When the band was expanded to 832 channels (covering 869.040 to 893.970 base, 824.040 to 848.970 mobile) the original channels remained with 33 channels being added at the bottom and 133 to the top. This led to an "A" band of channels (666-716) dividing the B band.
AMPS channels cannot be used on neighboring cells at the same time. In general, the allotment has been divided into seven pools with each cell or sector using one pool. This allows an approximate maximum of 56 users per cell sector.
Show CDMA Channels (Zoom)
CDMA carriers are 1.25MHz wide but in the arrangement above are allotted 1.23Mhz (or 41 channels) each. The CDMA carrier is numbered based on the center channel of the carrier, using the AMPS channel numbering. The display above shows how a CDMA system can be overlaid on the existing 800MHz bands. This use starts with the Primary CDMA channel, 283 or 384, and works outward until enough carriers have been added to handle the desired level of CDMA traffic. Channels 691 and 777 are the Secondary CDMA channel.
Note that the "A" carrier has room for eight contiguous CDMA carriers and one carrier at 691. The "B" carrier has room for seven contiguous CDMA carriers and one carrier at 777. While an additional CDMA carrier can be added at 739, it would interfere with the carrier at 777. Note also that a certain number of AMPS channels are not overlaid with CDMA channels. These are used for guard bands between services, AMPS service, and remain available for other possible uses.
(Note: EvDO carriers may be found centered on 037A, 078A, 589B, 636B and 770B on Verizon Wireless systems.)
Each CDMA carrier can carry multiple calls through Code Division of the signal. CDMA carriers are designed to be reused on neighboring cells and sectors, with a maximum 64 users per carrier per cell sector. 1x CDMA technology doubles the number of possible users on each cell. This allows for an 10 to 20 fold increase over AMPS technology. (Although some capacity is lost when a user is being handed off between cell sectors.)
TDMA carriers are 30kHz and follow the same assignments as AMPS channel numbers. As with AMPS, channels may not be reused on neighboring cells or sectors. Each TDMA channel can carry multiple calls through Time Division of the channel into slots. Normally this is 3 slots per channel, but the specification allows for 6 with reduced quality. This allows for a 3 or 6 fold increase over AMPS technlogy.
Show GSM Channels
GSM 850MHz band carriers are 200kHz and are numbered 128-251 in the cellular band. Both carriers have a maximum of 61 carriers. (The "A" carrier has 128-181 and 233-239, and the "B" carrier has 183-231 and 240-251.) Each GSM carrier can carry multiple calls, but cannot be used on neighboring cells. The GSM carrier is divided by time into frames, allowing up to 24 users per GSM channel. As each GSM carrier consumes 7 AMPS channels worth of bandwidth, this provides for an approximate 4 fold increase over AMPS.
US providers are currently required to offer AMPS service by the FCC (this requirement is scheduled to sunset in November of 2007). In addition, many carriers offer another technology - TDMA, CDMA, or GSM - to better use their bandwidth. Some carriers have decided to support multiple advanced technologies (such as CDMA and TDMA) in order to serve roamers or during their own transition to other advanced technologies.