Code Division Multiple Access (CDMA)

Code Division Multiple Access (CDMA)

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Multiple Access Techniques

Frequency Division Multiple Access (FDMA)

Time Division Multiple Access (TDMA)

Code Division Multiple Access (CDMA)


CDMA

In CDMA, a number of users occupy all of the channel bandwidth all of the time. Their signals are encoded so that information from an individual transmitter can be detected and recovered only by a properly synchronized receiving station that knows the code being used. Each receiving station has its own code, called its “address” and a transmitting station simply modulates its transmission with the address of the intended receiver whenever it wishes to send a message to that receiver.


CDMA


Features of CDMA

CDMA is Full DuplexEliminates need to engineer reuse patternIncreases Capacity by about 6 timesMultipath reception improves voice claritySecurity or Interference rejection : Cross-correlating the code signal with a narrowband signal spreads the power of the narrowband signal, thereby reducing the interfering information bandwidth.


Rake Receiver

Both the mobile and the BTS use rake receivers.


Power Control: Avoids near-far effect and increases mobile unit battery life

POWER CONTROL


Forward link Power Control

The BTS independently adjusts the power based on the information it receives from the mobile.


Reverse Link Power Control

Open Loop: Prior to any transmission, the mobile monitors the total power received by the base station.

Closed Loop: The base station provides continuous feedback to each mobile so that the mobile varies its power accordingly.


Block Diagram of CDMA


CDMA Architecture

Consists of the following components Mobile user/subscriber (MS) Base Trans/Receive Station (BTS) & Antennae Mobile Switching Center (MSC) Authentication Center (AC) Operation, Administration & Maintenance

(OAM)


CDMA Architecture

IOS: Interoperability Specification IWF: Interworking Function


CDMA Channels

Forward Link Channels


CDMA Channels

Reverse Link Channels


CDMA Call Processing

There are four stages or modes in CDMA call processing


CDMA Call Execution

Land to Mobile Call


CDMA Call Execution

Mobile to Land Call


CDMA Call Execution

Mobile to Mobile Call within same network


CDMA Call Execution

Mobile to Mobile Call in different network


CDMA Applications

In Bluetooth Bluetooth uses Fast FH-CDMA with a hopping

rate of 1,600 hops per second. Each piconet has its own spreading sequence. FH/CDMA in Bluetooth allows tens of piconets

to overlap in the same area providing an effective throughput of more than 1 Mbps.


CDMA Applications

In Global Positioning System (GPS): In GPS there are two satellite signals or carriers. A PN-sequence of freq. 1023 MHz is used to

modulate one of these carriers to generate coded signals that are needed for acquisition of the satellite by the receiver.

Different satellites use different sequences, which lets them all broadcast on the same frequencies while still allowing receivers to distinguish between satellites.


Limitations of CDMA

Synchronization Requirement

• Unavailability of large contiguous bandwidth

• Near-far Effect

• Self-Jamming


Conclusion

There are now different variants of CDMA, but the original CDMA is now known as cdmaOne. We now have cdma2000 and its variants like 1X EV, 1XEV-DO, and MC 3X. These refer to variants of usage of a 1.25Mhz channel. For example, 3X uses a 5 MHz channel. Over 35 countries have either commercial or trial activity ongoing regarding CDMA. There are already 43 Wireless Local Loop (WLL) systems in 22 countries using cdmaOne technology.


Code Division Multiple Access (CDMA)

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