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Unit 2: Mobile Communication SystemsLecture 8, 9:Performance Improvement Techniques in Cellular
RFMNRFMN--20092009 Umrani A. WaheedUmrani A. Waheed
Techniques in Cellular Systems
Today’s Lecture: Outline
Handover & RoamingHandover & RoamingH d d S ft H dH d d S ft H dHard and Soft HandoverHard and Soft Handover
Power ControlPower ControlCell SplittingCell SplittingSectorizationSectorization
RFMNRFMN--20092009 Umrani A. WaheedUmrani A. Waheed
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Learning ObjectivesStudents will be able to answer:
Wh t i th diff b tWhat is the difference between Handover & Roaming?How power control is helpful in cellular systems?How Cell splitting and Sectorization
RFMNRFMN--20092009 Umrani A. WaheedUmrani A. Waheed
How Cell splitting and Sectorization improves the cellular capacity of the system?
Summary of Wireless Systems
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Handover & Roaming
Call handover (handoff in US) is the switching of onon--going callgoing call to a different traffic channelof onon going callgoing call to a different traffic channel (frequency/ time/code) Must be transparent, uninterrupted, infrequent and successfulHandover inside cellinside cell11 – for performance or quality improvement
RFMNRFMN--20092009 Umrani A. WaheedUmrani A. Waheed
quality improvementHandover between cellsbetween cells22 – MS moves from one cell/BS within the same MSC (or operator) during an existing call.
RoamingRoaming – A MS is handed over to a service area (market) other than that from which the
Handover & Roaming
area (market) other than that from which the service has been subscribed RoamingRoaming – MS moves from one cell to another cell with different MSC in the samesame3 3
or differentdifferent44 network (?)Handover/Roaming: Involves identifying a
RFMNRFMN--20092009 Umrani A. WaheedUmrani A. Waheed
Handover/Roaming: Involves identifying a new BS, allocate new channels for both voice/data and control signals associated with new BS.
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Handover & Roaming
RFMNRFMN--20092009 Umrani A. WaheedUmrani A. Waheed
Handover Scenario
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Handover Scenario
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Types of Handover
Hard Handover:Perform ‘break before make’Perform break before make.Current channel with old BSold BS is released before the new channel with new BSnew BS is setup (may cause call dropping).
Remarks:Si l t i l t
RFMNRFMN--20092009 Umrani A. WaheedUmrani A. Waheed
Simple to implement.Cause a short interruption of communications.Used in TDMA/FDMA based systems (e.g., GSM, IS-136).
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Types of Handover
Soft Handover:Perform ‘make before break’Perform make before break.New channel with new BSnew BS is setup in parallel with the current channel with old BSold BS.Both channels (old and new) are used simultaneously for the communications.
R k
RFMNRFMN--20092009 Umrani A. WaheedUmrani A. Waheed
Remarks:Provide diversity to improve boundary conditions.No communication interruption during handover.Used in CDMA based systems (e.g., IS-95, WCDMA, CDMA2000).
Methods for Reducing Handover Call Dropping
Handover call dropping is less desirable compared to new call blockingnew call blocking.Handover calls are given higher priority than new calls in using network resources.
Methods to reduce Handover call dropping:Handover queuing:Handover queuing: If all channels are busy,
handover calls will be placed in a buffer, unless
RFMNRFMN--20092009 Umrani A. WaheedUmrani A. Waheed
handover calls will be placed in a buffer, unless the buffer is full.
Handover reservation:Handover reservation: A few channels, r, are reserved for handover calls. New calls will be blocked if number of free channels is <= r.
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Power Control (PC)
One of the limitation on cellular capacity is co-channel interference (CCI) To reduce CCI powerchannel interference (CCI). To reduce CCI, power control could be used.
Power Control:Power Control: The concept is to increaseincrease//decreasedecrease the transmitter power if it is lesser/morelesser/morethan enough. The idea is that no one will use enough power for transmission
RFMNRFMN--20092009 Umrani A. WaheedUmrani A. Waheed
Power Control: Figure 2.8
RFMNRFMN--20092009 Umrani A. WaheedUmrani A. Waheed
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Operation of Power Control (PC)
MS/BS will estimate its received SIR on DL/UL transmission.transmission.
If channel condition is good, then received SIR is better than required SIR. Thus, receiver (MS/ BS) will send a command to transmitter (BS/MS) to decrease the transmit power.
If channel condition is bad, then received SIR is worse than required SIR. Thus, receiver (MS/ BS) will send a
RFMNRFMN--20092009 Umrani A. WaheedUmrani A. Waheed
command to transmitter (BS/MS) to increase the transmit power.
A single bit command is sent. Typically, 1/01/0 to decrease/ decrease/ increaseincrease by about 1 dB.
Power Control (PC)
How often PC command is sent is based on PC raterate
GSM is 2 Hz. IS-95 is 800 Hz. WCDMA is 1500 Hz.
Advantages: PC not only helps extend the battery life of the device but also reduces the CCI to improve the cellular capacity.
PC i i ll i t t t CDMA b d ll l
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PC is specially important to CDMA based cellular networks because every user using the same frequency for transmission.
May not be able to compensate the fast Rayleigh fading.
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Cell Splitting
When a cell reaches to its maximum capacity, one solution is to split large cell into several smallone solution is to split large cell into several small cells to increase the capacity.
Each Cell has its own BS and a corresponding reduction in Antenna Height and Transmit power.
RFMNRFMN--20092009 Umrani A. WaheedUmrani A. Waheed
Cell Splitting
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Cell Splitting
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Trasmitter Power Reduction
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Sectorization
Keep the cell radius but reduce the total co-channel interference by using directional antennachannel interference by using directional antenna.
Each cell is divided into 3 (6) sectors with 3 120° (6 60°) directional antennas at the BS.
For N>=4The number of co-channel cells is reduced
f 6 2 ( )
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from 6 to 2 (1).Each sector is assigned a 1/3 (1/6) of the
available channel per cell.
Sectorization
For N = 3The number of co-channel cells is reducedThe number of co channel cells is reduced
from 6 to 3 (2) for 120° (60°) directional antenna
Each sector is assigned a 1/3 (1/6) of the available channel per cell.
SIR is improved thus cluster size is reduced
RFMNRFMN--20092009 Umrani A. WaheedUmrani A. Waheed
SIR is improved thus cluster size is reduced.
Number of handovers required ?
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120° Directional Antenna
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Co-Channel Reduction in 120°Directional Antennas (N>=4)
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Co-Channel Reduction in 60°Directional Antennas (N>=4)
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A simple analysis for Directional Antennas
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SIR (worst case) analysis for 120°Directional Antennas
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SIR (worst case) analysis for 120°Directional Antennas
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Example
What are the SIR (in dB) for N = 7, 4 and 3 using 120° directional antenna cellular system? Repeat it120 directional antenna cellular system? Repeat it for 60° directional antenna? Assume the path-loss exponent 4.
Home Work!Home Work!Using 120Using 120°° directional antennadirectional antennaSIRSIR == ?? SIRSIR == ?? SIRSIR == ??
RFMNRFMN--20092009 Umrani A. WaheedUmrani A. Waheed
SIRSIR77 = = ?? SIRSIR44 = = ?? SIRSIR33 = = ??
Using 60Using 60°° directional antennadirectional antennaSIRSIR77 = = ?? SIRSIR44 = = ?? SIRSIR33 = = ??
Reason to use Sectorization
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Unit 2: Mobile Communication SystemsLecture 10:Overview of Cellular Traffic Engineering
RFMNRFMN--20092009 Umrani A. WaheedUmrani A. Waheed
Engineering
Lecture 7: Outline
Definition of Traffic in Cellular SystemsDefinition of Traffic in Cellular SystemsT k d S tT k d S tTrunked SystemTrunked SystemTraffic Load CalculationsTraffic Load CalculationsTrunking EfficiencyTrunking Efficiency
RFMNRFMN--20092009 Umrani A. WaheedUmrani A. Waheed
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Learning ObjectivesStudents will be able to answer/learn:
Wh t b i d fi iti f T ffi iWhat are basic definitions of Traffic in Cellular System?What are Erlang B formula and Erlang C formulas ?How Traffic load and Trunking efficiency
RFMNRFMN--20092009 Umrani A. WaheedUmrani A. Waheed
How Traffic load and Trunking efficiency in Cellular System is computed?
Definitions
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Definitions
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Examples
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Traffic Load ?
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Examples
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Trunked SystemTwo types of Trunked Systems:Two types of Trunked Systems:
Blocked Calls Cleared (BCC)Blocked Calls Cleared (BCC)Blocked Calls Cleared (BCC)Blocked Calls Cleared (BCC)No queuing for call requests. The requesting user is granted a No queuing for call requests. The requesting user is granted a channel immediately if it is available. Otherwise, the user is channel immediately if it is available. Otherwise, the user is blocked and free to try again.blocked and free to try again.The grade of Service (GoS) for BCC is to find the blocking The grade of Service (GoS) for BCC is to find the blocking probability, Pprobability, Pbb, obtained by , obtained by Erlang B formulaErlang B formula..
Blocked Calls Delayed (BCD)Blocked Calls Delayed (BCD)
RFMNRFMN--20092009 Umrani A. WaheedUmrani A. Waheed
y ( )y ( )Queue for blocked call requests. A queue is to hold blocked calls. Queue for blocked call requests. A queue is to hold blocked calls. If no channel is available, the call is delayed until a channel is If no channel is available, the call is delayed until a channel is free.free.The grade of Service (GoS) for BCC is to find the P(delay>t), The grade of Service (GoS) for BCC is to find the P(delay>t), that a call is blocked after waiting for t seconds, obtained by that a call is blocked after waiting for t seconds, obtained by Erlang C formulaErlang C formula..
Erlang B Formula
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Erlang B Formula
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Erlang C Formula
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Erlang C Formula
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