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.