Why OFDM is better than CDMA in theory?

I heard that OFDM is better than CDMA in theory, e.g., from capacity
point of view. And I found the following from this group by James K.,
which I do not fully follow though (the two equations). Especially for
CDMA, where does the term 1/(1/r1+1/r2) come from?

Any idea ? Thanks.


Below is from James K,
http://groups.google.com/group/comp.dsp/browse_thread/thread/d976778329c3af6d/36dace632b34f503?hl=en&lnk=gst&q=cdma+ofdm#

`` Basically, look how much achievable capacities are different of
these systems? Having assume that the # of multipath is two equal
power paths, then the achievable capacities supposed to be represented
as

[OFDM] C_OFDM = 1/2*{Fc(r1*SNR/2)+Fc(r2*SNR/2)}

[CDMA with ZF receiver] C_CDMA_ZF = Fc(q(r)*SNR/2), q(r) =
((2*r1*r2)/(r1+r2)

where Fc(x) = 1/2*log(1+x), each of {r1,r2} is 2nd oder chi-square
distributed random variable (2*sigma^2 = 1), and SNR is Signal-to-
Noise
power Ratio.

Thus, as long as the external diversity is limited in the systems, the
capacity of OFDM seems quite larger than that of CDMA because of its
intrinsic diversity effect; on the other hand, with assumptions of
high-order diversity in {r1,r2,r} and high SNR, C_OFDM ~= C_CDMA_ZF.
Note
that in high SNR, the capacity of CDMA_ZF approaches to that of CDMA
with
MMSE receiver, which is the optimal linear receiver. For further
study, we
need to consider the ML case.

[Abbreviation]
ZF: Zero-Forcing
MMSE: Minimum Mean Square Error
ML: Maximum Likelihood

"phil" <philgo@gmail.com> wrote in message
news:86d28d10-277c-4a7a-9d5c-faa958884690@m34g2000hsf.googlegroups.com...
>
> I heard that OFDM is better than CDMA in theory, e.g., from capacity
> point of view.

I heard that RS-232 is worse then TCP/IP...
How do you compare a modulation and a communication protocol?

If you meant a comparison of the single carrier to multicarrier, they are
equivalent. The advantage of OFDM is a simple symbol-by-symbol receiver with
O(N) complexity, whereas single carrier requires MLSE with the exponential
complexity.

VLV

vladimir sir,
             good answer with loads of technicality,

thanks
particle (filter) reddy

On Jan 17, 5:56�pm, "Vladimir Vassilevsky"
<antispam_bo...@hotmail.com> wrote:
> "phil" <phi...@gmail.com> wrote in message
>
> news:86d28d10-277c-4a7a-9d5c-faa958884690@m34g2000hsf.googlegroups.com...
>
>
>
> > I heard that OFDM is better than CDMA in theory, e.g., from capacity
> > point of view.
>
> I heard that RS-232 is worse then TCP/IP...
> How do you compare a modulation and a communication protocol?
>
> If you meant a comparison of the single carrier to multicarrier, they are
> equivalent. The advantage of OFDM is a simple symbol-by-symbol receiver with
> O(N) complexity, whereas single carrier requires MLSE with the exponential
> complexity.
>
> VLV