In CDMA why walsh codes are used for downlink (BTS to mobile) and pn codes for uplink ( mobile to BTS) only ?

pn (psuedo noise) codes are actually used on both the DL and UL. This is what makes CDMA a spread spectrum technology as well as a multiple access technology. In CDMA everything transmits at the same time on the same wide band channel. The spectrum of a pn code looks like noise. A receiver that uses the same pn code (in demodulation) that was used to transmit (modulate) the signal, only some noise plus the original signal remains.

In CDMA, a Walsh code is used to identify the BTS. Each cell site or sector will have it's own Walsh Code. Walsh codes are orthogonal, i.e. don't interfere with one another, so that neighboring sites don't interfere with one another which goes a long way in allowing the mobile to tell them apart.

So why aren't walsh codes used in the uplink? That is because the mobiles transmitted signal gets received by more than one site and is processed by the site that transmitted the signal, plus the defined neighbors to that transmitter. The best received signal of those received by the sites is used.

So in general..

Walsh code identifies the transmitter (cell site).

Pn codes indicate which user is the intended recipient.

Short for Code-Division Multiple Access, a digital cellular technology that uses spread-spectrum techniques. Unlike competing systems, such as GSM, that use TDMA, CDMA does not assign a specific frequency to each user. Instead, every channel uses the full available spectrum. Individual conversations are encoded with a pseudo-random digital sequence. CDMA consistently provides better capacity for voice and data communications than other commercial mobile technologies, allowing more subscribers to connect at any given time, and it is the common platform on which 3G technologies are built. CDMA is a military technology first used during World War II by English allies to foil German attempts at jamming transmissions. The allies decided to transmit over several frequencies, instead of one, making it difficult for the Germans to pick up the complete signal. Because Qualcomm created communications chips for CDMA technology, it was privy to the classified information. Once the information became public, Qualcomm claimed patents on the technology and became the first to commercialize it.

Walsh Code

Because they are good in their job.