GSM

1
GSMGlobal System for Mobile
2
How does GSM handle multiple users

• The 1G cellular systems used FDMA. The first
  cellular standard adopting TDMA was GSM, a 2G
  system using digital technology.
• First deployment in 1992.

3
GSM is a TDMA/FDMA/FDD System

• A total of 124 carrier frequencies are available
  in the 25 MHz band in each direction (forward or
  reverse). This is FDMA.
• Each carrier frequency, occupying a frequency
  band of 200 kHz, has 8 time slots for 8 users.
  This is TDMA (TDMA overlaid on FDMA).
• The forward (downlink) and reverse (uplink) uses
  two different frequency bands 935-960 MHz for
  downlink and 890-915 for uplink. This is FDD
  (Frequency Division Duplex).

4
Mobile station

• Mobile Equipment (ME) the phone including
  transmitter, receiver, etc.
• Subscriber Identity Module (SIM) a smart card
  that stores the subscribers ID number, the
  networks the subscriber is authorized to use,
  encryption keys, etc.
• Thus a subscriber needs only carry his or her SIM
  to use a wide variety of subscriber devices in
  many countries. The SIMs roam, not the phones.
  (The US is different from most other countries.
  In the US, service providers lock the phones
  because they subsidize them.)

5
Base Station Subsystem (BSS)

• Base Transceiver Station (BTS) for a single cell
  an antenna, a transceiver (transmitter
  receiver), and a link to a base station
  controller
• A GSM cell can have a radius of 100m-35km.
• Base Station Controller (BSC) may control
  multiple BTS units and hence multiple cells. It
  reserves frequencies, manages the handoff of a
  mobile unit from one cell to another within the
  BSS.

6
Network Subsystem (NS)

• Provides the link between the cellular network
  and the public switched telecommunications
  networks (PSTN).
• The central element of the NS is the Mobile
  Switching Center (MSC), which
• Determines whether the subscriber is associated
  with the switching center
• Stores locations of the subscribers currently in
  the region covered by the switching center
• Stores authentication and encryption keys for all
  the subscribers in both the home and visitor
  location registers
• Keeps track of the type of equipment that exists
  at the mobile stations

7
GSM Frames and Data Rate

• Each time slot has 114 bits of data
• Each frame has 8 time slots, one for a user
• Each multiframe (duration120ms) has 24 data
  frames
• Data rate

8
Speech Coding

• Speech coding is used to compress the data
  (reduce the data sent). This will increase the
  speed of transmission.
• It uses previous data samples to predict current
  sample.
• Analog voice is transformed into 13 kbps digital
  signal.
• Adding error-detection and error-correction
  coding brings the transmission rate to 22.8 kbps.
• Data rate 9.6, 4.8 or 2.4 kbps.

9
Frequency Hopping

• A slow frequency hopping 216 hop/s
• To compensate multipath (frequency selective)
  fading
• To reduce the cochannel interference