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UNIT 4

Spread Spectrum and Multiple Access Techniques

INTRODUCTION TO SPREAD SPECTRUM TECHNIQUE:

• Spread spectrum communication systems are widely used today in a variety of applications for different purposes such as

▪ Access of same radio spectrum by multiple users (multiple access)

▪ Anti-jamming capability

▪ Interference rejection

▪ Secure communication

▪ Multipath rejection

• Primary advantage of a spread spectrum communication system is its ability to reject interference,

➢ whether it be the unintentional interference by another user simultaneously attempting to transmit through the channel, (or)

➢ Intentional interference by a hostile transmitter attempting to jam the transmission.

• In the field such as “military communication”, the information has to be highly secured.

▪ That means unauthorized user can’t access the information.

▪ Also third party cannot be allowed to interfere the communication by any means.

• Spread spectrum modulation can be defined in two parts.

▪ Spread spectrum is a means of transmission in which the data sequence occupies a BW in excess of the minimum BW required to send it.

▪ Spectrum spreading is accomplished before transmission through the use of a code that is independent of the data sequence.

✓ The same code is used in the receiver to despread the received signal so that the original data sequence may be recovered.

• Standard modulation techniques such as FM and pulse code modulation do satisfy part 1 of this definition, they are not spread spectrum techniques because they do not satisfy part 2 of this definition.

USES OF SPREAD SPECTRUM COMMUNICATION:

• Suppressing the determined effects due to jamming, interference arising from other users of the channel, and self interference due to multi path propagation.

• Achieving message privacy in the presence of other listeners.

ADVANTAGES OF SPREAD SPECTRUM MODULATION:

• Improved interference rejection and Anti jam capability.

• Code division multiplexing for CDMA application.

• Low density power spectra for signal hiding.

• Secure communications.

• Increased capacity and spectral efficiency in mobile communication systems.

• Lower cost of implementations.

DISADVANTAGES OF SPREAD SPECTRUM:

• There is no processing gain during signal acquisition.

• It is not useful option outside the US and Canada.

• Signal acquisition requires time and it has a complex circuit.

APPLICATIONS OF SPREAD SPECTRUM:

• Used in military communication systems.

• Used in satellite communication and LAN.

• Multiple access communications in which a number of independent users are required to share a common channel without external synchronization mechanism.

• Used in networking.

DIFFERENCE BETWEEN "JAMMING" AND "INTERFERENCE":

• Jamming ( Describe the careful use of radio noise or signals in an attempt to disrupt communications and also called as Intentional Interference.

• Interference (Describe unintentional forms of disruption.

CLASSIFICATION OF SPREAD SPECTRUM:

The spread spectrum techniques are classified as follows:

1. Direct Sequence Spread Spectrum (DSSS)

• First incoming data sequence is used to modulate a wide band code. This code transforms a narrow band data sequence into a noise like wide band signal.

• Then this signal undergoes second modulation using PSK.

2. Frequency Hop Spread Spectrum (FHSS)

• Spectrum of a data modulated carrier is widened by changing the carrier frequency in a pseudo random manner.

3. Time Hoping (TH)

• TH is a communications signal technique which can be used to achieve anti jamming or Low Probability of Intercept (LPI).

• Also refer to pulse position modulation.

4. Hybrid Spread Spectrum (HSS)

• In mobile radio and wireless LAI (Local Area Identity) direct sequence, frequency hopped SS or CSMA methods are widely used.

• All these techniques rely on the availability of a noise like spreading code called pseudo random or pseudo noise sequence.

PSEUDO-NOISE SEQUENCE:

Definition:

• It is a periodic binary sequence, with a noise-like wave form.

• It is usually generated by using a feedback shift register.

BLOCK DIAGRAM OF FEEDBACK SHIFT REGISTER:

[pic]

• Feedback shift register consists of an ordinary shift register made up of 1 to m - flip flops (two state memory stages) and a logic circuit (computer Boolean function) that are inter-connected to form a multi loop feedback circuit.

• All the flip-flops are regulated by a single timing clock.

• At each clock pulse, the state of each flip-flop is shifted to the next state.

• With each clock pulse, the logic circuit computes the Boolean function of the states of the flip-flops.

• Output of the logic circuit is then fed back as the input of the first flip flop, thereby preventing the shift register from emptying.

• The PN sequence to be generated is determined by the length m of the shift register, its initial state, and the feedback logic.

• Let Sj(k) denote the state of the jth flip flop after the kth clock pulse; this state may be represented by symbol 0 or 1.

• For the initial state, k is 0.

• For a specified length m, this Boolean function uniquely determines the subsequent sequence of states and therefore the PN sequence produced at the o/p of the final flip flop in the shift register.

• With a total number of m flip flops, the number of possible states of the shift register is at most 2m.

• The above concept can be explained with an example given below. Normally the logic circuit is modulo-2 adders.

CONDITIONS FOR CHECKING THE OUTPUT SEQUENCE:

• The Pseudo Noise (PN) sequence produced by a feedback shift register cannot exceed 2m - 1.

• The number of 1's is always one more than the number of 0's.

• When the period is exactly 2m-1, then the PN sequence is called a maximal-length sequence (or) simply m-sequence.

Example 1: Consider the linear feedback shift register shown in figure below, involves 3 flip-flops.

• Modulo 2 addition of S1, S3 is given to S0. i.e., S1 + S3. Output is given as feedback input to S0.

• Here number of flip flops = 3, therefore m = 3

[pic]

• Let us consider the initial state of the flip flop is 101.

• From the table, modulo 2 addition of S1 + S3 is given as input to the First state of Flip flop, and then each state is shifted one state as shown as in the above table.

• Similarly, the same steps to be followed till the original state 101 i.e., up to 2m- 1 status to be followed.

2m - 1 = 23 - 1 = 8 - 1 = 7

• The output sequence S3 is 1 0 1 0 0 1 1 1......... .

[pic]

PROPERTIES OF MAXIMUM LENGTH SEQUENCES:

• Maximal-length sequences have many of the properties owned by a truly random binary sequence.

• A random binary sequence is a sequence in which the presence of binary symbol 1 or 0 is equally probable.

Some of the properties are

i) Balance Property:

• The number of 1's is always one more than the numbers of 0's in each period of a maximal-length sequence.

ii) Run Property:

• Among the runs of 1’s and of 0’s in each period of a maximal length sequence, one half the runs 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.

• "Run" refers a subsequence of identical symbols; 1's or 0's within one period of sequence, i.e.,

Length of this subsequence = Length of Run

For a maximum-length sequence of Linear feedback shift register of length m, the total number of runs is (N + 1)/2; where N = 2m- 1.

iii) Auto Correlation Property:

• The auto correlation of a maximum length sequence is periodic and binary valued. This property is called correlation property.

• Period of a maximum length sequence is defined by N = 2m- 1.

Example:

00001010111011000111110011001

• Balance: 15 0’s ; 16 1’s

• Run: 2 runs of length 3; 4 runs of length 2; etc.

• Correlation:

– Auto ~ 1=31/31

– Cross ~ 0≈-1/31

Let us consider c(t), the resulting waveform of the maximal-length sequence, shown below for N = 7.

Binary sequence:

• From the figure,

N = 2m- 1; m = length of shift register m = 3.

N= 23-1= 8-1 = 7

• Let the binary symbols 0 and 1 of the sequence denoted by the levels..., -1 and + 1 respectively.

Waveform of maximal length sequence for length m = 3, N = 7.

[pic]

• The period of the waveform c(t) is given by

Tb = NTc

Where, Tc =Duration assigned to symbol 1 (or) 0

• The auto correlation function of a periodic signal c(t) of period Tb is given as

[pic]

• The lag [pic]lies in the interval (-Tb/2, Tb/2)

• Apply this to a PN sequence, then the above equation becomes

[pic]

Autocorrelation function is plotted below.

[pic]

How to choose a maximal-length sequence?

• We can generate the maximal length sequence using a linear feedback shift register.

• The key question we need to address is: How to find the feedback logic for a period N?

• The answer to this question is found in the theory of error-control codes.

• The following table shows the feedback logics pertaining to shift register lengths m = 2, 3, 4, 5, 6, 7, 8.

• When m increases, the number of alternative schemes (or) codes is enlarged.

• Also for every set of feedback connections, there is an image set that generates an identical maximal length code, reversed in time sequence.

Maximal length sequence of shift register lengths 2-8

[pic]

Example:

Consider a maximum sequence for a length m = 5.

From the above table the possible feedback logics are [5, 2], [5, 4, 3, 2], [5, 4, 2, 1]

i) Let us use [5, 2].

[pic]

ii) Next sequence [5, 4, 3, 2]

[pic]

Advantages of selection of feedback logics:

• The PN sequence will satisfy all the properties of maximum length sequences.

• Easy to implement the feedback.

Solution for the sequence [5, 2]

[pic]

NOTION OF SPREAD SPECTRUM:

• Important attribute of spread spectrum is that it can provide protection against externally generated interfering signals with finite power.

• The jamming signal may consist of a fairly powerful broadband noise that is directed to the receiver for the purpose of disrupting communications.

• Protection against this jamming waveform is provided by purposely making the information bearing signal occupy a bandwidth far in excess of the minimum bandwidth necessary to transmit it.

• This has the effect of making the transmitted signal assume a noise like appearance.

• The transmitted signal is thus enabled to propagate through the channel undetected by anyone who may be listening.

Modulation process:

• One method of widening the bandwidth of the data sequence involves the use of modulation

• Let {bk} denote a binary data sequence, and {ck} denote a pseudo noise (PN) sequence.

• b(t) refers to incoming data signal and c(t) denotes the pseudo noise signal.

• The desired modulation can be achieved by applying the data signal b(t) and the PN signal c(t) to a product modulator or multiplier.

• If the incoming message signal b(t) is narrowband and the PN signal c(t) is wideband, the product (modulated) signal m(t) will have a spectrum that is nearly the same as the wideband PN signal.

• In other words, the PN sequence performs a role of a spreading code.

• By multiplying the information message signal b(t) by the PN sequence c(t), each information bit is chopped up into a number of small time increments. The small time increments are commonly referred to as chips.

[pic]

• The product signal m(t) represents the transmitted signal. Thus it can be expressed as

m(t) = c(t)b(t)

• The received signal y(t) consists of the transmitted signal m(t) plus an additive interference denoted by i(t). Thus it can be expressed as

r(t) = m(t)+i(t)

= c(t)b(t) +i(t)

• To recover the original message b(t), the received signal y(t) is applied to a demodulator that consists of a multiplied followed by an integrator and a decision device.

• The multiplier is supplied with a locally generated PN sequence that is an exact replica of that used in the transmitter.

• We assume that the receiver operates in perfect synchronism with the transmitter, which means that the PN sequence in the receiver is lined up exactly with that in the transmitter.

• The multiplier output in the receiver is given by

z(t) = c(t)y(t)

= c2(t)b(t) + c(t)i(t)

• Here the data signal b(t) is multiplied twice by the PN signal c(t), whereas the unwanted signal i(t) is multiplied only once.

• The PN sequence alternates between +1 and -1, and the alteration is destroyed when it is squared. Then the value of c2(t) is always 1. So the multiplier output z(t) is given by

z(t) = b(t) + c(t)i(t)

• Finally the incoming component b(t) is narrowband, whereas the noise component c(t)i(t) is wideband.

• Hence, by applying the multiplier output to a LPF with a bandwidth just large enough to accommodate the recovery of the data signal b(t), most of the power in the noise component c(t)i(t) is filtered out.

• The effect of the interference i(t) is thus significantly reduced at the receiver output.

• The low pass filtering action is performed by the integrator.

• Integration is carried out for the bit interval 0≤t≤Tb providing the value v.

• Then the decision is made by the receiver.

o If v is greater than the threshold of zero, the receiver says that binary symbol 1 was sent in the interval 0≤t≤Tb.

o If v is lesser than the threshold of zero, the receiver says that binary symbol 0 was sent.

o If v is exactly 0, the receiver makes a random guess in favor of 1 or 0.

Summary:

• The use of PN code in the transmitter produces a wideband signal that appears noise like to a receiver that has no knowledge of the spreading code.

• The longer the period of PN sequence, the harder it is to detect.

• In order to provide more protection against interference, the price we may have to pay is the increased bandwidth, system complexity and processing delay.

Data signal b(t).

[pic]

Spreading code c(t).

[pic]

Product Signal m(t).

[pic]

DIRECT SEQUENCE (DS) SPECTRUM WITH COHERENT BINARY PSK:

Definition:

• Direct sequence spread spectrum, also known as direct sequence code division multiple Access (DS - CDMA), is one of the two approaches to spread spectrum modulation for signal transmission over the transmission medium.

• In this, the sequence of information to be transmitted is divided into small pieces, each of which is allocated across to a frequency channel across the spectrum.

• A data sequence at the point of transmission is combined with a higher data-rate bit sequence (also known a chirping code) that divides the data according to a spreading ratio.

• The redundant chirping code helps the signal resist interference and also enables the original data to be recovered if data bits are damaged during transmission.

• For long distance transmission (or) transmission over a satellite channel, BPSK can be used.

DS - BPSK TRANSMITTER:

[pic]

EXPLANATION:

• First, the binary data sequence bk is given to Non-Return to zero level encoder.

• This encoder converts bk into bipolar NRZ waveform b(t), which is followed by 2 stages of modulation.

• The first stage consists of a product modulator (or) multiplier.

• The second stage consists of BPSK modulator.

• The Pseudo-noise sequence generator generates and encodes this sequence in bipolar NRZ signal.

• The product modulator (or) multiplier multiplies the two signals b(t) and c(t). The output of multiplier is direct-sequence spread signal m(t).

• This output signal m(t) is given as modulating signal to Binary PSK Transmitter.

• The direct sequence BPSK (DS-BPSK) is generated at the Output x(t).

• The phase modulation θ(t) of x(t) has one of two values 0 and Π, depending on the polarities of the message signal b(t) and PN signal c(t) at time t in accordance with the truth table.

Truth Table for phase modulation:

| | |Polarity of data sequence b(t) at time t |

| | |+ |- |

|Polarity of PN sequence |+ |0 |Π |

|c(t) at time t | | | |

| |- |Π |0 |

When m(t) is positive; phase shift of 0°.

m(t) is negative phase shift of 180°.

• It can be represented in the waveforms shown in figure.

Product signal m(t)

[pic]

Carrier Signal c(t)

[pic]

Transmitted signal x(t)

DS - BPSK RECEIVER:

• Block Diagram of DS/BPSK Receiver consists of 2 stages of Demodulation.

• In the first stage, the received signal y(t) and a locally generated carrier are applied to a product modulator (or) multiplier.

• The output of the product modulator is then applied to the low pass filter. The BW of low pass filter is equal to the original message signal m(t).

• This stage of demodulation process reverses the phase shift keying which is applied to the transmitted signal.

• The second stage of demodulation performs spectrum de-spreading, that is obtained by multiplying the low pass filter output with locally generated PN code c(t).

• The Integrator integrates the product of detected message signal and pseudo noise signal over one bit interval 0≤t≤Tb.

• Finally decision is made by the receiver:

o If v is greater than the threshold of zero, the receiver says that binary symbol 1 was sent in the interval 0≤t≤Tb.

o If v is lesser than the threshold of zero, the receiver says that binary symbol 0 was sent.

o If v is exactly 0, the receiver makes a random guess in favor of 1 or 0.

[pic]

• Integration is carried out for the interval 0 ≤ t ≤ Tb, providing the sample value V.

• Finally, a decision is made by the Receiver. If V is greater than the threshold of zero, the receiver says that binary symbol '1' of the original data sequence was sent in the interval 0≤ t ≤Tb.

• If V is less than zero, the receiver says that symbol '0' was sent.

• If V is exactly zero than the receiver makes a random guess in favour of '1' (or) '0'.

PROCESSING GAIN:

DEFINITION:

• It can be defined as the ratio of the information bit duration to the PN chip duration.

Processing Gain = Tb/Tc

Which represents the gain achieved by processing a spread spectrum signal over an un spreaded signal. The longer the PN sequence, the larger will be the processing gain.

BANDWIDTH OF SPREADED SIGNAL:

[pic]

• From the figure it can be seen that anyone bit period in the message signal m(t) is same as of the spreading code c(t).

Bandwidth of Unspreaded Signal:

• In case of NRZ format, the bandwidth of the signal is equal to 1/one bit period.

[pic]

• By combining and substituting the formula, we obtain,

[pic]

• And also, from the figure, the one bit period 'Tb' of data signal is equal to 'N' bits of periods of spreaded pseudo noise signal.

[pic]

Substitute Tb = NTc in equation (3), we get,

[pic]

FREQUENCY HOP SPREAD SPECTRUM SIGNALS (FHSS):

Definition:

• It is a method of transmitting radio signals by means of quick switching of carrier among many frequency channels.

• Using PN sequence, this is known to both transmitter and Receiver, so that the transmitted information can be recovered.

• This is the type of spread spectrum in which the carrier hops (or) frequency slots in random fashion from one frequency to another.

• A common modulation format for FH systems is that of M-ary frequency shift keying (MFSK). In combination with frequency hopping it called as (FH-MFSK).

TYPES OF FREQUENCY HOPPING:

• Slow frequency hopping

• Fast frequency hopping

SLOW FREQUENCY HOPPING:

Definition:

• Here several symbols are transmitted on each frequency slots in which the symbol Rate Rs (rate at which k-bit symbols of data input sequence generated) of the MFSK is an integer multiples of the hop rate Rh (rate at which change of frequency slots).

FH-MFSK TRANSMITTER:

EXPLANATION:

• The Binary data is applied to the M-ary FSK Modulator.

• The output of FSK modulator is then applied to the mixer. A particular Frequency from the frequency synthesizer is also applied to the mixer.

• The mixer, which consists of a multiplier followed by a Band pass filter.

• The filter is designed to select the sum of frequency component results from the multiplication process of the transmitted signal.

• The output of mixer is FH/MFSK signal with large Bandwidth.

• FH Bandwidths of the order of several GHz is obtained, so that coherent detection is possible.

[pic]

FH-MFSK TRANSMITTER:

EXPLANATION:

• The frequency hopping (FH) is first removed by mixing the received sjgnal with the output of a frequency synthesizer.

• The mixing output is then applied to the band pass filter and it is subsequently processed by a non-coherent MFSK Detector.

• The M-ary Detector can be implemented by using a M-noncoherent matched filters, each of which is aligned to the matched tones.

• This non-coherent detector detects the particular symbol transmitted.

[pic]

CHIP RATE: (Rc)

• In FH-MFSK, the tone of shortest duration is referred as a "chip".

Definition

• The chip rate is defined as

Rc= max(Rb, Rc)

Where, Rb = hop rate

Rc = Symbol rate

• In slow FH-MFSK, multiple symbols are transmitted per hop. Each symbol is a chip.

Relation between Rb, Rs, Rc:

It can be related as

[pic]

Where, k = log2M

FREQUENCY HOPPING EXAMPLE:

Channel Assignment

[pic]

Channel use

[pic]

• In the Channel Assignment figure there are 3 bits of PN sequence are used to select a hop. Therefore totally 23 = 8 different hops over the complete Frequency Hopping Bandwidth.

• The next figure shows the uses of channels mentioned in the channel Assignment.

• Example of slow frequency Hop spread spectrum using MFSK (M = 4, k = 2)

• There will be total of M = 22 = 4 symbols. In a single frequency hop there are 4 different frequencies. Those 4 different frequencies correspond to 4 possible symbols.

• Two symbols will occupy any 2 frequencies in one hop out of four available.

This is explained in below figure.

From the figure we noted that

• Frequency is shifted for every Tc seconds.

• Duration of signal element is Ts seconds.

• In slow FHSS, Tc ≥Ts

[pic]

EXAMPLE OF FAST FREQUENCY HOP SPREAD SPECTRUM USING MFSK (M = 4, K = 2)

From the below figure we stated that

• Fast FHSS has Tc < Ts

• FHSS gives improved performance in noise.

[pic]

MULTIPLE ACCESS TECHNIQUES:

Definition:

• Multiple - many, it is a technique where many subscribers (or) many local stations can share the information through the same channel at the same time.

Difference between multiple access and multiplexing:

• Multiple Access ( sharing of a communication channel, such as satellite (or) radio channel by the users in a wide locations. User requirements can change with time.

• Multiplexing ( sharing of a channel such as telephone channel by the users to a local site. Here user’s requirements are fixed.

TYPES OF MULTIPLE ACCESS TECHNIQUES:

• Frequency - Division multiple Access (FDMA).

• Time - Division multiple Access (TDMA).

• Code - Division multiple Access (CDMA).

• Space - Division multiple Access (SDMA).

WIRELESS COMMUNICATIONS:

• Wireless communications is enjoying its fastest growth period in History. With the development of highly reliable, solid state radio frequency hardware in the 1970s', the wireless communication was born.

• Wireless communication which is similar meaning with mobile Radio. The term "mobile Radio" which covers indoor (or) outdoor forms of wireless communications.

CHARACTERISTICS OF MOBILE RADIO:

There are 2 factors which plays important role.

• Median signal strength

• Signal Variability

Median signal strength:

To predict the minimum power needed from the transmitter side.

To provide good quality of coverage over a pre-determined service area.

Signal Variability:

It deals with the fading nature of the channel.

CONCEPT OF CELLULAR RADIO:

[pic]

Cell:

• A Large geographical area is divided into smaller sections which is called cells.

Cellular Concept:

• Each area is divided into hexagonal shaped cells, with the base station located at the center of each cell.

• The cell has a radius of 1 to 12 miles.

Why Hexagon shape?

• It was chosen, because it provides most effective transmission.

• It eliminated the gaps between the adjacent circles.

Functions

i) Base Station

• It interfaces the mobile subscribers and the cellular radio system.

• It is connected to the switching center by means of wires.

ii) Mobile switching center (MSC)

• It acts as interface between cellular radio system and the public switched telephone network (PSTN).

• It performs overall supervision, monitoring, control of the mobile communication.

• It monitors the signal-to-noise ratio (SNR) of a call between the base station and the mobile subscriber.

Hand off (or) Hand over:

• One of the important feature of cellular concept is a technique in which a mobile subscriber can move from one base station to other base station, when the calls are already in progress (i.e.,) without any interruption of service.

Frequency Reuse:

• It is the process, in which the same set of frequencies (channels) can be allocated to more than one cell, provided that the cells are a certain distance apart.

Frequency Reuse concept:

• In the figure, cells with the same letter use the same set of channel Frequencies. These cells are placed at a particular distance to avoid the co-channel Interference.

[pic]

Cell splitting:

• It is the process of sub dividing the highly congested cells in to smaller cells, each have their own base station and set of channel frequencies.

• Macro cells are divided in to micro cells as the traffic density increases.

• Each time a cell is split, its transmit power is reduced.

[pic]

Co-channel cells:

• With the concept of frequency Reuse, several cells within a given coverage area use the same set of frequencies.

• Two cells using the same set of frequencies are called as co-channel cells and the interference between them is called co-channel Interference.

Steps to find the nearest co-channels is as follows:

• Move i cells through the center of successive cells.

• Turn 60° in a counter clockwise direction.

• Move j cells forward through the center of successive cells.

Cluster:

• The cells that collectively use the complete set of available channel frequencies are called a cluster.

• Formula for finding the number of cells in a cluster is

N = i2+ij+j2

Where N = Number of cells per cluster,

i+ j = Non negative integer values

This concept is shown in an example below.

Example:

• Determine the number of cells in a cluster and locate the co-channel cells for the following values. j =2, i = 3

Solution:

Formula for finding the number of cells in a cluster is

N = i2 + ij + j2

Substitute j = 2, i = 3 in the above equation.

N = (3)2 + (3)(2) + (2)2 = 9 + 6 + 4

= 19

Co-channel Location:

[pic]

The above figure clearly explains the six nearest co-channel cells for cell A.

• First i = 3 cells moved, through the center of successive cells.

• Then turned 60° in a anti-clockwise direction.

• Then j = 2 cells moved forward through the center of successive cells.

PROPAGATION EFFECTS:

• Propagation problems caused due to the antenna mobile unit, which lie well below the surrounding buildings. (i.e) there is no "Line of sight" (LOS) path to the base station.

• Instead of the above, radio propagation takes place by means of 2 factors.

➢ Scattering from the surrounding buildings.

➢ Diffraction.

[pic]

• From the figure it is noticed that the energy reaches the receiving antenna (via) more than one path.

MULTIPATH PHENOMENON:

DEFINITION:

• It can be defined as, the phenomenon in which the various radio ways reach their destination from different directions with different time delays.

CONCEPT OF MULTIPATH PHENOMENON

It can be explained by means of two factors

• Static environment

• Dynamic environment.

Static Environment:

• It involves a stationary Receiver and a transmitted signal. It consists of a narrow band signal.

• Let us assumed that we received two attenuated version of the transmitted signal.

• A Relative phase shift is introduced between the 2 components of the Received signal.

Then there arises two cases.

• Constructive form.

• Destructive form.

[pic]

• Figure (a) shows the constructive form, in which the relative phase shift is zero. The two signals added constructively.

• Figure (b), the phase shift is 180°, the two signals added destructively.

• It can also be explained in terms of phasor representations, shown below.

[pic]

Dynamic environment:

• Here Receiver is in motion.

• Two Versions of the transmitted signal (Narrow band signal) reaches the Receiver (via) different path lengths.

• The result is the continuous change in the length of each propagation path.

TDMA AND CDMA WIRELESS COMMUNICATION SYSTEMS

Definition

TDMA (Time Division multiple Access):

• The Radio spectrum is divided in to various Time slots.

• In each slot only one user can transmit (or) Receiver.

• In TDMA, each user occupies a repeating time slot.

• Channel may be allotted for each slot.

• N time slots comprise a frame.

[pic]

Code Division Multiple Access (CDMA):

Definition:

• In CDMA, the narrow band message signal is multiplied by a very large Bandwidth signal called spreading signal.

• The spreading signal is a pseudo noise code sequence that has a chip rate of magnitude greater than the data rate of the message.

[pic]

• In a CDMA system, use the same carrier Frequency and transmit simultaneously.

• Each user has its own pseudo random code word.

• The receiver performs a time correlation operation to detect the specific desired code word.

• For detection of the signal, the receiver needs to know the transmitted code word.

GSM: GLOBAL SYSTEM FOR MOBILE COMMUNICATION

• GSM uses TDMA.

• In TDMA, each user is allowed to access the radio channel, during a set of predetermined time slots.

GSM FRAME STRUCTURE:

[pic]

EXPLANATION:

• GSM composed of Eight 577 μs slots.

• 1 bit Flag is used to identify whether the data bits are digitized (or) not.

• The 3 tail bits are all logical zeros. Used in convolutional decoding of the decoder.

• 26 bit Training sequence is used for channel Equalization.

• Guard time of 8.25 bit size is used to prevent data bursts received at the base station.

• Each slot consisting of 156.25 bits, out of which 40.25 bits are overhead.

• The Frame efficiency of GSM is

[pic]

NEAR-FAR PROBLEM:

• This problem occurs, if the received signals from the mobile units do not have equal power at the base station.

• This problem can be rectified by means of using the "power control" at the base station.

RAKE RECEIVER:

• It is important for the CDMA application.

• It was developed in 1950s', to equalize the effect of multipath.

• Multipath means "linear combination of differently delayed echoes.

• The main Function of "Rake Receiver" is to prevent the multipath Effect by using correlation method, in which the echo signals can be detected individually and then added algebraically.

BLOCK DIAGRAM OF RAKE RECEIVER

[pic]

• There are number of correlators connected in parallel and operating synchronously.

• Each correlator has two inputs.

➢ Delayed Received signal Tc.

➢ Reference PN sequence (i.e.,) the PN code used at the Transmitter side, which could be used at the Receiver side, so that the original Information could be recovered.

Let the Nominal Bandwidth of the PN sequence is W = 1/ Tc

Where Tc = chip duration.

• The Bandwidth "W" must be large, so that the echoes could be identified in the receiver side.

PHASE GAIN ADJUSTORS (α, ϕ):

• It is used to make sure that the correlator outputs added constructively.

• A delay is introduced in each correlator output, so that the phase angles of the correlator output are aligned with each other.

• Correlator outputs are weighted in order to provide strong path in the multipath environment.

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• Here BPSK is used to perform spread spectrum modulation at the Transmitter.

• The output y(t) is integrated over the interval 0 to Tb.

• Final part of the Receiver is the Decision Device, in which the Receiver detects whether the binary symbol' I ' (or) '0' was sent, over the interval Tb.

• The Rake Receiver was so named, because the number of parallel correlator looks like the Fingers of a Rake.

SOURCE CODING OF SPEECH FOR WIRELESS COMMUNICATION:

• The main use of speech coding is to remove the natural redundancy in speech, and maintain a high quality speech during decoding.

One of the common approaches of source coding is Linear Predictive Coding (LPC).

Techniques for speech coding:

1. Multiple pulse excited LPC

2. Code excited LPC

These two techniques are mainly used in GSM & IS-95.

Multi-Pulse excited LPC:

• It is based on the "Principle of Analysis by synthesis", (i.e) the encoder is exactly the same of the decoder in its design.

Multi-Pulse Excited LPC Encoder:

It consists of three parts.

➢ Synthesis Filter

➢ Excitation generator

➢ Error Minimization

Synthesis Filter:

• It is used for the predictive modeling of speech.

• It consists of an "all-pole filter", which is a filter whose transfer function has only poles.

• It consists of short term and long term predictor.

• Short term predictor refers, the filter parameters are evaluated by predicting the present sample of the speech signal using 8 to 16 previous samples.

• Long term refers for modeling the fine structure of the speech spectrum.

• The main function of synthesis filter is to produce a version of original speech of high quality.

Excitation Generator:

• It provides the input applied to the synthesis filter.

• Input consists of a definite number of pulses every 5 to 15 ms, whose amplitude and positions are adjustable.

Error Minimization:

• Its main function is to optimize the amplitudes and position of the pulses used in the input.

• These three parts of the encoder form a closed-loop, which allows the encoder to operate at a bit rate below 16 kb/s to maintain high-quality speech.

Encoding Procedure:

• Input speech samples to evaluate the parameters of the synthesis filter.

• This evaluation can be performed outside the optimization loop over a period of 10 to 30 ms.

• The input to the synthesis filter is performed by minimizing the weighted error in the closed loop.

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MULTI-PULSE EXCITED LPC DECODER:

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The figure shows the Decoder part. It consists of 2 parts.

➢ Excitation Generator

➢ Synthesis Filter.

• The function of the decoder is to use the received signal to produce a synthetic version of the original speech signal.

• This can be done by passing the decoded input through the synthesis filter. The resulting output is the synthetic speech.

CODE-EXCITED LPC (CELP):

• The main feature of CELP is the predetermined code book of size N, used as the input for the synthesis filter.

• The synthesis filter performs short term prediction and Long term prediction.

• The parameter of the synthesis filter is computed first, using the original speech samples as input.

• The choice of a particular code stored in the Input code book and the gain factor G is optimized.

• The selected address from the code book and the gain G combined together with the filter parameters constitute the transmitted signal.

• An identical copy of the code book and synthesis filter is available to the decoder.

• With the received signal, the decoder detects its own synthesize filter thereby the original speech signal is reproduced.

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