- The IS-95 channels can be chop for the forward link and for the reverse link. The IS-95 channels vary between the forward and reverse links as a result of the different needs and the different way in which the links or connections work.
- Also Walsh codes need to be synchronized if they are to remain statistically independent. As the signals transmitted from the mobile stations travel over different distances because of the variety of locations of the mobiles they will all appear at slightly different times, and thus they will not be organized to one another.
- The IS-95 channels for the reverse link are quite different to those in the forward link. There are only two basic channels that are access channel and reverse traffic channels.
- The path in which these IS-95 channels are designed and fabricated is also different.This is because they are created within the mobile rather than the base station. In terms of the modulation, OQPSK is used where a half chip delay is launched onto the Q channel of the modulation.
- Although statistically independent modulation schemes are used. The different mobiles are individually identified by a mask on the long PN code which is based on the Equipment Serial Number (ESN). The long PN code is used to give the final expanding of the data to 1.228 Mcps.
The two IS-95 channels in the reverse link are encapsulated below:
Access channel (AC):
- This IS-95 channel is used by the mobile to communicate with the base station when no traffic channel has been layout. This IS-95 channel is hence used for gaining access to the network, call origination requests and also for sending responses to paging commands that might be sent by the network.
- There can be up to 32 Access Channels on the IS-95 reverse link for each Paging Channel on forward link. Each AC uses the same PN but they are time moved to allow the mobile to be uniquely recognized. Data is sent at 4800 bps in a 20 mS time frame so that each frame contains 96 bits.
Reverse traffic channel (RTC):
- It is used to convey voice, data and signaling information etc. When conveying voice, the coded voice data does not need a constant bit rate and IS-95 allows the rate of the frames to change lively (every 20 ms). When the rate is reduced it reduces the level of intrusion to other users.
- The following figure demonstrate the IS 95 reverse channel:
- The structure of the reverse traffic channel is alike to that of the access channel. Although this IS-95 also includes a data burst randomizer into which the orthogonal modulated data is fed. The data burst randomizer is the method used to consider for the variable rate voice data is lodged. It is not possible to use the same methods used on the forward link because they damage the channel power.
- As per above diagram of the reverse traffic channel is alike to that of the access channel. It contains the sub parts like CRC generator, encoder, repeater, interleaver, modulator and filter etc. where all are described below:
- CRC means cyclic redundancy check generator. It is an error detecting code which is commonly used in digital networks and storage devices to detect accidental changes to a raw data. In reverse traffic channel of IS-95, first of all CRC coded data to be transmitted from mobile unit to base stations.
- A convolutional code is a type of error correcting code that generates parity symbols via the sliding application of a Boolean polynomial function to a data stream. In reverse channel of IS-95, CRC generator transmits the coded data to convolutional encoder where a one third rate code is used for error correction for all information channels. Convolutional encoder transmit the encoded data to repeater.
- It is an electronic device which receives the encoded data from encoder and then retransmits it at a higher level or power so that the signal can cover longer distances and then transmits the data for interleaving.
- It is a technique for making forward error correction more robust with respect to burst errors. It receives the data from repeater and then correcting the error from data and again transmit the data for modulation.
Data burst randomizer:
Although IS-95 includes a data burst randomizer into which the orthogonal modulated data is fed. The data burst randomizer is the method used to consider for the variable rate voice data is lodged. It is not possible to use the same methods used on the forward link because they damage the channel power.
Spreading and modulation:
- In reverse channel IS-95A, modulation used is a 64-ary orthogonal modulation. Direct sequence spreading and filtering are used to obtain the modulation signal for transmission. Because the reverse traffic channel has a variable rate, randomized gating is used in the case of lower data rate frames to control the transmit power because the power control is done by gating, the power level of the transmitted symbols is kept constant. Here is no pilot available for the reverse link transmission. The Walsh modulator collects log2 64= 6 data bits and transmits one of 64 orthogonal Walsh functions over the channel.
- It uses the direct sequence spreading of coded signal to separate the components of received signal corresponding to different propagation delay paths. It derives the diversity gain from a potentially poor channel. After rake receiver dispreading, a demodulation routine detects the transmitted data from each delayed path component and combines the results.
- De-interleaving process uses the inverse mapping to store the original sequence of symbols.
- Viterbi decoder uses the Viterbi algorithm for decoding a bit stream that has been encoded using convolutional code or trellis code. In reverse channel of IS-95, the receiver used the Viterbi algorithm for optimal decoding of the decoded data.