W-CDMA
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W-CDMA (Wideband Code Division Multiple Access) è una tecnologia per reti cellulari di terza generazione (3G). W-CDMA è la tecnologia su cui si basa lo standard UMTS ed è alleata con lo standard 2G GSM tramite l' International Telecommunication Union.
Il W-CMDA è un'interfaccia wireless a banda larga e ampio spettro per la comunicazione mobile che utilizza il CDMA ("Code Division Multiple Access", lo schema generale di multiplexing, da non confondere con l'omonimo standard ameircano).
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[modifica] Storia
Il W-CDMA è stato sviluppatto dalla NTT DoCoMo come interfaccia wireless per il loro network 3G FOMA. In seguito NTT DoCoMo inviò le specifiche all'ITU per candidarlo allo standard internazionale 3G noto come IMT-2000. L'ITU, infine, accettò il W-CDMA come parte della famiglia di standard 3G dell'IMT-2000. In seguito, il W-CDMA fu scelto come interfaccia aerea per l'UMTS, la parte dati 3G del GSM. Sono stati compiuti tentativi per unificare gli standard W-CDMA (3GPP) e CDMA-1X (3GPP2) allo scopo di fornire un'unico standard mondiale ma requisiti divergenti causarono il mantenimento dei due standard incompatibili.
[modifica] Caratteristiche
Solamente le caratteristiche più importanti sono riportate di seguito.
- Supporta due modalità di comunicazione Duplex: FDD (frequency division duplex) e TDD (time division duplex)
- Usa rilevazione coerente (tanto in uplink quanto in downlink) basata sui simboli pilota
- Operazione inter-cella asincronata
- Missione varibile su frame a base di 10 ms
- Transmissione mulitcodice
- Controllo di potenza adattativa basata su SIR
- Rilevazione multiutenza e antenne intelligenti possono incrementare capacità e copertura
- Possibili varie tipologie di handover (handoff) tra le celle, tra le quali soft handoff, softer handoff e hard handoff
[modifica] Misconceptions
W-CDMA is based on the Direct Spread CDMA technique. Code Division Multiple Access has been developed by a number of companies, and in the US this is dominated by Qualcomm. However, the ITU standard is not a Qualcomm implementation, but one where Qualcomm has been invited to participate. Other standards, such as TD-SDMA/TD-CDMA has been developed by Siemens and Chinese companies independent of both the ITU and Qualcomm and bears no relationship with the US technology other than the name others have assigned to it.
In the mobile phone world, the term CDMA can refer to either the Code Division Multiple Access spread spectrum multiplexing technique, or the CDMA family of standards developed by Qualcomm, including cdmaOne (IS-95) and CDMA2000 (IS-2000 and IS-856).
The CDMA multiplexing technique existed long before Qualcomm used it for its IS-95 protocol. However, this protocol is now widely referred to as "CDMA" for its principal characteristic of using the CDMA multiplexing scheme to share multiple connections divided by different codes (PN sequences) over the same spectrum channel, as opposed to other spectrum division schemes
Qualcomm was able to introduce the first mobile protocol relying upon the CDMA multiplexing technique, associating the multiplexing technique with the name of the protocol.
W-CDMA is used in 3G as a political compromise made by the more advanced non-US companies to allow the US to participate in the rollout of 3G services. Since it now is part of the International Telecommunication Union's standards it is part of a larger suite of standards. W-CDMA is 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.
In summary:
- The term CDMA in the mobile world typically refers to the CDMA family of standards developed by Qualcomm. They are protocols, sets of defined specifications of mobile communications
- CDMA (the multiplexing technique) is used as the principle of the W-CDMA air interface protocol, as well as Qualcomm's CDMA protocols
- W-CDMA strictly refers to a mobile phone protocol with detailed specifications, as defined in IMT-2000
- The W-CDMA protocol was developed independently of the CDMA protocol developed by Qualcomm.
- The CDMA family of standards (including cdmaOne and CDMA2000) are not compatible with the W-CDMA family of standards that are based on ITU standards.
[modifica] Implementazioni
Il primo servizio commerciale W-CDMA al mondo,FOMA, è stato lanciato in Giappone dalla NTT DoCoMO nel 2001. FOMA non è compatibile con lo standard UMTS, ma lo sforzo per la adattare lo standard FOMA all'UMTS è stato cominciato dal Giappone stesso.
J-Phone Japan (ora confluita in Vodafone) inaugurò subito dopo il suo servizio W-CDMA, inizialmente chiamato "Vodafone Global Standard" e dichiarato compatibile con l'UMTS.Il nome del servizio è stato cambiato in "Vodafone 3G" nel dicembre 2004.
A partire dal 2003, Hutchison Whampoa introdusse gradualmente i suoi network UMTS.
Most countries have since the ITU approved of the 3G mobile service either "auctioned" the radio frequencies to the company willing to pay the most, or conducted a "beauty contest" - asking the various companies to present what they intend to commit to if awarded the licenses. This strategy has been critices to aiming to cash-drain the operators where some of the bigger ones have been close to bankruptcy to honour their bids or proposals. Most of them have a time constraint for the rollout of the service - where a certain "coverage" must be achieved within a given date or the license will be revoked.
Vodafone ha lanciato diversi networks UMTS: in Europa, nel febbraio 2004, in Nuova Zelanda nell'agosto 2005 ed in Australia nell'ottobre 2005.
AT&T Wireless (now a part of Cingular Wireless) has deployed UMTS in several cities. Though advancements in its network deployment have been delayed due to the merger with Cingular, Cingular began offering HSDPA service in December 2005.
Rogers in Canada is currently trialing HSDPA on W-CDMA at 1900MHz and plan the launch the service commercial in Q3, 2006.
TeliaSonera opened W-CDMA service in Finland October 13th 2004 with speeds up to 384 kbit/s. Availability only in main cities. Pricing is approx. 2€/MB. Template:Citeneeded
SK Telecom and KTF, two largest mobile phone service providers in South Korea, have each started offering W-CDMA service in December 2003. Due to poor coverage and lack of choice in handhelds, the W-CDMA service has barely made a dent in the Korean market which was dominated by CDMA2000.
In Norway, Telenor introduced W-CDMA in major cities by the end of 2004, while their competitor, NetCom, followed suit a few months later. Both operators have 98 % national coverage on EDGE, but Telenor has parallel WLAN roaming networks on GSM, where the UMTS service is competing with this. For this reason Telenor is dropping support of their WLAN service in Austria (2006).
Maxis Communications and Celcom, two mobile phone service providers in Malaysia, started offering W-CDMA services in 2005.
In Sweden, Telia introduced W-CDMA March 2004.
[modifica] Technology
W-CDMA may use unpaired or paired spectrum, though the current implementations of W-CDMA (i.e. FOMA and UMTS) all use a pair of 5MHz spectrum, one for uplink and one for downlink. See Spread spectrum for more information. FOMA uses 16 slots per radio frame, where as UMTS uses 15 slots per radio frame.
< da integrare
W indica WideBand che denota l'uso di wide carrier. CDMA indica che è basato sullo standard Code Division Multiple Access.
W-CDMA usa una portante di 5Mhz che è 25 volte quella di GSM.
È stato progettato per l'interwork con la rete GSM. La prima rete W-CDMA è stata sviluppata in Giappone.
Alcune volte UMTS viene anche chiamato W-CDMA. >
