Interference and system capacity


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Interference in wireless communication

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  • 17/10/201115/9/2011 15/9/2011
  • This is a channel overlap for 802.11b
  • This is a channel overlap for 802.11b
  • Channel = set of frequencies that can carry signal power
  • Adjacent channel interference contributes to background noise and cannot be handled in an explicit manner by channel contention techniques.
  • Interference and system capacity

    1. 1. Interference And System Capacity AJAL. A. J Assistant Professor –Dept of ECE,Federal Institute of Science And Technology (FISAT) TM 16/9/2011 16/02/2012 MAIL: Free Powerpoint Templates
    2. 2. Proofs of Wave Nature• Thomas Youngs Double Slit Experiment (1807) bright (constructive) and dark (destructive) fringes seen on screen• Thin Film Interference Patterns• Poisson/Arago Spot (1820)• Diffraction fringes seen within and around a small obstacle or through a narrow opening
    3. 3. Multi-channel real time environmentDVD Player Dual WiFi/cell PDA camera phone MP3 Player WiFi phone HDTV AP Desktop Multimedia games Laptop Printer Camcorder Camera
    4. 4. Interference Defined- Unwanted signals either entering your equipment or getting into equipment of other parties but generated by you.
    5. 5. Inter-Symbol-Interference (ISI) due to Multi- Path FadingTransmitted signal:Received Signals: Line-of-sight: Reflected:The symbols add up on the channel Delays Distortion! 5
    6. 6. Interference : Flavours- RFI - Radio Frequency Interference- - - Two or more signals competing for the same channel- EMI - Electromagnetic Interference- - - Appliances that are overloaded by strong EMI from nearby RF sources
    7. 7. Solving RFI Problems- - - Disconnect components to localize problem area- - - Check cable connections- - - Check for grounded polarized plugs- - - Ferrite cores around power cables- - - Hipass filter on 300 ohm TV feedline
    8. 8. Solving EMI Problems- Hard to track down appliance causing interference- Microprocessors often generate EMI- Enclose in grounded box- Ferrite cores on cables
    9. 9. EMI / EMC/ EMS• EMI is defined as the undesirable signal which causesunsatisfactory operation of a circuit or device.• EMC is defined as the ability of electronic and communicationequipment to be able to operate satisfactorily in the presence ofinterference and not be a source of interference to nearbyequipment.• EMS Electromagnetic susceptibility (EMS) is the capability of adevice to respond to EMI.
    11. 11. Electromagnetic Interference (EMI)• The effect of unwanted energy due to one or a combination of emissions, radiations, or inductions upon reception in a radiocommunication system, or loss of information which could be extracted in the absence of such unwanted energy
    12. 12. EMI depends on what? Emission Immunity (Offending EM Coupling (Victim apparatus) apparatus)Given interference criteria, EMI effects depend on 1. System emissions 2. System immunity 3. Degree of coupling
    13. 13. Electromagnetic Interference (EMI)• EMI: ‘quantification’ of degradation of the quality of an observation due to unwanted emissions, radiations, or inductions upon reception in a radiocommunication system• Information about EMI is obtained by inspection of observations
    14. 14. EMI in Cleanrooms – Example• Wafers are charged to the limit• Cart is charged by the wafers via capacitive coupling• Wheels are insulators – cart cannot discharge• EMI propagates throughout the fab causing lockup of wafer handlers
    15. 15. EMI from Mobile Phones• Frequency range: 800, 900 CREDENCE TECHNOLOGIES ©2002 and 1800MHz• GSM phones produce emission in bursts• High emission levels (~10V/m)• Easily creates disruption in sensitive equipment in immediate proximity 577µS Carrier: 900/1800MHz 4.6mS GSM Phone Transmission Pattern
    16. 16. EMC: what is it?• Electromagnetic compatibility (EMC): ability of an equipment or system to• (1) function satisfactorily in its EM environment (2) without introducing intolerable disturbance to anything in that environment – Criteria of ‘satisfactory’, and ‘intolerable’ and the definition of ‘anything’ and “environment” are all situation-dependent – Harmful (intolerable) interference - when the risk (probability) of interference and extent of its consequences exceed the acceptable levels
    17. 17. METHODS TO ELIMINATE EMI OR DESIGN METHODS FOR EMCThe effective methods to eliminate EMI are1. Shielding2. Grounding3. Bonding4. Filtering5. Isolation6. Separation and orientation7. Circuit impedance level control8. Cable design9. Cancellation techniques in frequency or timedomain10. Proper selection of cables, passive components11. Antenna polarization control12. Balancing
    18. 18. Elements of an EMI Situation– Source "Culprit"– Coupling method "Path"– Sensitive device "Victim" VICTIM SOURCE PATH
    19. 19. System & Environment For tests we separate the SYSTEM & ITS ENVIRONMENT system from its environment. SYSTEMIn emission testing we replace theenvironment by test equipment thatevaluate the level of emissions. ENVIRONMENTIn immunity tests we create aknown EM stress and observereactions.
    20. 20. CONDUCTED EMISSIONS TESTING• Measure Noise on Power Line Product Spectrum Analyzer Power Cord LISN
    21. 21. RADIATED EMISSIONS TESTING• Test Site: Measure Radiated Spectrum• Noise from Equipment Case Analyzer• and Cables Open Area Test Site Product 3 m or 10 m Turntable Measuring Antenna
    22. 22. RADIATED EMISSIONS TESTING• Test Site: Measure Radiated Spectrum• Noise from Equipment Case Analyzer• and Cables Open Area Test Site Product 3 m or 10 m Turntable Measuring Antenna Photos: EMC Test System, Austin, TX
    23. 23. RADIATED EMISSIONS TESTING• Test Site: Measure Radiated Spectrum• Noise from Equipment Case Analyzer• and Cables Open Area Test Site Product 3 m or 10 m Turntable Measuring Antenna
    24. 24. Anechoic Chamber
    25. 25. TEST CONFIGURATIONChamber Configuration 27
    26. 26. Chamber Configuration 28
    27. 27. AMS_02 Main AIR FLOTATION door PLATFORMCLEAN ROOM 1 Entry box Floor panels door CLEAN ROOM 2 29
    28. 28. Test room
    29. 29. EMC tests•
    30. 30. EMC tests
    31. 31. EMC tests
    32. 32. Test antennas
    33. 33. Test antennas
    34. 34. Tests from the air This photo shows a flying laboratory on manned helicopter I designed and supervised many years ago. Modern technology allows such measurements to be made at distance, using miniature unmanned radio-controlled airplanes and helicopters
    35. 35. Near field test
    36. 36. Summary on EMC• The aim of EMC is – to ensure the reliability of all types of electronic devices wherever they are used – and thus to ensure the reliable and safe operation of the systems in which they are employed.• EMC concerns all of us
    37. 37. Interference • Interference management is an central problem in wireless system design. • Within same system (eg. adjacent cells in a cellular system) or across different systems (eg. multiple WiFi networks) • Two basic approaches: 1. orthogonalize into different bands 2. full sharing of spectrum but treating interference as noise
    38. 38. Interference• Sources of interference –another mobile in the same cell –a call in progress in the neighboring cell –other base stations operating in the same frequency band –noncellular system leaks energy into the cellular frequency band• Two major cellular interference – co-channel interference – adjacent channel interference
    39. 39. 802.11b Channel OverlapRooms in Party (11 rooms) • Blue – noise from room 1 • Red – noise from room 6 • Yellow – noise from room 11 • Only 3 quite rooms available; 1, 6, and 11
    40. 40. 802.11b Channel Overlap Only 3 non-overlapping channels: 1, 6, and 11.
    41. 41. Types of Channel Interference• Adjacent channel interference: inversely proportional to the distance• Co-channel interference: directly proportional to the co- channel interference factor
    42. 42. Gaussian Network Capacity: What We Know Tx Rx point-to-point (Shannon 48) Tx 1 Rx1 Rx Tx Tx 2 Rx 2 multiple-access broadcast
    43. 43. Real time process
    44. 44. What We Don’t Know Unfortunately we don’t know the capacity of most other Gaussian networks. Tx 1 Rx 1 Tx 2 Rx 2 Interference Relay S D relay
    45. 45. Multiuser OpportunisticCommunication Multiple users offer new diversity modes, just like time or frequency or MIMO channels
    46. 46. Interference scenario : Real Time
    47. 47. It’s the model. • Shannon focused on noise in point-to-point communication. • But many wireless networks are interference rather than noise-limited. • We propose a deterministic channel model emphasizing interaction between users’ signals rather than on background noise. • Far more analytically tractable and can be used to determine approximate Gaussian capacity
    48. 48. Interference • So far we have looked at single source, single destination networks. • All the signals received is useful. • With multiple sources and multiple destinations, interference is the central phenomenon. • Simplest interference network is the two-user interference channel.
    49. 49. Main message: If something can’t be computed exactly, approximate. • Similar evolution has happened in other fields: – fluid and heavy-traffic approximation in queueing networks – approximation algorithms in CS theory • Approximation should be good in engineering-relevant regimes.
    50. 50. Interference It is a major limiting factor in the performance of cellular radiosystems. (In comparison with wired comm. Systems, the amountand sources of interferences in Wireless Systems are greater.) Creates bottleneck in increasing capacity Sources of interference are: 1. Mobile Stations 2. Neighboring Cells 3. The same frequency cells 4. Non-cellular signals in the same spectrum Interference in Voice Channels: Cross-Talk Urban areas usually have more interference, because of:a)Greater RF Noise Floor, b) More Number of Mobiles
    51. 51. MAJOR LIMITING FACTOR for Cellular System performance is the INTERFERENCEInterferences can cause:  CROSS TALK  Missed and Blocked Calls.SOURCES OF INTERFERENCE? Another mobile in the same cell (if distance & frequency are close) A call in progress in neighboring cell (if frequency is close). Other base stations operating in the same frequency band (from co-channel cells) Non-cellular systems leaking energy into cellular frequency band
    53. 53. 1.Co-Channel Interference
    54. 54. CO-CHANNEL INTERFERENCE Frequency Reuse  Given coverage area cells using the same set of frequencies  co-channel cell !!! Interference between these cells is called CO-CHANNEL INTERFERENCE. However, co-channel interference  cannot be overcome just by increasing the carrier power of a transmitter. Because increase in carrier transmit power increases the interference. How to Reduce co-channel interference? Co-channel cells must be physically separated by a minimum distance to provide sufficient isolation.
    55. 55. Co-Channel Interference Cell Site-to-Mobile Interference (Downlink) Mobile-to Cell-Site Interferences (Uplink)
    56. 56. Co-Channel Interference Intracell Interference: interferences from other mobile terminals in the same cell. – Duplex systems – Background white noise Intercell interference: interferences from other cells. – More evident in the downlink than uplink for reception – Can be reduced by using different set of frequencies Design considerations: – Frequency reuse – Interference – System capacity
    57. 57. 1.Co-Channel Interference• Cells using the same frequency cause interference to each other• Called co-channel interference (CCI)• CCI increases as the cluster size N decreases• Important factor for signal quality is the Carrier to Interference Ratio C/I• Most interference comes from the first tier of co-channel cells
    58. 58. Co-Channel Interference… 1 1 R Second tier 1 Interfering Cell First tier D 1 1 1 1 1 1 1 1 1 1
    59. 59. Cell GeometryR D RR D = q = 3N R
    60. 60. CALCULATION• Let i0 be the number of co-channel interfering cells, then the signal-to- interference ratio for a mobile receiver which monitors a forward channel is – where S is the desired signal power from desired BS and Ii is the interference power caused by ith interfering co-channel cell
    61. 61.  By increasing the ratio of D/R, ► separation between co-channel cells relative to coverage distance of a cell is increased. ► Thus interference is reduced. The parameter Q (co-channel reuse ratio) is related to cluster size. Thus for a hexagonal geometry A small value of Q provides larger capacity since N is cluster size Large value of Q improves transmission quality due to smaller level of co-channel interference A trade-off must be made between these two objectives
    62. 62.  Let i0 be the number of co-channel interfering cells, then the signal-to-interference ratio for a mobile receiver which monitors a forward channel is ► where S is the desired signal power from desired BS and Ii is the interference power caused by ith interfering co-channel cell
    63. 63.  Average received signal strength at any point decays as a power law of the distance of separation between transmitter and receiver Average received power Pr at a distance d from the transmitting antenna is approx ► Where Po is the power received at a close-in reference point at a small distance do from the transmitting antenna, n is path loss exponent ranging between 2 and 4
    64. 64.  Now consider co-channel cell interference If Di is the distance of ith interferer from the mobile, the received power will be proportional to (Di)-n When the transmit power of each BS is equal and the path loss exponent is same throughout coverage then S/I can be approximated as
    65. 65.  Considering only the first layer of interfering cells, which are equidistant D from the desired BS Eqn 4 implies to ► It relates S/I to cluster size N, which in turn determines the overall capacity of the system
    66. 66. INFERENCE For US AMPS system, tests indicate that for sufficient voice quality S/I should be greater or equal to 18 dB. By using Eqn 5, in order to meet this requirement, N should be at least 6.49 assuming n=4. Thus a minimum cluster size of 7 is required to meet S/I requirement of 18 dB It should be noted Eqn 5 is based on hexagonal cell geometry
    67. 67. Co-Channel InterferenceAn S/I of 18 dB is the measured value for the accepted voice quality from the present day cellular mobile receivers.Sufficient voice quality is provided when S/I is greater than or equal to 18dB.
    68. 68. Example: Co-Channel InterferenceIf S/I = 15 dB required for satisfactory performance for forward channel performance of a cellular system.a) What is the Frequency Reuse Factor q (assume K=4)?b) Can we use K=3?Assume 6 co-channels all of them (same distance from the mobile), I.e. N=7
    69. 69. Example:Co-Channel Interferencea) NI =6 => cluster size N= 7, and when κ=4The co-channel reuse ratio is q=D/R=sqrt(3N)=4.583 κ S q = = 1 ( 4.583) 4 = 75.3 6 I NIOr 18.66 dB  greater than the minimum required level  ACCEPT IT!!!b) N= 7 and κ=3 κ S q = = 1 (4.583)3 = 16.04 6 I NIOr 12.05 dB  less than the minimum required level REJECT IT!!!
    70. 70. Example: Worst Case Cochannel Interference (2) A cellular system that requires an S/I ratio of 18dB. (a) if cluster size is 7, what is the worst-case S/I? (b) Is a frequency reuse factor of 7 acceptable in terms of co-channel interference? If not, what would be a better choice of frequency reuse ratio? Solution(a) N=7  q = 3 N = 4.6. If a path loss component of κ=4, the worst- case signal-to-interference ratio is S/I = 54.3 or 17.3 dB.(b) The value of S/I is below the acceptable level of 18dB. We need to decrease I by increasing N =9. The S/I is 95.66 or 19.8dB.
    71. 71.  For 7-cell cluster, hexagonal cell geometry layout Mobile is at the boundary of the cell
    72. 72.  The worst case S/I ratio can be approximated using Eqn 4 The above Eqn can be rewritten in terms of co-channel reuse ratio Q as For N=7, the value of Q is 4.6 The worst case S/I is approximated as 49.56 (17 dB) using Eqn 7, where exact solution using Eqn 4 is 17.8 dB.
    73. 73. Example If S/I is required 15 dB for satisfactory forward channel performance, what is the frequency reuse factor and cluster size that should be used for maximum capacity if path loss exponent n = 4 and n = 3? Assuming 6 co-channel cells in first tier at same distance from desired BS ► n = 4, lets consider 7-cell reuse • Using Eqn. 1, reuse ratio is 4.583 • Using 5, S/I = 1/6 x (4.583)^4 = 75.3 = 18.66 dB • Since this is greater than min required, N=7 can be used ► n = 3, first consider 7-cell reuse • S/I = 1/6 x (4.583)^3 = 16.04 = 12.05 dB • Since this is less than min required, • Next possible value of N is 12-cell reuse (i = j = 2) • Using Eqn. 1, reuse ratio is 6.0 • S/I = 1/6 x (6)^3 = 36 = 15.56 dB • Since this is greater than min required S/I, So N=12 is used
    74. 74. Carrier to Interference Ratio C/I C C C/I is calculated as: = KI KI = # of interfering cells I ∑ Ik k =1The maximum number of K in the first tier is 6 and knowing that −γ −γ C∝R = αR Wanted signal I ∝ D −γ = αD −γ Interfering signal −γThe above equation becomes: C R = KI I ∑ Dk−γ k =1
    75. 75. Rearranging: C 1 1 = −γ = KI I KI  Dk  ∑( q ) −γ ∑ R ÷ k =1   k =1 k and Dk qk = RThe qk is the co-channel interference reduction factor with kthco-channel interfering cell.
    76. 76. Co-Channel Interference…• As N decreases the number of frequency channels per cell increases but C/I decreases• C/I is improved by different methods – Sectored antennas: reduces KI – Beam tilting: Reduces power to co-channel cells – Channel assignment: minimizes activation of co-channel frequencies, which reduces KI
    77. 77. Co-channel interference & system capacity• Co-channel cells use the same set of frequencies in a given coverage area.• Co-channel interference cannot be removed by increasing signal power.• They must be physically separated by certain distance to provide sufficient isolation for propagation.• Co-channel re-use factor is given by: Q = D/R = √3N where R – radius of the cell D – distance to the center of the nearest co-channel cells N – cluster sizeIncreasing D/R will give less interference, whereas decreasing Q value gives more capacity!
    78. 78. CCI Reduction: Cell Sectoring• Shown 120 sectored antennas• Channel per cell are divided among 3 sectors• CCI decreased. Sector 0 gets interference from sectors 4, 5 and 6 only• 60 degrees sectored also possible
    79. 79. Co-channel Interference and System Capacity• Frequency reuse - there are several cells that use the same set of frequencies – co-channel cells – co-channel interference• To reduce co-channel interference, co-channel cell must be separated by a minimum distance.• When the size of the cell is approximately the same – co-channel interference is independent of the transmitted power – co-channel interference is a function of • R: Radius of the cell • D: distance to the center of the nearest co-channel cell• Increasing the ratio Q=D/R, the interference is reduced.• Q is called the co-channel reuse ratio
    80. 80. • For a hexagonal geometry D Q= = 3N R• A small value of Q provides large capacity• A large value of Q improves the transmission quality - smaller level of co-channel interference• A tradeoff must be made between these two objectives
    81. 81. • Let i0 be the number of co-channel interfering cells. The signal-to- interference ratio (SIR) for a mobile receiver can be expressed as S S = i0 I ∑I i =1 i S: the desired signal power I i : interference power caused by the ith interfering co-channel cell base station• The average received power at a distance d from the transmitting antenna is approximated by −n d  close-in reference point Pr = P0   d   0 d0 or  d  P0 :measued power Pr (dBm) = P0 (dBm) − 10n log  d  TX  0 n is the path loss exponent which ranges between 2 and 4.
    82. 82. • When the transmission power of each base station is equal, SIR for a mobile can be approximated as S R −n = i0 I ∑ ( Di ) −n i =1 • Consider only the first layer of interfering cells S ( D / R)n = = ( 3N ) n i0 = 6 I i0 i0• Example: AMPS requires that SIR be greater than 18dB – N should be at least 6.49 for n=4. – Minimum cluster size is 7
    83. 83. • For hexagonal geometry with 7-cell cluster, with the mobile unit being at the cell boundary, the signal-to-interference ratio for the worst case can be approximated as S R −4 = I 2( D − R ) − 4 + ( D − R / 2 ) − 4 + ( D + R / 2 ) − 4 + ( D + R ) − 4 + D − 4
    84. 84. 2.Adjacent channel interference
    85. 85. 2. ADJACENT CHANNEL INTERFERENCEInterference resulting from signals which are adjacentin frequency to the desired signal is calledADJACENT CHANNEL INTERFERENCE.WHY?From imperfect receiver filters (which allow nearby frequencies) to leak into the pass-band.NEAR FAR EFFECT: Adjacent channel user is transmitting in very close range to a subscriber’s receiver, while the receiver attempts to receive a base station on the desired channel. Near far effect also occurs, when a mobile close to a base station transmits on a channel close to one being used by a weak mobile. Base station may have difficulty in discriminating the desired mobile user from the “bleedover” caused by the close adjacent channel mobile.
    86. 86. ADJACENT CHANNEL INTERFERENCEHow to reduce?• Careful filtering• Channel assignment no channel assignment which are all adjacent in frequency.• Keeping frequency separation between each channel in a given cell as large as possible.e.g., in AMPS System there are 395 voice channels which are divided into 21 subsets each with 19 channels.• In each subset, the closest adjacent channel is 21 channels away.• 7-cell reuse -> each cell uses 3 subsets of channels.• 3 subsets are assigned such that every channel in the cell is assured of being separated from every other channel by at least 7 channel spacings.
    87. 87. Adjacent Channel Interference• Adjacent channel interference: interference from adjacent in frequency to the desired signal. – Imperfect receiver filters allow nearby frequencies to leak into the passband – Performance degrade seriously due to near-far effect. receiving filter response signal on adjacent channel signal on adjacent channel desired signal FILTER interference interference desired signal
    88. 88. Adjacent Channel Interference• Adjacent channel interference: interference from adjacent in frequency to the desired signal. – Imperfect receiver filters allow nearby frequencies to leak into the passband – Performance degrade seriously due to near-far effect. receiving filter response signal on adjacent channel signal on adjacent channel desired signal FILTER interference interference desired signal
    89. 89. Adjacent channel interference can be minimized through1. careful filtering and2. channel assignment.• Keep the frequency separation between each channel in a given cell as large as possible• A channel separation greater than six is needed to bring the adjacent channel interference to an acceptable level.
    90. 90. Adjacent channel interference Receiver filter f1 f2 f3 interferenceAdjacent-site constraint: channels assigned toneighboring cells
    91. 91. Adjacent channel interference Interference resulting from signals which are adjacent in frequency It results from imperfect receiver filters which allow nearby frequencies to leak into passband It is more serious if the transmitter is more close to the user’s receiver listening to desired channel This is near-far effect ► A nearby transmitter captures the receiver of subscriber. ► Or mobile close to BS transmits on adjacent channel to one being used by a weak mobile
    92. 92.  Adjacent channel interference can be minimized by careful filtering and channel assignment A cell need not be assigned channels adjacent in frequency By keeping frequency separation in a given cell between channels as large as possible, interference can considerably minimized By sequentially assigning successive channels to different cells, channel allocation schemes are able to separate channels in a cell as many as N Some assigning strategies also avoid use of adjacent channels in neighboring cell sites.
    93. 93.  If reuse factor (1/N) is large i.e. N is small, the separation may not be sufficient to keep intf within tolerable limits. For example if a close-in mobile is 20 times as close to BS as another mobile and energy has leaked to passband, S/I at BS for weak mobile is approx S/I = (20)-n For n-4, this is -52 dB If filter of BS receiver has a slope of 20 dB/octave then intf must be displaced 6 times the passband bandwidth from the center to achieve 52 dB attenuation This implies more than 6 channels separation are needed for an acceptable S/ level I
    94. 94. (2) Adjacent Channel Interference Interference from channels that are adjacent in frequency, The primary reason for that is Imperfect Receiver Filters which cause the adjacent channel energy to leak into your spectrum. Problem is severer if the user of adjacent channel is in close proximity.  Near-Far Effect Near-Far Effect: The other transmitter(who may or may not be of the same type) captures the receiver of the subscriber. Also, when a Mobile Station close to the Base Station transmits on a channel close to the one being used by a weaker mobile: The BS faces difficulty in discriminating the desired mobile user from the “bleed over” of the adjacent channel mobile.
    95. 95. Near-Far Effect: Case 1 Uninte nded Tx Strong “bleed over” BS as Tx Mobile User Weaker signal RxThe Mobile receiver is captured by the unintended, unknowntransmitter, instead of the desired base station
    96. 96. Near-Far Effect: Case 2 BS as Rx Weaker signal Strong “bleed over” Desired Mobile Tx Adjacent Channel Mobile TxThe Base Station faces difficulty in recognizing the actualmobile user, when the adjacent channel bleed over is toohigh.
    97. 97. Minimization of ACI(1) Careful Filtering ---- min. leakage or sharp transition(2) Better Channel Assignment Strategy Channels in a cell need not be adjacent: For channels within a cell, Keep frequency separation as large as possible. Sequentially assigning cells the successive frequency channels. Also, secondary level of interference can be reduced by not assigning adjacent channels to neighboring cells. For tolerable ACI, we either need to increase the frequency separation or reduce the pass band BW.
    98. 98. Power Control in Mobile Com
    99. 99. What is power control ? Both the BS and MS transmitter powers are adjusted dynamically over a wide range. Typical cellular systems adjust their transmitter powers based on received signal strength.TYPES OF POWER CONTROLo Open Loop Power Control It depends solely on mobile unit, not as accurate as closed loop, but can react quicker to fluctuation in signal strength. In this there is no feed back from BS.o Closed Loop Power ControlIn this BS makes power adjustment decisions andcommunicates to mobile on control channels
    100. 100. Why power control ?  Near-far effect  Mechanism to compensate for “channel fading”  Interference reduction,  prolong battery life
    101. 101. Power Control for Reducing Interference• Ensure each mobile transmits the smallest power necessary to maintain a good quality link on the reverse channel – long battery life – increase SIR – solve the near-far problem
    102. 102. Thanks“You cant predict the future, but you can invent it.”
    103. 103. Spectral Bands and Channels • Wireless communication uses emag signals over a range of frequencies • FCC has split the spectrum into spectral bands • Each spectral band is split into channels Example of a channel
    104. 104. Typical usage of spectral band • Transmitter-receiver pairs use independent channels that don’t overlap to avoid interference. Channel A Channel B Channel C Channel D Fixed Block of Radio Frequency Spectrum
    105. 105. Ideal usage of channel bandwidth • Should use entire range of freqs spanning a channel • Usage drops down to 0 just outside channel boundary Channel A Channel B Channel C Channel D Power Frequency
    106. 106. Realistic usage of channel bandwidth • Realistically, transmitter power output is NOT uniform at all frequencies of the channel. Channel A Channel B Channel C Channel D Power Real Usage Wastage of spectrum • PROBLEM: – Transmitted power of some freqs. < max. permissible limit – Results in lower channel capacity and inefficient usage of the spectrum
    107. 107. Consideration of the 802.11b standard • Splits 2.4 GHz band into 11 channels of 22 MHz each – Channels 1, 6 and 11 don’t overlap • Can have 2 types of channel interferences: – Co-channel interference • Address by RTS/CTS handshakes etc. – Adjacent channel interference over partially overlapping channels • Cannot be handled by contention resolution techniques  Wireless networks in the past have used only non- overlapping channels
    108. 108. Focus • To examine approaches to use partially overlapped channels efficiently to improve spectral utilization Channel A Channel B Channel A’
    109. 109. Empirical proof of benefits of partial overlap Link A Ch 1 Ch 1 Ch 3 Ch 6 Link B Ch 3 Link C Ch 6 Amount of Interference• Can we use channels 1, 3 and 6 without interference ?
    110. 110. Empirical proof of benefits of partial overlap Link A Ch 1 Ch 1 Ch 3 Ch 6 Link B Ch 3 Link C Ch 6 Virtually non-overlapping• Typically partially overlapped channels are avoided• With sufficient spatial separation, they can be used