Cellular Wireless Networks Common issues for wireless solutions

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Cellular Wireless NetworksCommon issues for
wireless solutions
Kevin BoldingElectrical EngineeringSeattle
Pacific University
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Wireless Systems
Handsets Portable mobile devices
Base Station - Receiver
Network connects base stations
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Wireless (Voice) System Design Goals

• 1. Transmit the voice signal without errors

2. Minimize the handset power
3. Minimize the handset cost
4. Maximize the handset/base station ratio
5. Stay within the allocated band/power
1st Generation Analog (AMPS)
2nd Generation Digital (GSM, CDMA)
3rd Generation Data Services (WCDMA, CDMA2000)
4th Generation Higher Speed data (OFDM, MIMO)
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Second Generation Cell Phones

• GSM Global System for Mobile Communications
• Open international standard
• 213 countries
• 2-3 billion subscribers (80 of market)
• All of Europe, much of Asia, much of Americas
• Uses digital TDM / WDM

• CDMA Standard owned by QualComm
• Mostly replaced by GSM and 3rd gen standards
• Was largest in North America and Asia
• Uses digital CDMA

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Generic Cellular System
Base Station Controller
Mobile Switching Center
Base Station Controller
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Cells
Hexagonally tiled cells
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• Cannot reuse frequencies in six surrounding cells

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• Minimum of seven frequency sets required

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7
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Repeat tiling of seven-cell array

• Distance between like cells must be far enough to
  avoid interference

Smaller cells lead to better frequency reuse

• More calls per unit area

• Transmitted power must be smaller to avoid
  interference

• Requires careful power management

• Requires larger number of base stations

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Basic Issues for Mobile Communications
How do we manage handsets entering and leaving
communication?
How do we manage multiple handsets communicating
with one base?
How do we manage handsets moving from base
station to base station?
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Basic Issues for Mobile Communications
How do we manage multiple handsets communicating
with one base?

• Multiplexing (sharing the channel)
• Analog -
• Frequency-division

• Digital -
• Frequency-division and Time-division (GSM)
• Code-division (CDMA)

• We need at least
• 1 send channel for each mobile
• 1 receive channel for each mobile
• 1 control channel

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Basic Issues for Mobile Communications

• To initiate a call
• Mobile issues request on paging channel
• Receiving towers discuss who will answer

• To receive a call
• System must know where the mobile is
• Idle mobile periodically broadcasts on paging
  channel
• System broadcasts page signal on paging channel
  for all bases near mobile

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Basic Issues for Mobile Communications

• Mobile uses idle slots to monitor control
  channels of nearby bases
• Keeps sorted list of the most powerful ones

• If error rate increases, mobile can either
• Increase power on same channel, same base
• Switch to a new base

• Handoff from base to base managed at higher level

• May be a soft handoff

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GSM System Architecture - Handsets

• Subscriber is identified through a computer chip
  (SIM). The chip contains the subscriber ID,
  private key for encryption and can store billing
  information, like services or pre-paid account
  information.

SIM

• Handset is identified through a different unique
  ID, which identifies the capabilities of the
  handset.

• Subscribers can change handsets by simply moving
  their SIM to a new phone no need to re-provision
  the phone on the network.

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GSM System Architecture Base Stations

• Send and receive the signals from the handsets.

• Handles the coding and modulating of the signals,
  including removing the inter-channel
  interference.

• Controls the power correction to account for
  handsets moving away or toward the tower.

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GSM System Architecture - BSC

• Handles channel setup and teardown, including the
  channel code assignment for each session

Base Station Controller

• Handles the transfer of a handset to a new cell
  sector, called a handoff.

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GSM System Architecture - MSC

• Handles the transfer of the voice stream to the
  wired voice network

Mobile Switching Center

• Keeps track of subscribers in its domain that
  can receive calls

• Keeps track of roaming subscribers, so that call
  billing can be transferred to the home carrier

• Authorizes and executes service requests like
  caller ID and call forwarding.

• Keeps track of pre-paid time allotment.