Is-95 air interface
The Air interface for the CDMA was totally different from previous systems. The IS 95 forward link and the IS-95 reverse link are different in their construction in view of the fact that there are different requirements for each. The IS-95 air interface is based around the use of CDMA within a 1.25 MHz channel. Using this, the system can use the same channel for a large number of signals; each one allocated a different code to access the system.
IS-95 Air Interface links:
- The directions of the transmission within a cellular system, i.e. the links to and from the base station are defined. Shown in figure 8.18
- The names used within IS-95 and cdma2000 are different to those used for GSM and UMTS. For the IS-95 air interface, the convention is:
- Forward link: the link from the base station to the handset.
- Reverse link: the link from the handset to the base station.
IS-95 forward link:
Figure 8.18 IS-95 AIR INTERFACE
- 869 to 894 MHz (each channel is 1.25 MHz wide)
- Subdivided into 4 sub channels
b.) Phase Reference for Coherent Demodulation
c.) Means for Signal Strength Comparison
a.) Broadcasts future state of the long code register
a.) Call Control Information
Voice Data (includes power control sub channel)
IS-95 reverse link:
- In the reverse channel a form of modulation known as Offset Quadrature Phase Shift Keying (OQPSK). This is a form of quadrature phase shift keying, QPSK where the quadrature (Q) component of the signal is offset from the in-phase (I) component by half a symbol period.
- The IS-95 reverse link is generated in a different way to that of the forward link. Although the same voice encoder is used, the resulting data has a greater degree of error correction or protection applied. Accordingly the resulting data rate is brought up to 28.8 kbps. A more complicated method of spreading using a Walsh code is used. However this only results in 307 kbps data stream. To increase the bandwidth to the full 1.25 MHz, a further degree of spreading is provided by using an additional PN code. This is multiplied with the signal to increase its data rate by four to bring it up to the final data rate of 1.228 Mbps, the same as the downlink signal.