1

A Review on

Spread SpectrumTechniques and Their Applications

Presenter: Reza EbrahimiSpring 2010

2

Spread Spectrum (SS)

• Spread Spectrum is a technique in which pseudo-noise code, independent of the information data, is employed as a modulation waveform to spread the signal energy over a bandwidth much greater than the signal information bandwidth.

• Band expansion is achieved by a second modulation.• The term Spread Spectrum (SS) has been used in a

wide variety of military and commercial communication systems.

3

Advantages

The expanded bandwidth provides certain desirable features and characteristics that could otherwise be difficult to obtain.

• Common bandwidth usage• Improved interference rejection• Code division multiplexing for CDMA operation• Low-density power spectrum for signal hiding Anti jam

capability• Lower cost of implementation• Readily available IC components• Unlicensed operation in ISM band• Secure communication

4

Spread Spectrum modulations

• Direct Sequence Spread Spectrum (DSSS)• Frequency Hopping Spread Spectrum (FHSS)• Time Hopping Spread Spectrum (THSS)• Chirp Spread Spectrum (CSS)• Orthogonal Frequency Division Multiplexing

(OFDM)

5

Spread Spectrum in IEEE Standards and Protocols (1)

Protocol Modulation Description

802.11a-1999 OFDM 5 GHz54 Mbps

802.11b-1999 DSSS 2.4 GHz11 Mbps

802.11g-2003 OFDM 2.4 GHz6,9,12,18,24,36,48,54Mbps

802.11n-2009 OFDM 2.4 and 5 GHz54 Mbps

802.15.1-2002 FHSS(2005) Bluetooth1 Mbps

802.15.3-2003 OFDM and DSSS UWB11 to 54 Mbps

802.15.4-2003 CSS (2007) and C-UWB Includes ZigbeeLow data rate

6

Spread Spectrum in IEEE Standards and Protocols (2)

• IEEE 802.11 (Wi-Fi) for WLAN • IEEE 802.15 for WPAN• IEEE 802.16 (WiMax) for WMAN

Protocol Modulation Description

802.16.1 & 802.16.2-2001 Single Carrier (2001) Fixed broadband wireless access . 10 to 66 GHz

802.16a-2003 OFDM and OFDMA (2003) 2 to 11 GHz

802.16e -2005 OFDMA (2005) Mobile broadband wireless access

802.16d- … Single Carrier mode and OFDM

1.5 to 28 MHzUp to 26 Mbps

802.16k-2006 OFDM and OFDMA An amendment to 802.1d

802.16j-2005 OFDMA (2006)

7

DSSSMessage b(t)

PN sequence c(t)

B(t)c(t)

t

t

t

SNR psk = SNR dsss+psk

8

PN Sequences (1)

• Used for spreading spectrum of message signal• Pseudo-random• PeriodicMaximal length sequences or m sequences:

Rc(t)

t

Auto-correlation of a PN signal using M seq.

Tc

9

PN Sequences (2)

• Interference and multipath effectsSpread Spectrum multiple access sequences:Gold Sequence, Kasami Sequence, complex sequences

10

DSSS in IEEE 802.11• 1Mbps data signal → 11Mbps data signal• Using PSK modulation• Using 11-chip Barker sequence (+1 +1 +1 -1 -1 -1 +1 -1 -1 +1

-1) : provides good immunity against interference and noise, some protection against multipath propagation

• Operation in the 2.4 GHz ISM band, 11 channels in the US and 9 channels in Europe.

• At least 30MHz separation of center frequencies to avoid interfering with each other

• Also an option defined for 2Mbps using QPSK

11

DSSS in GPS

• Stubbornly uses DSSS• Low data rate (50bps)• 1023-chip pseudo-random spreading code (repeated every 1ms)• BPSK modulation (1.023 Mbps)• High level of redundancy in comparison with other wireless links• Power at receiver antenna ~ -130 dbm (noise power ~ -110 dbm)• At the receiver side, the decoding of the received signal is

performed before the demodulation. The decoder of the DSSS receiver is commonly implemented

as a despreading correlator.

12

FHSS

0 1 2 3 4 5 6 7 8-1.5

-1

-0.5

0

0.5

1

1.5

Transmitter :

Receiver :Wide-band BPF → × Frequency Generator → Narrow-band BPF → FSK Demodulator

Message b(t)

FHSS Signal (FSK Modulation)

t

t

13

FHSS in IEEE 802.11• Uses 79 non overlapping 1MHz channels to transmit a 1 Mbps

data signal over the 2.4 GHz ISM band. • A channel hop occurs every 224 microseconds• Each hopping pattern jumps a minimum of six channels in

each hop• Each 802.11 network must use a particular hopping pattern• An option provides for transmission at a rate of 2 Mbps• Hopping rate : 2.5 hop/s (hopping rate : 1600 hop/s in

Bluetooth)

14

THSS

t

Message

THSS Signal

t

15

THSS• There has been an interest in ultra wide band width (UWB)

time-hopping(TH) spread-spectrum(SS) multiple access techniques for both commercial and military applications

• The key motivations for using UWB TH-SS radio are the ability to highly resolve multipath, as well as the avail- ability of the technology to implement and generate UWB signals with relatively low complexity.• Since the signal is sent at different times no interference will

occur.• But sometimes we don’t use the channel

16

CSSA chirp is a sinusoidal signal whose frequency increases or decreases over a certain amount of time.

Chirp Signal

17

CSS

• Does not add any pseudo random elements to the signal

• Chirp Spread Spectrum is resistant to the Doppler effect

• Ideal for low power (difficult to detect) and low data rate applications.

• Currently, Nanotron Technologies, which produces real-time location devices and was the primary force behind getting CSS added to IEEE802.15.4a, is the only seller of wireless devices using CSS.

• It is not easy to build a chirp signal

18

OFDM

0 0.5 1 1.5 2 2.5 3

-1

-0.5

0

0.5

1

x

aaa

0 0.5 1 1.5 2 2.5 3

-1

-0.5

0

0.5

1

x

b

0 0.5 1 1.5 2 2.5 3

-1

-0.5

0

0.5

1

x

c

0 0.5 1 1.5 2 2.5 3

-1

-0.5

0

0.5

1

x

dd

0 0.5 1 1.5 2 2.5 3

-1

-0.5

0

0.5

1

x

ee

Message

t

t

t

t

t

t

PSK Modulation

19

Advantages and draw backs of OFDM

Advantages:• High spectral efficiency• Using FFT and IFFT• Low complex receivers (avoidance of ISI)• Different modulation schemes

Disadvantages:• More sensitive to Doppler shift• Accurate frequency and time synchronization needed• Phase noise

20

OFDM ApplicationsDAB DVB-T 802.11a 802.15.3a

(UWB)802.16d

Bandwidth 1.5 MHz 8MHz 20 MHz 528 MHz 1.5 to 28 MHz

Number of subcarriers

192 (256 FFT)384 (512 FFT)1536 (2k FFT)

1705 (2k FFT)6817 (8k FFT)

52 (64 FFT) 128 256 (OFDM mode)2048 (OFDMA mode)

Modulation D-QPSK QPSK,16-QAM, and 64-QAM

BPSK, QPSK, 16-QAM, and 64-QAM

QPSK QPSK, 16-QAM, and 64-QAM

Max. data rate

1.7 Mbps 31.7 Mbps 54 Mbps Up to 26 Mbps

21

References

[1] Theory and Applications of Spread-Spectrum Systems / Alex W.Lam and Sawasd Tantaranata / 2002

[2] OFDM-Based Broadband Wireless Networks Design and Optimization / Hui Liu and Guoqing Li / 2005

[3] Spread Spectrum and CDMA Principles and Applications / Valery P.Ipatov /2005[4] COMMUNICATION NETWORKS Fundamental Concepts and Key

Architectures / Alberto Leon-Garcia, Indra Widjaja/ 2006[5] Performance of Chirp Spread Spectrum in Wireless Communication Systems /

Xiaowei Wang Minrui Fei Xin Li / 2007[6] Multi-Carrie and Spread Spectrum Spectrum Systems from OFDM and MC-

CDMA to LTE and WiMAX / K.Fazel , S.Kasier / 2008[7] Ultra Wideband Circuits, Transceivers and Systems / Ranjit Gharpurey, Peter

Kinget/ 2008[8] Ultra-Wideband Communications Systems MULTIBAND OFDM APPROACH /

W. Pam Siriwongpairat, K. J. Ray Liu / 2008

22

Cont. References

[9] Ultra-Wideband Pulse-based Radio Reliable Communication over a Wideband Channel / Wim Vereecken , Michiel Steyaert / 2009

[10] Bit-Error-Rate Performance Analysis of an Overlap-based CSS System / Taeung Yoon, Dahae Chong, Sangho Ahn, and Seokho Yoon / 2009

[11] http://en.wikipedia.org/wiki/802.11[12] http://en.wikipedia.org/wiki/Chirp_Spread_Spectrum[13] http://cnx.org/content/m11762/latest/