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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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