Cellular Communications

Goal: encode information in a way that can be reliable decoded

Start with a carrier wave

• Must be much higher frequency than the information flow
• Examples: 2.4 GHz for WiFi, 900 Mhz for some GSM
• Normally sinusoidal

Amplitude modulation: vary the amplitude to carry the signal

• AM radio
• requires a good amplifier with a wide dynamic range

Frequency modulation: vary the frequency

• FM radio
• Analog: carrier freq proportional to signal
• Digital: discrete values for carrier frequency
  • called Frequency Shift Keying or FSK
  • a variation is used for modems (changing pitch)
  • Minimum-shift Keying (MSK) uses minimal delta from 0 to 1; used in GSM
• Same amplitude implies simpler amplifier

General advantages:

• more information in given spectrum
• can be lossless

Quadrature coding:

• Idea: two signals out of phase by 90 degrees are orthogonal
  • We can recover both signals simultaneously
  • Two orthogonal axes: I and Q

Phase-shift keying:

• Encode via phase shift for each bit
• Binary PSK (or 2-PSK):
• Quadrature PSK (QPSK)
  • Two bits per symbol
  • Used in CMDA

More general quadrature-amplitude modulation (QAM):

• 16-QAM

• 4-QAM == QPSK
• 256-QAM used for digital cable TV
• WiMAX uses adaptive modulation, a combination of 4- 16- and 64-QAM depending on the S/N ratio

Modulation gives us one communication stream

Next we need to multiplex many streams within the same spectrum

• Analogy: multiple rooms, one conversation per room
• This is the “cell” in cellular…
• But cells overlap some, creating background noise

• Divide the spectrum into channels
• 802.11b has 16 channels, but they overlap!
• GSM has 124 channels (each only 200 kHz, 270 kb/s)

• Analogy: take turns talking
• Can combine with channels, spatial multiplexing
• Divide time into slots
• Some overhead between slots
• Used in GSM, 8 slots per channel

• Spread spectrum: user transmitters use the whole frequency range
• Analogy: conversations in multiple languages simultaneously
• Start with a set of orthogonal vectors with one element for each “channel”
• Each sender uses one vector from this set
• All transmit at the same time
• Decompose signal into its orthogonol components to reconstruct

• Many orthogonal sub-carriers
• Each subcarrier uses relatively simple modulation
• Use the together for high bandwidth

AMPS (1G == analog)

• Analog system deployed by AT&T starting in 1983
• 800 Mhz carrier, 416 30-kHz channels in each of two blocks
• FM
• Neighboring cells use different channels

GSM (2G == digital)

• Largest digital standard
• Carriers vary: 900 MHz, 1800 MHz, US/Canada: 850 MHz, 1900 MHz
  • hence “quad band” phones
• 50 MHz allocation, 25 up and 25 down
• 124 200 kHz channels, 8-slot TDMA
• Gaussian MSK modulation
• GPRS is packet data encoded on these channels (2.5G)
  • can use spare channels dynamically for higher bandwidth
  • some sharing of these channels first-come first-served

UMTS

CDMA (Qualcomm):

• cdmaOne == IS-95: 2G version
  • duplex pair of 1.25 MHz channels
    • 64 channels
• CDMA2000 == IS-2000 is the 3G version
  • 1xRTT is 2.5G (like GPRS)
    • 128 channels at 144 kb/s
  • EV-DO is 3G
    • pair 3.75 MHz channels
• many carriers: 450 MHz, 700 MHz, 800 MHz, 900 MHz, 1700 MHz, 1800 MHz, 1900 MHz, and 2100 MHz

WiMAX (802.16e)

• Wide range of carriers
• OFDM
• 4- 16- or 64-QAM modulation
• up to 75 Mb/s total bandwidth (but not at 50 km)