adc material unit five cse

ANALOG AND DIGITAL COMMUNICATION /II/CSE/UNIT 5

Two Marks:

1. What are the three properties of PN sequence? (Nov/Dec 2009)**

Number of „1s‟ is always greater than number of „0s‟ in each period of maximum length

sequence.

For the shift register the length „m‟, there is total 2m-1 number of runs in pseudo – noise

sequence.

Autocorrelation of pseudo-noise sequence is periodic and it is binary valued.

2. Define Maximal length sequence. (Nov/Dec 2009)

The PN sequence is called maximum length sequence when the period is exactly 2m-1

, the PN sequence

is called as “Maximum Length Sequence”.

3. Define spread spectrum? (Nov/Dec 2009)

Spread spectrum is a techniques which a signal in a particular bandwidth is purposely spread in the

frequency domain, resulting in a signal with a wider bandwidth.

This technique is used to make a secured transmission of information signal.

4. List out the various applications of spread spectrum communication? (Nov/Dec 2009)

Applications of DS-SS System:

To avoid the intentional interference.

To reject the un-intentional interference.

To minimize the self interference due to multipath propagation.

To have secured transmission.

5. List out various speech coding techniques? (Nov/Dec 2008)

There are basically two types of speech coding techniques:

1. Multi pulse Excited LPC.

2. Code Excited LPC.

6. Define processing gain? (April/May 2007)

Processing Gain (PG) is defined as the ratio of the bandwidth of spread message signal to the bandwidth

of unspreaded data signal ie).

Processing Gain = BW (spreaded signal)

----------------------------

BW (Unspreaded signal)

UNIT V SPREAD SPECTRUM AND MULTIPLE ACCESS TECHNIQUES Introduction, Pseudo-noise sequence, DS spread spectrum with coherent binary PSK, processing

gain, FH spread spectrum, multiple access techniques – wireless communication, TDMA and CDMA in wireless communication systems, source coding of speech for wireless communications.


7. List out the different types of frequency hopping?(April/May 2007)

Frequency Hopping Spread Spectrum using FSK is of two types,

1. Slow Frequency Hopping.

2. Fast Frequency Hopping.

8. Compare DS-SS AND FH-SS? (Nov/Dec 2010)

S.No Parameters Direct Sequence Spread

Spectrum

Frequency HOP Spread

Spectrum

1. Definition PN sequence of large

bandwidth is multiplied with

the narrow band rate signal.

Data bits are transmitted in

different frequency slots

which are changed by PN

sequence

2. Chip Rate It is fixed Rc = T/Tc Rc = max(Rh,Rs)

3. Modulation Technique BPSK M-ary FSK

4. Processing Gain PG=Tb/Tc=N PG=2t

5. Acquisition Time Long Short

6. Effects of Distance This distance is distance

relative

Effect of distance is less.

9. List out major multiple access technique? (Nov/Dec 2010)

There are several methods in which the multiple users can send & receive their information through a

shared communication channel.

The methods are given below:

FDMA-Frequency Division Multiple Access.

TDMA- Time Division Multiple Access.

CDMA- Code Division Multiple Access.

10. List out any two features of TDMA? (Nov/Dec 2010)

TDMA Features:

TDMA is used to transmit data & digital voice channels.

Synchronization is necessary in TDMA.

Power efficiency of TDMA is better than FDMA.

11. Write the problems in CDMA? (April/May 2010)

Reduced efficiency due to coding.

Synchronization and precise timing is essential.

12. Why speech coding is done by wireless technique? (April/May 2010)

The wireless communication systems such as TDMA or CDMA use speech coding to ensure efficient

use of channel bandwidth.

The speech encoding removes almost all the redundancy in speech, while maintaining a high quality of

speech after decoding.

13. What is called frequency hop spread spectrum? (April/May 2010)

In frequency hop spread spectrum, the frequency of the carrier hops randomly from one frequency

to another frequency.


14. What is slow frequency hopping? (April/May 2010)

If the symbol rate of MFSK is an integer multiple of hop rate (multiple symbols per hop) then it is called

low frequency hopping

15. What is fast frequency hopping? (April/May 2009)

If the hop rate is an integer multiple of symbol rate (multiple hops per symbol) then it is called fast frequency

hopping.

16. What are the two function of fast frequency hopping? (April/May 2009)

1.Spread Jammer over the entire measure of the spectrum of Txed signal.

2.Retuning the Jamming signal over the frequency band of Txed signal.

17. What are the features of code Division multiple Accesses? (April/May 2009)

1.It does not require external synchronization networks.

2. CDMA offers gradual degradation in performance when the no. of users is increased But it is easy to

add new user to the system.

3. If offers an external interference rejection capability.

18. What is called multipath Interference? (April/May 2009)

The interference caused by the interfacing of the signal form the indirect path with the signal of direct path is

called multipath interference.

19. What is the advantage of a spread spectrum technique?(April/May 2009)

The main advantage of spread spectrum technique is its ability to reject interference whether it be the

unintentional interference of another user simultaneously attempting to transmit through the channel (or)

the intentional interference of a hostile transmitter to jam the transmission.

20. What is called frequency hop spread spectrum? (April/May 2008)**

In frequency hop spread spectrum, the frequency of the carrier hops randomly from one frequency to

another frequency.

21. What is slow frequency hopping? (April/May 2008)

If the symbol rate of MFSK is an integer multiple of hop rate (multiple symbols per hop) then it is called slow

frequency hopping.

22. Define pseudo-noise (PN) sequence. (April/May 2008)**

A pseudo-noise sequence is defined as a coded sequence of 1s and Os with certain autocorrelation

properties. It is used in spread Spectrum communications. It is periodic in that a sequence of 1s and 0s repeats

itself exactly with a known period.

23. What does the term catastrophic cyclic code represent ? (April/May 2008)*

„000‟ is not a state of the shift register sequence in PN sequence generator, since this results in a catastrophic

cyclic code i.e once the 000 state is entered, the shift register sequence cannot leave this state.

24. Define a random binary sequence. (April/May 2008)*

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


25. State the balance property of random binary sequence. (April/May 2008)*

In each period of a maximum length sequence, the number of 1s is always one more than the number of 0s.

This property is called the balance property.

26. Mention about the run property.(April/May 2008)

Among the runs of 1s and 0s in each period of a maximum 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 or as long as these function

represent meaningful numbers of runs. This property is called the run property.

27. Give the correlation property of random binary sequence. (Nov/Dec 2008)*

The autocorrelation function of a maximum length sequence is periodic and binary valued. This property is

called the correlation property.

28. Mention the significance of spread spectrum modulation. (Nov/Dec 2008)*

An important quality of spread-spectrum modulation is that it can provide protection against externally

generated interfering (jamming) signals with finite power.

The jamming signal may consist of a fairly powerful broadband noise waveform that is directed at the

receiver for the purpose of disrupting communications.

Protection against jamming waveforms is provided by purposely making the information bearing signal

occupy a bandwidth far in excess of minimum bandwidth necessary to transmit it.

29. What is called jamming effect? (Nov/Dec 2008)*

If someone mixes a unwanted frequency with the transmitted frequency then it makes a interference, the effect

due to this interference. Hence it becomes difficult to detect the required signals. This is called jamming effect.

30. What is Anti jamming? (Nov/Dec 2008)**

With the help of spread spectrum method, the transmitted signals are spread over the mid frequency band.

Hence these signals appear as noise. Then it becomes difficult for the jammers to send jamming signals.

This is called anti jamming.

31. What are the three codes used for the anti jamming application?(Nov/Dec 2008)*

1.Golay code (24, 12)

2.Expurgated Golay (24, 11)

3.Maximum length shift register code.

Sixteen Marks:

1. What is the need for Multiplexing? Discuss time division multiplexing system and explain its

advantages over a FDMA system. (16) (April May 2007) (Nov/Dec 2009)**

2. What is frequency hop spread spectrum? Explain the generation of slow frequency hop spread M-ary

FSK and Fast frequency hop spread M-ary FSK with appropriates diagram.(16) (April/May 2008)**

3. (a) Compare Time Division Multiple Access and Frequency Division Multiple Access. (8) (April/May

2008)

(b) A spread spectrum communication system has the following parameters: (8)

1. Information bit duration Tb: 4.095 ms.


2. PN chip duration Tc : 1 μs.

3. The energy to noise ratio Eb/N0=10.

Calculate the processing gain and jamming margin. (April/May 2008)

4. (a) Give the advantages associated with spreading a signal spectrum.(6)

(b) Describe the structure of feedback shift registers for generating PN sequences. (10)

5. (a) Explain FH-CDMA acquisition and tracking with neat sketches. (8). (Nov/Dec 2008)

(b) Compare TDMA, FDMA,CDMA multiple access techniques.(8) (Nov/Dec 2008)

6. Draw the block diagram of a simple PN sequence generator using shift register and obtain the

Output sequence. For this output sequence verify the properties of the PN sequence.(16) (Nov/Dec

2009)

7. (a) Draw the block diagram of RAKE receiver and explain the operation. (8) (Nov/Dec 2009)

(b) Explain the working of multi pulse excited LPC and code-excited LPC by suitable

diagrams.(Nov/Dec 2009)

8. (a) What are the properties of PN sequence random numbers? Draw the direct sequence spread

Spectrum system which employs BPSK and explain between the operation.

(b) Distinguish between FDMA and TDMA.(May/June 2009)

9. (a) Draw the block diagram of a DSS coherent binary PSK system and explain the transmitter and

Receiver block.(10) (Nov/Dec 2009) (April/May 2010)**

(b) Explain how the system is capable of resistance to interferences. (6) (Nov/Dec 2009)

10. Derive the equation for gain achieved by processing a spread spectrum signal over an un-spread

signal.(16) (April/May 2010)

11. Describe any two techniques for speech coding. (16) (April/May 2010)**

12. (a) A spread spectrum of chip rate 10MHz is used for ranging. If the reflected wave is received

after 0.1 ms. Find the probable distance of the target.(8) (Nov/Dec 2010)

(b) Explain the concept of frequency hopping spread spectrum.(8) (Nov/Dec 2010)

13. (a) A signal would have a bandwidth of 200 khz and a SNR of 20 db, if transmitted without spreading. It

is spread using a chipping rate of 50:1. Calculate its bandwidth and signal to noise ratio after

spreading.(8) (Nov/Dec 2010)

(b) Briefly explain the various source coding techniques of speech signal for wireless communication.(8).

(Nov/Dec 2010)

Classification of Spread spectrum Modulation Techniques:

The Spectrum modulation technique are broadly divided in to two categories,

1. Averaging Type System.

2. Avoidance Type System.

The Averaging system reduces reduce the interference by averaging it over a long period. The direct

sequence – spread spectrum (DS-SS) system is an averaging system.

The Avoidance System reduces the interference by making the signal avoid the interference in large

fraction of time.

The Avoidance System reduces can be classified as,

Frequency Hopping System.

Time Hopping System.

Chirp.


Hybrid Modulation System.

Draw the block diagram of a simple PN sequence generator using shift register and obtain the

Output sequence. For this output sequence verify the properties of the PN sequence.(16) (Nov/Dec 2009)

Generation of Pseudo-Noise (PN) Sequence:

The PN Sequence is used to encrypt the information signal.

The PN Sequence generator shown in the figure is basically a Shift Register.

The PN Sequence generator consists of a Parity Generator & „D‟ flip-flops.

The output of parity generator is connected to the input d0 of flip flop D0.

The Parity generator performs a EX-OR gate function.

The output of the parity generator is Logic „0‟ when both the inputs are same.

And the output is Logic „1‟ when both the inputs are different.

The inputs to the parity generator are output of flip-flops.

The character generated by a PN sequence generator depends on the number of flip-flops used.

The state of the each flip-flop changes & shifted for every clock pulse.

The shift register of „m‟ flip-flops will give 2N numbers of states.

The overall possible output patterns are 2N=M-1 patterns.

The parity generator shown in the figure is a deterministic structure.

Assume the initial state of the shift register is „001‟.

The output Q2 and Q3 are connected to the inputs of EX-or gate.

The PN Sequence is taken from the output of FF3 ie. Q3.


Describe any two techniques for speech coding. (16) (April/May 2010)**

Direct Sequence Spread Spectrum with Coherent BPSK:

The Direct Sequence Spread Spectrum is used in transmission for signals over baseband channels(

Satellite Channels).

In such application coherent BPSK is used in transmitter & receiver.

Transmitter:

The DS-BPSK transmitter is shown in the figure.

Operation:

The binary sequence is converted to NRZ(Non returned to Zero) signal using NRZ encoder.

The NRZ signal is then used to modulate the PN sequence generated by PN code generator.

The transmitter uses two stage of modulation.

The first stage uses a product modulator or multiplier.

The second stage consists of a BPSK modulator.

The output of the product modulator is used to modulate the carrier signal of BPSK modulator.

The transmitted signal is a Direct Sequence Spread Spectrum BPSK Signal.

The Waveforms are shown below.


Receiver:

The received signal & locally generated replica of the PN sequence are applied to the multiplier.

The multiplier performs the de-spreading operation.

Output of the multiplier is then applied to the coherent BPSK demodulator with locally generated carrier

signal.

At the output of the coherent BPSK detector, the original message signal is produced.

Applications of DS-SS System:

To avoid the intentional interference.

To reject the un-intentional interference.

To minimize the self interference due to multipath propagation.

To have secured transmission.


Advantages of DS-SS

The interference caused by multipath reception is minimum in DS-SS.

The performance of DS-SS with noise parameter is good.

DS-SS rejects the jamming most efficiently.

Disadvantages of DS-SS

With serial search system the DS-SS is slow.

The sequence generated by PN sequence generated should be long and random.

The Bandwidth requirement is large.

The synchronization is affected by the variable distance between the transmitter and receiver.

What is frequency hop spread spectrum? Explain the generation of slow frequency hop spread M-ary FSK

and Fast frequency hop spread M-ary FSK with appropriates diagram.(16) (April/May 2008)**

Frequency Hopping Spread Spectrum Signals:

Definition:

In Direct Sequence Spread Spectrum, the information signal is spread immediately which reduces the

processing gain.

The capacity of DS-SS system to reject jamming is depend on the “Processing Gain”.

But the physical devices to produce PN sequence puts a limitation on the maximum value of “Processing

Gain” and so the capacity to avoid interference in limited in DS-SS system.

To avoid this issue in Frequency Hop Spread Spectrum the data is modulated in two stages.

In first stage the date is modulated with the FSK carrier.

In second stage the modulated carrier signal is again modulated with the frequency HOP generated by

PN sequence.

Frequency Hopping Spread Spectrum using FSK is of two types,

3. Slow Frequency Hopping.

4. Fast Frequency Hopping.

Slow Frequency Hopping:

In Slow frequency hopping the Symbol Rate Rs of MFSK signal is an integer multiple of the hop

rate Rh.

It means one HOP carries more than one symbol slowly.


Slow Frequency Hopping Transmitter:

Block Diagram:

Transmitter Operation:

The Binary data is given as input to the MFSK modulator.

The output of the modulator is given as one of the input to the multiplier.

The second input to the multiplier is given by the Frequency Synthesizer.

The frequency produced by the Frequency Synthesizer is controlled by the PN sequence

generator which is random.

If the number of successive bits at the output of PN generator is „n‟ then the total number of

frequency hops will be 2n.

The output of the MFSK modulator is called SIGNAL and the output of the frequency

synthesizer is called HOP.

Each frequency HOP is mixed with the MFSK signal to produce the transmitted signal.

At the output of the multiplier we get sum and difference frequencies.

The band pass filter allows only the sum frequency and rejects the difference frequency.

The FH/MFSK signal is transmitted.

Slow Frequency hopping Receiver:


The received signal is applied to a multiplier. The other input to the multiplier is from the

Frequency synthesizer.

The PN sequence generator generated the exact sequence used in the transmitted side. This

makes the frequency synthesizer to produce the frequency used in the transmitter side.


The band pass filter allows only the difference frequency and rejects the sum frequency.

Thus the mixer removes the frequency hopping.

The difference is then applied to the MFSK demodulator where the information signal is

separated from the carrier signal.

Example:

Given Data:

1. Number of bits per MFSK symbol: K=2.

2. Number of MFSK tones M- 2k =2

2=4.

3. Length of PN sequence per hop : n=3.

4. Total number of frequency HOP : 2n= 2

3 = 8.

Draw the FH/MFSK signal showing the variation in frequency with respect to time for one

complete period of the PN sequence. Assume the PN sequence is

PN Sequence 0 0 1 1 1 0 0 1 1 0 0 1 0 0 1

Solution:

Here the number of bits per symbol is K=2, the carrier is hopped to a new frequency after

transmitting two symbols or four information bits.

Here the length of PN sequence is n=3, so there will be 8 output levels possible i.e,

000,001,010,011,100,101,110,111 which are names as FH1,FH2,FH3,FH4,FH5,FH6,FH7,FH8.

Each combination will generate a unique frequency called HOP.

The number of symbols transmitted in one frequency hop is 2.

The total number of symbols are M=4 ie. 00,01,10,11 which are named as f1,f2,f3,f4.

Symbol Transmitted Frequency

00 FH+f1

01 FH+f2

10 FH+f3

11 FH+f4


input binary data 01 11 11 10 00 10 01 11 10 10

PN Sequence 0 0 1 1 1 0 0 1 1 0 0 1 0 0 1

111

f4=11

f3=10

f2=01

f1=00

110

f4=11

f3=10

f2=01

f1=00

101

f4=11

f3=10

f2=01

f1=00

100

f4=11

f3=10

f2=01

f1=00

011

f4=11

f3=10

f2=01

f1=00

010

f4=11

f3=10

f2=01

f1=00

001

f4=11

f3=10

f2=01

f1=00

Fast Frequency Hopping:

In Fast frequency hopping the chip rate Rc is equal to rate of hopping Rh.

It means one symbol is transmitted by two HOP‟s.

The fast frequency hopping is used to defeat smart jammers.

Fast Frequency Hopping Transmitter:

Block Diagram:


Transmitter Operation:

The Binary data is given as input to the MFSK modulator.

The output of the modulator is given as one of the input to the multiplier.

The second input to the multiplier is given by the Frequency Synthesizer.

The frequency produced by the Frequency Synthesizer is controlled by the PN sequence

generator which is random.

If the number of successive bits at the output of PN generator is „n‟ then the total number of

frequency hops will be 2n.

The output of the MFSK modulator is called SIGNAL and the output of the frequency

synthesizer is called HOP.

Each frequency HOP is mixed with the MFSK signal to produce the transmitted signal.


The band pass filter allows only the sum frequency and rejects the difference frequency.

The FH/MFSK signal is transmitted.

Fast Frequency hopping Receiver:


The received signal is applied to a multiplier. The other input to the multiplier is from the

Frequency synthesizer.

The PN sequence generator generated the exact sequence used in the transmitted side. This

makes the frequency synthesizer to produce the frequency used in the transmitter side.


The band pass filter allows only the difference frequency and rejects the sum frequency.

Thus the mixer removes the frequency hopping.

The difference is then applied to the MFSK demodulator where the information signal is

separated from the carrier signal.

Example:

Given Data:

1. Number of bits per MFSK symbol: K=2.

2. Number of MFSK tones M- 2k =2

2=4.

3. Length of PN sequence per hop : n=3.

4. Total number of frequency HOP : 2n= 2

3 = 8.

Draw the FH/MFSK signal showing the variation in frequency with respect to time for one

complete period of the PN sequence. Assume the PN sequence is

PN Sequence 0 0 1 1 1 0 0 1 1 0 0 1 0 0 1

Solution:

Here the number of bits per symbol is K=2, the carrier is hopped to a new frequency after

transmitting two symbols or four information bits.

Here the length of PN sequence is n=3, so there will be 8 output levels possible i.e,

000,001,010,011,100,101,110,111 which are names as FH1,FH2,FH3,FH4,FH5,FH6,FH7,FH8.

Each combination will generate a unique frequency called HOP.

The number of symbols transmitted in one frequency hop is 2.

The total number of symbols are M=4 ie. 00,01,10,11 which are named as f1,f2,f3,f4.

Symbol Transmitted Frequency

00 FH+f1 and FH+f1

01 FH+f2 and FH+f2

10 FH+f3 and FH+f3

11 FH+f4 and FH+f4


input binary data 01 11 11 10 00

PN Sequence 001 110 011 001 001 001 110 011 001 001

111

f4=11

f3=10

f2=01

f1=00

110

f4=11

f3=10

f2=01

f1=00

101

f4=11

f3=10

f2=01

f1=00

100

f4=11

f3=10

f2=01

f1=00

011

f4=11

f3=10

f2=01

f1=00

010

f4=11

f3=10

f2=01

f1=00

001

f4=11

f3=10

f2=01

f1=00


Comparison of Slow Frequency Hopping & Fast Frequency Hopping:

S.NO Slow Frequency Hopping Fast Frequency Hopping

1. More than one symbol are

transmitted per frequency HOP.

More than one frequency HOP is required to

carry one symbol.

2. Chip Rate is equal to symbol rate. Chip rate is equal to HOP rate.

3. Symbol rate is higher that HOP rate. HOP rate is higher than symbol rate.

4. A jammer can detect this signal if the

carrier frequency of one HOP is

known.

A jammer can‟t detect this signal because one

symbol is transmitted using more than one

carrier frequency.

5. Same carrier frequency is used to

transmit one or more symbols.

One Symbol is transmitted over multiple

carrier in different HOP‟s

Advantages of FH-SS

The synchronization is not dependant on the distance.

The data acquisition in FH-SS is fast.

The processing gain PG is higher than that of DS-SS system.

Disadvantages of FH-SS

The bandwidth of FH-SS system is too large (in GHz).

Complex and expensive digital frequency synthesizer is required.

Comparison of Direct Sequence Spread Spectrum & Frequency HOP Spread Spectrum:

S.No Parameters Direct Sequence Spread

Spectrum

Frequency HOP Spread

Spectrum

1. Definition PN sequence of large

bandwidth is multiplied with

the narrow band rate signal.

Data bits are transmitted in

different frequency slots

which are changed by PN

sequence

2. Chip Rate It is fixed Rc = T/Tc Rc = max(Rh,Rs)

3. Modulation Technique BPSK M-ary FSK

4. Processing Gain PG=Tb/Tc=N PG=2t

5. Acquisition Time Long Short

6. Effects of Distance This distance is distance

relative

Effect of distance is less.


Wireless Communication System:

There are several methods in which the multiple users can send & receive their information through a

shared communication channel. The methods are given below:

FDMA-Frequency Division Multiple Access.

TDMA- Time Division Multiple Access.

CDMA- Code Division Multiple Access.

FDMA- Frequency Division Multiple Access:

One of the simplest method of multiple access technique is FDMA.

The figure shows how the bandwidth is subdivided in to a number of sub channels.

There are „K‟ non-overlapping sub channels.

FDMA is commonly used for voice and data transmission.

Features of FDMA:

The overall channel band width is divided in to multiple sub channels so that multiple users can

transmit & receive simultaneously.

The adjacent frequency band in the FDMA spectrum is likely to interfere with each other.

To avoid this interference it is necessary to include a GUARD BAND between each sub

channels.

No code word and synchronization is required.

Power efficiency is reduced.

Disadvantages of FDMA:

Due to simultaneous transmission of a large number of frequencies there is a possibility of inter-

modulation distortion.

It is suitable only for Analog Signals.

Storage enhancement of signals is not possible.


Large bandwidth is required for transponders.

What is the need for Multiplexing? Discuss time division multiplexing system and explain its advantages over

a FDMA system. (16) (April May 2007) (Nov/Dec 2009)**

TDMA-Time Divisional Multiple Accesses:

Another method of adopting multiple accesses is TDMA were the time duration is sub divided. Here the

subdivide time is called frame of Slot.

Each user is assigned a particular time slot where the used can transmit & receive.

TDMA Features:

TDMA is used to transmit data & digital voice channels.

While sub dividing the time it is necessary to include the “GUARD TIMES” between each time

slots.

Synchronization is necessary in TDMA.

Power efficiency of TDMA is better than FDMA.

Earth earth station transmits a short burst of digitally modulated carrier during the time slot

assigned to it in the TDMA time frame. Such a time slot is called EPOCH.

The burst of each earth station is synchronized so that at any instant of time one earth station is

services by the transponder.

The Transponder receives the signal, amplifies it and transmit it to back to all earth stations.

TDMA Frame:

The figure shows the structure of TDMA frame.


Reference burst or preamble: The first time slot in each TDMA frame is reference

burst or preamble.

Transmission from all the earth station is synchronized to this burst.

The reference burst is made of three slots,

1. Carrier Recovery Sequence (CRS).

2. Bit Time Recovery (BTR).

3. Unique Word (UW).

From the carrier recovery sequence (CRS) all the receiving stations recover a frequency

and phase coherent carrier. This carrier is used for PSK demodulation.

The Bit Time Recover (BTR) sequence is used by the receiving station for clock recovery.

The Unique Word (UW) sequence is used for establishing a precise time reference which

is used by each station to synchronize with the burst.

Unique Word correlator:

The figure shows the unique word correlated.

The UW is applied at the input of the integrator. It is a string of 20 consecutive

binary ‟1s‟.

The integrator input increases step by step and reaches the threshold voltage,

exactly at the end of the UW.

This generates the correlative spike at the time at the output of the threshold

detector.

Each earth station is synchronizes the transmission of carrier‟s at the instant of

correlation spikes arises.


Advantages of TDMA:

At a instant of time only one station is present at the transponder. This reduces the

intermediation distortion.

TDMA is suitable for transmission of digital information.

It is possible to store the digital information.

Disadvantages of TDMA:

Exact Synchronization is required.

Bit & Frame Timing must be achieved.

CDMA-Code Divisional Multiple Accesses:

The alternative to FDMA& TDMA systems is called Code Divisional Multiple Access.

In CDMA more than one user is allowed to share a channel or sub channel with the help of Direct

Sequence Spread Spectrum.

In CDMA each user is given unique code sequence. This code used by the user to spread the sequence.

At the receiver the signal is recovered by using the same code sequence.

At the receiver the signal receiver by various user are separated by using cross correlation of the

receiving signal with each possible user code.

In CDMA the user access the channel in a random manner.

In CDMA the user shares both the band width and time. So it is required to introduce the GUARD

BAND & GUARD TIME.

Encoding Rules for CDMA:

In CSMA each station is assigned a code which is simply a sequence of numbers called chips.

If a station want to send Logic‟0‟ then it sends a „-1‟.

If a Station want to send Logic „1‟ the nit sends a „+1‟.

If the station dose not wants to transmit it sends „0‟.


CDMA Encoder or multiplexer:

In the input to the encoder can be a digital data from a computer.

The data is multiplied with the unique code word.

The output of the balanced modulator is the product code. It is used to PSK modulate an IF

carrier.

The modulated signal is then up converted to RF band and used for transmission.

The high power antenna raises the power of this signal to very high level and the antenna transmits

the signal.

CDMA Decoder or De-multiplexer:

The decoder down converts the RF signal to IF. From IF the coherent PSK carrier is

obtained.

The receiver also acquires the chip code and uses it to synchronize the receiver station‟s code

generator.

The recovered chip code is multiplied with the recovered PSK carrier to generate a PSK

modulated signal which contains the PSK carrier plus the chip code.

The received IF signal contains the PSK carrier plus the chip code, PSK carrier and data.

The correlator compares these two signals and recovers the original data.


CDMA Multiplexer:

Let A, B,C, be three stations in a CDMA system with unique codes assigned.

Station Code

A +1,-1,-1,+1

B +1,+1,+1,+1

C +1,-1,+1,-1

Lets assume that station A want to transmit Logic „0‟, Station B do not want to transmit &

Station C wan to transmit Logic „1‟.

Operation:

The multiplexer receives one encoded number from each station(-1,0,+1)

Each bit is multiplied with the corresponding station.

The output of the multipliers is to obtain the multiplexed CDMA signal.

The multiplexed CDMA signal is then transmitted through the common link.

CDMA De-multiplexer:

The multiplexed CDMA signal is applied to all the multiplier.

It is multiplied with the code numbers assigned to the stations A,B,C.

The bit available at the output of each multiplier is divided by 4 to obtain the original transmitted data.


COMPARISON BETWEEN FDMA/TDMA/CDMA

S.NO PARAMETER FDMA TDMA CDMA

1. Technique

In FDMA the Overall

Band width is shared by

all uers.

In TDMA the time of the channel is

shared by all users.

In CDMA the time &

bandwidth of the

channel is shared by

multiple users

2. Structure

3. Interference

Effects

Adjacent frequency

bands interference

generated because of

non-linearity of satellite

transponders amplifiers.

Interference between the users of

adjacent time slots generated

because of incorrect

synchronization.

Performance is affected

due to adjacent time &

frequency slots.

4. Synchronization No Synchronization is

required.

Time Synchronization is required. No Synchronization is

required.

5. Code Word No code word is used. No code word is used. Code word or signature

waveforms are used.

6. Power Efficiency Power Efficiency is

required.

Full Power Efficiency is possible. Full Power Efficiency is

possible.

7. Guard Time &

Bands

Guard Bands are

required.

Guard times are required. Guard times & Bands

both are required.

Explain the working of multi pulse excited LPC and code-excited LPC by suitable diagrams.(Nov/Dec 2009)

Source Coding of Speech for Wireless Communications:

The wireless communication systems such as TDMA or CDMA use speech coding to ensure efficient

use of channel bandwidth.

The speech encoding removes almost all the redundancy in speech, while maintaining a high quality of

speech after decoding.

To enable speech coding Linear Predictive Coding of speech is used.

Linear Predictive Coding (LPC):

Linear Predictive Coding (LPC) is a new and totally different method of representing an analog

signal digitally.

The Analog signal is first synthesized. The parameters of the waveform synthesizer are then

encoded and transmitted inserted of the actual signal.

LPC- Transmitter:

The figure shows the block diagram of a LPC transmitter.

The voice input signal is first sampled to obtain the signal x (t) at the input of LPC transmitter.

The sampled values are applied to an analyzer which analyzes them to give the parameters for

synthesizer.


The synthesizer reconstructs the speech signal xq(k).

The reconstructed speech signal xq(k) and the sampled signal x(k) are compared to produce error

signal “ϵ(k)”.

The error signal and parameter are encoded by the encoder to form a digital signal which is

called as the LPC signal.

LPC Receiver:

The figure shows the LPC receiver.

From the received LPC signal the decoder separates out the error signal “ϵ(k)” and the

parameters.

The parameters are applied to the synthesizer. The synthesizer output is then added to the error

signal to produce speech signal xq(k).

A complete LPC code word consist of about 80 bits out of which 1 bit is used for the

voiced/unvoiced switch, 6 bits for pitch frequency of the voice and a few bits are used to

represent error.

LPC has a very low bit rate of about 3k to 8kbits/sec.

There are basically two types of speech coding techniques:

1. Multi pulse Excited LPC.

2. Code Excited LPC.

Multi Pulse Excited LPC:

This type of speech coding is base on the principle of analysis by synthesis.

The encoder here includes a replica of the decoder in its system.

Multi Pulse Excited LPC Encoder:

The block diagram of encoder consist of three blocks as follows,


1. Synthesis Filter.

2. Excitation Generator.

3. Error minimization.

Synthesis Filter:

This filter is used for the predictive modeling of speech.

Its output is synthetic speech signal.

The parameters of this filter are computed on the basis of predicting the present sample of the

speech signal using eight or sixteen previous samples. This is called short term samples.

The synthesis filter may also use long term sampling to get fine speech spectrum.

The work of synthesis filter is to produce high quality speech signal.

Excitation Generator:

This block is used to apply the excitation to the synthesis filter.

The excitation is a number of pulses for every 5 ti 15ms.

The amplitude and the position of these pulses are variable.

Error Minimization:

This block is used to reduce the perceptually weighted error.

This error occurs due to the difference between the original speech signal and the synthesized

speech signal.

The output of this block is applied to the excitation generator to excite the filter.

Hence the error is reduced and high quality speech signal is maintained.


Multi Pulse Excited LPC Decoder:

The decoder consist of two parts namely,

1. Excitation generator.

2. Synthesis Filter.

The received signal is applied to the excitation generator.

The synthetic filter produces the synthetic version of the original speech signal.

Synthesis Filter:

This filter is used for the predictive modeling of speech.

Its output is synthetic speech signal.

The parameters of this filter are computed on the basis of predicting the present sample of the

speech signal using eight or sixteen previous samples. This is called short term samples.

The synthesis filter may also use long term sampling to get fine speech spectrum.

The work of synthesis filter is to produce high quality speech signal.

Excitation Generator:

This block is used to apply the excitation to the synthesis filter.

The excitation is a number of pulses for every 5 ti 15ms.

The amplitude and the position of these pulses are variable.

Code Excited LPC:


The block diagram shows the code excited LPC. It is also called as CELP.

It uses predefined code block of stochastic vectors. They are used as source of excitation for the

synthesis filters.

The free parameters of the synthesis filter are computed first using the actual samples of the

speech at the input.

The error between the speech and synthetic speech is applied to the error minimization block

The error from its output is used to select any one of the code vectors to excite the filter.

The error signal is also used to choose the gain factor G.

The transmission signal consists of code word, gain factor G and quantized filter parameter.

At the decoder the same code is made available to decode the synthesis filter.

The synthesis filter then produces the synthesized version of the original signal.

CELP can produce good quality speech at bit rates below 8Kbps but it is a complex method.

Facility Course Material Coordinator HOD/ECE

adc material unit five cse

Documents