Advantages
 Some of the advantages that may be cited are the following :
1. Selective addressing capability
A single receiver or group of receivers may be addressed by assigning a
given reference code of them.
Selective addressing can then be as simple as transmitting the proper code
sequence as modulation.
http://www.visionsecurity.com.tw
/vs_products_page2_34_78_cht.html
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Wireless Communication Technologies 2.4.1
1
Advantages
2. Code division multiplexing is possible for multiple access
When codes are properly chosen for low cross correlation, minimum
interference occurs between users. Thus more than one signal can be
unambiguously transmitted at the same frequency and at the same time,
that is called code-division multiplexing.
x(t)
b1 (t )
z(t)
T
0 ( ) dt
v
判斷
bˆ1 (t )
c1 (t )
b2 (t )
VT  0
PN 1
c1 (t )
x(t)
PSK
Modulator
x(t)
z(t)
c2 (t )
b3 (t )
T
0 ( ) dt
v
判斷
bˆ2(t)
c2(t)
Carrier
VT  0
PN 2
c3 (t )
x(t)
z(t)
T
0 ( ) dt
v
判斷
bˆ3(t)
c3(t)
PN 3
教育部網路通訊人才培育先導型計畫
VT  0
Wireless Communication Technologies 2.4.1
2
Advantages
3. Low-density power spectral for signal hiding
Because of the wideband signal spectral generated by code modulation,
the power transmitted is low in any narrow region of frequency, that is, the
power spectral density is vary small.
This property is known as low probability of detection (LPD).
Noise
教育部網路通訊人才培育先導型計畫
Conventional
signal
Spread spectrum
signal
Wireless Communication Technologies 2.4.1
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Advantages
4. Message screening from eavesdroppers
Because of the coded signals employed, an eavesdropper cannot casually
listen to messages being sent.
Though the systems may not be secure, some conscious effort must be
made to decode the message.
This property is called low probability of interception (LPI).
http://4bluestones.biz/mtblog/2008/04/post-171.html
教育部網路通訊人才培育先導型計畫
Wireless Communication Technologies 2.4.1
4
Advantages
5. High-resolution ranging
Spread spectrum modulation has the inherent characteristic
(autocorrelation property ) that the codes may be used for ranging.
Resolution in ranging is afforded in accordance with the code rates used,
and the sequence length determines maximum unambiguous range.
http://zh.wikipedia.org/zh-hk/GPS
教育部網路通訊人才培育先導型計畫
Wireless Communication Technologies 2.4.1
5
Advantages
6. Interference rejection
Spread spectrum modulation has the gain advantage, referred to as
processing gain, to attenuate the interference.
Processing gain is approximately the ratio of the spread bandwidth to the
information rate.
t  t3
t  t1
Jamming
signal
t  t2
PG=7
教育部網路通訊人才培育先導型計畫
Wireless Communication Technologies 2.4.1
6
Advantages
7. Increased tolerance to multipath
In fading channels, if the multipath is treated as interference, then its
effect can also be reduced by the processing gain.
Through the use of a Rake receiver to constructively combine multipath
(diversity), it can effectively mitigate the effect of fading channel.
Delay
Re ceiver signal
rDS (t )
Tc
Tc
Tc
cos( 2 f 0 t )
C(t  nTc )
C (t  Tc )
C (t )
Rake Finger 0
LPF
Rake Finger 1
Diversity
Combiner
LPF
LPF
Path Selection
Rake Finger 2
I-finger combiner
Output
教育部網路通訊人才培育先導型計畫
Wireless Communication Technologies 2.4.1
7