Characteristics of a CDMA Wireless Network

 

LIMITATIONS
The network capacity (hard capacity) of a CDMA system is limited by two factors: code word length and maximum sender's power. Each code length may sustain only a limited number of orthogonal codes (for example, as determined by the Walsh Set). In addition a receiver sees other user transmissions on the system as noise. The transmitter that sends to a specific receiver must therefore transmit at a power high enough to overcome the noise; i.e. it must have a high enough signal-to-noise power ratio. Mobile users have only a finite source of power, thus limiting the noise level that they can overcome. If the noise of a system exceeds the value over which a user may successfully transmit, then the user cannot be added to the system.

SOFT CAPACITY
CDMA systems are able to accept users over its hard capacity limit. When a mobile user requests access to a system that has reached hard capacity, the cell may allow this user onto its network because it has a uniquely coded signal. To accommodate this new user, the quality of service of the cell is decreased to an allowable level. This alleviates the occurrence of dropped calls due to cells operating at hard capacity.

SOFT HANDOFFS
TDMA and FDMA suffer from hard handoffs. Since each user is designated a certain transmission slot within a cell, it must be reassigned to a new slot as it moves to another cell. If the new cell has reached capacity, then the call is dropped. CDMA eliminates the hard handoff through a technique logically labeled the soft handoff. Once a mobile user approaches the fringe of a cell, it may transmit to adjacent cells simultaneously because it has a uniquely encoded signal. Once it moves closer to one base station and farther from the others, the user transitions smoothly to just one base station. Because of the innovative soft capacity of CDMA cells, even if the adjacent cell is at hard capacity, it will still accept the new user.

POWER CONTROL
Rather than blindly broadcasting signals at full power, CDMA systems utilize power control to decrease system noise and interference, thus enhancing network quality. Mobile users are required to transmit at a minimum power level necessary for the signal to reach the base station. This eliminates interference into adjacent cells and decreases the amount of noise on the system.

INCREASED USER CAPACITY
CDMA has a higher user capacity than that of TDMA or FDMA. The full utilization of system resources (frequency bandwidth and time) allows more users access to the system. In addition, the code word length used to encode the signals most stringently limits user capacity. Thus, as long as adequate power is available to the mobile user, a system may have as many users as its code length can support.

SECURITY
CDMA inherently secures its signals by encoding the user transmission in a unique code. Eavesdropping listeners must have knowledge of that code in order to decipher the signal.

BANDWIDTH ON DEMAND
Since CDMA systems utilize the entire available frequency spectrum, each user that requires additional resources with which to transmit may get it without disrupting the quality of service for other users.