Notes 14 - ee.istanbul.edu.tr

Spread-Spectrum
Communication System
Wireless
Communication
CDMA
Assoc. Prof. Hakan DOGAN
Doç. Dr. Hakan DOGAN
FHSS / DSSS
There are two types of spread
spectrum techniques are used
• Direct Sequence
• Frequency Hopping.
Direct Sequence is adopted by CDMA
FDMA
➲
➲
➲
➲
Frequency
Division
Multiple
Access
FDMA is a multiple access method in
which users are assigned specific
frequency bands. The user has sole right
of using the frequency band for the entire
call duration.
TDMA
➲
➲
➲
➲
Time
Division
Multiple
Access
In TDMA an assigned frequency band
shared among a few users. However, each
user is allowed to transmit in
predetermined time slots. Hence,
channelization of user is achieved through
separation in time.
F/TDMA
CDMA
CODE DIVISION MULTIPLE ACCESS (CDMA)
LARGE NUMBER OF TRANSMISSIONS ARE COMBINED ON THE SAME
RF CHANNEL AT THE SAME TIME BUT ARE SEPERATED BY “CODES”.
DSSS
DSSS(Direct Sequence Spread Spectrum)
In telecommunications, direct-sequence spread spectrum (DSSS)
is a modulation technique. As with other spread spectrum
technologies, the transmitted signal takes up more bandwidth
than the information signal that modulates the carrier or
broadcast frequency. The name 'spread spectrum' comes from
the fact that the carrier signals occur over the full bandwidth
(spectrum) of a device's transmitting frequency
DSSS
Direct Sequence Spread Spectrum (DSSS) is a spread spectrum
technique whereby the original data signal is multiplied with a pseudo
random noise spreading code. This spreading code has a higher chip
rate (this the bitrate of the code), which results in a wideband time
continiuous scrambled signal.
Spreading
Spreading
Dar Band Girişime Karşı Çalışma
Prensibi
CDMA
Code division multiple access (CDMA) is a channel access method
used by various radio communication technologies.
One of the concepts in data communication is the idea of allowing
several transmitters to send information simultaneously over a single
communication channel. This allows several users to share a band of
frequencies (see bandwidth). This concept is called multiple access.
CDMA employs spread-spectrum technology and a special coding
scheme (where each transmitter is assigned a code) to allow
multiple users to be multiplexed over the same physical channel.
By contrast, time division multiple access (TDMA) divides access by
time, while frequency-division multiple access (FDMA) divides it by
frequency. CDMA is a form of spread-spectrum signalling, since the
modulated coded signal has a much higher data bandwidth than
the data being communicated.
CDMA
In Code Division Multiple Access (CDMA) systems all users transmit in
the same bandwidth simultaneously.
Ad
Bd
Data for User A
Data for User B
Bk
Spreading Sequence for
User B
Ak
Spreading Sequence for
User A
CDMA
Ak= 010011
0  -1
1  +1
BPSK Durumu
(-1,+1,-1,-1,+1,+1)
A kullanıcısının göndereceği data ‘Ad=1’ olsun (yani +1 sembol
nü yolluyor)
Vericisinden gönderilecek olan sinyal As olmak üzere
Ad=+1
=(+1)*(-1,+1,-1,-1,+1,+1)=(-1,+1,-1,-1,+1,+1)
Ak=(-1,+1,-1,-1,+1,+1)
CDMA
Bk= 110101
0  -1
1  +1
BPSK Durumu
(+1,+1,-1,+1,-1,+1)
B kullanıcısının göndereceği data ‘Bd=0’ olsun (yani -1 sembol
nü yolluyor)
Vericisinden gönderilecek olan sinyal Bs olmak üzere
Bd=-1
=(-1)*(+1,+1,-1,+1,-1,+1)=(-1,-1,+1,-1,+1,-1)
Bk=(+1,+1,-1,+1,-1,+1)
CDMA
R  As  Bs
(-1,+1,-1,-1,+1,+1) + (-1,-1,+1,-1,+1,-1) = (-2, 0, 0, -2, +2, 0)
Alıcı hangi kullanıcının sinyalini ayırt etmek istiyorsa ilgili kullanıcının
anahtar koduyla gelen sinyalin iç çarpımı yapılır ve çıkan sonucun <0
veya >0
olma durumuna göre gelen bilginin ‘1’ mi ‘0’ mi
olduğuanlaşılabilir.


Ae   As  Bs  * Ak


 R 
6>0
 Ad=1
(-2,0,0,-2,+2,0) * (-1,+1,-1,-1,+1,+1)= 2+0+0+2+2+0= 6


Be   As  Bs  * Bk


R


-6<0  Bd=0
(-2,0,0,-2,+2,0) * (+1,+1,-1,+1,-1,+1)= -2+0+0-2-2+0= -6
CDMA
Not: Güç norrmalizasyonu yapılmadığı için maksimum elde
edilebilecek sayılar 6 ve -6 dır.
(-1,+1,-1,-1,+1,+1) + (-1,-1,+1,-1,+1,-1) = (-2, 0, 0, -2, +2, 0)
= (-2, 0, 0, -2, +2, 0)
= (+1, 0, 1, 1, 0, -1) Hata
=(-1,0,1,-1,+2,-1) Alınan Dizi
(-1,0,1,-1,+2,-1) * (-1,+1,-1,-1,+1,+1)=
(-1,0,1,-1,+2,-1) * (+1,+1,-1,+1,-1,+1)=
FHSS
Frequency-hopping spread spectrum (FHSS) is a method of
transmitting radio signals by rapidly switching a carrier among
many frequency channels, using a pseudorandom sequence
known to both transmitter and receiver.
FHSS

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