Traffic Channel Definitions
In analog systems, the traffic channels (or voice channels) are synonymous with the physical transceiver hardware. The nature of CDMA technology implies that the effective traffic carrying capacity of a CDMA carrier varies in accordance with the interference density in the band and also depends on various CDMA system parameters. From the hardware perspective, the physical transceivers are the same for all channel types meaning that the sync, paging and the traffic channels are all supported by identical hardware.
The transceiver elements could be used for handling incoming traffic, supporting soft handoff or configured as sync and paging channels. Using the guidelines provided earlier in this section, an engineer/system planner is able to derive the total number of cell sites needed to support the traffic capacity of a planned CDMA system and estimate the total Erlangs supported by each cell.
However, for the purpose of equipment planning, it is desirable to convert the total Erlangs per cell to the number of transceiver elements required to support the calls (Physical TCHs). In order to convert Erlangs to Physical Traffic Channels (PTCH), the quantity of channels required to handle subscriber traffic, soft handoff and overhead messaging must be known. currently three principle traffic channel types.
Effective Traffic Channels (ETCH)
The Effective Traffic Channels (ETCH) are the quantity of channels required to support the primary traffic. The channel load associated with ETCH does not include the additional channel capacity required for soft handoff or overhead messaging. Note that in general, one ETCH corresponds to a single analog voice channel or one voice timeslot of a TDMA carrier.
As soft handoff is not included within the calculation for ETCH, the quantity of ETCHs can be considered the most relevant comparison of capacity with analog systems where no make-before-break handoff exists. The number of ETCHs are calculated by conversion from the total estimated Erlangs generated at a given Grade Of Service (GOS) using the standard Erlang conversion tables.
Actual Traffic Channels (ATCH)
The Actual Traffic Channels (ATCH) are the quantity of channels required for the primary traffic plus those Erlangs to support soft handoff (SHO). SHO corresponds to additional Erlangs generated by the primary traffic not additional channels.
For example, if the Effective Erlangs generated for a sector was 11.5 (using Erlang “B” at 2% GOS this converts to 18 ETCH), the conversion to ATCH assuming 35% SHO would be 11.5 x 0.35 = 4.025 + 11.5 = 15.52 Erlangs (using Erlang “B” at 2% GOS this converts to 24 ATCH).
Physical Traffic Channels (PTCH)
The Physical Traffic Channels (PTCH) are the total channels required for primary traffic, soft handoff (ATCH) plus OverHead (OH) messaging. The OH messaging corresponds to the channels required for paging and synchronization on the forward link and access on the reverse link. A total of two OH channels are required per CDMA sector.
Therefore, an Omni directional (single sector) site requires 2 OH channels, while a three sector configuration requires 6 OH channels (2 per sector). Additional carriers within a sector may require more OH channels to be dedicated.