N-GEN Communications Web-based Seminars ©NGCS

Seminar - An Overview of Communications Technologies

Module 9 - Wireless Systems and Technologies                                                                                 Slide 10 of 80                                                                                                    ________________

Code Division Multiplexing Access (CDMA)

                                     Improvement Over FDMA and TDMA

                                     Uses Both Frequency and Time Domains

                                     Multiple Users May Transmit Simultaneously

                                     Transmissions Distinguished from one another by using
                                       Different Pseudo-random Codes

                                                                                                   ________________

CDMA is an improvement on FDMA and TDMA and implemented for PCS cellular phones supporting more conversations in a given frequency bandwidth. It is a digital technology and suited for transmitting data, messages and e-mails.  

CDMA is implemented by using a long pseudorandom code.

CDMA capacity = 10 x TDMA
                             or
                         40 x FDMA

For example: CMDA Digital Cellular - EAI/TAI IS-95
                  - Converted from AMPS spectrum
                  - Uses one or more 1.25 M Hz bands of spectrum
                  - 1.25 M Hz band is 10% of the total AMPS spectrum
                  - Carries 90 - 130 conversations

Result: CDMA increases the system capacity (6 -12 times over AMPS).