Spread spectrum

SPREAD SPECTRUM AND MULTIPLE ACCESS

Consider a situation • There is a big gathering• You and your friend are attending• You want to communicate something to your friend alone• But all are noticing• How to do?• Speak in a language which no one knows in the gathering.

Uses of Spread Spectrum• Gained popularity by the needs of military communication• Proved to resistant against hostile jammers• Offers the following applications:• able to deal with multi-path• multiple access due to different spreading sequences• spreading sequence design is very important for

performance• low probability of interception• privacy• anti-jam capabilities

What is Spread Spectrum• A communication system is considered a spread spectrum

system if it satisfies the following two criteria:• Bandwidth of the spread spectrum signal has to be

greater than the information bandwidth• The spectrum spreading is accomplished before

transmission through the use of a code that is independent of data sequence. The same code is used in receiver to dispread the received signal.

• FM and PCM satisfy property 1 but not property 2

Functional block diagram

Types of spreading techniques

Pseudo-Noise Sequence• The pseudo-noise (PN) sequence is a periodic binary

sequence with a noise like waveform that is generated by means of a feedback shift register.

• How to generate?

Using feedback shift register• The feedback shift register consists of m-stage shift

registers and a logic circuit• Flip flops are regulated by clock• At each clock pulse state of the flip-flop change

PN Sequence generator

PN Sequence: Example• Three flip-flops form 3 bit shift register

• Logic circuit is modulo-2 adder

• Modulo-2 adder is nothing but 2 input X-OR gate

(inputs to this gate are s1 and s3,output of this gate is s0)

• Initial state is s1=1, s2=0 and s3=0

S1 S2 S3

1 0 0

1 1 0

1 1 1

0 1 1

1 0 1

0 1 0

0 0 1

1 0 0

• The PN sequence is the output sequence S3. Hence PN=0011101

• When we have m flip flops then the total number of possible states is 2m.

• If it is a linear feed back register then maximum length is 2m-1

• When the period is exactly 2m-1, the PN sequence is called maximal length sequence or simply m-sequence

Properties of maximal length sequence

• Balance property:• In each period of a maximal-length sequence, the number of 1’s is

always one more than the number of 0’s.

• Run property:• Among the runs of 1’s and of 0’s in each period of a maximal-

length sequence, one half the runs of each kind are of length one, one-fourth are of length two, one-eighth are of length three, and so on as long as these fractions represent meaningful numbers of runs.

• Auto correlation property:• The autocorrelation function of a maximal-length sequence is

periodic and binary valued.

Direct Sequence Spread Spectrum(DSSS)

• Transmitter• Channel• Receiver• Processing gain

Frequency Hopping SS• The Processing gain is dependent on the chip

duration.• Lesser the chip duration i.e., lengthier the PN

sequence better the PG.• Generating long PN sequence is complex. • PG achieved is not worth enough to combat jammer

interference• Hence Frequency Hopping Spread Spectrum is

used• Randomly hopping the data-modulated carrier from

one frequency to the next.

Frequency Hopping SS• The spectrum of the transmitted signal is

spread sequentially rather than instantaneously• The type of spread spectrum in which the

carrier hops randomly from one frequency to another is called frequency-hopping (FH) spread spectrum.

• M-ary frequency-shift keying (MFSK) is used along FHSS

• We are led to consider the rate at which the hops occur.

Frequency Hopping SS

• We are led to consider the rate at which the hops occur.

• Based on characterizations of frequency hopping it is classified as

• Slow-frequency hopping, in which the symbol rate Rs, of the MFSK signal is an integer multiple of the hop rate Rh

• That is, several symbols are transmitted on each frequency hop.

Frequency Hopping SS• We are led to consider the rate at which the hops occur.

• Based on characterizations of frequency hopping it is classified as

• Fast-frequency hopping, in which the hop rate Rh is an integer multiple of the MFSK symbol rate Rs.

• That is, the carrier frequency will change or hop several times during the transmission of one symbol

Slow Frequency Hopping SS | Transmitter

Slow Frequency Hopping SS | Receiver

Chip Rate• An individual FH/MFSK tone of shortest duration is

referred to as a chip.• Chip rate Rc, for FH/MFSK is defined as,

• For Slow FH/MFSK system, Chip rate = ??

Slow Frequency Hopping SS• For Slow FH/MFSK system,

Slow FHSS | Illustration

Fast FHSS/MFSK

• In Slow FHSS/MFSK , the jammer can measure the spectral component and tune the noise to corresponding frequency band

• Hence Fast FH is used.• For a single symbol, multiple hops are required• Chip rate = ??

Fast FHSS | Illustration

CONSIDER THIS…• Consider a large room with many people. May be like this

class room. Everyone wants to talk with someone. If all simultaneously speak then there will not be any communication at all.

• How to solve this problem?• Large room divided up into small rooms.

• Each pair of people takes different rooms.

• No division business• each pair gets 20 seconds to speak.

• Everyone is speaking in different languages.• No small rooms; no restriction on timings

Multi user radio communication• Types:

• Satellite communication- line of sight but global coverage• Wireless communication- mobility

• Both of them uses a special technique called as multiple access

• Definition of multiple access:• a technique whereby many subscribers or local stations can share

the use of a communication channel at the same time

• There is little difference between multiplexing and multiple access

Differences between Multiple access & multiplexingMULTIPLE ACCESS MULTIPLEXING

remote sharing of a communication channel such as a satellite or radio channel by users in highly dispersed locations

sharing of a channel such as a telephone channel by usersconfined to a local site.

a technique whereby many subscribers or local stations can share the use of a communication channel at the same time

A technique where many inputs are combined and a single output is transmitted over a single transmission medium

user requirements can change dynamically with time

user requirements are ordinarily fixed.

Example: FDMA, CDMA, TDMA, SDMA

Eg: 2:1 MUX, 4:1 MUX

Frequency-division multiple access (FDMA).

• In this technique, disjoint sub-bands of frequencies are allocated to the different users on a continuous-time basis.

FDMA channel structure

Time-division multiple access (TDMA).

• In this second technique, each user is allocated the full spectral occupancy of the channel, but only for a short duration of time called

a time slot.

TDMA channel structure

Code-division multiple access (CDMA).

• another technique for sharing the channel resources by using a hybrid combination of FDMA and TDMA, which represents a specific form of code-division multiple access (CDMA).

• To be specific, during time slot 1, user 1 occupies frequency band 1, user 2 occupies frequency band 2, user 3 occupies frequency band 3, and so on. During time slot 2, user 1 hops to frequency band 3, user 2 hops to frequency band 1, user 3 hops to frequency band 2, and so on.

• An important advantage of CDMA over both FDMA and TDMA is that it can provide for secure communications.

Wireless Communication• Synonymous with mobile radio• Concept of mobile radio• Basic terminologies used is mobile radio are:

• Cell• Base station, mobile station• Switching center

• Shape of cell-

Main feature of cellular radioFrequency re-use:

The term frequency reuse refers to the use of radio channels on the same carrier frequency to cover different areas, which are physically separated from each other sufficiently to ensure that co-channel interference is not objectionable.

Cell splitting:

When the demand for service exceeds the number of channels allocated to a particular cell, cell splitting is used to handle the additional growth in traffic within that particular cell. The new cells, which have a smaller radius than the original cells, are called micro cells.

The transmitter power and the antenna height of the new base stations are correspondingly reduced, and the same set of frequencies are reused in accordance with a new plan.

Source Coding of Speech forWireless Communications• For the efficient use of channel bandwidth, digital wireless

communication systems rely on the use of speech coding to remove almost all of the natural redundancy in speech, while maintaining a high-quality speech on decoding.

• The common approach is to use source coding, which, in one form or another, exploits the linear predictive coding (LPC) of speech.

• Techniques for speech coding:• Multi-pulse excited LPC• Code excited LPC

Multi-pulse Excited LPC• principle of analysis by synthesis: the encoder includes a

replica of the decoder in its design.• Encoder consists of 3 main parts

• Synthesis filter• Excitation generator• Error minimization

encoder• Synthesis filter: designed to model the short-term spectral

envelope of speech; the term short-term refers to the fact that the filter parameters are computed on the basis of predicting the present sample of the speech signal using eight to sixteen previous samples.

• Excitation generator: for producing the excitation applied to the synthesis filter. The excitation consists of a definite number of pulses every 5 to 15ms .

• Error minimization: for optimizing the weighted error between the original speech and synthesized speech. The aim of this minimization is to optimize the amplitudes and positions of the pulses used in the excitation.

Code excited LPC(CELP)• The distinguishing feature of CELP is the use of a

predetermined codebook of stochastic (zero-mean white Gaussian) vectors as the source of excitation for the synthesis filter.