* Soft handoff.
Since every cell uses the same radio frequency band, the only difference between user channels is the spreading code sequences. Therefore, there is no jump from one frequency to another frequency when a user moves between cells. The mobile terminal receives the same signal in one cell as it does in the next, and thus there is no harsh transition from one receiving mode to another. Two or more neighboring base stations can receive the signal of a particular user, because they all use the same channel. Moreover, two base stations can simultaneously transmit to the same user terminals. The mobile (rake) receiver can resolve the two signals separately and combine them (see diversity). This feature is called soft handoff.
* Soft capacity or graceful degradation.
In FDMA and TDMA, N channels can be used virtually without interference from other users in the same cell but potential users N+1, N+2, ..., are blocked until a channel is released. The capacity of FDMA and TDMA is therefore fixed at N users and the link quality is determined by the frequency reuse pattern.
In theory, it does not matter whether the spectrum is divided into frequencies, time slots, or codes, the capacity provided from these three multiple access schemes is the same. However, in CDMA, all the users in all cells share one radio channel and are separated by codes. Therefore, an additional user may be added by sacrificing somewhat the link quality, with the effect that voice quality is just slightly degraded compared to that of the normal N-channel cell. Thus, degradation of performance with an increasing number of simultaneous users is "graceful" in CDMA systems, versus the hard limits placed on FDMA and TDMA systems.
* Multipath tolerance.
Spread spectrum techniques are effective in combating the frequency selective fading that takes place in multipath channels. The underlying principle is that when a signal is spread over a wide bandwidth, a frequency selective fade will corrupt only a small portion of the signal's power spectrum, while passing the remaining spectrum unblemished. As a result, upon despreading there is a better probability that the signal can be recovered correctly. For an unspread signal whose spectral density happens to be misplaced in a deep fade, an unrecoverable signal at the receiver is virtually assured.
To optimally combine signals received over various delayed paths, a rake receiver can be used.
* No channel equalization needed.
When the transmission rate is much higher than 10 kbps in both FDMA and TDMA, an equalizer is needed for reducing the intersymbol interference caused by time delay spread. This is because when the bit period becomes smaller than about ten times the time delay spread, intersymbol interference becomes significant. However, in CDMA a correlatoris needed at minimum. To achieve good performance a rake receiver is needed combat delay spread.
* An important requirement of spreading signals is that they be "noise-like", or pseudorandom. Despreading the signal requires knowledge of the user's code, and for a binary code with spreading factor N there exist 2N possible random sequences. In military systems these codes are kept secret, so it is very difficult for an unauthorized attacker to tap into or transmit on another user's channel. Often it is even difficult to detect the presence of a spread-spectrum signal because it is below the noise that is present in the transmit bandwidth.
Note that in cellular systems, the codes are fully described in publicly available standards. In digital systems, security against eavesdropping (confidentiality) is obtained through encryption. This is a highly desirable alternative to the analog FDMA cellular phone system in wide use today, where with an inexpensive scanner one can tune in to the private conversations of unwary neighbors.
There are, of course, a number of disadvantages associated with CDMA; two of the most severe are the problem of "self-interference," and the related problem of the "near-far" effect.
* Self-interference arises from the presence of delayed replicas of signal due to multipath. The delays cause the spreading sequences of the different users to lose their orthogonality, as by design they are orthogonal only at zero phase offset. Hence in despreading a given user's waveform, nonzero contributions to that user's signal arise from the transmissions of the other users in the network. This is distinct from both TDMA and FDMA, wherein for reasonable time or frequency guardbands, respectively, orthogonality of the received signals can be preserved.
* The near-far problem arises from the fact that signals closer to the receiver of interest are received with smaller attenuation than are signals located further away. Therefore the strong signal from the nearby transmitter will mask the weak signal from the remote transmitter. In TDMA and FDMA, this is not a problem since mutual interference can be filtered. In CDMA, however, the near-far effect combined with imperfect orthogonality between codes (e.g. due to different time sifts), leads to substantial interference. Accurate and fast power control appears essential to ensure reliable operation of multi-user DS-CDMA systems.
The IS-95 System
Similar to most other digital vehicular cellular systems, IS95 uses speech coding at about 9.6 kbit/s.