The Airlink physical layer consists of two distinct one-way RF channels, as depicted in Figure 5.2. The forward channel is directed from the network to the mobile hosts. The reverse channel is directed from the mobiles to the network. These 30 KHz channels are the same as those used for analog cellular voice (AMPS) systems, as specified in EIA/TIA-553, located in the 800 MHz region of RF spectrum.
Digital transmission is used - the CDPD channels use Gaussian filtered Minimum Shift Keying (GMSK) modulation at a 19.2 Kbps transmission rate. This GMSK modulation uses a BT=.5 with a modulation index of 0.5. The power level used are consistent with EIA/TIA-54 and 55. GMSK is a form of frequency-shift keyeing (FSK) modulation
For most of us, these names and numbers are not meaningful. They aren't even information that we can easily work into a cocktail party conversation!5.2 Suffice it to say that the modulation scheme is different from typical cellular data modems. The selection of modulation scheme was based on the need to provide high data rate over the air while fitting the RF emission spectrum into the standard 30 kHz channel.
Furthermore, the RF modulation scheme must be robust enough to operate in metropolitan environments under typical cellular RF engineering constraints. Finally, the modulation method had to be easy to implement in terms of available hardware technology. This final limitation restricts the modulation scheme from being the "megabit per second" killer that requires multiple DSP chips and a 12 volt automotive battery.
The CDPD airlink physical layer is defined in Parts 400, 401, 408 and 409 of the CDPD System Specification [CDPD95] and remained largely unchanged between Release 1.0 and Release 1.1 of the specification. The reader interested in greater detail on the physical layer design should refer to [WONG95] and [PAHL95].