With a neat diagram, explain forward CDMA channel.

Mumbai university > Comp > SEM 8 > Mobile Communication

Marks: 10M

Year: Revised 2012

1 Answer
  • CDMA forward link uses same frequency spectrum as AMPS i.e. 869-894 MHz. One channel bandwidth is 1.25 MHz

  • Modulation scheme used is QPSK.

  • Orthogonal Walsh codes are used. Walsh codes are called Hadamard codes and they are used in all CDMA techniques.

  • Forward channels are separated from each other using different spreading codes. 64 Walsh codes are used to identify each channel.

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  • Type of forward channel:

A. Pilot channel:

  • It provides phase for coherent demodulation, time, signal strength, comparison with reference signal for determining when to hand off for all mobile stations.

  • It is used to uniquely identify sectors or cells.

  • It is 4-6 db stronger than all other channels. It is used to lock onto other channel.

  • It is obtained using all zero Walsh code i.e. it contains no information except the RF carrier.

B. Synch channel:

  • It is used to acquire initial time synchronization.

  • Synch messages include System ID (SID), Network ID (NID), the offset of the PN short code and the paging channel data rate.

  • It broadcasts synch messages to the mobile station and operates at 1200 bps.

  • It uses Walsh code 32 for spreading.

C. Paging channel:

  • There are 7 paging channels used to page the mobile station in case of an incoming call, or to carry the control messages for call set up.

  • It uses Walsh code 1-7. There is no power control.

  • It is additionally scrambled by PN long code, which is generated by LFSR of length 42.

  • It operates at the rate of 4.8 kbps or 9.6 kbps.

D. Traffic channel:

  • There are 55 traffic channels used to carry actual information.

  • It supports variable data rates-RS1={9.6. 4.8, 2.4, 1.2 kbps} and RS2={14.4, 7.2, 3.6, 1.8 kbps}

  • RS1 is mandatory for IS-95. But support for RS2 is optional.

  • It also carries power control bits for the reverse channel.

The forward channel modulation process is as follows:

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A. Convolution encoder and repetition:

  • Speech coded voice or user data is encoded using ½ rate convolution encoder with constraint length 9.

  • The speech coder exploits gaps and pauses in speech and reduces its output from 9600 bps to 1200 bps during silent period.

  • Whenever the user data rate is less than 9600 bps each bit is repeated to maintain a constant symbol rate of 19.2 kbps.

B. Block interleaver:

  • It makes data block of 20 ms in a random way i.e. consecutive bits are not in a same block.

  • It maps the data bits in a 24 by 16 matrix and then transmit it column wise.

  • This procedure is helpful in recovering the data back if a block is lost during channel transmission.

C. Long PN sequence:

  • In forward CDMA channel Direct Sequence is used for data scrambling.

  • Long PN sequence is user specific code of period $2^{42}-1$ chips.

  • PN sequence is generated from a 42 bit code also called as the public mask.

  • Public mask is specified as- M41 through M32 is set to 1100011000 and M31 through M0 is set to mobile station ESN bits. ESN= (E31, E30, E29, E28, ...... , E1, E0), permuted ESN= (E0, E31, E22, E13, E14, E26, E17, E8, ...... , E18, E9)

D. Data scrambler:

  • It is performed after block interleaver. The 1.2288 MHz PN sequence is applied to decimator which keeps only the first chip out of every 64 consecutive PN chips.

  • The data rate from the decimator is 19.2 ksps. The data scrambling is performed by modulo-2 addition of the interleaver output with the decimator output symbol.

E. Power control subchannel:

  • Power control measures are sent by base station every 1.25ms. Power control commands are sent to raise or lower its transmission power in 1 db steps.

  • If the received signal is low 0 is sent over power control subchannel instructing the mobile station to increase its mean output power level. If mobile’s power level is high 1 is sent to indicate that the mobile station should decrease the power level.

F. Orthogonal covering:

  • Orthogonal scrambling is performed following the data scrambling on the forward link.

  • Each traffic channel is transmitted on the forward CDMA channel is spread with a Walsh function at fixed rate of 1.2288 Mcps.

  • The Walsh functions consist of 64 binary sequences each of length 64 which are completely orthogonal to each other and provide orthogonal channelization.

  • After orthogonal covering Quadrature modulation is performed.

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