The CDMA Spread Spectrum Payoff

Would you like a lump-sum, or monthly payments?CDMA Spreading GainConsider a user with a 9600 bps vocodertalking on a CDMA signal 1,228,800 hzwide. The processing gain is 1,228,800/9600 = 128, which is 21 db. What happens if additional users are added?

1. Shannon's work suggests that a certain bit rate of information deserves a certain bandwidth
2. If one CDMA user is carried alone by a CDMA signal, the processing gain is large -roughly 21 db for an 8k vocoder.

• Each doubling of the number of users consumes 3 db of the processing gain
• Somewhere above 32 users, the signal-to-noise ratio becomes undesirable and the ultimate capacity of the sector is reached

Practical CDMA systems restrict the number of users per sector to ensure processing gain remains at usable levels