Wireless Communication
John Weber

Analog Mobile Phone Service (AMPS)

  • Bandwidth: 50MHz (A-side 25, B-side 25)
  • Allocation: Transmit 824-849 MHz, Receive 869-894 MHz
  • Modulation Scheme: FSK
  • Shared Channel Access Mechanism: FDMA
  • Each voice channel is allocated a 30-KHz portion of bandwidth within the AMPS frequency allocations. Since each carrier has 25 MHz of the frequency spectrum, this results in 832 cellular channels. AMPS uses two channels for each conversation -- one to communicate from the base station to the subscriber (forward channel) and another to communicate in the opposite direction (reverse channel). Therefore, 416 simultaneous full-duplex conversations can be supported. The first 21 of these channels (on each [AB] side) are used for control operations like call origination, call termination, etc. So each carrier has 395 channels available for voice.

Digital Advanced Mobile Phone Service (D-AMPS)

  • Bandwidth: 67 MHz
  • Allocation: 824-891 MHz
  • Modulation Scheme: PSK
  • Shared Channel Access Mechanism: TDMA
  • D-AMPS supports dual-mode operation. So when operating in the 800 Mhz range and in the analog mode of operation, frequency modulation is used with FDMA for compatibility with AMPS. In digital mode, D-AMPS adds TDMA support to AMPS to get three channels for each AMPS channel, tripling the number of calls that can be handled on a channel.

Global System for Mobile (GSM)

  • Bandwidth: 50 MHz
  • Allocation: Transmit 890-915 MHz, Receive 935-960 MHz
  • Modulation Scheme: GMSK
  • Shared Channel Access Mechanism: TDMA
  • The frequency band is divided into 124 pairs of frequency duplex channels each of 200KHz width. Each channel is divided into 8 time slots, and, therefore, each "frame" consists of 8 slots. Each call is assigned a channel and a slot. Under GSM, a sequence of 26 frames forms a multi-frame. Of these frames, 24 are used for traffic, 1 is used for control signals, and 1 is unused.

Cellular vs. PCS
PCS systems are those systems which operate in the Personal Communications Band (1850-1990 MHz); In contrast, cellular systems were assigned the frequencies 806-890 MHz. From this fact stem most of the other differences:

  1. PCS provides more bandwidth (this is directly derived from the greater frequency range)
  2. PCS cells are smaller (since higher frequency waves attenuate more rapidly the range is going to be lower leading to smaller cells)
  3. PCS uses less power at the tranceiver (PCS systems usually use either TDMA or CDMA for access which means that they are transmitting for at most 1/3 of the time which means that they are using less power).

PCS Frequency Allocation