Code Division Multiple Access - CDMA

The Unappreciated Inventors

CDMA - What is it?

Answer: Synchronous agreement on code books or code particulars!
Examples:
Military orders for the day.

The proposed "player piano" anti-jamming system of Hedy Lamarr and George Antheil
 

CDMA - Effectuating Modalities

 
Coding with Frequency Hops
The main parameter of importance in the discussion of spread spectrum systems is the so-call processing gain-- the ratio of the transmission bandwidth to the information bandwidth -- viz.,


This parameter is also refered to as  the spreading factorand it determines the number of users that can be allowed in a system, the amount of multi-path effect reduction, the difficulty to jam or detect a signal etc.  For spread spectrum systems it is advantageous to have a processing gain as high as possible!

Coding with Direct Sequences

PN code is an abbreviation for Pseudo Random Noise Code 


 

CDMA - What is it good for?

    Ten Top advantages of using CDMA

    1. Voice Activities Cycles. CDMA is the only access that takes advantage of the nature of human conversation. In conversation the "voice activity cycle" is 35%, the rest of the time we are listening.  In CDMA all the users are sharing one radio channel.  Because each channel user has active just 35% of the entire cycle, all others benefit with less interference in a single CDMA radio channel.  So, the mutual interference is reduced by 65%; and thus, the channel capacity is increased about three times.

    2.  
    3. No Hard Handoff. In CDMA, every cell uses the same radio, the only difference is the code sequences. As a result there is no handoff from one frequncy to another while moving between cells.

    4.  
    5. No Guard Time in CDMA. TDMA requires the use of guard time between time slots.  Of course, the guard time does occupy the time period for certain bits. This "waste" of bits does not exists in CDMA, because guard time is not needed in CDMA technique.

    6.  
    7. Less Fading. Less fading is observed in the wide-band signal while propagating in a mobile ratio environment.

    8.  
    9. Capacity Advantage. Within certain parameters, CDMA can have four times the TDMA capacity; and twenty times the FDMA capacity per channel/cell.

    10.  
    11. No frequency management or assignment needed. In both TDMA and FDMA the frequency management is always a critical task.  Since there is only one channel in CDMA, no frequency management is needed.

    12.  
    13. Soft Capacity. Since all CDMA traffic channels share a single CDMA radio channel, we can always add one additional user -- of course the voice quality is there by slightly degraded.

    14.  
    15. Coexistence. Both systems, analog and CDMA can operate in two different spectras, with no interference at all.

    16.  
    17. For Microcell and in-building Systems: CDMA is probably the best system for microcell and in-building systems.

    18.  
    19. No Equalizer Needed. When the transmission rate is much higher than 10kb/s in both FDMA and TDMA, an equalizer is required.  On the other hand, CDMA only needs a correlator, which is cheaper than the equalizer.

References:

The Principles of Spread Spectrum Communication: a brief, but quite insightful  tutorial by Jack Glas (original)
The CDMA Report from students at MTU

The ABCs of Spread Spectrum -- A Technology Introduction and Tutorial: an valuable tutorial and web link list (from the Spread Spectrum Scene) which includes:

 

Randy Katz' wonderful and downloadable introduction to wireless modulation: CS294-7: Digital Modulation

Phil Flikkema's concise and  downloadable introduction: Overview of Modulation Techniques for Wireless.


Amateur Radio Spread Spectrum Communications Page
Spread Spectrum Data Communcation:  a thesis by Darryl Smith at the University of Technology, Sidney.

Spread Spectrum Communication Papers - Tutorial Papers:  a collection of set of tutorials by Francisco Rodrigues-Henriquez.

The Hedy Lemarr - George Antheil Story

The Birth Of Spread Spectrum from MicroTimes (original)

The famous story ofHedy Lamarr's  "Secret Communications Technique" Patent.  (original)

 

This page was prepared and is maintained by R. Victor Jones
Comments to: jones@deas.harvard.edu.

Last updated November 10, 1999