Channels in is-95
To carry the data and the signaling various channels are used. The IS-95 channels can be split into those for the forward link and those for the reverse link. The IS-95 channels differ between the forward and reverse links as a result of the different requirements and the different way in which the links operate. Also Walsh codes need to be synchronized if they are to remain orthogonal.
Is-95 channel structure in forward link:
This is a total of 64 IS-95 code channels that can be accommodated on each RF channel. The IS-95 channels in the forward link are arranged in the following fashion:
- Pilot channel: The pilot channel is transmitted as a reference by the base station to provide timing and phase reference for the mobiles, and carries no real data. The pilot channel is effectively the PN sequence with its associated offset. A measurement of the signal-to-noise ratio of the pilot channel also gives the mobile an indication of which is the strongest serving sector.
- Paging channels (up to seven): It is used to carry information to enable mobiles to be paged. Data carried by this IS-95 channel includes system parameters, voice pages, SMS and other broadcast messages. It occupies Walsh codes 1 - 7 dependent upon the system requirements. The PCH carries data at either 4.8 or 9.6 kbps - a field in the Sync Channel indicates the data rate being transmitted.
- Sync channel: This IS-95 channel is used to provide the timing reference to access the cell . This IS-95 channel always uses Walsh code 32. Each base station has a fixed timing offset to reduce the interference between adjacent base-stations.
- The Sync channel incorporates an 80 mS super frame structure. This is divided into three 26.667 mS frames which correspond to the same length as the short PN sequences. This means that they align with the timing on the Pilot channel.
- Traffic channels: As the name implies, the Forward Traffic Channel is used to carry voice, user data, and also signaling information.
The IS-95 channels are associated with different Walsh codes - particular codes being used to support different functions as shown in the figure:
To generate the final signal, the data from the individual channels is multiplied with the Walsh codes to provide the individual IS-95 forward link channels. The output from this process is then further multiplied with the short PN codes. This provides a means of identifying the sector / cell from which the signal is coming for the handset / mobile
Figure 8.19 : IS-95 channels in the forward link with associated Walsh codes