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UNIT I Notes

UNIT – I

 2 – Marks

 1.What is Propagation?

 Radio propagation depends on the  frequency of operation. In case of frequency range from 300 MHz to 30 GHz, the type of  propagation is  Line of Sight.   The most important  impairment  for LOS propagation are attenuation, free space loss, noise, atmospheric absorption, multi path and refraction.

 

2. Define Attenuation

The strength of the  signal falls off with distance over any transmission  medium.  In case of  unguided media, the attenuation is a function of  distance and the made up of atmosphere.

The transmitted signal power must be sufficiently strong to be received at the receiver.  If however, the signal strength is   very large, it may saturate the receiver causing  distortion..   Also, higher the frequency the attenuation is  also higher.  It may be possible to have  higher gain at higher frequencies to overcome these problem.

 

3. What is Free Space Loss

If the attenuation  loss is assumed to be  nil,  still there will be  free space loss which is  expressed as the ratio of transmitted power to the received power.   The signal strength  falls inversely as square of distance.   This  can be overcome by   designing antenna with higher gain.

 

4. Define Noise and how to reduce it?

Unwanted signals – referred as noise  are inserted  or added to the signals between transmission and  reception.  Various source for noise are : thermal noise, inter modulation noise, cross talk and impulse noise.  Most of these noise are predictable and    it is possible to reduce the impact of these noise by proper design and operation

 

5.What is Atmospheric absorption?

Water vapour and oxygen in the atmosphere  introduces additional loss.  These are frequency  dependant with loss due to water vapour being maximum at 22 GHz and  attenuation loss maximum at 15 GHz.  Rain and fog (suspended droplets) cause scattering of radio waves that result in  attenuation.  Wherever, precipitation is higher  the path length is to be kept shorter  or lower frequency band should be used.

 

6. Define Multipath:

For most of the fixed communication system like satellite communication systems it is possible to locate the transmitter  and receiver  to have direct line of sight.   However in case of  mobile telephony, there can be abundant obstacles.  The Signal can be reflected   by such obstacles so that multiple copies of the signal with varying delays can be received. Sometimes, there maybe no direct signal. Depending on the  differences in the path lengths of the direct and reflected waves, the composite signal can be either larger or smaller than the direct signal.  A good reception under this condition requires positioning of the antenna at a  proper site.

 

7. What is Refraction:

The changes in the atmospheric condition, such as density causes the radio waves to bend instead of following pure  LOS propagation.   If the conditions are not favorable, no signal may reach the  receiver.

 

8. Define Multiplexing:

Multiplexing enables several user to share a medium with minimum interference  or no interference. In case of wireless communication, there can be  four types of multiplexing.  These are Space Division Multiplexing, Frequency Division Multiplexing, Time Division  Multiplexing and Code Division Multiplexing.

 

9. Explain Time Division Multiplexing:

 

 

 

 

 

 

f1   f2   f3   f4   f5   f6               T

In this the user is given  the hole bandwidth for a  given period of time.  That is many sends can use the  same frequency band , but at different point of time.  Here, there must be guard time, so as to avoid co channel   interference.  To reduce / avoid co channel interference, precise  synchronization  between senders is necessary.   Here, the receiver and transmitter  is to synchronize  precisely for proper communication.

 

The advantage with the TDM is that it is possible to allocate more time  to a user who has  heavy load.  This is one of the requirement in case of internet access by the mobile user where the requested traffic is much smaller than the downloaded traffic.

 

10. What is Cellular Concept😕

Cellular concept is a system level idea wherein a single high power transmitter (large cell) is replaced with  many low power transmitter covering a small portion of the service area. Each base station is allocated a portion of the total number of channels available to the entire system and nearby base stations are assigned different groups of channels.

 

16-Marks :

 

1.Explain Multiplexing and types of Multiplexing?

 

Multiplexing: Multiplexing enables several user to share a medium with minimum interference  or no interference. In case of wireless communication, there can be  four types of multiplexing.  These are Space Division Multiplexing, Frequency Division Multiplexing, Time Division  Multiplexing and Code Division Multiplexing.

 

Space Division Multiplexing:  In this,  physical space  is separated for each of the user so that the interference is minimum or nil as shown below:

S1…S6 are the space segments where  each pair of the user has separate  space for communicationin his own channel. As the space is separated there is likely to be no interference  among each pair. The transmitter power controls the radius of  action.  This is similar to the FM radio station thatuse the same frequency  at different location without  interference.   However, if several frequencies  (pair of user )are to be  used, this may need to be modified with other multiplexing techniques

.

        f

f5

f4

f3

f2

f1

T

Frequency Division Multiplexing:   In this the frequency is divided into several bands that are not overlapping. Each user can use the allotted  band of frequency without causing interference to other user.    Here also certain guard band is provided to minimize the  adjacent channel interference. The

drawback of this system is that the frequency remains allocated to the user  even during those times when  there is no communication. It is not an efficient method             as it ties up one channel to a user  and  also limits the number of users.

 

 f

 

 

 

 

 

f1   f2   f3   f4   f5   f6               T

Time Division Multiplexing:   In this the user is given  the hole bandwidth for a  given period of time.  That is many sends can use the  same frequency band , but at different point of time.  Here, there must be guard time, so as to avoid co channel   interference.  To reduce / avoid co channel interference, precise  synchronization  between senders is necessary.   Here, the receiver and transmitter  is to synchronize  precisely for proper communication.

 

The advantage with the TDM is that it is possible to allocate more time  to a user who has  heavy load.  This is one of the requirement in case of internet access by the mobile user where the requested traffic is much smaller than the downloaded traffic.

TDM and FDM Combined Multiplexing:

       f

f5

f4

f3

f2

f1

t1      t2       t3     t4      t5

 

It  is possible to combine the FDM and TDM wherein a certain user can use a certain frequency for certain  amount of time.  Now there is a need for guard band as well as better synchronization   to avoid both adjacent and co channel interference. One important advantage is that the system has better immunity  to  frequency  selective interference.   Also, this system provides protection against  channel tapping.  The disadvantage is that  there needs to be proper coordination  between  different sends and receivers both in  frequency and time domain.

 

 

      Code c

 

C5

 

c4

 

c3

 

c2

 

c1

 

Frequency f

Code Division Multiplexing:   CDM was first used in military application due to its inherent security features.  Currently it is extensively used in  civil  mobile communication applications.  In this  all the users trans receive the messages at the same frequency and at the same time.  This is made possible when the digital transmitted codes  modulates a pseudo random codes that are  different for each user. Similar to FDM and TDM,  to avoid interference  due to codes that are closer to each other, codes  are allotted to the user with a specific code space by using orthogonal code selection.  This technique facilitates  a   large number of  user to share the same frequency spectrum. Each user  having a separate code when communicates adds to the background noise. As  large users use the system, the noise increases thereby limiting its performance.  However it provides  good protection against interference and tapping.   The main disadvantage of the  system, is it is relatively quite complex.

 

 

 

2.   Explain Mobile and Wireless Devices and also explain the reference model

 

There are a number of mobile and wireless configurations.. Mobile or  mobility  means the user has the access to similar telecommunication services when he is on the move. Eg: ‘Call Forwarding’ solutions etc. Wireless means that the device is able to communicate in the absence of wired connection. This facility clubbed with mobility enhances the performance of the device to get connected at different location.  A number of  configurations  that are possible are Fixed and Wired,  Mobile and wired, Fixed and Wireless, Mobile and Wireless.  Out of these, the last one is the most challenging one.

 

Mobile and wireless devices are listed starting from simple one to more complex ones:

  • Pager: A simple receiver to receiver short text messages.
  • Mobile Phones: In addition to providing communication facility while on move,  these  devices have much advanced features.
  • Personal digital assistants (PDAs) are handheld computers that were originally designed as personal organizers, but became much more versatile over the years. PDAs have many uses: calculation, use as a clock and calendar, playing computer games, accessing the Internet, sending and receiving E-mails, video recording, typewriting and word processing, use as an address book, making and writing on spreadsheets. Newer PDAs also have both color screens and audio capabilities, enabling them to be used as mobile phones (smartphones), web browsers, or portable media players. Many PDAs can access the Internet, intranets or extranets via Wi-Fi, or Wireless Wide-Area Networks (WWANs). One of the most significant PDA characteristic is the presence of a touch screen
  • Palmtop/Packet computers:  These are the  Handheld PC that provide familiar applications from the PC in a mobile format with all the convenience and portability of a PDA. To be classed as a Handheld PC the device must be able to run Microsoft’s Windows CE (Handheld PC Platform Release), include a keyboard  and few other technical hardwares.
  • Notebook / Laptop: A laptop  is a small mobile computer, which usually weighs 2.2-18 pounds (1-6 kilograms), depending on size, materials, and other factors. The terms laptop and notebook are often used interchangeably. As of 2007, most manufacturers use the term “notebook” (or some variant thereof) for what most end-users call a “laptop”.  Laptops usually run on a single main battery or from an external AC/DC adapter which can charge the battery while also supplying power to the computer itself. Many computers also have a 3 volt cell to run the clock and other processes in the event of a power failure. As personal computers, laptops are capable of the same tasks as a desktop computer, although they are typically less powerful for the same price.
  • Sensor: A sensor is wireless devices having  one or more of sensors and having ability  to transmit sensor information.  Great deal of work is goin on these sensor that configured dynamically as network for transmission of information.

 

 

Applications: Over period of 30 years,  a number of application have been developed for  wireless application. Some of these are listed below:

  • Mobile communication on the move to any part of the world.
  • Seamless internet connectivity to the mobile device user through Wi-Fi and WiMax technologies.
  • Reception of broadband multimedia applications.
  • Reception of  music, news, road conditions, weather reports and other broadcast information .
  • Knowledge of location informtion  through  GPS

 

 

 

 

 

 

Referenc Model:

A generic reference model  for mobile wireless communication is shown below:

 

Mobile Unit                        Base Station                                                            Switching Center

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Radio link                                                                Media

The model shows the mobile unit connected to a base station through wireless media and  one or more base stations  may be connected to  Switching center through landline or  through optical fibre or through microwave link .   The protocol stack consists of application layer followed by data link  layer, MAC Layer and Physical Layer.  Call control manages the circuit switched call, including connection setup and release. Mobility Management handles, identity, authentication, access rights procedure etc. Additional services depends on the service provider.  The Data link layer  provides  for the reliable transmission of messages  using error detection and automatic repeat request  and MAC layer selects the physical channel and then establishes and releases the connection.  It also multiplexes the information in TDMA format.

Mobile Unit                                    Base Station                              Mobile Switching Center

 

 

Physical

Physical

Physical

Physical

Network

Data Link

Data Link

Network

Network

Transport

Application

Data Link

Radio                                                           UG/optic/Radio

 

In this the mobile unit and the MSC (Mobile Switching Center) form the end systems  that implement a full  protocol stack as shown  in the figure.  The intermediate  systems  do not necessarily  need the entire protocol stack. The functionalities of each  protocol stack is  described below:

 

  1. Application Layer:  Various applications include service location, support for multimedia  applications, GPRS, SMS, applications that can handle variations in the transmission characteristics etc.
  2. Transport Layer: This layer is responsible for  providing  end to end connection.  If TCP, UDP and  Internet are to  used, then the quality of service, flow and congestion control etc  come under this protocol stack.
  3. Network Layer: Network layer  is responsible for routing the packets through  two adjacent  entities that are intermediate to the end systems.  Important functions include, addressing, routing, device location and handover between different networks etc.
  4. Data Link Layer: Main function  of this layer is to access the medium, multiplexing the different data streams, correction of transmission errors, synchronization etc. This layer is responsible for  a reliable point to point connections between two devices or a point to multipoint connection.
  5.  Physical layer: This layer is responsible for  frequency selection, generation of the carrier frequency, signal detection, modulation of data on to carrier frequency and if required encryption.

 

3. a. Give an account Wireless Transmission: (08 marks)

 

300 MHz  to 30GHz  which  is suitable frequency for mobile operation is fully occupied for various  uses.  These are:  Television broad casts, satellite communication,  Microwave devices, modern radars- defense and airfield, wireless LAN, bluetooth, GPS, etc. As the spectrum is crowded with various requirements, it is difficult  to find  a large spectrum for  mobile communication.  As the spectrum allocation is decided by the country on its own depending  on its strength in related technologies, it becomes difficult to coordinates the spectrum issues for international  connectivity.

 

Selection and allocation  of  a frequency band  in a country depends on their own regulation.  However, in order to provide transnational communications, there needs to   certain standards in the  usage of frequencies  as well as  interface standards.  Towards achieving this, ITU – R(International Telecommunication Union- Radio communication   – a  sub organization of UN) coordinates the world wide telecommunication activities.  FCC – Federal Communication Commission of America, ETSI – European Telecommunication Standard Institute  of Europe, similarly TRAI – Telecommunication Regulatory Authority of India  are in turn are  responsible for  the general planning  and allocation of frequencies for mobile communication.  To achieve harmonization, the ITU – R holds periodic conferences, WRC- World Radio Conference  to discuss  and decide frequency allocations.

 

Access to radio spectrum is  critical requirement for all sectors using wireless technologies like media, mobile communication, defence  and  civil aviation.  For mobile network  equipment vendors, spectrum frequency harmonization has a major impact on cost at  terminal and    network levels. New technologies can  be deployed only  when the spectrum agencies and  regulators give the  go ahead and frequencies have been allocated.

 

Currently mobile operators are experiencing major increase in  mobile traffic every year Addition of new services like video services, ‘always – on’ internet services have put a severe strain on the  network capacity  requirements.

 

Following  table gives the frequencies that are used for analog and digital  mobile communications.

1 G Systems

Systems Base Tx Mobile Tx Spectrum Allocation Number of Channels Radio Channel Spacing
AMPS (US) 869-894 824 – 849 50 MHz 832 30 KIHz
NAMPS(Narrow band AMPS) (US) 869-874 824 – 849 50 MHz 2496 10KHz
NMT 450 (Europe) 463-468 453-458 10 MHz 200 25  KHz
NMT 900 461-466 451-456 50 MHz 1999 12.5 KHz

 

2 G Systems:

Systems Base Tx Mobile Tx Spectrum Allocation Number of Channels Radio Channel Spacing
IS 136 869-894 824 – 849 50 MHz 832 *3= 2496 30 KIHz
IS – 95 869-894 824 – 849 50 MHz 64 * 9=  576 1.25 MHz
GSM 925-960 880-915 50 MHz 124 * 8 200 KHz
           

 

 

3.b.  Define the Following terms (08 Marks)

            i) Attenuation

            ii) Free Space loss

            iii)Noise

            iv)Atmosperic absorption

 

Attenuation:  The strength of the  signal falls off with distance over any transmission  medium.  In case of  unguided media, the attenuation is a function of  distance and the made up of atmosphere.

The transmitted signal power must be sufficiently strong to be received at the receiver.  If however, the signal strength is   very large, it may saturate the receiver causing  distortion..   Also, higher the frequency the attenuation is  also higher.  It may be possible to have  higher gain at higher frequencies to overcome these problem.

 

Free Space Loss:   If the attenuation  loss is assumed to be  nil,  still there will be  free space loss which is  expressed as the ratio of transmitted power to the received power.   The signal strength  falls inversely as square of distance.   This  can be overcome by   designing antenna with higher gain.

 

Noise: Unwanted signals – referred as noise  are inserted  or added to the signals between transmission and  reception.  Various source for noise are : thermal noise, inter modulation noise, cross talk and impulse noise.  Most of these noise are predictable and    it is possible to reduce the impact of these noise by proper design and operation

 

Atmospheric absorption:  Water vapour and oxygen in the atmosphere  introduces additional loss.  These are frequency  dependant with loss due to water vapour being maximum at 22 GHz and  attenuation loss maximum at 15 GHz.  Rain and fog (suspended droplets) cause scattering of radio waves that result in  attenuation.  Wherever, precipitation is higher  the path length is to be kept shorter  or lower frequency band should be used.

 

 

 

4. Explain Frequency Reuse and Channel Assignment strategies:

 

Each  cellular base station is allocated a group of radio channels to be used within a small geographic area called a cell. The power of base station antennas are  limited to cover designated cell area only to avoid interference with other cells.  The design process of selecting and allocating channels groups for all of the cellular base stations within a system is called frequency reuse or frequency planning

 

For the ease of covering  the entire region without gaps, hexagonal shape is selected.  Also, the base station transmitter is normally kept either at the center or at the  corner.  Omni directional antennas are used in case of center location and  sectored directional antennas are used in case corner location.

 

Consider a  cellular system has a total of S duplex channels available for use.  Let each cell is allocated a group of k channels and if the S channels are divided among N cells into unique and disjoint channel groups, which each have the same number of channels, the total number  of  available ratio channels can be expressed as S=kN.  The N cells which collectively use the complete set of available frequencies is called a   cluster.  If a cluster is replicated M times within the system, the total number of duplex channels C can be used  as a measure of capacity and is given as  C = MkN = MS.

A cell cluster is outlined   in bold and replicated over the coverage area.

As can be seen, the capacity of a cellular system is directly proportional to  the number of times a cluster is replicated in a fixed service area.  The factor N is called cluster size and it is estimated for  hexagonal shape as N=i2 + ij + j2.  In this case for i= 1 and j = 1, N is 3; for i=1 and j=2, N is 7; for i=2 and j=2, N = 12 and so on.  If the cell area is kept and constant and the value of N is reduced,  more clusters are required to cover the entire area.  This gives rise to higher capacity to the system.  However, small value of N gives rise to co channels  repeated at closer distance compared  with that of larger clusters.   The frequency re use factor of a cellular system is  given by 1/N, as each cell in a cell is assigned only 1/N of available channels.

To find the nearest co channel neighbors of a particular cell one must do the following: Move i cells along any chain of hexagon and then turn 60 degrees counter clockwise and move j cells. This is illustrated in the following figure for i=3 and j=2.

 

Method of locating a co channel in cell system

 

Channel Assignment strategies:

Channel assignment strategies can be classified as Static and Dynamic.  When fixed number of channels are assigned to a cell, it is called  fixed channel assignment.   Sometimes, this may result in  traffic in some cells getting overloaded while  in others under loaded.  Hence, it is not  a method to manage traffic efficiently. Other variation is, a cell is allowed to borrow channels from neighboring cells  if its channels are filled up.  The mobile switching center supervises such activity.

 

In case of dynamic channel assignment, voice channels are  assigned by MSC based on request from the Base stations.  However, before the MSC need to keep a strict account on frequency use  in candidate cell, the reuse distance of the channel and other cost functions etc.  Although this method likely to avoid blocking of the channels and provides efficient use, the MSC need to tack a lot of data like, channel occupancy, traffic distribution, RSSI (Radio Signal Strength Indications of all channels) on a continuous basis.  This increases the storage and computational load on the system but provides the advantage of increased channel utilization.

 

5. How to Improve the  coverage and  the capacity of  cellular systems:

 

In order to enhance the capacity of the cellular system,  certain  techniques are followed.   These are cell splitting, sectoring and  zone microcell approaches etc.  Cell splitting allows the orderly  growth of the cellular system.  Sectoring uses directional antennas  to further control the interference and frequency reuse of  channel.  The micro cell zone concept distributes  the coverage of  a cell and extends  the cell boundary to hard to reach places.  While cell splitting increases the number of base stations, sectoring and zone micro cells rely on base stations  antenna placements to improve capacity by reducing co channel interferences.

 

 

Cell Splitting:

It is the process of  subdividing a congested cell into smaller cells, each with its  own base station and a corresponding reduction in antenna  height and transmitter power.  It increases the capacity of the system as the number of times the channels are reused.   BY defining new cells which  have a smaller radius and by installing these smaller cells between existing cells, capacity increases due to the additional number of channels per unit area.  Cell splitting is shown below:

If every cell were reduced  such that the

radius of every cell is reduced to half, then

it will required approximately four  smaller cells to cover the same area.  The increased number of cells would increase the number of clusters over the coverage region, which in turn would increase the number of channels and thus capacity in the coverage area.   While allocating the channels,   frequency reuse plan is preserved.

 

As can be seen  in the figure, microcell base station labeled G was placed half way between two larger stations utilizing the same channel as G.  Similar method is followed while allocating channels to other  micro cells.   In this case each cell is allowed to transmit only a power that is equal to  1/ 16th that of original transmitter power so as to cover the smaller area.

 

Essentially,  cell splitting achieves capacity improvement by essentially rescaling the system.   That is  by decreasing the cell radius  R and keeping the co channel reuse ration D / R unchanged, cell splitting increases the number of channels per unit area.

 

Sectoring:  IN this, the cell radius is kept unchangedbut methods are used to decrease the Ratio D / R.  In this , sectoring  improves the SIR so that the cluster size may be reduced.  It  replaces the omni directional  antenna  at the center  or  directional antennas at the corner with three directional antenna at the center if the cell is divided into three sectors. Each of the directional antennas cover a sector of 120 degrees as shown in the following figure

                 

 

 

 

 

When sectoring is employed, the channels used in particular cell are broken down into sectored groups and are used only within a particular sector.    Assuming a seven cell reuse, for the case of 120 deg sectors, the number of interferers in the  first tier  is reduced from six to two. This is because  only two of the six channels cells receive  interference with  a particular sectored channel group.    With this, the resulting SIR  is found to be 24.2 db instead of 17 db.

This enhancement in the SIR, allows one to  decrease the cluster size N  in order to improve the frequency reuse and thus the capacity of the system

In particular system, further improvement in SIR  is achieved by down tilting the sector antennas such that the radiation pattern in the vertical plane has a notch at the nearest co channels cell distance.   BY going in  for 7 cell reuse pattern instead of 12 cell pattern gives raise to a increase in capacity of 12 / 7 = 1.714 times.

 

However, the penalty for improved S/I  and the resulting capacity improvement from the shrinking cluster size is an increased  number of antennas at each base station.  Also as the sectoring reduces the coverage areas of a particular group of channels, the number of hand off increases.  This  increases the load on the switching and control link elements of the mobile system.

 

 

 

 

Microcell Zone :  In this each of the three zone sites are connected to a single base station and share the same radio requirement.  The zones are connected by coaxial  cable, fibre optic cable or microwave link to the base station.   As the mobile travels within the cell, it retains the same channel thus avoiding the handoff and associated complexity.  The  base station simply switches the channels to a different  zone site.    The advantage of  the zone cell technique is that while the cell maintains a particular coverage radius, the co channel interference in the cellular system is reduced such a large  central base station is replaced by several lower powered  transmitters on the edges of the cell.

Decreased co channel interferences improves the signal quality and also leads to an increase in capacity without the degradation in trucking efficiency caused by the sectoring.  For satisfactory performance, and SIR of 18 db is required.  For a system with  N= 7, a D/ R  of 4.6 was shown to achieve this.  With respect to Zone microcell, since the transmission is confined to  a particular  zone, this implies that the D / R ratio,  can be improved as the R value  is now equal to the  radius of the cell and is equal to the twice the  length of the hexagon radius.    Thus, instead of D / R ratio of   4.6 for cluster size of 7,  the D / R ratio becomes 3 with the microcell concept.  This  facilitates the  cluster size to be reduced from 7 to 3, thereby increasing the capacity   7 / 3 = 2.33 times.   Hence for the same SIR requirement of 18 db, the system provides significant increase in capacity over conventional  cellular planning.

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