CDMA vs WCDMA
Code Division Multiple Access (CDMA) and Wideband Code Division Multiple Access (WCDMA) are multiple access technologies used in telecommunication networks for users to access the network resources and services. Since the spectrum is a scarce resource, which is directly related to capacity, efficient use of the spectrum is a major concern in most theoretical proposals for air interface. Other than CDMA, different multiple access methods are used around the world in radio network. Even though, these access methods are developed in different time periods, combinations of these technologies are used for efficient use of spectrum. When comes to CDMA, North American version of third generation technology is called cdma2000, which is an extension of TIA/EIA-95B based CDMA, while European version of third generation CDMA is called WCDMA.
In general, CDMA is a multiple access technology that was introduced after the TDMA and FDMA. CDMA serves the different users with separate code sequences, while there are other multiple access technologies that use time, frequency, space, and polarisation for user access separation. When we consider the CDMA system design, multiple access and interference handling are totally different from the narrowband systems. In CDMA, each user spread his signal over the entire bandwidth using the direct sequence spread spectrum, whereas for other users, it is shown as pseudo white noise.
WCDMA was selected as the Universal Mobile Telecommunication Systems (UMTS) terrestrialair interface scheme for the Frequency Division Duplex (FDD) frequency bands by the European Telecommunications Standards Institute (ETSI) in 1998. WCDMA uses 5MHz, 10MHz or 20MHz channel bandwidth to send the data signals over the air interface. WCDMA mixes original signal with a pseudo random noise code, which is also known as Direct Sequence WCDMA. Therefore, each user ends up with a unique code, where only users with correct code can decode the message. Using the pseudo signal, original signal is modulated into a higher bandwidth, where original signal’s spectral components sink in the noise. Therefore, without the code, jammers can only see the signal as noise.
WCDMA uses the Quadrature Phase Shift Keying (QPSK) as the modulation scheme as per the original standard specified by the International Telecommunication Union (ITU) for 3G networks, which can support, 384 kbps in the mobile environment and 2Mbps in the stationary environment.
What is the difference between CDMA and WCDMA?
WCDMA is a 3G UTRAN proposed solution, while CDMA is an access technology. WCDMA uses direct spread (DS) as Forward link RF channel structure, while CDMA uses DS or multicarrier. Different versions of CDMA technologies are evolved from different continents, while WCDMA was the European evolved version of CDMA technology. Both technologies use spreading modulation as balanced QPSK in the forward link, and dual channel QPSK in the reverse link. Uniqueness of CDMA based access method is the universal frequency reuse where all the users in the same cell, and across different cells can transmit and receive on the same frequency. CDMA technology introduces core advantages, such as selective addressing capability for each user separately, message security and interference rejection. Proper selection of the codes with low cross correlation leads to minimum interference between users, where we can achieve higher spectral effectiveness in CDMA based technologies.
When compare the CDMA evolution in different European, US, and Japanese based systems, most of them have similar principle, while differ in the chip rate, and in channel structure. WCDMA is considered as a European evolution of CDMA technology for 3rd generation ITU specification.