Mumbai university > Comp > SEM 8 > Mobile Communication

Marks: 10M

Year: Revised 2012

• IS-95 reverse link frequency band is 824-849MHz and forward link frequency band is 869-894MHz.

• A PCS version of IS-95 has designed for international use in the 1800-2000MHz band

• A forward and reverse channel pair is separated by 45MHz for cellular band operation. Many users share a common channel for transmission. The maximum user data rate is 9.6kbps.User data in IS-95 is spread to a channel chip rate of 1.2288Mcps using a combination of techniques. This spreading process is different for forward and reverse links.

• On the forward link the user data stream is encoded using a rate ½ Convolutional code, interleaved and spread by one of 64 orthogonal spreading sequences. Each mobile in a given cell is assigned a different spreading sequence providing a perfect separation among the signals from the different users. To reduce interference between the mobiles that use the same spreading sequence in different cells all signals in a particular cell are scrambled using a pseudorandom sequence.

• Orthogonally among all forward channel users within a cell is preserved because their signals are scrambled synchronously. A pilot channel is provided on the forward link so that each subscriber within the cell can determine and react to the channel characteristics.

• On the reverse link a different spreading strategy is used since each received signal arrives at base station via different propagation path. The reverse channel user data stream is first convolutionally encoded with a rate 1/3 code. After interleaving each block of 6 encoded symbols is mapped to one of the 64 orthogonal Walsh functions providing 64-ary orthogonal signalling. A final fourfold spreading giving a rate of 1.2288Mcps is achieved by spreading the resulting 307.2kcps stream by user specific and base station specific codes.

• The rate of 1/3 coding and mapping onto Walsh functions result in a greater tolerance for interference. This added robustness is important on the reverse link due to the non coherent and the in-cell interference received at the base station.

• Another essential element of reverse link is tight control of each subscriber’s transmitter power to avoid the near-far problem that arises from varying received power of the users. Open and close loop power control is used to adjust transmitted power. Power controls are sent at a rate of 800 bps.

• At both base station and subscriber RAKE receivers are used to resolve and combine the multipath component thus reducing the degree of fading.