Code-Division Multiple Access
CDMA (Code-Division Multiple Access)
refers to any of several protocols used in second-generation (2G) and
third-generation 3G wireless communications. CDMA employs analog-to-digital
conversion (ADC) in combination with spread
spectrum
technology. Audio input is first digitized into
binary elements. The frequency of the transmitted signal is then made to vary
according to a defined pattern (code), so it can be intercepted only by a
receiver whose frequency response is programmed with the same code, so it
follows exactly along with the transmitter frequency. There are trillions of
possible frequency-sequencing codes, which enhances privacy and makes cloning
difficult.
CDMA Architecture elements
Mobile Station (MS):
The
MS is the mobile subscriber equipment, which can originate and receive calls
and communicate with the BTS.
Base Transceiver Station (BTS):
The
BTS transmits and receives radio signals, realizing communication between the
radio system and the mobile station
.
Base Station Controller (BSC):
The
BSC implements the following functions:
- Base Transceiver Station (BTS)
control and management
- call connection and
disconnection
- mobility management
- stable and reliable radio link
provision for the upper-layer services by soft/hard handoff
- power control
- radio resource
management.
Packet Control Function (PCF):
The
PCF implements the R-P connection management. Because of the shortage of radio
resources, some radio channels should be released when subscribers do not send
or receive data, but the PPP connection is maintained continuously. The PCF can
shield radio mobility for the upper-layer services via handoff.
Packet Data Service Node (PDSN):
The
PDSN implements the switching of packet data services of mobile subscribers.
One PDSN can be connected to multiple PCFs. It provides the interface between
the radio network and the packet data network
.
Home Agent (HA):
The
agent locates at the place where the Mobile Node opens its account; receive the
registration information from MN, Similar as HLR in mobile network. Broadcast
the accessible information of MN. Setup the tunnel between FA&HA. Transfer
the data from other computer to the MN via the tunnel.
The
MSC implements the service switching between the calling and called
subscribers. One MSC is connected with multiple BSCs. The MSC can also be
connected to the PSTN, ISDN or other MSCs. It provides the interface between
the radio network and PSTN.
Visitor Location Register (VLR):
It
is a dynamic database, stores the temporary information (all data necessary to
set up call connections) of the roaming subscribers in the local MSC area.
VLR
is used to store the subscriber information of all the MSs in its local area,
which can be used to establish the incoming/outgoing call connections, to
support basic services, supplementary services and mobility management.
Home Location Register (HLR):
It
is a database for mobile subscriber management, the HLR (Home Location
Register) is responsible for storing subscription information (telecom service
subscription information and subscriber status), MS location information, MDN,
IMSI (MIN), etc. The AC (Authentication Center) is physically combined with the
HLR. It is a functional entity of the HLR, specially dedicated to the security
management of the CDMA system. It stores the authentication information. It
also prevents unauthorized subscribers from accessing the system and prevents
the radio interface data from being stolen.
Advantages
- Frequency reuse
- Large coverage
- High spectrum capacity
- High privacy
- Soft handoff
- Good voice quality
- Perfect power control
