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Explain W-CDMA Forward channel structure in detail.
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Answer:

  • 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 in the forward link are presented in the following mode as shown below in figure.

This is a total of 64 IS-95 code channels that can be lodged on each RF channel which are associated with different Walsh codes. These particular codes being used to support different modes. As per above figure, we have to generate the final signal therefore the data from the single channel is multiplied with the Walsh codes to furnish the single IS-95 forward link channel. The output from this procedure is then further multiplied with the short PN codes. This furnishes a means of identifying the sector or cell from which the signal is coming for the mobile.

The IS-95 forward link channels and their tasks and make-up are encapsulated below:

  • Pilot channel (PC): The pilot channel is transmitted by the base station to furnish timing and phase reference for the mobiles which carries no real data. The data moved by the channel is a uninterrupted stream of zeros which is unroll by Walsh code zero where Walsh code zero itself a stream of zeros. This is further unroll by a pair of quadrature PN sequences. This means that the pilot channel is successfully the PN sequence with its associated offset. A measurement of the signal-to-noise ratio of the pilot channel also gives the indication to mobile about the strongest distributing sector.
  • Paging channels (PCH):This channel is used to carry information to authorize mobiles to be paged. Data carried by this channel includes system parameters, voice pages, SMS and other broadcast messages. It occupies Walsh codes that are from W0 to W7 which depends on the needs of system. As with other channels there are a number of stages taken to fabricate the final channel. First the base band information is error protected. After this the data is repeated if it is at a rate of 4.8 kbps, otherwise it is left as it is. Following this the data is interleaved and then crawled by the decimated long PN sequence and finally unroll by the Walsh code for the particular channel assignment. In this process the long PN code is itself masked with a code which is particular to the channel being used.
  • Synchronization channel (SC):This channel is used to furnish the timing reference to access the cell. It always uses Walsh code 32. Each base station has a fixed timing offset to reduce the interference between adjacent base-stations. This channel includes an 80 mS super frame structure. This is divided into three 26.667 ms frames which equate to the same length as the short PN sequences. This means that they join with the timing on the Pilot channel. This IS-95 channel is allotted the least power of the overhead channels in the overall CDMA transmission. In this channel, the transmitted data involves the system time, pilot PN of the base station, long code state, system ID, and the network ID.
  • Forward Traffic Channel (FTC): It is used to convey voice, data and signaling channels. 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 actively (every 20 ms). This channel use Walsh codes from W8 to W31 and Walsh codes from W33 to W63 as per above figure. When the rate is reduced then it reduces the level of intrusion to other users. The original vocoder particular used a set of rates based on divisions of 9.6 kbps. This is reflected in IS-95A. Later the vocoder was improved to give better voice quality and in IS-95B a vocoder was launched with a rate set based on 14.4 kbps. The 9.6 kbps rate set was labeled RS1 and the second based on 14.4 kbps was labeled RS2. Though data is always travelled at full rate.

Generation of cdma forward traffic channels:

 The below mentioned figure encapsulate an IS-95 channel where the transmitter portion includes the channel coding, modulation, spreading and filtering and on the receiver side, it includes filtering, de-spreading, demodulation and channel decoding.

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