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

DSSS Spreading: Time-Domain View

At Originating Site:

1. Input A: User’s Data @ 19,200 bits/second
2. Input B: Walsh Code #23 @ 1.2288 Mcps
3. Output: Spread spectrum signal

At Destination Site:

1. Input A: Received spread spectrum signal
2. Input B: Walsh Code #23 @ 1.2288 Mcps
3. Output: User’s Data @ 19,200 bits/second just as originally sent

Two Types of CDMA

There are Two types of CDMA:

1. Frequency-Hopping

• Each user’s narrowband signal hops among discrete frequencies, and the receiver follows in sequence
• Frequency-HoppingSpread Spectrum(FHSS)CDMA is NOTcurrently used in wireless systems, although used by the military

1. Direct Sequence

• narrowband input from a user is coded (“spread”) by a user-unique broadband code, then transmitted
• broadband signal is received; receiver knows, applies user’s code, recovers users’data
• Direct SequenceSpread Spectrum(DSSS)CDMA IS the method used in IS-95 commercial systems

CDMA : Using A New Dimension

• All CDMA users occupy the same frequency at the same time! Frequency and time are not used as discriminators
• CDMA operates by using CODING to discriminate between users
• CDMA interference comes mainly from nearby users
• Each user is a small voice in a roaring crowd --but with a uniquely recoverable code