A simple question about jamming margin equation in CDMA

[zeerum]

hey guys
i was studying cdma and there i came across an equation. it sorta confused me

Eb/No=Ps*Tb/Pn*W
Ps is the power of data signal, Tb is the bit interval, Pn is noise power, W is the bandwidth after spreading, Tc is chip interval

what i want to ask is as the signal after spreading is sent to reciever, so why didnt we use Tc instead of Tb. this calculation is for jamming margin. when the signal is spread shouldnt we be taking its energy over chip interval as this is the rate at which signal is varying?
or is it that we calculate for jamming margin after despreading and thats why they have used Tb and not Tc?

please clarify

 

[salem_eng1]

a simple question

They use Tb not Tc because the information bits is sent every Tb not every Tc.
so when we calculate Eb/No , Eb here is the Energy Per information bit so we multiply with Tb.
If we multiply the Ps with Tc it is not the information bit energy now it is the energy per chip bit.