||This article may be too technical for most readers to understand. (February 2012)|
W-CDMA (Wideband Code Division Multiple Access), UMTS-FDD, UTRA-FDD, or IMT-2000 CDMA Direct Spread is an air interface standard found in 3G mobile telecommunications networks. It is the basis of Japan's NTT DoCoMo's FOMA service and the most-commonly used member of the Universal Mobile Telecommunications System (UMTS) family and sometimes used as a synonym for UMTS. It uses the DS-CDMA channel access method and the FDD duplexing method to achieve higher speeds and support more users compared to most time division multiple access (TDMA) and time division duplex (TDD) schemes used before.
While not an evolutionary upgrade on the airside, it uses the same core network as the 2G GSM networks deployed worldwide, allowing dual mode mobile operation along with GSM/EDGE; a feat it shares with other members of the UMTS family.
In the late 1990s, W-CDMA was developed by NTT DoCoMo as the air interface for their 3G network FOMA. Later NTT DoCoMo submitted the specification to the International Telecommunication Union (ITU) as a candidate for the international 3G standard known as IMT-2000. The ITU eventually accepted W-CDMA as part of the IMT-2000 family of 3G standards, as an alternative to CDMA2000, EDGE, and the short range DECT system. Later, W-CDMA was selected as an air interface for UMTS.
As NTT DoCoMo did not wait for the finalisation of the 3G Release 99 specification, their network was initially incompatible with UMTS. However, this has been resolved by NTT DoCoMo updating their network.
Code Division Multiple Access communication networks have been developed by a number of companies over the years, but development of cell-phone networks based on CDMA (prior to W-CDMA) was dominated by Qualcomm. Qualcomm was the first company to succeed in developing a practical and cost-effective CDMA implementation for consumer cell phones: its early IS-95 air interface standard, which has since evolved into the current CDMA2000 (IS-856/IS-2000) standard. Qualcomm created an experimental wideband CDMA system called CDMA2000 3x which unified the W-CDMA (3GPP) and CDMA2000 (3GPP2) network technologies into a single design for a worldwide standard air interface. Compatibility with CDMA2000 would have beneficially enabled roaming on existing networks beyond Japan, since Qualcomm CDMA2000 networks are widely deployed, especially in the Americas, with coverage in 58 countries as of 2006[update]. However, divergent requirements resulted in the W-CDMA standard being retained and deployed globally. WCDMA has then become the dominant technology with 457 commercial networks in 178 countries as of April 2012. Several cdma2000 operators have even converted their networks to WCDMA for international roaming compatibility and smooth upgrade path to LTE.
Despite incompatibilities with existing air-interface standards, the late introduction of this 3G system, and despite the high upgrade cost of deploying an all-new transmitter technology, W-CDMA has become the dominant standard.
Rationale for W-CDMA 
W-CDMA transmits on a pair of 5 MHz-wide radio channels, while CDMA2000 transmits on one or several pairs of 1.25 MHz radio channels. Though W-CDMA does use a direct sequence CDMA transmission technique like CDMA2000, W-CDMA is not simply a wideband version of CDMA2000. The W-CDMA system is a new design by NTT DoCoMo, and it differs in many aspects from CDMA2000. From an engineering point of view, W-CDMA provides a different balance of trade-offs between cost, capacity, performance, and density; it also promises to achieve a benefit of reduced cost for video phone handsets. W-CDMA may also be better suited for deployment in the very dense cities of Europe and Asia. However, hurdles remain, and cross-licensing of patents between Qualcomm and W-CDMA vendors has not eliminated possible patent issues due to the features of W-CDMA which remain covered by Qualcomm patents.
W-CDMA has been developed into a complete set of specifications, a detailed protocol that defines how a mobile phone communicates with the tower, how signals are modulated, how datagrams are structured, and system interfaces are specified allowing free competition on technology elements.
Elsewhere, W-CDMA deployments are usually marketed under the UMTS brand. See the main UMTS article for more information.
See also 
- Cellular Frequencies
- Evolution-Data Optimized/CDMA2000
- PN sequences
- Spectral efficiency comparison table
- UMTS frequency bands
- 3GPP notes that “there currently existed many different names for the same system (eg FOMA, W-CDMA, UMTS, etc)”; 3GPP. "Draft summary minutes, decisions and actions from 3GPP Organizational Partners Meeting#6, Tokyo, 9 October 2001" (PDF). p. 7.
- Hsiao-Hwa Chen (2007), The Next Generation CDMA Technologies, John Wiley and Sons, pp. 105–106, ISBN 978-0-470-02294-8
- GSM Association HSPA Market update April 2012
- Qualcomm says it doesn't need Nokia patents
- 3GPP specification series 25—Radio aspects of 3G, including UMTS
- TS 25.201 Description—Describes basic differences between FDD and TDD.
- TS 25.211 Physical channels and mapping of transport channels onto physical channels (FDD)
- TS 25.212 Multiplexing and channel coding (FDD)
- TS 25.213 Spreading and modulation (FDD)
- TS 25.214 Physical layer procedures (FDD)
- TS 25.215 Physical layer - Measurements (FDD)