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North American Digital Standards

North America has two concurrent digital cellular standards. One is based on TDMA and has been in service since 1992. The other is based on CDMA and is imminent. The CDMA standard was accepted by the TIA as an interim standard in 1992, but has not yet been commercially deployed.

Having two incompatible digital standards in North America is ironic, considering the fact that the countries of Europe have long since united behind the common digital standard of GSM. Unless one of the two competing North American standards proves to be clearly superior over the other or cellular service providers agree to support only one of them, roaming subscribers will likely be restricted to the common denominator of AMPS whenever the service area they are visiting supports the "other" digital standard.

Each of TDMA and CDMA has advantages and disadvantages relative to the other, which are bandied about by their respective adherents and detractors. In general it is difficult to objectively compare TDMA and CDMA systems because their underlying assumptions differ and are not easily related to one another.

Advantages of TDMA-based systems include the capability for variable bit rates (by increasing or decreasing the timeslots in use for one or more users), less stringent power control requirements (time slots reduce mobile duty cycles and thus mobiles' ability to interfere with one another), the capability for half-duplex (less expensive) radios plus the ability to monitor alternative slots and frequencies for MAHO or MCHO operation (both due to offset transmit and receive time slots).

Advantages of CDMA-based systems include theoretically higher capacity per bandwidth, the ability to withstand noise and fading (due to the spreading of the channel) and the reduced frequency planning and complexity needed (due to shared channels and soft handoffs).

A comparison of AMPS, GSM, TDMA and CDMA transmission systems is displayed in Table 1.3.

 

 


 

 

Table 1.3: Cellular System Comparison
Cellular System Comparison