Unit 2: Mobile Communication Systems Performance ... · Performance Improvement Techniques in Cellular RFMNRFMN--20092009 Umrani A. Waheed ... To reduce CCI powerchannel interference

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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


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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


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


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


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


Handover Scenario


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Handover Scenario


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


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


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


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


Power Control: Figure 2.8


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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


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


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.


Cell Splitting


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Cell Splitting


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 ( )


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


SIR is improved thus cluster size is reduced.

Number of handovers required ?

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120° Directional Antenna


Co-Channel Reduction in 120°Directional Antennas (N>=4)


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Co-Channel Reduction in 60°Directional Antennas (N>=4)


A simple analysis for Directional Antennas


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SIR (worst case) analysis for 120°Directional Antennas


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 == ??


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


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


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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


How Traffic load and Trunking efficiency in Cellular System is computed?

Definitions


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Definitions


Examples


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Traffic Load ?


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)


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


Erlang C Formula


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Erlang C Formula


Documents

Unit 2: Mobile Communication Systems Performance ... · Performance Improvement Techniques in Cellular RFMNRFMN--20092009 Umrani A. Waheed ... To reduce CCI powerchannel interference