WALSH CODES and orthogonality

[hassantalal785]

we use walsh codes in cdma (synchronous) , because they are set be orthogonal , when we say orthogonal signals , what do we mean physically ? mathemetical iterpretation is clear that two walsh codes are orthogonal because they have equal no. of 1's and 0's , and scaled dot product of code is 1, and cross correlation is 0 between codes .......what is actually orthogonality ?

 

[Communications_Engineer]

cross correlation is zero, this is what is meant by orthogonality specific to CDMA (other definitions may also be correct)

The orthogonality is required to have no interference between physical channels

 

[hassantalal785]

PN sequences are also used in cdma for spreading , so if PN sequences also provide orthogonality and PN sequences also , then when to use Walsh codes and when to use PN sequences , i know read some where that Walsh codes are used in Synchronous CDMA and in Asynchronous CDMA PN sequences are used ????

 

[Communications_Engineer]

What you have read is correct. One thing that is wrong is that PN sequences provide orthogonality. They are actually not 'perfectly' orthogonal.

We use Walsh in CDMA2000 etc systems when we are sure of a synchronous link because if the link is asynchronous, then Walsh codes have poor auto-correlation i.e. they lose their orthogonality properties when you correlate a walsh code with its delayed version.

try to use matlab function xcorr and auto-correlate a walsh sequence, you'll understand better

 

[hassantalal785]

If the channel is asyncronous , walsh codes loose their orthogonal properties then why do we use them , we should always use PN sequences , i mean what are the advantages of using Walsh Codes and when to use them , similarly for Pn sequences ... there are also Gold codes , why arent they use widely ?