| Author |
Message |
Adameric
Joined: 30 Mar 2006
Posts: 2
|
 30 Mar 2006 3:29 What is Direct Sequencing and Frequecy Hopping ? |
|
|
|
Hi I'm new here.
Please help me as I am involved in a Digtal system that uses the above two method of transmission.
I may be backward and I tried to read up.
Still cannot understand how these 2 mode of freq works.
Advantages and advantages over normal Fix freq digital transmission ?
Thanks. 
Adameric
|
|
| Back to top |
|
 |
carpa
Joined: 12 Nov 2004
Posts: 78
Helped: 5
|
| 30 Mar 2006 9:14 What is Direct Sequencing and Frequecy Hopping ? |
|
|
|
| They are two kinds of techniques used in CDMA systems. DS(Direct Sequence)-CDMA modulate the signal by multiplying it with pseudo-random numbers directly. FH(Frequency Hopping)-CDMA modulate the signal using pseudo-random frequency subchannels. DS-CDMA occupy the band all the time and FH-CDMA only some certain point. The former is easier to implement and more popular.
|
|
| Back to top |
|
|
iffi84
Joined: 18 Mar 2006
Posts: 21
Helped: 2
|
| 30 Mar 2006 10:38 Re: What is Direct Sequencing and Frequecy Hopping ? |
|
|
|
http://en.wikipedia.org/wiki/Direct-sequence_spread_spectrum
go on the link and u will find a very simple and easy to understand description of DS, FH and if u need any further help u r most welcome
|
|
| Back to top |
|
|
singhmangal
Joined: 15 Feb 2006
Posts: 33
Location: India
|
| 30 Mar 2006 12:04 Re: What is Direct Sequencing and Frequecy Hopping ? |
|
|
|
| Adameric wrote: |
Hi I'm new here.
Please help me as I am involved in a Digtal system that uses the above two method of transmission.
I may be backward and I tried to read up.
Still cannot understand how these 2 mode of freq works.
Advantages and advantages over normal Fix freq digital transmission ?
Thanks.
Adameric |
try this man,
|
|
| Back to top |
|
|
nathan80
Joined: 23 Feb 2006
Posts: 68
Helped: 5
Location: Malaysia
|
| 31 Mar 2006 10:21 Re: What is Direct Sequencing and Frequecy Hopping ? |
|
|
|
Your question requires a very long answer with graphical explanations. However , I will try to explain as short as possible to give you an idea of direct sequencing and frequency hoping.
Signals(frequency might vary) that needs to be transmitted will be modulated with a carrier signal(fixed frequency) to ensure a fixed frequency to transmitt and receive the signal.
During signal modulation, designers will try to achieve power efficiency and bandwidth efficiency.
Using a technique known as spread spectrum modulation technique; a high powered low bandwidth spectrum is spread to produce a low powered high bandwidth spectrum thus achieving better bandwidth efficiency and power efficiency.
Spread spectrum signals are pseudorandom and have noise properties. This spread spectrum signals are controlled by pseudo-noise (PN) sequence.
When the signal is cross-correlated at receiver with the correct PN code, the signal will despread and restore the original modulated message in the same narrowband.
There are two types of spread spectrum techniques:
a) direct sequencing
- a PN sequence is used to modulate a signal (using phase shift keying); thus achieving instantaneous spreading of transmission bandwidth.
- the ability to combat jammers is determined by the PG of the system. The PG is a function of PN sequence length.
- the PG can be made larger by using longer PN sequence which requires a greater transmission bandwidth.
- however, sometimes certain physical devices have limitations on obtaining large PG.
b) frequency hoping
- when certain physical devices cannot obtain large PG, frequency hoping is used.
- using this technique, the jammer will cover a wider spectrum by randomly hopping the data modulated carrier from one frequency to the next. The manner of hoping is only known to the carrier.
|
|
| Back to top |
|
|
Naveed Alam
Joined: 06 Jan 2006
Posts: 280
Helped: 16
|
| 10 Apr 2006 10:27 Re: What is Direct Sequencing and Frequecy Hopping ? |
|
|
|
in frequency hopping data transmitted by different user is transmitted on more than one frequencies, and combination of frequencies used by users at different time may vary..
advantages include interference averaging etc...
direct sequence use single frequency or more than one frequency but its combination is fix...
its simple..algos are easy to implement,,,
frequency hopping is better..
Regards
|
|
| Back to top |
|
|
