Spectrum Problem

1
Spectrum Problem bandwidth compression in
relation toQuality of Service of Mobile
Telephony

• Lecture by
• Prof. Dr. P.J. Joglekar
• Friday 14 th March 2007
• Aset forum TIFR Mumbai

2
Lecture Scheme

• Cellular Mobile Operational details
• Spectrum Utilisation Efficiency
• Stake holders Users, Service Providers,Financiers,
  Government
• Signal Characteristics and bench marks for QoS
• Quality Appraisal, Subjective Objective
• W P C, TRAI, FCC, ITU
• Wireless Communication S/n and C/I ratios
• Proposed Re-allocations of Spectrum
• Telecom during disasters a matter of
  international concern.

3
Cellular Mobile Operational details

• Wireless communication
• Cell Coverage,
• hand off
• frequency reuse
• Control channel,
• Transmission Channel pairs
• Base station
• Broad casting

4
Cellular fundas

• The entire service area is divided into a number
  of cells. Each cell is served by a base station.
• Cellular mobile phone service is essentially a
  wireless communication from mobile handset to the
  base station in a cell.
• Connection of base station to the entire
  network is called backhaul which may be optical
  fibre etc.

5
Division of Service Area into Cells

6
Hand off

• .When a subscriber moves from cell to cell he is
  serviced by the respective cells which involves
  hand off.
• If Hand-off not possible, the call may drop.
• Reconnection on redialing depends upon congestion
  conditions in the new cell.

7
Wireless communication

• Radio wave communication between base station and
  mobile set is by line of sight propagation and
  reflections and scattering from other objects in
  the vicinity.
• The usable range of communication from a base
  station may be a few kilometers depending upon
  the base station antenna height and transmitter
  power.
• However, the weaker beyond that range van be
  source of interference.

8
Frequency Reuse

• A particular service operator is allocated a
  range or band of frequencies.
• Since the coverage area of a cell is small the
  same frequencies can be used in cells separated
  by a sufficiently large distance in a cellular
  mobile communication system. This point will
  become clear by the next slides.

9
Reuse Configurations

• Cells of same colour reuse same frequencies

10
Frequency Reuse

• Cells which are far apart reuse the same
  frequencies. Thus Navy nagar and Andheri may be
  using the same set of frequencies.
• Since the signal spreads in all directions it
  creates problems of interference from subscribers
  in other cells other of the same service provider
  depending upon terrain and weather conditions.

11
GSM Basic Radio aspects

• Uplink 890 915 MHz
• Downlink 935 960 MHz
• 124 carriers 200 KHz apart.
• Each carrier has 8 Time division Multiplex radio
  channels
• Each user transmits periodically in every eighth
  time slot in uplink radio carrier and receives in
  corresponding time slot on the downlink carrier.

12
Base Station System

• A base station is the radio interface between a
  mobile phone and the Network and switching
  subsystem.
• It continuously broadcasts its identifying signal
  which is received by all mobile sets in its range
  ( if they are switched on ).
• The mobile sets in turn automatically log on to
  the base station to enable communication.
• This involves updation of HLR VLR

13
Home Location Register (HLR )

• The basic data of a subscriber is maintained in a
  register of the service provider called home
  location register.
• This contains his billing status and other
  information and address of the cell where he is
  located at a particular time.
• When a subscriber moves from one cell to another
  this is updated.

14
Visitor Location Register (VLR)

• A base station maintains a register of all the
  subscribers who have at that time logged on to
  it.
• When a subscriber moves out of the cell this fact
  is reported to HLR.

15
Making a Call from a mobile

• If a mobile subscriber wants to make a call, he
  dials the wanted number,This dialing message is
  received by the base station and a reference is
  made to HLR to ascertain that he is entitled to
  get the service.
• Then the dialed number is accessed over a control
  channel ( CCH ) of small bandwidth. If it is
  available a pair of transmission channels ( TCH )
  involving larger bandwidth is allocated for two
  way communication by the base station.

16
Limitations

• The control channel uses a separate small
  bandwidth to access the dialed number Though the
  called party may be available, the base station
  may not have a free TCH.
• There could also be a congestion in the network
  because of which communication may not be
  possible as in case of point of interconnection.

17
Receiving a call on mobile

• When somebody wants to make a call to a mobile
  phone, he dials the number of the mobile
  subscriber from his phone. The callers service
  provider contacts the
• Home Location Register (HLR ) which directs to
  the Visitor Location Register (VLR ) of the cell
  where the called mobile subscriber is currently
  located.
• The base station ascertains availability on the
  control channel and then assigns a free
  transmission channel pair.

18
Transmission Channel pairs

• To establish a two way communication with the
  mobile subscriber a free pair of transmission
  channel is allocated by the base station.
• It is important to note that a subscriber does
  not have a captive frequency pair.
• Further as he moves from cell to cell these pairs
  keep changing.

19
Spectrum Utilization Efficiency

• How efficiently a cellular mobile service
  provider uses the spectrum allotted to him
  depends upon how efficiently he has reused the
  frequencies. So ITU has defined spectrum
  utilization efficiency as
• The number of simultaneous conversations per MHz
  per square kilometer.

20
Stake Holders

• Users
• Service Providers
• Financiers,
• Government
• Regulatory

21
Users want

• Competitive Rates
• good quality,
• latest facilities
• As the technology develops rapidly more
  facilities are possible and these are offered by
  the competing service providers. So the users
  demand these facilities and will change over to
  the other service provider if these are not
  provided.

22
Service Providers

• Market share,
• Brand Name,
• Average Revenue per User
• This is one of the most important aspect of
  running the business of any service. The service
  provider would like to provide better service to
  his more paying clients.

23
Financiers

• Security of investment
• Reasonable return on investment
• As the technical advancements take place rapidly
  so is the technical obsolescence therefore the
  money invested must be recovered and reasonable
  return obtained in a short life time of the
  product.

24
Government

• License Fee
• Any industry which is doing well attracts
  governments attention and a license fee both at
  entry level and gross revenue every year is
  levied.
• Tax on Revenue
• While tax on revenue is a legitimate demand from
  the government what constitutes revenue has been
  a subject matter of fierce legal battles.

25
Regulatory

• TRAI
• Telecom Regulatory Authority of India
• TDSAT
• Telecom Disputes Settlement and Appelate Tribunal
• Supreme Court
• Final legal authority

26
Signal Characteristics

• Voice
• Data
• Picture
• Video

27
Voice

• In public switched telephone networks an analog
  speech signal is converted into a digital signal
  by taking 8000 samples per second and using 8 bit
  code to represent the amplitude of each sample.
  This gives the standard 64 kilobits per second.
• Cellular mobile phones use only 13 kilo bits per
  second to economize the radio bandwidth.

28
Voice Signal Processing

• A 64 kbps signal fed to a speech coder to convert
  it to 13 kbps data rate.
• It also detects silent periods in speech and
  turns off the transmitter power to increase
  battery life.
• Such signal processing introduces delay of about
  80 milliseconds over and above the delay in the
  PSTN network.This requires echo control.

29
Quality of Servicein GSM

• TRAI benchmark
• The quality of voice in cellular mobile telecom
  services (GSM), is measured on a scale from 0 to
  7. As the quality deteriorates, this value
  increases. The quality of the voice is considered
  to be good, if this value remains between 0 and
  4.

30
Voice quality CDMA

• In case of CDMA, the fundamental performance
  measure for voice quality is the
• Frame Error Rate (FER).
• It is the probability that a transmitted frame
  will be received incorrectly. The frame includes
  signaling information and error detection bits as
  well as user voice/data. This metric includes the
  error detection/correction coding inherent in the
  system.
• Good voice quality is 0-4 FER value

31
Common Problem locations in Mumbai . Bharti
(Airtel), Hutch and BPL

• Navy Nagar / Army Cantonment, Colaba
• 2. HPCL Refinery, Mahul
• 3. Aarey Milk Colony, Goregaon
• 4. Bhabha Atomic Research Centre (BARC), Deonar
• 5. Air India Colony, Kalina
• 6. Vashi Check Naka
• 7. Mantralaya (Secretariat)

32
Navy Nagar

• Navy Nagar is in the southern tip of Mumbai. The
  Operators have sites outside the Defence area.
  The area is surrounded by sea from three sides
  and
• has civil area only on one side. The signal
  strength in some areas drops to 105 dBm due to
  thick vegetation. The interference due to
  reflections from the seawater is also resulting
  in poor voice quality. As such the operators are
  not able to provide coverage without actually
  planning sites inside the defence land.

33
Data

• SMS (Short Message Service) has been a useful
  innovation which is very commonly used by most of
  the cell phone users.
• MMS (Multi Media Message Service) is the next
  application which permits sending of still
  pictures as well as video clippings. This calls
  for much larger bandwidth or within a limited
  bandwidth will take longer time.

34
QoS for data

• Error rate
• Packet loss in next generation networks using IP
  technology.

35
Video Signal

• In TV or video a number of still pictures called
  frames are transmitted per second.
• Typically 25.So each frame takes 40 ms
• In each frame the picture is scanned from left to
  right and top to bottom in 625 lines.
• This makes band width requirement very large.

36
Pixel

• There are about 600 active horizontal lines in a
  PAL frame. So we can get a resolution of 600 in
  the vertical direction.
• Aspect ratio is 4 3 so we need resolution of
  800 in the horizontal direction.
• This results in 600 800 480000 cells called
  pixels. Thus there are about 0.5 million pixels
  per frame. See next slide

37
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38
Bandwidth Requirement

• The bandwidth required will be dependent on the
  picture content with respect to the scanning
  mechanism. In the next few slides we will show
  how different types of picture details will
  require different amounts of bandwidth.

39
A simple black and white picture

• In the next slide we show a picture in which top
  half is white where as bottom half is black. This
  will generate a one cycle of variation per frame.
  Thus by scanning this picture we will get a 25Hz
  square waveform.

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41
Another Picture

• In the next picture also we have half the picture
  white and remaining half black
• Difference is that it is left hand half
  portion is white and right hand half portion is
  black.
• However due to scanning of the picture from left
  to right and top to bottom in each line period we
  will get one cycle of variation. So we will get
  625/2 312.5 cycles of variation per frame or
  7812.5 cycles per second.

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43
Horizontal Vertical Straps

• In the following slide we show 8 horizontal
  straps. This will generate 4 cycles of white and
  black cycles per frame.
• In the subsequent slide there are 8 vertical
  straps which will generate 4 cycles of white and
  black cycles per horizontal scanning line.

44
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45
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46
Spectrum Allocations

• Before the cellular Mobile service was started,
  800 to 1000 MHz band was being used by defense
  services.
• They had to be given new allocations
• Now there is an international move to ask TV
  Broadcasting stations operating in 700 to 800 MHz
  band to shift down to lower frequencies by
  sharing channels by bandwidth compression

47
Surges in telecom traffic

• During fesival seasons etc.
• During natural Disasters dislocation of the
  service due to uprooting of base station antennas
  or power failure or flooding
• During manmade disasters like railway accidents
  or terrorist attacks every body wants to make
  calls and due to heavy demand the communication
  fails when it is needed most.

48
Solutions

• Call gapping only one call per subscriber
  during a specified interval of say 10 minutes
• Call duration limited to one minute.
• People to be informed about nature of calamity
  and advised to send SMS.
• Alternative media to be used.