About IS-95A

Introduction 

CDMA (Code Division Multiple Access) is a digital radio scheme designed to send voice, data and signaling data between mobile telephones and cellular base stations. IS-95A is a 2G mobile telecommunications standard that uses CDMA. It was developed to meet Revision A of the Telecommunications Industry Association / Electronic Industries Association Interim Standard - 95 (TIA/EIA IS-95). IS-95 is also known as cdmaOne.

An IS-95A system transmits streams of pseudo-random (PN) sequences of bits which allows several radios to share the same frequency band. IS-95 systems operate at a PN chip rate of 1.2288 Mcps. More mobile stations (MS) can be served by fewer of base stations (BS) because network capacity does not directly limit the number of active MS telephones.

Forward Link

The forward link for IS-95A has the four channel types:

• Pilot channel:

  The pilot channel provides a reference signal that helps the mobile station detect, synchronize, and demodulate the received signal. The pilot carries no information. The pilot channel is kept at a constant power approximately 4 to 6 dB greater than the carrier's traffic channels. The MS uses this constant signal to compare against the pilot channels of other BS so that an appropriate hand-off of the MS between the BS can be made. The pilot channel is always assigned a Walsh code of 0 (zero). The pilot channel operates at 4800 or 9600 bps.

• Sync channel:

  The sync channel may be used along with the pilot channel by the MS to establish initial time synchronization with the BS. Only a synchronization message is transmitted on the sync channel. The synchronization message contains BS and time information. This information is used by MS to synchronize to the BS network. The sync channel is always assigned a Walsh code of 32. The sync channel always operates at a data rate of 1200 bps but it is convolutionally encoded to 2400 bps and repeated to 4800 bps before being interleaved over the period of the pilot's PN sequence.

  System Identification Number Network Identification Number Pilot PN Sequence Index Long Code State Paging Channel Data Rate	System Time Leap Seconds Local Time Offset Daylight Savings Time Indicator

• Paging channel:

  The paging channel is used to transmit:

  • system control messages to all MS on the BS network, and

  • dedicated call-setup information to individual MS on the BS network. The carrier contains at least one paging channel but it may contain as many as seven paging channels (assigned the code channel number of 1 through 7).

• Traffic channel:

  Forward traffic channels can use the remaining code channels. A traffic channel is used to carry user information (such as speech or data) to individual mobile phones during a call. It also carries signaling messages (such as requests to change frequency) to individual mobile phones during a call. Like the paging channel, traffic channels have a frame time of 20ms. Since voice and user data are intermittent, the traffic channels support variable-rate operation. Every 20 ms frame may be transmitted at a different rate (1200, 2400, 4800, or 9600 bps), as determined by the service in use (voice or data).

  Traffic channels may also carry circuit-switch data calls in IS-95. The variable-rate traffic frames are generated using the IS-95 Radio Link Protocol (RLP). RLP provides a mechanism to improve the performance of the wireless link for data. Where voice calls might tolerate the dropping of occasional 20 ms frames, a data call would have unacceptable performance without RLP.

Reverse Link

In the reverse link direction, no pilot is used because a pilot channel would be required for each signal. The total number of MS that the BS can support is usually limited by the maximum number of MS that can be supported in the reverse link direction. The reverse link channels are divided into two channel types:

• Traffic channels:

  The reverse traffic channel is used by the MS to carry user information (such as speech or data) to the BS during a call. The traffic channel can use a data rate of 9600 bps, 4800 bps, 2400 bps, or 1200 bps. The traffic channel has a transmitting duty cycle that is proportionate with the data rate used as shown in the following table:

  Data Rate	Duty Cycle
  9600 bps	100%
  4800 bps	50%
  2400 bps	25%
  1200 bps	12.5%

• Access channels:

  The reverse access channel is used by the MS to communicate information such as call origination and paging response to the BS. The access channel has a fixed access rate of 4800 bps. Each access channel is identified by a unique access-channel long code sequence with an access number, an associated paging channel number, and additional system data. The BS can identify an individual MS because each MS has a different PN code. All transmitted reverse link data is grouped into 20-ms frames.