Contents
Code Channels
A
channel is a stream of data designated for a specific use or person and is
separated by a code. A channel may be voice data or overhead control data.
3
.1 Forward Link Channels
On the forward link there are 4 channels used to transmit control and voice data
to the mobile. These code channels are:
1. One Pilot Channel
2. One Sync Channel
3. One to seven Paging Channels
4. Traffic Channels assigned as needed in the Forward link.
Channels are divided by assigning each channel a unique Walsh code:
1. Walsh code 0 reserved for Pilot Channel.
2. Walsh codes 1 through 7 reserved for Paging Channel. Unused Walsh
codes may be assigned to Traffic Channels.
3. Walsh code 32 reserved for Sync Channel.
4. Remaining Walsh codes assigned to subscriber Traffic Channels as
required.
3.1.1 Pilot Channel
Walsh code 0 is reserved for the Pilot Channel. Every cell or sector must transmit
a Pilot Channel for each frequency that is supported. The pilot channel is
constantly transmitted. The mobile uses the pilot signal to acquire the system.
After the mobile has acquired the system the pilot is used for signal strength
measurement. The strength of the pilot is used to determine the power required
for mobile transmits.
Pilot signals contain no messages; however, they are used by mobiles for Traffic
Channel demodulation. it is the strongest signal on the forward link.
3.1.2 Sync channel
The Sync Channel is used by mobiles to obtain timing and cell specific
information. Mobiles must acquire the Sync Channel and decode its message in
order to synchronize with the system. The Sync message is spread with Walsh
code 32 and is broadcast continuously.
The mobile will decode the sync channel message during the power up
sequence. Once the mobile is synchronized with the base station the sync
channel is ignored.
The Sync message includes the following information:
1. System time
2. System ID
3. Network ID
4. Paging Channel data rate
3.1.3 Paging Channels
A Paging Channel is used by the CDMA system to transmit overhead messages
and mobile directed messages. Walsh codes 1 through 7 may be used for
Paging Channels.
When a call is being set up the commands and traffic channel assignment are
sent on the paging channel. Once a traffic channel is established the paging
channel is ignored by the mobile.
3.1.4 Forward Traffic Channel
The forward traffic channel is used when there is a call. Voice data and control
overhead information, normally sent on the paging channel, are transmitted to
the mobile on the traffic channel.
The Forward Traffic Channels are separated by their unique Walsh code
assignments.
Once the mobile is assigned a Walsh code in an omni cell (or sector), the code
cannot be assigned to any other mobile in that omni cell (or sector) for the entire
duration of the call.
3.2 Reverse Link Channels
On the reverse link there are 2 types of channels used to transmit control and
voice data to the mobile. These channels are:
Access
Traffic
3.2.1 Access Channel:
The access channel is used by the mobile when not assigned to a traffic channel.
The access channel is used by the mobile to register with the network, originate
calls, respond to pages and commands from the base station, and transmit
overhead messages to the base station.
3.2.2 Reverse Traffic Channel:
T
he reverse traffic channel is used when there is a call. Voice data and control
overhead information, normally sent on the access channel, are transmitted to
the base station on this channel.
4. Call Processing
4.1 Initialization Mode: Mobile acquires system via Pilot code channel.
Mobile synchronizes with system via Sync code channel.
4.2 Idle Mode
Mobile and base station communicate over Access and Paging code channels when NOT involved in a call.
Mobile obtains overhead information via the Paging code channel.
4 .3 Access Mode Mobile accesses the network via the Access code channel
4.4 Traffic Mode
1.4.1 Land to mobile call
1. When a call for a mobile is placed the mobile will receive a page on the Paging channel.
2. The mobile responds on the Access channel and Traffic channels are established.
1.4.1 Mobile to land call
1. When a call is placed from a mobile the call is placed using the Access channel.
2. The base station responds on the Paging channel and Traffic channels are established.
Roaming: Wireless phones are programmed to operate in a specific system, called the home system. A phone operating in a non-home system is said to be a roamer.
5.Error Detection
Cyclic Redundancy Checking (CRC): CRC provides a high level of error detection. The price paid for this detection capability is overhead bits. These overhead bits are appended to the data stream prior to transmission. In CDMA systems, for example, a Traffic Channel data block (frame) is generated once every 20 milliseconds. When transmitted at a rate of 14,400 bps, each frame contains 288 bits; twelve of these bits are CRC bits. Voice transmissions require real time transmission. Delays are unacceptable to the human ear. It is impractical to attempt to retransmit a portion of a conversation that arrives at the receiver in error. For this reason, the signal must be transmitted reliably. One method to increase reliability is to simply transmit at higher power. An alternative method is to use some form of coding that enables the receiver to correct some detection errors.FEC Coding: Coding that enables the receiver to correct errors is referred to as Forward Error Correction Coding. Redundancy is introduced simply by repeating the digits. The receiver can use a simple majority rule device to make a detection. More sophisticated techniques introduce redundancy through other means. Digital systems, including CDMA systems, commonly use Convolutional Coding to introduce redundancy and enable the receiver to correct some detection errors without increasing transmission power.
