
1X
IS-95B
No 3X
2G
2.5G
3G Phase 1
3G Phase 2