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.
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:
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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