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lecture 8,9, Performance improvement Techniques in Cellular Systems

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Unit 2: Mobile Communication Systems Lecture 8, 9: Performance Improvement Techniques in Cellular Systems
RFMNRFMN-2009 Umrani A. Waheed

Today’s Lecture: Outline
Handover & Roaming
Hard and Soft H d H d d S ft Handover

Power Control Cell Splitting Sectorization

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Learning Objectives
Students will be able to answer: What i th diff Wh t is the difference b t between Handover & Roaming? How power control is helpful in cellular systems? How Cell splitting and Sectorization improves the cellular capacity of the system?
RFMNRFMN-2009 Umrani A. Waheed

Summary of Wireless Systems

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Handover & Roaming
Call handover (handoff in US) is the switching of on-going call to a different traffic channel on(frequency/ time/code) Must be transparent, uninterrupted, infrequent and successful Handover inside cell1 – for performance or quality improvement Handover between cells2 – MS moves from one cell/BS within the same MSC (or operator) during an existing call.
RFMNRFMN-2009 Umrani A. Waheed

Handover & Roaming
Roaming – A MS is handed over to a service area (market) other than that from which the service has been subscribed Roaming – MS moves from one cell to another cell with different MSC in the same3 or different4 network (?) 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

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Handover Scenario

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Handover Scenario

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Types of Handover
Hard Handover: Perform ‘break before make’. break make Current channel with old BS is released before the new channel with new BS is setup (may cause call dropping). Remarks:
Simple t i l Si l to implement. t 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’. make break New channel with new BS is setup in parallel with the current channel with old BS BS. Both channels (old and new) are used simultaneously for the communications. Remarks: R k
Provide diversity to improve boundary conditions. No communication interruption during handover. Used in CDMA based systems (e.g., IS-95, WCDMA, CDMA2000).
RFMNRFMN-2009 Umrani A. Waheed

Methods for Reducing Handover Call Dropping
Handover call dropping is less desirable compared to blocking. new call blocking Handover calls are given higher priority than new calls in using network resources.

Handover queuing: If all channels are busy, handover calls will be placed in a buffer, unless the buffer is full. Handover reservation: A few channels, r, are reserved for handover calls. New calls will be blocked if number of free channels is <= r.
RFMNRFMN-2009 Umrani A. Waheed

Methods to reduce Handover call dropping:

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Power Control (PC)
One of the limitation on cellular capacity is cochannel interference (CCI) To reduce CCI, power (CCI). CCI control could be used. Power Control: The concept is to increase/ increase/ decrease the transmitter power if it is lesser/more than enough. The idea is that no one will use enough power for transmission

RFMNRFMN-2009

Umrani A. Waheed

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. 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, 1/0 to decrease/ increase by about 1 dB.
RFMNRFMN-2009 Umrani A. Waheed

Power Control (PC)
How often PC command is sent is based on PC rate
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 is specially important to CDMA based cellular i i ll i t tt b d ll l networks because every user using the same frequency for transmission. May not be able to compensate the fast Rayleigh fading.
RFMNRFMN-2009 Umrani A. Waheed

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Cell Splitting
When a cell reaches to its maximum capacity, one 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.

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Umrani 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 cochannel interference by using directional antenna. antenna Each cell is divided into 3 (6) sectors with 3 120° (6 60°) directional antennas at the BS. For N>=4 The number of co-channel cells is reduced from 6 to 2 ( ) f (1). Each sector is assigned a 1/3 (1/6) of the available channel per cell.
RFMNRFMN-2009 Umrani A. Waheed

Sectorization
For N = 3 The number of co-channel cells is reduced co channel 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. 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 120 120° directional antenna cellular system? Repeat it for 60° directional antenna? Assume the path-loss exponent 4. Home Work! Using 120° directional antenna 120° SIR7 = ? SIR4 = ? SIR3 = ? Using 60° directional antenna 60° SIR7 = ? SIR4 = ? SIR3 = ?
RFMNRFMN-2009 Umrani A. Waheed

Reason to use Sectorization

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Unit 2: Mobile Communication Systems Lecture 10: Overview of Cellular Traffic Engineering

RFMNRFMN-2009

Umrani A. Waheed

Lecture 7: Outline
Definition of Traffic in Cellular Systems Trunked S t T k d System Traffic Load Calculations Trunking Efficiency

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Learning Objectives
Students will be able to answer/learn: What Wh t are b i definitions of Traffic in basic d fi iti f T ffi i Cellular System? What are Erlang B formula and Erlang C formulas ? How Traffic load and Trunking efficiency in Cellular System is computed?

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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 System
Two types of Trunked Systems:
Blocked Calls Cleared (BCC)
No queuing for call requests. The requesting user is granted a channel immediately if it is available. Otherwise, the user is blocked and free to try again. The grade of Service (GoS) for BCC is to find the blocking probability, Pb, obtained by Erlang B formula. formula.

y ( ) Blocked Calls Delayed (BCD)
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 free. 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 Erlang C formula. formula.
RFMNRFMN-2009 Umrani A. Waheed

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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