Code division multiple access (CDMA)
Code division multiple access (CDMA) is a form of multiplexing and a method of multiple access that divides up a radio channel neither by time nor by frequency but instead by using different pseudo-random code sequences for each user.
Features of CDMA
- CDMA is a form of "spread-spectrum" signaling, since the modulated coded signal has a much higher bandwidth than the data being communicated
- In CDMA each user is assigned a unique code sequence (spreading code), which it uses to encode its data signal
- The receiver, knowing the code sequence of the user, decodes the received signal and recovers the original data
- The bandwidth of the coded data signal is chosen to be much larger than the bandwidth of the original data signal, that is, the encoding process enlarges (spreads) the spectrum of the data signal
- If multiple users transmit a spread-spectrum signal at the same time, the receiver will still be able to distinguish between users, provided the receiver will still be able to distinguish between users, provided that each user has a unique code that has a sufficiently low cross correlation with the other codes.
- This technology is used in ultra-high-frequency (UHF) cellular telephone systems in the 800-MHz and 1.9-GHz bands.
Direct Sequence CDMA (DS-CDMA)
The original data signal is multiplied directly by the high chip rate spreading code
Frequency Hopping CDMA (FH-CDMA)
The carrier frequency at which the original data signal is transmitted is rapidly change according to the rapidly changed according to the spreading code
Time Hopping CDMA (TH-CDMA)
The original data signal is not transmitted continuously. Instead, the signal is transmitted in short bursts where thetimes of the bursts are decided by the spreading code
- CDMA systems utilize a spread spectrum technique in which a spreading signal, which is uncorrelated to the signal and has a large bandwidth, is used to spread the narrow band message signal.
- Unlike TDMA, CDMA does not require time synchronization between the users.
- A CDMA system experiences a problem called self-jamming which arises when the spreading codes used for different users are not exactly orthogonal
- If the power of the multiple users in a CDMA system is unequal, then the user with the strongest signal power will be demodulated at the receiver
- The strength of the received signal raises the noise floor for the weaker signals at the demodulators. This reduces the probability that weaker signals will be received. This problem, known as the near-far problem can be taken care of by using power control.