Basics of CDMA

8/10/2019 Basics of CDMA

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Basics of CDMA

Sunil S. Ksheersagar

6/5/06


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History of Coding First coding was used as Cipher code, in which the letter,

numbers and punctuations were used in different sequence.

The decoding was available with the receiving station. Thiswas manually done.

The next step was signal with Pilot carrier, in which the firstpart of the transmitted signal was with some code as a part ofsignal. This was automatic. This pilot signal is transmittedevery time the Tx is put on.

Then came the revolution of Scrambler. In this the signal itselfis coded or mixed in random sequence. The coding &decoding is sequential as defined by the OEM.

Then advanced version of Scramble came into existence. The

sequence is random & the probability of repeating the samesequence becoming very very less, thereby increasing theprivacy in communication between the two parties.

Now with the mobile communication, the coding hasimproved, advanced & now the identification of the caller and

the information is employing advance techniques of Coding.


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Starting of CDMACDMA is a military technology first used during World War II

by English allies to foil German attempts at jammingtransmissions. The allies decided to transmit over several

frequencies, instead of one, making it difficult for the

Germans to pick up the complete signal.

CDMA Technology is being used in many communicationssystems, including the Global Positioning System and in the

OmniTRACS satellite system for transportation logistics.

Qualcomm is pioneer in designing and building the latter

system, this further lead to invent Soft Handover and fastpower control, the necessary technologies that made CDMA

practical and efficient for terrestrial cellular communications.


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Basic Elements of CDMA

Occupies more bandwidth than required, helpingto reduce the interference & jamming in Multi-user Access.

Bandwidth spread by means of code independent of data.

The use of an independent code and synchronousreception allows multiple users to access the samefrequency band at the same time.

The receiver synchronizes to the code to recoverthe data.

Pseudo-noise (PN) Code is used to protect thesignal. Though it appears random, it is

Deterministic

able to reconstruct theinformation.


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CDMA features

Narrowband message signal multiplied by

wideband spreading signal or Pseudo noise code.Each user has his own pseudonoise (PN) code

Cell frequency reuse: no frequency planning

needed.Soft handoff increases capacity.

Wide bandwidth induces diversity: rake receiver is

used.

Interference limited: power control is required.

Soft capacity limit: system performance degrades

for all users as number of users increases.

Near far problem.


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Three Types of Spread

Spectrum Communications

Frequency hopping.

Time hopping.

Direct sequence.


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ACCESS SCHEMES

For radio systems there are two resources, frequency and time.

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 (FDMA).

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 (TDMA).

In Code Division Multiple Access (CDMA), every communicator will

be allocated the entire spectrum all of the time. CDMA uses codes to

identify connections.


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Direct sequence.

The digital data is directly coded at a much

higher frequency. The code is generatedpseudo-randomly, the receiver knows how to

generate the same code, and correlates the

received signal with that code to extract thedata.


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Generation of Pseudo Noise Signal

The Pseudo-Noise signal is generated with thehelp of linear

filters & combining them. The output signal is

random & in the amplitude range of +1 & -1.

The Pseudo Noise signal is imposed on the

Noisy signal to give

Out put of Pseudo Noise Code signal.


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Further illustration of Coding


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Signal transmission steps

1. A pseudo-random code is generated, different

for each channel and each successive

connection.

2. The Information data modulates the pseudo-

random code (the Information data is spread).

3. The resulting signal modulates a carrier.

4. The modulated carrier is amplified and

broadcast


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Signal reception steps

1. The carrier is received and amplified.2. The received signal is mixed with a local

carrier to recover the spread digital signal.

3. A pseudo-random code is generated, matchingthe anticipated signal.

4. The receiver acquires the received code and

phase locks its own code to it.5. The received signal is correlated with the

generated code, extracting the Information data.


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In Telecommunication, Forward Error

Correction(FEC) is a system of error control

for data transmission.

The receiver is designed to automatically correct

some errors detected in the Received signal upto

some extent without asking for retransmission ofthe signal. The provision to extent of capacity for

correction is predefined by the OEM.

Ch l it


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Channel capacity

where

Cis the channel capacity in bits per second, net of error correction;

BWis the bandwidth of the channel in hertzSis the total signal power over the bandwidth and

Nis the total noise power over the bandwidth.

Example:

If the S/N is 20 dB, and the bandwidth available is 4 kHz, which is

appropriate for telephone communications,

then C = 4 log2(1 + 100) = 4 log2(101) = 26.63 kbit/s.

Note that the value of 100 is appropriate for an S/N of 20 dB


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How it worksBy adding redundancy in the transmitted signal as

per the predetermined Algorithm FEC is achieved.Each redundant bit is invariably a complex function

of many original information bits.

An extremely simple example would be to transmit each bit ofinformation three times over, and for the receiver to assume

the correct output is given by the most frequently occurring

value in each group of three.


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ExampleTriplet received Interpreted as

000 0001 0

010 0

100 0

111 1

110 1

101 1

011 1

In this case maximum number of the bits are considered

for the Correction, even if either bit is flipped or

unreadable.


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Types of FEC Coding

Block Coding

fixed-size blocks

(packets) of bits orsymbols of

predetermined size.

Convolutional Coding

work on bit or symbol

streams of arbitrarylength

can be turned into a

block code

C t t FEC d t d


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Concatenate FEC codes to reduce errors

. Block and Convolutional codes are frequently

combined in concatenatedcoding schemes inwhich the convolutional code does most of

the work and the block code (usually Reed-

Solomon) "mops up" any errors made by the

Convolutional decoder.

. This has been standard practice in

satellite and deep space communications

since Voyager 2 first used the technique

in its 1986 encounter with Uranus.


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HANDOVER

Handover is a process where a call is passed from one cell toanother as the user moves between cells. There are two types

"hard" handover & soft handover.

In HARD Handover, the connection to the current cell is broken,

and then the connection to the new cell is made. (Break before

make)

In SOFT Handover, the connection is made with the new cell site

before breaking from the old site. (Make before break).

In CDMA all sites use the same frequency, hence it is possible to

make the connection to the new cell before leaving the current cell(Make before break).

Soft handovers require less power, which reduces interference and

increases capacity. Mobile can be connected to more that two BTS

during the handover.


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N F P bl & S l ti


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Near Far Problem & Solution

It is a problem of hearing ability problem. This can be

described as below.

Consider a receiver and two transmitters (one close to the receiver;

the other far away). If both transmitters transmit simultaneously

and at equal powers, then due to the inverse square law, the

receiver will receive more power from the nearer transmitter.

In CDMA systems or other cellular phone-like networks, this is

commonly solved by dynamic output power adjustmentof the

transmitters. That is the closer transmitters use less power so that

the SNR for all transmitters at the receiver is roughly the same.


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Thanks a lot.

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Basics of CDMA

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