Code Division Multiple Access


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Code Division Multiple Access : 

Code Division Multiple Access

What is CDMA? : 

What is CDMA? A multiple access method in which the radio spectrum is divided into channels and how channels are allocated to the many users of the system For radio systems there are two resources, frequency and time


FDMA &TDMA Division by frequency, so that each pair of communicators is allocated part of the spectrum for all of the time, results in Frequency Division Multiple Access . Division by time, so that each pair of communicators is allocated all (or at least a large part) of the spectrum for part of the time results in Time Division Multiple Access . In Code Division Multiple Access (CDMA), every communicator will be allocated the entire spectrum all of the time. CDMA uses codes to identify connections.


TECHNOLOGIES USED CDMA uses a technology called direct sequence spread spectrum to provide more conversations for a given amount of bandwidth and digital service. Unique digital codes, rather than separate RF frequencies or channels, are used to differentiate subscribers. The codes are shared by both the mobile station (cellular phone) and the base station, and are called "Pseudo-Random Code Sequences." All users share the same range of radio spectrum.


SPREAD SPECTRUM FREQUENCY HOPPING– this technology works by transmitting data on ever-changing frequencies in a pseudo-random pattern . DIRECT SEQUENCE– this technology divides each bit of information into smaller units called chips which are then broadcast simultaneously on a large number of channels.

Different Codes used in CDMA : 

Different Codes used in CDMA Walsh codes: Walsh codes provide a means to uniquely identify each user on the forward link. Walsh codes have a unique mathematical property--they are "orthogonal." PN codes : Pseudorandom Noise (PN) codes uniquely identify users on the reverse link. The PN codes used in CDMA yield about 4.4 trillion combinations of code. This is a key reason why CDMA is so secure.

Slide 8: 

Mobile switch center (MSC) MSC is a functional entity that performs control and switching to the mobile stations within the area that it serves, and an automatic connecting device for the subscriber traffic between the CDMA network and other public networks or other MSCs. MSC is the kernel of the CDMA cellular mobile communication system, and it is different from a wired switch in that an MSC must consider the allocation of the wireless resources and the mobility of subscribers, and at least it must implement the follows processing activities: 1. Location Registration processing; 2. Handoff.

Gateway MSC (GMSC) : 

Gateway MSC (GMSC) When a non-CDMA subscriber calls a CDMA subscriber, the call will first be routed to an MSC, which will inquires the corresponding HLR and further route the call to the called party’s MSC. This kind of MSC is called Gateway MSC (GMSC). It is up to the network operator to select which MSCs as GMSCs.

Visitor location register (VLR) : 

Visitor location register (VLR) When the mobile station enters a new location area, the MSC will notice the VLR, which will initiate registration processing to the HLR to update the subscriber location information. The VLR also stores necessary information for the establishment of calls in the database for the MSC to search. One VLR can cover one or more MSC areas.

No Frequency Planning: 

No Frequency Planning A CDMA system requires no frequency planning as the adjacent cells use the same common frequency. A typical cellular system (with a repetition rate of 7) and a CDMA system is shown in the following figures which clearly indicates that in a CDMA network no frequency planning is required.

CDMA Frequency : 

CDMA Frequency

GSM Frequency : 

GSM Frequency

Hand Offs in CDMA : 

Hand Offs in CDMA As the phone moves through a network the system controller transfers the call from one cell to another, this process is called “handoff”. Handoffs maybe done with the assistance of the mobile or the system controller will control the process by itself. Handoffs are necessary to continue the call as the phone travels. Handoffs may also occur in idle state due to mobility. Types of Handoffs in CDMA: There are primarily three types of Handoffs in CDMA. They are Soft Hard Idle.

Soft Handoff  : 

Soft Handoff Soft handoffs are also called "make-before-break." Soft handoff can take place only when the serving cell and target cell are working in the same frequency.


HARD HANDOFF HARD handoffs are also called "make-after-break.“ A Hard handoff establishes a connection with the new BTS after breaking the connection with the old one.

Advantages of CDMA : 

Advantages of CDMA Improved Call Quality and Reduced Audible Effects of Multipath Fading Greater Coverage for Lower Cost Less Power Required for Transmission Improved Security and Privacy Increased Data Throughput

Disadvantages of CDMA : 

Disadvantages of CDMA The only disadvantage at this point to CDMA that has been brought to light is the fact that it is not in use as widely as GSM so roaming can be a problem, especially internationally.