What is CDMA ?

CDMA (Code division multiple access) is a channel access method utilized by various radio communication technologies. It should not be confused with the mobile phone standards called cdmaOne and CDMA2000 (which are often referred to as simply "CDMA"), that use CDMA as their underlying channel access methods.

CDMA is a spread spectrum multiple access technique. In CDMA a locally generated code runs at a much higher rate than the data to be transmitted. Data for transmission is simply logically XOR (exclusive OR) added with the faster code.

Each user in a CDMA system uses a different code to modulate their signal. Choosing the codes used to modulate the signal is very important in the performance of CDMA systems. The best performance will occur when there is good separation between the signal of a desired user and the signals of other users.

• Universal frequency reuse
  
  The frequency spectrum is a limited resource. Therefore, wireless telephony, like radio, must reuse frequency assignments.
  
  For example, two radio stations might transmit at 91.3 FM. There is no interference as long as the radio stations are far enough apart.
  
  
• Fast and accurate power control
  
  Power control is a CDMA feature that enables mobiles to adjust the power at which they transmit. This ensures that the base station receives all signals at the appropriate power.
  
  The CDMA network independently controls the power at which each mobile transmits.
  
  Both forward and reverse links use power control techniques.
  
  
• Rake receiver
  
  CDMA's rake receiver is multiple receivers in one. The rake receiver identifies the three strongest multi-path signals and combines them to produce one very strong signal.
  
  The rake receiver therefore uses multipath to reduce the power the transmitter must send.
  
  Both the mobile and the BTS use rake receivers.
  
• Different types of handoff
  
  Handoff is the process of transferring a call from one cell to another. This is necessary to continue the call as the phone travels.
  
  CDMA is unique in how it handles handoff.
  
  
  Types of CDMA handoff
  
  CDMA has three primary types of handoff:
  
  • hard
  • soft
  • idle

Advantages of using CDMA

• 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.
  
• 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.
  
• 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.
  
  
• Less Fading
  Less fading is observed in the wide-band signal while propagating in a mobile ratio environment.
  
• Capacity Advantage
  Within certain parameters, CDMA can have four times the TDMA capacity; and twenty times the FDMA capacity per channel/cell.
  
• 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.
  
• 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.
  
• Coexistence
  Both systems, analog and CDMA can operate in two different spectras, with no interference at all.
  
• For Microcell and in-building Systems
  CDMA is probably the best system for microcell and in-building systems.
  
• 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.